Nowadays, transonic axial flow compressors are very common for aircraft engines in order to obtain maximum pressure ratios per single-stage, which will lead to engine weight and size reduction and therefore less operational costs. Although the performance of these compressors is already high, a further increment in efficiency can result in huge savings in fuel costs and determine a key factor for product success. Therefore, the manufacturers put a lot of effort towards this aspect, while trying to broaden the operating range of the compressors at the same time.
The creation of shocks, strong secondary flows and other phenomena increases the complexity of the flow field inside a transonic compressor and challenges the designers who need to face many negative flow characteristics such as, high energy losses, efficiency decrease, flow blockage, separation and many more. As the compressor operates from peak to near-stall, the blade loading increases and flow structures become stronger and unsteady. Despite the presence of such flow unsteadiness, the compressor can still operate in a stable mode. Rotating stall arises when the loading is further increased, i.e. at a condition of lower mass flow rate. There are several possible techniques to limit the negative effect of the flow features mentioned above. Here we will present only two. The first one is related to the blade shape generation, while the second one is linked to flow control techniques.
First, the designer should focus on the optimization of the blade geometry to maximize the aerodynamic performance not only at the design point but at the off-design operating conditions too. The development of optimization tools coupled with accurate CFD is helping towards that aspect. AxSTREAM® platform is using design of experiment (DoE) approach which significantly cutes down the computation time, while using meanline or streamline solvers for direct task simulations. However, creating an automatic closed loop optimization routing taking into account CFD is also possible using ION™ module and AxCFD™ tool. Such a design process is particularly successful in the field of transonic compressors, where performance is highly sensitive to little changes in airfoil design. The three-dimensional shape of the blade is of equal importance great importance with the two-dimensional section generation, especially in transonic compressor rotors where an optimization of shock structure and its interference with secondary flows is required.
Regarding the flow control mechanisms, one of the ideas to receive considerable attention in gas turbine applications is the flow injection and bleeding concept. On one hand, air injection near the blade tips helps increase the stall margin, leading to higher engine operability, while bleeding can be used to delay blade separation. Although this can increase the work per stage and consequently reduce the number of stages, using bleeding does not necessarily result in a proportional reduction of compressor weight due to additional system complexity. The best way to model the secondary flow path and explore how this would affect the overall performance of the compressor is by using AxSTREAM NET™. Data exchange between AxSTREAM NET™ and other software enables a tight integration useful to perform multi-run calculations as well as multi-disciplinary batch mode optimization tasks.
“State-of-Art of Transonic Axial Compressors”, DOI: 10.5772/25257