On the Second Explosion Limits of Hydrogen, Methane, Ethane, and Propane

In this work, we have first investigated the explosion limit behaviors from hydrogen to propane through numerical simulations and validated with the available experimental data. The shape of the explosion limit curves and the possible turning points (P1-2, T1-2), first to second limit transition, and (P2-3, T2-3), second to third limit transition that bound the second explosion limit as a function of the fuel carbon number, have been examined. Results show that with an increase of methane mole fraction in the hydrogen/methane system, the upper turning point (P1-2, T1-2) remains almost unchanged and the lower transition point (P2-3, T2-3) rotates counterclockwise around (P1-2, T1-2). With a further increase of carbon number, (P1-2, T1-2) moves to the lower-pressure and -temperature region and (P2-3, T2-3) gradually moves to the lower-pressure and higher-temperature region. The slope of the second explosion limit is inversely proportional to the carbon number, kPT = 0.0069 - 0.005/(Xc - 0.7), approximately. Second, a sensitivity analysis has been conducted to study the elementary reaction on the second explosion limits. The results show that the chain branching and termination reactions governing the explosion limit of hydrogen have a little effect on the second explosion limit of methane. The C2H5O2H decomposition to form OH radicals is dominant in controlling the nonmonotonic behavior of the second explosion limit of C2H6. The second explosion limit behavior of propane is governed by three sets of reactions in the low-temperature oxidation process.