A MODEL FOR NUMERICAL SIMULATION OF VARIABLE STATOR AXIAL FLOW COMPRESSORS Jesuino Takachi Tomita ITA - Instituto Tecnológico de Aeronáutica Pr. Marechal Eduardo Gomes, 50 - Vl. das Acácias 12228-900 - SJCampos jtakachi@mec.ita.cta.br João Roberto Barbosa ITA - Instituto Tecnológico de Aeronáutica Pr. Marechal Eduardo Gomes, 50 - Vl. das Acácias 12228-900 - SJCampos barbosa@mec.ita.cta.br Abstract. Axial flow compressors are usually found in gas turbines, both for aero and industrial applications due to their higher flow capacity, pressure ratio and efficiency when compared to the centrifugal counterpart. Due to the wide range of ambient conditions at which the gas turbines are required to work, compressor designers have to foresee its performance capability and possible points of malfunction. To explore all points of actual compressor operation it is required extensively and time-consuming tests, in addition to high costs. To cut time and costs, numerical simulations have been employed extensively, through simple to more complex mathematical models. Capability to predict the compressor map is required at the beginning, since is the map characteristics an indication of the success of the designed compressor. At design point compressor efficiency is optimized due to the alignment of flow with blade passage areas. At off-design point the flow misalignment at the various rows causes losses to increase sharply, therefore decreasing pressure ratio and efficiency. To bring the flow to alignment with the blade passages it is required to re-stagger the blades. To avoid mechanical complexities it is generally accepted to re-stagger only the stators. This work deals with a numerical approach to the simulation of an axial flow compressor equipped with variable stators. Improvement to the compressor performance is demonstrated with calculated compressor maps for different blade staggers, using a specially developed computer program to simulate virtually any axial flow compressor having variable stators. Keywords. Compressor, Axial-flow Compressor, Gas Turbine, Variable Geometry, Performance improvement, Part-load. 1. Introduction An axial compressor is usually made of many stages each one composed by a rotor and a stator (rotating cascade and fixed cascade, respectively - Figure 1). The air properties at the stage outlet are calculated from the air properties at the stage inlet. Losses are calculated from cascade data (Abbott - 1959) or from data obtained from rig-tested compressors, for fixed geometry (Miller - 1971; Casey - 1987; Carter - 1948/1949; Barbosa - 1987/2001, NASA SP -36 - 1956; Saravanammuttoo - 2001; Mattingly - 1996) and for variable geometry (Walsh - 1998; Bobula - 1983; Serovy - 1968). Rotor Stator Rotor and stator Figure 1: Examples or rotor and stator blade rows of a 9-stage axial flow compressor. The compressor is the component that most strongly influences the gas turbine performance, either for the peculiar characteristic of operation instability or for the high consumption of energy during the air compression Flow properties at the stage outlet are calculated combining a rotor with a stator, thus whole compressor characteristics are calculated stacking the compressor stages: the inlet conditions of a previous stage are obtained from the outlet conditions of the previous stage. Friction, shock waves, secondary flow and levels of velocities (Mach numbers), influence the stage performance most. .The high performance compressors, those that promote a high compression per stage, require a flow with high velocity in the blade channels. The flow velocity is associated with energy losses when the flow direction does not match the blade channels direction. Thus, at off-design operation the compressor efficiency can vary significantly, causing high losses and deterioration of the gas turbine performance as result and to decrease the losses in the compressor operation, use is made of variable geometry when the position of the stators (angles) is altered during