Pore Network Simulations of Heat and Mass Transfer inside an Unsaturated Capillary Porous Wick in the Dry-out Regime

Abstract

In this work, a two-dimensional pore network model is developed to study the heat and mass transfer inside a capillary porous wick with opposite replenishment in the dry-out regime. The mass flow rate in each throat of the pore network is computed according to the Hagen-Poiseuille law, and the heat flux is calculated based on Fourier's law with an effective local thermal conductivity. By coupling the heat and the mass transfer, a numerical method is devised to determine the evolution of the liquid-vapor interface. The model is verified by comparing the effective heat transfer coefficient versus heat load with experimental observations. For increasing heat load, an inflation/deflation of the vapor pocket is observed. The influences of microstructural properties on the vapor pocket pattern and on the effective heat transfer coefficient are discussed: A porous wick with a non-uniform or bimodal pore size distribution results in a larger heat transfer coefficient compared to a porous wick with a uniform pore size distribution. The heat and mass transfer efficiency of a porous wick comprised of two connected regions of small and large pores is also examined. The simulation results indicate that the introduction of a coarse layer with a suitable thickness strongly enhances the heat transfer coefficient. Area fraction of the vapor region H evp Evaporation latent heat (J/kg) L Throat length (m) M Mass flow rate (kg/s) n Outward unit normal vector p Pressure (Pa) Q Heat rate (W) q Heat load (W/m 2 ) r Throat radius (m) r 0 Mean throat radius (m) T Temperature ( • C) v Superficial velocity (m/s) v 0 Interstitial velocity (m/s) W Network thickness (m) Greek symbols α Heat transfer coefficient (W/m 2 K) ε Porosity ξ Parameter used in the stopping criterion of the algorithm (Sect. 3.5) λ Thermal conductivity (W/m K) μ Dynamic viscosity (Pa s) σ Surface tension (N/m) σ 0 Throat radius standard deviation (m) υ Kinematic viscosity (m 2 /s) ρ Liquid mass density (kg/m 3 )

Kinematics in a slowly drying porous medium: Reconciliation of pore network simulations and continuum modeling

“…It is still a very active field of research not only because of the continuous emergence of new applications, such as evaporation in fuel cells 4 or in loop heat pipes, 5 for example, but also because evaporation in porous media is not yet completely understood. Many aspects are still to be described, e.g., Refs.…”

Enhanced thermal fingering in a shear-thinning fluid flow through porous media: Dynamic pore network modeling

“…i , effective heat capacity c eff p , and the product of the effective thermal conductivity and effective area, A eff i j λ eff i j , were volume-averaged values of the solid and fluid phases based on the local porosity 22,60 .…”

“…They require, however, geometrical simplifications, as the PNMs are not exact representations of the pore-scale morphology of porous media. Despite this, the PNMs have been highly successful in studies of various phenomena in porous media, and have been widely leveraged in recent years for simulating thermal transport in porous media, especially as a tool to study phase-change of water as a Newtonian fluid 7,[20][21][22] .…”

Visualization Study on Operating Performance of a Dual Compensation Chamber Loop Heat Pipe Under Acceleration Condition

“…The working fluid such as water, ethanol and acetone has the low vapor pressure. Moreover, the water has the largest latent heat of evaporation and is a widely used working fluid [63][64][65]. In the current visualization study, deionized water was selected as the working fluid.…”