Multiple wall temperature peaks during forced convective heat transfer of supercritical carbon dioxide in tubes

摘要

Heat transfer deterioration (HTD) is defined as having one or multiple wall temperature peaks along the flow length for supercritical heat transfer. The single-peak phenomenon has been widely reported in the literature, but the multi-peak phenomenon is seldomly studied and not well understood. Here, the convective heat transfer of supercritical carbon dioxide in vertical tubes is investigated, with pressure, mass flux, and heat flux in the ranges of 7.5～23 MPa, 400～1500 kg/m~2s, and 25～450 kW/m~2, respectively, and inner tube diameter covering 8, 10, and 12 mm. Sudden changes are observed between normal heat transfer (NHT) without wall temperature peak and HTD by crossing a global critical supercritical boiling number (SBO) based on the pseudo-boiling theory. For HTD, we show that inner diameter and pressure have significant influences on whether the multi-peak phenomenon occurs: it takes place for inner diameter of 12 mm only and under pressure of ～8 and ～12.5 MPa. It is found that friction factors for multi-peak cases are obviously larger than those for single-peak or NHT cases, which is successfully explained by the orifice contraction effect due to the strong "vapor" expansion at the tube cross-section corresponding to the sharp wall temperature peak. The multi-peak phenomenon is shown to be governed by the competition between the local evaporation momentum force and inertia force along the flow length. Our work enhances the understanding of supercritical heat transfer and verifies the validity of pseudo-boiling assumption, and can also benefit engineering applications in advanced power cycles.