The Effect of Fin Array Height and Spacing on Heat Transfer Performance during Pool Boiling from Extended Surfaces

The use of heat sinks is a promising approach to extend the range of immersion cooling in dielectric fluids to higher heat fluxes for thermal management of next-generation electronics. However, the effects of extended surface area enhancement on the heat transfer performance under pool boiling conditions are not well understood, even in the simple case of straight fins. Further investigation of the heat-flux-dependent variation of boiling modes that can manifest along the fin height is required. Although approaches exist to predict the extended surface pool boiling heat transfer coefficient, they have been developed from single fins. As a result, when applied to fin arrays, they are rarely accurate across the full operating range up to the critical heat flux, particularly if height or spacing varies drastically. To better understand the effects of fin arrays on heat transfer, pool boiling experiments are performed using copper fins in water, varying fin height and spacing. These variations span a range from much larger to less than half of the scale of the capillary length scale, L
b
. The pool boiling data, complemented with high-speed visualization of the boiling regimes and bubble dynamics, strongly support a hypothesis that L
b
is the key length scale. Heat transfer from fin array heat sinks with heights and spacings above L
b
are shown to be accurately predicted using existing fin analysis approaches from the literature. However, spacings smaller than L
b
affect the nucleate boiling superheat while heights shorter than L
b
are unable to support multiple boiling regimes along the fin sidewall, both leading to disagreement between the experiments and predictions. These aspects, coupled with observation of vapor entrapment between closely spaced fins, indicate that new predictive methods must be developed. The valuable insights offered into the effects of fin array height and spacing on pool boiling provide a pathway toward heat sink design optimization for immersion cooling applications.