Qinghua Chen

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.

In this paper, a new kind of evaporative heat transfer experiment for the cooling process of coolers/condensers is conducted. The design of the test coils is immersed in an airwater bubbling layer. The air-water two-phase flow passes through the tubes of the coils. Due to the motion of the air bubbles in the water, a thin water film forms on the surface of the tube. As the air bubbles pass by the tube this water film is evaporated into the air. The tubes of coil reject heat to the water film, and the evaporation of the water film rejects heat to the air bubble stream. This heat transfer mode significantly increases the heat transfer coefficient between tubes and air. The consumption of the power of a water pump can be decreased. Moreover, the airflow rate required is less than that of an aircooled condenser. The pressure drop of air through air-water bubbling layer and the heat transfer between the tube and water are experimentally investigated in this paper. The results show that the factors affecting the pressure drop and the heat transfer coefficient involve the pore geometry of sieve plate, the height of the air-water bubbling layer, the air flow rate through the sieve plate and the heat flux of tubes. The heat transfer coefficient between tube and water is two times larger than that of falling film of water on the outer surface of tube.