This paper presents a numerical simulation on the heat transfer of liquid sodium in a solar receiver tube, as the liquid sodium is a promising heat-transfer candidate for the next generation solar-power-tower (SPT) system. A comparison between three mediums—solar salt, Hitec and liquid sodium—is presented under uniform and nonuniform heat-flux configurations. We studied the effects of mass flow rate (Qm), inlet temperature (Tin), and maximum heat flux (qomax), on the average heat-transfer coefficient (h) and the friction coefficient (f) of the three mediums. The results show that the h of liquid sodium is about 2.5 to 5 times than other two molten salts when Tin is varying from 550 to 800 K, Qm is 1.0 kg/s, and qomax is 0.1 MW/m2. For maximum heat fluxes from 0.1 to 0.3 MW/m2, the h of liquid sodium is always an order of magnitude larger than that of Hitec and Solar-Salt (S-S), while maintaining a small friction coefficient.

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The present paper is devoted to investigating the difference of heat transfer characteristics between horizontal and vertical upward flows of supercritical pressure water. Experimental study is conducted with both horizontal and vertical upward tubes (32 mm3 mm), covering a range of mass fluxes (G) from 200 to 600 kg·m -2 ·s -1 , heat fluxes (q) up to 400 kW·m -2 , and pressure (P) from 23 to 28 MPa.Heat transfer characteristics are analyzed in detail for selected parameters. The results show at low q/G, an apparent heat transfer enhancement and insignificant difference in the two arrangements. However, when the q/G increases to a higher value (i.e. q/G>0.5), heat transfer deterioration occurs and a noticeable heat transfer discrepancy is detected, where the inner-wall temperature of vertical flow far exceeds that of horizontal flow. Dimensionless parameters, Bo + , K, and BTH are adopted to analyze the effects of buoyancy force and thermal acceleration for both flows. The analysis suggests that mechanisms governing horizontal and vertical flows of supercritical © 2016. This manuscript version is made available under the Elsevier user license http://www.elsevier.com/open-access/userlicense/1.0/ pressure water are different at high q/G or in deteriorated heat transfer mode. For the vertical flow, thermal acceleration play a leading role, while for the horizontal flow, the effect of buoyancy plays a larger effect than that for vertical flow.  dynamic viscosity, Pa s λ Thermal conductivity, W/K/m  density, kg/m 3 Non-dimensional Numbers Bo Bo number BTH Combined effect of buoyancy and thermal acceleration Gr Grashoft number J Acceleration effect Kv Flow-acceleration parameter Pr Prandtl number; ( ) Re Reynolds number; ( )