Experimental study on the influence of airflow velocity on frosting and defrosting performance of compact precooler under actual air humidity condition

•The effects of mainstream flow velocity on the frosting and defrosting performance of the compact precooler are studied.•The influence of methanol mass ratio on the defrosting performance of the compact precooler has been studied.•The approximate optimal mass ratios of methanol for achieving the best defrosting performance under different main flow velocity are obtained.•The main defrosting mechanism of methanol has been analyzed in detail.•The mechanism of the condensate structural morphology changes with the variation of methanol mass ratio has been revealed. Due to the precooler of hypersonic vehicle is frosted during the process of operation, the flow channel is blocked and the performance of engine is affected, which resulting in the decline of thrust and specific impulse. Therefore, for the purpose of maintain the performance of aero-engine, it is necessary to defrost the precooler. In order to study the influence of the airflow velocity on the frosting and defrosting performance of the compact precooler under actual air humidity condition, the frosting and defrosting performance of the microtubule precooler are experimentally studied in the wind tunnel under the conditions of air humidity of 1.8 g/kg and main flow velocities are 10 m/s, 20 m/s and 30 m/s, respectively. Methanol is utilized as the defrosting solvent, the mass ratio of methanol ranges from 0.8 to 1.2. The experimental results reveal that under the frosting experimental conditions, frost layer quickly condenses on the outside of tube bank, the wall surface temperature and heat transfer rate decrease dramatically, but the pressure drop coefficient increases sharply. It indicates that the frosting has an adverse effect on throughflow characteristics and heat transfer performance of the precooler. However, for the defrosting experiments, the wall temperature increases significantly, the defrosting effect and heat transfer rate are improved obviously and the pressure loss coefficient drops distinctly. In addition, with the increase of airflow velocity, the pressure drop coefficient increases sharply, but the coverage area of the frost layer gradually decreases, it manifests that increasing the main flow velocity is beneficial to suppress the formation of the frost layer. Through the analysis of experimental results, it can be found that within the range of experimental study parameters, the optimal defrosting performance can be obtained when the mass ratio of methanol is about 1.1. Moreov