A study of heat transfer scaling of supercritical pressure water in horizontal tubes

Understanding the role of the heat transfer scaling in supercritical utilities can help us obtain a better design for supercritical single phase thermosiphons. In this paper, heat transfer scaling analysis of supercritical pressure water in horizontal tubes with differential inside diameters are experimentally investigated. The operating conditions included mass flux of 100–600 kg/m² s and heat flux of up to 400 kW/m². The uneven circumferential temperature distributions are discussed firstly over a broad range of heat fluxes. A significant temperature discrepancy exists between the top and bottom surface in each pipe, deteriorated heat transfer occurs on the top surface but enhanced heat transfer happens in the bottom region. With the rise of heat flux, more severe deterioration appears on the higher domain. Then a qualitatively comparison on the heat transfer characteristic of supercritical flow in different pipes are implemented. Evidently, with the increase of tube diameter, the inner-wall temperature peak is much pronounced and the degree of deterioration at the top surface much severe due to the intensification of buoyancy force. The heat transfer mechanism of horizontal pipes is further discussed by comparing the effect of buoyancy and thermal acceleration. With the augment of diameter, free convection gradually intensifies due to its steeper density variation and expanded flow space, which leads to larger temperature deviation between the bottom and top region along the circumferential direction. On the contrary, thermal acceleration plays a relatively minor role in deteriorated heat transfer. Considering the scaling effect in various pipes, two heat transfer correlations are proposed respectively in both the bottom and top region of the horizontal flows.