A Large Design Space Multidisciplinary Optimization of a Mixed Flow Micro Gas Turbine Compressor Stage

A novel method of parametrization and determining of parameter influence for a large design space optimization of a micro gas turbine compressor stage is presented. A compact, high efficiency, compressor for use in a 600 N thrust UAV engine is developed. The compressor consists of a mixed flow impeller and a continuous vane diffuser that negates the need for dedicated de-swirling vanes. 48 free parameters were used to control the meridional channel, blade camber, and structural geometric features. The optimization focused on determining the optimal impeller meridional discharge (mixed flow) angle, for a predetermined set of constraints. The number of parameters required to achieve a large geometric diversity was greatly reduced by coupling the endwall Bézier control points with user-defined functions that modified multiple control points with a single parameter as input (such as mixed flow angle).

The influence of key geometric features on design performance was assessed using a Pearson correlation coefficient map. From the correlation map, it was observed that stage total-to-static pressure ratio, and efficiency, were strongly influenced by diffuser outlet passage height and diffuser vane wrap angle. This was due these parameters’ control of flow separation magnitude at the diffuser hub in the radial-to-axial bend. The proposed method of assessing parameter influence was found to be very helpful in the free parameter selection process due to the ease with which a large number of parameters can be compared by their performance influence.

A multidisciplinary workflow was scripted to incorporate the CalculiX CrunchiX structural analysis into the NUMECA FINE^TM/Design3D aerodynamic optimization package. A multipoint and multiobjective optimization was performed using the design of experiments method, which was accelerated by using a surrogate model that was constructed using the MINAMO tool to increase design convergence rate. The impeller and diffuser blade rows were simultaneously optimized due to the strong influence of impeller wake on diffuser leading edge design. Structural feasibility constraints were placed on maximum von Mises stress, blade tip displacement, and the resonance frequencies of the impeller. A three-dimensional Pareto front was constructed to assist in the selection of the final design. The final design achieved a total-to-static pressure ratio of 4,15 and efficiency of 86,24%, at a design mass flow rate of 1,089 kg/s. Choke and stall margins of 7,4% and 11,8% were achieved at the design speed of 73 000 RPM. The continuous vane diffuser successfully achieved a high diffusion rate, which allowed for a compact compressor stage of 180 mm maximum diameter and 80 mm axial length, as desirable for aircraft propulsion.