A characteristic of radial turbines for turbocharger applications subject to pulsating flow is the deviation of the turbine performance compared with corresponding continuous flow conditions, typically associated with gas-stand experiments. The performance deviations under pulsating flow generate a hysteresis loop that encloses the performance line obtained in gas-stand conditions and their intensity is demonstrated to grow with increasing pulse amplitude and frequency. Predicting the performance deviations is of great interest to improve the predictive capabilities of reduced-order models and enhance engine-turbocharger matching.

In this work, the performance response of a turbocharger radial turbine is studied with respect to variations of the normalized pulse amplitude (between 0.4 and 1.6) and the pulse frequency (between 20Hz and 100Hz). Results show that the hysteresis loop expands with increasing pulse amplitude and frequency, so that the turbine cannot be treated as a quasi-steady device. The characteristic trends of the turbine performance are also highlighted with respect to pulse amplitude and frequency variations. The expansion ratio is registered to improve by +4.0% with increasing pulse amplitude and decrease by -1.3% with increasing pulse frequency. An opposite trend is otherwise registered for the isentropic efficiency, which decreases by -6.5% for increasing pulse amplitude and increases by +6.5% for increasing pulse frequency. Finally, through a simple model, the deviations of the turbine performance from quasi-steady to pulsating flow conditions are demonstrated to depend on the time derivative of the pressure pulse and the residence time of the fluid particle rather than pulse amplitude and frequency.

Under on-engine operating conditions, a turbocharger turbine is subject to a pulsating flow and, consequently, experiences deviations from the performance measured in gas-stand flow conditions. Furthermore, due to the high exhaust gases temperatures, heat transfer further deteriorates the turbine performance. The complex interaction of the aerothermodynamic mechanisms occurring inside the hot-side, and consequently the turbine behavior, is largely affected by the shape of the pulse, which can be parameterized through three parameters: pulse amplitude, frequency, and temporal gradient. This paper investigates the hot-side system response to the pulse amplitude via detached eddy simulations (DES) of a turbocharger radial turbine system including the exhaust manifold. First, the computational model is validated against experimental data obtained in gas-stand flow conditions. Then, two different mass flow pulses, characterized by a pulse amplitude difference of ≈5%≈5%⁠, are compared. An exergy-based post-processing approach shows the beneficial effects of increasing the pulse amplitude. An improvement of the turbine power by 1.3%1.3%⁠, despite the increment of the heat transfer and total internal irreversibilities by 5.8%5.8% and 3.4%3.4%⁠, respectively, is reported. As a result of the higher maximum speeds, internal losses caused by viscous friction are responsible for the growth of the total internal irreversibilities as pulse amplitude increases.

Due to the reciprocating engine, a pulsating flow occurs in the turbine turbocharger, which experiences conditions far from the continuous flow scenario. In this work, the effects of the characteristics of the mass flow pulse, parameterized through amplitude, frequency and temporal gradient, are decoupled and studied via unsteady Computational Fluid Dynamics calculations under on-engine operating conditions. Firstly, the model is validated based on comparisons with experimental data in steady flow conditions. Then, the effect of each parameter on exergy budget is assessed by considering a +/-10% variation with respect to a baseline pulse. The other factors defining the operating conditions (e.g. mass flow, shaft speed and inflow exergy) are kept the same as the baseline. The adopted approach enables to completely isolate the effects of each parameter in contrast with previous literature studies. Based on the results observed, pulse amplitude is identified as the primary parameter affecting the hot-side system response in terms of turbine performance, heat transfer and entropy generation, while frequency and temporal gradient show a smaller influence compared to it. As the pulse amplitude increases, the turbine work is reported to improve up to 9.4%. Smaller variations are observed for the frequency and temporal gradient analysis. With a 10% increase of the pulse frequency the turbine work is registered to improve by 5.0%, while the same percentage reduction of the temporal gradient leads to an increase of turbine work equal to 3.6%.