Experimental Evaluation of Turbocharger Turbine Performance Under Pulsating Flow Conditions

Abstract

The steady and pulsating performance results of a turbocharger mixed-flow turbine are presented. The results are taken at an equivalent speed of 70% (42,000rpm) for a pulse frequency range of 20 to 80 Hz. All instantaneous parameters required for unsteady performance evaluation are measured and discussed. Significant improvements to the measurement of instantaneous actual power have been carried out. Large variations in the operating point of the turbine occur in each pulse cycle, a velocity ratio range of 0.43 to 1.28 is seen for a 20 Hz pulse, this range reduces as the pulse frequency increases and unsteady effects become more prominent. During periods of turbine freewheeling, negative efficiencies can arise due to momentum transfer from the turbine to the working gas, although detrimental to the efficiency the energy content in these regimes are low. The use of a modified Strouhal number (MSt.) and a pressure modified Strouhal number (PMSt.) has proved useful in assessing when the onset of unsteadiness of the flow will become significant, a value of 0.1 has been used as an appropriate limit to steadiness. The results suggest that for a typical engine speed range the rotor may be considered quasi-steady whilst the turbine stage is predominately operating in an unsteady regime. Inference from the experimental data would suggest it is adequate to capture the performance of a turbine under pulsating flow using a ‘quasi-steady’ model when the MSt. < 0.1, and a ‘filling and emptying’ code when a PMSt. < 0.1 and above this value a ‘wave action’ model is more appropriate.

Unsteady performance analysis of a twin-entry variable geometry turbocharger turbine

“…Similar results were also shown by Karamanis and Martinez-Botas [22], Szymko et al [17] and Rajoo and Martinez-Botas [21], who measured higher cycle-averaged efficiency in a single-entry turbine for higher speeds and frequencies. However, it must be noted that no consistent trend was observed in the cycle-averaged efficiency variation.…”

“…properties requires a degree of phase shifting to enable evaluation on common time frame. This is achieved by phase-shifting all the measurement to a common reference location using bulk + sonic flow velocity as described by Szymko et al [17]. The instantaneous pressure ratio is given in eq.…”

“…A good correlation is observed between the isentropic and actual power, given the separate deriving technique for both. It can be seen that at some instances the actual power of the turbine goes negative; this is explained in Szymko et al [17] as the conditions where the turbine rotor imparts momentum on the flow due to a very low energy content of the free stream. Interestingly such condition does not occur for out-of-phase flow, indicating better flow momentum with 2 pulses per cycle.…”

The introduction of a tilted volute design for operation with a mixed flow turbine for turbocharger applications

“…This is of importance, as during the turbine inlet pulsations, the blade speed remains almost constant [7], but a significant variation in inlet pressure and velocity is experienced. The maximum energy in the exhaust pulse is available at higher pressure ratios that result in low values [8].…”

The Sensitivity of Transient Response Prediction of a Turbocharged Diesel Engine to Turbine Map Extrapolation

“…This dataset was needed in order to compare simulation results for cases with different levels of extrapolation. The turbine loading device was a bespoke eddy current dynamometer [16], enabling measurement over a wide range of velocity ratio conditions since it is not constrained by compressor surge and choke, as would be the case for a traditional turbocharger hot gas bench. This results in performance characteristics that are typically at least three times as wide as those normally supplied, with consequently much less need for map extrapolation.…”