New journal papers about to be published

28 April 2010

The close liason between Stuttgart University and PCA Engineers continues to provide benefits to both parties. Professor Michael Casey of Stuttgart University has recently published 2 new papers with his students related to the performance of radial compressors in turbochargers. Both are based on student projects suggested by PCA. The first describes how to extrapolate characteristics of compressors to zero pressure rise and beyond, using a physically based method. The second describes the effect of heat transfer on the efficiency of turbocharger compressors at low speed and shows that the conventional use of an adiabatic efficiency is fundamentally flawed for this purpose and should be replaced by the polytropic efficiency.


A third paper describes the development of the new 2D throughflow code (Vista TF) which can be applied to a wide range ot turbomachinery, such as centrifugal and axial compressors, radial inflow and axial turbines, fans and pumps.


The papers will appear during the next month or so in journals published by the leading American and British engineering organisations, ASME and the IMechE. Journal publication is reserved for papers felt to have a long-term academic relevance, only a fraction of papers presented in conferences are deemed to meet this requirement.


Estimation of the performance of turbocharger compressors at extremely low pressure ratio

M V Casey and M Schlegel (Institute of Thermal Turbomachinery (ITSM), University of Stuttgart, Stuttgart, Germany) 

Constant speed lines on turbocharger compressor performance maps usually have no test data at low pressure ratio and high flow. The engine, however, can force the turbocharger compressor to operate in this region and whole-engine simulation systems need to extend the test data in some way to include this region of the map. A physically based method of estimating the performance at low pressure ratios from measurements made in the rest of the map has been developed and is described in this article. The method requires no detailed geometrical information on the impeller and the stage but extracts the necessary information from the test data available, so it can be applied to any measured compressor data. The test data over the measured range of speed and flow are used to estimate the work transfer and loss characteristics of the impeller, and the impeller throat area.

The Euler equation justifies a linear extrapolation of the work input to higher outlet flow and a new technique based on the density ratio is used to extrapolate the losses. This accounts for choking at the impeller inlet in a similar way to choking of a one-dimensional duct of varying area, where losses are also a function of density ratio. The stage performance at low pressure ratios can then be obtained by recombining the extrapolated loss and work characteristics at higher flows. The method allows the measured performance map to be extrapolated to low pressure ratios on a sound physical basis and to identify physical aspects of the flow in this extreme off-design region, such as the location of increased risk of oil-blow-by.

Proceedings of the IMechE Vol 224 Part A: Journal of Power and Energy pp239-250

Journal of Power and Energy: DOI: 10.1243/09576509JPE810


The Efficiency of Turbocharger Compressors With Diabatic Flows

Michael V. Casey and Thomas M. Fesich (Institute of Thermal Turbomachinery (ITSM), University of Stuttgart, Stuttgart, Germany)

In most compressors the flow is adiabatic, but in low-speed turbochargers, the compression process has both heat transfer and work input. This paper examines different compressor efficiency definitions for such diabatic flows. Fundamental flaws in the use of the isentropic efficiency for this purpose are identified, whereas the polytropic efficiency can be used with or without heat transfer without ambiguities. The advantage of the polytropic approach for a practical application is demonstrated by analysing the heat transfer in a turbocharger compressor. A simple model of the heat transfer allows a correction for this effect on the polytropic efficiency at low-speed to be derived.

Compressor characteristics that have been corrected for this surprisingly large effect maintain a much higher efficiency down to low-speeds.

Journal of Engineering for Gas Turbines and Power: DOI: 10.1115/1.4000300


A new streamline curvature throughflow method for radial turbomachinery

M V Casey and C J Robinson (Institute of Thermal Turbomachinery (ITSM), University of Stuttgart, Stuttgart, Germany; PCA Engineers Limited, Lincoln UK)

This paper describes a newly developed streamline curvature throughflow method for the analysis of radial or mixed flow machines. The code includes curved walls, curved leading and trailing edges and internal blade row calculating stations. A general method of specifying the empirical data provides separate treatment of blockage, losses, and deviation. Incompressible and compressible fluids are allowed, including supersonic relative flow in blade rows. The paper describes some new aspects of the code. In particular, a numerical model for spanwise mixing is derived, the calculation method for prescribed pressure ratio in compressor bladed rows is described, and the method used to redistribute the flow across the span due to choking is given. Examples are given of the use and validation of the code for many types of radial turbomachinery and these show it is an excellent tool for preliminary design.

Transactions of the ASME, Journal of Turbomachinery, July 2010, Vol 132 / 031021-1