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Two-phase microgap heat sink has a large potential to minimize the drawbacks associated with two-phase microchannel heat sink, especially flow instabilities, flow reversal and lateral variation of flow and wall temperature between channels. This new concept of the two-phase microgap heat sink is very promising due to its high heat transfer rate and ease of fabrication. However, comparison of the performance of the microgap heat sink (heat transfer and pressure drop characteristics) with some conventional heat sink has not been investigated extensively.

The current chapter presents the heat transfer and pressure data of three different dimension microgap heat sinks collected during the experimental program. Experimental results are discussed to characterize the two phase flow boiling heat transfer and pressure drop behaviors in microgap channel. Vapor confinement criteria was adopted from Harirchian and Garimella [1]. and experimental data was plotted based on this criterion to predict the vapor confinement inside the microgap channel under different operating conditions and microgap dimension. The heat transfer mechanisms are explained based on the vapor confined criteria and flow boiling regime. High speed visualizations are shown to validate the explanation.

Hotspots can be generated by non-uniform heat flux condition over the heated surface due to higher packaging densities and greater power consumption of high-performance computing technology in military systems designs. Because of this hotspot within a given chip, local heat generation rate exceed the average value on the chip and increase the peak temperature for a given total power generation which degrades the reliability and performance of equipments. This chapter presents the comparison of hotspot mitigation ability of microgap heat sink with conventional straight microchannel heat sink at the beginning of the chapter. The comparison is done with the same footprint, the same inlet mass flux, and the same effective and wall heat flux supplied based on the footprint. In the later section, this chapter presents the influencing factors of hotspots and its mitigation in microgap heat sink.

The current chapter presents the heat transfer and pressure data of ten different dimension microgap heat sinks collected during the experimental program. Experimental results are presented and discussed to optimize the microgap heat sink dimension and flow condition based on two-phase flow boiling heat transfer and pressure drop characteristics in microgap channel. Vapor confinement criteria was adopted from Harirchian and Garimella [1] and experimental data was plotted based on this criterion to predict the vapor confinement inside the microgap channel under different operating conditions and microgap dimension. The heat transfer mechanisms are explained based on the vapor confined criteria and high speed visualizations are shown to validate the explanation.

Flow boiling instabilities induce mechanical vibration in the system and deteriorate the heat transfer performances, for example- premature dryout, critical heat flux limitation, etc. The two-phase microgap heat sink has novel potential to mitigate these undesirable flow boiling instabilities and flow reversal issues inherent with two-phase microchannel heat sink. This chapter presents the comparison of instabilities in microgap heat sink with conventional straight microchannel heat sink at the beginning of the chapter. The comparison is done with the same footprint, the same inlet mass flux, and the same effective heat flux supplied based on the footprint. In the later section, this chapter presents the influencing parameters that affect the instabilities in microgap heat sink. High speed visualizations are shown to validate the experiment results and explanation.

Understanding the influence of surface characteristics on flow boiling heat transfer behavior in microgap is necessary to enhance the performance of microgap heat sink. This chapter presents the heat transfer and pressure data of three different dimension microgap heat sinks with varying surface roughness collected during the experimental program. Experimental results are presented and discussed to investigate the influences of surface roughness on flow boiling heat transfer and pressure drop in microgap heat sink. High speed visualizations are shown to validate the explanation.

The experiment facility, test section, design, and operating parameters are briefly described in this chapter. More details of test procedure, calibration procedure, and heat loss calculation are available in Alam et al. [1, 2].