Mixed flow turbines: Inlet and exit flow under steady and pulsating conditions

The performance and detailed pow characteristics of a high pressure ratio m ixed pow turbine has been investigated under steady and pulsating pow condi tions. The rotor has been designed to have a nominal constant incidence (ba sed on free vortex flow in the volute) and it is for use in an automotive h igh speed diesel turbocharger. The results indicated a departure from the q uasi-steady analysis commonly used in turbocharger turbine design. The puls ations from the engine have been followed through the inlet pipe and around the volute; the pulse has been shown to propagate close to the speed of so und and not according to the bulk pow velocity as stated by some researcher s. The pow entering and exiting the blades has been quantified by a laser D oppler velocimetry system. The measurements were performed at a plane 3.0 m m ahead of the rotor lending edge and 9.5 mm behind the rotor trailing edge . The turbine test conditions corresponded to the peak efficiency point at 29,400 and 41,300 rpm. The results were resolved in a blade-to-blade sense to examine in greater detail the nature of the pow at turbocharger represen tative conditions. A correlation between the combined effects of incidence and exit flow angle with the isentropic efficiency has been shown. The unst eady flow characteristics have been investigated at two flow pulse frequenc ies, corresponding to internal combustion engine speeds of 1600 and 2400 rp m. Four measurement planes have been investigated: one in the pipe feeding the volute, two in the volute (40 deg and 130 deg downstream of the tongue) and one at the exit of the turbine. The pulse propagation at these planes has been investigated; the effect of the different planes on the evaluation of the unsteady isentropic efficiency is shown to be significant. Overall, the unsteady performance efficiency results indicated a significant depart ure from the corresponding steady performance, in accordance with the inlet and exit flow measurements.