Development of a Reduced Computational Model to Replicate Inlet Distortion in an APU-Style Inlet of a Centrifugal Compressor
The purpose of this research was to determine what components of a complex centrifugal compression system inlet needed to be modelled to accurately predict the swirl and total pressure distortions at the compressor face. Two computational models were developed. A full-fidelity case where all the inlet geometry was modelled and a reduced model where a small portion of the inlet was considered. Both the numerical cases were compared with experimental data from a research compressor rig developed by Honeywell Aerospace. The test apparatus was designed with a modular inlet system to develop swirl distortion patterns. The modular inlet system utilized transposable baffles within the radial-to-axial section of the inlet and blockage plates of varying sizes and geometries at the inlet to this section. Discerning the dominant inlet component that dictates distortion behavior at the compressor face would allow the reduced modelling of inlet components for compression systems and would allow coupling with more tortuous systems. Furthermore, it would reduce the design iteration and simulation time of the inlet systems. Several investigations utilizing a reduced model only considering a radial-to-axial inlet are available in literature, but no comprehensive justification has been presented as to the impact this has on the distortion behavior. Experimental surveys of flow conditions just upstream of the inducer of the centrifugal compressor were conducted at several operating conditions. The highest and lowest mass flow rates of these operating points were simulated using ANSYS CFX 2020R1 for both the computational models. Multiple inlet configurations were simulated to test the robustness of the reduced model in comparison to the full fidelity. The numerical simulations highlighted shortcomings of the instrumentation used to characterize the experimental flow field at the inducer, particularly with respect to total pressure distortion. Furthermore, transient pressure data were measured in experiment and indicated unsteady fluctuations in the inlet that would not be captured by steady computational fluid dynamic simulations. These data matched locations of disagreement with swirl distortion behavior at high mass flow rates. This suggested that transient vortex movement occured at the aerodynamic interface plane in certain configurations. The total pressure distortion metrics between the two models were remarkably comparable. Furthermore, the simplified model accurately predicted the mixing losses associated with the blockage plates at the inlet to the radial-to-axial inlet using a simple inlet extension. Swirl 18 distortion was dictated by the radial-to-axial inlet. The reduced model data trends were comparable with experiment for both the baffle and blocker plate configurations. The swirl intensities for all configurations were comparable between the two models. The reduced model swirl directivity trends matched those of experiment. The most notable deviations between the full-fidelity model and the reduced model were observed with swirl directivity numerics.