Hydraulic Resistance of Subcritical and Supercritical Water Flowing in a Rifled Tube

Abstract

Large capacity supercritical boiler is at the leading edge of efficiency boost for thermal power plant. Water wall design is a key issue for a supercritical boiler. To ensure successful design and safe operation of water wall, studying hydraulic resistance of water is significant. Considerable work on frictional pressure drop of gas-liquid two-phase flow in tubes has been done and various correlations have been proposed to predict it. However, these correlations are restricted to particular rib geometries and flow conditions. Because of significant variations in thermo physical properties near the critical and pseudo-critical points, pressure drop at supercritical pressures is different from that at subcritical pressures. However, limited studies have been devoted to estimate hydraulic resistance of supercritical water. More work need be conducted to develop prediction method for pressure drop at supercritical pressures. Therefore, to accumulate fundamental experimental data for the design of a supercritical boiler, an experiment on hydraulic resistance of water was performed in a vertical upward rifled tube. The experiment was carried out in the high-temperature and high-pressure steam-water test loop at Xi’an Jiaotong University. Based on the experimental data, the two-phase frictional multiplier was calculated to analyze the two-phase frictional pressure drop. At low to moderate vapor quality, the two-phase frictional multiplier increases rapidly and reaches a peak. When the vapor quality exceeds a certain value, the two-phase frictional multiplier starts to decrease with increasing vapor quality. It is because the tube wall is covered by liquid film at low to moderate vapor quality. Within the high vapor quality region, the high-speed vapor tears the liquid film and the flow pattern turns to mist flow with lower frictional pressure drop. Increasing pressure decreases the two-phase frictional multiplier and when the pressure approaches the critical pressure, the multiplier is close to 1. The effect of mass flux on the multiplier is so weak that it can be neglected. At supercritical pressures, the pressure drops due to frictional resistance and flow acceleration both increase with bulk fluid enthalpy. Increasing pressure decreases the frictional pressure drop. This result is mainly attributed to pressure approaching the critical point. Frictional pressure drop is significantly affected by fluid property variations; in particular, severe density decreases with increasing bulk fluid enthalpy. Acceleration pressure drop increases with decreasing pressure and increasing heat flux. When heat flux increases, the density difference between the inlet and the outlet increases with the same mass flux, which results in a considerable acceleration pressure drop. Decreasing pressure results in a similar acceleration pressure drop variation because of the same reason. The frictional resistance coefficient was calculated to analyze the supercritical frictional pressure drop. In the large specific heat region, the frictional resistance coefficient peaks at a certain enthalpy in the vicinity of the pseudo-critical point, and increasing mass flux reduces the magnitude of the peak value.