ENHANCEMENT OF HEAT-TRANSFER DUE TO BUBBLES PASSING THROUGH A NARROW VERTICAL RECTANGULAR CHANNEL (CHANGE IN HEAT-TRANSFER ALONG FLOW)

The objective of this paper is to make clear how heat transfer coeffic ient changes along the flow with the passing bubbles through a narrow vertical rectangular channel (20 mm wide, 2 mm deep and 450 mm long). The experiments were done using subcooled water of 80, 60, and 40 K at atmospheric pressure in which the air bubbles were injected into the channel at a designated period from 0.125 to 1.0 s and their length wa s controlled to be equal to 0.03, 0.02, and 0.01 m. The experiment sho ws that the heat transfer coefficients decrease along the flow and the n reach a constant value beyond a certain distance from the leading ed ge of the heated surface where the flow becomes fully developed in bot h the velocity and the thermal conditions. Under the fully developed c onditions, the heat transfer coefficients are predicted well by the ex isting theoretical analysis in which both the convective term and evap oration on the interface are ignored.