Validation of a Numerical Model for Predicting Stalled Flows in a Low-Speed Fan

Abstract

This paper investigates the rotating stall for a low-speed/low-pressure ratio fan blade. The type of stall studied in this paper is similar to the multi cell, part span rotating stall, which can occur on the front stages of the core compressors. Although this type of rotating stall may not result in surge or massive loss of power, it can cause excessive vibration and noise. It was found that the standard Spalart-Allmaras (SA) model implemented in the CFD solver used in this work predicts premature stall, which is in line with the observation of other researchers who use the SA model. Therefore, to improve the prediction of the stall boundary, the standard SA model was modified by scaling the source term in the model based on the local pressure gradient and the velocity helicity of the flow. Furthermore, a generalized wall function valid for non-zero wall pressure gradient was implemented to improve the accuracy of boundary conditions at the wall. The turbulence modelling part of this work aims to produce a turbulence model which can be used to model the flows near the stall boundary for the transonic fan blades on relatively coarse grids of around 300k points per passage. Initially, two fan blades with different design and operating speeds were used to optimize the new parameters in the modified turbulence model. The optimization was based on improving the correlation between measured and numerical radial profiles of the pressure ratio. Thereafter, steady computations were performed for two other blades (with the same parameters) and the predictions were compared with the experimental data for all the four fan blades. Numerical results showed a significant improvement over those obtained with the original SA model, when compared against the measured data. In the second part, the modified turbulence model was used to study the flow near the stalled region. Three-dimensional, whole assembly unsteady simulations were performed for a modern low speed fan rig for which extensive measured data were available. Stall simulations were conducted at 80% speed, which contained a part with positive slope on the measured constant speed characteristic. The results indicated that by using an unsteady whole assembly approach and the modified turbulence model, it is possible to predict the flow for all the points on the measured constant speed characteristic (including those on the positive slope part).