• Transactions of the Korean Society of Mechanical Engineers
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• v.14 no.3
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• pp.702-715
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• 1990

This paper describes behaviors of two-phase open channel flow inside the flash chamber of a horizontal Multi-Stage-Flash evaporator numerically along with the experimental observations. Bubble trajectories and the velocity and temperature distributions of the liquid phase were predicted by using the particle-source-in-cell(PSI-Cell) method with the appropriate bubble motion/growth equations. Size and number of bubble nuclei embedded in the incoming liquid(brine) were taken into account as important parameters in addition to the conventional ones such as the velocity, degree of inlet superheat, inlet opening height, and the liquid level. Bubble motions, which are unsteady, appeared to be mostly determined by the buoyancy and the drag forces. The calculations, though a number of simplifying assumptions were made, reasonably simulated the hydrodynamic behaviors of the two-phase horizontal stream observed in the experiments. The simulated temperature distributions also agreed fairly well with the other's measurements. Non-equilibrium allownaces, evaluated from the simulated temperature distributions, were within the range of those obtained from the existing correlations, and reduced with the increases of the number and size of incoming bubble nuclei due to vigorous flashing.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.4
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• pp.833-846
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• 1988

Two-phase flow heat transfer phenomena with flash evaporation inside a vertical tube were studied experimentally. Void fractions were measured using electrical probes, and the flow patterns were identified from the output voltage signal itself. The flow pattern as well as the beat transfer rates were changing along the axial distance from the tube inlet with the system pressure. As the pressure inside the tube decreases with fixed inlet temperature, the overall heat transfer coefficient through the tube wall and the boiling heat transfer coefficient inside the tube increase whereas the condensation heat transfer coefficient outside the tube decreases. The boiling heat transfer coefficient inside the tube measured by the experiments appeared to be somewhat larger than the value obtained from the Chen's correlation. Also, the flow patterns identified from present experiments are at the larger quality region of the low pattern map based on the transition criteria of Mishima and Ishii. This may be due to the non-equilibrium flashing phenomenon occurred at the nozzle exit and the tube inlet ; this also implies that the flow pattern of the two-phase flow depends strongly on the inlet conditions.

• Transactions of the Korean Society of Mechanical Engineers
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• v.12 no.6
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• pp.1399-1406
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• 1988

Flow behaviors of the open channel type flow with its geometric boundary conditions being similar to that of the Multi-Stage-Flash evaporator were studied qualitatively by measuring the velocity distribution. Without a baffle, the flow was in the shape of a simple submerged plane wall jet. At the downstream of this flow, the jet boundary made sharp curve toward the free surface ; this is because the entrainment of the ambient liquid is restricted by the free surface boundary, similar to the Coanda effect. According to the experimental results the level of the free surface appeared to be the most important parameter. The flow with a baffle was in much complicated shape ; especially the recirculating region at the downstream free surface was detected according to the experimental conditions imposed. Inlet liquid velocity, heights of the liquid level and the baffle, and the opening heights of sluice gate of the entrance were the most important parameters in the baffle flow.