Condensation Flow Pattern And Heat Transfer Characteristics Of Propylene Loop Heat Pipes In 150-290 K

Loop heat pipes(LHP)can transfer heat efficiently through the phase change of their working fluid.They have no moving parts and can effectively isolate the electromagnetic interference and mechanical vibration from the refrigeration device to the optical system.Meanwhile,more reasonable space layout of thermal control system can be realized through the flexible pipelines,which can effectively meet the temperature control requirements of space detection systems.With the development of space detection technology and the diversification of detection targets of optical systems,more and more requirements are put forward for the working temperature zones of temperature control components,and higher requirements are put forward for the more stable and accurate of temperature control.LHP is a complex system where phase change,heat transfer and flow take place simultaneously.As one of the core components,the structural design of the condenser will affect the operation mode of LHP and determine the working temperature of the LHP in the constant conductance mode.Generally,the temperature difference between the evaporator wall and the condenser wall is an important index to judge the performance of LHPs.The smaller the temperature difference,the better the performance of the LHP.The existing experiments show that the main factor causing the large temperature difference of LHP is the large temperature difference between the condensation temperature and the wall temperature.The condensation temperature is determined by the condensation flow pattern and the condensation heat transfer mode.How to reduce the temperature difference between the condensation temperature and the condensation wall temperature by adjusting the condensation heat transfer process is the core problem of improving the performance of LHP.However,research on condensation of LHPs mostly focus on their application field.The mechanism of condensation heat and mass transfer is still not clear,and the change of operation mechanism caused by the change of physical properties at low temperature is rarely reported.Based on the research background,this paper mainly considers the condensation flow pattern and condensation heat transfer characteristics of the LHP working between 150-290 K.By means of indium sealing,the visualizing condenser can withstand a pressure of 1.5 MPa and a low temperature of 150 K.At the same time,the seal failure problem in the cooling process is solved,which meets the needs of observation of condensation flow pattern of LHP at low temperature.The working fluid filling system,vacuum experimental platform and condensation visualization experiment system were built.The effects of different heating rates on the performance and condensation flow pattern of the LHP working at both high and low temperature ones were studied,and the effects of different condenser temperatures from 150 K to 290 K on the performance and condensation characteristics of the LHP were studied.The experimental results under different heating powers show that the performance of the loop heat pipe in the room temperature region was better than that in the low temperature region.Affected by the heat leakage from the evaporator to the compensation chamber,the transition from the stable conductance mode to the variable conductance mode would be delayed at low temperature,which also led to the lower temperature heat transfer limit of low temperature region comparing to the room temperature region.The length of the two-phase zone was closely related to the operation mode of the LHP.In the variable conductance mode,the length of the two-phase zone was positively related to the heat transfer.In the stable conductance mode,the length of the two-phase zone did not change.The visualization results show that low mass flow rate of the LHP led to flow patterns dominated by gravity,such as wavy flow,stratified flow and intermittent flow.Through the analysis of flow pattern results,it was found that the Cavallini flow map can better predict the flow pattern under different heating rates.It is suggested that the Cavallini correlation be used in the design and simulation of cryogenic LHP condenser.The experimental results under different condenser temperatures show that with the decrease of condenser temperature,the condensation flow pattern changed from stratified flow to wavy flow,pseudo intermittent flow and semi-annular flow.The change of physical properties of cryogenic fluid is the main reason for the difference of flow patterns between cryogenic and room temperature.At low temperatures,larger liquid-vapor density ratio and viscosity ratio will produce larger shear force,which will change the flow pattern into semi-annular flow and increase the heat transfer coefficient of the two-phase region.The condensation heat transfer coefficient at low temperature was greater than that at normal temperature,but the condensation heat transfer coefficient can not determine the performance of the LHP unilaterally.The performance was also closely related to the condensation temperature.Reasonable heat transfer enhancement method can improve the performance of the LHP by reducing the condensation temperature.In the range of 180-290 K,the predicted value of the Cavallini correlation is in good agreement with the experimental value,but the deviation increases when the semi-annular flow occurs.The condensation heat transfer characteristics of semi-annular flow and the influence of the LHP oscillation on condensation heat transfer at low temperature need to be further studied.After the analysis of the experimental results,the selection of the condensation flow pattern and the reasonable selection of the condensation heat transfer correlation in the numerical simulation were refined,and the numerical simulation results were compared with the experimental results.The length of the condensation two-phase zone and the working temperature of the LHP were well verified.It is proved that the performance of the LHP can be more effectively predicted by selecting a reasonable condensation heat transfer correlation,and the improvement direction of condensation optimization in the LHP was pointed out.Through the numerical simulation and experimental results analysis,a parametric design of condenser length at 203 K was carried out,and LHPs experiments using 90 mm,190 mm and 420 mm condensers working at 223 K,203 K and 183 K were conducted.The results show that the reasonable design of the condenser length was beneficial to reduce the temperature difference of the LHP,and an appropriate condenser length can make the LHP operate in the stable conductance mode under the target conditions.On the other hand,the effect of condensation heat transfer enhancement on the performance of LHP was verified.The results show that the performance of LHP can be improved by condensation enhancement,but the flow resistance will also be increased by condensation enhancement.The balance between the two needs to be reached.The verification work in this study provides a theoretical support for the structural design and numerical simulation of the LHP condenser.