Tamanna Alam

Experimental investigation of microgap cooling technology for minimizing temperature gradient and mitigating hotspots in electronic devices

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. Two phase microgap cooling technology is promising to minimization of temperature gradient and reduction of maximum temperature over the heated surface of the device because of unique boiling mechanism in microgap: confined flow and thin film evaporation. The present study aims to experimentally investigate the applicability of microgap cooling technology for minimizining temperature gradient and mitigating hotspots from the heated surface of electronic device. Experiments are performed in silicon based microgap heat sink having a range of gap dimension from 200 µm – 400 µm. Encouraging results have been obtained using microgap channel cooler for hotspots mitigation as it maintain uniform and low wall temperature over the heated surface.

Investigation of flow boiling instabilities in silicon microgap heat sink

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 work is an experimental study of boiling instabilities in microgap heat sink for different microgap depths ranging from 80μm–1000μm, mass fluxes from 390kg/m²s–900kg/m²s, heat fluxes up to 85W/cm² and different microgap surface roughnesses, Ra=0.6−1μm. A series of systematic experiments have been carried out to investigate the inlet pressure and wall temperature oscillations during two phase flow boiling condition under uniform heating, with deionized water as a cooling liquid. Experimental result shows that pressure oscillation increases with the decreasing microgap depth. Temperature oscillation is observed lower for smaller gap than larger gap up to a certain heat flux condition before the dryout phase. In addition, inlet pressure instabilities increase with increasing heat flux and decreasing mass flux. Moreover, surface roughness has an adverse effect on the inlet pressure instability at larger depth microgap heat sink and inlet pressure fluctuation increases with increasing surface roughness.

Experimental Study and High Speed Visualization of Flow Boiling Characteristics in Silicon Microgap Heat Sink

Flow boiling in microgap heat sink is very attractive for high-performance electronics cooling due to its high heat transfer rate and easy fabrication process. In absence of thermal interface material between the active electronic component and a microgap cold plate, significant reduction in interface thermal resistance and enhancement in heat transfer rate can be achieved. In earlier studies by these authors, encouraging results have been obtained using microgap heat sink as it can potentially mitigate flow instabilities, flow reversal and maintain uniform wall temperatures over the heated surface. So, more work should be carried out to advance the fundamental understanding of the two-phase flow heat transfer associated with microgap heat sink and the underlying mechanisms. In this study, local flow boiling phenomena in different microgap sizes have been investigated experimentally. Experiments are performed in silicon based microgap heat sink having microgap depth ranging from 80 μm to 500 μm, using deionized water with 10 °C subcooled inlet temperature. The effects of mass flux and heat flux on heat transfer coefficient and pressure drop characteristics are examined by using different mass fluxes ranging from 400 kg/m2s to 1000 kg/m2s and effective heat flux varying from 0 to 100 W/cm2. Apart from these experimental investigations, simultaneous high speed visualizations are conducted to observe and explore the mechanism of flow boiling in microgap. Confined slug and annular boiling are observed as the two main heat transfer mechanisms in microgap. Moreover, experimental results show that flow boiling heat transfer coefficients are dependent on gap size, and the lower the gap size, higher the heat transfer coefficient.Copyright

Surface Roughness Effect on Microgap Channel

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.

Comparison of Flow Boiling Characteristics Between Microgap and Microchannel

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.

Design and Operating Parameters

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].

Characteristics of Two-Phase Flow Boiling in Microgap Channel

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.

Optimization of Microgap Channel Dimension and Operating Condition

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.

Two-Phase Microgap Channel Cooling Technology for Hotspots Mitigation

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.

Two-Phase Microgap Channel in Mitigating Flow Instabilities and Flow Reversal

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.