STUDIES OF CONVECTIVE HEAT TRANSFER COEFFICIENT ENHANCEMENT IN TWO-PHASE, NON-BOILING FLOW

An experimental investigation has been conducted to determine the heat transfer coefficient in a non-boiling, two-component, two-phase flow. Both upflow and downflow of air-water mixtures were studied in a 1 inch pipe. The superficial water velocities ranged from 0.40 to 3.24 m/s with the corresponding Reynolds numbers from 15,000 to 116,000. The superficial air velocities ranged from 0.39 to 1.95 m/s with the corresponding Reynolds numbers from 540 to 2,700. The two-phase flow patterns observed in the experiments were bubbly, slug and froth in upflow and bubbly, slug and annular in downflow. Heat fluxes up to 54 kw/m('2) were applied to the test section.;In this study a variety of sensors were employed in the flow loop. A conductivity probe was used to measure the void fraction profile. It was calibrated using a radiation attenuation technique. A void fraction readout device and digital counter were designed and assembled for use with this probe. A T.S.I. conical hot-film probe, driven by a constant temperature anemometer, was used to measure local water velocity and void fraction profiles in the air-water flow. Wall temperatures were measured with thermocouples embedded in the test section wall, which was electrically heated. All probes were calibrated for both relative and absolute levels.;The local void fraction profile is known to be a strong function of the flow pattern and flow orientation. It was observed in the upflow tests, that the profile is a saddle shape distribution in bubble flow and changes to a bullet shape distribution in slug flow. However, the profile in downflow showed a peak at the pipe center in bubbly flow and became a uniform distribution in slug and annular flows. Consequently, the water velocity profile also was found to strongly depend on the flow orientation. In the bubbly flow pattern, it retained the 1/7-power low profile in upflow and had a saddle shape profile in downflow.;The measurement of the convective heat transfer coefficient in the two-phase flow showed that the injection of air causes a significant increase in this coefficient. At higher liquid flow rates, and with the same rates of air injection, the increases of heat transfer coefficient were less and tended to level off at the highest water flow rates. The heat transfer coefficient was higher in downflow than in upflow with the same air and water flow rates. A modified Sieder-Tate correlation was developed for each flow direction studied, i.e. upflow and downflow, to collapse the experimental data within (+OR-) 15 percent.;Previously published experimental results and correlations of the heat transfer coefficient in the nonboiling, two-phase flow systems have reported enhancement of the two-phase heat transfer coefficient, as a result of injection of a gas phase into the liquid flow. This was suggested to have been caused by the resulting increase of liquid velocity and the associated increase of turbulent intensity. Also, it was reported that the two-phase heat transfer coefficient was a strong function of flow pattern. Few theoretical studies have been carried out to investigate the heat transport phenomena in the two-component, two-phase flow due, in no small part, to the complexity of the problem. However, correlations based on the modified Sieder-Tate model have been widely used to collapse the experimental results with varying degrees of success.