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In the present study direct contact heat transfer between hot water and cold air in vertical pipes with cocurrent downward flow is investigated. Experiments are performed in turbulent condition using 1, 1.5, 2 meter pipes lengths with 0.021 meter diameter. The hot water temperature was varied between 35 to 50 degrees of Celsius and the air temperature was varied between 25 to 35˚C. Air velocities were chosen to be 3, 5, 7, 9 and 11 m/s and the rates of flow rate of water was selected as 5, 8 and 10 lit/min. Two phase annular flow pattern of water and air is formed in the vertical pipe. The interface area between water and air was measured by Image Processing taken from shining laser light sheet on the transparent tubes. Over 80 percent of heat was transferred by evaporation and less than 20 percent was transferred by convection. In all three pipes, Nusselt number decreased with increase in flowrate of liquid phase. In all the verified temperatures, Nusselt number was decreasing by increasing pipe length. Experimental values for Nusselt numbers were compered to empirical values for smooth surfaces. Finally, some adjustments were suggested for Nusselt number correlation by fitting experimental data into existing correlations for pipes with smooth surface. Proposed correlations for heat transfer resulted in Nusselt number with ±15.26% average error.

Keywords: Direct contact heat exchanger, Downward flow, two-phase flow, Heat Transfer

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