The flow and heat transfer rates inside a condenser depend on the specification of inlet, wall, and exit conditions. For steady/quasi-steady internal condensing flows (that involve compressible vapor at low Mach Numbers), the vapor’s ability to change its density — and hence interfacial mass transfer rates and associated locations of the interface — allows the flow to have a rather significant dependence on exit conditions. Both experimental and direct computational simulation results presented here show that this is indeed the case for flows of pure vapor experiencing film condensation on the inside walls of a vertical tube. In applications, the totality of boundary conditions are determined not only by the condenser; but also by the flow-loop (or the system) — of which the condenser is only a part. Therefore, the results outlined here should contribute towards a better understanding of the behavior (particularly the extent to which vapor compressibility effects affect the flow regimes of operation — i.e. annular, plug/churn, etc.) and response (transients due to start-up, system instabilities, etc.) of condensers in application systems (e.g. Rankine Cycle power plants, Capillary Pumped Loops, Looped Heat Pipes, etc.). In this connection, an experimental example of a relevant system instability is presented here. In summary, the experimental results presented here, and computational results presented elsewhere, reinforce the fact that there exist multiple steady solutions (with different heat transfer rates) for different exit conditions and that there also exists a “natural” steady solution for straight vertical condensers (circular and rectangular cross-sections).

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