Study On The Dynamics Of Gas Explosion Process In Confined Space

Flammable gases are widely used in industries especially in chemical or petroleum chemical industry. Accidental gas explosion in confined space often occurs because of defection of installations or mistaken operations. High pressure and rate of pressure rise caused by confinement of explosion energy are intolerable for most industrial equipment. Many injuries and much financial loss are caused, and even environmental damage. So importance of the research work on gas explosion in confined space is obvious, which is valuable and referable for prevention and control of such hazardous industrial disaster.Classification and statistics of gas explosion accidents in chemical equipment happened from 1949 and 2000 in our country are analyzed. The results show that gas explosion is the major source of risk in chemical industry. So gas explosion in confined space is decided as the object of the research work in this thesis. Firstly computation methods of gas explosion pressure and temperature are presented based on analysis of chemical engineering thermodynamics. The thermodynamics models of local combustible gas explosion pressure and temperature in enclosed space are also investigated. Considering the characteristic of multicomponent for the mixture and effect of high pressure and high temperature on fundamental properties for the mixture of flammable gas and air, calculation methods of thermal and transport properties for single component and multicomponent are recommended. The precision of mathematical model of gas explosion is thus improved by the use of state equations and the mixture regulation for real gas.Physical model and mathematical model of gas explosion in enclosure are established on the basis of theoretical investigation and previous studies. Then initial and boundary conditions are presented. EBU-Arrhenius model is used to simulate the reaction of gas mixture which can solve the problem of not calculating turbulent combustion rate accurately. In order to obtain the information of flame propagation, shock wave and gas flow turbulence in explosion process, adaptive grid technique is adopted. The gas explosion models are validated by experimental data in literatures and calculated data by typical theoretical models previously. Fair agreement between simulation results and experimental data or calculated data by theoretical models shows that it is applicable and accurate to use the mathematical models to simulategas explosion in confined space.The behavior of gas explosion partially and fully filled with methane-air mixture in pipe, gas explosion with obstacles and by multi-resources have been revealed. Explosion pressure variation and flame propagation is built up after ignition in enclosed pipe. Pressure and temperature rise appears in the pipe. And dilatant flow appears ahead of flame front. Density of resultants is lower than reactants. Flame propagates from the ignition end to another end of the pipe which stops when the flame arrives at another end of the pipe and the maximum pressure is obtained. Gas explosion pressure partially with methane-air mixture in pipe is lower than fully filled with methane-air mixture. However very high pressure is still caused by lower filling rate of flammable gas. Acceleration mechanism of flame transmission in methane-air explosion is due to mutual feedback of turbulent combustion and gas flow which is induced by obstacles in pipe. More obstacles cause higher explosion pressure, rate of pressure rise and gas velocity. Different from single explosion resource, gas explosion pressure is not uniform with multi-sources in pipe by the influence of interaction of explosion waves. Two explosion waves develop ahead of flame front which move towards each other. When the two waves encounter, gas flow in two directions piled up where gas density and temperature rise rapidly. Pressurized gas flow piled up starts to flow in two directions and pressure in pipe is gradually uniform.Dynamics of unrestricted and restricted venting of gas explosion in pipe are numerically investigated. Explosion pressure rises slowly with unrestricted venting. Vent area is one of the most important influence factors. Explosion pressure decreases with increment of vent area. Unlike unrestricted venting, two stages are involved in restricted venting of gas explosion in pipe, including gas explosion in enclosed pipe and vented deflagration. Restricted venting of gas explosion is influenced by vent area and relief pressure.Effects on gas explosion behavior in enclosed pipe are simulated. Ignition position, concentration of flammable gas, gas type, initial pressure, initial temperature, ratio of length and diameter of pipe, obstacles, water content and inert gases play a great role in gas explosion in pipe. More over, fineness of calculation grids also affect the simulation results.Gas explosion in two linked vessels are numerically simulated. Dynamics characteristics and the mechanism of higher explosion strength compared to isolated vessel are investigated. Characteristics of flame propagation, pressure variation andgas flow are obtained. After ignition in the first vessel, a laminar flame front develops which, when it enters the pipe, is accelerated and becomes turbulent. The jet of flame which then enters the second vessel, is related to precompressed mixture of unburned gas, since the displacement of unburned mixture ahead of the flame front caused precompression and turbulence in the second vessel. This is a local dynamic effect called as pressure piling which can cause high local explosion pressure. The flame is accelerated during gas explosion especially when it enters the pipe from ignition vessel and leaves the pipe to enter the second vessel. Gas mixture in the vessels starts to flow backward because of the difference of pressure induced by gas combustion in the vessels. Transition of flame propagation from laminar combustion to turbulent combustion occurs in the course of gas explosion in such vessels. So compression, backflow and jet flame of unburned gas mixture represent a major factor affecting the explosion violence. Influence factors of gas explosion in linked vessels with the same volumes and different volumes are investigated, including ignition position, diameter and length of pipe, volume of vessels, transmission directions of flame and ratio of volumes of the two vessels.Strength analysis of pressure shock resistant vessel subjected to different gas explosion loads is made by using the finite element analysis software ANSYS. The maximum equivalent stress of the vessel subjected to detonation load is much higher than under deflagration load because of higher peak pressure and rate of pressure rise. Detonation load can cause bigger deformation than deflagration load. And the vessel and connecting pipe firstly expands outward under dynamic explosion load. The support plates also deforms outward and bends which cause the vessel falls. At last the vessel is inclined due to plastic deformation. Based on strength analysis of pressure shock resistant vessel under gas explosion, a simulating method for safety design of such vessels is put forward.