Subcooled Water Flow Boiling Transition and theL/DEffect on CHF for a Horizontal Uniformly Heated Tube

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Abstract
Steady-State subcooled water flow boiling experiments were carried out in a uniformly heated horizontal circular channel with a 0.45-MPa exit pressure and with the mass velocity varying from 1.56 to 8.55 Mg/m2·s. Measurements of critical heat flux (CHF), local heat transfer, and pressure drop were made for a smooth-wall 1.02-cm-diam copper test section with a heated length-to-diameter (L/D) ratio of 49.0. For the same inlet temperature near 20.0°C, comparisons are made with previous data with L/D = 33.0, from 30.0 to 50.0, 96.6 (two cases), and 115.5. The exit pressures for the above data are 0.1, 0.45, 0.77, 1.59, and 1.67 MPa, respectively. When L/D is between 49.0 and 115.5, the L/D influence on CHF is found to be significant for a 1.02-cm channel diameter in subcooled flows for mass velocities above 4.0 Mg/m2·s. This finding is important since most researchers and designers assume minimal L/D influence when L/D is >30. Further, the present CHF and local heat transfer data extend the data base (CHF near 1000.0 W/cm2 and heat transfer coefficient near 70 000 W/m2·K) for large channel diameters near 1.0 cm and low exit pressures. These results will assist in preventing catastrophic conditions from occurring in future systems where the L/D influence might inappropriately be neglected. Finally, Gambill’s correlation predicts CHF significantly above the present data for CHF below 500 W/cm2. Above 500 W/cm2, however, his correlation agreed well with both the present data and the data for L/D = 96.6.