Numerical investigation into the effects of multiple bubbles in microchannel flow boiling

Abstract

Recent developments in microelectronics have produced higher heat fluxes that are beyond the capabilities of current heat exchangers. An increase in computing power coupled with decreasing processor size requires high thermal management on a smaller contact area. Microchannel heat sinks utilising flow boiling have been shown to produce heat fluxes orders of magnitude higher than those of their macroscale counterparts. Several factors influence the high heat transfer capabilities of the systems such as taking advantage of both the sensible and latent heat of the working fluid and the evaporation of the thin liquid film present between the channel walls and the vapour bubbles. Many researchers have investigated a wide range of microchannel geometries, orientations and different working fluids and applied heat fluxes. The correlations developed between confined boiling, heat flux and pressure drop are for macroscale flow and are ill-suited to microscale analysis. Heat transfer correlations are generally derived from experimental results conducted over a range of parameters and from evaluation of the influence of these varying parameters on the system. Because the scales of these phenomena are extremely small, visualisation and measurement during experimentation are difficult and inaccurate. Numerical modelling through computational fluid dynamics allows researchers to simulate and investigate these small-scale phenomena. This study focused on numerically modelling the interaction between multiple bubbles during flow boiling of refrigerant R245fa. The two-dimensional numerical domain had a length of 36 mm, consisting of three sections, and a height of 0.5 mm. The first section was adiabatic to allow the patched bubbles to develop in shape before phase change was present. The middle section had an applied heat flux of 5 kW/m2 and was the main focus. The last section was also adiabatic and was used to retain the leading bubbles. An interface-tracking mesh refinement method was used in all the cases. This method refined the liquid-vapour interface and a set distance around the interface, reducing the computational cost of the simulations. The results from Magnini, Pulvirenti & Thome (2013) were recreated with less than 4% of the required mesh elements. A set of three-dimensional simulations was attempted using the same method, but the simulations have not yet been completed. The bubbles were patched into the domain, instead of simulating bubble departure, to have better control over the positions of the bubbles. In all the cases, the heat flux improved from the first to the second bubble by at least 25%. A further 20% improvement was observed from the second to the third bubble at the end of the heated section. An increase in phase change was observed as the distance between bubbles were decreased, suggesting better heat transfer. This study illustrated the advantages of flow boiling over single-phase cooling, and the results corresponded to the findings of Magnini et al. (2013) and Magnini & Thome (2016).