Experimental research on natural convective heat transfer of water near its density maximum in a horizontal annulus

In this paper, the heat transfer characteristics of the natural convection of water near its density maximum in a horizontal annulus had been experimentally investigated. The annulus was formed with two copper cylindrical walls, which were concentric or eccentric. The temperature of the outer cylinder was kept at around 0 °C and that of the inner cylinder was ranged from 0 to 12 °C. In order to enhance heat transfer, three-dimensional external fins were applied on the inner wall. The effects of the gap width and the eccentricity of annulus and the fin geometric parameters on heat transfer were studied. The results show that the two turning points of the average Nusselt number on the inner wall appear at the temperature differences close to 4 °C and 8 °C, respectively. With the increase of temperature difference, the average Nusselt number firstly increases, and then decreases, finally increases again. The minimum value of the average Nusselt number locates near the temperature difference 8 °C. Additionally, it increases with the increase of the gap width of annulus and the eccentricity. Furthermore, when three-dimensional external fins were applied, the average Nusselt number increases with the increase of the fin axial pitch, whereas it decreases with the increase of the fin width. In the range of experimental values, the influences of the fin height and circular pitch are slight. Finally, the experimental empirical correlations of heat transfer on the inner wall are obtained.

► Natural convection of water near its density maximum in a horizontal annulus had been investigated.
► Effects of the gap width and the eccentricity on heat transfer were analyzed.
► Enhancement effect of three-dimensional external fins on the inner wall was exhibited.
► Experimental empirical correlations of heat transfer were obtained.