Pore scale simulation of evaporation in a porous wick of a loop heat pipe flat evaporator using Lattice Boltzmann method

Abstract

The evaporation heat transfer of a loop heat pipe (LHP) flat evaporator with porous wick is numerically simulated at pore scale using an advanced phase-change Lattice Boltzmann Method (LBM). The effects of heat flux and surface wettability (contact angle) on the patterns and dynamics of liquid-vapor interface, liquid volume fraction, temperature distribution at wick-fin and wick-groove interfaces, as well as effective heat transfer coefficient of the evaporator are investigated. It is found that liquid-vapor interface will appear five kinds of patterns or dynamics sequentially with the increase of heat flux, i.e., (a) entirely saturated wick, (b) partially saturated wick with periodical nucleation and vanishing of bubbles, (c) partially saturated wick with periodical growth and shrink of bubbles, (d) partially saturated wick with stable liquid-vapor interface, and (e) fully dry-out at wick-fin interface. Generally, the higher the heat flux and the larger the contact angle, the smaller the steady-state liquid volume fraction in the wick. However, at a certain range of heat flux, the liquid volume fraction oscillates periodically corresponding to periodical oscillations of liquid-vapor interface, and its amplitude and period increases with increasing heat flux and contact angle. Due to the random pore size distribution of porous wick, the temperature distributions are not symmetric between the left and the right outlets. This discrepancy is especially significant for partially saturated wicks, revealing the remarkable impacts of local pore structure on evaporation characteristics. With the increase of heat flux, the effective heat transfer coefficient at first increases owing to the inception and development of bubble nucleation within the wick, and then decreases due to the occurrence and enlargement of dry out. The larger the contact angle, the smaller the heat flux to trigger the bubble nucleation, and therefore the earlier to achieve the maximum effective heat transfer coefficient. Nevertheless, contact angles studied in this paper have no obvious effect on the maximum value of effective heat transfer coefficient.