Sensitivity of shear-driven internal condensing flows to pressure fluctuations and its utilization for heat flux enhancements

The reported experimental results are for annular zones of fully condensing flows of pure FC-72 (perfluorohexane) vapor. The flow condenses on the bottom surface (316 stainless steel) of a horizontal, rectangular cross-section duct The sides and top of the duct are made of clear plastic. The annular portion of the flow in the test-section is driven, under negligible to zero gravity effects along the flow direction, by pressure-difference and cooling conditions. Since the annular regime condensate motion is primarily driven by an effective interfacial shear stress, all such flows are termed shear-driven flows. The experimental system in which this condenser is used is able to control quasi-steady (termed quasi-steady) values of inlet mass flow rate, inlet (or exit) pressure, and wall cooling conditions. For the experimental results reported here, the mean (time-averaged) inlet mass flow rate, mean inlet pressure, and condensing-surface cooling conditions were held fixed at their quasi-steady values. Under these conditions, it was found that the imposition of small inlet pressure fluctuations (relative to the mean inlet pressure) induces significant mass flow rate fluctuations at the condenser inlet, and that there is a change in the very nature of the quasi-steady annular condensing flow regime. The resulting phenomena change the mean local heat flux values with significant (>200%) enhancements. There are accompanying time-varying changes in the liquid-vapor configurations within the annular and the non-annular regimes. This changes the mean and fluctuation amplitude values (with induced harmonics) in the pressure at any interior location within the annular regime. It is shown here that the heat flux enhancement phenomenon is real and occurs regardless of the method of cooling for a suitable range of fluctuation frequencies and amplitudes. This paper experimentally investigates how the strength of this sensitivity varies with amplitude and frequency of pressure or mass flow rate fluctuations imposed at the inlet of the condenser. Associated theory and rudimentary experiments (not reported here) suggest that similar enhancement may be observed in annular flows which do not completely condense before the exit, provided that suitable arrangements at the condenser exit allow similar or equivalent liquid-vapor interfacial wave structures with the help of similar acoustic wave reflections in the vapor phase. (C) 2012 Elsevier Ltd. All rights reserved.