Performance and Exit Flow Characteristics of Mixed-Flow Turbines

C Arcoumanis,J M Nouri,C C Su,R F Martinez-Botas

doi:10.1155/s1023621x97000262

C Arcoumanis, J M Nouri + Show 2 more

Open Access

https://doi.org/10.1155/s1023621x97000262

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Abstract

The performance and exit flow characteristics of two mixed-flow turbines have been investigated under steady-state conditions. The two rotors differ mainly in their inlet angle geometry, one has a nominal constant incidence (rotor B) and the other has a constant blade angle (rotor C), but also in the number of blades. The results showed that the overall peak efficiency of rotor C is higher than that of rotor B. Two different volutes were also used for the tests, differing in their cross-sectional area, which confirm that the new larger area volute turbine has a higher efficiency than the old one, particularly at lower speeds, and a fairly uniform variation with velocity ratio.The flow exiting the blades has been quantified by laser Doppler velocimetry. A difference in the exit flow velocity for rotors B and C with the new volute was observed which is expected given their variation in geometry and performance. The tangential velocities near the shroud resemble a forced vortex flow structure, while a uniform tangential velocity component was measured near the hub. The exit flow angles for both rotor cases decreased rapidly from the shroud to a minimum value in the annular core region before increasing gradually towards the hub. In addition, the exit flow angles with both rotors were reduced with increasing rotational speeds. The magnitude of the absolute flow angle was reduced in the case of rotor C, which may explain the improved steady state performance with this rotor. The results also revealed a correlation between the exit flow angle and the performance of the turbines; a reduction in flow angle resulted in an increase in the overall turbine efficiency.

Highlights

Mixed-flow turbines for automotive turbochargers have been recently investigated both experimentally and theoretically
The exit flow characteristics of the two rotors were quantified by laser Doppler velocimetry for 50% and 70% equivalent design speed
The main findings can be summarised as follows: 1. The overall total-to-static efficiencies of both rotors increased with speed up to 70% equivalent design speed after which they remained almost constant except close to the surge conditions

Summary

Introduction

Mixed-flow turbines for automotive turbochargers have been recently investigated both experimentally and theoretically. A mixed-flow turbine overcomes one of the principal limitations of radial-inflow turbines, where stress and material considerations dictate that the blade angle must be zero to maintain radial blade sections and keep the centrifugal load in the blades purely tensile (see Fig. 1) It permits the use of non-zero blade angles without departure from the requirement of radial blade fibres, introducing an extra degree of freedom in the rotor design. A mixed-flow rotor is capable of achieving a peak efficiency at lower velocity ratios (or higher pressure ratios) when compared with its radial counterpart This provides a better utilisation of the engine exhaust gas energy since the maximum energy in the exhaust gases is available at high exhaust manifold pressure (Watson and Janota [1982]) which, improves the engineturbocharger matching.

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