3916 (RP-668) -- Effects of Surface Enhancement, Film-Feed Supply Rate, and Bundle Geometry on Spray Evaporation Heat Transfer Performance PDF

Testing was conducted with R134a through an overfeed ratio range of 1.4 to 7.9 in order to evaluate the effects of Reynolds number on shell-side heat transfer performance in the spray evaporation environment. The overfeed ratio is defined as the ratio of the refrigerant flow rate supplied to the tube bundle to the refrigerant flow rate that vaporises. Data were taken with a fixed refrigerant supply rate while varying the shell-side heat flux from 40 kW/m² (12,688 Btu/[h×ft²]) to 19 kW/m² (6,027 Btu/[h×ft²]). Both triangular and square-pitch tube bundles were tested to determine the effects of bundle geometry on heat transfer performance. Two enhanced condensation surfaces, one enhanced boiling surface, and one low-finned surface tube were used in this study. Plain-surface bundle testing was conducted in parallel with the enhanced surface testing to determine the degree of improvement obtained with the different surface enhancements relative to that of a smooth tube. In addition, the effect of bundle depth on heat transfer performance was evaluated. Refrigerant was introduced into the test section with wide-angle, solid-cone nozzles. To determine the amount of refrigerant contacting the tube bundle, collector testing was performed in parallel with the heat transfer analysis experiments. Using results from the collector tests, bundle overfeed ratios were calculated and are reported. Heat transfer performance showed dependence on film-feed supply rate (i.e., overfeed ratio) to varying degrees, depending on the type of surface enhancement. Those surfaces that limited axial flow of the liquid film yielded poor heat transfer performance in lower rows of the bundle. The spray evaporation heat transfer performance for one of the enhanced condensation surfaces was better than the flooded evaporator performance for the enhanced boiling surface.

KEYWORDS: year 1995, evaporators, refrigerants, fluid flow, testing, heat flow,performance, tubes, R134a, Reynolds numbers, geometry, finned tubes, comparing