Unsteady Effects of Blade Row Interaction on Flow Field and Aerodynamic Performance of a Transonic Centrifugal Compressor Impeller

This paper discusses the unsteady effects associated with the impeller/diffuser interaction on the internal flow field and aerodynamic performance of a centrifugal compressor. In centrifugal compressors with a vaned diffuser, the flow field is inherently unsteady due to the influence of interaction between the impeller and the diffuser, and the unsteadiness of the flow field can often have a great influence on the aerodynamic performance of the compressor. Especially in high-load compressors, it is considered that large unsteady effects are produced on the compressor performance with a strong flow unsteadiness. The unsteady effect on aerodynamic performance of the compressor has not been fully revealed yet, and sometimes the steady-state RANS simulation finds it difficult to predict the compressor performance. In this study, numerical simulations have been conducted for a transonic centrifugal compressor with a vaned diffuser. The unsteady effects were clarified by comparing the numerical results between a single-passage steady-state RANS analysis and a full-annulus unsteady RANS analysis. The comparison of simulation results showed the difference in entropy generation in the impeller. The impingement of diffuser shock wave with the impeller pressure surface brought about a cyclic increase in the blade loading near the impeller trailing edge. Accordingly, with increasing tip leakage flow rate, a second tip leakage vortex was newly generated in the aft part of the impeller, which resulted in additional unsteady loss generation inside the impeller.

Applicability of Dynamic Modeling for Turbulent Schmidt and Prandtl Numbers on Density Stratification Breakup in Several Flow Conditions

Many experiments on density stratification breakup in several flow conditions have been performed with the large- and small-scale experimental facilities to understand the mechanism underlying hydrogen behavior in a nuclear containment vessel during a severe accident. To improve the predictability of the RANS (Reynolds-averaged Navier Stokes) approach, we implemented the dynamic modeling for turbulent Schmidt Sct and Prandtl Prt numbers. In this paper, the capability of the RANS analysis with dynamic Sct modeling is assessed with several experimental data obtained by using the MISTRA (Commissariat à l’énergie atomique et aux énergies alternatives, CEA, France), CIGMA and VIMES (Japan Atomic Energy Agency, Japan). For the quantitative assessment, the completion time of the stratification breakup, defined as when helium concentration in the upper region decreases to the same value in the lower region, is focused. The comparison study shows the good performance of the dynamic modeling for Sct and Prt. Besides, in the case with the low jet Froude number, the CFD accuracy declines significantly, because the jet upward bending is over-estimated.

Evaluation of Rotor–Stator Interface Models for the Prediction of the Hydraulic and Suction Performance of a Centrifugal Pump

The effects of a rotor–stator interface model on the hydraulic and suction performance of a single-stage centrifugal pump have been evaluated. A three-dimensional Reynolds-averaged Navier–Stokes (RANS) analysis was performed using the shear-stress transport turbulence model. The cavitating flow was simulated using a homogeneous two-phase mixture model and a simplified Rayleigh–Plesset cavitation model. Three performance parameters were selected to compare different cases: the hydraulic efficiency, head coefficient, and critical cavitation number for a head-drop of 3%. Frozen-rotor and stage models were evaluated for the rotor–stator interface. The evaluation was done using three different computational domains: one with a single passage of the impeller with a vaneless diffuser, one with a single passage of the impeller with the whole shape of volute casing, and another with the whole passage of the impeller with the whole shape of volute casing. Two different volute shapes were also tested. The results show that it is desirable to use the whole domain of the impeller and volute with the frozen-rotor model for accurate prediction of the suction performance. The stage model is not recommended for the prediction of the suction performance of the centrifugal pump with the volute in severe off-design conditions.

Redistribution of Total Temperature Through an Annular Turbine Nozzle Cascade

Flow exiting the combustor is highly turbulent and contains significant spatial gradients of pressure and temperature. The high pressure turbine nozzle vanes operating in this environment redistribute these spatial gradients and impact the inflow characteristics of the turbine rotor blades. The present study investigates the redistribution of total temperature through a turbine nozzle vane. Numerical investigation was performed using three-dimensional RANS analysis. Simulations were conducted using the Wilcox k–ω turbulence model and Shear Stress Transport (SST) with and without γ–Reθ transition model. Experimental measurements were obtained in an annular nozzle cascade facility. Two sets of inlet conditions were considered. The first was a nominally uniform total temperature. The second had a span-wise variation of total temperature. Both sets of inlet conditions had nominally the same inlet total pressure and inlet Mach number. Span-wise redistribution was evaluated using the circum-ferentially averaged total temperature profile at a plane downstream of the nozzle. Physical arguments about the influence of nozzle secondary flows on this redistribution are presented.

Numerical Study on the Effects of Freestream Turbulence on Antivortex Film Cooling Design at High Blowing Ratio

An advanced, high-effectiveness film cooling design, the antivortex hole (AVH) has been investigated by several research groups and shown to mitigate or counter the vorticity generated by conventional holes and increase film effectiveness at high blowing ratios and low freestream turbulence levels. The effects of increased turbulence on an AVH geometry were previously investigated in a preliminary steady computational fluid dynamics (CFD) study by Hunley et al. on the film effectiveness and net heat flux reduction (NHFR) at high blowing ratio. The current paper presents the results of an extended numerical parametric study, which attempts to separate the effects of turbulence intensity and length scale on film cooling performance of the AVH concept at high blowing ratio (2.0) and density ratio (2.0). In this extended study, steady Reynolds-averaged Navier–Stokes (RANS) analysis was performed with turbulence intensities of 5, 10, and 20% and length scales based on cooling hole diameter of Λx/dm = 1, 3, and 6. Increasing turbulence intensity was shown to increase the centerline, span-averaged, and area-averaged adiabatic film cooling effectiveness and NHFR. Larger turbulent length scales in the steady RANS analysis were shown to have little to no effect on the centerline, span-averaged, and area-averaged adiabatic film cooling effectiveness and NHFR at lower turbulence levels, but moderate effect at the highest turbulence levels investigated. Heat transfer results were in good agreement with the findings from adiabatic cases from previous work. Unsteady RANS results also provided supplementary flow visualization for the AVH film cooling flow under varying turbulence levels.