Study On Internal Refrigerant Boiling State And Heat Transfer Performance Of The Recycling Evaporator

Recycling evaporator driven by gravity is mainly dependent on the height difference between gas-liquid separator and inlet of the evaporator to realize recycling of refrigerant in the evaporator.Qualitative analysis,recycling for the evaporator can improve the flow rate of the refrigerant,improve the nonuniformity of heart exchange across horizontal evaporator tube,increase the average heat transfer coefficient of recycling evaporator,thus recycling evaporator of the heat transfer performance was improved.Experiment set up a set of experimental rig which can switch between recycling and direct expansion liquid supplying cooling system.The liquid height for recycling evaporator in trials is 2350 mm.The evaporator,used for visualization research is composed of 16 glass tube,respectively,at 0 ℃,-5 ℃,-10 ℃,-15 ℃,-20 ℃ under the condition of temperature,the state of the flow boiling heat transfer of refrigerant inside the recycling evaporator and the heat transfer performance of the recycling evaporator are studied.Recycle at first,this paper studied the state of the flow boiling heat transfer of refrigerant inside the evaporator by the method of visual,and according to the test flow chart acquired by the experiment,the heat transfer mechanism of recycling evaporator are analyzed.Experimental results show that the in the recycling evaporator,refrigerant in the glass tube,in turn,the flow patterns are: single phase liquid refrigerant,bubble flow,plug flow,wave flow and "air flow".From time to time in the evaporator export,“air flow”and wavy flow have poured out of evaporator together.When achieving stable working conditions of experiment,the flow pattern will appear changes periodically,the formation of the flow pattern and flow rate of refrigerant are cyclical.The proportion of the flow pattern can be roughly determined according to the flow pattern of the length of the section.Inside the recycling evaporator,refrigerant boiling heat transfer mechanism can be divided into single-phase liquid and saturated boiling heat transfer and convection heat transfer process forced convection evaporation.Second,the heat transfer performance of the recycling evaporator is studied by experimental research.The research contents mainly contain the changes of evaporator surface temperature,heat transfer temperature difference and heat transfer coefficient when the evaporation tube section and the working condition of the temperature change which will influence recycling evaporator heat transfer performance.Experiment results show that under the condition of-20℃ ~ 15℃,the evaporation of refrigerant inside the evaporator is less,the evaporation tube is filled with liquid refrigeration,dryness of the evaporator outlet is small,the refrigerant in the evaporator and the evaporation duct between the wetted perimeter is larger,the utilization rate of recycling evaporator of the heat transfer area islarge.Under the condition of-10℃ ~ 0℃,evaporation of refrigerant in the evaporator is large and gas-liquid two-phase flow of refrigerant dryness of the evaporator export is so larger that the utilization rate of the heat exchange area of the evaporator is small.Under the condition of-20℃ ~ 0℃,there has relatively flat section of the heat transfer coefficient and in the section "tipping point",heat transfer coefficient started to unstable increasing.With the increase of working temperature,the "tipping point" is ahead of time.Throughout all the test points,the heat transfer coefficient of recycling evaporator changes between24W/ ㎡.K ~ 57W/ ㎡.K and within 0 ℃ ~-10 ℃,they presents inverse curve change trend.Finally,according to the knowledge of recycling the flow of refrigerant inside the evaporator and heat transfer mechanism,the mathematical model containing single-phase region and gas-liquid two-phase area has been built and the mathematical model has been carried on the theoretical calculation.Theoretical calculation results show that,according to the recycling of evaporator of the heat transfer mechanism,mathematical model is established to solve the results coinciding with experimental data,can reflect experiment results well.