Computation of Flow Past a Turbine Blade With and Without Tip Clearance

Three-dimensional solutions of the ensemble-averaged Navier-Stokes equations have been computed for a high-turning turbine rotor passage, both with and without tip clearance effects. The geometry is Pratt & Whitney’s preliminary design for the Generic Gas Generator Turbine (GGGT), having an axial chord of 0.5 inch and turning angle of about 160 degrees. The solutions match the design Reynolds number of 3x 106/inch and design inflow/outflow distributions of flow quantities. The grid contains 627,000 points, including 20 radial points in the clearance gap of 0.015 inch, and has a minimum spacing of 10−4 inch adjacent to all surfaces. The solutions account for relative motion of the blade and shroud surfaces and include a backstep on the shroud. Computed results are presented which show the general flow behavior, especially near the tip clearance and backstep regions. The results are generally consistent with experimental observations for other geometries having thinner blades and smaller turning angles. The leakage flow includes some fluid originally in the freestream at 91 percent span. Downstream, the leakage flow behaves as a wall jet directed at 100 degrees to the main stream, with total pressure and temperature higher than the freestream. Radial distributions of circumferentially-averaged flow quantities are compared for solutions with and without tip leakage flow. Two-dimensional solutions are also presented for the mid-span blade geometry for design and off-design inflow angles.

Influence of Unsteady Effects on the Measurements in a Transonic Axial Compressor

Interaction phenomena between rotor and stator are unavoidable in advanced compressors and their effects increase with the performances of the turbomachines. Until now, it was not possible to quantify the interaction effects, but with the development of 3-D unsteady computation codes in a complete stage, it is possible to know, in detail, the flow field through the machine and to make evident and to explain the difficulties encountered in measuring the flow parameters. A study has been conducted in this way at ONERA, on an axial transonic compressor stage. The computations have been made with a simulation of the losses; in this manner, the overall computed and measured performances of the compressor are the same. A detailed analysis of the unsteady computation results makes evident, between rotor and stator, large variations of some parameters of the flow as a function of time but also as a function of the axial and tangential relative position of steady probes and stator blades. Unsteady measurements made on another transonic machine confirm the indications given by these computations.

The Role of Tip Clearance in High-Speed Fan Stall

A numerical experiment has been carried out to define the near stall casing endwall flow field of a high-speed fan rotor. The experiment used a simulation code incorporating a simple clearance model, whose calibration is presented. The results of the simulation show that the interaction of the tip leakage vortex and the in-passage shock plays a major role in determining the fan flow range. More specifically, the computations imply that it is the area increase of this vortex as it passes through the in-passage shock, which is the source of the blockage associated with stall. In addition, for fans of this type, it is the clearance over the forward portion of the fan blade which controls the flow processes leading to stall.

A Unified Variable-Domain Variational Approach to Hybrid Problems of Compressible Blade-to-Blade Flow

A unified theory of various hybrid problems for blade-to-blade compressible flow is developed herein via the functional variation with variable domain. Two variational principle (VP) families for three typical hybrid problems are derived, following a systematic approach (Liu, 1990a). Full advantage is taken of the natural boundary condition and suction/blowing along the blade surface are accommodated. This theory is aimed at offering a new theoretical basis for the finite element method (FEM) and various ways for blade design and/or modification, and it also constitutes an important part of optimal cascade theory (Liu,1987b). Based on these VPs, a new FEM with self-adjusting nodes is also suggested, and the numerical tests yield good results.

Design and Analysis of a High Pitch to Chord Ratio Cascade Representative of Ducted Propfans

Efforts to reduce the specific fuel consumption of a modern aero engine focus in particular on increasing the by-pass ratio beyond the current level of around 5. One concept is the counterrotating shrouded propfan operating at low overall pressure ratio and having only very few fan blades of extremely high pitch/chord ratios. The relative inlet Mach numbers cover a range from 0.7 at the hub to 1.1 at the tip section of the first rotor. A propfan cascade was designed by taking into account two characteristic features of a propfan blade-blade section: • a very high pitch/chord ratio of s/c = 2.25 • an inlet Mach number of M1 = 0.90 which leads to transonic flow conditions inside the blade passage In the design process a profile generator and a quasi-3D Euler solver were used iteratively to optimize the profile Mach number distribution. Boundary layer behavior was checked with an integral boundary layer code. The cascade design was verified experimentally in the transonic cascade wind tunnel of DLR at Cologne. The extensive experimental results confirm the design goal of roughly 5 degree flow turning. A total pressure loss coefficient of less than 1.5% was measured at design conditions. This validates the very high efficiency level the propfan concept is calling for. A 2D Navier-Stokes flow analysis code yields good results in comparison to the experimental ones.

Self-Induced Flow in a Stepped Rotating Tube

Self-induced flow occurs when a tube, open at one end and sealed at the other, is rotated about its central axis: fluid flows along the axis from the open end towards the sealed end and returns in a layer adjacent to the inner surface of the tube. This mechanism, which can occur under isothermal or nonisothermal conditions, is believed to be responsible for the so-called “hot-poker effect” that was observed during anti-icing tests on the nose bullet of an aeroengine. This paper describes a combined theoretical and experimental study of self-induced flow. It is shown that, for the length-to-diameter (L/D) ratios and rotational Reynolds numbers associated with the anti-icing tubes of aeroengines, the laminar flow near the sealed end of the tube is similar to that of the so-called free disc. Swirl in the air outside the open end reduces the self-induced flow, but flow can reach the sealed end of a stepped tube which has either a sudden contraction or a sudden enlargement.

An Intelligent/Learning Axial Fan Design System

A computer-based axial fan design system has been developed that allows the designer to rapidly obtain a preliminary axial fan design. Program FANDES allows the designer two options to determine the preliminary design parameters for a single-stage axial fan. The first option allows the designer the ability to design an axial fan using conventional blade-element design techniques. The second option enables the designer to search a database of previously designed fans for a set of scaled fans that will satisfy the current design point requirements. The designer can then refine one of the fans in this set to possibly improve the selected fan’s performance. The database of fans is utilized and maintained by FANDES and new fans are added at the user’s request. This allows for an intelligent program that is constantly learning from previous designs. As more fans are designed and saved to the database the design process becomes more of a selection and refinement process of previously designed fans.

Measurement and Prediction of Heat Transfer From Compressor Discs With a Radial Inflow of Cooling Air

An integral theory is used to model the flow, and predict heat transfer rates, for corotating compressor discs with a superposed radial inflow of air. Measurements of heat transfer are also made, both in an experimental rig and in an engine. The flow structure comprises source and sink regions, Ekman-type layers and an inviscid central core. Entrainment occurs in the source region, the fluid being distributed into the two nonentraining Ekman-type layers. Fluid leaves the cavity via the sink region. The integral model is validated against the experimental data, although there are some uncertainties in modelling the exact thermal conditions of the experiment. The magnitude of the Nusselt numbers is affected by the rotational Reynolds number and dimensionless flowrate; the maximum value of Nu is found to occur near the edge of the source region. The heat transfer measurements using the engine data show acceptable agreement with theory and experiment. This is very encouraging considering the large levels of uncertainty in the engine data.

Turbulence Measurements in a Centrifugal Pump With a Synchronously Orbiting Impeller

Turbulence profiles were measured in a centrifugal pump with an impeller with backswept blades using a two directional laser velocimeter. Data presented includes radial, tangential, and cross product Reynolds stresses. Blade to blade profiles were measured at four circumferential positions and four radii within and one radius outside the four bladed impeller. The pump was tested in two configurations; with the impeller running centered within the volute, and with the impeller orbiting with a synchronous motion (ε/r2 = 0.016). Flow rates ranged from 40% to 106% of the design flow rate. Variation in profiles among the individual passages in the orbiting impeller were found. For several regions the turbulence was isotropic so that the cross product Reynolds stress was low. At low flow rates the highest cross product Reynolds stress was near the exit. At near design conditions the lowest cross product stress was near the exit, where uniform flow was also observed. Also, near the exit of the impeller the highest turbulence levels were seen near the tongue. For the design flow rate, inlet turbulence intensities were typically 9% and exit turbulence intensities were 6%. For 40% flow capacity the values increased to 18% and 19%, respectively. Large local turbulence intensities correlated with separated regions. The synchronous orbit did not increase the random turbulence, but did affect the turbulence in the individual channels in a systematic pattern.

Computations of Unsteady Multistage Compressor Flows in a Workstation Environment

High-end graphics workstations are becoming a necessary tool in the Computational Fluid Dynamics (CFD) environment. In addition to their graphics capabilities, the latest generation of workstations have powerful floating point operation capabilities. As workstations become common, they could provide valuable computing time for applications, such as turbomachinery flow calculations. This paper discusses the issues involved in implementing an unsteady, viscous multistage turbomachinery code (STAGE-2) on workstations. The workstation version of STAGE-2 has then been used to study the effects of axial-gap spacing on the time-averaged and unsteady flow within a 2 1/2-stage compressor. Results include force polar plots, time-averaged pressure contours, standard deviation of pressure contours, time-averaged surface pressures and pressure amplitudes.