Compressor Variable Geometry Schedule Optimisation Using Genetic Algorithms

Abstract

Variable geometry blade rows in axial compressors are devised to fulfil different requirements. Main objectives include their role as a “part speed crutch” to push the front stages out of surge at low spool speeds, modulation of the power output in industrial machines — given the fact that the spool needs to run at synchronous speed with the electric generator frequency — and they can also be re-staggered to attain a modified capacity (usually upflowed) of the same baseline compressor. The operating schedule of the variable vanes is typically obtained from expensive and time consuming performance rig tests in which a large number of possible combinations are compared. In principle, the final choice is dictated by the pursuit of high efficiency at high rotational speeds and increased surge margin at low speeds where large excursions away from the design point are expected. The aim of this work is to integrate a validated genetic algorithm optimiser within an industry proprietary mean line compressor performance prediction code to maximise the machine efficiency while keeping an adequate user-defined value of the surge margin. In so doing, an optimised variable geometry schedule is derived, together with a modified range of rotational speeds for each given operating point. Nevertheless, aware of the detrimental consequences to the whole engine performance that the new arrangement can cause, the whole engine response for the new settings has been investigated. In this regard and to a first order, the working line on the compressor map is considered unaffected by the setting of the variable vanes and the effect of the spool speed variation on the turbine operation is accounted for by a reduction in turbine efficiency proportional to any fall in the shaft speed. Results for a state of the art eight stage compressor show a marked improvement for the coupled compressor-turbine efficiency particularly at low spool speeds for a sensible value of the surge margin. Free from the surge margin constraint the efficiency is further increased at the expense of a hindered compressor operational stability. The work is intended to continue with the incorporation of bleeds and power off take in the calculations for the sake of a greater applicability of the tool.