Heat transfer in a compact heat exchanger containing rectangular channels and using helium gas

: We have constructed a compact heat exchanger consisting of 12 parallel, rectangular channels in a flat piece of commercially pure nickel. This channel specimen was radiatively heated on the top side at heat fluxes of up to 77 W/sq cm, insulated on the back side, and cooled with helium gas flowing in the channels at 3.5 to 7.0 MPa and Reynolds numbers of 1400 to 28 000. The measured friction factor was lower than that of the accepted correlation for fully developed turbulent flow, although our uncertainty was high due to uncertainty in the channel height and a high ratio of dynamic pressure to pressure drop. The measured Nusselt number, when modified to account for differences in fluid properties between the wall and the cooling fluid, agreed with past correlations for fully developed turbulent flow in channels. Flow nonuniformity from channel-to-channel was as high as 12% above and 19% below the mean flow.