Experimental and numerical investigations of double pulse laser energy deposition in air

Abstract

This study focuses on the laser energy deposition characteristics through single and double pulse at short and long pulse intervals in quiescent air, both experimentally and numerically. A frequency-doubled and Q-switched Nd: YAG laser is used to create the spark in the air. High-speed schlieren imaging technique is used to visualize the shock-wave evolution and plasma kernel expansion. To estimate the energy available for ignition, the energy absorbed is measured and the energy loss through the shock-wave is estimated by using Jones blast wave model. Two-dimensional numerical simulations are carried out by solving the Navier-Stokes equations along with the species conservation equations in finite volume framework. After cessation of the laser pulse, the breakdown zone assumed to consists of seven species (O2, O, N2, N, NO, NO+ and e−). For double pulse at short pulse interval, the second pulse absorbed more energy than the first pulse, and the shock loss is relatively higher. The lifetime of plasma is found to have increased by 13.56 % due to the jump in temperature and pressure after the introduction of the second pulse. Same total energy two successive laser pulses at short time interval are found to be a promising alternative of single pulse for lean limit of combustion. The amount of energy left in the kernel for ignition is 16.02 % and 15.78 % of the absorbed energy in 50 + 50 mJ and 100 + 100 mJ double pulse respectively. In case of double laser pulse deposition at long pulse interval, along with multiple layers of shock-wave, a third lobe and a fourth lobe are observed on either side (upstream and downstream) of the plasma kernel enhancing the kernel surface area and mixing process.