Bleed Airflow CFD Modeling in Aerodynamics Simulations of Jet Engine Compressors

Abstract

Several types of bleed airflow in jet engine compressors are required for both aircraft and engine operations. Since accessory, discharge or ventilation bleeds divert a significant percentage of the total input flow rate, it is compulsory to take these variations into account both in through-flow and in 3D Navier-Stokes compressor computations. While the former is straightforward to implement, which has been done for some time, several options are available for the three-dimensional case. The details of such models are most likely to have a local effect on the pressure and velocity fields distribution near the bleed section. Nevertheless, for the purpose of modeling the global effect of bleed flow magnitude on stage matching and overall aerodynamic stage performance, it is thought that a simple geometrical model of the bleed section combined with a simple outlet boundary condition will be accurate enough to provide a realistic representation of the actual compressor characteristics under various bleed flow rate conditions. Thus, our purpose in this study was to develop, implement and validate a simple model based upon both analytical geometry descriptions and outlet flow boundary conditions without any mesh modification or refinement with respect to the standard no-bleed calculations. The analytical description of bleed section is restricted to the intersection of the hub or casing with a set of radial cylinders or axisymetric slots, covering within manufacturing tolerance the known range of bleed section shapes. The physical boundary condition is applied at the intersection of the main flow path with the bleed duct or cavity outlet, ideally imposing a predefined exit flow rate. However, without access to an experimental database of engine bleed flow section measurements, only a qualitative comparison of the models with academic bleed flow experiments has been achieved. In order to assess the modeling scatter due to the details of the methods, two different models have been tested and validated against meridional flow computations, based upon compressor test results.