Martyn Jupp

The influence of secondary flow structures in a turbocharger turbine housing in steady state and pulsating flow conditions

This paper presents a computational investigation into the effect of volute secondary flow structures on turbine inlet flow conditions. The steady state results show Dean type vortices exist early in the volute. As a result a substantial variation in absolute flow angle at the volute exit was observed. Pulsed flow simulations showed that the size and position of the secondary flow structures are time dependent. The resulting volute exit flow conditions were also found to be time dependent with the absolute flow angle at the volute exit varying with pulse pressure. This paper shows that that the secondary flow structures that exist in the volute as a result of cross sectional shape can have significant downstream effects on rotor performance.

Analysis of leading edge flow characteristics in a mixed flow turbine under pulsating flows

Current trends in the automotive industry towards engine downsizing means turbocharging now plays a vital role in engine performance. A turbocharger increases charge air density using a turbine to extract waste energy from the exhaust gas to drive a compressor. Most turbocharger applications employ a radial inflow turbine. However, to ensure radial stacking of the blade fibers and avoid excessive blade stresses, the inlet blade angle must remain at zero degrees, creating large incidence angles. Alternately, mixed flow turbines can offer non-zero blade angles while maintaining radial stacking of the blade fibers and reducing leading edge separation at low velocity ratios. Furthermore, the physical blade cone angle introduced reduces the blade mass at the rotor outer diameter reducing rotor inertia and improving turbine transient response. The current paper investigates the performance of a mixed flow turbine under a range of pulsating inlet flow conditions. A significant variation in incidence across the LE span was observed within the pulse, where the distribution of incidence over the LE span was also found to change over the duration of the pulse. Analysis of the secondary flow structures developing within the volute shows the non-uniform flow distribution at the volute outlet is the result of the Dean effect in the housing passage. In-depth analysis of the mixed flow effect is also included, showing that poor axial flow turning ahead of the rotor was evident, particularly at the hub, resulting in modest blade angles. This work shows that the complex secondary flow structures that develop in the turbine volute are heavily influenced by the inlet pulsating flow. In turn, this significantly impacts the rotor inlet conditions and rotor losses.

Ported shroud flow processes and their effect on turbocharger compressor operation

The ported shroud (PS) self-recirculating casing treatment is widely used to delay the onset of the surge by enhancing the aerodynamic stability of the turbocharger compressor. The increase in the stable operation region of the turbocharger compressor is achieved by recirculating the low momentum fluid that blocks the blade passage to the compressor inlet through a ported shroud cavity. While the ported shroud design delays surge, it comes with a small penalty in efficiency. This work presents an investigation of the flow processes associated with a ported shroud compressor and quantifies the effect of these flow mechanisms on the compressor operation. The full compressor stage is numerically modelled using a Reynolds Averaged Navier-Stokes (RANS) approach employing the shear stress transport (SST) turbulence model for steady state simulations at the design and near surge conditions. The wheel rotation is modelled using a multiple reference frame (MRF) approach. The results show that the flow exits the PS cavity at the near surge condition in the form of three jet-like structures of varying velocity amplitudes. Net entropy generation in the compressor model is used to assess the influence of the ported shroud design on the compressor losses, and the results indicate a small Inlet-PS mixing region is the primary source of entropy generation in the near surge conditions. The analysis also explores the trends of entropy generation at the design and the near surge condition across the different speed lines. The results show that the primary source of entropy generation is the impeller region for the design condition and the inlet-PS cavity region for the near surge condition.

Analysis of a tilted turbine housing volute design under pulsating inlet conditions

Radial inflow turbines are widely used in the automotive turbocharger industry due to the greater amount of work that can be extracted per stage and their ease of manufacture compared with equivalent axial designs [1]. The current industry trend towards downsized engines for lower emissions has driven research to focus on improving turbine technologies for greater aero-thermal efficiency. Consequently, mixed flow turbines have recently received significant interest due to a number of potential performance benefits over their radial counterparts, including reduced inertia and improved performance at low velocity ratios. This paper investigates the performance of a tilted volute design compared with that of a radial design, under steady state and pulsating flow conditions. The tilted volute design was introduced in an attempt to improve inlet flow conditions of a mixed flow turbine wheel and hence improve performance. The investigation is entirely computational and the approach used was carefully validated against gas stand test results. The results of the study show that under steady state conditions the tilted volute design resulted in stage efficiency improvements of up to 1.64%. Under pulsating flow conditions, the tilted housing design resulted in a reduction in incidence angle and a maximum cycle averaged rotor efficiency improvement of 1.49% while the stage efficiencies resulted in a 1.23% increase. To assess the loss mechanisms within the rotor, the entropy flux generation through the blade passage was calculated. The tilted housing design resulted in reductions in leading edge suction and shroud surface separation resulting in the improved efficiency as observed.

The introduction of a tilted volute design for operation with a mixed flow turbine for turbocharger applications

This paper introduces a tilted volute design for operation with a mixed flow turbine rotor. CFD results show an efficiency gain of up to 1.2% over the standard radial design at the highest tested turbine rotational speed. The efficiency gain was found to be the result of a reduction in separation from the blade suction surface. A reduction in the flow cone angle was also observed for the tilted housing, as a result an increase in negative incidence angles at the blade LE was observed. This work shows that optimization of the turbine housing specifically for mixed flow applications can yield significant performance benefits.