Investigation On Flow And Phase Change Heat Transfer Characteristics In Porous Wick Evaporator With Alkali Metal As Working Fluid

Alkali Metal Thermal-to-Electric Converter(AMTEC)has many advantages,such as absence of moving parts,high efficiency,maintenance-free,high power density and so on.The normal operation of the porous wick evaporator is a precondition for stable operation of AMTEC.Porous wick evaporator based on capillary pumped loop(CPL)principle can greatly reduce pressure drop during the working fluid flow,and increase the mass flow rate and heat transfer efficiency.The paper studied the flow and phase change heat transfer process of alkali metal working fluid in the porous wick evaporator based on CPL principle,and explored the impact of the evaporator parameters in order to improve its performance.The study provides a theoretical basis for the future AMTEC applications under higher power operating conditions,and it is also of great academic value for the choice of porous wick evaporator working fluid and the theoretical study on phase change characteristics of working fluid with high thermal conductivity.An invariable temperature phase change model of porous wick evaporator based on CPL principle was established to obtain the physical field distribution of the vapor volume fraction,the temperature and the pressure inside the evaporator by solving the governing equations of calculation region numerically.The flow and phase change heat transfer characteristics of alkali metal,such as sodium,in porous wick evaporator was analyzed.The role of bayonet tube and the effects of thermal resistance,dimensionless length and dimensionless diameter of the bayonet tube were discussed.The material,porosity and effective pore diameter of porous wick on the performance of porous wick evaporator were also investigated.Meanwhile,the flow and phase change heat transfer characteristics between potassium and sodium were discussed and compared.The results showed that for the evaporator using sodium as working fluid,a stable vapor-liquid interface is maintained at wick-groove interface.The pressure loss inside the evaporator mainly occurs in the porous wick and the pressure at the phase change interface changes abruptly,which can generate a capillary force to guarantee the circulation of working fluid.The bayonet tube structure in the evaporator can make the temperature distribution on the inner surface of porous wick more accordant and the peak temperature decrease rapidly.With the increasing of thermal resistance per unit area RA of bayonet tube,the sodium temperature at the front part of the evaporator outside bayonet tube will be increased and the temperature at the rear part decreased.With the increasing of the dimensionless length Lb*of bayonet tube,the sodium temperature can be increased at the middle section and decreased at the rear part of the evaporator outside bayonet tube.When the value of dimensionless diameter db*of bayonet tube is between 0.16 and 0.3,with the increasing of the dimensionless diameter db*of bayonet tube,the sodium temperature at the front part of the evaporator outside bayonet tube will be decreased and the temperature at the rear part increased.When the value db* is between 0.4 and 0.7,with the increasing of the dimensionless diameter db*of bayonet tube,the sodium temperature at the front part of the evaporator outside bayonet tube will be increased and the temperature at the rear part decreased.And the acceptable range of RA will change along with the variation of Lb* and db*.Under the calculated operating conditions,using the porous wick material with larger thermal conductivity and increasing the porosity can both lead to the increase of the effective thermal conductivity of porous wick,in this way both the heat leak of the evaporator and the temperature on the inner surface of porous wick can be increased.Reducing the porosity and the effective pore diameter of porous wick will lead to the increase of pressure difference between the inner and outer surfaces of the porous wick.In the meantime,decreasing the effective pore diameter can increase the capillary force at the phase change interface.For the evaporator using potassium as working fluid,a stable vapor-liquid interface is also maintained at wick-groove interface.Compared with porous wick evaporator using sodium as working fluid,the evaporator with potassium has a wider acceptable range of bayonet tube parameters but a worse circulation performance for it requires smaller porous wick pore diameter.