Noncontinuum gas-phase heat transfer in two microscale geometries is investigated using two computational methods. The motivation is microscale thermal actuation produced by heating-induced expansion of a near-substrate microbeam in air. The first geometry involves a 1-μm microgap filled with gas and bounded by parallel solid slabs. The second geometry involves a heated I-shaped microbeam 2 μm from the adjacent substrate, with gas in between. Two computational methods are applied. The Navier-Stokes slip-jump (NSSJ) method uses continuum heat transfer in the gas, with temperature jumps at boundaries to treat noncontinuum effects. The Direct Simulation Monte Carlo (DSMC) method uses computational molecules to simulate noncontinuum gas behavior accurately. For the microgap, the heat-flux values from both methods are in good agreement for all pressures and accommodation coefficients. For the microbeam, there is comparably good agreement except for cases with low pressures and near-unity accommodation coefficients. The causes of this discrepancy are discussed.
For a thin film heat flux gage designed to provide both rapid response and long use time, initial calibrations using standard techniques indicated differences between the literature and the estimated properties. In order to estimate thermophysical properties and subsequently the gage sensitivity, an analytical model of the response to a step change in heating current was developed. Starting from a Green’s function description, the model is reduced to three algebraic expressions, which correspond to the early, middle, and late time regimes. These expressions provide a framework for least-squares estimates of gage parameters. This provides an in-situ, nondestructive measurement of the thermal impedance of the substrate. There is very good agreement between the model and the experimental data. The estimated parameter values demonstrated good to excellent repeatability and good agreement with both new literature data and results from destructive property measurements.
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