Enhanced wickability of single-columnar, non-uniform pore-size wick using Lattice Boltzmann method

Abstract

Optimal wick designs are essential to develop high heat flux two-phase thermal management systems in various applications including miniaturized power electronics, energy, high power battery, and spacecraft systems, as they require both large permeability and improved capillary pressure, i.e., enhanced wickability. The enhanced wickability is examined using non-uniform pore size wicks, while the larger pores increase the permeability and the smaller pores improve the capillary pressure. A two-phase single component free-energy-based Lattice Boltzmann Method (LBM) is employed to study the enhanced wickability, i.e., pore-scale rate-of-rise through wicks. The rate-of-rise is predicted for uniform and non-uniform pore size wicks having a single-column-particle between the two parallel plates for given porosities (ε = 0.67 and 0.8) and pore size ratios (lr = 1.3 and 2.6). The study shows that the non-uniform pore size wicks enhance the rate-of-rise and capillary pressure up to 298 and 157%, respectively, compared to those of the uniform pore size wicks, by improving the permeability through larger pores and increased capillary pumping capability through smaller pores. Also, the wickability enhances as the pore size ratios increase at given porosity or the porosity decreases at given pore size ratio. The cumulative enhancements of the maximum/minimum dimensionless liquid heights and the liquid saturation of non-uniform pore size wick are found to be up to 90, 114, and 112%, respectively, at ε = 0.67 and lr = 2.6. The capillary pressure enhancement of non-uniform particle size wicks results from the presence of the small pores. Also, the vertically graded wicks increase the capillary pressure due to the smaller pores at the top of the wicks, while they marginally decrease the rate-of-rise compared to the non-uniform pore size wicks at given porosity and pore size ratio. The simulation results provide insights into the optimal thin wick structures for high heat flux two-phase thermal management system by enhancing the wickability through the non-uniform pore sizes.