ABSTRACT

Highly efficient and reliable electronic devices are a key factor in further electrification as part of the greater adoption of renewable energy sources and drive to a net-zero carbon economy. The rapid advances in performance and miniaturization of these electronic devices resulted in huge heat fluxes that need to be dissipated effectively, while maintaining low and uniform system temperatures. Traditional air-cooled heat sinks have reached their limits, and the research community has now focused on single-phase and flow boiling in heat sinks, applied directly to the heat dissipating device. These systems can achieve high heat transfer rates, low and uniform substrate temperature (component to be cooled), low power consumption, high thermal performance and use eco-friendly and dielectric refrigerants. Applications include the possible dissipation of high heat fluxes in modern central processing units, graphics processing units, memory cards, data storage devices, insulated gate bipolar transistors, switch-mode power supplies and use in systems such as fast charging vehicle stations and data centres, fuels cells and photovoltaics. Three different geometries were investigated, namely: a simple microgap (single wide channel), rectangular multi-channels and a heat sink having micro-pin fins in a staggered arrangement. Fundamental issues that need to be considered in the drive towards a wider adoption of these heat sinks include the definition of micro scales, the prevailing flow regimes, the effect of system pressure, mass and heat flux, channel aspect ratio and length and surface characteristics on the resulting heat transfer rates, critical heat flux (CHF) and pressure drop. A high-speed, high-resolution camera was used to record the flow regimes. The microgap provided benchmark results with lower maximum heat transfer rates and lower pressure drop. Both the micro-channels and the micro pin-fin heat sinks provided better heat transfer results, with the latter showing superior performance. A large data bank for flow boiling in micro-channels was used to design new well-validated heat transfer and pressure drop correlations suitable for the design of thermal management systems. The CHF in these systems was compared with existing correlations and again design recommendations were made for safe operational limits. The pressure drop and heat transfer results obtained with the micro-pin fin heat sink were compared with correlations and final recommendations, including modifications for the heat transfer correlations, were made for system design.

BIOGRAPHICAL NOTE