Using a miniature wet-bulb/dry-bulb probe, detailed measurements of noncondensable-gas-concentration profiles were made in a gas-loaded two-phase reflux thermosyphon to investigate the effects of orientation and gas/vapor molecular weight combinations. Natural convection and radial diffusion were found to have a large influence on the noncondensable-gas distribution, even when the gas and vapor molecular weights are equal. An axisymmetric two-dimensional diffusion model is presented to allow evaluation of the relative effects of natural convection versus diffusion. Gas recirculation, axial stratification, regions of unstable flow, and their effects on performance are discussed.

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