3843 (RP-668) -- Heat Transfer of R134a in Single-Tube Spray Evaporation Including Lubricant Effects and Enhanced Surface Results PDF

Single-tube spray evaporation experimental tests were conducted in order to evaluate the average wall heat transfer coefficients for seven different commercially available tubes. Liquid film supply rates were held constant in order to evaluate the effects of the enhancement on shelf-side heat transfer under similar conditions. Because the spray evaporation phenomenon is so different from pool boiling, both condensation-type and evaporation-type enhanced surfaces were evaluated. A comparison of the results for all of the tubes showed that the enhanced condensation surfaces performed better than the enhanced boiling surfaces. In addition the 26-fpi surface tested marginally better than the 40-fpi surface. Small concentrations of a polyol-ester lubricant cause a foaming effect that increases the heat transfer performance. This tendency was seen with both 32-cs and 68-cs polyol-ester oils. The 68-cs lubricant was tested at concentrations of 0.0, 0.5, 1.0, 3.0, and 5.0 with the W-40 fpi and Tu-Cii surfaces. Results with this lubricant show the performance continues to increase through the 3% concentration for most of the heat flux range tested. At the upper end of the range tested the 1.0% mass fraction yielded the best performance. The 32-cs lubricant generated trends similar to those of the 68-cs lubricant. Lubricant concentrations of 1.0%, 2.0% and 3.0% were evaluated with plain, W-40 fpi, and Tu Cii surfaces. The 2.0% concentration, not the 1.0%, generated the best performance at the highest heat flux tested. This difference must be attributed to the difference in the lubricant viscosity.

KEYWORDS: year 1995, heat flow, R134a, refrigerants, lubricants, tubes, evaporation, sprays, evaporators, performance, testing, heat transfer coefficient, comparing, copper, viscosity