Investigation of Cross Flow in Double Entry Turbocharger Turbines

Abstract

The downsizing of combustion engines has become the major strategy within the automotive industry to meet the increasing demands in terms of fuel economy and harmful emissions. Furthermore, it is important to fulfil the customers expectations in terms of drivability by increasing the power density and transient performance of the engines. The key technology to reach these ambitious targets is the enhanced utilization of exhaust pulses on turbocharged engines. In four cylinder gasoline engine applications this is mainly realized by the use of double entry turbines or variabilities in the exhaust valve train. During the designing and matching process of double entry turbines it is still a major challenge to predict the turbine power output and accurately model its interaction with the engine. In the past few years, several authors have published measurement and simulation technologies aimed at enhanced modelling quality. Most of these approaches are based on the introduction of different flow conditions which help to describe the thermodynamic performance under various pulsating boundary conditions. Within an average engine cycle, the turbine operates under equal, single and unequal admissions. Furthermore, the evaluation of a turbine interacting with a four cylinder gasoline engine shows that cross flow between both turbine scrolls can occur during the blow-down phase of the cylinders. In this phase, the temperature and pressure upstream of the turbine reach their peak values within the complete engine cycle. Therefore, this phase is most crucial for the conversion of the exhaust energy into mechanical energy, which drives the compressor impeller of the turbocharger. This work focuses on the results of stationary hot gas measurements and 3D CFD simulations of the cross flow phenomena to gain a deeper understanding of the scroll interaction in double entry turbines and its impact on engine performance. The findings were used to improve the modeling quality of double entry turbines in 1D engine process simulations, especially during the exhaust blow down where cross flow between the dividing wall and the turbine wheel occurs. The new methodology to quantify the amount of cross flow with a hot gas test has shown that the cross flow rate of a twin scroll turbine can reach values as high as 35% of the overall flow rate entering the turbine housing, whereas this value can be significantly reduced by using a segment turbine housing. The new map based turbine model, which enables predictive simulations, covers all engine relevant flow conditions of the turbine including cross flow. This is important because the cross flow has a large impact on the exhaust pulse separation and thus on the residual gas fraction of the cylinders after the gas exchange.

Attempt to correlate simulations and measurements of turbine performance under pulsating flows for automotive turbochargers

“…25 In addition, the degree of complexity is increased in fully unsteady turbine models, and performance could be poorly predicted under unequal flow admissions for twin- and double-entry turbocharger turbines. 26 Whereas extended mapping of twin- and double-entry turbocharger turbines is able to capture unsteady performance, 27 a quasi-steady assumption for the turbine rotor could be able to model turbocharger turbine behaviour. 28…”

Experimental characterization and mean line modelling of twin-entry and dual-volute turbines working under different admission conditions with steady flow

“…Few authors [33,34,35] suggested that, if twin-entry and dual-volute turbines are modelled as in the view of two turbine approach (TTA) attached to a single compressor, then the effective turbine efficiency can be calculated as the ratio between the compressor power to the sum of two individual isentropic turbine powers of the individual turbine flows; as shown in the Equation 2. 13.…”

“…The rotor outlet geometrical area (A geom ) is calculated by taking the arithmetic diameter between the turbine rotor shroud (D 4 ) and rotor hub (D nut ) diameters as shown in Equation 2. 33.…”

“…This means that a part of the mass flow going through the turbine branch with greater pressure levels does not arrive at the turbine wheel, but recirculates into the following branch in front of the wheel. This phenomenon is generally described as back-flow/cross-flow [33]. If one can quantify the impact of cross-flow through experimental works or CFD simulations, it will provide better insights for improving the 1D double-entry turbine model and also helps in obtaining the accurate predictions in the 1D engine simulation process.…”