Investigation On Heat Transfer Of Spray Icing

Spray freezing is a common technique in artificial snowmaking、icing wind tunnels and solidification heat pump.During the liquid solidification process,the latent heat of solidification will be released,which will have a certain heating effect on the ambient air.The three-phase heat transfer process of spray freezing was numerically and experimentally investigated in this thesis.An effervescent atomizer was designed for spray freezing in this thesis.A good atomization quality can be obtained at low pressure and small flow rate.The sauter mean diameter(SMD)was less than 16 microns and the atomization consumption of water per kilogram is only 5.93 W.The spray freezing process based on the discrete phase(DPM)model and the equivalent volume method was numerically simulated by software ANSYS-fluent.The effect of the cold air temperature,spray speed,spray mean diameter on the freezing process and heat transfer was obtained and accumulated some data for experimental device design and test.In the spray freezing process,the lower the cold air temperature,the better the spray freezing.The smaller the drpolet size,the shorter the freezing time required,and the higher the spray freezing efficiency.Therefore,when the ambient temperature were-10°C and-5°C,the droplet size were respectively below 100μm and 50μm,the spray freezing efficiency could be improved by multiple atomizers or reverse spray method.The test system of the spray freezing was bulit based on the results of numerical simulations.The research focused on the effect of two-phase heat transfer in different flow modes and different parameters on the spray freezing heat exchanger.The results showed the distribution mode of the atomizers was important parameter for the temperature change,when the ambient temperature was high.The effect of the heat transfer was the worst when the single nozzle was sprayed.But the effect of the heat transfer was the best when the double nozzles were reversely sprayed.The effect was obvious when the water volume was large and the air volume was small.However,the effect of the atomizer distribution mode on the temperature change became gradually small.When the ambient temperature was-5℃,the volume of water freezing was 8g/m~3.When the ambient temperature was-10℃,the volume of water freezing was 15g/m~3.Finally,the energy efficiency ratio of the spray freezing heat exchanger coupled with the air source heat pump at low temperature was analyzed in this thesis.When the ambient temperature was increased from-13°C to-8°C by the spray freezingmethod,the comprehensive energy efficiency ratio of the coupled system was higher 60%than that of the original air source heat pump.The research work of this thesis has certain guiding significance to the engineering application of spray freezing technology.