EFFECTS OF ENHANCED SURFACES AND SURFACE ORIENTATION ON NUCLEATE AND FILM BOILING HEAT-TRANSFER IN R-11

A study of pool boiling heat transfer in R-11 is reported as a function of surface characterization and orientation. Two specially prepared metal coated surfaces (UNB#1, UNB#2) and a flat copper surface were subjected to heat fluxes up to 180 kw m** minus **2 with surface orientations varying from horizontally facing upward (0 degree ), to vertical (90 degree ), to horizontally facing downward (180 degree ). The resulting nucleate boiling curves display considerable boiling hysteresis and enhanced surfaces show 2-3 times better heat transfer than a plain surface. Rohsenow's nucleate to boiling equation is used to correlate the data and modified to account for the effects of surface characterization and orientation. In film boiling, enhanced surfaces also reveal better heat transfer characteristics and the role of surface orientation on the motion and stability of the vapor film is clarified.