Heat Transfer in Flow Boiling Over a Bundle of Horizontal Tubes

The paper describes tests boiling R 113 at atmospheric pressure in upward flow over a 17 row column of square pitched electrically heated tubes. Uniform heat fluxes of 10-65 kW m-2 and maximum Reynolds numbers, Remax, between 7800 and 27 000 were used. The data at the lower heat fluxes of 10 and 20 kW m-2 consistently exhibited a linear increase of heat transfer coefficient, h, with quality, the slope lower at 20 kW m-2, so that h at 10 kW m-2 exceeded that at 20 kW m-2 and both reached h at 40 kW m-2 for the maximum quality tested. At the higher heat fluxes a nucleate boiling controlled region was observed at low quality, with h equal to the value at the same heat flux as observed with an isolated tube in a pool, followed by a rise in h at higher quality. The results were compared with the work of other researchers. Heat transfer coefficients for q ≥ 40 kW m-2 were predicted to an average r.m.s. deviation of 7%, using the asymptotic flow boiling model. However, the sensitivity of h to change of quality and flow rate predicted was much lower than measured and in some cases exhibited opposite trends.