Experimental and Numerical Investigation of an Evaporating Meniscus in a Capillary Slot : Microscale and Pore Scale Studies

Abstract

An evaporating meniscus formed by a wetting °uid in a heated capillary slot with capillary driven °flow is numerically and experimentally investigated at the microscale and pore scale. In the microscale analysis, the contact line region of an extended evaporating thin ¯lm meniscus is numerically investigated to study the influence of °uid properties on the heat transfer. The governing equations to describe the fluid °flow, heat and mass transfer phenomena in an evaporating extended meniscus are grouped uniquely as function of °uid dependent parameters, namely the interline heat flow number and heat pipe ¯figure of merit. A physical interpretation of these parameters is presented. Numerical experiments conducted with different working °fluids show that a °uid with a high interline heat °flow parameter and heat pipe ¯figure of merit also has a high cumulative heat transfer in the micro region encompassing the evaporating thin ¯lm. In the pore scale analysis, the evaporation from a pentane meniscus in a heated capillary slot is experimentally and numerically investigated to study how the wetting characteristics are influenced with heat input. In the experimental investigation, a test set up is fabricated with a heated glass capillary slot that is partially immersed in a constant temperature bath with constant °uid level. A novel aspect of this experiment is that both the wicking height and steady state evaporation mass flow rate are measured simultaneously. Based on a macroscopic force balance, the apparent contact angle of the evaporating meniscus is experimentally estimated from the wicking height and mass flow rate. This is compared with the results obtained using evaporating thin ¯lm theory. The experimentally estimated contact angle is higher than that obtained from the thin ¯lm model but both experiment and theory show similar trends. In the numerical study, a ¯finite volume numerical model of an evaporating meniscus in a heated capillary slot (simulating the above experimental condition) is developed for predicting the wicking height and mass flow rate. This model includes: (i) one-dimensional heat transfer and °uid °flow in the liquid and vapour regions of the capillary slot, (ii) one{dimensional evaporating thin ¯lm model for the meniscus region, and (iii) two-dimensional conduction heat transfer in the capillary wall. Correlations obtained from the evaporating thin{¯lm theory in terms of cumulative heat transfer and apparent contact angle are applied to the pore level problem. The problem is solved iteratively between the micro and pore scales till convergence is achieved. The wicking height is influenced by the change in apparent contact angle and the pressure drops to flow of liquid and vapor in the capillary slot that is a function of evaporation mass °ow rate. Heat input to the capillary slot increases both the contact angle in the evaporating meniscus and the frictional pressure drops in the liquid and vapor regions. In the present study, the influence of increased contact angle is more significant and the liquid and vapor pressure drops are negligible. The trends in the wicking height, mass flow rate and conductance are similar to the experimental data. The proposed numerical approach using correlations from thin ¯lm theory to link the micro and macro scales yields results that are consistent with experimental data. The results show that the change in contact angle can degrade the ability of the liquid to wet the pore and hence result in a lower heat transfer coefficident.