Many factors come into play in a successful turbomachinery design, but peak aerodynamic performance, maximum operating range, stress levels, and manufacturability are generally the dominant concerns. Depending on the application, other considerations such as acoustics, size, weight, and environmental issues can also come into the picture. The most effective designers take a holistic approach, which tries to incorporate all of these concerns at the beginning of the design process. One of the challenges facing aerodynamicists is a basic understanding of how these downstream issues can be accommodated in the initial design process. Typically, an aerodynamic engineer has minimal knowledge of manufacturing methods such as 5-axis machining. This paper will provide a basic understanding of the subsequent manufacturing impact of various decisions typically made during the preliminary and detailed aerodynamic design process. The paper will focus on radial turbomachinery, but much of the information provided is common for axial turbomachinery produced by 5-axis Computer Numerical Controlled (CNC) machining.
In the initial phase of the design, basic layout decisions generally dictate the overarching manufacturing process. For components produced with 5-axis machining, these would typically be: flank milled or point milled, open or covered impeller, and for covered impellers, integral or welded shroud. The relative costs of these processes are considered in this work along with first-order estimates of their typical impact on aerodynamic performance and stress levels. Once a general layout (and accompanying manufacturing process) is determined, other geometric parameters of the components drive secondary costs in the manufacturing process. The secondary parameters include main blade count, the presence of splitter blades, the blade length, thickness, curvatures, leading and trailing edge shapes, and fillet radii. The use of Computational Fluid Dynamics (CFD) and numerical optimization has driven even more choices into blade shapes, such as splitter blades with different shapes and offsets from the main blade, bowed blades, irregular blade patterns, and non-axisymmetric hub shapes. In many cases, the aerodynamicists and CFD analysts have pushed geometry further than manufacturing capabilities are ready to accept. These secondary costs, and the aerodynamic compromises needed to reduce them, are also considered here. This paper attempts to lay out the basic principles of cost-effective manufacturing, and how these can be considered as early in the design process as possible. Specific examples are considered, and quantitative information is provided which can help guide the designer from the beginning and avoid expensive reworks resulting from downstream revisions. This paper will provide a framework for collaboration between aerodynamic engineers and the manufacturing teams assigned to produce the parts they have designed.
