Investigation On Heat And Mass Transfer Characteristics In Capillary Porous Wick Evaporator Based On AMTEC Solar Thermal Power Generation

A parabolic dish/ Alkali Metal Thermal-to-Electric Converter (AMTEC) solar thermal power system using the capillary porous wick evaporator based on capillary pumped loop (CPL) principle has many inherent advantages over conventional forms of electric generation, such as high efficiency, high density, good reliability, no noise, flexibility of structure, absence of moving parts, high temperature control capacity and so on. As the power source and energy receiver of the loop, the design and operation of the capillary porous wick are very important. It has important significance and value to find the parameters affecting the performance of the evaporator and optimize the system for wide range of application of the AMTEC, especially for the parabolic dish/ AMTEC solar thermal power system.An axial symmetric invariable temperature phase change interface model of capillary porous wick evaporator with or without bayonet tube based on the parabolic dish/ AMTEC solar thermal power system was established to simulate flow and heat transfer characteristics in capillary porous wick and liquid channel by solving the mass and heat transport controlling equations numerically. The effects of the working fluid flow rate, inlet temperature, porous wick thickness, porosity and effective pore diameter on the distributions of pressure, velocity and temperature were analyzed. Meanwhile, flow and heat transfer characteristics of some other kinds of alkali metal such as potassium, Na-K alloy were also discussed and compared with sodium's. Besides these, the influences of length, diameter, and thermal resistance of the bayonet tube on the distributions of pressure, velocity and temperature in capillary porous wick and liquid channel were investigated.The results showed that:(1) Reducing the outlet temperature of AMTEC condenser or the length-radius ratio or increasing the mass flow rate can reduce the temperature on the inner surface of capillary wick. The axial flow in the wick can be ignored, and the radial flow is uniform along the axial direction. The pressure drop in the wick is only directly proportion to mass flow rate of working fluid when the material and structure of porous wick have been determined. Increasing the porosity can reduce the pressure drop in the wick, but also increase the temperature on the inner surface of capillary wick. The available effective pore size of the porous wick and the measures to improve the performance of the porous wick were given by analyzing the relationship between the maximum available capillary pressure and the loop pressure drop. The flow and heat transfer characteristics of the capillary porous wick evaporator with the alkali metal liquid as working fluid are different from that with the conventional working fluid, and the temperature in the liquid channel is much higher which is unfavorable to safety and effective operation.(2) The bayonet tube plays a significant role in the performance improvement of capillary porous wick evaporator. The bayonet tube can not only make the surface temperature inside the capillary porous wick be uniform, but also reduce the peak surface temperature. The effect of bayonet tube on the flow in the liquid channel is noticeable while the effect on the flow in the wick is slight. The axial flow in the wick can be ignored, and the radial flow is uniform along the axial direction. The optimal bayonet tube parameters are dependent on various working conditions of capillary porous wick evaporator. Thermal resistance, length and radius of bayonet tube should be carefully chosen; the inappropriate bayonet tube can even worsen the temperature distribution and boost the peak temperature on the inner surface of wick.