Study on Two-Phase Thermal Hydraulics in Porous Structure and Design Method of Capillary Evaporator

The liquid–vapor thermal hydraulics and phase displacement in the wick of a loop heat pipe (LHP) are key phenomena affecting the operating characteristics. At low heat flux, the wick is saturated with liquid, on the other hand, two-phase state appears at high heat flux. This paper investigates design method of the evaporator with the either phase state. On the saturated wick, because the applied heat flux concentrates along the three phase contact line (TPCL) within the case, wick, and grooves, method to optimize wick shape using only the length of the TPCL as a design variable of wick dimension is developed. To validate the method, some wicks machined with a lot of micro-grooves are manufactured and tested in the LHP. In addition, effect of micro-miniaturization of the grooves on the evaporator performance is investigated by 3D numerical simulation using OpenFOAM of the open source CFD toolbox. On the two-phase wick, optical observation in the transparent cylindrical evaporator during the LHP operation is conducted and then the phase displacement at the contact surface between the case and wick is observed. Static behavior of the liquid–vapor thermal hydraulics is investigated by the self-build pore-network simulation taking into account void disorder of the wick. This simulation reveals phase characteristics required for heat-transfer enhancement and effect of disorder of the pore size and biporous structure.