Review of pool boiling enhancement by surface modification

Review of boiling heat transfer enhancement on micro/nanostructured surfaces

Effect of open microchannel geometry on pool boiling enhancement

One of the promising liquid cooling techniques for microelectronics is attaching a microchannel heat sink to, or directly fabricating microchannels on, the inactive side of the chip. A stacked microchannel heat sink integrates many layers of microchannels and manifold layers into one stack. Compared with single-layered microchannels, stacked microchannels provide larger flow passages, so that for a fixed heat load the required pressure drop is significantly reduced. Better temperature uniformity can be achieved by arranging counterflow in adjacent microchannel layers. The dedicated manifolds help to distribute coolant uniformly to microchannels. In the present work, a stacked microchannel heat sink is fabricated using silicon micromachining techniques. Thermal performance of the stacked microchannel heat sink is characterized through experimental measurements and numerical simulations. Effects of coolant flow direction, flow rate allocation among layers, and nonuniform heating are studied. Wall temperature profiles are measured using an array of nine platinum thin-film resistive temperature detectors deposited simultaneously with thin-film platinum heaters on the backside of the stacked structure. Excellent overall cooling performance (0.09 ° C/W cm 2 ) for the stacked microchannel heat sink has been shown in the experiments. It has also been identified that over the tested flow rate range, counterflow arrangement provides better temperature uniformity, while parallel flow has the best performance in reducing the peak temperature. Conjugate heat transfer effects for stacked microchannels for different flow conditions are investigated through numerical simulations. Based on the results, some general design guidelines for stacked microchannel heat sinks are provided.

Experimental and Numerical Study of a Stacked Microchannel Heat Sink for Liquid Cooling of Microelectronic Devices

Pool boiling and flow boiling on micro- and nanostructured surfaces

High-speed visualization of boiling from an enhanced structure

The current study involves two-phase cooling from enhanced structures whose dimensions have been changed systematically using microfabrication techniques. The aim is to optimise the dimensions to maximize the heat transfer. The entranced structure used in this study consists of a stacked network of interconnecting channels making it highly porous. The effect of varying the pore size, pitch and height on the boiling performance was studied, with fluorocarbon FC-72 as the working fluid. While most of the previous studies on the mechanism of enhanced nucleate boiling have focused on a small range of wall superheats (0-4 K), the present study covers a wider range (as high as 30 K) A larger pore and smaller pitch resulted in higher heat dissipation at all heat fluxes. The effect of stacking multiple layers showed a proportional increase in heat dissipation (with additional layers) in a certain range of wall superheat values only. In the wall superheat range 8-13 K, no appreciable difference was observed between a single layer structure and a three layer structure. A fin effect combined with change in the boiling phenomenon within the sub-surface layers is proposed to explain this effect.

Effects of Varying Geometrical Parameters on Boiling From Microfabricated Enhanced Structures

Semi-analytical model for boiling from enhanced structures

The heat dissipation rates at the chip level are projected to reach the 50-100 W/cm/sup 2/ mark for some future high performance electronic systems. Liquid cooling with phase change has been demonstrated to be a very efficient technique for thermal management of such high heat dissipation rates. Past work on liquid immersion cooling using fluorocarbons has shown the advantage of using enhanced structures to reduce boiling incipience excursion and raise the critical heat flux (CHF). Thermosyphons, employing these enhanced structures are an alternative to liquid immersion and are suitable for point cooling applications, where very compact evaporators are needed. This study investigates the combined effect of sub-cooling and pressure on the performance of an enhanced microstructure based thermosyphon, which has shown very high heat transfer rates (up to 100 W/cm/sup 2/ with a wall superheat of 27.8/spl deg/C). The pressure levels tested were partial vacuum (40-101.3 kPa), atmospheric pressure (101.3 kPa) and high pressure (101.3-370 kPa). The experiments were initiated at room temperature, and hence the sub-cooling corresponded to the difference in the liquid saturation temperature at the starting system pressure and room temperature. The results show a reduction in wall superheat values at higher pressures, at a given heat flux. The performance of the system was evaluated by defining a surface-to-ambient resistance. Results show that a partial vacuum at all heat fluxes results in better performance compared to higher pressures. The combined effect of pressure and sub-cooling was also tested for a compact evaporator and the results obtained were similar to the baseline case (larger evaporator).

Combined effects of sub-cooling and operating pressure on the performance of a two-chamber thermosyphon

This report presents a study of microfabricated enhanced structures for enhancing the thermal performance of a two-phase thermosyphon loop. The structures used in this study have an array of channels and pores that make it highly porous. The structures were fabricated in silicon using wafer dicing, wet-chemical etching, and laser milling, and details are included in this report. The structures were employed inside the evaporator section of a thermosyphon loop for enhancing the boiling performance. Preliminary results show that the heat dissipation with these structures is at least three times better than that of a plain polished silicon surface, at a wall superheat of 30 degrees C. The results also indicate that increasing the channel width resulted in better thermal performance.

W.B. Johnson

Papers

High-speed visualization of boiling from an enhanced structure

Effects of Varying Geometrical Parameters on Boiling From Microfabricated Enhanced Structures

Semi-analytical model for boiling from enhanced structures

Combined effects of sub-cooling and operating pressure on the performance of a two-chamber thermosyphon

Compact thermosyphons employing microfabricated components