A Comparison of the Performance of Three Model Axial Flow Turbines Tested under Both Steady and Pulse Flow Conditions

Abstract

Three turbines, covering a range of reaction of +20 per cent to − 10 per cent at their blade roots, have been tested under both steady and ‘pulse flow’ conditions in order to assess their degree of suitability for turbo-charger application. The results have been analysed in a way which enables efficiencies to be plotted against a mean non-dimensional blade speed irrespective of the level of pulsation. This method of presentation enables the results to be used directly for design purposes and in matching calculations. Thus the efficiency penalty due to pulse flow, the effect of pulse flow on the choice of turbine size, and the preferred degree of reaction under pulse flow may be assessed. For the present tests the range of variation in gas speed under pulsing conditions is thought to compare in severity with that produced when a single turbocharger is connected to three cylinders of a large two-stroke marine engine. For this case the design having the highest degree of reaction is shown to give an efficiency four percentage points higher than either of the other designs. It also has a clear advantage under steady flow. The intermediate design, having zero root reaction, corresponds to the type of turbine conventionally used in turbo-chargers and is shown to be the one most sensitive to losses in efficiency resulting from pulsation. It appears therefore that substantial improvements could be made in turbines for future turbocharger designs by increasing the reaction. Further testing of these turbines under modified pulse conditions, with significant windage period, is advocated. The advantage of the high reaction design must be demonstrated for such conditions before a change of design reaction could be regarded as fully justifiable.