Heat transfer and pressure drop experimental correlations for air-water bubbly flow

In this work a uniquely designed air-water bubbly flow heat transfer experiment is accomplished to investigate the characteristics of pressure drop of airflow and heat transfer between water and tubes for its potential application in evaporative cooling. The attempts to reduce the pressure drop at the condition of maintaining higher heat transfer coefficient have been done by decreasing the bubble layer thickness though the water pump circulation. Pressure drops of air passing through the sieve plate and the bubbling layer are measured for different heights of bubble layer, hole area ratio of the sieve plate and the air velocity through the holes. Experimental data show that the increase of bubble layer height and air velocity both make the pressure drop larger while the effect of the hole area ratio of the sieve plate is relatively sophisticated. A correlation including the effects of all the tested parameters is proposed, which has a mean absolute deviation of 14.5% to that of the experimental data. Heat transfer coefficients of the water and the outside tube wall are measured and the effect of heating flux is also considered. The results show that even at a considerably lower height of bubbling layer the heat transfer coefficient can exceed 5 kW/m²-K. A heat transfer coefficient correlation with a mean absolute deviation of 9.7% is developed based on the experimental data.