Flow Boiling in Expanding Microchannels

This chapter presents the need of flow boiling microscale heat sink cooling technique for thermal management in miniaturized electronic devices. The current understanding on this topic and the inhibiting factors associated with it (instabilities, hotspots) for practical applications are discussed. Potential methods, namely, expanding microgap, sloping fin microchannels, and stepped fin microchannels to mitigate drawbacks associated with flow boiling in microscale heat sinks, are highlighted. The aims and objectives of this brief are presented.

The current chapter presents the heat transfer and pressure drop data of three different microgap heat sinks collected during the experimental program. The aims of this study are to understand the parametric effects including heat flux, mass flux, and channel dimension on heat transfer and pressure drop in expanding microgap experimentally. High-speed flow visualizations are conducted simultaneously along with the experiments to explore the bubble behavior in expanding microgap heat sink. Results are presented for wide range of heat and mass fluxes, validated by high-speed visualizations and compared with literatures. Enhanced heat transfer performances and reduced pressure drop are reported in flow boiling expanding microgap heat sinks.

In Chap. 2, the flow boiling heat transfer and pressure characteristics of three different expanding microgap heat sinks have been reported. This present chapter aims to understand the parametric effects including heat flux, mass flux, and channel dimension on flow boiling instabilities in expanding microgap visually and experimentally. High-speed flow visualizations from the top of the microgap heat sink are conducted simultaneously along with the experiments. The different flow patterns are presented and discussed to illustrate the bubble characteristics in the flow boiling in expanding microgap heat sinks. The measured flow boiling instability data are presented in this section for three different microgap configurations: straight microgap of depth 200 μm and two expanding microgaps having the same inlet depth 200 μm with gradually increasing exit depth 300 μm and 460 μm. Results are presented for wide range of heat and mass fluxes. Results show that two-phase expanding microgap heat sink has novel potential to mitigate the flow instabilities and flow reversal issues as the generated vapor has room to expand downstream and also due to reducing shear force along the direction of expansion of the test section. Key details of the experimental setup, procedure, test section, and experimental conditions are described in Chap. 2.

Expanding microchannels, namely, sloping fin microchannels (straight microchannels having a gradual slope at the fins from inlet to outlet), have been introduced in this chapter. Straight fin microchannels experience vapor confinement and vapor blockage at the downstream of the channel which in turn introduce vapor reversal and system instabilities and result in early partial dryout. The motivation of introducing sloping fin microchannel geometry is to mitigate vapor blockage as experienced by the straight microchannels by allowing vapor to expand downstream. The flow boiling heat transfer, pressure, and instability characteristics of sloping fin microchannels have been reported, and comparison of performances with straight microchannels has been performed. High-speed flow visualizations are conducted simultaneously along with the experiments, and different flow patterns are presented and discussed. Results are presented for wide range of heat and mass fluxes. Details of the experimental procedure, key findings, and significance of this work are reported in this chapter.

Stepped fin microchannels showed significant improvements in stability, pressure drop reduction, as well as heat transfer performance enhancement in earlier studies [1, 2]. Improved system stability and heat transfer imply reliable operation, and hence, this technology may be useful in practical applications. The aim of this study is to develop stepped fin minichannel heat sink prototypes along with a closed-loop system to demonstrate the technology for case-specific applications, such as IGBTs. The flow boiling heat transfer, pressure, and instability characteristics of stepped fin minichannels have been reported, and comparison of performances with straight minichannels has been performed. Results are presented for wide range of heat and mass fluxes. Details of the experimental procedure, key findings, and significance of this work are reported in this chapter.

The aims of these extensive studies were focused on: