Experimental investigation of inclination effect on subcritical and supercritical water flows heat transfer in an internally ribbed tube

The effect of various inclination angles on heat transfer of water at subcritical and supercritical operating pressures is investigated experimentally. The test section was a SA213T12 steel six-headed internally ribbed tube with minimum inner diameter of 19.5 mm. The operating test pressures were 15, 21.5, 22.5, 25 and 28 MPa, the mass flux was 800 kg/m2 s and the heat flux was 400 kW/m2. To keep the mass flux to heat flux ratio equal to 2 kg/kJ. These operating conditions covered subcritical, near critical and supercritical water flows and also refers to low mass flux conditions. The inclination angles were 5, 20, 30, 45 and 90 (vertical) degrees respecting to horizontal plane. The heat flux was kept constant along the test tube by controlling of electric heating. As a result the inner wall temperature and convective heat transfer coefficient variations with respect to heated length and bulk enthalpy of fluid were considered in order to study the heat transfer characteristics of various flows at different inclinations. The corresponding correlation for heat transfer coefficient was developed which is applicable for wide range of inclination angles. The heat transfer enhancement was obvious for inclination angles other than 90°, however, this effect was more obvious in 5° and 20° in some operating conditions. It was also concluded that the effect of inclination on heat transfer of water was more considerable in subcritical flow conditions than supercritical ones. Also, it was observed that angle of 20° seems to be the best for subcritical flows from heat transfer point of view, but for supercritical flows 5 or 45  seem to be more advantageous. These differences could be related to different heat transfer mechanisms of subcritical and supercritical flows.