An aeroacoustic study of industrial gas turbine intake and exhaust systems.

Abstract

The purpose of this study was to seek a means of numerically simulating and analyzing the aerodynamic and acoustic behaviour of industrial gas turbine intake or exhaust systems. The method was developed in an effort to advance the current state of the art employed for the prediction of intake and exhaust noise. Current methods rely upon the summation of experimentally gathered insertion loss data for individual system components. This approach requires that physical testing of chosen components be undertaken which can be both costly and time consuming. The proposed numerical method is based upon a two stage finite volume solution of the two dimensional non linear Euler equations whereby an aerodynamic solution for the mean flow is first obtained followed by a solution for the acoustic field. The method has been found to be both accurate and easy to implement, furthermore being numerically based it eliminates the need for the costly and time consuming procurement and testing of physical prototypes. In this respect it is considered to advance the present state of the art. The two step Euler solver developed herein employs a multiblock finite volume formulation wherein the hyperbolic Euler equations are solved using a cell centred finite volume technique employing a second order central differencing method for spatial discretisation on a multiblock grid and with temporal integration being undertaken using an explicit time marching two stage second order Runge Kutta scheme. In the first step the mean flow is solved using the finite volume method to obtain a transient or pseudo steady state solution. This provides both the general aerodynamic characteristics of the system and a baseline steady state solution for the acoustic analysis. In the second step a perturbation representing an acoustic disturbance is introduced to the mean flow and the non linear Euler equations are again solved using the finite volume scheme. Spurious non physical numerical reflections at the domain boundaries are prevented by the application of the characteristic based non reflecting boundary conditions. The resulting unsteady pressure field is then post processed to generate the unsteady pressure disturbance due to the applied acoustic perturbation. The interaction of the acoustic disturbance can then be evaluated by examination of the pressure amplitudes and frequency spectra at any location within the domain. The scheme was applied to the study of the aerodynamic and acoustic characteristics of an industrial gas turbine emergency shutdown compressor bleed system. It was found to accurately predict the acoustic attenuation characteristics of the system, furthermore it was shown that it could also be usefully applied as a design tool and as an example of this a change of attenuation performance of the bleed system is demonstrated by a simple relocation of the baffle silencer within the system ducting. It is concluded that the method developed herein demonstrates that a numerical solution to predict the acoustic attenuation characteristics of an industrial gas turbine silencing system is possible and as such offers an improved means for undertaking the acoustic design of products for noise control in industrial gas turbine applications. In this respect it is considered to advance the present state of the art.