Improving Efficiency in a Turbocharger Centrifugal Compressor with TURBOdesign1

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Introduction

Cummins Turbo Technologies is a leading designer and manufacturer of turbochargers for diesel and gas-derivative engines above 2.3 litres. Headquartered in the UK with global manufacturing facilities in; Brazil, China, Europe, India and USA along with dedicated technical centres in UK, China and USA. Cummins Turbo Technologies have over 2000 employees globally. The company started as Holset in 1952. In 1973 ownership changed to Cummins Inc (known as Cummins Engine Company Inc at that time). In 2006 Holset Turbochargers changed its name to Cummins Turbo Technologies, an autonomous business unit of Cummins Inc.

Cummins Turbo Technologies has developed advanced turbocharging technologies to enable engine manufacturers to meet strict emissions criteria and engine performance requirements.

The application of TURBOdesign1 to improve the efficiency of a turbocharger centrifugal compressor wheel will be discussed

A Need for Efficiency

A turbocharged diesel engine’s fuel consumption is directly influenced by the turbocharger efficiency through the engine’s pumping cycle work. The more efficient the turbomachinery, the lower the work done by the engine during the pumping cycle, resulting in better engine BSFC (Brake Specific Fuel Consumption). The overall turbocharger efficiency is the product of the compressor, turbine and rotor system efficiencies:

Overall Efficiency = Compressor Efficiency X Turbine Efficiency X Rotor System efficiency


Compressor efficiency therefore has a direct impact on engine BSFC and is one of the areas recently targeted by Cummins Turbo Technologies for improvements. In this article the application of TURBOdesign1 to improve the efficiency of a turbocharger centrifugal compressor wheel will be discussed.

Baseline Impeller

The baseline impeller already had very high efficiency and previous attempts to improve its efficiency by using conventional (direct) design, based on iterative changes to blade angle had failed to produce a design with improved efficiency. The purpose of this project was to see whether its possible to improve this compressor’s efficiency by using inverse design code TURBOdesign1.

Standard design (left) compared to TURBOdesign1 (right)

 

The initial plan was to design a straight filament geometry. However, the detailed study using TURBOdesign1 and 3D CFD confirmed that by using straight filament geometry it was very difficult to improve the performance of the compressor. However, the inversely designed impeller, which has 3D geometry, was found to show significant improvement in efficiency based on stage CFD, using a similar mesh to that shown below, which is using about 7 Million nodes.

 

CFD computational mesh

 

The 3D impeller designed by TURBOdesign1 results in a stiffer blade with first mode frequency that is 20% higher than the conventional impeller

FEA Analysis

Structural analysis of the new design was carried out using Ansys Workbench FEA package, using a 3D model including, blade root radii and leading / trailing edge treatment. This model was used for both vibration and stress analysis. The resulting contours of principal stress are shown in Fig. 3. The resulting stress distributions are different but the maximum principal stress is actually similar between the 3D geometry designed by TURBOdesign1 and the conventional straight filament design, resulting in similar durability of the inverse designed impeller. Furthermore, modal analysis confirmed that the 3D impeller designed by TURBOdesign1 results in a stiffer blade with first mode frequency that is 20% higher than the conventional impeller.

 

Comparison of predicted maximum principal stress between inverse designed impeller (left) and baseline (right)

Performance Improvement

Efficiency improvement with inverse design compressorThe performance map shows the comparison of the measured performance of the new inverse design compressor stage to the conventional design hardware. The performance map overplot shows compressor pressure ratio and normalised efficiency plotted against flow rate. We can see that the inverse designed hardware is delivering up to 3% higher efficiency.

This work confirms that in many turbocharger compressor applications higher performance is only possible by using 3D blade profiles. TURBOdesign1 provides the best means of rapidly designing 3D centrifugal compressor blades with superior performance.ig. 4, shows the comparison of the measured performance of the new inverse design compressor stage to the conventional design hardware. The performance map overplot shows compressor pressure ratio and normalised efficiency plotted against flow rate. We can see that the inverse designed hardware is delivering up to 3% higher efficiency.

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