Experimental Research Of Frost And Defrost On Fine Tube Bundle

When the reusable air breathing engine is flying at high Mach number,the stagnation temperature of the mainstream air is higher after the air is compressed in the inlet,which not only limits the further increase of the compression ratio,but also reduces the heating ratio of the combustor.Therefore,an air precooler is needed between the inlet and the compressor to cool the temperature of mainstream air.However,when the mainstream air passes through the air precooler,the surface temperature of the precooler is lower than the three-phase point temperature of water,the water vapor in the mainstream air frost,and the frost layer formed on the surface of the precooler grow rapidly,which increase the pressure loss of the mainstream and increase thermal resistance.At the same time,the inlet of the fan is greatly deformed,the fan efficiency and surge margin is reduced.Therefore,it is necessary to overcome the serious harm caused by frosting.In order to study the reliable defrosting method,a visualization test bench is used to investigate the flow and heat transfer characteristics,frosting characteristics and defrosting characteristics of fine tube bundle under various working conditions.In order to realize the defrosting effect of ethanol on fine tube bundle,a light weight and efficient defrosting system for condensable substances is designed.The whole system consists of five parts: mixing injection unit,air compression unit,ethanol flow monitoring unit and mixing injection flow-path,which can realize the injection regulation of ethanol with different mass fraction.The main research contents and results are listed as follows:Pressure loss is proportional to the square of wind velocity under the condition of no heat transfer.In the case of large temperature difference,the pressure loss is still approximately proportional to the square of wind velocity,and the air-side heat transfer coefficient is approximately proportional to the 0.6 power of wind velocity.The temperature difference has little effect on pressure loss and air-side heat transfer coefficient.Under condensation conditions,the pressure loss increases by 30%~40% and the air-side heat transfer coefficient decreases by more than 50% because of the formation of flow resistance and thermal resistance due to the existence of liquid film.With the increase of wind velocity,the liquid film is blown off the surface of heat exchanger,so the influence of liquid film on the air-side heat transfer coefficient of heat exchanger decreases when the wind velocity reaches a certain value.Under the condition of ultra-low temperature,liquid nitrogen is introduced into the fine tube the heat exchanger.The surface of fine tubes frosts rapidly,and the effective heat transfer time is only dozens of seconds.The higher relative humidity,the more latent heat is released by phase change of water vapor,and the larger heat transfer happens,and the larger frost form,and the more the pressure loss of fine tube bundle increases because of the increase of frost thickness.The shearing effect of high-speed air flow can blow off the liquid film and frost crystals under the condition of ultra-low temperature and high wind velocity,which can significantly increase the effective heat transfer time of heat exchanger.Spraying ethanol which is ultrafine atomized in the mainstream can defrost and greatly improve the performance of heat exchanger due to frost.The defrosting effect is very obvious when the mass ratio of water vapor to ethanol is 1:0.3.Frost formation is completely inhibited,air-side heat transfer coefficient is no longer reduced and maintain a stable level,and pressure loss is no longer continuously increased and maintain a stable level.There are different types of air-side heat transfer coefficient fitting formulas and pressure loss fitting formulas under different wind velocity and different mass ratio conditions.There are mainly two kinds of air-side heat transfer coefficient fitting formulas: Nelder and exponential function.Pressure drop fitting formulas are mainly exponential function.