Experimental heat-transfer results in the low Reynolds modulus range of 1000 to 10,000 are reported for two different liquid metals, lead-bismuth eutectic and mercury. The single heat exchanger used in both series of tests was a ¾-in., 18-gage, 4-ft-long mild-steel tube. Similar heat-transfer tests with water in laminar and turbulent flow were performed with this same tube before and after each liquid-metal series, and the agreement of the turbulent-water tests with previous published results supports the liquid-metals test procedure. The data for both liquid metals, including flow in the upward and downward directions, are correlated on a Nusselt-Peclet basis with a single curve representing all experimental results with a maximum deviation of 20 per cent. The experimental Nusselt values decrease from approximately 6 at Reynolds number of 10,000 down to 1 at a Reynolds number of 1200, and consequently are considerably lower than the theoretical constant heat-rate laminar-flow Nusselt value of 4.36 (6). Possible causes for this behavior which have been investigated and found to be inadequate when considered independently include: (a) Insulating gas-layer or oxide-film fouling at the heated surface; (b) nonwetting; (c) uncertainties in property values; (d) axial-conduction effects. Although no supporting data are available, a distortion of the velocity profile from the parabolic distribution appears to be the most plausible explanation.

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