Development Of Numerical Simulation Program And Performance Analysis For High Temperature Lithium Heat Pipe

The exploration of traditional energy forms used in deep sea and deep space has reached the limit capacity level.Heat pipe nuclear reactor has become an important solution to break through this development limit because of its small and light weight,long-term safety,silence and concealment.High temperature heat pipes are the key heat transfer components in the heat pipe nuclear reactor.Its performance affects the reactor safety and the energy conversion efficiency.Therefore,it is very important to study the startup and operation characteristics of the high temperature heat pipes.Among all the working fluids for high-temperature heat pipe,the lithium has the advantages of low saturated vapor pressure,high working temperature,and good heat transfer effect,which makes the high-temperature lithium heat pipe become a good choice for heat transfer of space heat pipe nuclear reactor.In the dissertation,a series of study on the high temperature lithium heat pipe were conducted,including the development of the startup and operation models,and the analysis of the startup and operation characteristics of the heat pipes.Firstly,the design optimization of high temperature lithium heat pipe is carried out,and the improved startup program of high temperature heat pipe is developed.The program has the advantages of simplicity,stability and short calculation time;and considers the influence of vapor compressibility on flow.Through this startup program,the startup process prediction of high temperature lithium heat pipe is realized.The results show that before the continuous flow is completely established,the temperature distribution of lithium heat pipe remains almost constant as a steep temperature front moved down the pipe.At the beginning of the continuous flow is completely established,the axial temperature difference of lithium heat pipe is relatively large,and it first decreases and then rises along the axial direction;With the increase of the working temperature,the axial temperature difference and rebound of lithium heat pipe will continue to decrease and finally reach stability.In addition,it is found that the free molecular flow has little effect on the start-up of the lithium heat pipe.Secondly,the steady-state characteristics of high temperature lithium heat pipe is analyzed.An improved lumped parameter analysis model of high temperature heat pipe is developed.The model establishes the overall cycle of vapor and liquid flow,and takes into account the Marangoni effect,which was previously ignored.The model can directly predict the effect of capillary force and the change of liquid in the wick.Through this model,the effects of different heating power,different heat pipe structure and Marangoni effect on the flow and heat transfer performance of high temperature lithium heat pipe were studied.In addition,a three-dimensional hightemperature heat pipe numerical model is established,which takes into account the influence of evaporative condensation suction effect on vapor flow in heat pipe,and realizes the prediction of three-dimensional temperature field,pressure field and velocity field of lithium heat pipe through software COMSOL.The results show that the total temperature difference and total equivalent thermal resistance of lithium heat pipe decrease rapidly with the increase of heating power at low working temperature;and the opposite happens for the high working temperature.Increasing the diameter of vapor chamber is conducive to reducing the total temperature difference and total equivalent thermal resistance,especially for the low temperature.Marangoni effect will increase the liquid pressure difference and the dryness of the wick.When the working temperature is low,the maximum liquid pressure difference increases by 78%,and the total pressure difference increases by 10%.However,with the rise of the working temperature,the influence of Marangoni effect decreases rapidly and can be ignored eventually.The increase of the length of the insulation section will lead to the increase of the total temperature difference and the total equivalent thermal resistance of the lithium heat pipe,while the influence of the length of the evaporation section and the condensation section on the total temperature difference and the total equivalent thermal resistance of the lithium heat pipe is related to the working temperature;and then increasing the length of the condensing section will reduce or even eliminate the rebound of the axial temperature of the lithium heat pipe.Finally,the influence of bending on the operation performance of high temperature lithium heat pipe is analyzed.Based on COMSOL software,a threedimensional numerical simulation model of high temperature lithium heat pipe with bending structure is established.By analyzing the temperature field,pressure field,velocity field and Mach number field,the influence mechanism of bending on the operation performance of lithium heat pipe is revealed;The effects of different heating power,working temperature and bending structure on the heat transfer performance and sonic limit of lithium heat pipe were studied.The results show that under the same heating power and condensation conditions,the existence of bending structure will cause the temperature rise in the evaporation section and the temperature rebound in the condensation section to decrease,resulting in an increase in the overall axial temperature difference of the lithium heat pipe.The effect of bending structure mainly causes the increase of vapor axial differential pressure,but it has little effect on liquid differential pressure.And then the existence of bending structure will reduce the vapor Mach number in the heat pipe due to temperature rise,and the highest vapor Mach number will offset outward from the vapor central axis.The existence of bending structure will cause a large increase in the total equivalent thermal resistance under low temperature operation,but the effect can be basically ignored when the temperature is higher than 1600K.The total equivalent thermal resistance increases with the increase of bending angle,decreases with the bending position away from the evaporation section,and decreases with the increase of bending radius;but when the ratio of bending radius to heat pipe diameter is greater than 2.5,bending radius has no effect on the total equivalent thermal resistance.In addition,the existence of bending structure will also cause the reduction of sonic limit.Therefore,the lithium heat pipe with bending structure should be started up by slowly increasing the power.