51. Outflow velocity in a narrowing channel, mass velocity of flow movement. Outflow velocity in a narrowing channel. Let us consider the process of adiabatic outflow of matter. Let us assume that the working fluid with a certain specific volume (v1) is in a tank under

Lubricants

The main goal in developing environmentally friendly lubricants is to create a product with high biodegradability and low ecotoxicity. In developed Western countries

Currently, public and private companies are beginning to create a market for environmentally friendly lubricants. Most studies focus on the chemical composition of the product and assessing its biodegradability. When creating environmentally friendly lubricants, two main directions are considered: the production of base oils, the chemical nature of which determines the nature of the impact on the environment, and the synthesis of new additives - environmentally friendly, biodegradable and effective.

Currently, and likely in the future, three groups of base oils obtained from various raw material sources are of particular importance: hydrocracking petroleum oils (HC), polyalphaolefins (PAO) and esters, which are susceptible to rapid biodegradation in the environment. Base petroleum oils of traditional flow schemes will undoubtedly remain of great importance for an indefinitely long period, especially taking into account the fact that lubricants obtained on the basis of PJSC. polyalcohol esters, polyalkylene glycols and diesters have a cost 2-10 times more than petroleum products. Increased biodegradability is not an incentive to overcome price differences.

High performance characteristics and environmental friendliness of mineral oils are ensured by a set of certain qualities. First of all, this is their narrow fractional and favorable group chemical composition with a minimum amount of sulfur and nitrogen-containing compounds in base oils. The selection of raw materials, sorting of oils used in the production of high-index oils, and their separate processing are of paramount importance. In obtaining base mineral oils that meet environmental requirements, selective purification plays an important role,

significant carcinogenicity of the product. Currently, in the USA and Canada, over 70% of base oils are obtained through selective refining. The use of such modern processes as hydrocracking, hydrodewaxing, and hydroisomerization opens up wide possibilities. These technologies are described in detail in the work. The use of hydrocatalytic processes in combination with traditional methods of purifying oil raw materials with selective solvents improves the performance and environmental properties of base oils.

In table Table 1.4 provides comparative data on the chemical composition of base oils obtained using selective purification and hydrotreating. The latter significantly reduces the content of arenes, sulfur and nitrogen in oils.

Table 14

Effect of Hydrotreating on Chemical Composition

base oils

The introduction of hydrocracking and hydroisomerization processes into the production of base mineral oils makes it possible to obtain products that are highly biodegradable and do not contain arenes. Hydrocracking oils, according to the results obtained using modern testing methods, are non-toxic; the practical absence of arenes in them indicates a very low carcinogenicity and an insignificant probability of its growth through the formation and accumulation of polycyclic arenes during operation; absence of arenas and predominant

The use of isoparaffins ensures fairly high biodegradability.

In the USA, hydrocracking base oils have been produced since the end of 1996. . The installation in Finland is ready for start-up.

In Russia, VNIINP, together with the scientific and engineering center of OJSC LUKOIL and JSC LUKOIL - Volgogradneftepe-rerabotka, are conducting research on organizing the production of a number of scarce oils and bases using hydrogenation technologies, in particular, aviation oil MS-8 and aviation hydraulic fluid AMG -10.

Compared to mineral oils, synthetic oils in some cases have better environmental characteristics. The most important classes of synthetic oils from the point of view of environmental safety include oils made on the basis of synthetic esters, polyalphaolefins and polybutenes. They are non-toxic, non-carcinogenic, and characterized by low emissions of harmful substances.

Synthetic oils based on esters with additives have been widely used in gas turbine engines of civil and military aircraft since the 60s. At CIAM, together with VNIINP and the 25th State Research Institute of the Ministry of Defense of the Russian Federation, work is being carried out to create a high-temperature-high (up to 240 ° C) ester oil using effective additive compositions that are not inferior in quality to the best foreign oils. Analysis of scientific, technical and patent information on oils for aviation gas turbine engines shows that polyol esters remain the main class of compounds for use as base stocks [PO]. However, the situation is changing with the next generation of aircraft engines, as improvements in design and the need to reduce fuel consumption lead to increases in pressure, temperature and oil load.

The latter contributes to the risk of local carbon deposits. Therefore, for military aviation in the future, it is necessary to eliminate the use of ester-based oils. For this purpose, the most promising oils are a new type - based on perfluoroalkyl polyethers. According to modern data, these compounds are non-toxic and are even used abroad in perfumery and for the conservation of marble monuments of art and architecture.

Additives have a great influence on the environmental properties of lubricants. In aviation oils, traditional antioxidants and corrosion inhibitors such as dioctyldiphenylamine, phenyl-α-naphthylamine, benzotriazole, K-51 succinimide type additive and others that have proven themselves are widely used as additives.

All over the world, work has been underway for a long time to create new non-toxic and biodegradable products. In particular, since the 90s, the development of substitutes for chlorine-containing additives has been carried out. The issue of replacing lead compounds is important. Bismuth compounds are a substitute for lead. The development of a bismuth dithiocarbamate additive has begun.

Such additives have been developed as Mif-1 (an additive of a complex composition of the benzene type), Irganox L-57 (an antioxidant additive from Shiba, octylated and butylated diphenylamine), additive “X” (a fluorine-containing compound with functional groups of oxysulfite and hydroxycarbamate), etc.

The properties of known additives are improved. Thus, in tricresyl phosphate the content of the neutrotoxic ortho isomer is reduced to 3% (Russia), and in the USA tricresyl phosphate is produced that does not contain the ortho isomer.

Fire and explosion hazard of avnafuels and lubricants

Currently used aviation fuels and lubricants are fire hazardous products. In terms of fire, gas fuels are especially dangerous. Hydrocarbon fuels (jet fuels, gasoline, etc.) are classified as flammable liquids (flammable liquids). They are characterized by high heat production (-2000 ° C) and evaporation, they easily create flammable mixtures with air, which during combustion form a large amount of combustion products (large stoichiometric coefficient), which are good dielectrics and, therefore, can accumulate charges of static electricity.

Based on fire hazard, flammable liquids are divided into three categories. Flash point is used as a determining indicator (it is determined according to GOST 12.1.044-89):

Depending on the auto-ignition temperature (determined according to GOST 12.1.044-89), hydrocarbon fuels belong to one or another group of explosive mixtures of vapors with air:

We dare vapors of hydrocarbon fuels with air belong to the TTA explosion hazard category: it is determined according to GOST 12.1.011-78. This indicator is used when choosing the type of explosion-proof electrical equipment and when designing fire extinguishers.

The fire hazardous properties of the fuel are also determined by the concentration ignition limits (CFL) - the minimum and maximum content of fuel vapor in a mixture with air (oxidizer), at which a flame can spread through the mixture to any distance from the ignition source (GOST 12.1.044-89). An important characteristic of fuel is the temperature limits of ignition - temperatures at which saturated fuel vapors in the air are in concentrations equal to the lower or upper CPV, respectively. The minimum electrical discharge energy required to ignite the vapor-air mixture is important.

When assessing the fire hazard when handling fuels, the burnout rate is also determined - the amount of fuel burned per unit time from a unit surface; minimum ignition energy - to ensure electrostatic intrinsic safety. The interaction of burning fuel with water-foam extinguishing agents is assessed (according to GOST 12.1.044-89).

A fire is often preceded by an explosion of a gas-air mixture. When air mixtures explode in pipes of large diameter and length, detonation combustion can occur, propagating at a speed of 1100-1400 m/s. The pressure can increase to 0.8 MPa or more. A fast-acting shock wave causes a sharp increase in pressure, temperature and density of the combustible mixture, which, in turn, accelerates the chemical combustion reactions and enhances the destructive effect.

Explosive concentrations of fuel vapors with air can form over a wide range of temperatures and especially in enclosed spaces and containers. The nature and content of precautionary measures are regulated by special departmental instructions. The essence of the precautions is to prevent the occurrence of a heating source, especially a source of open fire, in places where explosive mixtures are formed. One of the most dangerous sources of open fire is the discharge of electrostatic potentials through a vapor-air environment and the formation of a spark upon impacts of solid bodies. The occurrence of high electrical potentials in fuel is explained by its electrophysical properties. They can be characterized by their ability to accumulate charges in a volume (electrolysability) and charge relaxation properties (the electrical wire is on them).

In table 1.5. indicators characterizing the fire hazardous properties of aviation fuels are given.

Table 1.5

Fire hazardous properties of aviation fuels

1 Calculated by additivity.

^Calculated using equations (47) and (48) GOST 12.1.044-89 based on the initial boiling point -10/-4°C.

°In the numerator - in a closed crucible, in the denominator - in an open crucible. a ‘Flame propagation limits according to GOST 10277-89.

Normal flame propagation speed

The speed of flame propagation in a combustible mixture depends on the conditions of its definition and reference. For a comparative assessment of fuels according to this characteristic, the normal flame propagation speed is accepted - this is the linear speed of movement of the combustion zone in relation to the fresh homogeneous combustible mixture in the direction normal to the flame front. The speed of flame propagation under such conditions for a given composition of the combustible mixture can be considered as a physicochemical characteristic that depends only on pressure and temperature.

Experimentally, the normal flame propagation speed is determined according to GOST 12.1.044-89.

At a temperature of 20° C and a pressure of 0.101 MPa in hydrocarbon-hydro-air mixtures, the maximum speed u is achieved at a fuel concentration in the mixture of ~1.15 C st x (Fig. 1.24), i.e.

at a - 0.87 and at the number of carbon atoms in the hydrocarbon n > 7, it is -39-40 cm/s (Fig. 1.25). The minimum normal flame propagation speed and mass combustion speed achieved at the concentration limits of flame propagation under normal conditions are 4-6 cm/s and (5-7) 10° g/(cm 2 s), respectively.

In the absence of experimental data, the normal flame propagation speed should be selected by interpolation from the values ​​of u for mixtures with similar physicochemical properties, or empirical equations should be used. Simple and convenient equations were proposed by A.S. Pre-driver:

• (1.3)

t=t p +B(St-C^(C in -C t),

where u is the propagation speed in cm/s; t - mass combustion rate of the mixture, g/(cm 2 s); and 11P, t„ - limiting (minimum) values ​​of flame propagation speed; С„ and Сн - concentration of fuel in the mixture at the lower and upper concentration limits of flame propagation; A and B are coefficients determined from one experimental point.

Rice. 1.24.

flame propagation depending on the molar stoichiometric coefficient of excess air Lm:

• - paraffin; * - olefinic; ° - acetylene; D - neftene; © - dpolefnovye; ° hydrocarbons with C p 11 2 „ cycles
• 1 2 3 4 5 b 7 p

Rice. 1.25. The maximum normal speed of flame propagation in a fuel-air mixture depending on the number of carbon atoms in a hydrocarbon molecule (P=0.101 MPa, 1=20°C, open glass pipe: length 57 cm, diameter 2.5 cm): - paraffin; * - olefinic;

° - acetylene; D - naphthenic; c - dnolfipovye; o cyclic (C P P2„);

1 - gasoline [116]; 2 - benzene

The functional relationship between the flame propagation speed and the fuel concentration C t at C t C* t (but given by EMIN) can be represented by the equation:

• - = 11 p

/ s g -s; l

"s t -s "t"

where m and, and p- normal flame propagation speed

at fuel concentrations in the mixture C t and S*t, cm/s; and pp- Same,

at the lower concentration limit of flame propagation, cm/s.

Approximate course of the curve and n - /(S t) in a mixture of complex

composition can be constructed using three reference points corresponding to the lower and upper concentration limits and the maximum flame propagation speed. Fuel concentrations and flame propagation rates must be known for these points.

S t i values and and for the specified points are calculated

according to the following method. Each complex mixture of flammable gases is represented as consisting of a corresponding number of simple mixtures. The calculation of the composition at concentration limits and at the point of maximum speeds is carried out according to the mixing rule, based on the concentration limits and the composition of “maximum mixtures”. The corresponding design equation has the form:

C] + C* 2 + Su-y....

• -I---r...
• (1.5)

Where b- fuel concentration at the CPRP or in the mixture with the maximum flame propagation speed, % (vol.); C, C 2, C 3,... - concentration of simple gases in a complex mixture,

(c, + C 2 + C 3 +... = 100%); b|, b 2, b 3> ... - concentration of gases in simple mixtures at the CPRP or in mixtures with And and, % (vol.).

The value of the maximum normal flame propagation speed in the mixture is calculated by the equation;

C, g/, + C2i2 + C3i3 +

С, + С 2 + с 3 4-...

• (1.6)

where C*, C 2, C 3 - the content of simple mixtures in a complex mixture with a maximum flame propagation speed,% (vol.); And*, and 2 , and 3 - maximum flame propagation speeds in simple mixtures, cm/s.

To calculate other curve points and and= /(C; .) you should set several arbitrary values ​​of the flame speed, find the concentration b in a complex mixture using equation (1.5), in which C, C 2, C 3 are given by the composition of the mixture.

This calculation method is applicable to mixtures of gases of related nature (for example, methane-propane). This technique is not applicable to a mixture of S P N Sh with Nz and CO.

The mass combustion rate is directly proportional to the absolute preheating temperature of the mixture and can be calculated using the equation:

where w, then and t „ R e o- mass combustion rate of the mixture at temperatures T, To and T Prev, respectively, g/(cm -s).

If T»T is pre D, then

The dependence of the maximum normal flame propagation speed on temperature and pressure is approximately described by the equation:

And' =u1(T/273) 2 ?(/’/10 5)", (19)

where i'o is the maximum normal flame propagation speed at a temperature of 293 K and a pressure of 0.101 MPa, cm/s; T is the flame temperature l, in K; P - pressure, in Pa; n - exponent, ns dependent on pressure in the range MO 4 + 5-10 5 Pa; for the air-fuel mixture n = -0.3 -*? -0.4; for hydrocarbon-oxygen mixtures P = -0.1 -5- 0.

Maximum normal flame propagation speed depending on the oxygen concentration in the oxidizer P R P Uu P

giil = \%ig" 0 + B-

where Г„ I! But - at y, n y^0, cm 2 /s; B is the coefficient determined from experimental data (for propane B ~ 0.22); u/t- extremely low concentration of oxygen in the oxidizer.

The value of u*„ at different concentrations of oxygen in the oxidizer 1 //"P when the mixture preheating temperature changes from 310 to 422 K, it can be determined by the equation:

":=at; (sch, -s), (MO

where u*„ - in cm/s; T - in K; A, C ip - are found according to experimental data, their values ​​for propane, isooctane and ethylene are given below:

Concentration and temperature limits of flame propagation

Concentration limits of flame propagation (CFLP) in a combustible mixture are the maximum minimum and maximum concentrations of fuel in the mixture at which flame propagation is still possible (lower and upper limits, respectively). They depend on the chemical activity of the fuel, the concentration of the oxidizer and inert impurities, thermal conductivity and heat capacity of the mixture, temperature and pressure. CPRP for suspension fuels, based on their physical and chemical properties, are determined by the dispersion medium. Determination of CPRP for homogeneous combustible mixtures is carried out according to GOST 12.1.044-89: according to clause 4.11 experimentally and according to clause 4.12 - by calculation.

According to GOST 12.1.044-84, the concentration limits of flame propagation are defined as

where C„ (i) is the lower (upper) KPRP, % (vol.); R- stoichiometric coefficient (number of moles of oxygen per mole of fuel); A And b- universal constants, their meanings are given below:

For fuels S P N Sh

P = p + t/ 4.

Calculation error: for the lower limit 0.12; for the upper 0.40 at (3 p > 7.5. Data on KPRP depending on R(% vol.) are given in table. 1.6 (GOST 12.1.044-84).

Table 1.6

Concentration limits of flame propagation (lower and upper) of vapors and gases in air

There are other known equations for calculating the CPRP, namely:

• 4.76-(N-1) + ! ’
• (1.14)

• 4.76/U +4 '
• (1.15)

where C„ and C in - in about.); N is the number of oxygen atoms required for complete oxidation of the fuel.

For fuel С„Нт

• (1.17)
• 3,74 10 5

where C„ - in % (vol.); ()n- lower molar heat of combustion, kJ/kmol.

For hydrocarbon fuels SpN t at 3 p 10, the calculation error is ±15%.

If the CPRP for individual fuel components is known, then its lower CPRP is recommended to be calculated using the equation:

where C and C„ are the concentrations of the 1st component in the mixture and at the lower limit, % (vol.).

For fuels C p N t as a first approximation a k ~ a p - 1.42. Recalculation, and C in in a n And a n produced:

where C„(th) is the concentration of fuel at the lower (upper)

KPRP, % (vol.); Mt and Mo-molecular weight of fuel and oxidizer; Lо - in kg of oxidizer/kg of fuel; b m - molar stoichiometric coefficient, mol of fuel/mol of fuel.

Recalculation of the lower CPRP for different temperatures can be carried out using the equation:

L II l

T - 293

where T„ is the temperature (in K) of the combustion products of the mixture, in which the fuel concentration at 293 K corresponds to the lower CPRP (to a first approximation, T„ for a hydrocarbon-air mixture is 1600-1650K); C„ and C„ - fuel concentrations corresponding to the lower concentration limit at temperatures T and 293 K, % (about.).

Equation (1.20) is valid over a wide temperature range, but it cannot be used at temperatures close to the auto-ignition temperature.

The temperature of combustion products at the lower CPRP can also be calculated using the equation

• (A.+1)-s_s
• (1.21)

stech

where T„ in K; Tc is the temperature of the mixture before combustion, K; Cstskh - concentration of fuel in a mixture of stoichiometric composition, % (vol.);

Срш - average isobaric heat capacity of combustion products at temperature T,„ kJ/(kg °C).

CPRP practically do not depend on the size of a cylindrical reaction vessel if its diameter is more than 50 mm, and for a spherical one - if the volume exceeds 2000 cm 3.

To determine the CPRP and the optimal composition of the hydrocarbon-air mixture, the graphs shown in Fig. 1.26.

С„,с,%(ov.)

Rice. 1.26. Concentration limits of flame propagation in hydrocarbon-air mixtures (Cb and C") and hydrocarbon concentration in mixtures of stoichiometric composition (Cc, ") depending on the molar stoichiometric coefficient 1^ m at I20 ° C P = 0.101 MPa:

• - paraffin; a - olefinic;
• ? - naphthenic; ? - aromatic

Combustible mixtures of fuel vapor and air in the space above the fuel can only form in a certain temperature range. The minimum temperature at which a combustible mixture capable of stationary combustion when ignited from an external source can still form in a closed volume of the space above the fuel is called the lower temperature limit; it corresponds to the lower CPRP. The highest temperature at which the mixture of vapors with air in the space above the fuel still retains the ability for stationary combustion is called the upper temperature limit; it corresponds to the upper CPRP. Experimental determination of the temperature limits for the formation of explosive mixtures is carried out in accordance with GOST 12.1.044-89 (clause 4.12), calculation - according to the appendix of the same standard.

The temperature at which the lower temperature limit for the formation of an explosive mixture at atmospheric pressure is reached is usually identified with the flash point. At the flash point, only the resulting steam-air mixture burns, but the combustion process does not stabilize.

Calculation of temperature limits for the formation of flammable mixtures is reduced to the following operations. Initially, at a given total pressure P and known values ​​of the oxidizer (air) excess coefficient corresponding to the lower and upper CPRP (A n and a c), using equation (1.22) they determine

partial pressure of fuel vapor Р t:

X | 0.232 o? 0 Mt " ?« -

where P is the total pressure, Pa; C - stoichiometric coefficient, kg oxidizer/kg fuel; A - oxidant excess ratio; Mt is the mass of a mole of fuel, kg/kmol; Mo is the mass of a mole of the oxidizing agent, for air Mo = 28.966 kg/kmol; at/ 0 - concentration of oxygen in the oxidizer by mass.

Rice. 1.27.

Then, using tables or graphs Pts.p.=^(0 (where P is the saturated vapor pressure of the fuel), temperatures corresponding to the calculated values ​​of Pt-

If the concentration limits for the formation of flammable mixtures are unknown, then the temperature limits can be approximately calculated using the equation:

1,15 1*(7,5 R d) - 0.239 3.31

where I - at 0 C; 15% - boiling point of 5% fraction, 0 C; RT - fuel vapor pressure at the CPRP (Р„ or Р„), kPa; 8„с„ is the entropy of evaporation at a temperature of 15% and atmospheric pressure (accepted according to the graph in Fig. 1.28).

Rice. 1.28.

60 80 100 120 140 160 180 1,°С

Ignition energy and flammability concentration limits

The flammability of a homogeneous combustible mixture by an external heat source is characterized by concentration limits and the energy required for its ignition.

Concentration ignition limits (CFL) are those limiting concentrations of fuel in a mixture at which a local ignition source (electric discharge, heated body, flame) is capable of ensuring the propagation of the combustion process throughout the entire volume of the mixture. By analogy with KG1RP, lower and upper CPV are distinguished. They depend on the physicochemical properties of the fuel and oxidizer, the energy and type of ignition source, its location, etc.

According to Ya.B. Zeldovich, the energy required to ignite a homogeneous combustible mixture is determined by:

R1-T with g (T 2 -T s)

where рс and Тс are the density and temperature of the mixture; T g - temperature of combustion products in the initial combustion source; L 7 - coefficient of thermal conductivity of combustion products at Тg; u - normal flame propagation speed; S RT - average

mass isobaric heat capacity of gas in a spherical layer of 8 T surrounding the spherical initial combustion site; 5, - thermal width of the flame front.

Equation (1.24) is also applicable to the case of ignition of a moving mixture if the thermal conductivity coefficient L 7 replace with the turbulent exchange coefficient IV/"(/ - scale

turbulence, V/*- pulsation speed), and the value cn - the speed of flame propagation in a turbulent flow.

Mixture composition corresponding to the minimum of the O = curve KS,), is usually called optimal. For normal paraffin hydrocarbons, the fuel concentration in a mixture of optimal composition at 25°C can be determined from the relationship:

• 1 - methane; 2 - ethane; 3 - propane;
• 4 - n-butane; 5 - n-hexane; 6 - n-heptane;
• 7 - cyclopropane: 8 - diethyl ether;
• 9 - benzene

As the oxygen concentration in the oxidizer increases, the optimal composition of the combustible mixture shifts to the region of lower fuel concentration.

The dependence of the optimal (minimum) ignition energy on the pressure and temperature of the combustible mixture is described by the equation [114]:

O-opt

where Oopt is the ignition energy at R and T, J; Cb is the ignition energy at T = 273 K and P = 10 5 Pa.

Equation (1.26) has a good correlation with experimental data.

The relationship between the optimal ignition energy and the oxygen concentration in the oxidizer is described by the equation

where (С? 0 „„,) у/ =/ is the optimal value of the ignition energy of the fuel-oxygen mixture; ~ volume concentration

oxygen in the oxidizer; n is an exponent, it is close to unity (n ~ 0.8).

Experienced data for methane, ethane and propane when changing c/x, from 0.1 to 0.21 and pressures from 0.98 to 19.6 kPa confirm equation (1.27). Apparently, it remains valid for mixtures of hydrocarbons.

Fuel concentrations at the ignition limits can be calculated if the CPRP and the values ​​of () opx and C opt are known using the equations

o.5(s; + s;)=C_ +0.15(C.(1.29)

Equations (1.28) and (1.29) are valid for --

Denoting the right-hand sides of these equations, respectively, B and 0.5A, we obtain

WITH" - WITH" = B and C"+ C" = A . (1.30)

C" = 0.5(L-B) and C; =0.5 (A + B). (1.31)

In the given equations: C in and C n are the concentrations of fuel in the mixture at the upper and lower CPRP; C in and C", - the concentration of fuel in the mixture at the upper and lower CPV with the igniting energy of a capacitive electric charge; C opt - the concentration of fuel in the mixture corresponding to O opx.

Equations (1.28) and (1.29) are based on the results of experimental studies shown in Fig. 1.30.

• (s;-s > ;)-2s opt

Rice. 1.30. The ignition region of mixtures C p N P1 +02+^ depending on the ignition energy

The concentration limits of ignition depend on the flow rate, approaching each other as it increases (Fig. 1.31 and 1.32).

The effect of flow speed on ignition energy is correctly described by the equation:

(2 = (?o + Au"k (1.32)

where (Zo is the ignition energy of the stationary mixture, 10" 3 J; XV is the flow velocity, m/s; A is a coefficient established experimentally.

Rice. 1.31.

Rice. 1.32. Excess air coefficient a at the CPV of a gasoline-air mixture depending on the flow speed? and pressure P [114]:

Flash point and auto-ignition temperature

The flash point is the minimum temperature at which the resulting steam-air mixture can be ignited by an external heat source, but the combustion process does not stabilize. The flash point is determined experimentally in an open or closed crucible according to GOST 12.1.044-84 (clauses 4.3 and 4.4). The calculated determination of flash point is carried out according to GOST 12.1.044.84 (clause 4.5).

The flash point is 10-15°C below the temperature limit for the formation of a flammable mixture capable of spreading flame.

To approximately determine the flash point, you can use the dependence presented in Fig. 1.33.

Rice. 1.33. Flash point 1 V cf of jet fuels and B-70 gasoline depending on the saturated vapor pressure P„ p at 1 = 40 ° C in a closed crucible (62]: o - fuels of different compositions; - generalizing curve

Self-ignition is the process of igniting a combustible mixture without contact with a flame or a hot body. The minimum initial temperature sufficient for self-ignition of a combustible mixture is called the self-ignition temperature. It depends on the chemical nature of the fuel, the composition of the air-fuel mixture, pressure, the adiabatic nature of the self-ignition process, the presence of catalysts and oxidation inhibitors and other factors.

The time interval between the moment the combustible mixture reaches the auto-ignition temperature and the appearance of a flame is called the auto-ignition delay period. When supplying liquid fuel, it covers the process of atomization, heating and evaporation of fuel droplets, diffusion of fuel vapor and oxygen, and finally chemical reactions.

The temperature and the auto-ignition delay period are related to each other by the relationship:

Where E- effective activation energy, kJ/kmol; E=8.31419 kJ/(kmol K) - universal gas constant; T- auto-ignition delay period at temperature T.

The tendency of hydrocarbons and their mixtures to self-ignition is characterized by the minimum temperature of self-ignition obtained under adiabatic conditions, when the duration of exposure of the combustible mixture at given initial conditions does not limit the process of self-ignition.

The minimum auto-ignition temperature is uniquely determined by the structure of the molecule. So, for example, for paraffin hydrocarbons, 1 св is in direct connection with the effective length of the carbon chain bc, which is calculated by the equation:

• 21>GLG,
• (1.34)

where r is the number of CH 3 groups in the molecule; k is the number of carbon chains beginning and ending with the CH 3 group, m* is the number of possible chains containing b^ carbon atoms. The dependence 1 sv = A(bts) is shown in Fig. 1.34.

Rice. 1.34.

• 1 - CH 4; 2 - C 2 H 6; 3 - C 3 H"; 10 - n - C 4 H 10; 11 - n - C 5 H 12;
• 14 - n - S L N M; 15 - n - C7H16; 16 - n - SkNsch; 17 - n - SdN 2 o;
• 18 - n - S| 0 H 22 ; 19 - n - S, 2 N 2Y; 21 - n - C14H30; 22 - n - C|^H 3 4

The self-ignition temperature of hydrocarbon mixtures does not obey the additivity rule; it is, as a rule, lower than calculated based on this rule.

Data on the self-ignition temperature of air-fuel mixtures of optimal composition depending on the number of carbon atoms in the hydrocarbon molecule (for jet fuels in the given formula) are presented in Fig. 1.35. The influence of pressure and oxygen concentration in the oxidizer is illustrated by the data shown in Fig. 1.36.

Rice. 1.35. Dependence of the self-ignition temperature of fuel-air mixtures of optimal composition on the number of hydrocarbon atoms n in the molecule at P = 0.101 MPa [124]; t - auto-ignition delay period; t L - “o; R.T. - jet fuels (in the given formula) - paraffinic; a-olefinic; ? - naphthenic hydrocarbons

Rice. 1.36. Dependence of the self-ignition temperature of T-6 fuel on pressure P and oxygen concentration in the oxidizer f 0 2 (according to V.V. Malyshev):

2 = 0 2/(°2+L, g)

The auto-ignition temperature is determined by the ability of the fuel to form combustible mixtures in the vapor phase. It follows from this that the auto-ignition temperature of the suspension

of fuels is determined by the dispersion medium and thickener. The dispersed phase takes part in the self-ignition process only in terms of heat absorption when the suspension is heated to the self-ignition temperature of the liquid phase.

Explosion pressure in a closed volume

Explosion pressure is the highest pressure that occurs during a deflagration explosion of a steam-air mixture in a closed volume at an initial pressure of 0.101 MPa. The rate of pressure increase during an explosion is the derivative of the explosion pressure with respect to time (s1P/(1t) on the ascending section of the P=Y dependence T).

Experimentally, the maximum explosion pressure and the rate of pressure increase during the explosion of steam-air mixtures are determined according to GOST 12.1.044-89 (Appendix 8). The calculated determination of the rate of pressure increase during an explosion is carried out according to GOST 12.1.044-89 (Appendix 12).

The explosion pressure is determined by:

where Рвзр - explosion pressure, Pa; Р„ - initial pressure, Pa; T„, and T p.s. - initial temperature and temperature of combustion products. TO; spike - the number of moles of combustion products and the initial mixture.

The maximum rate of pressure rise (in Pa/s) is calculated using the equation

where Po is the initial pressure. Pa; u„ - normal flame propagation speed at Po and To m/s; T is the initial temperature of the mixture, K; r - bomb radius, m; P -Р m /Р 0 - reduced maximum explosion pressure; k is the adiabatic index for the test mixture; e- thermokinetic indicator, depending on and n, pressure and temperature; if value e unknown, it is taken equal to 0.4.

The average rate of pressure rise (in Pa/s) is calculated using the equation:

"s1R _ ZR 0 and ‘(i-)-i k * e ^t) with r/(l,k,e)

Where ^tg,k 7 e)-function, its value is found using the nomogram in Fig. 1.37.

Rice. 1.37. Function Dependency /(p, k.s) from reduced pressure n=R/R K,„ adiabatic index To and thermokinetic indicator With test mixture (appendix to GOST 12.1.044-84)

Values tg and k are found by thermodynamic calculation or. in case of impossibility of calculation, accept To= 9.0 and k = 1.4.

Emergencies and emergencies

Accident is a dangerous man-made incident that creates a threat to the life and health of people at an object, a certain territory or water area and leads to the destruction of buildings, structures, equipment and vehicles, disruption of the production or transport process, as well as damage to the environment (GOST R 22.0 .05-94).

An accident is a destructive uncontrolled release of energy or chemically (biologically, radiationally) active components. Depending on the source of occurrence, emergencies of a natural, man-made and natural-technogenic nature are distinguished. In Fig. Figure 1.38 shows the relative increase in the number of natural, man-made and natural-man-made accidents and disasters in Russia. In Fig. Figure 1.39 shows the dynamics of the number of all man-made accidents in Russia for the period 1990-94. The figure shows that the increase in the number of emergencies does not occur smoothly, but spasmodically, with surges occurring in periods immediately after social upheavals (August 1991, October 1993).

The number of man-made emergencies, including in aviation, has increased especially sharply in recent years.

Potential objects of accidents are aircraft, as well as storage facilities and warehouses for explosive and fire-hazardous petroleum products located on the territory of the airport, refueling and maintenance points, and repair points. The cause of emergencies may be oil leaks

products through sealing units of shut-off valves, transfer pumps, pipelines and filling devices; through ventilation of the gas space of tanks; overflowing tanks, cisterns and tanks; tank cleaning; corrosive destruction of tanks and communications.

Various containers are used for storing and transporting petroleum products. The safe operation of containers is determined by their strength. However, accidents at such facilities can occur due to shortcomings of the existing control and monitoring system for the condition of structures, as well as the lack of regulatory and technical documentation.

The safety of operation of petroleum product storage facilities must be ensured during design, construction and operation. This approach is dictated by an analysis of acceptance and operational documentation, as well as the causes of emergency situations. An important task, the solution of which will improve the reliability of operating storage facilities, is to carry out scientifically based comprehensive technical examinations and equip them with a system for diagnostics and operational monitoring of the condition of metal, foundation, heat-insulating structures and technological equipment.

For the safe management of petroleum product flows, the serviceability of pipeline process fittings is of great importance: shut-off, throttle, and safety devices; control valves; reverse action fittings (to prevent the possibility of movement of the product opposite to the working one); emergency and shut-off valves (for automatically shutting off the flow to the emergency area or shutting it off), condensate drainage, etc.

Number of accidents

Rice. 1.38.

• 1 - pg "relatives;
• 2 - natural-technogenic;
• 3 - man-made

Rice. 1.39.

When the equipment is depressurized, the product leaks out and quickly evaporates to form a concentrated

tions of explosive and fire-hazardous gas-vapour-air mixtures. Emergency emissions or leaks of vapor-gas mixtures lead to the formation of clouds that can detonate. The detonation of steam-gas and air-dispersed systems is considered in the work. The occurrence of detonation in large clouds is explained by the following mechanisms. The first of them takes into account the possible effect of intense thermal radiation from a long flame in clouds previously mixed by turbulent gas flows.

The second mechanism for the occurrence of detonation involves the acceleration of flames in large clouds due to the difference in acceleration of elementary volumes of burnt gas and fresh mixture in a turbulent flame. This difference arises under the influence of average pressure gradients in the flame due to the different buoyancy of elementary volumes of gas of different densities, which leads to additional turbulization of the flow and the appearance of feedback. This positive feedback mechanism, determined by the density difference in different zones of the cloud, can significantly intensify the acceleration of the flame.

Ignition is accompanied by a bright high-temperature flash. The most acceptable geometric figure of an ignited vapor-gas mixture is the figure of an irregular ball or ellipse (fireball). A fireball (FB) is understood as a product of sudden evaporation or leakage of gasified fuel (or gas), accompanied by its flash and subsequent normal or deflagration combustion. For numerous hydrocarbon combustible linear and cyclic discharges in the density range from 700 to 1000 kg/m 3 in, the following ratios are given for the diameter of the fireball:

where M is the mass of fuel in fuel capacity, kg;

Tf - actual temperature in the OS (in the cloud), 0 C;

Trep - reference (reference) temperature, °C.

The range of the coefficient 4.2n-5.3 depends on the type of fuel and the conditions of cloud formation.

For the lifetime of a cloud during its natural combustion, the expression has the form:

t = 0M-*1m-1±.

These dependencies are shown in Fig. 1.40 and 1.41.

Rice. 1.40.

Rice. 1.41.

There is a great danger of explosion of steam-gas mixtures in a closed volume. In table 1.7 shows the limits of detonation of hydrocarbons in air in a closed volume and open space, which indicate a greater danger of an explosion of gas or vapor-gas mixtures in a closed volume. This is explained both by the processes of accelerating the reaction due to the enhancement of autocatalysis, and by the enhancement of reflected waves when the ary process has begun and due to a number of always existing kinetic reasons. The increased ease of excitation of detonation in vessels is due to the ability of the walls to generate turbulence in the flow in front of the flame, which accelerates the transition of combustion to detonation.

Detonation limits of hydrocarbons in air

An explosion of the accumulated gas mixture can occur under the influence of an accidental spark. When openly loading oil products, an explosion due to a static discharge is also possible, in particular, in the absence of a grounding device. The most common cause of explosion is a spark, including as a result of the accumulation of static electricity. An electric spark can occur without any conductors or networks at all. It is dangerous because it appears in the most unexpected places: on the walls of tanks, on car tires, on clothing, during impact, during friction, etc. Another reason for the explosion is the negligence and indiscipline of workers.

Where the formation of steam-gas mixtures is possible, it is necessary to provide reliable lightning protection, protection from static electricity, and take measures against sparking of electrical appliances and other equipment.

In accidents involving explosions, surrounding objects are destroyed and people are injured. The destruction is a consequence of the phantom action of the explosion products and the air shock wave. In this case, the main damaging factors are the shock wave, light-thermal radiation and toxic loads (carbon monoxide). People located at a distance of 5 m receive 1st degree burns and other injuries.

Accidents involving explosions are often accompanied by fires, which can cause catastrophic consequences and subsequent more powerful explosions and greater destruction. The causes of fires are usually the same as explosions. In this case, an explosion can be a cause or a consequence of a fire, and vice versa, a fire can be a cause or a consequence of an explosion.

A fire is a spontaneously developing fire not provided for by technological processes. Combustion of petroleum products can occur in tanks, production equipment and during spills in open areas. In the event of a fire of petroleum products in tanks, bursts, boiling and release may occur, and as a result, spills of hot liquid. The greatest danger is represented by emissions and boiling of petroleum products, which is associated with the presence of water in them and is characterized by violent combustion of the foamed mass of products. During boiling, the temperature (up to 1500° C) and flame height increase sharply.

To assess the degree of damage to an object, they usually use the so-called threshold curve, which connects the flux of heat and light energy μ (heat flux) and the total energy O falling per unit surface (Fig. 1.42).

Rice. 1.42.

For long times of thermal exposure, exceeding the time of possible undamaged existence of the object, the threshold of damage will be determined exclusively by the thermal (thermoluminous) flux I. With pulsed effects of short exposure, the threshold will be determined mainly by the energy O. Values ​​of I and O exceeding the threshold will cause unconditional damage to the object.

If either I or O is less than their threshold values, then there is no typical lesion and only mild discomfort is possible. For example, when the radiation exposure time increases from 0.5 to 2 s, i decreases from 120 to 30 units, i.e. with a slight increase in O even with an increase in exposure time by 4 times, damaging injuries

are absent, and a person can only feel a slight discomfort.

However, the amount of total O energy incident on the target during the same period of time increases from approximately 10 to 25 units. (^.

Thus, line K, responding to interrelated changes in I and O, forms a zone (area) of damage, indicated in the figure to the right of line K.

One of the most unpleasant consequences of radiation damage is a burn to the “rods” and “cones” of the eye.

In Fig. Figure 1.43 shows the dependence of I on m, as well as T on m, which determines the areas of tolerable and intolerable pain during the formation of thermal light burns of varying degrees. The criterion implemented in the figure below is based on the fact that during thermal irradiation, unbearable pain occurs when the temperature of the skin layer with a thickness of about 0.14-0.15 mm (under the surface of the upper epithelial layer) reaches or exceeds a temperature of 45 ° C.

After eliminating the radiation (but not more than 20-30 s), the sharp pain subsides and then, as a rule, disappears altogether. An increase in the temperature of this layer by 4-10 degrees or more causes painful shock and obvious skin burns.

The area of ​​tolerable pain shown in the graph is determined by the fact that at the moment of exposure to radiation, a biological protective reflex occurs, causing an increase in blood flow from the peripheral parts of the body, which prevents a local increase in temperature to a threshold level. When exposed to a high dose of thermal pressure, this physiological mechanism can no longer provide the necessary heat removal, and the body undergoes pathological and sometimes extreme thermal loads. From the nature of the lines in Fig. 1.42 it is clear that there is a certain quantitative

dose of radiation q and temperature T, which causes thermal injury and unbearable pain when this dose is provided with the necessary exposure time.

Duration of exposure, s Fig. 1.43. Limits of heat-light injury

Accidents with aircraft (aircraft) occur mainly due to unit malfunctions, primarily engine failure, terrorist attacks, fires, and are accompanied by explosions. The explosion can occur in the air or upon impact with the ground. When an aircraft falls on residential areas, people, structures, etc. may be harmed. Examples of aviation emergency situations and their analysis are given in the works.

One of the main dangers in aviation is the possibility of a fire during an emergency landing. Fuel leaking from damaged tanks can be ignited by a spark caused by friction or hot

surfaces or open flames. The resulting combustion center quickly spreads across all zones in which the steam/fuel air ratio is within the flammability range. One method of reducing fire hazards is to use thickened fuels, which flow more slowly and are less volatile than conventional liquid fuels. If a tank with thickened fuel is damaged, both the rate of fuel spreading and the rate of formation of flammable aerosols are sharply reduced. This allows you to increase the period of time during which passengers can be evacuated.

Emergencies and emergency situations cause great material damage and aggravate environmental problems. In accidents accompanied by explosions and fires, there is a strong mechanical, thermal and chemical impact on the environment. At the same time, emissions of pollutants increase sharply; the surface of the earth becomes clogged with LL debris, fuel residues, and combustion products; significant damage is caused to the natural landscape, flora, and fauna; pastures and fertile soils are dying.

Mechanical impact is characterized by disruption of the top (fertile) layer of soil due to surface and deep destruction, exposure to explosion energy (shock wave); disruption of grass cover, damage or death of bushes, trees and other vegetation. The structure of the upper fertile layer, gas and water exchange, and capillary structure change.

Measures aimed at improving safety in emergency situations are usually divided into two categories. The first includes activities carried out after the emergence of

emergency situations. El1 measures are usually called operational, and they essentially boil down to protecting the population and eliminating the consequences of emergencies. The second group of measures includes activities carried out in advance. These include increasing the reliability of process equipment, reducing stocks of hazardous substances at sites, removing a hazardous facility, and taking early measures to protect people.

Of great importance is the active flight safety system (AFS), which is an element of the on-board “intelligent” pilot support system, known in aviation practice as the “pilot assistant”, designed to work in both normal and abnormal flight situations. ASOBP issues warning signals about a threat to flight safety, as well as promptly advising information in the form of “tips” for controlling the aircraft and its onboard complex in order to prevent the aircraft from entering critical flight modes. To prevent collisions with the earth's surface and between aircraft, ASOBP forms spatial “disengagement” trajectories.

One of the effective areas of work to prevent aviation accidents is a complete, in-depth and objective investigation of events that have already occurred and, on this basis, the development of recommendations to prevent their recurrence.

The effectiveness of such work depends not only on a sufficient level of resources, but also on the exhaustive powers of the body conducting an independent investigation, allowing it to influence any area of ​​the air transport system (production, design, testing, certification, operation, repair, regulatory framework, etc.) .

Standard 5.4. Annex 13 to the Convention on International Civil Aviation states: “The Aircraft Accident Investigation Authority shall be granted independence in the conduct of the investigation and unrestricted powers to conduct it.” This requirement is also implemented in the Russian Investigation Rules, approved by the Government of the Russian Federation. The Interstate Aviation Committee (IAC), formed by the Agreement, received from the heads of state and government of the CIS the right to independently investigate aviation accidents. Since 1992, IAC specialists have investigated more than 270 aviation accidents, including more than 50 international ones, including investigations into events involving Western-made aircraft.

There are currently seven such specialized aviation accident investigation centers in the world (USA, France, UK, Canada, Germany, Australia and IAC).

Of no small importance is the provision of information to states with data on failures and malfunctions of aircraft and erroneous actions of crews. Using this data, the aviation authorities of each state can take preventive measures.

Above the surface of a liquid or solid substance at any temperature there is a vapor-air mixture, the pressure of which in a state of equilibrium is determined by the pressure of saturated vapors or their concentration. With increasing temperature, the saturated vapor pressure will increase exponentially (Clapeyron - Clausis equation):

where Р n „ - saturated vapor pressure, Pa; Q„ C11 - heat of evaporation, kJ/mol; T - liquid temperature, K.

For any liquid, there is a temperature range in which the concentration of saturated vapors above the mirror (liquid surface) will be in the ignition region, i.e. NKPV

To create LTPV of vapor, it is enough to heat not the entire liquid, but only its surface layer, to a temperature equal to LTPV.

In the presence of an ignition source, such a mixture will be capable of ignition. In practice, the concepts “flash point” and “ignition temperature” are more often used.

Flash point is the minimum temperature of a liquid at which a concentration of vapor forms above its surface that is capable of ignition from an ignition source, but the rate of vapor formation is insufficient to maintain combustion.

Thus, both at the flash point and at the lower temperature limit of ignition, a lower concentration limit of ignition is formed above the surface of the liquid, but in the latter case, the LFL is created by saturated vapor. Therefore, the flash point is always slightly higher than the LTPV. Although at the flash point there is a short-term ignition of vapors that is not capable of developing into a stable combustion of a liquid, nevertheless, under certain conditions, a flash can cause a fire.

The flash point is taken as the basis for classifying liquids into flammable liquids (FLL) and flammable liquids (CL). Liquids with a flash point in a closed container of 61 °C or lower are classified as flammable liquids, while flammable liquids include those with a flash point of more than 61 °C.

The flash point is determined experimentally in open and closed type devices. In closed vessels, the flash point values ​​are always lower than in open ones, since in this case liquid vapors are able to diffuse into the atmosphere and a higher temperature is required to create a flammable concentration above the surface.

In table 2.4 shows the flash point of some liquids determined by open and closed type instruments.

Table 2.4

Flash point of different types of liquid using different determination methods

Ignition temperature is the minimum temperature of the liquid at which, after ignition of the vapors from the ignition source, steady combustion is established.

For flammable liquids, the ignition temperature is 1-5° higher than the flash point, while the lower the flash point, the smaller the difference between the ignition and flash points.

For flammable liquids with a high flash point, the difference between these temperatures reaches 25-35°. There is a correlation between the flash point in a closed crucible and the lower temperature limit of ignition, described by the formula

This relation is valid for ГВ(.

The significant dependence of flash and ignition temperatures on experimental conditions causes certain difficulties in creating a calculation method for estimating their values. One of the most common of them is the semi-empirical method proposed by V. I. Blinov:

where G sun is the flash (ignition) temperature, K; R np - partial pressure of saturated vapor of a liquid at the flash (ignition) temperature, Pa; D()- liquid vapor diffusion coefficient, s/m 2 ; b- the number of oxygen molecules required for the complete oxidation of one molecule of fuel; IN - determination method constant.

When calculating the flash point in a closed vessel, it is recommended to take IN= 28, in an open container IN= 45; to calculate the ignition temperature take IN = 53.

Flammable temperature limits can be calculated:

Based on known boiling point values

where ^н(в)’ 7/ip - respectively lower (upper) temperature limit of ignition and boiling point, °C; k, I - parameters whose values ​​depend on the type of flammable liquid;

Based on known values ​​of concentration limits. To do this, first determine the concentration of saturated vapors above the surface of the liquid

where (p„ p is the concentration of saturated vapors, %; R n n - saturated vapor pressure, Pa; P 0 - external (atmospheric) pressure, Pa.

From formula (2.41) it follows

Having determined the saturated vapor pressure from the value of the lower (upper) flammability limit, we find the temperature at which this pressure is achieved. It is the lower (upper) temperature limit of ignition.

Using formula (2.41), you can also solve the inverse problem: calculate the concentration limits of ignition based on the known values ​​of the temperature limits.

The property of a flame to spontaneously spread is observed not only during the combustion of mixtures of flammable gases with an oxidizer, but also when burning liquids And solids. When exposed locally to a heat source, for example an open flame, the liquid will warm up, the evaporation rate will increase, and when the surface of the liquid reaches the ignition temperature at the point of influence of the heat source, the steam-air mixture will ignite, a stable flame will be established, which will then spread at a certain speed along the surface and the cold part liquids.

What is the driving force behind the spread of the combustion process, what is its mechanism?

The propagation of a flame over the surface of a liquid occurs as a result of heat transfer due to radiation, convection and molecular thermal conductivity from the flame zone to the surface of the liquid mirror.

According to modern concepts, the main driving force for the propagation of the combustion process is heat radiation from the flame. The flame, having a high temperature (more than 1000°C), is known to be capable of emitting thermal energy. According to the Stefan-Boltzmann law, the intensity of the radiant heat flux given off by a heated body is determined by the relation

Where ts i- intensity of radiant heat flow, kW/m 2 ; 8 0 - degree of blackness of the body (flame) (e 0 = 0.75-H.0); a = = 5.7 10 11 kJ/(m 2 s K 4) - Stefan-Boltzmann constant; G g - body (flame) temperature, K; G 0 - medium temperature, K.

The heat, radiating in all directions, partially reaches the areas of the surface of the liquid that have not yet ignited, warming them up. As the temperature of the surface layer above the heated area increases, the process of liquid evaporation intensifies and a steam-air mixture is formed. As soon as the concentration of liquid vapor exceeds the LVEL, it will ignite from the flame. Then this section of the liquid surface begins to intensively heat up the neighboring section of the liquid surface, etc. The speed of flame propagation through the liquid depends on the rate of heating of the liquid surface by the radiant heat flux from the flame, i.e. on the rate of formation of a flammable vapor-air mixture above the surface of the liquid, which, in turn, depends on the nature of the liquid and the initial temperature.

Each type of liquid has its own heat of evaporation and flash point. The higher their values, the longer the time required to warm it up until a flammable steam-air mixture is formed, and, consequently, the lower the flame propagation speed. With an increase in the molecular weight of a substance within one homologous series, the elastic vapor pressure decreases, the heat of evaporation and flash point increase, and the speed of flame propagation decreases accordingly.

Increasing the temperature of the liquid increases the speed of flame propagation, since the time required to warm the liquid to its flash point before the combustion zone decreases.

During a flash, the speed of flame propagation across the liquid surface will be (in the physical sense) equal to the speed of flame propagation through a steam-air mixture of composition close to the LCPV, i.e. 4-5 cm/s. When the initial temperature of the liquid increases above the flash point, the speed of flame propagation will depend (similarly to the speed of flame propagation) on the composition of the combustible mixture. Indeed, with an increase in the temperature of the liquid above its flash point, the concentration of the vapor-air mixture above the surface of the mirror will increase from LVVP to 100% (boiling point).

Consequently, initially, when the temperature of the liquid increases from the flash point to the temperature at which saturated vapors are formed above the surface, with a concentration equal to the stoichiometric (more precisely, slightly higher than the stoichiometric), the speed of flame propagation will increase. In closed vessels, as the temperature of the liquid increases further, the speed of flame propagation begins to decrease, down to a speed corresponding to the upper temperature limit of ignition, at which the spread of the flame-steam-air mixture will no longer be possible due to the lack of oxygen in the steam-air mixture above the surface of the liquid. Above the surface of an open reservoir, the vapor concentration at different levels will be different: at the surface it will be maximum and correspond to the concentration of saturated vapor at a given temperature; as the distance from the surface increases, the concentration will gradually decrease due to convective and molecular diffusion.

At a liquid temperature close to the flash point, the speed of flame propagation along the surface of the liquid will be equal to the speed of its propagation through the mixture of vapors in the air at the LCPV, i.e. 3-4 cm/s. In this case, the flame front will be located at the surface of the liquid. With a further increase in the initial temperature of the liquid, the speed of flame propagation will increase similarly to the increase in the normal speed of flame propagation through the steam-air mixture with increasing its concentration. At maximum speed, the flame will spread through the mixture with a concentration close to stoichiometric. Consequently, with an increase in the initial temperature of the liquid above Gstx, the flame propagation speed will remain constant, equal to the maximum value of the combustion propagation speed through the stoichiometric mixture or slightly greater than it (Fig. 2.5). Thus,

Rice. 25.

1 - combustion of liquid in a closed container; 2 - combustion of a liquid in an open container, when the initial temperature of the liquid in an open container changes over a wide temperature range (up to the boiling point), the speed of flame propagation will vary from several millimeters to 3-4 m/s.

At maximum speed, the flame will spread through the mixture with a concentration close to stoichiometric. As the temperature of the liquid increases above Gstx, the distance above the liquid at which a stoichiometric concentration will form will increase, and the speed of flame propagation will remain the same (see Fig. 2.5). This circumstance must always be remembered, both when organizing preventive work and when extinguishing fires, when, for example, there may be a danger of air leaking into a closed container - its depressurization.

After the liquid ignites and the flame spreads, its surface becomes diffusion mode of its burnout, which is characterized by specific mass W rM and linear W V Jl speeds.

Specific mass velocity is the mass of a substance burned from a unit area of ​​a liquid mirror per unit time (kg/(m 2 *s)).

Linear speed is the distance by which the level of the liquid surface moves per unit time due to its burnout (m/s).

Mass and linear burnout rates are interrelated through the liquid density p:

After the liquid ignites, its surface temperature rises from the ignition temperature to boiling, and a heated layer is formed. During this period, the rate of liquid burnout gradually increases, the height of the flame increases depending on the diameter of the tank and the type of flammable liquid. After 1-10 minutes of combustion, the process stabilizes: the burnout rate and flame size remain unchanged in the future.

The height and shape of the flame during diffusion combustion of liquid and gas are subject to the same laws, since in both cases the combustion process is determined by the mutual diffusion of fuel and oxidizer. However, if during the diffusion combustion of gases the speed of the gas stream does not depend on the processes occurring in the flame, then during the combustion of a liquid a certain burnout rate is established, which depends both on the thermodynamic parameters of the liquid and on the conditions of diffusion of air oxygen and liquid vapor.

A certain heat and mass transfer is established between the combustion zone and the surface of the liquid (Fig. 2.6). Part of the heat flow reaching the surface of the liquid q 0y is spent on heating it to the boiling point q ucn. Moreover, it is warm qCT The liquid is supplied to heat from the flame through the walls of the tank due to thermal conductivity. With a sufficiently large diameter of qCT can be neglected, then q() = K „ n +

It's obvious that

where c is the heat capacity of the liquid, kJDkg-K); p - liquid density, kg/m3; Wnc- growth rate of the heated layer, m/s; W Jl - linear burnout speed, m/s; 0 and SP - heat of vaporization, kJ/kg; G kip is the boiling point of the liquid, K.

Rice. 2.6.

Г () - initial temperature; G boil - boiling point;

T g- combustion temperature; q KUW q Jl - convective and radiant heat flows, respectively; q 0 - heat flow arriving at the surface of a liquid

From formula (2.45) it follows that the intensity of the heat flow from the flame zone determines a certain rate of supply of fuel to this zone, the chemical interaction of which with the oxidizer, in turn, affects the value #0. This is what mass- And heat exchange between the flame zone and the condensed phase during the combustion of liquids and solids.

Estimation of the proportion of heat from the total heat release during combustion of a liquid that is spent on preparing it for combustion q 0 can be done in the following sequence.

Taking for simplicity W rjl= W nx , we get

The rate of heat release per unit surface of the liquid surface (specific heat of fire qll7K) can be determined by the formula

where Q H is the lower heat of combustion of the substance, kJ/kg; R p - combustion efficiency coefficient.

Then, taking into account state (2.44) and dividing expression (2.45) by formula (2.46), we obtain

Calculations show that about 2% of the total heat release during liquid combustion is spent on the formation and delivery of liquid vapor to the combustion zone. When the burnout process is established, the surface temperature of the liquid increases to the boiling point, which subsequently remains unchanged. This statement applies to individual liquid. If we consider mixtures of liquids with different boiling points, then the low-boiling fractions come out first, then increasingly higher-boiling ones.

The rate of burnout is significantly influenced by the heating of the liquid in depth as a result of heat transfer from the liquid heated by the radiant flow q 0 the surface of the liquid into its depth. This heat transfer is carried out due to thermal conductivity And convention.

Heating of the liquid due to thermal conductivity can be represented by an exponential dependence of the form

Where T x - temperature of the liquid layer at depth X, TO; G kip - surface temperature (boiling point), K; k- proportionality coefficient, m -1.

This type of temperature field is called temperature distribution of the first kind(Fig. 2.7).

Laminar convention arises as a result of different temperatures of the liquid at the walls of the tank and in its center, as well as due to fractional distillation in the upper layer during combustion of the mixture.

Additional heat transfer from the heated walls of the tank to the liquid leads to heating of its layers near the walls to a higher temperature than in the center. The liquid that is more heated near the walls (or even steam bubbles if it is heated at the walls above the boiling point) rises, which contributes to intensive mixing and rapid heating of the liquid at great depths. The so-called homothermal layer, those. a layer with an almost constant temperature, the thickness of which increases during combustion. This temperature field is called temperature distribution of the second kind.

Rice. 2.7.

1 - temperature distribution of the first kind; 2 - temperature distribution of the second kind

The formation of a homothermic layer is also possible as a result of fractional distillation of near-surface layers of a mixture of liquids having different boiling points. As such liquids burn out, the near-surface layer becomes enriched with denser, high-boiling fractions, which sink down, facilitating convective heating of the liquid.

It has been established that the lower the boiling point of a liquid (diesel fuel, transformer oil), the more difficult it is for a homothermic layer to form. When they burn, the temperature of the tank walls rarely exceeds the boiling point. However, when burning wet high-boiling oil products, the probability of the formation of a homothermic layer is quite high. When the walls of the tank are heated to 100°C and above, bubbles of water vapor are formed, which, rushing upward, cause intense movement of all the liquid and rapid heating in depth. The dependence of the thickness of the homothermal layer on the combustion time is described by the relation

Where X - thickness of the homothermic layer at some point in the combustion time, m; x pr - maximum thickness of the homothermal layer, m; t is the time counted from the moment the layer begins to form, s; p - coefficient, s -1.

The possibility of the formation of a sufficiently thick homothermal layer during the combustion of wet petroleum products is fraught with the occurrence of boiling and ejection of liquid.

The burnout rate significantly depends on the type of liquid, initial temperature, humidity and oxygen concentration in the atmosphere.

From equation (2.45) taking into account expression (2.44), the mass burnout rate can be determined:

From formula (2.50) it is obvious that the burnout rate is influenced by the intensity of the heat flow coming from the flame to the liquid surface and the thermophysical parameters of the fuel: boiling point, heat capacity and heat of evaporation.

From the table 2.5 it is obvious that there is a certain correspondence between the rate of burnout and the heat consumption for heating and evaporation of the liquid. Thus, in the series of benzenexylene glycerols, with an increase in heat consumption for heating and evaporation, the burnout rate decreases. However, when moving from benzene to diethyl ether, heat costs decrease. This apparent discrepancy is due to differences in the intensity of heat flows coming from the torch to the surface of the liquid. The radiant flux is large enough for the smoky flame of benzene and small for the relatively transparent flame of diethyl ether. As a rule, the ratio of the burnout rates of the fastest burning liquids and the slowest burning ones is quite small and amounts to 3.0-4.5.

Table 25

Dependence of burnout rate on heat consumption for heating and evaporation

From expression (2.50) it follows that with increasing Г 0 the burnout rate increases, since the heat consumption for heating the liquid to the boiling point decreases.

The moisture content in the mixture reduces the rate of liquid burnout, firstly, due to additional heat consumption for its evaporation, and secondly, as a result of the phlegmatizing effect of water vapor in the gas zone. The latter leads to a decrease in the flame temperature, and therefore, according to formula (2.43), its emissivity also decreases. Strictly speaking, the rate of burning of a wet liquid (liquid containing water) is not constant; it increases or decreases during the combustion process depending on the boiling point of the liquid.

Wet fuel can be represented as a mixture of two liquids: fuel + water, during the combustion process of which their fractional distillation. If the boiling point of a flammable liquid is less than the boiling point of water (100°C), then preferential combustion of the fuel occurs, the mixture is enriched with water, the burnout rate decreases and, finally, combustion stops. If the boiling point of a liquid is more than 100°C, then, on the contrary, moisture first evaporates predominantly and its concentration decreases. As a result, the rate of liquid burning increases, up to the burning rate of the pure product.

As a rule, as wind speed increases, the rate of liquid burnout increases. The wind intensifies the process of mixing fuel with the oxidizer, thereby increasing the temperature of the flame (Table 2.6) and bringing the flame closer to the combustion surface.

Table 2.6

Effect of wind speed on flame temperature

All this increases the intensity of the heat flow supplied to heat and evaporate the liquid, therefore leading to an increase in the burnout rate. At higher wind speeds, the flame may break, which will lead to the cessation of combustion. For example, when tractor kerosene burned in a tank with a diameter of 3 m, the flame failed at a wind speed of 22 m/s.

Most liquids cannot burn in an atmosphere with less than 15% oxygen. As the oxygen concentration increases above this limit, the burnout rate increases. In an atmosphere significantly enriched with oxygen, combustion of a liquid proceeds with the release of a large amount of soot in the flame and intense boiling of the liquid phase is observed. For multicomponent liquids (gasoline, kerosene, etc.), the surface temperature increases with increasing oxygen content in the environment.

An increase in the burnout rate and liquid surface temperature with increasing oxygen concentration in the atmosphere is due to an increase in the emissivity of the flame as a result of an increase in combustion temperature and a high soot content in it.

The burnout rate also changes significantly with a decrease in the level of flammable liquid in the tank: the burnout rate decreases, until combustion stops. Since the supply of air oxygen from the environment into the tank is difficult, when the liquid level decreases, the distance increases h np between the flame zone and the combustion surface (Fig. 2.8). The radiant flow to the liquid mirror decreases, and consequently, the burnout rate decreases, even to the point of attenuation. When burning liquids in large-diameter tanks, the maximum depth /g at which combustion attenuation occurs is very large. So, for a tank with a diameter of 5 m it is 11 m, and with a diameter of Im it is about 35 m.

3. SPREAD OF FLAME IN GAS MIXTURES

The speed of flame propagation during the combustion of solid, liquid and gaseous substances is of practical interest in terms of preventing fires and explosions. Let us consider the speed of flame propagation in mixtures of flammable gases and vapors with air. Knowing this speed, it is possible to determine safe gas-air flow rates in a pipeline, shaft, ventilation unit and other explosive systems.

3.1. FLAME SPREAD RATE

As an example in Fig. Figure 3.1 shows a diagram of exhaust ventilation in a coal mine. From the drifts of mine 1, via pipeline 2, a dusty mixture of air and coal dust is removed, and in some cases, methane released in the coal seams. If a fire occurs, the flame front 3 will spread towards the drifts 1. If the speed of movement of the combustible mixturew will be less than the speed of propagation of the flame frontAnd relative to the walls of the tube, the flame will spread into the shaft and lead to an explosion. Therefore, for normal operation of the ventilation system, it is necessary to comply with the conditions

w > u.

The speed of removal of the explosive mixture must be greater than the speed of propagation of the flame front. This will prevent flames from entering the mine shafts.

Rice. 3.1. Scheme of flame propagation in the mine:

1 – shaft; 2 – pipeline; 3 – flame front

The theory of flame propagation, developed in the works of Ya.B. Zeldovich and D.A. Frank-Kamenetsky, is based on the equations of thermal conductivity, diffusion and chemical kinetics. Ignition of a combustible mixture always begins at one point and spreads throughout the entire volume occupied by the combustible mixture. Let's consider a one-dimensional case - a tube filled with a combustible mixture (Fig. 3.2).

If the mixture is ignited at one end of the tube, then a narrow flame front will spread along the tube, separating the combustion products (behind the flame front) from the fresh combustible mixture. The flame front has the form of a cap or cone, with its convex part facing the direction of the flame movement. The flame front is a thin gas layer (10 -4 ÷10 -6) m wide. Chemical combustion reactions take place in this layer, which is called the combustion zone. The temperature of the flame front, depending on the composition of the mixture, is T= (1500÷3000) K. The released heat of combustion is spent on heating the combustion products of the fresh combustible mixture and the walls of the tube due to the processes of thermal conductivity and radiation.

Rice. 3.2. Scheme of flame front propagation in a tube

When the flame front moves in the tube, compression waves arise in the combustible mixture, which create vortex movements. Swirls of gases bend the flame front without changing its thickness and the nature of the processes occurring in it. On a unit surface of the flame front, the same amount of substance always burns per unit time . The value is constant for each combustible mixture and is called the mass burning rate . Knowing the flame front areaS, you can calculate the mass of a substance M, burned in the entire combustion front per unit time:

Each element of the flame front dSalways moves relative to the fresh mixture in the direction of the normal to the flame front at a given point (Fig. 3.2), and the speed of this movement:

where is the density of the fresh combustible mixture.

Magnitude is called the normal flame propagation speed and has the dimension m/s. It is a constant value of the combustion process of a given mixture and does not depend on the hydrodynamic conditions accompanying the combustion process. The normal speed of flame propagation is always less than the observed speed And, that is, the speed of movement of the combustion front relative to the walls of the tube:

u n< u .

If the flame front is flat and directed perpendicular to the axis of the tube, then in this case the observed and normal flame propagation speed will be the same

u n = u .

Area of ​​convex flame frontS issuealways greater than the area of ​​the flat frontS pl, That's why

> 1.

Normal flame propagation speedu nfor each combustible mixture depends on the admixture of inert gases, the temperature of the mixture, humidity and other factors. In particular, preheating the combustible gas increases the speed of flame propagation. It can be shown that the speed of flame propagationu nproportional to the square of the absolute temperature of the mixture:

u n .= const · T 2.

In Fig. Figure 3.3 shows the dependence of the speed of flame propagation in the combustible mixture “air – carbon monoxide” depending on the concentration of CO. As follows from the graphs above, the speed of flame propagation increases with increasing temperature of the mixture. For each temperature value, the flame propagation speed has a maximum in the region of carbon monoxide CO concentration equal to ~ 40%.

The speed of flame propagation is affected by the heat capacity of the inert gas. The greater the heat capacity of an inert gas, the more it reduces the combustion temperature and the more it reduces the speed of flame propagation. So, if a mixture of methane and air is diluted with carbon dioxide, then the speed of flame propagation can decrease by 2–3 times. The rate of flame propagation in mixtures of carbon monoxide with air is greatly influenced by the moisture contained in the mixture, the presence of soot particles and admixtures of inert gases.

Rice. 3.3. Dependence of flame propagation speed

on the concentration of carbon monoxide in the mixture