Transition between thin film boiling and evaporation on nanoporous membranes near the kinetic limit

Abstract

Nanoporous structures including single nanopores and nanoporous membranes have beenutilized as a platform to study fundamental liquid-vapor phase change heat transfer (PCHT) processes as well as a promising candidate for high flux heat dissipation. Previously, we implemented nanoporous membranes to support a thin liquid film for boiling, which was termed "thin film boiling", and realized high heat transfer performance. Besides thin film boiling, thin film evaporation through nanoporous structures have also been demonstrated to achieve high heat flux, but these two mechanisms are usually considered two mutually exclusive regimes operated under vastly different conditions, and the factors dictating how close the PCHT process is to the kinetic limit are elusive. In this work, we utilized a unique transition between thin film boiling and evaporation through nanoporous membranes to clarify the factors determining the heat flux and heat transfer coefficient (HTC) with respect to the kinetic limit conditions. We unambiguously showed the controllable transition from boiling to evaporation, when the liquid receded into the nanopores and provided additional driving force from capillary pumping sustained in the nanoscale pores. We showed that this transition is universal and can be understood from a simple fluid transport model for all the four types of fluids we studied, which cover a wide span of surface tension (water, ethanol, IPA, FC-72). More importantly, PCHT conditions at the transition points between boiling and evaporation were close to those of the kinetic limit of all these fluids. However, further increase of the heat flux beyond the transition points led to decreasing HTC and deviation from the kinetic limit, which can be attributed to the increasing vapor resistance in the vapor space and inside the nanopores. This increasing vapor resistance was also confirmed by experiments on IPA with different vapor pressures. Our work could shed light on PCHT on nanoporous structures with respect to the kinetic limit, and could advance the development of high heat-flux heat dissipation devices, especially using dielectric fluids.

New Model for Liquid Evaporation and Vapor Transport in Nanopores Covering the Entire Knudsen Regime and Arbitrary Pore Length

“…An evaporative heat flux of 500 W/cm 2 was achieved which is promising for electronics cooling applications. Wang et al − studied the thin-film boiling phenomena of liquids on the surface of nanoporous membranes. It was found that the evaporative flux was highest in the pure evaporation stage.…”

“…Laboratory experiments were also conducted using an anodic aluminum oxide (AAO) membrane evaporator. The experiments leveraged the advanced fabrication of an integrated heater and a resistance temperature detector, ,− overcoming the difficulty in measuring the liquid surface temperature. − The adoption of a thin membrane with nanopores ,, provides opportunities for studying liquid evaporation and vapor transport in nanoscale space. Through modeling, simulation, and experimental investigation, we aim at deriving a unified theory that captures the vapor transport resistance under the whole Knudsen regime, completing the modeling of evaporative heat transfer from nanopores …”

Theoretical and numerical study of nanoporous evaporation with receded liquid surface: effect of Knudsen number

“…Li, Wang & Xia (2021b) introduced the transitional gas flow formulation into the calculation of vapour flow resistance and extended the nanoporous evaporation model to a wider range of Knudsen numbers. The theoretical studies on nanoporous evaporation dynamics were validated by experimental data since nanoscale manufacturing and measurement became possible (Lu et al 2019;Wang, Shi & Chen 2020;Xia et al 2020).…”

The Critical Diameter for Continuous Evaporation Is between 3 and 4 nm for Hydrophilic Nanopores

“…The past two decades have seen a plethora of research activities in utilizing nanoscale surface structures to enhance the heat transfer from a solid surface to liquid. − The studies ranged from theoretical modeling of nanopore evaporation using continuum methods , for d = 10 – 100 nm, to experimental studies of nanopore evaporation , for d = 24 – 100 nm, suggesting that high heat fluxes are possible through nanoporous evaporation. However, these enhancements are frequently limited by the critical heat flux (CHF) or contamination or due to the lack of continuous evaporation at nanopores and structures called dry out or burnout conditions. The evaporation characteristics of nanopores with d < 10 nm are not studied through simulations or experiments.…”