Vladislav Shevkunenko

Electrotechnical faculty

The Department of Mining Transport and Logistics

«Speciality «Electromechanical Equipment of Power-Consuming Industries»»

Justification of parameters and development of recommendations for improvement explosion protected electrical equipment

Scientific supervisor: Aleksander Tkachuk

About author

Summary of research and developments

Introduction

To date, the coal industry of Ukraine is one of the most difficult in regard to ensuring safety.
To avoid the likelihood of accidents all electrical equipment, used in mines, must be explosion-proof [1].

Topical issue

Analysis of explosion protected electrical equipment showed that explosion-proof shell is the most common conception of explosion safety.
Animated drawing, made in gif animator, the number of frames - 12, the number of repetitions - 5, volume 184 КБ

Animated drawing, made in gif animator, the number of frames - 12, the number of repetitions - 5, volume 184 КБ

Explosion in explosion-proof equipment         The concept implies that the electrical equipment is placed in a special shell with a small gap. This does not prevent contact of electrical circuits with an explosive mixture and the possibility of inflammation, but it is guaranteed that the shell constrains resulting from blast overpressure, and flash does not go beyond the restrictions of shell.
        However, this type of protection increases the weight and dimensions of electrical equipment, degrades the thermal regime[1,2,3].
        An urgent task is to develop recommendations for improvement explosion protected electrical equipment.

The aim and objectives

The aim of this work is to develop recommendations for improving the consumer properties of explosion protected electrical equipment, such as reliability, usability, etc.
To achieve the above objectives the following main tasks:
1.To do review and analysis of explosion–proof construction of electro equipment.
2.To do analysis of claims submitted to the constructions of explosion–proof electro equipment.
3.To Develop a mathematical model of an explosion inside a flameproof.

Main results

To construct a mathematical model of the explosion is expedient divided into 3 phases:
Stage 1 — initialization of the explosion. Instantaneous release of energy from a point source. The source can be:
a) a spark from an electrical discharge.
b) chemical reaction in the gas mixture, which began as a result of the presence of a catalyst.
In this case, the amount of energy released by the source should be sufficient to increase the velocity of the molecules (temperature of the mixture) to a level sufficient to initiate a chemical reaction. The chemical reaction rate depends on the concentration of reacting elements. If the concentration is sufficient, then the rate of chemical reaction can be so high that the reaction proceeds in the form of an explosion. A sharp increase in the volume of gas, which explains the decrease of temperature, revealed by the experiments[4,5].
Stage 2 — the motion of the blast wave. The emergence of a spherical wave formed by the reacting molecules to produce heat of the gas mixture. The wave front moves from the source, acting as a piston compresses the gas in front of him. The energy of a chemical reaction in the wave of costs of carrying out work on gas compression in the shell and the message of the kinetic energy of molecules of gas in front of the wave.

where p2 is the pressure of the gas after the explosion; V1–envelope volume; V2–volume of the mixture after compression; v–velocity wave; At the second stage of the explosion, the gas inside the shell is a perfect
The process of reducing the temperature described by equation in polar coordinates takes the form:

where r–distance from the center of the sphere.
Ie excess heat between the heat, which came through the interior of the sphere and emerging through the outer surface of the elementary concentric layer is heat accumulated in the element Fig.1

Figure 1. The process of heat distribution.
If the combustible mixture, then the cooling process is slowed down due to allocation of additional heat due to the development of a chemical reaction. When the temperature drops to T–temperature of combustion of the mixture, further lowering the temperature will stop due to compensation of the heat of exhaust into the surrounding layers of the mixture of heat produced during combustion. Ie creates a spherical flame front, which consists of the preheating zone and reaction zone (Fig. 2).

Fig.2 The model of explosion of combustible mixture.
In the preheat zone, constituted the bulk of the flame front, a fresh mixture gets warm, you need to heat it to the combustion temperature Tr. This heat is due to the heat from an area in which chemical reactions occur. The smaller the radius of the initial core of burning gas, the greater the ratio of the amount of heat, leaving a spherical volume (it is proportional to the square of the radius) to the amount of heat generated in this volume (it is proportional to the cube of the radius)[6,7].
Thus, the critical conditions of ignition spark can be reduced to heat the gas sphere whose radius is nearly four times the width of the zone of laminar flames in this combustible mixture to the flame temperature due to thermal energy discharge. Under this condition, the surrounding layers of the fuel mixture will have time to ignite before the spark to cool the heated volume. The forces acting on the molecules of gas, the second phase of the explosion are the central (Fig. 3). That is:

where F–the force acting on an element of gas at the source of explosion, p–density of gas inside the shell; mi–mass of the gas molecules, and E–the internal energy of gas in the shell; That is, the flow forces acting on the molecules of gas from the source of the explosion through any closed surface is proportional to the mass of the fuel–air mixture contained in a volume bounded by this surface and does not depend on its shape.

Fig.3 Model of the explosion of combustible mixture.
Stage 3 — extinguishing blast.
Meeting of the wave with the surface of the shell, the problem is to absorb the energy of a chemical reaction and to prevent the penetration of the explosion to the environment. In this condition should be satisfied:

where: Ehim — the energy of a chemical reaction; p2–pressure in the shell after the explosion; V1–volume of the shell; m–mass of gas inside the shell; v–speed of the wave front; Erazr — fracture energy of the shell;
Thus, using equations, we can get the system to determine the velocity, pressure, density and temperature atboundary of the shell .

Conclusion

Study the mechanism of ignition of combustible mixture showed that there are currently two approaches for modeling this process, heat and electric. For the thermal approach, there are differences in the wording of the conditions of the explosion. When an electric approach experimentally detected effect of reducing the temperature at the time of the explosion, which contradicts the theory of heat. In this paper, a model of an explosion, taking into account the contradiction between the thermal and electrical theory. The forces acting on the molecules of gas in the explosion, are central, and the flow of forces acting through any closed surface does not depend on its shape. The resulting mathematical model of the explosion gas mixture inside the shell provides a system of equations to determine the parameters of the explosion and refinement of design calculations for the shell, which is the direction for further research. The correctness of the proposed method was confirmed by tests conducted at the expert — Technical Center Institute UkrNIIVE.

References

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