The implementation of increasingly stringent emissions and efficiency targets has seen engine downsizing and other complementary technologies increase in prevalence throughout the automotive sector. In order to facilitate ongoing improvements associated with the use of these strategies, delivering enhancements to the performance and stability of the turbocharger compressor when operating at low mass flow rates is of paramount importance. In spite of this, a few concepts (either active or passive) targeting such aims have successfully transitioned into use in automotive turbochargers, due primarily to the requirement for a very wide compressor-operating range. In order to overcome the operational limitations associated with existing pre-swirl generation devices such as inlet guide vanes, this study developed a concept comprising of an electrically driven axial fan mounted upstream of a standard automotive turbocharger centrifugal compressor. Rather than targeting a direct contribution to compressor boost pressure, the fan was designed to act as a variable pre-swirl generation device capable of being operated completely independently of the centrifugal impeller. It was envisioned that this architecture would allow efficient generation of the large pre-swirl angles needed for compressor surge margin extension and efficiency enhancement at low mass flow rate-operating points, while also facilitating the delivery of zero pre-swirl at higher mass flow rates to ensure no detrimental impact on performance at the rated power point of the engine. Having progressed through 1-D and 3-D aerodynamic modelling phases to understand the potential of the system, detailed component design and hardware manufacture were completed to enable an extensive experimental test campaign to be conducted. The experimental results were scrutinized to validate the numerical findings and to test the surge margin extension potential of the device. Compressor efficiency improvements of up to 3.0% pts were witnessed at the target-operating conditions.
It is widely accepted that large swirl angles (>60°) are required to deliver meaningful surge margin extension at the relatively low pressure ratios typical of automotive turbocharger compressors. However, in order to maintain performance towards choke (and hence the rated power point of the engine), the requirement is for delivery of zero pre-swirl. These constraints cannot be met using traditional variable inlet guide vane systems, as whether flat plate or cambered vanes are chosen, significant losses are to be expected at low and high mass flow rates respectively.Taking the above into account, the primary intention of this study was to develop a device capable of efficiently generating large swirl angles for surge margin extension and compressor efficiency enhancement at low mass flow operating points, without adversely affecting performance at other areas of the compressor map.The chosen concept involved placing an axial fan upstream of a standard automotive turbocharger compressor stage. The fan was designed to act as a variable pre-swirl device, which due to being driven by an electric motor, was capable of operating com pletely independently of the centrifugal impeller. The chosen concept was progressed through 1-D and 3-D design phases to understand the feasibility of the system, before committing to hardware manufacture and an extensive experimental test campaign to validate the numerical findings and test the surge margin extension potential of the device.
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