The off-highway industry is facing many of the same challenges as the commercial vehicle and automotive industries, which include drives to reduce fuel consumption, production costs, and design time, as well as a shortage of skilled designers. Each component on a vehicle plays a critical role in addressing the challenges listed.
As TDT sees it, there are basically two approaches to the aerodynamic design of turbomachinery: direct design and inverse design. In the direct method the blade geometry is specified and the resulting flow field calculated via CFD. The designer evaluates the analysis and determines whether the design meets requirements: pressure rise, flow, and efficiency. If the design does not meet conditions, the designer modifies the geometry to improve the performance.
This process requires extensive experience to know what impact geometry modifications have on performance and may require a large number of iterations to meet the basic design requirements of flow and pressure rise. Then, the designer has to spend additional time trying to improve the efficiency.
Experienced designers can achieve good performance. However, they tend to stay within their comfort zones, and, as a result, this can limit the design space, making it difficult to meet today’s challenges for better efficiency or broader operating range.
In many turbocharger or torque converter applications it is becoming more difficult to achieve better performance improvement based on conventional or the direct design approach.
In the inverse design method, the desired flow field (via blade loading) is specified along with the total work required, and the geometry that produces that flow field is generated. Specifying the blade loading gives direct control over the 3-D pressure and velocity distributions, which allow for direct control of the 3-D flow field.
Thus, there is an intuitive connection between the design input and the resulting performance. This allows the designer to explore a large part of the design space and hence arrive at breakthrough designs.
Consider a compressor designed by conventional design methods by a leading heavy-duty diesel engine turbocharger manufacturer. Despite continuous attempts to improve the performance of this compressor, no improvements had been achieved in more than 10 years. By using the inverse design approach, a new impeller was designed. Test data showed 2.5- to 3.0-point improvement in efficiency over most of the map. This improvement in performance was particularly significant given the failure of all attempts by direct design to improve the compressor's efficiency.
The resulting geometry has 3-D blades and had to be manufactured by a point milling process. This, in fact, did not pose a huge problem, even in relatively high volume applications such as turbochargers. Many turbocharger manufacturers have begun using the point milling process as it provides better structural reliability as compared to the casting process.
Another interesting feature of the new impeller design obtained by the inverse design process was the fact that, despite its 3-D shape, the stresses in the impeller were comparable to the original design, and its vibration response was better than the baseline impeller.
Application of inverse design in other turbomachinery applications related to off-highway, such as torque converters, has also resulted in significant improvement in efficiency. The inverse design approach can provide a significant improvement in performance as well as reduction in design and development times.
The other advantage is that new designers do not have to have years of experience to use the inverse design approach. A fundamental understanding of the fluid flow allows them to start designing higher performing turbomachinery in a shorter amount of time.
Mehrdad Zangeneh, Ph.D., Founder and Managing Director, Turbodesign Technology Inc., wrote this article for SAE Off-Highway Engineering.