The present paper treats the backflow vortex structure observed at the turbomachinery inlet at reduced flow rate. It is caused by the roll-up of the shear layer between swirling backflow and axial main flow. In order to verify this, a simple model test was carried out in which the effect of impeller was represented by an axisymmetric swirling backflow. In the present paper, the flow field of a simplified model test is simulated by using LES calculations to investigate detailed flow structure. The computed results are compared with experimental results.
Abstract In the flow control valve used in the upper or the lower stream of a turbomachinery, when opening is small, the self-induced oscillation resulting from a clearance gap flow may occur. In this study, the mechanism of self-induced oscillation which occurs on the small valve opening condition was examined. And a concept to control the self-induced oscillation was built. The air model test of the flow control valve was carried out, and it was confirmed that self-vibration occurs under specific conditions. Moreover, stability evaluation using CFD was carried out, and it checked that the calculation results become the almost same tendency as a test results. The area where the instability force is generated on a valve was grasped from the simulation results, and it checked that stability was changed with flow field differences. Based on these results, stabilization of the flow control valve was attained by improving clearance gap geometry of the valve sheet area. It checked that the intended flow field was obtained in the geometry in CFD analysis, and it confirmed that fluid additive damping became positive in CFD.
In order to analyze the occurrence mechanism of the backflow at the inlet of inducers, a series of numerical calculations were carried out including the different sizes of the tip clearance and the different geometry of the leading edges. Discussions are made based not only on the flow field at the inlet but also on the on the balance of the flow into/out from control volumes. The results show that the radial direction flow near the blade suction surface plays the main role in the formation of backflow. It is also shown that the leading edge gives an important influence for the backflow region
In most hydraulic applications (turbines, pumps, water intakes, propellers) the appearance of gas filled vortices, either caused by cavitation or air entrainment from a free surface, is usually associated with increase of losses, vibrations, noise and erosion risk. However, a correct prediction of the vortex characteristics (most importantly, of the pressure at the core) by numerical simulations may be challenging. A common example is the over-prediction of the vortex dissipation, which leads to wrong estimation of the gas core collapse location. In the present paper we assess the numerical requirements necessary to compute vortex characteristics comparable to experimental results. As a first step, we evaluate the influence of the mesh resolution for different turbulence models (SST, SAS and RMS), in the case of a vortex generated by an elliptical wing. Secondly, we compare the efficiency of several popular vortex identification techniques (helicity, Q, λ, Δ, etc…) to designate the mesh refinement regions, thus adapting the mesh to successfully compute the vortex characteristics, in the case of a vortex created in a cylindrical container with tangential inflow and central outflow. Therefore, we are able to present effective guidelines for the correct computation of the above mentioned two phase problems, that can also be applied to leakage flow in gas turbomachines, wing-tip vortices, and more generally all computations where a high quality resolution of the vortices is necessary.
A numerical study of the cavitation performance of 3-bladed inducer is performed. Calculations are made for two types of leading edge geometry using the commercial CFD code CFX-TASCflow, Cavity zone and cavitation behavior under different conditions are presented. It is shown that the cavitation zone and suction performance can be reasonably predicted.
In order to analyze the occurrence mechanism of the backflow at the inlet of inducer, a series of numerical calculations were carried out, Discussions are made based not only on the flow field at the inlet but also on the balance of the flow into/out from two control volumes. The analysis results show that the radial direction flow near the blade suction surface play a main role in the formation of backflow
Turbopump inducers are designed with a certain atack angle even at the design point in order to improve suction performance. For this reason, the swirling backflow always occurs at the inlet of the inducer. As the flow rate is lower, it extends upstream and may cause various problems if it interacts with upstream elements. In this study, we investigated the backflow vortex structure at the inlet of the inducer by Large eddy simulation in order to elucidate the fundamental characteristics.
Abstract In order to enhance the efficiency and reduce the manufacturing cost of steam turbine, combined main steam valve consisting of main stop valve and control valve in the same casing has been developed. Combined main steam valve with improved flow path at internal structure will reduce pressure loss. A downscale model test on air condition was conducted to verify pressure distribution characteristics. Experimental result and numerical result obtained by computational fluid dynamics (CFD) showed reasonable agreement in pressure loss and static pressure at each flow point within the design operation range. Main stop valve (SV) and control valve (CV) vibration characteristics tests were performed simultaneously for the stability verification of governing operation from start up to full load. We confirmed that CV aerodynamic added damping ratio was positive under all measured condition of CV stroke and pressure ratio, and self-excited vibration was not generated. We also confirmed that pressure fluctuation acting on main stop valve body was sufficiently small within the operation range. Meanwhile, in the case where pressure ratio or CV stroke deviated from the operation range, pressure fluctuation around main stop valve increased. Based on CFD result and detailed analysis of experimental result, it was found that steam flow along valve seat separates periodically, which revealed the mechanism of increasing pressure fluctuation around main stop valve. By reflecting on these results, the reliability of new combined main steam valve has substantially been enhanced.
