Laminar burning rate – the speed with which the flame front moves in the direction perpendicular to the surface of the fresh fuel assembly.
– laminar combustion zone;
– speed of laminar combustion.
Turbulent combustion.
Turbulent flame speed – the speed with which the flame front moves in a turbulent flow.
– turbulent combustion zone;
– normal speeds of small particles.
Laminar combustion does not provide the required rate of heat release in the engine, so turbulence of the gas flow is required.
Arrhenius equation:
- speed chemical reaction.
– chemical reaction constant, depending on the composition of the mixture and the type of fuel;
– chemical reaction pressure;
– the order of the chemical reaction; 
–universal gas constant;
– temperature of the chemical reaction;
– activation energy is the energy required to break intramolecular bonds.
The influence of various factors on the combustion process in an internal combustion engine with spark ignition.
Composition of the mixture.
– upper concentration limit;
–lower concentration limit;
– normal combustion;
–power composition of the mixture
– maximum power developed by the engine.
–economic composition of the mixture
– maximum efficiency.
Compression ratio.
With an increase in speed, the ignition phase increases, which leads to a late development of the combustion process and a decrease in the amount of heat released per cycle. Therefore, when changing 
regulation of the ignition timing (IPA) is required.
Ignition timing.
Ignition timing – the angle of rotation of the crankshaft from the moment the spark is supplied to TDC.
P 
under load
understand the angle of rotation of the throttle valve - this is what regulates the load on the engine.
– angle of rotation of the throttle valve.
The main disturbances in the combustion process in spark-ignition combustion engines. Detonation.
D 
etonation
– explosive combustion of the mixture, accompanied by shock waves of pressure propagating throughout the volume of the combustion chamber. Detonation occurs as a result of self-ignition of parts of the mixture remote from the spark plug, due to intense heating and compression during the propagation of the flame front.
Upon detonation: 
Reflecting from the walls of the combustion chamber, the shock wave forms secondary flame fronts and sources of self-ignition. Externally, detonation manifests itself in the form of dull knocks when the engine is running under heavy loads.
Consequences of engine operation with detonation:
Overheating and burnout of individual engine components (valves, pistons, head gaskets, spark plug electrodes);
Mechanical destruction of engine parts due to shock loads;
Reduced power and operating efficiency.
That. Prolonged work with detonation is unacceptable.
P 
Here are the factors that cause detonation:
The ability of a fuel to self-ignite characterizes detonation resistance , and the detonation resistance is estimated octane number (OC) .
VERY – is numerically equal to the volume fraction of poorly ditonating isooctane in a mixture with easily ditonating normal heptane, which is equivalent in detonation properties to this gasoline.
Isooctane – 100 units, normal heptane – 0 units.
For example: An octane rating of 92 means that this gasoline has the same knock resistance as a reference mixture of 92% isooctane and 8% normal heptane.
A 
– motor gasoline;
and – research method for obtaining gasoline;
m – motor method (the letter is usually not written).
In the motor research method, the compression ratio is adjusted until detonation begins, and the octane number is determined from the tables.
Motor methods simulate driving at full load (truck outside the city).
Research method simulates driving at partial load (in the city).
If the octane number is excessively high, then the speed of flame propagation decreases. The combustion process is delayed, which leads to a decrease in efficiency and an increase in exhaust gas temperature. The consequence of this is a drop in power, increased fuel consumption, engine overheating and burnout of individual elements. Maximum engine performance is achieved when the fuel octane number is close to the detonation threshold.
Ways to combat detonation:
Propagation of the zone of chemical transformations in an open combustible system
Combustion begins with the ignition of the combustible mixture in the local volume of the combustible system, then spreads in the direction of the moving mixture. The burning zone in which redox chemical reactions visible to the observer take place is called a flame. The surface separating the flame and the still unburning mixture serves as the flame front. The nature of flame propagation depends on many processes, but the determining process is the heating of the combustible mixture. Depending on the method of heating the combustible mixture to the ignition temperature, normal, turbulent and detonation flame propagation is distinguished.
Normal flame propagation is observed during combustion in a combustible system with a laminar moving mixture. With normal flame spread thermal energy from the burning layer to the cold one is transferred mainly by thermal conductivity, as well as molecular diffusion. Thermal conductivity in gases is low, so the speed of normal flame propagation is low.
During the turbulent movement of a combustible mixture, the transfer of thermal energy from the burning layer to the cold layer occurs primarily by molar diffusion, as well as thermal conductivity. Molar transfer is proportional to the scale of turbulence, which is determined by the speed of the mixture. The speed of turbulent flame propagation depends on the properties of the mixture and the gas dynamics of the flow.
The propagation of flame in a combustible mixture from the combustion zone to the cold layers through molecular and molar processes is called deflagration.
Physico-chemical combustion processes are accompanied by an increase in temperature and pressure in the flame. In flammable systems, under certain conditions, high-pressure zones may arise that can compress adjacent layers, heating them to the point of ignition. Flame propagation by rapid compression cold mixture up to the ignition temperature is called detonation and is always explosive in nature.
In flammable systems, vibration combustion can occur, in which the flame front moves at a speed that varies in both magnitude and direction.
The speed of propagation of the combustion front in a laminar moving or stationary mixture is called the normal or fundamental speed of flame propagation. The numerical value of the normal speed is determined by the speed of the mixture that has not yet ignited, normally directed towards the combustion front.
The value of u n for a flat combustion front can be determined from the condition of dynamic equilibrium between the rate of heating of the mixture by thermal conductivity to the ignition temperature and the rate of the chemical reaction. As a result, the following formula is obtained
where l is the coefficient of thermal conductivity of the gas mixture, c p is the coefficient of heat capacity of the mixture at constant pressure, T initial is the initial temperature of the mixture, T a is the adiabatic combustion temperature, Arr is the Arrhenius criterion, k 0 is the coefficient of Arrhenius’ law.
The normal speed can be determined experimentally by the speed of movement of the front in a tube with a stationary mixture or by the height of the combustion cone in a Bunsen burner. A Bunsen burner is a laboratory burner with partial pre-mixing of gas and air. At the exit from the burner, a flame is formed with a combustion front in the form of a cone of regular shape (Fig.).
Fig.7. Combustion front in a Bunsen burner
With a stable position of the combustion front, the flame propagation speed u n is balanced by the component W n of the movement speed normal to the surface of the combustion cone gas-air mixture W, i.e.
where j is the angle between the velocity vector of the gas-air mixture and the vector of its component normal to the surface of the combustion cone.
The speed of movement of the gas-air mixture at the nozzle exit with a combustion cone of regular shape is determined by the formula
where d 0 is the diameter of the burner nozzle, V is the flow rate of the gas-air mixture through the burner.
The value of cos j can be expressed in terms of the height of the combustion cone
Taking into account the fact that the combustion surface is side surface correct cone
the normal speed value is determined
The normal flame propagation speed is affected by:
1. Initial temperature of the mixture. At low temperatures u n is directly proportional to the square of the absolute temperature of the mixture entering combustion. At temperatures above the ignition temperature, the concept of normal speed loses its meaning, since the mixture becomes capable of self-ignition.
2. Temperature of the channel walls, provided that the flame propagates inside this channel. Cold walls break chain reactions and slow down the spread of flame.
3. Channel diameter. For each combustible mixture there is a critical value of diameter dcr, starting from which the spread of flame inside the channel is impossible. The value of the critical diameter can be determined by the formula
where a cm is the coefficient of thermal diffusivity of the mixture.
4. Pressure. As pressure increases, u n decreases.
5. Composition of the mixture. For a mixture with a composition close to stoichiometric, the normal speed has a maximum value. In addition, there are lower and upper limits for fuel concentration, beyond which the flame cannot spread.
1) Humidity of the material.
2) The influence of the orientation of the sample in space.
At negative angles of inclination (direction of flame movement from top to bottom), the speed of flame propagation either does not change or slightly decreases. When the positive angle of inclination (direction of flame movement from bottom to top) increases above 10-15 0, the speed of flame propagation increases sharply.
3) Influence of speed and direction of air flows.
With increasing tailwind speed, gas exchange improves and the angle of inclination of the flame to the sample decreases. The speed of spread is increasing.
Air flow directed against the direction of flame movement has a dual effect on the speed of flame propagation.
As a result of aerodynamic braking and cooling of heated areas of the surface in front of the flame front, the speed of flame propagation decreases. On the other hand, the air flow intensifies the mixing of pyrolysis products with the oxidizer, the formation of a homogeneous combustible mixture occurs faster, the flame tip 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 geometric dimensions sample.
There are thermally thick and thermally thin samples.
Thermal thickness is the thickness of the layer of solid material heated in front of 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 flammable material comes into contact with a material (substrate) whose thermophysical properties differ from air, this will also affect the speed of flame propagation (pasted paper, wire insulation, etc.). If l low > l high. mat. , then heat will be intensively removed from the sample, and the propagation speed will be lower than in the case of the absence of a substrate.
6) Effect of oxygen content in environment.
As the oxygen content in the environment increases, the speed of flame propagation increases.
7. Influence of the initial temperature of the sample.
For wood, an increase in the initial temperature to 230–250 o C (temperature range of pyrolysis) leads to a sharp increase in u l.
Burnout of solid materials
Simultaneously with the spread of the flame over the surface of the material, the process of burning out begins. The patterns of burnout of solid materials significantly depend on the nature of the transformation of the solid phase into gaseous products.
If the decomposition of the solid phase occurs in a narrow near-surface layer without the formation of a carbon 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 mechanism of combustion of solids, which occurs with the formation of a carbonaceous residue on the combustion surface, is more complex. This is how almost all substances of plant origin burn, some plastics containing non-flammable or slow-burning fillers (talc, soot, etc.). The most common flammable substances of plant origin of this type are wood. At the moment of ignition due to heat flow from the flame zone, the temperature of the surface layer of wood quickly increases to 450-500 o C. Intensive decomposition of substances occurs with the formation of volatile products and charcoal, while the temperature on the surface rises to 600 o C.
According to the depth of burning wood, there are areas with different physical and physicochemical 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 wood combustion, the material is re-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 wood layers that have not yet decomposed, and the rate of flaming combustion gradually decreases. The flaming combustion of wood stops when the mass rate of volatile emission decreases to 5 g/(m 2 s). The thickness of the coal layer reaches 15-20 mm.
The cessation of flaming combustion of wood opens up access of air oxygen to coal heated to a temperature of 650-700 o 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 o C, and the process heterogeneous combustion coal is further intensified.
The real picture of the transition from homogeneous combustion to heterogeneous combustion is somewhat different from that shown.
The main quantitative parameter characterizing the process of burning out of solid materials is the mass burnout rate, which is one of the parameters that determines the dynamics of the fire.
The reduced mass burnout rate is the amount of substance burned per unit time per unit area of the fire.
Combustion of 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 Tm >1000 K, Tbp >2500 K. The combustion mechanism is largely determined by the properties of the metal oxide. The melting point of volatile metals is lower than the melting point of their oxides. Moreover, the latter are quite porous formations.
When IR is brought to the surface of the metal, it evaporates and oxidizes. When the vapor concentration reaches the lower concentration limit ignition, they ignite. Zone diffusion combustion is installed near the surface, a large portion of the heat is transferred to the metal and it is heated to T boil. 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, promoting the formation of an oxide crust, but also into the surrounding space, where they condense and form solid particles in the form of white smoke. The formation of dense white smoke is a visual sign of burning of volatile metals.
In non-volatile metals with high phase transition temperatures, when burned, a very dense oxide film is formed on the surface, which adheres well to the metal surface. As a result of this, the rate of diffusion of metal vapor through the film is sharply reduced and large particles, for example, aluminum and beryllium, are not able to burn. As a rule, fires of such metals occur when they are in the form of shavings, powders and aerosols. They burn without producing dense smoke. The formation of a dense oxide film on the metal surface leads to the explosion of the particle. This phenomenon, especially often observed when particles move in a high-temperature oxidizing environment, is associated with the accumulation of metal vapors under the oxide film, followed by its sudden rupture. This naturally leads to a sharp intensification of combustion.
Dust burning
Dust is a dispersed system consisting of a gaseous dispersion medium (air, etc.) and a solid dispersed phase (flour, sugar, wood, coal, etc.).
Factors influencing 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 speed occurs for mixtures slightly higher than the stoichiometric composition. For peat dust it is 1.0-1.5 kg/m3.
2) Ash content.
As the ash content increases, the concentration of the flammable component decreases and, accordingly, the speed of flame propagation decreases.
As the oxygen content decreases, the speed of flame propagation decreases.
Classification of dusts according to fire and explosion hazard.
Based on the fire and explosion hazard, dusts are divided into classes:
Class I - the most explosive - j n up to 15 g/m 3;
Class II - 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 o C;
IV class - fire hazardous - j n > 65 g/m 3 ; T St > 250 o C.
DYNAMICS OF FIRE DEVELOPMENT
Fire dynamics is understood as a set of laws and patterns that describe changes in the main parameters of a fire in time and space. The nature of the fire can be judged by the combination of a large number of its parameters: the area of the fire, 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, and in others they are secondary. It all depends on what goals are set for the study of a particular type of fire.
To study 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 fire spread as the main parameters that change over time. These fire parameters are most accessible to measurement, analysis, and calculations. They serve as initial data for determining the type necessary equipment and calculation of forces and means when extinguishing fires, designing automatic systems fire extinguishing, 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 arises from an external ignition source in a small area and spreads slowly. A convective gas flow is formed around the combustion zone, which ensures 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 ranges from 1 to 3 minutes.
II. Fire start phase.
The ambient temperature in the room is slowly increasing. 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- a rapid process of growth of all the listed parameters. The room temperature reaches 250 -300°C. The “volumetric” phase of fire development and the phase of volumetric fire propagation begins. When the gas temperature in the room is 300°C, the glazing is destroyed. Afterburning can also occur outside the room (the 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 influx fresh air into the combustion zone.
IV.Fire phase.
During this phase, the room temperature may decrease briefly. But in accordance with the change in gas exchange conditions, such fire parameters as the completeness of combustion, the rate of burnout and the spread of the combustion process increase sharply. Accordingly, the overall heat release during a fire increases sharply. The temperature, which decreased slightly at the moment of destruction of the glazing due to the influx of cold air, increases sharply, reaching 500 - 600 ° C. The process of fire development is rapidly intensifying. Increases numerical value all previously mentioned fire parameters. The area of the fire, the average volumetric temperature in the room (800-900 °C), the intensity of fire load burnout and the degree of smoke reach a maximum.
V. Stationary combustion phase.
Fire parameters are stabilizing. This usually occurs 20-25 minutes into the fire and, depending on the size of the fire load, can last 20-30 minutes.
VI. Decay phase.
The combustion intensity gradually decreases, because the bulk of the fire load has already burned out. A large amount of combustion products had 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 maximum value. The intensity of radiative heat transfer to the combustible material decreased due to a decrease in temperature in the combustion zone. Due to an increase in the optical density of the medium, the burning intensity slowly decreases, which leads to a decrease in all other fire parameters. The fire area does not shrink: it can grow or stabilize.
VII. Afterburning phase.
This final phase of the fire is characterized by slow smoldering, after which after some, sometimes quite long, time, the burning stops.
Basic fire parameters
Let us quantitatively consider some of the basic 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=βQ р n V m ’Sp, (kJ/s)
where β and Q р n are constants (underburning coefficient and 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 fire development, fire temperature, gas exchange rate, etc.
The reduced mass burnout rate V m ¢ is determined by the formula:
v m ¢ = (a×T p +b×I g) v m o ¢
where a, b are empirical coefficients;
v m o ¢ - reduced mass rate of fire load burnout for a given type of combustible material;
T p - average fire temperature;
I g - intensity of gas exchange.
The dependence of the fire area on the main parameters of its development has the form:
S p = k (v p ∙ τ) n
where k and n are coefficients depending on geometric shape fire area;
v р – linear speed of fire spread;
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 temperature of the fire and the intensity of gas exchange:
v p = (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 spread 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 combustion propagation for a given type of fuel.
As the fire develops, the fire temperature and gas exchange rate will increase, increasing the linear rate of combustion propagation and the reduced mass burnout rate.
Thermal conditions during a fire
The occurrence and speed of thermal processes depend on the intensity of heat release in the combustion zone, i.e. from the heat of the fire. Quantitative characteristics of changes in heat release in a fire depending on various conditions combustion is controlled by temperature. Under temperature conditions fires understand the change in temperature over time. Determining the fire temperature using 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 of a fire is compiled not only to determine the temperature of the fire, but also to identify the quantitative distribution of thermal energy. In general, the heat balance of a fire for a given point in time can be presented as follows:
Q p = Q pg +Q k +Q l
where Q p is the heat released in a fire, kJ;
Q pg - heat contained in combustion products, kJ;
Q к - heat transferred from the combustion zone by convection to the air washing 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 combustion products. Thus, 60-70% of the theoretical combustion temperature of a given combustible material will give an approximate value of the flame temperature. The temperature of open fires depends on calorific value combustible materials, their burnout rate and meteorological conditions. On average, the maximum temperature of an open fire for flammable gases is 1200 - 1350°C, for liquids - 1100 - 1300°C and for solid combustible materials of organic origin - 1100 - 1250°C.
In an internal fire, the temperature is influenced by more factors: the nature of the combustible material, the magnitude of the fire load and its location, the combustion 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.
A fire can be divided into three characteristic periods based on 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 is burned. The end of this period occurs when the average volume temperature reaches highest value or decreases to no more than 80% of the maximum value.
Final period- characterized by a decrease in temperature due to burnout of the fire load.
Figure 9.1. Change in the temperature of an internal fire over time: 1 - curve of a specific fire; 2 - standard curve
Since the growth rate and absolute value of the fire temperature in each specific case have their own characteristic values and features, the concept of a standard temperature curve was introduced (Fig. 21.2), which summarizes the most characteristics temperature changes in internal fires. The standard temperature is described by the equation.
Combustion- these are intense chemical oxidative reactions that are accompanied by the release of heat and glow. Combustion occurs in the presence of a flammable substance, an oxidizer, 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, pyrites, most free metals, carbon monoxide, hydrogen, etc.
In a real fire, the oxidizing agent in the combustion process is usually air oxygen. The external manifestation of combustion is a flame, which is characterized by glow and heat release. When burning systems consisting only of solid or liquid phases or mixtures thereof, a flame may not occur, i.e. flameless burning or smoldering.
Depending on the state of aggregation of the initial substance and combustion products, they are distinguished homogeneous combustion, combustion of explosives, heterogeneous combustion.
Homogeneous combustion. With homogeneous combustion, the starting materials 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) /
Combustion of explosives associated with the transition of a substance from a condensed state to a gas.
Heterogeneous combustion. In heterogeneous combustion, the starting 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, engines internal combustion, combustion chambers of rocket engines.
The movement of a flame through a gas mixture is called spread of flame. Depending on the speed of propagation of the combustion flame, it can be deflagrative at a speed of several m/s, explosive at a speed of the order of tens and hundreds of m/s, and detonative at a speed of thousands of m/s.
Deflagration combustion is divided into laminar and turbulent.
Laminar combustion has a normal flame propagation speed.
Normal flame propagation speed, is the speed of movement of the flame front relative to the unburned gas, in a direction perpendicular to its surface.
Temperature increases the normal speed of flame propagation relatively weakly, inert impurities reduce it, and increasing pressure leads to either an increase or decrease in speed.
In a laminar gas flow, 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 vortex of gas jets improves the mixing of the reacting gases, since the surface area through which molecular diffusion occurs increases.
Indicators of fire and explosion hazard of gases. Their characteristics and scope
Fire hazard technological processes is largely determined by the physicochemical properties of raw materials, intermediate and final products used in production.
Indicators of fire and explosion hazard are used when categorizing premises and buildings, when developing 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 more than 300 kPa or whose critical temperature is less than 50 °C.
The following indicators are applicable for gases:
Flammability group-an indicator that is applicable for all states of aggregation.
Flammability is the ability of a substance or material to burn. Based on flammability, substances and materials are divided into three groups.
Non-flammable(non-combustible) - substances and materials that are incapable of combustion in air. Non-flammable substances can be fire hazardous (for example, oxidizers, as well as substances that release flammable products when interacting with water, air oxygen, or with each other).
Low-flammability(hard to combust) - substances and materials that can ignite in air from an ignition source, but are not capable of burning independently after its removal.
Flammable(combustible) - substances and materials capable of spontaneous combustion, as well as ignite from an ignition source and burn independently after its removal. From the group of flammable substances and materials, flammable substances and materials are distinguished.
Flammable are flammable substances and materials that can ignite from short-term (up to 30 s) exposure to a low-energy ignition source (match flame, spark, smoldering cigarette, etc.).
The flammability of gases is determined indirectly: a gas that has concentration limits of flammability in air is classified as flammable; if a gas does not have concentration limits for flammability, but spontaneously ignites at a certain temperature, it is classified as flame retardant; in the absence of concentration limits of ignition and auto-ignition temperature, the gas is classified as non-flammable.
In practice, the flammability group is used to subdivide materials by flammability, when establishing classes of explosive and fire hazardous areas according to the PUE, when determining the category of premises and buildings according to explosion and fire hazard, when developing measures to ensure fire and explosion safety of equipment and premises.
Auto-ignition temperature- the most low temperature a substance in which, under special test conditions, there is a sharp increase in the rate of exothermic reactions ending in flaming combustion.
Concentration limits of flame propagation (ignition) - that concentration range in which combustion of mixtures of flammable 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 and oxidizing medium” at which the flame can spread through the mixture to any distance from the ignition source. Within these limits the mixture is flammable, but outside of them the mixture is incapable of burning.
Temperature limits of flame propagation(ignition) - such temperatures of a substance at which its saturated vapors form concentrations in a specific oxidizing environment equal to the lower (lower temperature limit) and upper (upper temperature limit) concentration limits of flame propagation, respectively.
The ability to explode and burn when interacting with water, air 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 carrying out technological processes and conditions for joint storage and transportation of substances and materials.
Normal flame propagation speed is the speed at which the flame front moves relative to the unburned gas in a direction perpendicular to its surface.
The value of the normal flame propagation speed should be used in calculating the rate of increase in explosion pressure of gas and steam air mixtures in closed, leaky equipment and premises, critical (extinguishing) diameter in the development and creation of fire arresters, area of easily resettable structures, safety membranes and other depressurizing devices; when developing measures to ensure 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 a reaction vessel, ignite the mixture in the center with a point source, record changes in pressure in the vessel over time and process the experimental pressure-time relationship 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 pressure-time relationship, the optimization of which using a similar experimental relationship results in a change in the normal speed during the development of an explosion for a specific test.
The normal burning rate is the speed of propagation of the flame front relative to the unburned reagents. The burning rate depends on the row physical and chemical properties reagents, in particular thermal conductivity and the rate of chemical reaction, and has a very specific value for each fuel (with constant conditions combustion). In table Table 1 shows the combustion rates (and flammability limits) of some gaseous mixtures. Fuel concentrations in mixtures were determined at 25°C and normal atmospheric pressure. With noted exceptions, flammable limits are obtained using flame propagation in a pipe with a diameter of 0.05 m, closed on both sides. Fuel excess coefficients are defined as the ratio of the volumetric fuel content in a real mixture to the stoichiometric mixture (j1) and to the mixture at maximum speed combustion (j2).
Table 1
Burning rates of condensed mixtures (inorganic oxidizer + magnesium)
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As can be seen, when burning air gas mixtures at atmospheric pressure u max lies in the range of 0.40-0.55 m/s, and - in the range of 0.3-0.6 kg/(m2-s). Only for some low molecular weight unsaturated compounds and hydrogen u max lies in the range of 0.8-3.0 m/s, and reaches 1–2 kg/(m2s). By increase And max of the studied combustibles 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 significantly higher than that of 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 N. A. Silin and D. I. Postovsky) the combustion rates of compacted mixtures of nitrates and perchlorates with magnesium. To prepare the mixtures, powdered components with particle sizes of nitrates 150-250 microns, perchlorates 200-250 microns and magnesium 75-105 microns were used. The mixture was filled into cardboard shells with a diameter of 24-46 mm to a compaction coefficient 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 mass and inferior to them in linear combustion rate. The burning rate of mixtures with perchlorates is less than the burning rate of mixtures with nitrates, and mixtures with nitrates alkali metals burn with more high speed than mixtures with alkaline earth metal nitrates.
table 2
Limits of ignition and burning rate 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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| TGiV 20.05.01.070000.000 PZ |
; (16)
Where M- dimensionless burnout rate;
; (17)
M F- molecular weight of the liquid, kg mol -1;
d- characteristic size of the burning liquid mirror, m. 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 k- boiling point of the liquid, K.
The calculation procedure is as follows.
The combustion mode is determined by the value of the Galileo 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 mode:
; 
, (22)
M0- molecular mass of oxygen, kg mol -1;
n 0- stoichiometric coefficient of oxygen in the combustion reaction;
n F- stoichiometric coefficient of liquid in the combustion reaction.
B- dimensionless parameter characterizing the intensity of mass transfer, calculated by the formula
, (23)
Where Q- lower heat of combustion of the liquid, kJ kg -1;
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| TGiV 20.05.01.070000.000 PZ |
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, assumed to be 293 K;
H- heat of vaporization of liquid at boiling point, kJ kg -1;
c e- average isobaric heat capacity of a 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 rate of turbulent combustion is calculated using experimental data according to the formula
Where m i- experimental value of the burnout rate in the transitional combustion mode, kg m --2 s -1 ;
d i- diameter of the burner in which the value was obtained m i, m. It is recommended to use a burner with a diameter of 30 mm. If laminar combustion is observed in a burner with a diameter of 30 mm, a larger diameter burner should be used.
