Abstr:High speed centrifugal compressor
Application Area 6: Turbomachinery Internal Flows
Application Challenge AC6-08
The SRV2 is a high speed centrifugal impeller, with a very high total pressure ratio (close to 6 which has been designed and investigated in an industrial research project of German and Swiss turbo-compressor manufacturers. The main focus of this work, as outlined in Eisenlohr et al. , was to measure and calculate the global flow field structure to provide indications for necessary blade design changes that could lead to a further improvement of the impeller efficiency. High quality performance measurements and extensive laser measurements (Laser 2-Focus L2F) of the flow field in the rotating impeller were carried out at the DLR, and CFD simulations were carried out by the project partners.
The companies involved in this project together with the DLR performed 3D calculations with different codes, involving different grids and using different turbulence models. In addition to comparing the measured and predicted pressure ratio characteristics, the measured and calculated flow fields at design point were compared. This earlier work was unable to come to any final conclusions with regard to the requirements for best practice for these simulations, although some simulations were found to be fairly inadequate. In this current work, more recent simulations with CFX-TASCflow are described, whereby high standards of grid definition and resolution have been applied. In addition, simulations have been carried out for different tip clearance levels, as a common feature of all high speed open impellers including this one is that there is inevitably some uncertainty related to the level of running tip clearance in the machine.
This test case is a highly relevant and extremely difficult test case for assessing the use of CFD in high speed turbomachinery. The high pressure ratio (of nearly 6) in this centrifugal impeller is at the upper end of typical applications in radial compressors for gas turbines (in helicopter engines and APU’s) and is substantially higher than that currently used in radial compressors for turbochargers (typically 3 to 4.5) or other industrial gas compression applications (HVAC and industrial gas processing). The test case is thus representative of extremely highly loaded compressor applications.
The geometry of this impeller is typical of most modern centrifugal compressor stages with splitter vanes and with about 38 degrees of back-sweep at the impeller outlet. The impeller is well-designed and has typical modern values for all of the key global geometrical parameters (axial length, inlet hub diameter, inlet tip diameter, blade inlet angles, blade wrap angle, blade thickness, etc.) which make it highly relevant as a typical industrial test case.
One of the major difficulties in turbomachinery simulations at these high pressure ratios is the compressible nature of the medium being compressed, as this means that the prediction of the flow field becomes strongly dependant on the correct prediction of the performance. A small error in the prediction of performance (losses, efficiency, work input, pressure ratio and temperature ratio) will lead to an error in the predicted density of the gas, and this automatically causes an error in the volume flow leading to a change in the predicted mean flow velocities. This type of error is typical of high speed turbomachinery (in radial machines and in multistage axial compressors and axial turbines). It does not occur in incompressible flows as for these applications the density is constant and the mean flow velocities are not a function of the predicted performance. Because of this, this high speed test case is highly relevant with an added degree of difficulty compared to the low-speed centrifugal compressor test case (see application challenge submitted by Numeca – the NASA low speed centrifugal impeller).
As a result of the sensitivity of the mean flow velocity predictions to small changes in gas density, high speed turbomachinery simulations are also quite sensitive to small changes in geometry, so that an exact representation of the geometry is needed for accurate simulations. This case provides all the relevant geometrical information needed for good simulations, but as in all such high speed radial compressors there remains an uncertain area with regard to the high sensitivity of the results with regard to tip clearance flows. This is a highly relevant application uncertainty for such industrial applications that is well documented in this test case.
For turbo-compressor design, it is primarily the correct prediction of the flow field that is required as a main application of CFD. The accurate calculation of the flow field, with the prediction of the location and extent of separations and shocks, is usually considered more important than a precise prediction of the over-all performance. From the simulations the change in flow-field (such as reduction in the extent of a separated zone or the shift in the position of the shock) due to a change in geometry can be assessed. This is a key element in the design of centrifugal impellers where new versions are often based on modifications of existing impellers that have already been proven in tests. The availability of the detailed flow field measurements with L2F in the SRV2 impeller make this a highly suitable test case by which the competency of CFD for the sector can be judged with respect to such flow patterns.
Contributors: Beat Ribi; Frank Sieverding - MAN Turbomaschinen AG Schweiz; Sulzer Innotec AG