Heat and mass transfer enhancement of thin film evaporative cooling by nanoelectrospray-induced gas jets

Abstract

Evaporation cooling is enhanced by forming a thin liquid film with low resistance for heat conduction and using an impinging gas jet for effective removal of the vapor from the interface. Our previous experiments show that when nanoelectrospray (nES) is directed onto a nearby surface, ultra-thin liquid films can be formed with thicknesses less than 260 nm. Our previous experiments and simulations also show that the stream of high-velocity liquid droplets emanating from the nES capillary entrain the surrounding ambient air, forming a narrow gas jet with speeds of tens of meters per second resulting in a means for vapor advection from the evaporating interface.

With promising results from the previous work, the current work considers the problem of hotspot thermal management using the nES-generated evaporating films of methanol and water as coolants. A comprehensive model, which considers the charged droplet transport, liquid/gas momentum exchange, fluid film evaporation, vapor transport, and heat transfer by evaporation, convection, and conductive spreading is used to evaluate the theoretical performance of nES evaporative cooling. The key demonstrated result is that hotspots on the order of tens of µm in diameter with heat fluxes of over 1000 W/cm² (or larger hotspots of a few hundred µm in diameter with heat fluxes of a few hundred W/cm²) can be effectively cooled while keeping the surface temperatures below the boiling point of the working fluid at atmospheric pressure. The effects of the key nES parameters (emitter positioning, applied potential, droplet size, liquid mass flowrate) and thermophysical properties of the coolants (mass density, maximum stable electric charge density, saturated vapor density, latent heat of vaporization, thermal conductivity) are analyzed, resulting in fundamental guidelines for heat and mass transfer enhancement in thin film evaporative cooling with application to microelectronics thermal management.