Experimental Study And Numerical Simulation Of Negative Pressure Boiling Heat Transfer Characteristics In Inclined Annular Tubes

Low-grade energy can be recovered in the production process of food,medicines,etc.,and it can be effectively recovered by using a low-temperature heat pipes.The heat pipe with a annular tube heat exchanger can more easily carry out energy recovery transformation of the existing production line,so further research is needed on the boiling phenomenon in the annular tube.In order to systematically describe the heat transfer characteristics of the negative pressure boiling phenomenon of the water in the annular tube,and the feasibility of using a hydraulic fluid gravity type separation heat pipe with a stainless steel annular tube evaporator in low-grade energy recovery,the heat transfer characteristics of the negative pressure boiling phenomenon of water in the annular tube were studied combined with numerical simulation in this paper.In this paper,an annular tube negative pressure boiling test bench based on a gravity-type separation heat pipe is built.The experimental research on the negative pressure boiling process of water in the annular tube under different evaporation pressures,different inclination angles,and different system charging amounts is carried out.The results show that: When the evaporation pressure of the annular tube is maintained,the average surface heat transfer coefficient of the main phase transition area of negative pressure boiling in the annular tube increases with the inclination angle of the annular tube from 0 °,and reaches a pole at 10-20 °,then decreases,reaching a trough at 60 ? 75 °,after it increases again,and reaches another pole value at 90 °;when the inclination angle is large,a local high temperature area is formed,and it is easy to damage heat-sensitive materials in production;the change of average surface heat transfer coefficient and the formation of local high temperature area are related to the change of boiling flow pattern.At 0 °,the boiling phenomenon in the annular tube appears as a weak surface boiling,with the inclination angle increase,the flow pattern evolve into a slug flow at 10 °,a “surge phenomenon”occurs,and then nuclear boiling begins to develop,the flow pattern changes to a slug-bubble flow,and a complete nuclear boiling bubble flow is formed at 90 °;the optimal filing ratio amount corresponding to different inclination angle and evaporation pressure is different,when the inclination angle is 20 °,the optimal filing ratio corresponding to the evaporation pressures of 26000 Pa and 22000 Pa are 48.3%and 43.3%,respectively.In this paper,the negative pressure boiling process in the annular tube with 0 °and 90 ° inclination angle was numerically simulated using ANSYS Fluent 15.0,and a three-stage numerical simulation program was developed.The results show that:with the increase of the factor r in the Lee model,the average surface heat transfer coefficient of the main phase transition area increases at 0 ° inclination angle conditions,and decreases at 90 ° inclination angle conditions,and the reason is that at90 °,bubbles are more likely to form a gas film and cover the outer wall of the innertube and cause heat exchange deterioration;the factor r is recommended to be 0.01 for0 ° inclination angle conditions,and 0.1 to be recommended for 90 ° inclination angle conditions;the average surface heat transfer coefficient of the main phase transition area can be matched with the experiment;the simulation reproduces the local high temperature area in the experiment phenomenon,the reason for its formation is closely related to the disturbance of the boiling area.