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Condenser Performance, Control, and Heat Transfer Enhancement Issues Resulting from Elliptic-Sensitivity of Shear Driven Internal Condensing Flows
Shantanu Kulkarni, Amitabh Narain, Michael Kivisalu, Soumya Mitra and Mohammad M. Hasan

This paper presents unsteady computational simulation results and supporting experimental evidence that show a certain fundamental feature of a purely shear driven annular/stratified internal condensing flow with respect to its sensitivity to boundary conditions. This feature is termed “elliptic sensitivity.” Shear driven condensing flows occur in 0g, horizontal channels, and micro-meter scale ducts of any orientation and they often have, or are designed to have, a significant annular/stratified regime. This fundamental feature of the flow allows imposition of several possible values of the mean pressure-difference (unlike the usual situation of having only one pressure difference value) for a given set of quasi-steady values of mass-flow rate, inlet or outlet pressure, and a steady cooling approach for the condensing-surface. By a quasi-steady time-varying flow variable, it is meant that the variable exhibits a steady-in-the-mean value with suitable time periodic fluctuation (s) superposed on it. For most common cooling approaches, when a quasi-steady value of the pressure-difference is changed (even by an amount in the range of 5 – 200 Pa) in time to another quasi-steady value, it often triggers significant changes in the mean condensate thickness, heat transfer rates which induces significant thermal transients, and system characteristics outside the condenser. However if the system always allows the flow to self-seek a “natural” pressure-difference across the condenser, then purely shear driven flows behave like gravity driven and dominated flows in the sense that they are able to achieve a unique and stable realization of a quasi-steady flow with a unique (termed “natural”) value of the pressure-difference. The reported “elliptic” sensitivity feature is absent for gravity dominated flows for which gravity is so strong that it determines the condensate motion as well as its mean interface location.

Keywords: Sensitivity, shear Driven Internal Condensing flows, Heat Transfer Enhancement, Controllability of Condensing flows.

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