Computational analysis of surface tension effects in microchannel flow boiling with self-rewetting fluids

Abstract

Continual developments in computing technologies have caused processors and micro-processors to reduce in size while at the same time, operating at higher power densities and increased heat flux demands. Current cooling methods are quickly becoming less effective as they struggle to remove heat at the required rates. Microchannel flow boiling is at the forefront for cooling of high heat flux applications due to its combined convection heat transfer and latent heat transfer mechanisms. However, innovation for flow boiling in microchannels is needed to keep up with the continual developments, particularly because the typical coolant fluids often tend to experience local dry-out regions. There exists a class of fluids, known as self-rewetting fluids (SRF), which possess unique surface tension characteristics that reduce the local dry-out regions. This reduction in local dry-out regions is caused by the fluid motion at the two-phase interface being driven by surface tension gradients as described by the Marangoni effect. Limited information is available to study this phenomenon and therefore any additional information regarding the heat transfer capabilities and fluid dynamics of these fluids is crucial. In this numerical investigation, a 5% v/v 1-butanol-water solution was used as the SRF and studied in a thin horizontal channel at different heat fluxes. This was achieved by conducting two-dimensional (2D) simulations using a domain with a length of 5 mm and a height of 0.3 mm for various applied heat fluxes and an inlet mass flux of 15 kg/m²s. The numerical study investigated the flow of vapour slugs in the channel without the influence of surface wettability by not modelling any contact between vapour slugs and the wall. The results from the SRF were compared to those of water. It was found that the SRF, which has a unique surface tension gradient profile, drew the fluid surrounding the two-phase interface into the hotter region between the heated wall and the vapour slug. The water on the other hand experienced fluid being drawn out of this heated region resulting in thinner liquid films. The difference in the liquid films meant that the hotter fluid near the heated wall was evaporated in the case of water whereas it was trapped in the case of the SRF. Due to the hotter fluid being trapped in the SRF, higher surface temperatures were recorded than in the water case. The higher surface temperatures resulted in the SRF having lower heat transfer coefficients than the water. These results were observed at all the applied heat fluxes. Interestingly, these results are opposite to what was experienced in a similar study with a 0.2wt% heptanol-water mixture as the SRF, where surface wettability was a factor. Here surface wettability refers to the vapour bubble contacting the heated surface, such as during bubble departure, which was not modelled in the current study, instead the bubble was initialised in the fluid stream. The dry-out regions formed were smaller in the case of the SRF than in the water, which caused lower heat transfer coefficients in the water due to the poor heat transfer of vapour. On the other hand, an experimental investigation of the 5% v/v 1-butanol-water mixture as the SRF yielded similar results to the current numerical study in that the SRF experienced lower heat transfer coefficients to the water for a mass flux of G = 15 kg/m2s. The numerical study observed that in the case of slug flow where surface wettability and surface dry-out is not relevant, and where a mass flux of G = 15 kg/m2s is used, the self-rewetting fluid does not provide any clear heat transfer benefits over pure fluids like water. The current work was presented at the following conferences: - The 4th ThermaSMART Workshop titled: “Thermal Management for Net-Zero: Sustainability in Earth and Space Environments”. The workshop was hosted by the University of Pretoria from the 10th – 11th August 2023 at Lagoon Beach Conference Centre in Cape Town, South Africa. The work was presented by Mr André Pienaar. - The 76th Annual Meeting of the Division of Fluid Dynamics by the American Physical Society (APS/DFD). The APS/DFD meeting was held at the Washington Convention Center in Washington DC, United States of America from the 19th – 21st November 2023. The work was presented by Mr André Pienaar.