The design and test of a two-stage, vaneless, aspirated counter-rotating fan is presented in this paper. The fan nominal design objectives were a pressure ratio of 3:1 and adiabatic efficiency of 87%. A pressure ratio of 2.9 at 89% efficiency was measured in the tests. The configuration consists of a counter-swirl-producing inlet guide vane, followed by a high tip speed (1450 feet/sec) non-aspirated rotor, and a counter-rotating low speed (1150 feet/sec) aspirated rotor. The lower tip speed and lower solidity of the second rotor results in a blade loading above conventional limits, but enables a balance between the shock loss and viscous boundary layer loss, the latter of which can be controlled by aspiration. The aspiration slot on the second rotor suction surface extends from the hub up to 80% span, with a conventional tip clearance, and the bleed flow is discharged at the hub. The fan was tested in a short duration blowdown facility. Particular attention was given to the design of the instrumentation to obtain efficiency measurements within 0.5 percentage points. High response static pressure measurements were taken between the rotors and downstream of the fan to determine the stall behavior. Pressure ratio, mass flow, and efficiency on speedlines from 90% to 102% of the design speed are presented and discussed along with comparison to CFD predictions and design intent. The results presented here complement those presented earlier for two aspirated fan stages with tip shrouds, extending the validated design space for aspirated compressors to include designs with conventional unshrouded rotors and with inward removal of the aspirated flow.Copyright © 2006 by ASME

Design and Test of an Aspirated Counter-Rotating Fan

Nomenclature e = internal energy e = e{ + ¥211-u e^ = specific internal energy / =force F, G, H = vector fluxes J = Jacobian of coordinate transformation k = molecular heat conductivity n — outward normal P = pressure Pr = Prandtl number Prt = turbulent Prandtl number q =heat exchange Q = heat release t = time T = temperature u, v, w = velocity components in Cartesian frame x, y, z = Cartesian coordinates 7 = ratio of specific heats e = eddy viscosity coefficient X = bulk viscosity coefficient fji = molecular viscosity coefficient £, 17, f = transformed coordinates p = density T = stress tensor

An Assessment of Numerical Solutions of the Compressible Navier-Stokes Equations

Progress in computational physics

Introduction B the term transonic compressor we mean an axial flow compressor in which the inlet flow Mach number, relative to the rotating blades, varies from below unity at radii near the inner casing or hub to values substantially above unity at radii near the blade tips. Usually the flow is diffused to subsonic velocities in the flow passages formed by the rotating blades, generating part of the desired pressure rise. A further pressure rise is generated in a stationary blade row downstream of the rotor in the process of removing the swirl imparted by the rotor, but the flow Mach numbers relative to the stator blades are generally below unity. Thus, the flows of principle concern here are those in the rotor passages. Axial flow compressors have been the subject of research and development efforts for at least 40 years, and the transonic compressor for nearly 30. The earliest systematic studies in the United States were carried out by the Lewis Research Center of the NACA about 1950, and the technology was quickly developed for application in the first generation of turbofan engines. Because it offers higher pressure ratios per stage and larger airflows per unit of engine frontal area than the subsonic compressor, the transonic compressor has become a major component of all modern aircraft engines. It has received a proportionately large share of the huge research and development effort expended on these engines. The opinion has often been proffered that it represents a highly developed technology which will not yield significant gains from additional research and development. I argue to the contrary, that there is much scope for improvement in the design of these important components, and that the capability for making these improvements is rapidly coming to hand. This position rests on several facts. First, there are large inefficiencies in transonic compressors due to the three-dimensional character of the inviscid and viscous flows and their coupling. Second, the design methods currently in use do not deal rationally with these loss mechanisms. Third, until recently there has not been a capability for measuring the actual flow in the rotor passages to provide feedback to the design process. The designer has had to rely on overall performance measurements and timeaveraged measurements between the blade rows for verification of the design process. We now have several diagnostic methods capable of providing detailed information about the flow in the blade passages. These include laser velocimetry, gas fluorescent density measurements, and timeresolved pressure measurements, as well as the time-honored hot-wire anemometer. We also have a capability for numerical computation of the three-dimensional transonic flow, inviscid at present but with viscous effects in the near future. Turbomachinery should be one of the major beneficiaries of the advances in this area as was discussed by Chapman in last year's Dryden Lecture. Taken together, the problem of the transonic compressor and these new engineering tools represent an opportunity quite comparable to that presented by recent improvements in high-speed airfoil sections, together with three-dimensional computational techniques in external aerodynamics. My objectives in this paper are to describe the role of the technology and its status, discusss some recent progress toward understanding the flowfield, and indicate the directions which I feel future work should take. I do so with the full knowledge that some of my colleagues in industry and government have much more experience in the design of compressors than I, and that I may not fully represent their viewpoint. My hope is that the view expressed here will prove complementary to that of practicing engineers, and have some beneficial effect on future fan and compressor designs. This review will deal mainly with the question of efficiency. There are many other important problems in the design of transonic compressors, including flutter and forced vibration, stall and surge, foreign object damage, and noise. Some will be mentioned as they impinge on the issue of design for high efficiency, but only the latter will be discussed in detail. Apart from the necessity for focusing on one of these many issues in order to achieve the depth appropriate to a Research Lecture, this choice is motivated by the perception that efficiency is the

Flow in Transonic Compressors

An assessment of numerical solutions of the compressible Navier-Stokes equations

A blowdown compressor test facility has been developed which allows time-resolved aerodynamic testing of full-scale transonic compressor rotors at low cost. The rotor is brought to speed in vacuum, a diaphragm is opened, and the test gas is allowed to flow for a time of the order of 0.1 sec, during which the rotor is driven by its own inertia. Both 'steady-state' performance evaluation and detailed time resolution of the flow on the blade-passing time scale have been demonstrated for a two-ft-diam transonic rotor with a tangential Mach number of 1.2 and a nominal pressure ratio of 1.6. The steady-state performance as determined in the experiments includes an efficiency of 0.92 and a pressure ratio of 1.55 at design speed. The time-resolved measurements include the combination tone structure in the upstream flow field, resolved both axially and radially, and the wake structure downstream of the rotor, also resolved both radially and axially.

/pdf/the-m-i-t-blowdown-compressor-facility-j77j43u72u.pdf

The M.I.T. Blowdown Compressor Facility

In recent years, much progress has been made in solving fluid dynamical problems using finite difference methods. Solving inviscid compressible problems in two and three dimensions has become almost routine with many suitable methods, explicit or implicit, available. The problem of compressible, viscous flows in complicated geometries remains, however, a major challenge. Here fine mesh spacing in the viscous flow regions makes the explicit methods with their simple boundary conditions extremely costly. Existing implicit methods can make use of large time steps, but require inversions of large block tridiagonal matrices. A method recently developed by MacCormack eliminates this disadvantage by introducing a predictor-corrector scheme requiring the inversion of only block bidiagonal matrices. It is the aim of present work to extend this method to allow solution of viscous, compressible problems in general coordinates for arbitrary two-dimensional geometries.

An Implicit, Bidiagonal Numerical Method for Solving the Navier-Stokes Equations

The inadequacy of axisymmetric theory in modeling the structure of the shock system in a transonic compressor rotor is shown with a simple model. A more accurate model for the flow near the sonic radius is proposed, which is supported by some flow visualization data and three-dimensional computations.

Shock Structure in Transonic Compressor Rotors

An implicit, bi-diagonal numerical method for solving the Navier-Stokes equations

The time resolved flowfield in a low aspect ratio transonic compressor stage has been studied using a high frequency response sphere probe with a bandpass of D.C. to 20 kHz in a blowdown compressor facility. Averaged over the compressor annulus, the data agree well with those measured with standard pitot type probes on the same stage in a conventional test rig. Not all the spanwise distributions agree, however. These differences are explained as errors in the pitot probe readings due to fluctuations in the flow. The experimental data are compared to the results of a three-dimensional inviscid Euler calculation.

/pdf/time-resolved-measurements-in-a-low-aspect-ratio-transonic-4fvybnp8l9.pdf

Time Resolved Measurements in a Low Aspect Ratio Transonic Compressor Stage

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