Experimental Study of Flame Acceleration in Closed Pipe

Hydrogen is regarded as one of the most promising fuels in the future, which offers carbon free emissions at the point of use. However, the transportation of the hydrogen from manufacturing to end users could affect the integrity or durability of the pipeline network as the existing gas pipeline network are designed, constructed and operated using natural gas. The hydrogen would mix with natural gas once introduced into the pipeline. Hydrogen has unique chemical and physical properties, which may adversely affect on pipeline network, hence increase the potential risk of explosion [1]. For the safer use of hydrogen fuels, it is essential to fully characterize and quantify their explosion behaviour. In particular, knowledge is required about maximum pressure, maximum rate of pressure rise (i.e., deflagration index) and burning velocity, which are among the most important parameters for the assessment of process hazards and the safe design of process equipment. Even though the effects of enriching natural gas with hydrogen on flame stabilities, flammability regions and burning velocity have been widely studied [2-4], yet the issues of explosion safety for hydrogen/natural gas mixtures are still unclear. 

Hence, this study aims to investigate the physical and dynamic characteristics of hydrogen/natural gas explosion, particularly on the effect of pipe configuration, i.e. straight and 90 degree bending. Pure natural gas /air mixtures will be used as a baseline to study the influence of hydrogen enrichment on the fuel/air mixtures. For this purpose, several different mixture compositions (e.g. 3%, 4 %, 6% and 8% hydrogen) are considered. A key objective is to determine factors that could quantify the effect of hydrogen enrichment on combustion behavior such as the hydrogen concentration, flame speed and rate of pressure rise. It can be said that an order of magnitude in overpressure of hydrogen/natural gas mixtures was observed on bending pipe compared to natural gas/air mixtures. This is due to bending that acts similar to obstacles. This mechanism could induce and create more turbulence, initiating the combustion of unburned pocket at the corner region, causing high mass burning rate and hence, increasing the flame speed. In this paper, comparison between pure hydrogen/air mixtures with hydrogen/natural gas mixtures was also made and factors enhancing the combustion behavior on hydrogen/natural gas mixtures were highlighted.  

References

[1] Bjerketvedt, D., J.R. Bakke, and K. van Wingerden, Gas explosion handbook. Journal of Hazardous Materials, 1997. 52(1): p. 1-150.

[2]. Xiao, H., et al., Experimental and numerical study on premixed hydrogen/air flame propagation in a horizontal rectangular closed duct. International Journal of Hydrogen Energy, 2010. 35(3): p. 1367-1376.

[3]. Cammarota, F., et al., Combined effects of initial pressure and turbulence on explosions of hydrogen-enriched methane/air mixtures. Journal of Loss Prevention in the Process Industries, 2009. 22(5): p. 607-613.

[4]. Salzano, E., et al., Explosion behavior of hydrogen–methane/air mixtures. Journal of Loss Prevention in the Process Industries, 2012. 25(3): p. 443-447.