Design of low inertia and high performance radial compressor and turbine for high-speed turbomachinery

This Ph.D. thesis presents research into small size centrifugal compressors and radial inflow turbines for improved performance applicable to turbochargers. The underlying proposition of this work is to develop a compressor stage with the 44 mm diameter impeller, delivering the performance of the compressor stage having up to 20% larger diameter impeller alongside a wide flow operating range. This work also includes designing the radial turbine wheel with a 39 mm inlet diameter having an 11% lower inlet diameter than the compressor impeller outlet diameter to drive the compressor stage. The encouragement for this research and development work comes from increasing demands for lower inertia and higher performance wheels in the turbocharger industry. The need for a rapid spooling of turbocharger wheels and better transient response in vehicle engines endorse these demands, especially in urban driving conditions. Besides, progressively expanding range of engine speed demands a wide flow range in compressors and hence paves a way for the development of new turbocharger wheels. The proposed compressor impeller features state-of-the-art design such as a transonic inducer and blade lean at the blade leading-edge demonstrating a novelty. In the context of performance evaluation, the comparison of the measured and predicted performance of the 44 mm compressor stage exhibits a good agreement. Furthermore, when incorporated with the well-matched diffuser width and relatively larger volute, the improved 44 mm compressor stage shows a good overlap of performance with the performance of the 52 mm compressor stage. To improve the surge margin, a novel casing groove treatment is proposed. The numerical investigation reveals that the compressor stage with the proposed casing treatment improves the operating range by 8% on average but penalize the peak efficiency by up to 5%. Likewise, the development of the radial turbine is focused on achieving lower wheel inertia by utilizing a relatively smaller diameter and differs from the usual turbine design in terms of the blade loading distribution. The numerical analysis reveals that the proposed turbine design can produce the requisite shaft power to drive the compressor. Moreover, as the functionality of the cold flow test rig was affected due to oil leakage, the measured results are not used to verify the performance of the turbine stage. However, the measured turbine performance agrees with the trend of the predicted turbine performance. The mechanical integrity of the compressor (Aluminium alloy 2024-T6) and turbine (Inconel 713LC) impeller is examined at rotating speed up to 230,000 rpm and under the working temperature of 1800C and 8000C, respectively. When applied with centrifugal and thermal loading, the compressor impeller exhibits the highest stress of 341 MPa in the bore, 10% lower than the yield stress of the material. The maximum stress in the blade root and backdisk is observed to be 11% and 20% lower than the yield strength of the material, respectively. The modal analysis of the compressor impeller predicts that a first bending mode will occur at the blade inducer with a natural frequency four times higher than the exciting frequency of the blade. Likewise, the mechanical analysis of the turbine impeller reveals the maximum stress in the backdisk and shaft attachment region, however, less than the yield strength of the material. Other locations with high stress are blade pressure side fillet and blade exducer root, but the stress magnitudes are lower than the yield strength of the material. The frequency analysis of the turbine wheel predicts that a first bending mode will occur at the turbine blade exducer at a natural frequency 3.7 times higher than the exciting frequency of the blade. The durability of the compressor and turbine wheel is assessed in terms of fatigue life, damage, and safety factor. With the stress, vibration, and durability assessed, the present compressor and turbine impellers are deemed to be mechanically viable