Types of combustible mixtures flame propagation speed. The concept of combustion
The normal speed of flame propagation is the speed of movement of the flame front relative to unburned gas in a direction perpendicular to its surface.
The value of the normal speed of flame propagation should be used in calculating the rate of increase in the pressure of an explosion of gas and vapor mixtures in closed, leaky equipment and premises, the critical (extinguishing) diameter in the development and creation of flame arresters, the area of easily dropped structures, safety membranes and other depressurization devices; when developing measures to ensure the fire and explosion safety of technological processes in accordance with the requirements of GOST 12.1.004 and GOST 12.1.010.
The essence of the method for determining the normal speed of flame propagation is to prepare a combustible mixture of known composition inside the reaction vessel, ignite the mixture in the center with a point source, record the change in pressure in the vessel with time, and process the experimental pressure-time dependence using a mathematical model of the gas combustion process in closed vessel and optimization procedures. The mathematical model makes it possible to obtain a calculated dependence “pressure-time”, optimization of which according to a similar experimental dependence results in a change in the normal velocity during the development of an explosion for a particular test.
The normal burning rate is the rate at which the flame front propagates relative to the unburned reactants. The burning rate depends on a number of physicochemical properties of the reagents, in particular, thermal conductivity and the rate of a chemical reaction, and has a well-defined value for each fuel (under constant combustion conditions). In table. 1 shows the burning rates (and ignition limits) of some gaseous mixtures. Fuel concentrations in mixtures were determined at 25°C and normal atmospheric pressure. The flammability limits, with exceptions noted, were obtained with flame propagation in a 0.05 m diameter tube closed on both sides. The fuel excess coefficients are defined as the ratio of the volumetric fuel content in the real mixture to the stoichiometric mixture (j1) and to the mixture at the maximum burning rate (j2).
Table 1
Burning rates of condensed mixtures (inorganic oxidant + magnesium)
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As can be seen, during the combustion of air gas mixtures at atmospheric pressure u max lies within 0.40-0.55 m/s, and - within 0.3-0.6 kg/(m2-s). Only for some low molecular weight unsaturated compounds and hydrogen u max lies within 0.8-3.0 m/s, and reaches 1-2 kg/(m2s). By magnification and max the studied fuels in mixtures with air can be
arrange in the following row: gasoline and liquid rocket fuels - paraffins and aromatics - carbon monoxide - cyclohexane and cyclopropane - ethylene - propylene oxide - ethylene oxide - acetylene - hydrogen.
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The linear combustion rate of oxygen mixtures is much higher than air mixtures (for hydrogen and carbon monoxide - 2-3 times, and for methane - more than an order of magnitude). The mass combustion rate of the studied oxygen mixtures (except for the CO + O2 mixture) lies in the range of 3.7–11.6 kg/(m2 s).
In table. Table 1 shows (according to the data of N. A. Silin and D. I. Postovsky) the burning rates of compacted mixtures of nitrates and perchlorates with magnesium. For the preparation of mixtures, powdered components were used with particle sizes of nitrates 150–250 μm, perchlorates 200–250 μm, and magnesium 75–105 μm. The mixture was filled into cardboard shells with a diameter of 24-46 mm to a compaction factor of 0.86. The samples were burned in air at normal pressure and initial temperature.
From a comparison of the data in Table. 1 and 1.25 it follows that condensed mixtures are superior to gas mixtures in terms of mass and are inferior to them in terms of linear burning rate. The burning rate of mixtures with perchlorates is less than the burning rate of mixtures with nitrates, and mixtures with alkali metal nitrates burn at a higher rate than mixtures with alkaline earth metal nitrates.
table 2
Flammability limits and burning rates of mixtures with air (I) and oxygen (II) at normal pressure and room temperature
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Methods for calculating the burnout rate of liquids
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; (16)
where M is the dimensionless burnout rate;
; (17)
M F- molecular weight of the liquid, kg mol -1 ;
d- characteristic size of the burning liquid mirror, m. It is determined as the square root of the combustion surface area; if the combustion area has the shape of a circle, then the characteristic size is equal to its diameter. When calculating the rate of turbulent combustion, one can take d= 10 m;
T to is the boiling point of the liquid, K.
The calculation procedure is as follows.
The combustion mode is determined by the value of the Galilean criterion Ga, calculated by the formula
where g- free fall acceleration, m·s -2 .
Depending on the combustion mode, the dimensionless burnout rate is calculated M. For laminar combustion mode:
For transient combustion mode:
if , then 
, (20)
if , then , (21)
For turbulent combustion regime:
; 
, (22)
M0- molecular weight of oxygen, kg mol -1 ;
n 0- stoichiometric coefficient of oxygen in the combustion reaction;
n F- stoichiometric coefficient of the liquid in the combustion reaction.
B- dimensionless parameter characterizing the intensity of mass transfer, calculated by the formula
, (23)
where Q- lower calorific value of liquid, kJ·kg -1 ;
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c- isobaric heat capacity of combustion products (assumed to be equal to the heat capacity of air c = 1), kJ·kg -1 ·K -1 ;
T0- ambient temperature, taken equal to 293 K;
H- heat of vaporization of the liquid at the boiling point, kJ·kg -1 ;
c e is the average isobaric heat capacity of the liquid in the range from T0 before T to.
If the kinematic viscosity of the vapor or the molecular weight and boiling point of the liquid under study are known, then the turbulent combustion rate is calculated using experimental data by the formula
where m i- experimental value of the burnout rate in the transient combustion mode, kg·m -2 ·s -1 ;
d i- the diameter of the burner in which the value is obtained m i, m. It is recommended to use a torch with a diameter of 30 mm. If a laminar combustion regime is observed in a burner with a diameter of 30 mm, a burner with a larger diameter should be used.
Above the surface of a liquid or solid at any temperature there is a vapor-air mixture, the pressure of which in the state of equilibrium is determined by the pressure of saturated vapors or their concentration. With an increase in temperature, the saturated vapor pressure will increase but exponentially (Clapeyron - Clausis equation):
where P n „ - pressure of saturated steam, Pa; Q„ C11 - heat of vaporization, 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
In order to create LCVV of vapors, it is sufficient to heat not the entire liquid, but only its surface layer, to a temperature equal to the LTPV.
In the presence of an ignition source, such a mixture will be capable of ignition. In practice, the concepts of "flash point" and "ignition temperature" are more often used.
Flash point - the minimum temperature of a liquid at which a concentration of vapor forms above its surface, capable of being ignited by an ignition source, but the rate of vapor formation is insufficient to sustain combustion.
Thus, both at the flash point and at the lower temperature limit of ignition above the surface of the liquid, a lower concentration limit of ignition is formed, however, in the latter case, LEL is created by saturated vapors. Therefore, the flash point is always somewhat higher than the LTLW. Although at the flash point there is a short-term ignition of the vapor, which is not capable of turning into a stable combustion of the liquid, nevertheless, under certain conditions, the flash can cause a fire.
The flash point is taken as the basis for the classification of liquids into flammable (flammable liquids) and combustible liquids (FL). Flammable liquids include liquids with a flash point in a closed vessel of 61 ° C and below, combustible liquids with a flash point of more than 61 ° C.
Experimentally, the flash point is determined in open and closed devices. In closed vessels, flash points are always lower than in open vessels, because in this case liquid vapors have the opportunity to diffuse into the atmosphere and a higher temperature is required to create a combustible concentration above the surface.
In table. 2.4 shows the flash point of some liquids, determined by devices of open and closed type.
Table 2.4
Flash point of different types of liquid with different methods of determination
Ignition temperature - the minimum temperature of a liquid at which, after ignition of vapors from an ignition source, stationary combustion is established.
In flammable liquids, the ignition temperature is higher than the flash point by 1-5 °, while the lower the flash point, the smaller the difference between the ignition and flash points.
For combustible 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 ignition temperature limit, described by the formula
This relation is valid for Г В(.
The significant dependence of the flash and ignition temperatures on the 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 - flash point (ignition), K; R np - partial pressure of saturated vapor of liquid at flash point (ignition), Pa; D()- diffusion coefficient of liquid vapors, s/m 2 ; b- the number of oxygen molecules required for the complete oxidation of one fuel molecule; AT - definition method constant.
When calculating the flash point in a closed vessel, it is recommended to take AT= 28, in an open vessel AT= 45; to calculate the ignition temperature, take AT = 53.
The flammable temperature limits can be calculated:
According to the known values of the boiling point
where ^n(v)' 7/ip - lower (upper) temperature limit of ignition and boiling point, respectively, °C; k, I- parameters, the values of which depend on the type of combustible liquid;
According to known values of concentration limits. To do this, first determine the concentration of saturated vapors above the surface of the liquid
where (р„ n is the concentration of saturated vapors, %; R n p - saturated vapor pressure, Pa; P 0 - external (atmospheric) pressure, Pa.
From formula (2.41) it follows
Having determined the pressure of saturated vapor by the value of the lower (upper) ignition limit, we find the temperature at which this pressure is reached. It is the lower (upper) temperature limit of ignition.
Using formula (2.41), one can also solve the inverse problem: calculate the concentration limits of ignition from known values of the temperature limits.
The property of a flame to spontaneous propagation is observed not only during the combustion of mixtures of combustible gases with an oxidizing agent, but also when burning liquids and solids. Under local exposure 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, the vapor-air mixture will ignite at the site of exposure to the heat source, a stable flame will be established, which will then spread at a certain speed over the surface and the cold part. liquids.
What is the driving force behind the propagation of the combustion process, what is its mechanism?
Flame propagation over the liquid surface proceeds as a result of heat transfer due to radiation, convection and molecular heat conduction from the flame zone to the surface of the liquid mirror.
According to modern concepts, the main driving force for the spread 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 radiating 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 c i- intensity of radiant heat flow, kW/m 2 ; 8 0 - degree of blackness of the body (flame) (e 0 \u003d 0.75-H.0); a = = 5.7 10 11 kJ / (m 2 s K 4) - Stefan-Boltzmann constant; Г g - temperature of the body (flame), K; Г 0 - medium temperature, K.
Heat, radiating in all directions, partially enters the areas of the surface of the liquid that have not yet caught fire, warming them up. With an increase in the temperature of the surface layer above the heated area, the process of liquid evaporation is intensified and a vapor-air mixture is formed. As soon as the liquid vapor concentration exceeds the NKVP, it will be ignited from the flame. Then, this section of the liquid surface begins to intensively heat up the adjacent section of the liquid surface, and so on. The rate 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 combustible vapor-air mixture above the liquid surface, which, in turn, depends on the nature of the liquid and the initial temperature.
Each type of liquid has its own heat of vaporization and flash point. The higher their values, the longer the time required for its heating to form a combustible vapor-air mixture, the lower the flame propagation speed. With an increase in the molecular weight of a substance within the same homologous series, the vapor pressure of elasticity decreases, the heat of evaporation and the 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 for the liquid to warm up to the flash point in front of the combustion zone is reduced.
During a flash, the speed of flame propagation along the liquid mirror will be (in physical terms) equal to the speed of flame propagation through the vapor-air mixture of a composition close to the LCV, i.e. 4-5 cm/s. With an increase in the initial temperature of the liquid above the flash point, the flame propagation rate will depend (similarly to the flame propagation rate) on the composition of the combustible mixture. Indeed, as the temperature of the liquid rises above its flash point, the concentration of the vapor-air mixture above the surface of the mirror will increase from NKVP to 100% (boiling point).
Therefore, initially, as the temperature of the liquid rises 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, somewhat higher than the stoichiometric), the flame propagation rate will increase. In closed vessels, as the temperature of the liquid rises further, the flame propagation rate begins to decrease, down to the speed corresponding to the upper temperature limit of ignition, at which the propagation of the flame and the vapor-air mixture will no longer be possible due to the lack of oxygen in the vapor-air mixture above the surface of the liquid. Above the surface of an open reservoir, the concentration of vapors 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 over the surface of the liquid will be equal to the speed of its propagation through the mixture of vapors in air at the LIP, i.e. 3-4 cm/s. In this case, the flame front will be located near the surface of the liquid. With a further increase in the initial temperature of the liquid, the flame propagation velocity will increase similarly to the growth of the normal flame propagation velocity in the vapor-air mixture with an increase in its concentration. At maximum speed, the flame will propagate through the mixture with a concentration close to stoichiometric. Consequently, with an increase in the initial temperature of the liquid above G stx, the flame propagation rate will remain constant, equal to the maximum value of the combustion propagation rate in the stoichiometric mixture or somewhat greater than it (Fig. 2.5). In this way,
Rice. 25.
1 - burning liquid in a closed container; 2 - combustion of a liquid in an open container with a change in the initial temperature of the liquid in an open container in a wide temperature range (up to the boiling point), the flame propagation velocity will vary from a few millimeters to 3-4 m / s.
At maximum speed, the flame will propagate through the mixture with a concentration close to stoichiometric. With an increase in the temperature of the liquid above Гstx, the distance above the liquid will increase, at which the stoichiometric concentration will form, and the flame propagation speed 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 being sucked into a closed container - its depressurization.
After the ignition of the liquid and the spread of the flame, but its surface is established diffusion mode of its burnout, which is characterized by the specific mass WrM and linear W V Jl speeds.
Specific mass velocity - the mass of a substance that burns out from a unit area of a liquid mirror per unit time (kg / (m 2 * s)).
Linear speed - the distance over which the level of the liquid mirror moves per unit time due to its burnout (m / s).
The mass and linear burnout rates are interconnected through the liquid density p:
After ignition of the liquid, its surface temperature rises from the ignition temperature to boiling, and a heated layer is formed. During this period, the rate of burning out of the liquid gradually increases, the height of the flame grows depending on the diameter of the tank and the type of combustible liquid. After 1–10 minutes of combustion, the process stabilizes: the burnout rate and flame dimensions remain unchanged in the future.
The height and shape of the flame during diffusion combustion of liquid and gas obey the same laws, since in both cases the combustion process is determined by the mutual diffusion of the fuel and oxidizer. However, if during diffusion combustion of gases, the speed of the gas jet 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 liquid surface (Fig. 2.6). Part of the heat flux arriving at the surface of the liquid q 0y is spent on heating it to the boiling point q ucn . In addition, warm q CT for heating the liquid comes from the torch of the flame through the walls of the tank due to heat conduction. With a sufficiently large diameter q CT can be neglected, then q() = K „ n +
It's obvious that
where c is the heat capacity of the liquid, kJDkg-K); p is the density of the liquid, kg / m 3; Wnc- growth rate of the heated layer, m/s; W Jl- linear burnout rate, m/s; 0i SP - heat of vaporization, kJ/kg; G kip - the boiling point of the liquid, K.
Rice. 2.6.
Г () - initial temperature; G kip - boiling point;
T g- combustion temperature; q KUW q Jl - convective and radiant heat fluxes, respectively; q 0 - heat flux entering the surface of the liquid
It follows from formula (2.45) that the intensity of the heat flow from the flame zone determines a certain rate of fuel supply to this zone, the chemical interaction of which with the oxidizer, in turn, affects the value # 0 . This is what it consists the relationship of mass and heat exchange between the flame zone and the condensed phase during the combustion of liquids and solids.
Estimation of the share of heat from the total heat release during the combustion of the liquid, which is spent on its preparation for combustion q 0 , can be carried out in the following sequence.
Taking for simplicity wrijl= W nx , we get
The rate of heat release per unit surface of the liquid mirror (specific heat of fire qll7K) can be determined by the formula
where Q H is the lowest calorific value of the substance, kJ/kg; P p - coefficient of completeness of combustion.
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 temperature of the liquid surface increases to the boiling point, which subsequently remains unchanged. This statement refers to an individual liquid. If, however, we consider mixtures of liquids having different boiling points, then at first the release of light-boiling fractions occurs, then - increasingly higher-boiling ones.
The burn-up rate is significantly affected by the heating of the liquid in depth as a result of heat transfer from the liquid heated by the radiant flow q0 the surface of the liquid to its depth. This heat transfer is carried out by thermal conductivity and conventions.
The heating of a 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- coefficient of proportionality, m -1 .
This type of temperature field is called temperature distribution of the first kind(Fig. 2.7).
The laminar convention arises as a result of different liquid temperatures at the walls of the tank and in its center, as well as due to fractional distillation in the upper layer during the combustion of the mixture.
Additional heat transfer from the heated walls of the reservoir to the liquid leads to heating of its layers near the walls to a higher temperature than in the center. The liquid heated near the walls (or even steam bubbles if it is heated near the walls above the boiling point) rises, which contributes to intensive mixing and rapid heating of the liquid at a great depth. The so-called homothermal layer, those. a layer with a practically constant temperature, the thickness of which increases during combustion. Such a 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 homothermal 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 is enriched in denser, high-boiling fractions, which sink down, contributing to the most 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 to form a homothermal layer. 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 homothermal layer is rather high. When the tank walls are heated to 100°C and higher, water vapor bubbles are formed, which, rushing upward, cause an intensive movement of the entire liquid and rapid heating in depth. The dependence of the thickness of the homothermal layer on the burning time is described by the relation
where X - thickness of the homothermal layer at a certain moment of combustion time, m; x pr - limiting thickness of the homothermal layer, m; t is the time counted from the beginning of the layer formation, s; p - coefficient, s -1.
The possibility of the formation of a sufficiently thick homothermal layer during the combustion of wet oil products is fraught with the occurrence of boiling and liquid ejection.
The burn-out 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), it is possible to determine the mass burnout rate:
It is obvious from formula (2.50) that the rate of burnout is affected by the intensity of the heat flux coming from the flame to the liquid mirror and the thermophysical parameters of the fuel: boiling point, heat capacity and heat of evaporation.
From Table. 2.5 it is obvious that there is a certain correspondence between the burnout rate and the heat costs for heating and evaporating the liquid. Thus, in the series of benzenexyleneglycerols, with an increase in heat consumption for heating and evaporation, the burnout rate decreases. However, when passing from benzene to diethyl ether, the heat costs decrease. This apparent discrepancy is due to the difference in the intensity of heat fluxes coming from the flame to the liquid surface. The radiant flux is large enough for a smoky benzene flame and small for a relatively transparent diethyl ether flame. As a rule, the ratio of the burnout rates of the fastest burning liquids and the slowest burning liquids is quite small and amounts to 3.0-4.5.
Table 25
Dependence of the burn-out rate on heat consumption for heating and evaporation
It follows from expression (2.50) that with an increase in Г 0 the burnout rate increases, since the heat costs for heating the liquid to the boiling point decrease.
The moisture content in the mixture reduces the burnout rate of the liquid, 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 temperature of the flame, and therefore, according to formula (2.43), its radiant power 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 of which their fractional dispersal. If the boiling point of a combustible liquid is less than the boiling point of water (100°C), then the fuel burns out preferentially, the mixture is enriched with water, the burnout rate decreases, and, finally, combustion stops. If the boiling point of the liquid is more than 100 ° C, then, on the contrary, moisture primarily evaporates first and its concentration decreases. As a result, the burnout rate of the liquid increases, up to the burning rate of the pure product.
As a rule, with an increase in wind speed, the rate of burnout of the liquid increases. The wind intensifies the process of mixing the fuel with the oxidizer, thereby raising 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 the heating and evaporation of the liquid, therefore, leads to an increase in the burnout rate. At higher wind speeds, the flame can break off, which will lead to the cessation of combustion. So, for example, when tractor kerosene burned in a tank with a diameter of 3 m, flameout occurred at a wind speed of 22 m/s.
Most liquids cannot burn in an atmosphere with less than 15% oxygen. With an increase in the oxygen concentration above this limit, the burn-up rate increases. In an atmosphere significantly enriched with oxygen, the combustion of the 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 an increase in the oxygen content in the environment.
An increase in the burn-out rate and liquid surface temperature with an increase in the oxygen concentration in the atmosphere is due to an increase in the emissivity of the flame as a result of an increase in the 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, up to the cessation of combustion. Since the supply of air oxygen from the environment inside the tank is difficult, when the liquid level decreases, the distance h np between the flame zone and the combustion surface (Fig. 2.8). The radiant flux to the liquid mirror decreases, and, consequently, the burnout rate also decreases, up to attenuation. When burning liquids in tanks of large diameter, the limiting depth /g pr at which combustion is attenuated is very large. So, for a tank with a diameter of 5 m, it is 11 m, and with a diameter of Im - about 35 m.
3. FLAME PROPAGATION 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. Consider the speed of flame propagation in mixtures of combustible gases and vapors with air. Knowing this speed, it is possible to determine the safe speed of the gas-air flow in a pipeline, mine, ventilation installation and other explosive systems.
3.1. FLAME SPEED
As an example, in fig. 3.1 shows a diagram of exhaust ventilation in a coal mine. From the drifts of mine 1 through pipeline 2, a dusty mixture of air and coal dust is removed, and in some cases, methane released in coal seams. When a fire occurs, the flame front 3 will spread towards the drifts 1. If the speed 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 mine and lead to an explosion. Therefore, for the normal operation of the ventilation system, it is necessary to comply with the condition
w>u.
The rate of removal of the explosive mixture must be greater than the rate of propagation of the flame front. This will prevent flames from entering the shaft drifts.
Rice. 3.1. Scheme of flame propagation in the mine:
1 - mine; 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 heat conduction, diffusion and chemical kinetics. The ignition of a combustible mixture always starts at one point and spreads over the entire volume occupied by the combustible mixture. Consider a one-dimensional case - a tube filled with a combustible mixture (Fig. 3.2).
If the mixture is ignited from one end of the tube, then a narrow flame front will propagate 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 a cone with its convex part turned towards the flame movement. The flame front is a thin gaseous layer (10 -4 ÷10 -6) m wide. In this layer, which is called the combustion zone, chemical combustion reactions take place. 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 tube walls due to the processes of heat conduction 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 motions. Gas swirls 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 per unit time always burns. 
. The value is constant for each combustible mixture and is called the mass burning rate .
Knowing the area of the flame frontS, you can calculate the mass of a substance M, combustible in the entire combustion front per unit time:
Each element of the flame front dSmoves relative to the fresh mixture always 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.
Value is called the normal speed of flame propagation 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 velocity of the combustion front relative to the tube walls:
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 speed of flame propagation will be the same
u n = u .
The area of the convex flame frontS issuealways greater than the area of the flat frontS pl, that's why
> 1.
Normal Flame Speedu nfor each combustible mixture depends on the admixture of inert gases, mixture temperature, humidity and other factors. In particular, the preheating of the combustible gas increases the rate of flame propagation. It can be shown that the speed of flame propagationu nis proportional to the square of the absolute temperature of the mixture:
u n .= const T 2.
On fig. 3.3 shows the dependence of the flame propagation speed in the combustible mixture "air - carbon monoxide" depending on the concentration of CO. As follows from the above graphs, the speed of flame propagation increases with increasing temperature of the mixture. For each temperature value, the flame propagation velocity has a maximum in the region of carbon monoxide concentration CO equal to ~ 40%.
The heat capacity of the inert gas affects the rate of flame propagation. 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 with 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 impurities of inert gases.
Rice. 3.3. Flame propagation velocity dependence
on the concentration of carbon monoxide in the mixture
1) Humidity of the material.
2) Influence of sample orientation in space.
At negative angles of inclination (direction of flame movement from top to bottom), the flame propagation velocity either does not change or slightly decreases. With an increase in the positive angle of inclination (direction of flame movement from bottom to top) over 10-15 0, the speed of flame propagation increases sharply.
3) The influence of the speed and direction of air flows.
With an increase in the tailwind speed, gas exchange improves, and the angle of inclination of the flame to the sample decreases. The propagation speed is increasing.
The flow of air directed against the direction of flame movement has a twofold effect on the speed of flame propagation.
As a result of aerodynamic deceleration and cooling of the heated surface areas in front of the flame front, the flame propagation speed decreases. On the other hand, the air flow intensifies the mixing of the pyrolysis products with the oxidizing agent, the formation of a homogeneous combustible mixture occurs faster, the flame nose approaches the surface of the solid material, which, in turn, leads to a further increase in intensity, and this accelerates the spread of the flame.
4) Influence of the geometric dimensions of the sample.
There are thermally thick and thermally thin samples.
Thermal thickness is the thickness of a layer of solid material heated before the flame front above the initial temperature by the time the flame spreads to a given surface area.
5) Influence of the substrate material.
If a combustible material comes into contact with a material (substrate) whose thermophysical properties differ from air, then this will also affect the speed of flame propagation (pasted paper, wire insulation, etc.). If l low > l mountains. mat. , then the heat will be intensively removed from the sample, and the propagation velocity will be less than in the case of the absence of a substrate.
6) Influence of oxygen content in the environment.
As the oxygen content in the environment increases, the rate of flame propagation increases.
7. Influence of initial sample temperature.
For wood, an increase in the initial temperature to 230–250 ° C (temperature range of pyrolysis) leads to a sharp increase in u l.
Burnout of hard materials
Simultaneously with the spread of the flame over the surface of the material, the process of its burnout begins. The patterns of burnout of solid materials essentially depend on the nature of the transformation of the solid phase into gaseous products.
If the decomposition of the solid phase proceeds in a narrow near-surface layer without the formation of a carbonaceous layer, then in this case combustion proceeds at a constant rate. After ignition, a constant temperature is established on the surface of the solid phase, equal to the boiling or sublimation temperature of the substance.
The combustion mechanism of solids, proceeding with the formation of a carbonaceous residue on the combustion surface, is more complex. Almost all substances of plant origin burn in this way, some plastics containing non-combustible or slow-burning fillers (talc, soot, etc.) in their composition. Wood is one of the most common combustible substances of vegetable origin of this type. At the moment of ignition, due to the heat flow from the flame zone, the temperature of the surface layer of wood quickly increases to 450-500 ° C. Intensive decomposition of substances occurs with the formation of volatile products and charcoal, while the temperature on the surface rises to 600 ° C.
In the depth of burning wood, there are areas with different physical and physico-chemical characteristics. Conventionally, they can be divided into 4 zones:
I - charcoal, consisting of 99% carbon;
II - wood with varying degrees of pyrolysis;
III - non-pyrolyzed, dry wood;
IV - original wood.
As volatile products are released from the solid phase during the combustion of wood, the material is charred to an ever greater depth. An increase in the thickness of the carbonaceous layer causes an increase in its thermal resistance and, consequently, reduces the rate of heating and pyrolysis of the not yet decomposed layers of wood, and the rate of flame combustion gradually decreases. The fiery combustion of wood stops when the mass rate of volatile emission decreases to 5 g/(m 2 s). The thickness of the coal layer in this case reaches 15-20 mm.
The cessation of the fiery combustion of wood opens the access of air oxygen to coal heated to a temperature of 650-700 ° C. The second stage of wood combustion begins - heterogeneous oxidation of the carbon layer mainly by the reaction C + O 2 ® CO 2 + 33000 kJ / kg, the temperature of the carbon layer increases to 800 ° C, and the process of heterogeneous combustion of coal is even more intensified.
The real picture of the transition from homogeneous to heterogeneous combustion is somewhat different from the one presented.
The main quantitative parameter characterizing the process of burning out solid materials is the mass burning rate, which is one of the parameters that determine the dynamics of a fire.
The reduced mass burnout rate is the amount of substance that burns out per unit time per unit fire area.
Burning metals
According to the nature of combustion, metals are divided into two groups: volatile and non-volatile.
Volatile metals have T pl< 1000 К, Т кип < 1500 К. К ним относятся щелочные металлы (литий, натрий, калий и др.) и щелочноземельные (магний, кальций).
Non-volatile metals have Tmelt >1000 K, Tboil >2500 K. The combustion mechanism is largely determined by the properties of the metal oxide. The melting temperature of volatile metals is lower than that of their oxides. In this case, the latter are rather porous formations.
When IS is brought to the metal surface, it evaporates and oxidizes. When the concentration of vapors is equal to the lower concentration limit of ignition, they ignite. The diffusion combustion zone is established near the surface, a large proportion of heat is transferred to the metal and it heats up to T kip. The resulting vapors, freely diffusing through the porous oxide film, enter the combustion zone. Boiling of the metal causes periodic destruction of the oxide film, which intensifies combustion. Combustion products (metal oxides) diffuse not only to the metal surface, contributing to the formation of an oxide crust, but also to the surrounding space, where, condensing, they form solid particles in the form of white smoke. The formation of white dense smoke is a visual sign of burning volatile metals.
For non-volatile metals with high phase transition temperatures, during combustion, a very dense oxide film is formed on the surface, which adheres well to the metal surface. As a result, the rate of diffusion of metal vapor through the film is sharply reduced and large particles, such as aluminum and beryllium, are not able to burn. As a rule, fires of such metals occur when they are in the form of chips, powders and aerosols. Their combustion occurs without the formation of dense smoke. The formation of a dense oxide film on the metal surface leads to particle explosion. This phenomenon, which is especially often observed when a particle moves in a high-temperature oxidizing medium, is associated with the accumulation of metal vapors under the oxide film, followed by its sudden rupture. This, of course, leads to a sharp intensification of combustion.
burning dust
Dust is a dispersed system consisting of a gaseous dispersion medium (air, etc.) and a solid dispersed phase (flour, sugar, wood, coal, etc.).
Factors affecting the speed of flame propagation through dust-air mixtures:
1) Dust concentration.
As in the case of combustion of a homogeneous gas-air mixture, the maximum flame propagation velocity occurs for mixtures somewhat higher than the stoichiometric composition. For peat dust, this is 1.0-1.5 kg / m 3.
2) Ash content.
With an increase in ash content, the concentration of the combustible component decreases and, accordingly, the speed of flame propagation decreases.
As the oxygen content decreases, the flame propagation speed decreases.
Classification of dusts according to fire and explosion hazard.
According to the explosion hazard, dust is divided into classes:
I class - the most explosive - j n up to 15 g / m 3;
II class - explosive - 15 g / m 3< j н < 65 г/м 3 ;
Class III - the most fire hazardous - j n > 65 g / m 3; T St up to 250 about C;
Class IV - fire hazardous - j n > 65 g / m 3; T St > 250 o C.
FIRE DYNAMICS
The dynamics of a fire is understood as a set of laws and regularities that describe the change in the main parameters of a fire in time and space. The nature of a fire can be judged by the combination of a large number of its parameters: by the area of the fire, by the temperature of the fire, the speed of its spread, the intensity of heat release, the intensity of gas exchange, the intensity of smoke, etc.
There are so many fire parameters that in some types of fires some of them are primary, while in others they are secondary. It all depends on what goals are set in the study of a particular type of fire.
As the main time-varying parameters, for studying the dynamics of a fire, we take the area of the fire, the temperature of the fire, the intensity of gas exchange and smoke, and the speed of the spread of the fire. These fire parameters are the most accessible for measurement, analysis, and calculations. They serve as initial data for determining the type of equipment required and calculating the forces and means for extinguishing fires, designing automatic fire extinguishing systems, etc.
From the moment a fire occurs, with its free development, until its complete cessation, a fire in a room can be divided into phases.
Fire phases
I. Ignition phase.
The flame originates from an external source of ignition in a small area and slowly spreads. A convective gas flow is formed around the combustion zone, which provides the necessary gas exchange. The surface of the combustible material warms up, the size of the torch increases, gas exchange increases, and the radiant heat flux increases, which enters the surrounding space and onto the surface of the combustible material. The duration of the tanning phase varies from 1 to 3 minutes.
II. phase of the fire.
The room temperature rises slowly. The entire previous process is repeated, but with greater intensity. The duration of the second stage is approximately 5-10 minutes.
III. Volumetric fire development phase- rapid process of growth of all listed parameters. The temperature in the room reaches 250 -300°C. The "volume" phase of fire development and the phase of volumetric spread of the fire begin. At a temperature of the gaseous medium in the room of 300 ° C, the glazing is destroyed. In this case, afterburning can also occur outside the premises (fire escapes from the openings to the outside). The intensity of gas exchange changes abruptly: it increases sharply, the process of outflow of hot combustion products and the influx of fresh air into the combustion zone intensify.
IV.Fire phase.
During this phase, the room temperature may decrease for a short time. But in accordance with the change in the conditions of gas exchange, such fire parameters sharply increase as the completeness of combustion, the rate of burnout and the spread of the combustion process. Accordingly, the total heat release in the fire also increases sharply. The temperature, which slightly decreased at the time of the destruction of the glazing due to the influx of cold air, rises sharply, reaching 500 - 600 ° C. The fire development process is rapidly intensifying. The numerical value of all previously mentioned fire parameters is increased. The area of the fire, the average volumetric temperature in the room (800-900 ° C), the intensity of the burnout of the fire load and the degree of smoke reach a maximum.
V. The phase of stationary combustion.
The fire parameters are stabilizing. This usually occurs within 20-25 minutes of a fire and, depending on the magnitude of the fire load, may last 20-30 minutes.
VI. Decay phase.
The intensity of combustion gradually decreases, because. the bulk of the fire load has already burned out. A large amount of combustion products has accumulated in the room. The average volume concentration of oxygen in the room decreased to 16-17%, and the concentration of combustion products that prevent intense combustion increased to the limit value. The intensity of radiant heat transfer to the combustible material decreased due to a decrease in temperature in the combustion zone. Due to the increase in the optical density of the medium, the intensity of combustion slowly decreases, which leads to a decrease in all other fire parameters. The fire area is not shrinking: it can grow or stabilize.
VII. Burnout phase.
This final phase of the fire is characterized by slow smoldering, after which, after some, sometimes quite a long time, the burning stops.
Basic fire parameters
Let us quantitatively consider some of the main parameters of a fire that determine the dynamics of its development. Let us determine the intensity of heat release in a fire, since this is one of the main parameters of the combustion process:
Q \u003d βQ r n V m ’Sp, (kJ / s)
where β and Q p n are constants (the underburning coefficient and the lower calorific value of the fire load);
V m ¢ - reduced mass burnout rate;
S p - fire area;
V m ¢ and S p depend on the time of development of the fire, the temperature of the fire, the intensity of gas exchange, etc.
The reduced mass burnout rate V m ¢ is determined by the formula:
v m ¢ \u003d (a × T p + b × I g) v m o ¢
where a, b are empirical coefficients;
v m o ¢ is the reduced mass burnout rate of the fire load for a given type of combustible material;
T p - the average value of the temperature of the fire;
I g - intensity of gas exchange.
The dependence of the fire area on the main parameters of its development has the form:
S p \u003d k (v p ∙ τ) n
where k and n are coefficients depending on the geometric shape of the fire area;
v p - linear speed of fire propagation;
τ is the time of its free development.
k = π; n = 2 k = ; n = 2 k = 2a; n=1
k = ; n = 2 k = 2a; n=1
The linear speed of fire propagation depends on the type of combustible load, the average fire temperature and the intensity of gas exchange:
v p \u003d (a 1 T p + b 1 I g) v po
where a 1 and b 1 are empirical coefficients that establish the dependence of the linear speed of fire propagation on the average temperature and intensity of gas exchange, the numerical value of which is determined empirically for each specific type of fuel;
v p o - linear speed of propagation of combustion for a given type of fuel.
As the fire develops, the fire temperature and the intensity of gas exchange will increase, increasing the linear rate of combustion propagation and the reduced mass burnout rate.
Thermal regime on fire
The occurrence and rate of occurrence of thermal processes depend on the intensity of heat release in the combustion zone, i.e. from the heat of the fire. The quantitative characteristic of the change in heat release in a fire, depending on various combustion conditions, is the temperature regime. Under the temperature regime of a fire understand the change in temperature over time. Determining the temperature of a fire by both experimental and computational methods is extremely difficult. For engineering calculations, when solving a number of practical problems, the fire temperature is determined from the heat balance equation. The heat balance on a fire is compiled not only to determine the temperature of the fire, but also to identify the quantitative distribution of thermal energy. In the general case, the heat balance of a fire for a given point in time can be represented as follows:
Q p \u003d Q pg + Q to + Q l
where Q p is the heat released in the fire, kJ;
Q pg - heat contained in the combustion products, kJ;
Q to - heat transferred from the combustion zone by convection to the air surrounding the zone, but not participating in combustion, kJ;
Q l - heat transferred from the combustion zone by radiation.
For open fires, it has been established that the share of heat transferred from the combustion zone by radiation and convection is 40-50% of Q p. The remaining share of heat (60-70% of Q p) is used to heat the combustion products. Thus, 60-70% of the theoretical combustion temperature of a given combustible material will give an approximate value for the flame temperature. The temperature of open fires depends on the calorific value of combustible materials, the rate of their burnout and meteorological conditions. On average, the maximum temperature of an open fire for combustible gases is 1200 - 1350°C, for liquids - 1100 - 1300°C and for solid combustible materials of organic origin - 1100 - 1250°C.
During an internal fire, temperature is influenced by more factors: the nature of the combustible material, the magnitude of the fire load and its location, the burning area, the dimensions of the building (floor area, room height, etc.) and the intensity of gas exchange (size and location of openings). Let us consider in more detail the influence of these factors.
The fire can be divided into three characteristic periods according to temperature changes: initial, main and final.
Initial period- characterized by a relatively low average volume temperature.
Main period- during it, 70-80% of the total load of combustible materials burns out. The end of this period occurs when the average volume temperature reaches its maximum value or decreases to no more than 80% of the maximum value.
Final period- characterized by a decrease in temperature due to the burnout of the fire load.
Figure 9.1. Temperature change of an internal fire in time: 1 - curve of a specific fire; 2 - standard curve
Since the growth rate and the absolute value of the fire temperature in each specific case have their own characteristic values and features, the concept of a standard temperature curve (Fig. 21.2) has been introduced, generalizing the most characteristic features of the change in the temperature of internal fires. The standard temperature is described by the equation.
Combustion- these are intense chemical oxidative reactions, which are accompanied by the release of heat and luminescence. Combustion occurs in the presence of a combustible substance, an oxidizing agent and an ignition source. Oxygen and nitric acid can act as oxidizing agents in the combustion process. As fuel - many organic compounds, sulfur, hydrogen sulfide, pyrite, most metals in free form, carbon monoxide, hydrogen, etc.
In a real fire, the oxidizing agent in the combustion process is usually atmospheric oxygen. The external manifestation of combustion is a flame, which is characterized by luminescence and heat release. When burning systems consisting only of solid or liquid phases or their mixtures, a flame may not occur, i.e., occurs flameless burning or smoldering.
Depending on the state of aggregation of the initial substance and combustion products, homogeneous combustion, combustion of explosives, and heterogeneous combustion are distinguished.
Homogeneous combustion. In homogeneous combustion, the initial substances and combustion products are in the same state of aggregation. This type includes the combustion of gas mixtures (natural gas, hydrogen, etc. with an oxidizing agent, usually air oxygen) /
Burning explosives associated with the transition of a substance from a condensed state to a gas.
heterogeneous combustion. In heterogeneous combustion, the initial substances (for example, solid or liquid fuel and gaseous oxidizer) are in different states of aggregation. The most important technological processes of heterogeneous combustion are the combustion of coal, metals, the combustion of liquid fuels in oil furnaces, internal combustion engines, combustion chambers of rocket engines.
The movement of a flame through a gas mixture is called flame spread. Depending on the speed of propagation of the combustion flame, it can be deflagration at a speed of several m/s, explosive at a speed of the order of tens and hundreds of m/s, and detonation at thousands of m/s.
Deflagration combustion is subdivided into laminar and turbulent.
Laminar combustion is characterized by a normal flame propagation velocity.
Normal flame propagation speed called the speed of movement of the flame front relative to unburned gas, in a direction perpendicular to its surface.
Temperature increases the normal speed of flame propagation relatively little, inert impurities reduce it, and an increase in pressure leads either to an increase or decrease in the speed.
In a laminar gas flow, the gas velocities are low. The burning rate in this case depends on the rate of formation of the combustible mixture. In a turbulent flame, the swirling of gas jets improves the mixing of the reacting gases, since the surface through which molecular diffusion occurs increases.
Indicators of fire and explosion hazard of gases. Their characteristics and scope
The fire hazard of technological processes is largely determined by the physical and chemical properties of raw materials, intermediate and final products circulating in the production.
Fire and explosion hazard indicators are used in the categorization of premises and buildings, in the development of systems to ensure fire safety and explosion safety.
Gases are substances whose absolute vapor pressure at a temperature of 50 °C is equal to or greater than 300 kPa or whose critical temperature is less than 50 °C.
For gases, the following values apply:
Flammability group- an indicator that is applicable to all aggregate states.
Flammability is the ability of a substance or material to burn. According to the combustibility of substances and materials are divided into three groups.
non-combustible(fireproof) - substances and materials that are incapable of combustion in air. Non-combustible substances can be flammable (for example, oxidizing agents, as well as substances that release combustible products when interacting with water, atmospheric oxygen, or with each other).
slow-burning(flammable) - substances and materials that can ignite in the air from an ignition source, but are not able to burn independently after its removal.
combustible(combustible) - substances and materials capable of spontaneous combustion, as well as ignite from an ignition source and burn independently after its removal. Flammable substances and materials are distinguished from the group of combustible substances and materials.
Flammable substances and materials that can ignite from a short-term (up to 30 s) exposure to a low-energy ignition source (match flame, spark, smoldering cigarette, etc.) are called flammable.
The combustibility of gases is determined indirectly: a gas that has concentration limits of ignition in air is referred to as fuel; if the gas does not have concentration limits of ignition, but ignites spontaneously at a certain temperature, it is classified as slow-burning; in the absence of concentration limits of ignition and autoignition temperature, the gas is classified as non-combustible.
In practice, the combustibility group is used to subdivide materials by combustibility, when establishing classes of explosive and fire hazardous zones according to the PUE, when determining the category of premises and buildings according to explosion and fire hazard, and when developing measures to ensure fire and explosion safety of equipment and premises.
Auto ignition temperature- the lowest temperature of a substance at which, under the conditions of special tests, there is a sharp increase in the rate of exothermic reactions, ending in fiery combustion.
The concentration limits of flame propagation (ignition) - that the range of concentrations in which combustion of mixtures of combustible vapors and gases with air or oxygen is possible.
Lower (upper) concentration limit of flame propagation - the minimum (maximum) content of fuel in a mixture of combustible substance-oxidizing medium "at which flame propagation through the mixture is possible at any distance from the ignition source. Within these limits, the mixture is combustible, and outside of them, the mixture is unable to burn.
Temperature Limits of Flame Propagation(ignition) - such temperatures of a substance at which its saturated vapors form in a particular oxidizing environment concentrations equal, respectively, to the lower (lower temperature limit) and upper (upper temperature limit) concentration limits of flame propagation.
The ability to explode and burn when interacting with water, atmospheric oxygen and other substances- a qualitative indicator that characterizes the special fire hazard of certain substances. This property of substances is used when determining the category of production, as well as when choosing safe conditions for conducting technological processes and conditions for joint storage and transportation of substances and materials.