Mike Eastwood, group leader for structural analysis at Cummins Turbo Technologies, explains the process that the team took and how it overcame challenges.
Why did you conduct this research?
There is a safety requirement for turbochargers, in that if a wheel burst occurs, all fragments of the wheel and housing must be contained within the housing without puncture of the outer surface this is called ‘containment’. Cummins Turbo Technologies has used Ansys analysis products for almost 25 years. As part of this ongoing relationship, it was suggested that we may be able to use the Autodyn explicit package, as supplied by Ansys, to perform turbine end burst simulation, to allow optimization of design to achieve containment.
As part of the Cummins Analysis Led Design (ALD) initiative, we thought there might be an opportunity to reduce the concept design cycle time by optimizing the turbine housing design using analysis to enable containment. The second reason was to optimize the weakening slot to cause wheel burst at the required speed, again cutting down on the amount of testing required. Replacing testing with analysis helps cut down costs.
What was your role on the project and how long did it take?
My role was supervisor to the paper’s author (Lin Wang). I have had some experience in the area of impact analysis using LS-Dyna for turbine housing containment prediction around 17 years ago. Wang took advice from Alex Pett at Ansys on explicit analysis. It took a few months to develop and correlate the turbine housing containment technique and a further few months to develop and correlate the wheel burst with weakening slot technique.
What were the project’s goals?
The aim of the project was to use analysis to simulate the turbine end burst test in order to cut down on the amount of physical testing required. There would be two savings at the turbine end. The first saving was using analysis to help design a turbine housing that could pass the burst test first time by absorbing enough kinetic energy and not allowing turbine wheel fragments to escape.
This would prevent a ‘design test design’ trial-and-error approach and its associated cost implications. The second saving was to create the dimensions of the required weakening slot to allow the turbine wheel to burst at the required speed on test, again preventing a trial-and-error approach.
What did you do, and what challenges did you encounter?
The turbine housing containment model was built gradually, considering geometry; meshing; material properties; material plasticity and fracture models; wheel speed; and thermal and structural boundary conditions. This was carried out until we were satisfied in a number of cases that simulating was correlating well to real burst test data. We also showed the method and results in a number of Cummins functional excellence forums along the way to gain feedback.
For a highly non-linear problem, a large number of iterations have to be carried out before finding the equilibrium, thus the global stiffness matrix has to be assembled and inverted many times during the analysis. Time steps during solution also have to be very small, therefore, the computation is extremely expensive and memory requirements are also very high. It is difficult to predict how long it will take to solve the problem or even if convergence can be achieved.
What was the end result?
We have achieved good correlation to test over a number of simulations, allowing us to standardize analysis and test techniques. The next step for the company is to look at compressor end containment and other areas of the turbocharger where explicit analysis may be helpful, such as parts of the manufacturing process requiring non-linear explicit analysis for simulation.
Uploaded 22 July 2016
