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An Experimental Study of Enhanced Thermal Conductivity Utilizing Columnated Silicon Microevaporators for Convective Boiling Heat Transfer at the Microscale


In this work, silicon-based two-layer microfluidic evaporators were designed, fabricated, and tested in an experimental study designed to examine the effects of out-of-plane vaporization and subsequent lateral convection on the effective in-plane thermal conductivity of the device. The evaporator region in all devices measured 10 mm x 10mm, with an overall device size of 24 mm x 10mm. A columnated structure within the evaporator was utilized to mimic both structurally and functionally the fluid delivery and heat transfer characteristics of the microColumnated Loop Heat Pipe ("&mu"C-LHP) design. In-plane heat flux was provided by a high-power ceramic heater, while working fluid was pumped through the system using a servo-controlled syringe pump.

Temperature data was collected primarily using infrared thermal imaging, and detailed analysis was conducted to estimate convective heat losses from external surfaces and calculate the effective in-plane thermal conductivity. The veracity of this analysis was verified experimentally by analyzing the well-established case of pure conduction and correctly predicting the intrinsic solid conductivity of silicon (k ~ 130 W/m-K) over a wide range of input heat fluxes and surface temperatures.

Several distinct performance regimes were observed as the input flux was increased and more vigorous vaporization occurred. Most significantly, subsequent to the onset of stable vaporization, the effective thermal conductivity of a device typically increased by a factor of more than ten (k ~ 1000-2000 W/m-K). This conductivity was easily maintained and extremely insensitive to further increases in flux. The highest effective thermal conductivities were observed just prior to dryout. Single data point spikes as well as short (3-5 second) intervals of 10,000-20,000 W/m-K were regularly observed.

Peripheral experimental studies related to the author's previous work on self-nucleating surfaces were performed, as was a brief study on the hermetic sealing of microfluidic devices.

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