Development of a Methodology for Performance Optimisation and Manufacturing Sensitivity Studies for Radial Flow Turbocharger Compressors for 21st Century Legislation

The use of algorithmic optimisation techniques whereby several designs are evaluated automatically inbatches using Computational Structural Mechanics (CSM) and or Computational Fluid Dynamics (CFD)modelling to improve performance, has become an integral part of turbomachinery design process.Designing radial compressors for better performance as well as manufacturing the impeller such that thereare no discrepancies between the designed surface and machined surface represents a significantchallenge for the industry. Accounting for geometric variability (to model manufacturing errors) duringthe design/optimisation phase where hundreds of candidate geometries are evaluated is costly due to thelarge number of calculations required to analyse the possible combinations of manufacture errors for eachnew geometry design. This thesis addressed the problem by separating the design phase from themanufacture uncertainty calculations phase, focusing on a common 5-axis milling type error – undercut;and using a low cost high throughput computing cluster to meet the computational requirements of bothphases.A bespoke parametric CAD algorithm was developed to automate the geometry creation during theoptimisation phase. The Differential Evolution for Multi-Objective Optimisation (DEMO) algorithm wasused to drive the optimisation calculations. In-house meshing software from Napier Turbochargers Ltd,subsequently referred to as Napier, was used to mesh the computational domain, which was then solvedusing a commercial CFD solver. The compressor in the high-pressure (HP) stage of a two-stageturbocharger was optimised, and shows significant improvements in measured parameters - up to 1.6points of efficiency gain and 20% increase in map width, respectively. The calculations were carried outon a HTCondor cluster of 8 Linux workstations.Moreover, a study on the sensitivity of radial compressor aerodynamic performance to the presence of anundercut on the impeller surface was also carried out. In-house software from Napier was used to createan undercut on the impeller surface by modifying the CAD geometry file. The impact of the undercut onperformance was quantified using detail 3D CFD analysis. Various undercut height and width levels at 13different locations on the blade surface were analysed for three compressor designs. A unique sensitivitydistribution for each compressor impeller is calculated and used to create a variable tolerance map on theimpeller surface. This approach was shown to facilitate savings in cost by reducing scrap rate.In addition, a bespoke 1-D algorithm for estimating the size of a radial compressor impeller required tomeet a design mass flow and pressure ratio at a given rotor speed was developed. The model can be used PhD Thesis – O. F. Okhuahesogie Page 5as a preliminary tool when designing a new compressor (where there is no previous experimental ornumerical data). The algorithm is based on a combination of fundamental turbomachinery physicsequations, correlations extracted from literature and statistical modelling.Finally, an algorithm for calculating the flow area and air mass flow of the low pressure (LP) and highpressure (HP) compressors and turbines in a two-stage turbocharger required to meet a diesel enginespecification was developed. The algorithm was used to validate the flow specification of a two-stageturbocharger for a test diesel engine.