Researches On The Flow, Heat Transfer And Frosting For Air Cooling By Mini-Tube Bundle At Very Low Temperature

Reusable hypersonic vehicles are an important the trend of future aerospace technology.Some hypersonic vehicles adopted low temperature pre-cooler to increase thrust,such as "SABER" engine used by "SKYLON" plan of Britain,and ATREX in Japan.It is required that the pre-cooler should be of extreme capacity of heat exchange,low flow resistance,light mass weight,and the ability of anti-frost.The pre-cooler is one of the key device for those engines and the purpose of this dissertation is to investigate characteristics of the flow and heat transfer,as well as the frost,of mini-tube bundle at low temperature.This dissertation set up a numerical model for calculating the flow resistance and heat transfer characteristics of the two-dimensional mini-tube bundle by referring the working parameters of "SABER" pre-cooler.In the range of Reynolds number of 2500-7000,the flow resistance and heat transfer characteristics of the staggered tube bundle were numerically simulated for the tube diameter less than 6 mm.The correlations for the flow of the heat exchanger with staggered tubes were revised according to the simulation results with large cooling temperature difference in the range of Reynolds number.A system for testing frosting on tube bundle surface was designed referring the flow velocity,coolant temperature range and humidity of SABER pre-cooler.The inlet temperature range of coolant was from-15 to-188℃.The experiments with different air velocity,humidity were carried out.The experiments showed that the frost thickness on the tube surface became thicker with decreasing cooling temperature and gradually approached to a constant.It was found that the frost thickness was thinner for higher inlet air humidity at constant coolant temperature.A one-dimensional theorectical model was developed and it recealed that the amount of released latent heat played a dominant role in determining the final frost thickness.This model also demonstrated the other factors affecting the frost.A spray nozzle sub-system was added to the system and different solvents could be sprayed into the inlet air to observe their effect on anti-frosting.Spraying ethanol was first tested and it could effectively depress the frost by 1:1 mass fraction of ethanol to humidity at constant coolant temperature of-88℃.The effects of spraying methanol,ethanol,n-propanol and isopropanol were experimentally compared and the results showed that the methanol had the best anti-frosting effect.The reasons for the anti-frost of solvents are summarized: 1)the solvent can reduce the freezing point of mixture of solvent and water;2)the addition of solvent adds more latent heat for depressing frost thickness.A two-dimensional numerical model with VOF method was used to simulate the ice on a single tube surface.The effects of inlet air velocity and humidity,and surface temperature of the tube were investigated and the distributions of ice,velocity,temperature and humidity were given.It was found that the ice thinckness on the front surface of the tube was the thinnest and the higher the inlet air velocity the thinner the ice thickness.The humidity distribution is a key parameter that determines the ice thickness.The other factors had the similar influence on icing as observed in the experiments and the one-dimensional theoretical model.