Graduate Thesis Or Dissertation
 

Evaluation of the predictive capability of two-phase flow maps for microchannel condensation of R-134a at low mass flux conditions

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/z316q6548

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  • Predicting condensation flow regimes and the associated heat transfer and pressure drop in microchannels is critical for designing terrestrial and space systems for heating, cooling, power generation, and advanced manufacturing. It is well established that in flows approaching the microscale (Dh < 1 mm) gravity-dominated flow regimes become less relevant, with stratified and wavy flows being replaced by the intermittent and annular regimes. While a large body of work has been developed to investigate fluid behavior in microscale channels, past research has focused primarily on adiabatic air-water studies at relatively high superficial velocities or condensing refrigerants at large mass fluxes (G > 100 kg-m-2-s-1). Fluid visualization results from these studies have been used to develop many of the flow regime maps currently available for estimating heat transfer and pressure drop. Review of these flow maps shows that at microscale channel diameters and low mass fluxes, common flow regime predictions begin to break down.In the current research, two-phase flow regime data is obtained via high-speed visualization of condensing flows of R-134a at mass fluxes from 80 to 150 kg-m-2-s-1 in square microchannels (Dh = 840 μm) at quality ranging from 0.01 to 0.60. A relatively small experimental uncertainty in thermodynamic quality (Uavg ≈ 3%) was maintained by enforcing a large temperature difference across the water-side (ΔT > 10°C). To mitigate maldistribution in the channels, vapor enters the test section superheated and is condensed to the desired quality before entering the viewing section. All data points were found to be either wavy or wavy-annular flow, and did not compare well to predictions made by commonly used flow regime maps for both micro and macro channel flow. Additional research is required to better understand the characteristics of low mass-flux two phase flows in microchannel geometries.
  • Keywords: R134a, Two-Phase, Microchannel, Visualization
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