Experimental studies on the frosting and defrosting performance of compact heat exchanger under moderate humidity condition

Abstract

As an important component of the air-breathing combined precooling cycle engine of the hypersonic aircraft, the compact heat exchanger plays an important role in precooling the incoming air. However, the compact heat exchanger is easy to frosting during the precooling operation, which leads to an extremely unfavorable result, that is, the specific impulse and specific thrust of the engine significantly decrease. Therefore, in order to maintain the performance of the engine, some measures should be taken to defrosting the compact heat exchanger of the engine. In order to carried out the frosting and defrosting experiments on the compact heat exchanger that operating at relatively low altitudes, the frosting and defrosting performance of the compact heat exchanger are experimentally studied in the wind tunnel under the conditions of moderate airflow humidity value (6.4 g/kg), as well as the flow velocity and the temperature of the mainstream are 10 m/s and 50 °C, respectively. In the defrosting experiments, anhydrous methanol and anhydrous ethanol are sprayed into the mainstream as the defrosting solvents, respectively, and the mass ratios of anhydrous methanol to water and anhydrous ethanol to water are 0.75, 1.0 and 1.25. The experimental results indicate that for the frosting experiment, after coolant flows through the inside of the heat exchanger tube bundles, the frost layer quickly condenses on the outside of tube bundles. However, in the defrosting experiments, once a certain mass ratio of anhydrous methanol or anhydrous ethanol is sprayed into the main flow, the wall temperature of the heat exchanger tube bundles increases significantly, and the wall temperature is close to or higher than the freezing point of water. Furthermore, the defrosting effect and the heat transfer rate are obviously improved, and the pressure loss coefficient drops sharply. Through the analyzation of the defrosting experimental results, it is found that the defrosting performance of anhydrous methanol and anhydrous ethanol are obvious different, and the defrosting effect of anhydrous methanol is significantly higher than that of anhydrous ethanol with the same mass ratio. Within the range of experimental study parameters, for anhydrous methanol, the best defrosting performance can be obtained when the mass ratio is 1.0; and for anhydrous ethanol, the best defrosting effect can be realized when the mass ratio is 1.25. However, the defrosting performance of anhydrous ethanol with the mass ratio of 1.25 is still slightly lower than that of anhydrous methanol with the mass ratio of 0.75. Moreover, in order to achieve the best defrosting performance of the compact heat exchanger, the optimal mass ratio of anhydrous methanol to water may be in the range of between 1.0 and 1.25; and the optimal mass ratio of anhydrous ethanol to water should be greater than 1.25.