Examination: Theory of combustion and explosion. Combustion of gas and steam-air mixtures. Explosion, detonation Calculation of the pressure rise rate of explosion of gas-vapor mixture
1 The method consists in determining the upper limits for the maximum and average rate of increase in the pressure of the explosion of gas and vapor-air mixtures in a spherical reaction vessel of constant volume.
The upper limit for the maximum rate of pressure rise in kPa s -1 is calculated by the formula
where p i- initial pressure, kPa;
S And. i- normal speed of flame propagation at initial pressure and temperature, m·s -1 ;
a- radius of the spherical reaction vessel, m;
Dimensionless maximum explosion pressure;
R - maximum absolute explosion pressure, kPa;
And- adiabatic index for the mixture under study;
is a thermokinetic exponent as a function of normal flame propagation velocity as a function of pressure and temperature. If the value unknown, it is taken equal to 0.4.
The upper limit for the average rate of pressure rise in kPa s -1 is calculated by the formula
,
(98)
where is a function of the parameters e , And , , the values of which are found using the nomograms shown in Fig. 26 and 27.
Values e And And are found by thermodynamic calculation or, in case of impossibility of calculation, are taken equal to 9.0 and 1.4, respectively.
The relative root-mean-square error of calculation by formulas (97) and (98) does not exceed 20%.
2. The maximum rate of increase in the explosion pressure of gas and vapor-air mixtures for substances consisting of atoms C, H, O, N, S, F, Cl is calculated by the formula
,
(99)
where V- volume of the reaction vessel, m 3 .
The relative root-mean-square error of calculation by formula (99) does not exceed 30%.
Method for experimental determination of the conditions of thermal spontaneous combustion of solid substances and materials
1. Hardware.
The equipment for determining the conditions of thermal spontaneous combustion includes the following elements.
1.1. Thermostat with a capacity of the working chamber of at least 40 dm 3 with a thermostat that allows you to maintain a constant temperature from 60 to 250 ° C with an error of not more than 3 ° C.
1.2. Baskets made of corrosion-resistant metal of cubic or cylindrical shape 35, 50, 70, 100, 140 and 200 mm high (10 pieces of each size) with lids. The diameter of the cylindrical basket should be equal to its height. The wall thickness of the basket is (1.0 ± 0.1) mm.
1.3. Thermoelectric transducers (not less than 3) with a maximum working junction diameter of not more than 0.8 mm.
2. Preparation for the test.
2.1. Carry out a calibration test to determine the correction ( t T) to the readings of thermoelectric converters 2 And 3 . To do this, a basket with a non-combustible substance (for example, calcined sand) is placed in a thermostat heated to a given temperature. Thermoelectric converters (Fig. 2) are installed in such a way that the working junction of one thermoelectric converter is in contact with the sample and is located in its center, the second one is in contact with the outer side of the basket, the third one is at a distance of (30 ± 1) mm from the basket wall. The working junctions of all three thermoelectric converters must be located at the same horizontal level, corresponding to the middle line of the thermostat.
1 , 2 , 3 - working junctions of thermoelectric converters.
A basket with a non-combustible substance is kept in a thermostat until a stationary regime is established, in which the readings of all thermoelectric
transducers for 10 minutes remain unchanged or fluctuate with a constant amplitude around average temperatures t 1 , t 2 , t 3 . Amendment t T is calculated by the formula
,
(100)
2.2. Samples for testing should characterize the average properties of the test substance (material). When testing sheet material, it is collected in a pile corresponding to the internal dimensions of the basket. In samples of monolithic materials, a hole with a diameter of (7.0 ± 0.5) mm is pre-drilled to the center for a thermoelectric converter.
The movement of the flame through the gas mixture called flame propagation. Depending on the speed of flame propagation, combustion 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 - thousands of m/s.
For deflagration or normal flame propagation characteristic is the transfer of heat from layer to layer, and the flame that occurs in the mixture heated and diluted with active radicals and reaction products moves in the direction of the initial combustible mixture. This is explained by the fact that the flame, as it were, becomes a source that releases a continuous flow of heat and chemically active particles. As a result, the flame front moves towards the combustible mixture.
deflagration combustion subdivided into laminar and turbulent.
Laminar combustion is characterized by a normal flame propagation velocity.
The normal flame propagation speed, according to GOST 12.1.044 SSBT, is called flame front speed relative to unburned gas, in a direction perpendicular to its surface.
The value of the normal speed of flame propagation, being one of the indicators of the fire and explosion hazard of substances, characterizes the danger of industries associated with the use of liquids and gases, it is used in calculating the rate of increase in the explosive pressure of gas, vapor-air mixtures, critical (extinguishing) diameter and in the development of measures providing fire and explosion safety technological processes in accordance with the requirements of GOST 12.1.004 and GOST 12.1.010 SSBT.
The normal speed of flame propagation - the physicochemical constant of the mixture - depends on the composition of the mixture, pressure and temperature and is determined by the speed chemical reaction and molecular thermal conductivity.
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, and the combustible mixture is formed as a result of molecular diffusion. The burning rate in this case depends on the rate of formation of the combustible mixture. turbulent flame It is formed with an increase in the speed of flame propagation, when the laminarity of its movement is disturbed. In a turbulent flame, the swirl of gas jets improves the mixing of the reacting gases, since the surface through which molecular diffusion occurs increases.
As a result of the interaction of a combustible substance with an oxidizing agent, combustion products are formed, the composition of which depends on the initial compounds and the conditions of the combustion reaction.
With the complete combustion of organic compounds, CO 2, SO 2, H 2 O, N 2 are formed, and with the combustion of inorganic compounds, oxides are formed. Depending on the melting temperature, the reaction products can either be in the form of a melt (Al 2 O 3, TiO 2), or rise into the air in the form of smoke (P 2 O 5, Na 2 O, MgO). The molten solid particles create the luminosity of the flame. During the combustion of hydrocarbons, the strong luminosity of the flame is provided by the glow of carbon black particles, which are formed in large quantities. A decrease in the content of carbon black as a result of its oxidation reduces the luminosity of the flame, and a decrease in temperature makes it difficult to oxidize carbon black and leads to the formation of soot in the flame.
In order to interrupt the combustion reaction, it is necessary to violate the conditions for its occurrence and maintenance. Usually, for extinguishing, violation of two basic conditions of a steady state is used - a decrease in temperature and a mode of movement of gases.
Temperature drop can be achieved by introducing substances that absorb a lot of heat as a result of evaporation and dissociation (eg water, powders).
Gas movement mode can be changed by reducing and eliminating the supply of oxygen.
Explosion, according to GOST 12.1.010 " Explosion proof”, - a fast transformation of matter (explosive combustion), accompanied by the release of energy and the formation of compressed gases capable of doing work.
An explosion, as a rule, leads to an intense increase in pressure. IN environment a shock wave is generated and propagated.
shock wave has a destructive capacity if the excess pressure in it is higher than 15 kPa. It propagates in the gas ahead of the flame front at a sound speed of 330 m/s. During an explosion, the initial energy is converted into the energy of heated compressed gases, which is converted into the energy of movement, compression and heating of the medium. Possible different kinds the initial energy of the explosion - electrical, thermal, elastic compression energy, atomic, chemical.
The main parameters characterizing the danger of an explosion in accordance with GOST 12.1.010 are the pressure at the shock wave front, the maximum explosion pressure, the average and maximum rate of pressure increase during an explosion, crushing or high-explosive properties of an explosive environment.
General explosion action manifests itself in the destruction of equipment or premises caused by a shock wave, as well as in the release harmful substances(explosion products or contained in equipment).
Max Burst Pressure(P max) - the highest pressure that occurs during a deflagration explosion of a gas, vapor or dust-air mixture in a closed vessel at an initial mixture pressure of 101.3 kPa.
Explosion pressure rise rate(dР/dt) is the derivative of the explosion pressure with respect to time in the ascending section of the dependence of the explosion pressure of a gas, steam, dust-air mixture in a closed vessel on time. In this case, the maximum and average rates of pressure increase during the explosion are distinguished. When establishing the maximum speed, the pressure increment is used in the straight-line section of the dependence of the explosion pressure on time, and when determining the average speed, the section between the maximum explosion pressure and the initial pressure in the vessel before the explosion is used.
Both of these characteristics are important factors for explosion protection. They are used in establishing the category of premises and buildings in terms of explosion and fire hazard, in the calculation of safety devices, in the development of measures for fire and explosion safety of technological processes.
Detonation there is a process of chemical transformation of the oxidizer-reductant system, which is a combination of a shock wave propagating at a constant speed and exceeding the speed of sound, and following the front of the zone of chemical transformations of the initial substances. chemical energy, released in the detonation wave, feeds the shock wave, preventing it from decaying. The speed of the detonation wave is a characteristic of each specific system.
The theory states that the explosion of a gas or vapor-air mixture is not an instantaneous phenomenon. When the ignition source is introduced into the combustible mixture, the oxidation reaction of the fuel with the oxidizer begins in the area of the ignition source. The rate of the oxidation reaction in some elementary volume of this zone reaches a maximum - combustion occurs. Combustion at the boundary of the elementary volume with the medium is called the flame front. The flame front looks like a sphere. The thickness of the flame front, according to Ya.B. Zeldovich , equal to 1-100 microns. Although the thickness of the combustion zone is small, it is sufficient for the combustion reaction to proceed. The temperature of the flame front due to the heat of the combustion reaction is 1000-3000°C and depends on the composition of the combustible mixture.
When the flame front moves, the temperature of the unburned part of the combustible mixture increases, as the pressure of the mixture increases. Near the flame front, the temperature of the mixture also rises, due to the non-
heat transfer by thermal conduction, diffusion of heated molecules and radiation. On the outer surface of the flame front, this temperature is equal to the self-ignition temperature of the combustible mixture.
After ignition of the combustible mixture, the spherical shape of the flame is very quickly distorted and more and more drawn towards the still unignited mixture. The extension of the flame front and the rapid increase in its surface is accompanied by an increase in the speed of the central part of the flame. This acceleration lasts until the flame touches the walls of the pipes or, in any case, does not come close to the wall of the pipe. At this moment, the size of the flame decreases sharply, and only a small part of it remains from the flame, covering the entire section of the pipe. Pulling the flame front,
and its intense acceleration immediately after ignition by a spark, when the flame has not yet reached the walls of the pipe, are caused by an increase in the volume of combustion products. Thus, at the initial stage of the formation of the flame front, regardless of the degree of combustibility of the gas mixture, acceleration and subsequent deceleration of the flame occurs, and this deceleration will be the greater, the greater the flame speed.
The process of development of the subsequent stages of combustion is influenced by the length of the pipe. Elongation of the pipe leads to the appearance of vibrations and the formation of a cellular structure of the flame, shock and detonation waves.
The width of the heating zone (in cm) can be determined from the dependence
1 = a / v
where but- coefficient of thermal diffusivity; v- flame propagation speed.
Linear travel speed v(in m/s) can be determined by the formula
V = V t /
where V t- mass burning rate, g / (s m 3); - density of the initial combustible mixture, kg/m 3 .
The linear velocity of the flame front is not constant, it varies depending on the compositions. Mixtures and impurities of inert (non-combustible) gases, mixture temperature, pipe diameter, etc. The maximum flame propagation velocity is observed not at a stoichiometric mixture concentration, but in a mixture with an excess of fuel. When inert gases are introduced into the combustible mixture, the flame propagation speed decreases. This is explained by a decrease in the combustion temperature of the mixture, since part of the heat is spent on heating the inert impurities that do not participate in the reaction.
With an increase in the diameter of the pipes, the speed of flame propagation increases unevenly. With an increase in the diameter of the pipes to 0.1-0.15 m, the speed increases quite quickly. The temperature rises until the diameter reaches a certain limiting diameter,
above which there is no increase in speed. With a decrease in the diameter of the pipe, the flame propagation speed decreases, and at a certain small diameter, the flame does not propagate in the pipe. This phenomenon can be explained by an increase in heat losses through the walls
pipes.
Therefore, in order to stop the spread of flame in a combustible mixture, it is necessary in one way or another to lower the temperature of the mixture by cooling the vessel (in our example, a pipe) from the outside or by diluting the mixture with cold inert gas.
The normal speed of flame propagation is relatively small (no more than tens of meters per second), but under certain conditions, the flame in pipes propagates at a tremendous speed (from 2 to 5 km / s), exceeding the speed of sound in a given environment. This phenomenon has been called detonation. Distinctive features detonations are as follows:
1) constant burning rate regardless of the pipe diameter;
2) high pressure a flame caused by a detonation wave, which may exceed 50 MPa, depending on the chemical nature of the combustible mixture and the initial pressure; moreover, due to the high burning rate, the developed pressure does not depend on the shape, capacity and tightness of the vessel (or pipe).
As the flame accelerates, the amplitude of the shock wave also increases, and the compression temperature reaches the self-ignition temperature of the mixture.
The increase in the total amount of gas burning per unit time is explained by the fact that in a jet with a velocity variable in cross section, the flame front bends, as a result of which its surface increases and the amount of the burning substance increases proportionally.
When gas mixtures are burned in a closed volume, the products of combustion do not do work; the energy of the explosion is spent only on heating the products of the explosion. In this case, the total energy is defined as the sum of the internal energy explosive mixture Q ext.en.cm and the heat of combustion of a given substance ΔQ g. The value of Q vn.en.sm. is equal to the sum of the products of the heat capacities of the components of the explosive mixture at a constant volume and the initial temperature
mixture temperature
Q ext.en.cm \u003d C 1 T + C 2 T + ... + C p T
where C 1, C 2, C p - specific heat capacities of the components that make up
explosive mixture, kJ/(kg K); T - initial temperature of the mixture, K.
The explosion temperature of gas mixtures at constant volume is calculated by the same method as the combustion temperature of a mixture at constant pressure.
Explosion pressure is found from the explosion temperature. The pressure during the explosion of a gas-air mixture in a closed volume depends on the temperature of the explosion and the ratio of the number of molecules of combustion products to the number of molecules in the explosive mixture. During the explosion of gas-air mixtures, the pressure usually does not exceed 1.0 MPa, if the initial pressure of the mixture was normal. When the air in the explosive mixture is replaced by oxygen, the pressure of the explosion increases sharply, since the combustion temperature increases.
Explosion pressure of stoichiometric mixtures of methane, ethylene, acetone and
methyl ether with oxygen is 1.5 - 1.9 MPa, and their stoichiometric mixtures with air is 1.0 MPa.
The maximum explosion pressure is used in calculations of the explosion resistance of equipment, as well as in the calculations of safety valves, explosive membranes and shells of explosion-proof electrical equipment. Explosion pressure R vzr (in MPa) of gas-air mixtures is calculated by the formula
R vzr =
where p 0- initial pressure of the explosive mixture, MPa; T 0 And T vzr- the initial temperature of the explosive mixture and the temperature of the explosion, K;
The number of molecules of gases of combustion products after the explosion;
is the number of gas molecules of the mixture before the explosion.
