1990-02-01

Multidimensional Numerical Modeling of Two-Phase Flow and Heat Transfer Processes in a Plate Type A/C Evaporator 900726

A model developed to analyze steady, two-dimensional, two-phase flow of Freon-12 with simultaneous heat and mass transfer in a plate type A/C evaporator is described. The results obtained for specified flow rates and qualities at entry indicate that, for given heat input at the walls, dryout conditions prevail in certain regions of the evaporator with correspondingly higher wall temperatures there. This happens particularly when the quality is high and the flow rate is low at evaporator entry. Though the model predictions are plausible, an improved understanding of the evaporation process of Freon-12 will assure greater realism in the predictions.THIS PAPER IS CONCERNED WITH numerical modeling of steady, two-phase flow with heat and mass exchange in the plate type A/C evaporator. In the geometry considered (Figure 1), Freon-12 of specified quality i.e. a two-phase mixture of known mass fraction of vapor (or gas), flows through the unit while receiving heat from the ambient air blowing past the outer walls. This causes further evaporation of the liquid with consequent rise in the quality as the fluid approaches the outlet. Such an increase in quality, especially when it is high initially, may affect the flow and the boiling regimes, and the heat transfer processes [1]1 as shown in Figure 2. The transition from an annular flow with entrainment pattern to a drop flow pattern reduces the inner wall heat transfer coefficient sharply to bring about a significant rise in wall temperature.Such a condition called ‘dryout’ arises when the liquid exists like a mist away from the wall in a liquid-vapor mixture. While more information is available on the onset of dryout phenomenon in water-steam flow fields, not much is known for other fluids. Robertson [2] suggests that dryout at modest heat fluxes occurs at qualities above 95% for fluids other than water-steam mixture.In the present model it is assumed that the evaporation process is dominated by (a) forced convective heat transfer through liquid film when the quality x ≤0.95; and (b) dryout heat transfer in the liquid deficient region at x > 0.95. The respective heat transfer correlations [3, 4] adopted here for the above two situations are (A1) and (A2) given in Appendix A. All fluid properties in those correlations were obtained from [5]. To account for the frictional losses, as recommended in [6] and [7], the pressure drop correlation (A3) was used for two-phase cross-flow past the dimples in the evaporator.In what follows, the objectives of the paper are indicated first, and they are followed by the flow specifications and boundary conditions used by the model. The mathematical formulation and computational details are then given before presenting the results and the discussions. Thereafter, the conclusions are drawn and the recommendations are made. These are followed by the references and the appendix.

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