Abstract—Gas release from the pipelines is one of the main factors in the gas industry accidents. Released gas ejects from the pipeline as a free jet and in the growth process, the fuel gets mixed with the ambient air. Accordingly, an accidental spark will release the chemical energy of the mixture with an explosion. Gas explosion damages the equipment and endangers the life of staffs. So due to importance of safety in gas industries, prevision of accident can reduce the number of the casualties. In this paper, natural gas leakages from the low pressure pipelines are studied in two steps: 1) the simulation of mixing process and identification of flammable zones and 2) the simulation of wind effects on the mixing process. The numerical simulations were performed by using the finite volume method and the pressure-based algorithm. Also, for the grid generation the structured method was used. The results show that, in just 6.4 s after accident, released natural gas could penetrate to 40 m in vertical and 20 m in horizontal direction. Moreover, the results show that the wind speed is a key factor in dispersion process. In fact, the wind transports the flammable zones into the downstream. Hence, to improve the safety of the people and human property, it is preferable to construct gas facilities and buildings in the opposite side of prevailing wind direction. Keywords—Flammable zones, gas pipelines, numerical simulation, wind effects. I. INTRODUCTION AS transporting pipelines are vital parts of worldwide countries for the operation of all economic and social activities. The functional defeat of these networks can have severe human and financial losses in numerous ways [1], [2]. In recent years, due to importance of the problem, lots of researches have been performed to improve safety of people and human property in the case of natural gas accidents. In these studies, besides considering accident preventions, lots of efforts were also prepared on the mitigation of accident consequences [3]-[6]. Natural gas accidents can be started with small leakage and then by combination of combustion triangle (fuel, oxygen and spark), get followed by devastating explosions [4]. One of the key points in prevention of gas accidents is mixture process of fuel and oxygen where lots of analytical, experimental and numerical studies have been performed in this field. Mixing process begins after pipeline failure and is O. Adibi is PhD Candidate, N. Najafpour is PhD Candidate, and B. Farhanieh is Professor at School of Mechanical Engineering, Sharif University of Technology, Azadi Ave., Tehran, Iran (e-mail: oadibi@mech.sharif.edu, najafpour@mech.sharif.edu, bifa@sharif.edu). H. Afshin is Associate Professor at School of Mechanical Engineering, Sharif University of Technology, Azadi Ave., Tehran, Iran (corresponding author, phone: +98-21-66165530; fax: +98-21-66000021; e-mail: afshin@sharif.edu). followed by pollution problems or fire creation [7], [8]. In one of these analytical-numerical studies, Meysami et al. [9], by using “PHAST” (a commercial risk management package), provided a scheme to select the most appropriate conditions for gas dispersion modeling. This scheme approaches modeling was based on the worst-case scenario. Borujerdi and Rad [10] numerically studied transient turbulent gas flow in a ruptured pipe by using a combined finite element-finite volume method. In the simulations to predict the turbulent viscosity, for the near wall region and compressibility correction, a modified model with a two-layer equation was used. Results showed that, the released mass flow rate from the rupture area reaches 2.4 times of its initial value and then becomes constant. Liu et al. [11] determined the source strength and dispersion of CO 2 releases from high pressure pipelines using real gas equation of states. In this study, the results were compared with experimental data and the results of “PHAST”. The results indicated that PHAST can predict slightly better release flow rate but may considerably underpredict the dispersion concentration. Adibi et al. [12] numerically investigated heat and mass transfer through ruptures of high pressure gas reservoirs. In this study, numerical simulations were discretized based on the finite volume method, and flow variables were calculated using density-based algorithm. The results of parametric studies showed that the exhausted mass flow rate has direct relation with reservoir’s pressure. Also, the results illustrated that in the rupture area of high pressure tanks, chocking phenomena occur, and the structure of exhausted gas is similar to under- expanded free jets. In this paper, following earlier studies, computational fluid dynamic methods are used to investigate the natural gas and air mixing process in ruptured pipelines. Besides, by performing parametric studies, the wind effects on mixing process and flammable zones are also explored. In the next section the model and methodology are introduced. Then, by analyzing concentration and flammability contours, the discussions on results are presented. II. MODEL AND METHODOLOGY A. Problem Description Flammables zones around gas pipelines are vital areas which should be protected from any probable spark (e.g. sparks from cables’ short circuit, sparks from workers lighter). Since, a tiny spark can release combustion energy and lead to terrible explosions. In this study, by simulating gas leakages through a 40 cm rupture, the mixing processes are investigated, and potential flammable zones around a pipeline Omid Adibi, Nategheh Najafpour, Bijan Farhanieh, Hossein Afshin Numerical Simulation of Natural Gas Dispersion from Low Pressure Pipelines G World Academy of Science, Engineering and Technology International Journal of Mechanical and Mechatronics Engineering Vol:12, No:2, 2018 133 International Scholarly and Scientific Research & Innovation 12(2) 2018 scholar.waset.org/1307-6892/10008547 International Science Index, Mechanical and Mechatronics Engineering Vol:12, No:2, 2018 waset.org/Publication/10008547