Date of Award

Spring 5-15-2022

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Mechanical and Aerospace Engineering

Advisor(s)

Maroo, Shalabh

Keywords

CFD simulation, heat transfer, nanochannels, nanoscale flow, phase change, thermal management

Subject Categories

Aerospace Engineering | Engineering | Mechanical Engineering | Nanoscience and Nanotechnology

Abstract

Superior wettability of porous medium marks their potential to be used in the field of thermal management employing phase-change heat transfer. Comprehending the phenomena of wicking and liquid-vapor phase-change in micro/nano structured surfaces are key aspects towards advancing heat transfer solutions. In this work, fundamental understanding of droplet wicking, thin-film evaporation, and their subsequent application of heat-flux removal for cooling technology is first reported. The latter part of the dissertation is related to the disjoining pressure driven flow of nanoscale liquid film and liquid-vapor phase change in nano confinement. First, experimental and numerical investigation of droplet wicking in ∼728 nm height cross-connected buried SiO2 nanochannels, with micropores of diameter ∼2 μm at each intersection, is accomplished. The micropores allow water from a droplet placed on the surface to wick into the channels as well as allow thin-film evaporation from a meniscus. Experimental data in wicking-dominant regime are found to be in good agreement with analytical models and can be used to predict the wicking distance evolution in such nanochannels. Later, numerical technique of computational fluid dynamics (CFD) is employed to understand the dynamics of evaporating menisci in nanochannels and micropores. Evaporation flux at the meniscus interface of channels/pores is estimated over time. Local contact line regions are found to form underneath the pores when the meniscus recedes in the channels, thus rapidly enhancing evaporation flux as a power-law function of time. Temporal variation of wicking flux velocity and pressure gradient in the nanochannels is also independently computed, from which the viscous resistance variation is estimated and compared to the theoretical prediction. Further, to comprehend the effect of high-temperature on droplet spreading and evaporation over the nanochannels sample, experiments are conducted on a heated surface at temperatures ranging from 35°C to 90°C. Evaporation flux from the nanochannels/micropores is estimated from the droplet experiments but is also independently confirmed via an independent set of experiments where water is continuously fed to the sample through a microtube so that it matches the evaporation rate. Heat flux as high as ∼294 W/cm2 is achieved from channels and pores. The experimental findings are applied to evaluate the use of porous nanochannel geometry in spray cooling application and is found to be capable of passively dissipating high heat fluxes up to ∼77 W/cm2 at temperatures below nucleation, thus highlighting the thermal management potential of the fabricated geometry. Next, the porous nanochannels device capable to dissipate high heat flux is employed to regulate the temperature of a commercial PV panel by numerically integrating the device on the back face of the panel. The spatial and temporal variation of the PV surface temperature is obtained by solving the energy balance equation numerically and the extent of cooling and the resulting enhancement in the electrical power output is studied in detail. The nanochannels device is found to reduce the PV surface temperature significantly with an average cooling of 31.5°C. Additionally, the enhancement in the electrical power output by ~33% and the reduction in the response time to 1/8th demonstrating the porous nanochannels as an efficient thermal management device. In the later part of the work, an expression is developed for the disjoining pressure in a water film as a function of distance from the surface from prior experimental findings, which is key to understand water transport and liquid-vapor phase change in nanoscale confinement. The expression is implemented in a commercial CFD solver and the disjoining pressure effect on water wicking in nanochannels of height varying from 59 nm to 1 micron is simulated. The simulation results are in excellent agreement with experimental data, thus demonstrating and validating that near-surface molecular interactions can be integrated in continuum numerical simulations. Following the implementation, transpiration process and the passive water transport in trees of over a height of 100 m is simulated by using a domain comprising of nanopore connected to a tube with a ground-based water tank, thus mimicking the stomata-xylem-soil pathway in trees. In addition, the implementation of disjoining pressure in CFD simulation enabled the study of homogeneous bubble nucleation in nanochannel filled with liquid water. The bubble nucleation temperature was found to be ~125°C which closely matches with the experimental observation (~123°C) providing the evidence on incorporation of the disjoining pressure term to account for the effect of nanoscale confinement. By means of nucleation simulation, lesser-known parameters of homogeneous nucleation including the heat-flux supplied, the liquid film thickness underneath the bubble, etc. are identified which otherwise would been challenging to achieve experimentally.

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