EXPERIMENTAL STUDY OF NUCLEATE POOL BOILING OF R134a ON A STAINLESS STEEL TUBE (DFG Joint Research Project: Thermo- and Fluiddynamics in Boiling)

摘要

Nucleate pool boiling of R134a is studied experimentally in the reduced pressure range 0.03≤p/p_(c)≤0.5 (1.2 bar≤p≤20.3 bar) for heat fluxes from q=100 000 W/m~(2) down to single phase natural convection. Additionally to the heat transfer measurements, nucleation site density, up to (N/A)_(max)≈6000 sites/cm~(2) is measured by an optical method. The test specimen is a horizontal stainless steel tube with an outer diameter d_(o)=15.0mm and a heated length l_(h)=142mm. The heating surface is sandblasted (roughness R_(a)=0.18 μm). The test tube is heated electrically by a cartridge heater with 6.5 mm diameter and a heating power of 1500 W. The wall temperatures are measured by 8 thermocouples, 4 of them arranged 0.9mm below the heated surface and the other 4 arranged 2.8mm below the heated surface. This arrangement allows for the direct measurement of the local radial heat fluxes in four directions (upward, downward, horizontal forward and horizontal backward). The stainless steel pool boiling apparatus ("German standard apparatus") is contained in a precisely temperature-controlled cell with a temperature stability of Δpartial deriv_(air)=0.02 K between the top point of the test installation (the condenser), and its lowest point where the condensed liquid is fed back into the liquid pool. Experimental results: For the highest pressure investigated (p/p_(c)=0.5) and for high heat fluxes (q≥10 000 W/m~(2)), 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 boiling heat transfer around the test tube is no more influenced by the direction of the gravity field. For lower average heat fluxes, first the local heat flux from the upper side of the tube and then also from the lower side fall below the ratio q_(loc)/q=1; the heat fluxes from the sides rise to higher values. This indicates that two-phase convective heat transfer and/or sliding bubbles effects around the flanks of the tube are dominating. For the lower pressures, the convection dominated region extends to higher heat fluxes. The circumferential variations of the local heat flux observed are mainly attributed to the low thermal conductivity of the wall material. The data for high heat fluxes (where no circumferential variation occurs) are used for in situ evaluation of the thermal conductivity of the stainless steel wall material, resulting in k≈14 W/m K. The thermal conductivity slightly increases with increasing temperature, showing the common behaviour for stainless steel materials in this temperature range. 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 stable nucleation site can be observed.