KEYWORDS:Turbocharger; mixed flow variable geometry turbine, turbocharger test rig, steady state performance, transient response, CFD.

Turbocharger is a critical equipment for supplying compressed air to automotive engines, which currently face a significant demand for optimum downsizing. The turbocharger with radial flow variable geometry turbine (RFVGT) is widely used in the industry. However, if peak efficiencies are achieved at a lower velocity ratio, it will be quite beneficial for improved transient response of the engine. This characteristic can be achieved with mixed flow turbines. Additionally, a mixed flow turbine is capable of handling higher mass flow rates and providing higher efficiency with higher turbine stage loading. This forms the motivation for the present research on mixed flow variable geometry gas turbine (MFVGT).
In the present work, the mixed flow variable geometry turbine is developed for a 1.37 L diesel engine with a 74 kW power rating. For the purpose of benchmarking as well as comparison, the radial flow variable geometry turbine is also developed. The required turbocharger for both the VGTs along with the experimental test facility for steady and transient response tests were developed at Turbo Energy Private Limited (TEPL), Chennai.
The steady state performance parameters evaluated are: turbine mass flow parameter, combined turbine efficiency, velocity ratio, and turbine specific speed. The turbine mass flow parameter (MFP) of the MFVGT is found to be higher than the RFVGT by about 13% for the similar turbine size. For similar MFP conditions, the combined turbine efficiency of the MFVGT is about 14% to 27% higher than the RFVGT at an expansion ratio of 1.5. The velocity ratio at which peak combined turbine efficiency occurs with MFVGT is 0.78 against the RFVGT with 0.83. The specific speed at which peak combined turbine efficiency occurs is 0.88 for MFVGT against 0.73 for RFVGT.
The transient performance is evaluated by observing the time for the turbine to reach 95% of the target compressor outlet pressure (or boost pressure). For the entire turbine operating regime, i.e. various mass flow parameters and turbine inlet pressures, the transient response time for the MFVGT is lower than the RFVGT. At 50% MFP and the turbine inlet pressure of 1.5 bar (abs), the transient response time for the RFVGT is 0.858 s. The same is 0.770 s for the MFVGT. Thus the superior steady, as well as the transient performance of the MFVGT over RFVGT is established.
The computational fluid dynamic (CFD) simulations are conducted for steady state operating conditions at different turbine mass flow parameters and turbine expansion ratios. These studies reveal that the flow is more uniform in MFVGT than in the RFVGT. The rotor loss coefficient for the MFVGT is also found to be lower than the RFVGT. This conforms to the higher combined turbine efficiency of MFVGT noticed in the experimental study.