Experimental Study of Differences in Single-Phase Forced-Convection Heat Transfer Characteristics between Upflow and Downflow for Narrow Rectangular Channel

Abstract

The differences in the single-phase forced-convection heat transfer characteristics between upflow and downflow were investigated experimentally with a narrow vertical rectangular channel. The objectives of the experiment were to investigate in both laminar and turbulent flow regions the applicability of existing correlations to and the effects of buoyant force on the heat transfer characteristics in the narrow vertical rectangular channel, which is simulating a subchannel of 2.25 mm in gap and 750 mm in length in the fuel element of the research reactor, JRR-3 to be upgraded at 20 MWt. As the results, it was revealed that (1) by use of equivalent hydraulic diameter, existing correlations are applicable to a channel as narrow as 2.25 mm in gap for turbulent flow though the precision and critical Reynolds number are different among the correlations, and (2) in the laminar flow, the difference in heat transfer characteristics arises between upflow and downflow with Reynolds number less than about 700 and Grashof number larger than about 1,000, giving smaller Nusselt number for down-flow than for upflow as the effect of buoyant force. New heat transfer correlations for channel heated from both sides are proposed as lower limits for upflow and downflow, respectively, in the laminar flow.