Simulation Study On Leakage And Diffusion Of Parking Lot Under CNG Bus

With the progress and development of human, natural gas is gradually becoming an important part of urban energy. Compressed Natural Gas (CNG) buses which use the compressed natural gas and have a complex progress of using gas have benn widely put into use, and many places of the buses may leak. Once the leaking happen, it will threat the safety of surrounding people and lead to property damage. At the same time, overpass came into developing as the product of modern city. The space under the overpass have a lot of using, including the parking of a large number CNG buses. Natural gas will accumulate and form an explosive atmosphere under the overpass after the leaking accident, because its density is samller than air. Therefore, the research and simulation of leakage and diffusion of natural gas under the overpass parking is great significant.Firstly, the principle and process of diffusion gas are analysised. According to it, we know the gas diffusion is a very complex process. The diffusion of gas was due to the effection of power factor and thermal factor. And its process is generally divided into three steps:release of gas, forming the gas cloud and diffusing of gas cloud. Secondly, according to the results of domestic and foreign scholars, summed the main effect factors of diffusion are the source of the leak, the density of the diffusion fluid, ground conditions and weather conditions. Then using the software of Gambit establish corresponding geometric model and mesh, with the prototype of overpass of Xinxing, Yuantong and Dabei in Beijing. Finally, the grid file was imported to Fluent to simulate, and the effection of different leakage rates, bridge structures and wind speeds to the gas concentration distribution and variation explosion hazardous areas was discussed. According to the picture of concentration contours, under the same conditions, natural gas was easily accumulated in the place which is between the two buses and explosion region would be more widely when the wind speed is 4m/s. When comparing the leaking under three kinds overpass, the DaBeiYao overpass more easily accumulate a large explosion area, and YuanTong overpass more likely cause the accumulation of high concentration gas. The explosion area is broader compared to the leakage rate of 30m/s,40m/s,50m/s, when the leakage rate is 20m/s. Through the concentration contours, the variation explosion of explosion hazardous areas and the concentration of some key points can provide recommendations to the safety management of overpass and important basis to the early warning and analysis of accident.