Condensation Heat Transfer Characteristics Of AMTEC Porous Wick Condenser

Alkali metal thermal to electric converter?AMTEC?,characterized by high efficiency,high power density and no moving parts,is a kind of new thermoelectric conversion device.It has a broad application prospect.As an important part of AMTEC,the porous wick condenser controls the circulation and heat transfer of the working medium.However,little research has been carried out on its internal flow and heat transfer.In fact,the condensation of alkali metal vapor in the porous wick condenser is the guarantee of the self-circulation of AMTEC,and it is also an important prerequisite for further utilization of the heat rejection from AMTEC condenser.The motivation behind our work is to provide theoretical guidance for the design,operation,optimization and evaluation of AMTEC itself or AMTEC cascading with other heat utilization device.An axisymmetric two-dimensional mathematical model was established and the volume of fluid?VOF?method was adopted to investigate the condensation heat transfer process of alkali metal in AMTEC porous wick condenser numerically.The steady-state distributions of phase volume fraction,temperature,velocity and pressure of working medium were obtained.Moreover,the impacts of some relevant parameters including operational,material and geometrical parameters on condensation heat transfer characteristics have been discussed.The results show that the thickness of the liquid film increases along the radial direction.Under an ideal working situation,the vapor-liquid interface can stabilize near the surface of the porous wick condenser and the thickness of the liquid film increases very little along the radial direction.The thermal conduction dominates the heat transfer process in the porous wick condenser,thus the temperature distribution in the liquid phase is approximately linear.The condensation rate at the phase-change interface decreases along the radial direction,and the cold end heat flux q increases initially and then decreases along the radial direction.The peak value of heat flux q appears in the ring region near r=7mm.The porous wick condenser has very limited self-adaptive ability.Once the M_in exceeds some critical value,the porous wick condenser can reach a steady condition by adjusting the position and shape of the interface.In addition,there is a corresponding T_w for a certain M_in to get an ideal phase-change interface.And the greater the mass flow rate M_in becomes,the smaller the corresponding T_w is.Under the same working condition,the heat transfer characteristics of the condenser using the sodium or potassium as woring fluid is very similar,and the difference of liquid film thickness is very small.The liquid film thickness of the condenser increases with the increase of thermal conductivity of the porous wick material.The porosity and effective effective pore size of porous wick have great influence on the flow process of the condenser.It is beneficial for the performance of condenser to increase the porosity and effective pore size.Decreasing the distance between the outlet and the cold end not only can significantly reduce the outlet temperature,but also improve the self-adaptive ability of the condenser in the case of large M_in.Finally,the outlet diameter r_out has little effect on the thickness of the liquid film,but the increase of r_out can reduce the average temperature of the outlet and the pressure drop of the porous wick zone.