59997 - Turbocharger Radial Turbine Response to Pulse Amplitude 

Under on-engine operating conditions, a turbocharger turbine is fed by a pulsating flow. Due to the periodic opening/closing sequence of the exhaust valves, to which the turbine is connected to a series of bended pipes (exhaust manifold), the turbine is not fed by a constant mass flow rate and operates in non-optimal conditions.

Pulse shape, determined by exhaust valves geometry and engine operating conditions, has a large impact on the entire hot side system, both in terms of global performance and turbulent flow structures. Understanding the turbine response to the different pulse parameters, i.e. amplitude, frequency and temporal gradient, is of great importance in order to identify an ideal pulse shape that maximizes the power output.   

In this work, two different valve strategies, characterized by a difference in pulse amplitude around 5% and same mass flow rate, temporal gradient and frequency, are considered at an engine operating points equal to 1500rpm. The particular boundary conditions imposed are of particular interest since they effectively let to decouple the effects of pulse amplitude with respect to the temporal gradient ones, which is often overlaped in previous pulse characteristics studies present in the literature. 

Numerical solution is obtained via an unsteady RANS on a single scroll, vaneless radial turbine from a commercial turbocharger for a 2.0L, four cylinders and spark-ignition (SI) engine for passengers car. Furthermore, the rotation of  the rotor is modeled though the sliding mesh method (SMM) by imposing time-varying boundary conditions obtained from an experiments-calibrated GT-POWER model.  

In the post-processing phase, an exergy-based analysis, which enables to quantify the total internal irreversibilities  is employed to assess the global performance of various components in the computational domain, i.e. exhaust manifolds, scroll, turbine, diffuser and outlet pipe.

Preliminary results show that, despite the increment of total irreversibilities caused by higher velocity gradients in all the system components, a higher pulse amplitude has beneficial effects in terms of power extracted by the turbine. In this work, we are able to purely investigate the effect of pulse amplitude because mass flow rate, temporal gradient, and frequency are kept constant between the two pulses considered. Furthemore the inflow exergy is kept constant, so that the turbine capacity is not affected by the available amount of energy. This result is also of great applicability value from a manufacturer point of view, since a larger power output is achievable by designing suitable exhaust valves in order to produce a pulse shape characterized by a larger amplitude.