Transient heat transfer characteristics of a thermal microdevice

Abstract

The present work was designed to obtain a comprehensive understanding of the mechanisms governing unsteady heat transfer at the microscale by identifying the important parametric trends governing the system temperature response to transient heat loads. To meet these objectives, a detailed experimental study was conducted to investigate transient heat transfer characteristics of a microchannel heat sink for single-phase flow of air and HFE-7000 and flow boiling of HFE-7000.

For the flow boiling experiments, the heater temperature response was correlated to the bubble dynamics for different operating parameters. Conditions at which onset of boiling occurred were identified and the repeatability of the boiling process was studied in detail. Onset of boiling and the subsequent bubble dynamics were recorded with the help of a high-speed video camera. Boiling was initiated at very high wall superheats due to the smoothness of the heater surface and low surface tension of HFE-7000. Discrete bubbles resulted in a temperature drop whereas a vigorous boiling regime resulted in localized dryout and subsequent temperature rise. For short pulses, very high heat fluxes were applied and the heating rate was high. At these heat fluxes, onset of boiling resulted in the formation of a vapor film on the surface and rapid heater temperature rise was observed. Time taken to initiate boiling decreased rapidly with increasing heat flux and then reached a constant value. The wall superheat at which boiling started increased with increasing heat flux and subsequently reached a constant limit.

The transient heat load was applied in the form of a step change in heat flux and that of a rectangular pulse. The power supplied to the heater was dissipated by fluid convection and by conduction in the Pyrex substrate. Conjugate heat transfer had a significant effect on the heater temperature response. CFD simulations were carried out in ANSYS FLUENT to obtain a qualitative estimate of conduction heat transfer in the solid substrate. In the single-phase experiments, the effects of Reynolds number, heat flux (pulse amplitude), pulse width, pulse waiting period, and fluid properties on the transient temperature response of the heater were identified. The heater temperature response consisted of a rapid temperature rise during the initial transients, slower convection dynamics, and finally steady state. Higher heat fluxes or pulse amplitude resulted in higher temperatures. The effects of mass flux on the heater temperature were not observed initially but took some time to come into play. The convection dynamics could be modeled as a first order system. Time constant of the convection dynamics decreased with increasing Reynolds number and was lower for HFE-7000 when compared to air.