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).

Reported experimental and computational results confirm that both the flow features and heat-transfer rates inside a condenser depend on the specification of inlet, wall, and exit conditions. The results show that the commonly occurring condensing flows’ special sensitivity to changes in exit conditions (i.e., changes in exit pressure) arises from the ease with which these changes alter the vapor flow field in the interior. When, at a fixed steady mass flow rate, the exit pressure is changed from one steady value to another, the changes required of the interior vapor flow toward achieving a new steady duct flow are such that they do not demand a removal of the new exit pressure imposition back to the original steady value—as is the case for incompressible single phase duct flows with an original and “required” exit pressure. Instead, new steady flows may be achieved through appropriate changes in the vapor/liquid interfacial configurations and associated changes in interfacial mass, heat-transfer rates (both local and overall), and other flow variables. This special feature of these flows has been investigated here for the commonly occurring large heat sink situations, for which the condensing surface temperature (not heat flux) remains approximately the same for any given set of inlet conditions while the exit-condition changes. In this paper’s context of flows of a pure vapor that experience film condensation on the inside walls of a vertical tube, the reported results provide an important quantitative and qualitative understanding and support an exit-condition-based categorization of the flows. Experimental results and selected relevant computational results that are presented here reinforce the fact that there exist multiple steady solutions (with different heat-transfer rates) for multiple steady prescriptions of the exit condition—even though the other boundary conditions do not change. However, for some situations that do not fix any specific value for the exit condition (say, exit pressure) but allow the flow the freedom to choose any exit pressure value within a certain range, experiments confirm the computational results that, given enough time, there typically exists, under normal gravity conditions, a self-selected “natural” steady flow with a natural exit condition. This happens if the vapor flow is seeking (or is attracted to) a specific exit condition and the conditions downstream of the condenser allow the vapor flow a range of exit conditions that includes the specific natural exit condition of choice. However, for some unspecified exit-condition cases involving partial condensation, even if computations predict that a natural exit-condition choice exists, the experimental arrangement employed here does not allow the flow to approach its steady natural exit-condition value. Instead, it only allows oscillatory exit conditions leading to an oscillatory flow. For the reported experiments, these oscillatory pressures are induced and imposed by the instabilities in the system components downstream of the condenser.

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This paper presents computational simulations for internal condensing flows over a range of tube/channel geometries — ranging from one micro-meter to several millimeters in hydraulic diameters. Over the mm-scale, three sets of condensing flow results are presented that are obtained from: (i) full computational fluid dynamics (CFD) based steady simulations, (ii) quasi-1D steady simulations that employ solutions of singular non-linear ordinary differential equations, and (iii) experiments involving partially and fully condensing gravity driven flows of FC-72 vapor. These results are shown to be self-consistent and in agreement with one another. The paper demonstrates the existence of a unique solution for the strictly steady equations for gravity and shear driven flows. This paper also develops useful correlations for shear driven and gravity driven annular stratified internal condensing flows (covering some refrigerants and common operating conditions of interest). A useful map that marks various transitions between gravity and shear dominated annular stratified flows is also presented. For the micro-meter scale condensers, computations indentify a critical diameter condition (in non-dimensional terms), below which the flows are insensitive to the orientation of the gravity vector as the condensate is always shear driven. Large pressure drop, importance of surface tension, and vapor compressibility for μm-scale flows are also discussed. With the help of comparisons with 0g flows, the paper also discusses effects of transverse gravity on the solutions for horizontal channel flows.

in Paraguay, where he got his BS in Electromechanical Engineering. After graduation, he spent some time in academia working as faculty. During this tenure he taught courses on heat transfer, fluid mechanics and physics. In 2004 Dr. Kurita was granted the Fulbright scholarship to attend a graduate program on Mechanical Engineering at Michigan Technological University. He has finished his MS and then continued with a doctorate program. His doctorate research was funded by NASA and the NSF. Dr. Kurita's contribution to his field was well published in several papers from high impact journals. From 2011 Dr. Kurita worked as a development engineer II, in the competitive automotive industry, Filtran LLC, located in Des Plaines Illinois. His experience as an experimental researcher helped Filtran to develop special testing techniques never implemented before on filtration systems. In addition, Dr. Kurita worked in the CAE group, contributing to develop simulation techniques to help develop state of the art filtration systems. From 2016 Dr. Kurita is back to his alma mater as an assistant professor in Universidad Nacional de Asuncion. Later the same year, he is appointed to lead the research department of the School of Engineering. From 2017 he is appointed to be the head of the Mechanical Engineering Department at Universidad Nacional de Asuncion. He is currently working as the director of the Planning Directorate of the Paraguayan Space Agency. For his contributions to the Paraguayan society in the field of science and engineering, he was acknowledged as the "Exceptional Protagonist of 2017" by the Ultimahora news, a major newspaper in Paraguay. Another distinction, the "Outstanding Citizen Award," was granted by the city council of the city of Asuncion in 2017.