Investigation on Heat and Mass Transfer in a Evaporator of a Capillary-Pumped Loop with the Lattice Boltzmann Method: Pore Scale Simulation

Abstract

A two-dimensional transient numerical model based on the lattice Boltzmann method (LBM) for the global evaporator of a capillary-pumped loop (CPL) is proposed to describe heat and mass transfer with evaporation in the porous wick, heat conduction in the cover plate, and heat transfer in the vapor groove. To indicate the stochastic phase distribution characteristics of most porous wick, the quartet structure generation set (QSGS) is introduced for generating more realistic microstructures of porous media. By using the present lattice Boltzmann algorithm along with the porous structure, the heat and mass transfer of an evaporator on pore scale can be predicted without resorting to any empirical parameters determined case by case. The energy equations for entire evaporator are solved as a conjugate problem, which are solved by means of a spatially varying relaxation time in the lattice Boltzmann model and the liquid flow is driven via the interfacial mass flux. A convective boundary condition considering the latent heat during the evaporation on the interface is introduced into the lattice Boltzmann model based on the nonequilibrium extrapolation rule. Especially, the bounce-back rule and the equilibrium rule of the LBM are, respectively, introduced to deal with the momentum boundary conditions inside the porous wick and on the evaporation interface in order to ensure the stability and the efficiency of the LBM model. Numerical results corresponding to different working conditions and different working fluids are presented, which provide guidance for the evaporator design of a CPL system.

Three dimensional liquid and vapour distribution in the wick of capillary evaporators

“…One can then again distinguish two types of works as regards the modelling of heat and mass transfers in the porous wick of evaporator: the works where the wick is assumed to be liquid saturated, e.g. [8][9][10][11][12], and the works where the wick can be partially invaded by the vapour. The latter is the situation of primary interest for the present study.…”

Numerical simulation of heat and mass transfer in bidispersed capillary structures: Application to the evaporator of a loop heat pipe

“…An iterative home-made code written in Fortran90 was developed in order to solve Eqs. (1)- (19). A finite volume method [44] is used to discretize the governing equations with grid points centred on pores.…”

“…This value is compared for each meniscus to the capillary pressure threshold defined by Eq. (19). If DP > DP cap , the meniscus is then displaced into the adjacent pore and all the equations are solved again.…”

“…Several parameters and/or properties have been investigated numerically and/or experimentally in order to improve the evaporator performances. Amongst other the geometrical dimensions and the position of vapour grooves [10], the choice of the working fluid ( [18,19]) or the use of a bilayer wick ( [20][21][22]) were investigated. In this context, the use of a bimodal capillary wick has been considered in the literature and seems to be promising to improve LHP performance.…”