Normal flame propagation speed. Flammable energy and concentration limits of ignition. Water hammer wave propagation speed

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

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 flame, 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 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 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 on their own 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 are combustible 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 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 environment "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 concentrations in a particular 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, 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.

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 flame spread rate should be used in calculating the rate of increase in the pressure of an explosion of gas and vapor-air 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 depressurizing 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 volumetric fuel contents 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)

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.

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

Methods for calculating the burnout rate of liquids

; (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;

nF- 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 ;

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.