Heat transfer and pressure drop in microchannels with isotropically etched pillars at sub-ambient temperatures

•Temperature- and pressure-dependent flow behavior of nitrogen gas in microchannels with isotropically etched pillars was studied.•Staggered pillar arrays provided a higher Nusselt number than aligned pillar arrays.•Correlations for the Nusselt number and friction factor as a function of Reynolds and Prandtl number were developed.•Correlations were validated by using Joule-Thomson microcoolers as test platforms. Glass microfluidic devices are often manufactured using micromachining techniques that involve the use of wet etching. In applications where high-pressure fluids are used, the microchannels in the microchip are filled with pillar structures for mechanical strength. Owing to the isotropic nature of the wet etching process, the pillars resulting from this process are shaped as truncated cones. In this paper, we present the results of a numerical study for predicting the flow and the heat transfer characteristics in a microchannel with truncated cone-shaped pillar arrays at sub-ambient temperatures. In order to verify the developed correlations, we use Joule-Thomson microcoolers that contain a counter-flow heat exchanger (CFHX) as test platforms and operate these with nitrogen gas. The performance of the microcoolers predicted with the new correlations matches well with the experimental data. Using these correlations, the CFHX is optimized and the CFHX losses are reduced by more than 30%.