Effect of Tube Size on Flame and Pressure Wave Propagation in a Tube Closed at One End: A Numerical Study

Deflagration of fire and explosion caused by ignition of a methane-air mixture is a major safety concern in industrial settings, such as the mining and petrochemical industries. The aim of this study is to investigate numerically the effect of tube size on the flame and pressure wave propagations for methane-air deflagration in a tube closed at one end. A partially premixed combustion model that avoids the need to specify the flame speed is deployed based upon the Flamelet Generated Manifold (FGM) model. Good agreement is achieved between the predicted results and the benchmark experimental data collected using a large-scale detonation tube (L/D = 66). Subsequently, the explosion behaviour immediately after the ignition of methane-air mixtures and the propagation characteristics of the flame front and pressure wave through the tube are examined, covering a broad range of L/D, that is from 26 to 526, in which the diameter is changed and the length is kept fixed. The results show that the pressure wave propagates significantly faster in narrower tubes and hence decouples from the flame front shortly after ignition, which in turn results in a low overpressure at the flame front. Moreover, abrupt changes in gas properties are observed in narrow tubes with L/D >= 132. The peak overpressure increases as the tube diameter increases; however, the local maximum pressure decreases substantially in large tubes when approaching the tube vent but remains almost constant throughout in the narrow tubes. Similarly, the flame propagates faster in narrower tubes. A correlation that estimates the distance the flame propagates in the exponential acceleration stage is proposed as a function of tube size and time. Deviations of less than 7% are obtained when comparing the predicted results using the correlation against the experimental data. The results provide local information that aids the theoretical interpretation of experimental observations and the understanding of the fuel combustion and explosion phenomena in different sized tubes.