Experimental Study And Numerical Simulation Of Heat And Mass Transfer Characteristics Of High Efficiency Medium Temperature Cesium Heat Pipe

As an efficient heat exchange device,the medium-temperature heat pipe has the characteristics of large heat transfer capacity,good isothermal performance,and heat flux variability.Cesium has a high figure of merit and suitable saturation pressure in medium temperature zone.At the same time,cesium,as an alkali metal,does not decompose,has no toxicity,and is not easy to corrode.Therefore,cesium heat pipe has great application value in the field of heat transfer in the medium temperature zone.However,the chemical properties of the cesium metal are very active,and the preparation of the cesium heat pipe is extremely difficult.The reports on the thermal properties of the cesium heat pipe are very limited.It is of great significance to explore the preparation of the cesium heat pipe,supplement the experimental data of the cesium heat pipe,and improve the cesium heat pipe theory model.Based on the design principle and manufacturing process of the cesium heat pipe,a series of high-performance cesium heat pipes were designed and prepared.The frozen startup characteristics and steady state heat transfer performance of the cesium heat pipe at different inclination angles were experimentally studied.In addition,the vapor-liquid flow and distribution in the cesium heat pipe were obtained through the CFD numerical simulation.The main research conclusions are as follows:Firstly,the frozen startup characteristics and heat transfer performance of the cesium heat pipe in the horizontal state were systematically studied.The vapor flow state is divided into the free molecular vapor flow,the transition flow,and the continuum vapor flow by the Kn number.Then the frozen startup process is divided into the free molecular vapor flow stage,the transition stage,and the continuum vapor flow stage by the change of vapor flow distribution with time.Under the frozen startup with constant heating power,the cesium heat pipe has a fast frozen startup speed and good temperature uniformity in the range of 772.5 W to 940.9 W.Especially,the cesium heat pipe has the best temperature uniformity at 827.5 W with a total thermal resistance of 0.042 K/W.The cesium heat pipe starts the fastest at 940.9 W,and its value is 1175 s.Since the effect of gravity on the liquid reflux can be ignored in the horizontal state,the cesium heat pipe has a capillary limit phenomenon at 980.7 W or more.In addition,the operating temperature range of the cesium heat pipe can be broadened by using the heating mode of progressive increasing power and step heating.Under this startup mode,the operating temperature of the cesium heat pipe can be increased to 500℃ and the effective length can be increased to 92%of the total length.Secondly,the combined effects of different inclination angles(0°,30°,60°,68°,and 80°)and different heating power(317.2 W～1416.2 W)on the heat transfer performance of cesium heat pipe were experimentally studied.The applicability of the theoretical calculation of the transition temperature to the cesium heat pipe is verified by the maximum wall temperature variation rate during the frozen startup.It can be seen that the "flat-front" model and the two-region model also apply to the cesium heat pipe.In the process of gradually increasing the heating power.the phenomenon of sonic limit is obtained,and the theoretical model of sonic limit is quantitatively verified.Through the performance test of the cesium heat pipe under different inclination angles,it can be seen that the frozen startup time increases with the increase of inclination angle due to the joint influence of the gravity component and the distribution position of the working medium before startup.Then.the evaluation method of steady-state temperature uniformity is established with the standard deviation of axial temperature as the evaluation index.When the inclination angle is 68°,the temperature uniformity is the best with a standard deviation of axial temperature within 9.44 K.While,the temperature uniformity is the worst at 80°.with a standard deviation of axial temperature up to 44.33 K.To improve the temperature uniformity and heat transfer distance of the heat pipe at 80°,the heat shield can be arranged in the adiabatic section.Finally,based on the Fluent VOF model.a new adaptive phase transition heat transfer model was established,and the phase change heat transfer process of the cesium heat pipe was numerically simulated.The accuracy of the model was verified according to the experimental data of wall temperature.The results show that CFD simulation can accurately describe the vapor-liquid phase transition process from the beginning to the quasi-steady state of the cesium heat pipe.It cannot be ignored that due to the simplification of the capillary wick,nucleate boiling occurs in the liquid pool,leading to periodic "geyser boiling" in the heat pipe,which increases the error of the simulation results.In addition,the effects of filling ratio(8.8%,12%.15%)and evaporator length(120 mm,200 mm)on cesium heat pipe steady-state heat transfer performance were compared by numerical simulation.It is found that excellent temperature uniformity requires the matching of evaporator length and filling ratio.The 15%filling ratio matches the 200 mm evaporator length,and the 12%filling ratio matches the 120 mm evaporator length.The wettability of the inner wall of the cesium heat pipe is excellent under the above two conditions.When the mismatch between the filling ratio and the evaporator length,heat transfer deterioration such as single-phase heat transfer,dry-out of the evaporator,and reduction of the effective length is easy to occur.