Investigation of wall chemical effect using PLIF measurement of OH radical generated by pulsed electric discharge

Abstract

Near-wall distribution of OH radical concentration provides a measure of the wall chemical effect near a solid surface. However, it is not a straightforward process to isolate the wall chemical effect from the effect of gas-phase reactions because the OH radical distribution can be influenced by both the wall chemical effect and the effect of heat and radicals released by gas-phase reactions. In the present study, instead of the flame, OH radicals generated by a pulsed electric discharge is used in order to eliminate the interference from the gas-phase reaction. OH field with comparable concentration to a methane/air premixed flame has been achieved by tuning the input electric power of pulsed electric discharge. The wall chemical effect is investigated by comparing OH distributions near the quartz wall and the quartz wall with 100-nm-thick alumina coating, while the wall thermal boundary condition is kept identical. High-resolution near-wall measurement of OH distribution was carried out by microscopic planar laser-induced fluorescence (PLIF), and the result was analyzed with the aid of numerical simulations with a surface reaction mechanism. It is found that the initial sticking coefficients estimated on the quartz/alumina surfaces are almost the same with the results in our previous methane/air flame experiment (Saiki and Suzuki, 2013). In the present OH field with electric discharge, it is easier to investigate the radical quenching effect, as the wall chemical effect on OH is decoupled from the gas-phase reaction. The chemical action defined as the wall-normal OH concentration gradient divided by the local OH concentration, which is an index of the wall chemical effect, increases with increasing wall temperature in the OH field generated by pulsed electric discharge.