Investigation On Characteristic Of Heat Transfer And Flow For Small Scale Flat Plate CPL/LHP

The capillary pumped loop (CPL) and the loop heat pipe (LHP) are the two-phase thermal control devices with the latent heat of evaporation of a working fluid to transfer heat and the capillary action for fluid transport, which are capable of transport large heat density and passively transporting heat over large distances with minimal temperature losses, and contain no moving parts. As a result, CPL/LHP becomes more active and interesting in many engineering domains including thermal management of satellites and spacecrafts as well as cooling of electronic devices.The research progress in the CPL/LHP technology is briefly reviewed, and the operational principles and characteristics of CPL/LHP systems are introduced in this paper. CPL/LHP is especially well suited for thermal management of dissipating high heat flux, at the same time, due to the flat plate evaporator has the advantages of perfect thermal contact between evaporator surface and electronics devices surface, and low thermal resistance and isothermal heated surface, our primary interest in this investigation is small scale flat plate CPL/LHP.The total pressure loss along small scale flat plate CPL loop and maximum capillary pressure of the evaporator wick for different working fluids are obtained The analysis results showed that ammonia working fluid had better performance and higher capillary limit, and capillary limit is affected significantly by vapor line radius and working fluid density. The working fluid transmission coefficient is introduced as an important criterion of choosing working fluid in the CPL/LHP system.The speediness, simpleness and robustness should be the startup characteristics of advanced CPL/LHP system, but many investigations indicate that there are some problems which plague the applications of CPL/LHP, such as startup difficulty and not being robust. Few researchers focus on the startup process of CPL/LHP, especially on the startup process of small scale flat plate evaporator, which appears the new characteristics during startup process. The features of startup process and the reasons which result in the failure of small-scale flat-plate type capillary pumped loop(CPL)evaporator during the preheating stage are analyzed in the present work. The unsteady heat transfer model of evaporator in the fully-flooded startup stage is presented, and the model is solved by SIMPLE algorithm. The influences of heat flux, wick material and metallic wall material on the evaporator performance are discussed in detail.In the previous works on the capillary evaporator, the computational domain adopted is a single segment of wick structure in the evaporator, which is only a very small part of wick, and it leads to some shortages in evaluating the overall evaporator performance by those models. Firstly, these models can not predict the influences of metallic wall, vapor grooves and compensation cavity on heat and mass transfer of the capillary evaporator, the heated surface temperature which is very important in estimating the performance of thermal management system can not obtained, too. Two-dimensional mathematical model of the small-scale flat plate CPL/LHP evaporator is presented to simulate heat and mass transfer in the capillary porous structure and heat transfer in the vapor grooves and metallic wall for the first time. The overall evaporator is solved with SIMPLE algorithm as a conjugate problem. The influences of heat flux, liquid subcooling and metallic wall material on the evaporator performance are discussed in detail. The numerical results show that there exists a side wall effect heat transfer limit for small scale flat plate CPL/LHP evaporator, and the evaporator with combined wall or fins in the bottom can improve the heat transfer capacity remarkably. The conjugate model offers a numerical investigation in the explanation of the robustness of the flat plate CPL/LHP operation.Due to dissipating high heat flux requirements of small system, new miniature flat plate CPL/LHP evaporator have been designed, which utilizes the compensation cavity replacing the liquid grooves, uses the metallic sintered wick with smaller effective capillary radius and utilizes the high pure ammonia to improve the heat transfer capacity of CPL/LHP. The overall numerical model for the global evaporator of miniature flat plate CPL/LHP is developed to describe the heat and mass transfer with phase change in the porous wick , liquid flow and heat transfer in the compensation cavity and heat transfer in the vapor grooves and metallic wall. The governing equations for different zones are solved as a conjugate problem. The influences of uniform heat flux, liquid subcooling, wick material, metallic wall material and non-uniform heat flux on the evaporator performance are studied in detail. The results are useful for the evaporator design and performance optimization of miniature flat plate CPL/LHP.The dynamic operational characteristics of small scale flat plate LHP has been analyzed, and the mathematical models of flat plate evaporator, condenser, vapor liquid transport line and liquid transport line are developed based on the nodal method. MATALAB/Simulink is used to perform the simulation to obtain the operational parameters. The dynamic parameters are useful for the design of subsequent experiment of CPL/LHP.