Computational investigation into jet impingement boiling on pin-fin surfaces

Abstract

Thermal management of densely packed chips is critical for developing prevailing chips. For years, conventional air-cooling techniques have been utilised for numerous microsystems where fans and heat sinks were used in high-power computing systems due to their low cost and high reliability. Unfortunately, recent developments have exceeded the heat dissipation capability of these conventional techniques, leading to a shift towards liquid-to-vapour phase-change cooling techniques. Various multiphase cooling techniques have been reported in the literature. Over the last few decades, jet impingement has been shown to be an effective and attractive way to transfer energy from high heat flux components by the substantial amount of thermal energy transferred between the solid and the liquid. Surface enhancement techniques have also gained traction due to the increased average surface heat transfer coefficient and surface area by disrupting boundary layer growth and improving turbulent transport. This research combined jet impingement with phase change or boiling and surface area enhancement to improve heat transfer from a surface.

Different boiling types arise in boiling jet impingement on pin-fin surfaces due to the various flow patterns caused by the pin-fin layout, thereby increasing the numerical complexity. All relevant numerical studies documented in the literature focused on boiling jet impingement on flat surfaces, whereas no studies were found on pin-fin surfaces. Therefore, conducting a well-documented numerical study of pin-fin surfaces formed an essential part of the current work. The complex flow patterns and boiling types between the pin fins had to be better understood before they could be widely implemented in electronic cooling applications.

In this study, the heat transfer effect of pin-fin surfaces in boiling jet impingement was investigated using the RPI boiling model embedded in the Eulerian multiphase framework, which is an option in ANSYS Fluent. The numerical results of wall surface temperature in the stagnation area of the jet correlated well with experimental data reported in the literature. Not measured in the reference experiment, the pool-boiling areas caused by flow obstruction led to the cyclic behaviour of vapour formation and condensation. Furthermore, the cyclic behaviour was linked to the dry-out behaviour in the pool-boiling regions. An automatic mesh adaption tool allowed cell refinement at cells experiencing unrealistically high vapour velocities and increased numerical stability. The temperature distribution over the pin-fins formed cool regions corresponding to the flow-boiling regions; and warmer pockets corresponding to the pool-boiling regions. The turbulent kinetic energy followed the formation and condensation of the vapour column in the pool-boiling regions. The highest turbulent kinetic energy was produced as the liquid was forced into the staggered-facing pin-fins. These results highlighted the advantage of a validated numerical study to understand the detailed jet impingement boiling behaviour.

Finally, a parametric study was conducted on a single jet impinging on a pin-fin surface to comprehend the effect of the inlet Reynolds number, pin-fin height, spacing and distribution on the heat transfer characteristics. The study of the inlet Reynolds number considered a lower and higher inlet velocity than for the validation case. An increase in jet velocity increased heat transfer at the stagnation region but had a limited effect on eliminating the dry-out areas at the outer regions of the domain. The study of pin-fin height and spacing suggested that heat transfer was mainly linked to surface augmentation. However, the decrease in pin-fin height allowed the liquid to spread to the outer regions of the domain and eliminated dry-out. The height and spacing study also suggested that the pressure drop over the domain was mainly linked to the stagnation pressure drop of the jet, while the pin-fin height and spacing had a negligible influence on the pressure drop for the parameter variation considered. The change in pin-fin configuration allowed the liquid to reach the outer regions of the domain while keeping the surface augmentation factor at a maximum. A star arrangement eliminated dry-out at 23.2 𝑊��/𝑐��𝑚��^2 and increased the average surface heat transfer.

Therefore, the RPI boiling model, along with the use of a 𝑦��+ insensitive near-wall treatment model could accurately predict the heat transfer of a single jet boiling on pin-fin surfaces. The findings of the parametric study aligned well with expectations to eliminate dry-out at the outer regions of the domain while increasing the overall surface heat transfer. The CFD model suggested that researchers would have to measure local dry-out if pin-fins were used in boiling jet impingement. Furthermore, the influence of pin-fin shape, distributions and the working fluid needs further investigation to allow for heat transfer at higher heat fluxes, which align with modern-day electronic applications.