Unsteady Aerodynamics and Forced Response Studies on Aeroderivative Gas Turbine Exhaust System

Abstract

Industrial and aeroderivative gas turbines use exhaust systems for flow diffusion and pressure recovery. These processes result in a three-dimensional, unsteady, turbulent, and complex flow in the exhaust diffusers. The downstream balance-of-plant systems such as heat recovery steam generators or selective catalytic systems require, in general, a steady, uniform flow out of the exhaust system. Aeroderivative gas turbines for power generation application have a wide operational envelope. Even though the exhaust systems are designed for 70% load to 110% load, its performance is significantly altered at low power operations. Application of gas turbines at low power can increase exhaust diffuser vibrations because of diffuser flow separations and wakes from the last stage of the power turbine. Aerodynamic excitations which result in excessive structural vibration can cause the units to trip and the power plant to stop, resulting in customer revenue loss. The primary motivation for this research is to investigate an aerodynamic mechanism to ensure reliable operation of the exhaust system by identifying the regimes where aerodynamic instabilities can occur. In-house and university supported initiative to predict unsteady aerodynamics at low power conditions shows the presence of turbulent and time dependent flow. The frequency spectrum results are discussed for low power and high power gas turbine operating conditions. The numerical predictions are in good agreement with test results.