Investigation Of Scalloping Effects Of Radial Turbines With Computational Fluid Dynamics

Abstract

Turbochargers should both have high efficiency and rapid engine response. By removing material from the rotating parts in the turbocharger, decreases the inertia. Most suited is the turbine in the turbocharger, which has high density because of nickel-based alloys. The material is removed between the radial blades in a shape that looks like a scallop and therefore called scalloping. Volvo Powertrain AB has developed prototypes of non-scalloped turbines and now wants to evaluate if it is possible to design and investigate the performance of different scalloped rotors.
This thesis presents a study on a mixed-flow prototype rotor. The rotor has been scalloped in five different models and covers a range where the inertia has been reduced by 35%. The models were simulated with the Computational Fluid Dynamics program STAR-CCM+ and a single blade passage geometry were designed with Creo Parametrics. Because of the time limit of this study, the simulations have been performed with steady state simulations compared to time-consuming transient simulations. Therefore, the exhaust pulse flow has been simplified as four different points covering the velocity ratios 0.51 to 0.77. A comparison between the scalloped and reference rotor have been made with the total to static efficiency. Also, a visualisation and evaluation of main stream flow through the rotors have been performed. The reference rotor was redesigned to be more comparable to the scalloped models with an outcome of increased efficiency.
The results from the simulations show that a lightly scalloped rotor has similar efficiency as the new reference rotor but with a reduction in transient response time to 2.5%.

Popular Abstract

Improving a radial turbine in a turbocharger with a shape from nature


By removing material from a radial turbine with the shape of a scallop and simulate the turbine with help from 200 computers, it is possible to investigate and look deep inside the turbine of what is happening when scalloping is introduced. Despite previous studies, has this thesis showed that a light scalloping actually improves the turbine.

The thesis has focused on how scalloping influences the expected performance degradation and how the flow field looks like inside the turbine. Scalloping a radial turbine is actually a quite simple design method; it is done by simply removing material between the blades. This action decreases the inertia and may introduce disturbances in the flow field with a result of decreased efficiency. Even if scalloping is a quite common method, there are few previous studies. All of them have shown that a scalloped turbine suffers a decrease in efficiency. However, the studies are quite old and Volvo Powertrain AB has been curious of study scalloping with advanced computer program in an earlier stage of the design process instead of developing and manufacturing expensive prototypes. Therefore, a turbine was selected by Volvo Powertrain AB, and scalloped in five different designs. The result showed that a light scalloped turbine, Figure 1: example B, was equal in efficiency compared to the original turbine but had an improved acceleration. This result is unique and shows that scalloping does not always decreases the performance as previous studies has stated. This result gives Volvo and other turbocharger designers a foundation of proof to consider a scalloped turbine for their next prototype.

Figure 1: Shows the original turbine and two examples of scalloped turbines. Scalloping is introducing losses due to different pressures on each side of the blades in the turbine.
One other mayor surprise and achievement of this study was that the original turbine could be improved. It was noticed when comparing the first results, that the efficiency of the light scalloped turbine was better compared to the original turbine. A deeper analyse of the flow field, showed that the original turbine was introduced to a disturbance at the leading edge. By making the edge smoother with a rounding, it was possible to improve the original turbine.

A simplification was needed to be done to actually manage to evaluate the scalloped turbines within the time limit of the thesis. Because a turbine feels a pulse flow from the engine, with different pressure and mass flow rate, when the piston moves up and down. The pulse flow was divided into 4 different points and each scalloped turbine were simulated and evaluated against each other with the efficiency.


Figure 2: The figure shows an example of how the colourful pictures can look like. In this figure, streamlines and section planes show vortices and leakage for one blade in the turbine.
Finally, something that was really interesting was that, the evaluation of the three-dimensional flow field in a radial turbine is not an easy task, it can simply not be done by using a pencil and a paper. Therefore, a powerful Computational Fluid Dynamics program was used and the calculations were computed on 200 computers to compromise the computational time from a proximally 48 hour to 1 hour. In the thesis there are a huge amount of colourful pictures that are calculated with the program STAR-CCM+, Figure 2 shows an example of one of the scalloped turbine. If this looks interesting, there are more to read about it in the thesis and maybe you decide to study more about the huge world of turbines and Computational Fluid Dynamics.

• author: Jacobsson, Anders
• supervisor: Magnus Genrup
• organization: Department of Energy Sciences
• course: MVK920 20161
• year: 2016
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• keywords: Radial turbine, scallop, inertia, turbocharger, STAR-CCM+, CFD
• report number: LUTMDN/TMHP-16/5378-SE
• ISSN: 0282-1990
• language: English
• id: 8891981
• date added to LUP: 2016-10-31 08:26:22
• date last changed: 2016-10-31 08:26:22