Experimental study of nucleate pool boiling of R134a on a stainless steel tube

Nucleate pool boiling of R134a is studied experimentally in the reduced pressure range 0.03⩽p/pc⩽0.5 (1.2 bar⩽p⩽20.3 bar) for heat fluxes from q=100 000 W/m2 down to single phase natural convection. Additionally to the heat transfer measurements, nucleation site density, up to (N/A)max≈6000 sites/cm2 is measured by an optical method. The test specimen is a horizontal stainless steel tube (sandblasted) with an outer diameter do=15.0 mm. The arrangement of the wall thermocouples allows for the direct measurement of the local radial heat flow. Experimental results: For the highest pressure investigated (p/pc=0.5) and for high heat fluxes (q⩾10 000 W/m2), the ratio of the local heat fluxes to the average heat flux is found to be in the range between 0.95 and 1.05, indicating that the boiling process around the test tube is only weakly influenced by the direction of the gravity field. For lower average heat fluxes, the angular heat flux variation increases, indicating the occurrence of two-phase convective effects. For the lower pressures, this convection dominated region extends to higher heat fluxes. The data for high heat fluxes (where nearly no angular variation occurs) are used for in situ evaluation of the thermal conductivity of the stainless steel wall material. The nucleation site density measurements are performed on the vertical flank of the test tube. The active nucleation sites are found to exhibit a rather strange behaviour on the sandblasted surface. Most of the sites emit only a series of few bubbles and then become inactive for a longer period; seldom a really stable nucleation site can be observed.