Nucleate pool boiling on structured enhanced tubes having pores with connecting gaps

In this study, pool boiling test results are provided for the structured en hanced tubes having pores with connecting gaps. The surface geometry of the present tube is similar to that of Turbo-B. Three tubes with different por e size (0.20, 0.23 and 0.27 mm) were manufactured and tested using R-ll, R- 123 and R-134a. The pore size which yields the maximum heat transfer coeffi cient varied depending on the refrigerant. For R-134a, the maximum heat tra nsfer coefficient was obtained for the tube having 0.27 mm pore size. For R -ll and R-123, the optimum pore size was 0.23 mm. One novel feature of the present tubes is that their boiling curves do not show a 'cross-over' chara cteristic, which existing pored tubes do. The connecting gaps of the presen t tubes are believed to serve an additional route for the liquid supply and delay the dry-out of the tunnel. The present tubes yield the heat transfer coefficients approximately equal to those of the existing pored enhanced t ubes. At the heat flux 40 kW/m(2) and saturation temperature 4.4 degreesC, the heat transfer coefficients of the present tubes are 6.5 times larger fo r R-ll, 6.0 times larger for R-123 and 5.0 times larger for R-134a than tha t of the smooth tube. (C) 2000 Elsevier Science Ltd. All rights reserved.