Pore network simulation of loop heat pipe evaporator with different pore size distribution

Abstract

To investigate the relationship between the loop heat pipe evaporator heat-transfer coefficient and wick pore size distribution, this work simulated the two−phase thermal hydraulics in the evaporator by pore network model in three dimensions. The simulation considered a liquid–vapor (L–V) two-phase state in the wick, which is induced by nucleate boiling and local pore-scale fluid dynamics. Some wick samples, while having the same macroscopic porous characteristics such as permeability, were created with different pore size distributions. Heat transfer and fluid flow of the evaporator were solved with ammonia and R134a as working fluids, along with stainless-steel and polytetrafluoroethylene (PTFE) porous materials. Each evaporator showed a different evaporator heat-transfer coefficient and maximum applied heat flux. Some characteristics with respect to the L–V phase distribution in the wick were selected to investigate their correlation with heat-transfer coefficients. As a result, the correlation coefficients with the three-phase contact line (TPCL) length ranged from 0.70 to 0.91. Only the TPCL showed a correlation with the heat-transfer coefficients in all conditions, as opposed to the vapor pocket maximum depth and liquid saturation. It was discovered that the TPCL length is proportional to the heat-transfer coefficients. In all cases, the TPCL length and heat-transfer coefficients reached a maximum at moderate pore size dispersion. In addition, The heat-transfer coefficients of the PTFE wick were more sensitive to the TPCL length than that of the stainless-steel one. The discovery helps enhance the thermal performance of the evaporator porous structural design by additive manufacturing.