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ISSN Online: 2377-424X

International Heat Transfer Conference 12
August, 18-23, 2002, Grenoble, France

Second order effects in an experimental study of nucleate pool boiling of R134a

Get access (open in a dialog) DOI: 10.1615/IHTC12.4650
6 pages

Аннотация

Nucleate pool boiling of Refrigerant R134a has been examined at heat fluxes ranging from natural convection up to 100 000 W/m2 in the reduced pressure range 0.1 ≤ p/pc ≤ 0.5. The test specimen is a gold-plated horizontal copper tube with a diameter d = 15mm. The gold-plated surface is sandblasted (roughness Ra = 0 .3 µm). The stainless steel pool boiling apparatus is contained in a precisely temperature-controlled cell with a temperature stability of Δϑair = 0.02 K.
The temperatures inside the boiling apparatus are measured with three thermocouples, two of them arranged in the liquid below and above the test tube and the third arranged in the vapour space. For the unheated test tube, the three thermocouples coincide within Δϑ = ±0.001 K.
When the test tube is heated, the liquid temperature below the tube is nearly always higher than the saturation temperature. This infers that the test tube is surrounded by superheated liquid, which originates from a largespaced vortex flow generated by the rising vapour bubbles. The liquid superheat depends on the boiling pressure and on the heat flux, it decreases with increasing pressure and increasing heat flux.
Long-time experiments at constant pressure and constant heat flux reveal, that the boiling process is stable on a short-term scale (hours) but shows a continuous drift on a longer time scale (days, weeks). For experiments performed with decreasing heat flux, the heat transfer coefficient increased with time, whereas for experiments with increasing heat flux the heat transfer coefficient decreased. Therefore, in experimental boiling studies, the "history", i.e. the relevant time intervals of the experiments, seem to play an important role and should be better documented in future.