Xianliang Lei

Study on the Minimum Drag Coefficient Phenomenon of Supercritical Pressure Water in the Pseudocritical Region

With the development of supercritical (and even ultra-supercritical) pressure boilers (SCBs) with high capacities, and at the same time, with the consideration of supercritical pressure water-cooled reactors (SCWRs) as one of the six most promising reactor concepts accepted in the Generation IV International Forum (GIF), flow and heat transfer of supercritical water becomes more and more important for both the design and operation safety of the related facilities. Thermo-hydraulic characteristics are among the issues, which are of special significance for the SCBs and SCWRs. It has been found that at supercritical pressures, the hydraulic resistance of water exhibits special characteristics in regions near its pseudo-critical point, which is hereafter called the minimum drag coefficient phenomenon. Experimental investigation was carried out in the present study to investigate further the characteristics of drag coefficient of supercritical pressure water under different conditions. The total pressure drop characteristic of water flowing in smooth tube and internally ribbed tube under the supercritical pressures was measured in experiments with a wide range of operational parameters, such as the system pressures ranging from 23 to 28 MPa, the average heat fluxes varied from 100 kW/m2 to 500kW/m2, and the mass fluxes of water in a range of 600 ∼ 1050 kg/m2s. The experimental data were compared with prediction results calculated by existing common correlations for single phase pressure drops, and large discrepancies were observed between the experimental data and the prediction results. Furthermore, the pressure drops characteristics of supercritical pressure water in cases with different tube arrangement and test conditions were compared with each other, such as that in horizontal tubes and vertical tubes, and that in isothermal flows and in non-isothermal flows. Additionally, this phenomenon observed in the present studies was also analyzed by using computational fluid dynamics technology, and the mechanism of pressure drop variation was reasonably explained. It was found that the deviation appeared between the previously proposed drag coefficient correlations and the present experimental data was mainly owning to ignoring the variation of an existence of the minimum drag coefficient in the pseudo critical region in previous studies, and based on the data obtained in this study, a new correlation for drag coefficient for supercritical pressure water was presented.Copyright

Study of the Effect of Buoyancy on Flow and Heat Transfer of Supercritical Pressure Water in Horizontal Pipes

The present paper is devoted to clarify the effect of buoyancy on the flow and heat transfer of supercritical pressure water flowing in horizontal pipes at supercritical pressures. A series of experiments have been designed and carried out in Xi’an Jiaotong University, Xi’an, China to obtain data in relation to flow and heat transfer of supercritical pressure water in pipes with different arrangements. The experimental parameters are as follows: pressures ranging from 23 to 28MPa, heat flux being up to 600 kW/m2 , and the fluid mass fluxes being in the range from 100 to 1000kg/(m2 s). In this study, distributions of the local wall temperatures and the local heat transfer coefficients around the circumference of the tube are measured at different cross-sections along the flowing direction. On the basis of the experimental data obtained in the study, some criteria available in open literatures, including Gr/Re2.7 , Gr/Re2 , and Grq/Grth, are employed to estimate the magnitude of buoyancy and the effect of buoyancy on the flow and heat transfer behavior of the supercritical fluid. It is showed that buoyancy is of particular importance for horizontal flows, but play significantly different role in different regions having different characteristics of the specific heat capacity. Strong buoyancy effect exists in the large specific heat region, but in the enthalpy region which is far away from the LSHR, the discrepancy between the temperature of the top wall and that of the bottom wall is small, indicating that the buoyancy effect can be negligible. Based on the present study, it was found that the criteria Grq/Grth is better than others in terms of the capability of evaluating the effect of the buoyancy on the flow and heat transfer of supercritical water.Copyright