Cooling technique based on evaporation of thin and ultra thin liquid films

Abstract

The fast development in semiconductor technology is leading to very high chip power dissipation and greater non- uniformity of on-chip power dissipation with the result of localized hot spots, often exceeding lkW/cm2 in heat flux, which can degrade the microelectronics performance and reliability. Thin and very thin liquid films driven by a forced gas/vapor flow (stratified or annular flows), i.e. shear-driven liquid films in a narrow channel are promising candidate for an innovative cooling technique optimizing the tradeoffs between performance and cost. The paper focuses on the recent progress that has been achieved by the authors through conducting theory and experiment of locally heated shear- driven liquid films. Experiments with water and FC-72 in flat channels (height 0.3-2 mm) show that a liquid film driven by the action of a gas flow is stable in a wide range of liquid/gas flow rates. Maps of isothermal flow regimes were plotted and the lengths of smooth region were measured. Breakdown of liquid film was investigated and it was found that scenario of film breakdown differs widely for different flow regimes. The critical heat flux is about 10 times higher than that for a falling liquid film and exceeds 250 W/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> in experiments with water at atmospheric pressure. Procedures to organize a gas shear- driven liquid film flow under variable gravity conditions (parabolic flights) have been verified. It was found that the flow dynamics in normal gravity differs significantly from that in microgravity. The film is wavier under low gravity conditions. The effect of slip at the wall and evaporation effect on liquid film dynamics and heat transfer was analyzed numerically. The macroscopic interface shape is found to be sensitive to slip length comparable with the initial film thickness. Calculations reveal that the maximum of the slip velocity is located in the transition region. All transport phenomena (convection to liquid and gas, evaporation) are found to be important for relatively thin films, and the thermal entry length is a determining factor for heaters of finite length. The thermal entry length depends on film thickness, which can be regulated by gas flow rate or channel height.