Study On Heat Transfer Characteristics And Two-phase Flow Instability In A Two-phase Closed Loop Thermosyphon

It allows higher heat flux density in the two-phase closed loop thermosyphon?TPCLT?,because there is no capillary limit and carrying limit,but higher boiling limit and viscosity limit,compared with traditional heat pipes.TPCTL are receiving more and more attention,and the application prospects are very broad.Relevant research shows that,for the heat transfer characteristics of the TPCLT,it is affected by many factors such as the internal two-phase flow pattern,boiling and cooling modes,and external heat transfer conditions.Importantly,it is very obvious of the effect of the filling ratio on heat transfer characteristics,such as thermal resistance,temperature uniformity,start-up and ultimate heat transport capacity,etc.Seriously,there is instability under a certain range of filling ratio,which will seriously affect the normal heat transfer and temperature control performance.Therefore,the effects of filling ratio and main external conditions on the heat transfer characteristics and unstable characteristics of the TPCLT are studied in this paper.The main work is as follows:The heat transfer experiment platform was designed and built in order to study the heat transfer characteristics of the TPCLT.The filling ratio was divided into three categories:high,medium and low,according to the different heat transfer phenomena and characteristics in the TPCLT.And The influence of heat flux,inclination angles,and cooling water temperature on the temperature uniformity???,thermal resistance(R_loop,R_sys),and flow characteristics?W?of the TPCLT under different filling ratios were analyzed.The stability distribution map was established,based on the two dimensionless quantity phase change number(N_pch)and the filling ratio?FR?.The results show that,there are single-phase flow,intermittent boiling and local near-dryout characteristics at high,medium and low filling ratios,respectively.The proportion of latent heat and sensible heat is affected by heat flux,inclination angles and cooling water temperature,which will further affect the temperature uniformity,thermal resistance and flow characteristics of the TPCLT.The visual experiment platform was designed and built in order to study the mechanism of instability and fluctuation characteristics in the TPCLT.Through the statistics of the results,the conditions for the instability of the two-phase flow in the TPCLT were qualitatively analyzed.By comparing the flow pattern and pressure fluctuation in the TPCLT,the type and mechanism of the two-phase flow instability phenomenon were revealed.Through the analysis of the pressure fluctuation at the outlet of the evaporation section,using the PSD and SD methods,the unstable fluctuation characteristics of the two-phase flow were studied,such as the two parameters of period and amplitude.And the unstable characteristics of the two-phase flow under different working fluids were compared.The results show that,under the conditions of medium filling ratio and moderate heat flux,there is the two-phase flow instability in the TPCLT.In the process of instability,there is a periodic change of"single-phase flow-bubbly flow-agitated flow-bubbly flow-single-phase flow",which proves that the unstable fluctuations in the TPCLT are caused by intermittent boiling.There are intermittent boiling phenomena in water,absolute ethanol and R134a,and the intermittent boiling is more complicated in water.The flow velocity measurement experimental platform based on PIV technology was built to study the flow characteristics of the TPCLT under high filling ratio.The start-up and flow heat transfer characteristics of the TPCLT at high filling ratio are studied based on temperature and pressure parameters.The bubble pump effect under different flow conditions was compared,through the flow field image,and the relationship between the bubble size and the bubble pump effect was quantified.The results show that under high filling ratio,there are mainly two kinds of flow modes:bubbly flow and single-phase flow.The average velocity of bubbly flow(0.144m·s^-1)is twice that of single-phase flow(0.073m·s^-1),and the bubble pump effect in the tube is obvious.When the bubble diameter is doubled,the average velocity of the flow field increases by about 32%,the enhancement of convective heat transfer by bubble pump effect is the main reason for the high heat transfer performance of the TPCLT under high filling ratio.In addition,it is found that there are severe pressure fluctuations and propagation during cold start-up under high filling ratio,and there is weak thermoacoustic heat transfer in the TPCLT during start-up and steady-state.The numerical simulation of the TPCLT was carried out in this paper based on the VOF model.The flow and heat transfer model of the TPCLT under three filling ratio?83%,64%and 38%?was established,the heat transfer and flow characteristics were studied,and the velocity field,temperature field and gas-liquid distribution of the TPCLT were obtained,the mechanism of two-phase flow instability at a moderate filling ratio was explained.The results show that,under 64%filling ratio,the intermittent boiling cycle in the TPCLT is 7.7s,and the evolution of flow pattern is consistent with the experimental results;under 38%filling ratio,the evaporation section and the condensation section are mainly churn flow and bead condensation,respectively,and the thermal resistance is less than the former two.In summary,the comprehensive study on the heat transfer characteristics of the TPCLT was carried out in this paper.The influence of the filling ratio and external heat transfer conditions on the heat transfer performance and unstable characteristics was obtained.The interaction mechanism of the two-phase flow pattern change and geyser boiling in the TPCLT under different filling ratio was revealed.It can provide theoretical guidance for further improving the heat transfer performance and safety reliability of the TPCLT,and promoting the application development of the TPCLT.