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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 ( P 1–2 , T 1–2 ), first to second limit transition, and ( P 2–3 , T 2–3 ), 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 ( P 1–2 , T 1–2 ) remains almost unchanged and the lower transition point ( P 2–3 , T 2–3 ) rotates counterclockwise around ( P 1–2 , T 1–2 ). With a further increase of carbon number, ( P 1–2 , T 1–2 ) moves to the lower-pressure and -temperature region and ( P 2–3 , T 2–3 ) gradually moves to the lower-pressure and higher-temperature region. The slope of the second explosion limit is inversely proportional to the carbon number, k PT = 0.0069 – 0.005/( X c – 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 C 2 H 5 O 2 H decomposition to form OH radicals is dominant in controlling the nonmonotonic behavior of the second explosion limit of C 2 H 6 . The second explosion limit behavior of propane is governed by three sets of reactions in the low-temperature oxidation process.

In this study, the effects of ozone addition on the cool flame and NTC (negative temperature coefficient) regions of stoichiometric C 3 H 8 /O 2 mixtures are computationally studied through the explosion limit profiles. The results show that with minute quantities of ozone addition (the mole fraction of ozone is 0.1%), the cool flame area is enlarged to the low-temperature region. Further increases in the mole fraction of ozone gradually weaken the NTC behavior, and a monotonic explosion limit is eventually achieved. The sensitivity analysis of the main reactions involving ozone reveals that the explosion limit is mainly controlled by the ozone unimolecular decomposition reaction O 3 (+M) = O 2 + O (+M). However, as its reverse reaction is a third-body reaction, this reaction will lose its effect on the explosion limit in the high-pressure region. On the contrary, the reaction O 3 + HO 2 = OH + O 2 + O 2 has a significant effect on the explosion limit in the high-pressure and low-temperature region, as the concentration of HO 2 increases through the rapid third-body reaction H + O 2 + M = HO 2 + M.

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