Mathematical Model & Numerical Investigation Of Heat Transfer With Multi-Phenomena In The Capillary Wick Of Loop Heat Pipe

In the last decades, because of the demand in thermal management of advanced satellites and spacecrafts as well as cooling of electrical and electronic devices, two auto-driving and two-phase heat transfer devices have been developed and tested, so-called Capillary Pumped Loop (CPL) and Loop Heat Pipe (LHP) which depend on evaporation to absorb heat load and capillary force developed on menisci to drive a working fluid to flow and transfer heat over large distance in a direction. In a CPL/LHP, the evaporator with the capillary porous structure, i.e. the primary wick, is the key functional component which accepts heat fluxes, organizes evaporation and produces driving force of the working fluid flowing circularly in the whole device. Heat transfer with capillary-driven convection and evaporation occurs just in the primary wick. So the physical phenomenon in the primary wick is complicated and interesting for researchers/engineers. In this paper, heat transfer with capillary-driven convection and evaporation in the wick is investigated numerically and theoretically. The full paper consists of two topics. One includes the mathematical model, the numerical simulation, the validation and discussion of results. The other is the application of the mathematical model, including evaluation of wick's performances, effects of both single parameter and the interaction between parameters, the operating simulation and dynamic response of the wick.In this paper, an axisymmetric two-dimensional mathematical model of the cylindrical evaporator's wick with azimuthal vapor grooves of LHPs is developed to simulate transient /steady heat transfer with capillary-driven convection and evaporation in the capillary porous structure. The effect of the interaction between the flowfield and the liquid-vapor interface on the flowfield, the position of the interface and the curvature of menisci, i.e. the capillary force, is adequately considered in this model, as well as the effect of conductive, convective and evaporative heat transfer. Due to the capillary effect, the superheating liquid-vapor interface is also considered in this model, and the capillary force is matched to superheat and heat load. In order to track continuously the interface, the velocity of the liquid-vapor interface is demanded before the displacement of the interface is determined. And the calculation of the interfacial velocity considers the combined effect of mass and heat through the interface based on the conservation equation of energy on the interface.In this model, the staggered mesh with local thin grids in the vicinity of heated fins and finite volume method are introduced. The transient/steady flowfields at different heat fluxes are obtained. The results and auto-driving mechanism of LHPs are validated. Both the effect of convection and evaporation in capillary porous media are discussed, as well as the behavior of the liquid-vapor interface. And some reasonable assumptions of the model are validated.Wick's performances are evaluated in three aspects: the driving performance, the heat transfer performance and the capillary effect of evaporation. The effects of single parameter, such as heat flux and porous structure parameters including porosity, effective pore radius, permeability and effective thermal conductivity, are investigated in detail. As results of them, wick's operating modes are respectively defined as the mode of complete heat conduction, the mode of incomplete heat conduction and the mode of convection. All operating modes are discussed in detail. The effects of multi-parameters as well as the interaction between heat flux and permeability/effective thermal conductivity are also discussed, as results of which, the phase-diagrams on the heat flux-effective thermal conductivity plane and the heat flux-permeability plane are obtained.Finally, the operating simulation and dynamic response of the wick in the power-step process and power-cycle process are investigated. Both effect of the initial state in the wick and delay of temperature are discussed, as well as the anti-jamming and auto-resuming capability of the wick in operation. And it is also discussed why temperature oscillation and temperature hysteresis did not occur in operation.The mathematical model and results introduced in this paper are helpful to comprehend the complicated physical phenomena in the capillary wick and design the finer wick of a loop heat pipe.