| Summary: | Approved for public release; distribution is unlimited. === An investigation on the effect of oil on the onset of nucleate pool boiling of R-124 from a single horizontal tube was conducted at saturation temperature of 2.2 deg C. Pure R-124 and R-124/oil mixtures of 3% and 10% (by weight) miscible alkylbenzene oil were used. The tubes tested were: (1) smooth tube, (2) 19 fins per inch and (3) 26 fins per inch low-integral finned (GEWA-K) tubes, and (4) porous-coated (HIGH FLUX) tube. The effect of dissolved gases, subcooling and pressure on the onset of nucleate boiling were also investigated. An oil concentration of 3% tends to delay the onset of nucleate pool boiling (compared to pure R-124) on the smooth tube and GEWA-K 19 fins per inch tube, but not on the GEWA-K 26 fins per inch and HIGH FLUX tubes. The reason for this is not known precisely but is very repeatable. A 10% oil concentration tends to delay the onset of nucleate pool boiling on all tubes tested. This is due mainly to the increase of surface tension, and saturation temperature with increase in oil concentration. The presence of dissolved gases in the pool tends to lower the onset of nucleate pool boiling by increasing the number of entrapped vapor nuclei. Pressurization and subcooling tend to increase the onset of nucleate pool boiling by deactivating potential nucleation sites.
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