Application of a Multi-Block CFD Code to Investigate the Impact of Geometry Modeling on Centrifugal Compressor Flow Field Predictions

Abstract

CFD codes capable of utilizing multi-block grids provide the capability to analyze the complete geometry of centrifugal compressors including, among others, multiple splitter rows, tip clearance, blunt trailing edges, fillets, and slots between moving and stationary surfaces. Attendant with this increased capability is potentially increased grid setup time and more computational overhead—CPU time and memory requirements—with the resultant increase in “wall clock” time to obtain a solution. If the increase in “difficulty” of obtaining a solution significantly improves the solution from that obtained by modeling the features of the tip clearance flow or the typical bluntness of a centrifugal compressor’s trailing edge, then the additional burden is worthwhile. However, if the additional information obtained is of marginal use, then modeling of certain features of the geometry may provide reasonable solutions for designers to make comparative choices when pursuing a new design. In this spirit a sequence of grids were generated to study the relative importance of modeling versus detailed gridding of the tip gap and blunt trailing edge regions of the NASA large low-speed centrifugal compressor for which there is considerable detailed internal laser anemometry data available for comparison. The results indicate: (1) There is no significant difference in predicted tip clearance mass flow rate whether the tip gap is gridded or modeled. (2) Gridding rather than modeling the trailing edge results in better predictions of some flow details downstream of the impeller, but otherwise appears to offer no great benefits. (3) The pitchwise variation of absolute flow angle decreases rapidly up to 8 percent impeller radius ratio and much more slowly thereafter. Although some improvements in prediction of flow field details are realized as a result of analyzing the actual geometry there is no clear consensus that any of the grids investigated produced superior results in every case when compared to the measurements. However, if a multi-block code is available, it should be used, as it has the propensity for enabling better predictions than a single block code, which requires modeling of certain geometry features. If a single block code must be used, some guidance is offered for modeling those geometry features that cannot be directly gridded.

Effect of tip clearance on impeller discharge flow and vaneless diffuser performance of a centrifugal compressor

“…Chriss et al 3 and Harthaway and Wood 4 emphasized the importance of throughflow wake, which is largely fed by the tip clearance flow when the centrifugal impeller is throttled. Eisenlohr and Chladek 5 described the possibilities for optimizing passive tip clearance control by design features and suggested that transient expansion processes must be considered when using a thermal tip clearance control.…”

A First-Principles Based Methodology for Design of Axial Compressor Configurations

“…Also, with the availability of reliable experimental test data by AGARD 21 , researchers now have access to a plethora of data for validation of turbomachinery codes. Since then a number of three dimensional CFD codes capable of analyzing single and multiple blade passage turbomachinery flow have been developed by many researchers [22][23][24][25][26][27][28][29] .…”

Computational Investigation of Flow in a Centrifugal Impeller with Chamfered Blade Tips: Effects of Stage Loading and Tip Clearance

“…Ubaldi et al [9] concluded that turbulence kinetic energy is very high on the pressure side of the blade wake, where the jet has originated, and is lower on the suction side. Hathaway and Wood [10] used three-dimensional viscous code in their computational investigation of NASA low speed centrifugal compressor flow field. They concluded that increasing grid density in the tip gap by a factor of 6.5 resulted in no difference in predicted tip clearance overall flow nor in the chord wise distribution of the clearance flow.…”