Impacts of turbulence on explosion characteristics of methane-air mixtures with different fuel concentration

To study the effects of turbulence on the typical explosion characteristics (the maximum explosion pressure p(max), the explosion duration time tau(e), the maximum rate of pressure rise (dp/dt)(max) and the deflagration index K-G) of methane-air mixtures, experiments are conducted in a standard 20-L explosion spherical vessel, the concentration of methane varies from 0.06 to 0.15, the mixtures are ignited by a spark in the end-wall of the vessel. The purpose of this study aims to further probe the relationship between the positive turbulence-enhancement effect and the negative inert gas inhibition effect in methane-air mixture with the conditions that covering the entire flammability limit. The turbulence is generated when a N-2 jet is promptly injected into the chamber from a high-pressure storage tank via a solenoid valve, and the initial pressure of the storage tank p(J0) is set as 500 kPa. The explosion behavior of methane-air mixture under the effect of turbulence is firstly compared with that in quiescent conditions. For quiescent explosions, adding a small amount of N-2 into the methane-air mixture has no notable impact on the explosion behavior, while a relatively large amount of N-2 significantly slows down the explosion rate, but it has little influence on pmax. Furthermore, the influence of turbulence associated with different concentration of methane on the explosion behavior is systematically analyzed. The results of quiescent explosions indicate that, the values of explosion parameters increase with the increasing of p(0), by adding N-2 dilution, it has a notable effect on reducing (dp/dt)(max) and this effect is weak on pmax, which is due to the competing mechanism between the positive effect from increased initial pressure and the negative effect of inert gas being achieved to an equilibrium state. By introducing turbulence, results clearly show that the turbulence has a prominent enhancement effect on the explosion, especially as t(J0) within a short time; longer t(J0) does not further contribute in promoting the explosion hazard because of the suppression effect of N-2 dilution balances a part of turbulent kinetic energy. The impact of jet turbulence on pmax at the condition near flammability limits is more prominent than that near stoichiometric condition, this behavior is attributed to the mixture being relatively well-reacted when near stoichiometric condition, and thus the positive effect of turbulence on reducing heat loss is weak due to the short reaction time in this case, resulting in a minor change in p(max). The experimental results illustrate that the turbulence-enhancement on explosion is greatly dependent on t(J0) and t(d), at a critical state when heat loss and turbulence intensity achieve to a best condition during the explosion process, and this state is related to the optimal value of t(J0) and t(d).