Heat transfer enhancement of single-phase flow and flow boiling in microchannels : an experimental study

Abstract

Subcooled flow boiling of an engineering fluid -- HFE-7000 -- downstream a single pin fin was also studied. A liquid secondary jet was introduced into the flow to examine its merits pertinent to heat transfer enhancement. It was found that for HFE- 7000 high wall superheats ( ~40 °C) were required for the onset of nucleate boiling (ONB). Once boiling started, nucleate boiling dominated. Heat transfer coefficient increased monotonically with heat flux, independent of mass flux and jet injections. Secondary flow injection, which was previously found to be an affective single phase heat transfer enhancement technique, showed limited potential for fully developed nucleate boiling.

Single phase heat transfer and fluid flow downstream a single pin fin experiments were conducted with both air and an engineering fluid -- HFE-7300. A secondary jet flow was issued from slits formed along the pillar. A comparison of the thermal performances of a plain microchannel, a microchannel with a pillar, and a microchannel with a jet issued from a pillar was performed to elucidate the merits of this heat transfer enhancement technique. It was found that the presence of a pillar upstream the heater enhanced the heat transfer, among the three geometric shapes of pillar studied, triangular pillar performed the best; the addition of jet flow issued from a pillar further enhanced the heat transfer. At a Reynolds number of 730, an improvement of spatially averaged Nusselt number of 80% was achieved due to the combined effect of the pillar and the jet compared with the corresponding plain channel.

μPIV measurements provided planar velocity fields at two planes along the channel height, and allowed flow structure visualization. Turbulent kinetic energy (TKE) was used to measure flow mixing and to quantify the hydrodynamic effect of pin fins as well as the injection of the secondary jet. It was shown that the TKE is closely related to the Nusselt number.

To meet the requirement of extremely high heat flux removal in the future generation electronic devices, heat transfer enhancement techniques, specifically pin-fins entrenched in microchannel and jet injection to crossflow, were experimentally investigated in microchannel in the current study.

Microchannel devices with different pin fin geometries were designed and fabricated using MEMS techniques in a cleanroom environment. A 1 x 1 mm2 heater, made of Titanium film, was positioned behind the pin fin, used as a resistance temperature detector (RTD) to measure the area-averaged temperature at one side of the microchannel wall. Micro particle image velocimetry (μPIV) technique was used to observe flow structure downstream of a pillar; furthermore, a normalized parameter, turbulent kinetic energy (TKE) representing the intensity of velocity fluctuation, was defined, analyzed, and compared to the heat transfer trends.