Effects of microwave radiation on laser induced plasma ignition of n-butane/air mixture under atmospheric conditions

This study presents the effect of the microwave radiation control parameters (i.e. pulse width and pulse repetition frequency (PRF)) on laser-induced plasma (LIP) ignition in a constant volume combustion chamber (CVCC) of the n-butane-air mixture ( ? = 0.7) under atmospheric conditions. The ignition behaviors were investigated using ignition success rate, high-speed imaging, net heat release, and plasma temperatures. The result shows that the ignition success rate (ISR) defined in this study gets increased with an increase in the pulse width. Moreover, ISR was found insensitive to PRF at higher pulse width. This implies that the combustion advancement occurred at a faster rate on the coupling of high PRF and longer pulse width microwave radiation with the LIP. The ignition delay evaluated based on the net heat release gets reduced; consequently, the ignition advances much faster with an increase in the pulse width as compared to the small pulse width. The direct images of early combustion revealed an expansion of the plasma when the coupling between microwave radiation and the LIP occurs. This expansion of the plasma is the generation of active radicals due to the direct collisions of electrons with ionized molecules via interaction between the electromagnetic field and free electrons. Besides, the combustion advancement mechanism was investigated using plasma emission spectroscopy. The plasma temperatures which are closely related to the chemical reaction rates were calculated by fitting the homo-nuclear N 2 (2 + ) system. The results show that the ignition success rate enhances with an increase in the rotational temperature. This might be due to much faster rotational-transnational energy transfer as compare to vibrational-transnational energy transfer. This implies that the bulk gas temperature increases quickly since the rotational temperature is approximately equal to the gas temperature. ? 2020 The Combustion Institute. Published by Elsevier Inc. All rights reserved.