Heat Transfer And Frictional Resistance Of Vertical-upward Internally-ribbed Tubes In Supercritical Pressure Region

In pace with the acceleration of China’s modernization process,the demand for energy is also increasing. Due to coal’s dominance in China’s energy consumption, it’s necessary to reduce coal consumption for electricity generation, develop supercritical power generation technology for energy saving and emission reduction.To adapt to the adjustment of the power grid load, modern supercritical and ultra-supercritical pressure boilers always adopt the sliding pressure operation. So the water-wall design is one of the key technologies in supercritical pressure boilers. In the sliding pressure operation, with the change of load the system pressure was also constantly changing, the form of flow and heat transfer were became complicated and the phenomenon of heat transfer enhancement or deterioration might occur. The experiments were carried out with ?35×8mm,?35×6mm six-starts internally-ribbed tubes, the pressure at the inlet of the test section varied from 23.5 to 28 MPa, and the mass flux was from 600 to 1000 kg·m-2·s-1, and the inside wall heat flux ranged from 300 to 500 k W·m-2.Based on the experimental results, the influences of mass fluxes, heat fluxes, pressure and buoyancy force on the heat transfer characteristics of internally-ribbed tubes was analyzed in detail, and the abnormal heat transfer phenomenon in the large specific heat area near the pseudo-critical temperature was investigated. According to analysis of several existing heat transfer mechanism, comparison with the previous classic heat transfer correlation, gives a new formula for the calculation of heat transfer. It was considered that the decrease of heat transfer in the large specific heat area was caused by many factors, and the macroscopic factor was that the wall was covered by a thin layer of low density fluid, and the microscopic factor was due to buoyancy force. The main reason of heat transfer enhancement in the large specific heat region was the increase of the specific heat.In order to find out the frictional resistance characteristics of these internally-ribbed tubes, analysis of the variation of frictional resistance with mass fluxes, heat fluxes and pressure under supercritical pressure has been conducted. Five types of resistance correlations were collected and sorted out, and the results show that there is a large error between the calculated value and the test value. Based on the experimental results, the influences of Re, the rib height e, the rib pitch P and helix angle α were analyzed, the new resistance correlation composed of tube’s geometric parameter and Re number, and the average relative error of the test values are less than 10%. Mass flux is the main factor that influences the friction pressure drop and total pressure drop.