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ISSN Online: 2377-424X

ISBN CD: 1-56700-226-9

ISBN Online: 1-56700-225-0

International Heat Transfer Conference 13
August, 13-18, 2006, Sydney, Australia

HEAT TRANSFER AND PRESSURE DROP DURING FLOW BOILING OF R134a AND R404A IN A MICROFIN TUBE AT LOW MASS FLUXES

Get access (open in a dialog) DOI: 10.1615/IHTC13.p28.240
13 pages

Resumo

An experimental investigation of flow boiling heat transfer in a microfin tube with 9.52 mm outer diameter has been carried out. The microfin tube is made of copper with a total fin number of 55 and a helix angle of 15°. The fin height is 0.24 mm and the inner tube diameter at fin root is 8.95 mm. The test tube is 1 m long and is electrically heated. The experiments have been performed at saturation temperatures between 0°C and −20°C. The mass flux was varied between 25 and 150 kg/m2s, the heat flux from 15000 W/m2 down to 1000 W/m2. All measurements have been performed at constant inlet vapour quality ranging from 0.1 to 0.7. To evaluate the local heat transfer coefficients the wall temperatures have been measured at 5 positions along the test tube with a distance of 200 mm. At each measuring section 4 miniature thermocouples are located along the circumference.
The measured heat transfer coefficients range from 1300 to 15700 W/m2K for R134a and from 912 to 11451 W/m2K for R404A. The mean heat transfer coefficient of R134a is in average 1.5 times higher than for R404A. The heat transfer coefficient of the microfin tube is about 2 to 6 times higher than for the smooth tube. The mean heat transfer coefficient has been compared with the correlations by Koyama et al. and by Kandlikar. The deviations are within ±30% and ±15%, respectively. The influence of the mass flux on the heat transfer is most significant between 25 and 62.5 kg/m2, where the flow pattern changes from stratified wavy flow to almost annular flow. This flow pattern transition is shifted to lower mass fluxes for the microfin tube compared to the smooth tube. The pressure drop data have been compared with the modified correlation by Müller-Steinhagen & Heck. The deviations are within ±30% for pressure losses >10 mbar/m and within ±15% for pressure losses >40 mbar/m.