Numerical Investigation of the Fundamental Mechanism for Entropy Noise Generation in Aero-Engines

Abstract

In the present work, the generation and propagation of entropynoise is computed using acompressible URANS approach in combination with appropriate acoustic boundary conditions. The EntropyW aveG enerator (EWG) experiment is best suited for validating the proposed approach and for investigating the acoustic sources of entropyn oise. The EWG involves an on-reactive tube flow, where entropym odes are imposed to an incoming air flowbyusing aheating module. The generated temperature non-uniformities are accelerated with the mean flowdownstream in aconvergent divergent nozzle, thus producing entropynoise. Simulation results of pressure fluctuations and their spectra for adefined standard configuration as well as for different operating points of the EWG agree very well with the respective experimental data. Additionally,a na nalysis of the acoustic sources wasperformed. Forthis purpose the acoustic sources caused by the acceleration of density inhomogeneities were calculated. Fort he first time, an umerical method is introduced for the localization of the acoustic sources of entropynoise in acceleration/deceleration regions. PACS no. 43.28.Kt, 43.28.Py 470 ©S.Hirzel Verlag · EAA Mühlbauer et al.:N umerical investigation of entropynoise ACTA ACUSTICA UNITED WITH ACUSTICA Vol. 95 (2009)

Acoustic and Entropy Waves in Nozzles in Combustion Noise Framework

“…[16,33]). For an axisymmetric configuration, Leyko et al [16] using the compact nozzle approximation of Marble & Candel [6] and considering reflection from EWG rig outlet found excellent agreement between their analytical solution and experimental measurements.…”

On the thermo-acoustic Fant equation

“…We begin writing the momentum equation 2.9 in subscript form 25) whereσ ij is the viscous stress tensor, σ ij = −pδ ij +σ ij , and δ ij is the Kronecker delta function (= 1 for i = j and 0 for i = j).σ ij is the antisymmetric part of the velocity gradient and vanishes for the case of rigid motion, where there is no deformation. For a Newtonian fluid, 27) where π ij = ρv i v j + pδ ij −σ ij is the momentum flux tensor.…”

“…where 25) which is the Green's function corresponding to a source located at the flame-holder with the constriction assumed to be closed. The implied Green's function conditions above, G → α −¯ , α respectively as y 1 → ∓0, yield consistency conditions which can be used to determine the following functional forms of α and β:…”

“…There is also a secondary 'indirect' combustion dipole source producing 'entropy noise' and is due to unevenly heated combustion products accelerating unevenly within the mean flow [11][12][13][14][15][16][17][18][19][20][21]. Although interest in this type of source has recently revived [22][23][24][25][26][27][28][29][30], understanding factors governing feedback on the direct monopole flame source (i.e. aerodynamics of the oscillating mean flow, structural vibrations, flame flashback, flame-vortex interactions, burn rate fluctuations produced by flame-area oscillation, saturation of nonlinear heat release rates, etc) are probably more important for the control of the system instabilities [31][32][33][34][35][36][37].…”

A nonlinear model for indirect combustion noise through a compact nozzle

“…The acoustic levels were measured in the downstream section for variable operating conditions, varying from subsonic to supersonic diffusers with the presence of a shock. The shocked configuration was investigated numerically and analytically by Leyko et al (2011) using large eddy simulation (LES) and Mühlbauer, Noll & Aigner (2009) with an unsteady Reynoldsaveraged Navier-Stokes (URANS) approach. The subsonic configuration has been reproduced numerically by Mühlbauer et al (2009), Giauque et al (2012) and Durán, Moreau & Poinsot (2013b) and has also been investigated analytically by Howe (2010).…”