Experiment And Simulation On Pressure And Impulse Field Of Gas Cloud Deflagration

The cloud deflagration has made great damage to people and property loss. Investigation into flammable gas cloud deflagration law is the basis of deflagration suppression and prevention technologies. The correlative experimental research especially gas cloud built-in obstructions focuses on some experimental results under particular circumstances. It is very difficult to draw general and reliable regulations by means of experimental research completely due to the complicated factors affecting deflagration. On engineering, TNT Equivalency Method, Multi-energy Model and Scale Rule Method, etc., brought forward belong to empirical models, which are limited to make a rough estimation for explosion power and possess strong subjectivity. In theory, Self-Resemblance Method is applied only to steady flame propagation. CFD mehod is the main research means in the recent years. Because of deflagration complexity and subject cross, the dealing with obstruction is still the hotpot problem facing the learning. Moreover, impulse field must be emphasized besides pressure field. Some damage to structures is likely caused by pressure, also is likely caused by impulse and is likely the result of synthetical actions of them. So it not only possesses profound engineering application background but also has significant academic value and social meaning to investigate the deflagration pressure field and impulse field systematically adopting the method combining experiments with numerical simulation.Firstly, gas cloud deflagration regulations are concluded from a series of experiments and at the same time the physical and mathematical models are put forward by means of thermodynamics , combustion dynamics and gas dynamics analysis. Secondly, the general rules and ways are abtained applying CFD to solve the maths model. The main work and conclusion are as follows.1. Experimental investigation into uniform flammable gas cloud deflagration induced by built-in obstructionsThe gas cloud deflagration experiments are carried out by means of acetylene/air premixed gas. The deflagration pressure field and impulse field induced by built-in spherical-zone obstructions is studied sysmatically. The obstructions enhancement on deflagration is discussed. (1) A flammable gas cloud deflagration dynamic measuring system consisting of pressure transducers, AD conversion card, data acquisition card, computer, electric spark ignition device and over-loading protecting device is devised and set up.(2) Through mass spectrum and chromatogram analysis, the components of the acetylene/air premixed gas were analysed. The volume concentration of mixture leading to the largest deflagration power is confirmed as 13.3% by means of corresponding deflagration experiments.(3) A series of perpendicular experiments and deviation analysis were conducted on the built-in spherical zone obstruction. The results show that the character parameter (radius r₀ and shielding angleθ₀) of obstructions play a prominent role on deflagration pressure field and impulse fields, and the effect of the interaction between them is little. The deflagration overpressure and impulse induced by obstructions is increased by 8-10 times respectively.(4) The multi-rectangle-hole hemisperical shells and the multi-circle-hole hemispherical shells were used as obstructions inside the gas cloud to study the effects of geometry on gas cloud deflagration pressure field and impulse field. It is showed that the overpressure and impulse induced by multi-rectangle-hole obstructions is larger than that of multi-circle-hole obstructions.(5) Based on the clearup, regression and deviation analysis of experimental data, the fitting equation is obtained between the overpressure, impulse and obstruction character parameters(r₀,φ), gas cloud radius R₀, measuring distance d and obstruction geometry:namely, deflagration pressure and impulse increase with the increasing of the gas cloud radius R₀, obstruction radius r₀, and they decrease with the increase of measuring distance and opening ratio.2. Numerical simulation of uniform flammable gas cloud deflagration(1) Based on the time-averaged equations of hydrodynamics,κ-εturbulent model and EBU-Arrhenius combustion model, the mathematical model of flammable gas cloud deflagration has been formed. The acetylene/air premixed gas deflagration mathematical model has been solved through the high accuracy difference scheme and the SIMPLE algorithm.(2) The relation curves between deflagration overpressure, pressure rise velocity and fuel activity, fuel concentration, gas cloud radius and gas cloud initial pressure were obtained by means of numerical simulation of unconfined gas cloud deflagration. The deflagration pressure-time history is also obtained. The calculated result is in good agreement with the previous experiment. It is evident that the mathematical model can be used to simulate the gas cloud deflagration.(3) The numerical character of the program was discussed. It is concluded that the numerical solution is grid-independent when the calculated area is equal to the 8 times size of original gas cloud radius, grid size is 1mm and time step size is 2×10^-5s.(4) The numerical simulation of the built-in obstruction gas cloud deflagration shows that the disturbance induced by obstruction on the flame front and pressure wave front is negligible originally. When the flame approaches obstructions, the flame front and the pressure wave front change greatly. The flame velocity increases rapidly near the obstruction. The turbulent reaction section is induced behind the obstruction leading to the rapid rise of the flame velocity at first, then the flame velocity decreases rapidly and in the end it rises rapidly. The larger the radius and shielding angle of obstructions is, the smaller the opening ratio is, the faster the flame velocity is and the larger is the deflagration power. It is concluded that the obstruction enhances the deflagration power.(5) The mechanism of flammable gas cloud deflagration enhanced by obstructions is as follows: The obstruction distorts the flame front. At the same time a large amout of unreacted gas is captured and a large amount of vortex is induced behind the obstruction because of pressure wave. Thus the volume combustion velocity increases leading to the increase of flame propagation as well as the flow field velocity and the deflagration power. The larger the radius and the shielding angle of the obstruction, the smaller the opening ratio is, the more the unreacted gas behind obstruction is, the larger the deflagration power is.3. Numerical simulation of Gaussian distribution flammable gas cloud deflagrationDeflagration of acetylene/air premixed gas cloud in Gaussian distribution was simulated, and deflagration pressure and impulse were obtained. The deflagration grade and the damage effect of industry Gaussian cloud were forcast according to Schumacher pressure-impulse criterion. It showed that the built-in obstructions Gaussian cloud deflagration has stronger damage power. The deflagration power induced by the acetylene/air premixed gas cloud of 60m radius built in 30m radius obstruction can reach grade 6 in Multi-Energy Model which can make brick walls and reinforced concrete housetop collapse.The comparison of deflagration calculated results of Gaussian gas cloud built in obstructions with unconfined Gaussian gas cloud and uniformed gas cloud built in obstructions shows that the difference in them is large. The uniform gas cloud built in obstructions leads to the largest deflagration power. Because it is in the most hazardous concentration leading to self-accelerating mechanism and turbulence accelerating mechanism induced by obstructions. Unconfined gas cloud results in the lowest deflagration power owing to the self-accelerating time is very short with the decrease of gas cloud concentration. Gaussian gas cloud built-in obstructions makes the larger deflagration power due to its shorter duration of flame self-accelerating mechanism and the turbulence accelerating mechanism.