Compositional inhomogeneities as a source of indirect combustion noise

Abstract

The generation of indirect combustion noise by compositional inhomogeneities is examined theoretically. For this, the compact nozzle theory of~\cite{MARBLE_CANDEL_JSV1977} is extended to a multi-component gas mixture, and the chemical potential function is introduced as an additional acoustic source mechanism. Transfer functions for subcritical and supercritical nozzle flows are derived and the contribution of compositional noise is compared to entropy noise and direct noise by considering an idealized nozzle downstream of the combustor exit. It is shown that compositional noise is dependent on the local mixture composition and can exceed entropy noise for fuel-lean conditions and supercritical nozzle flows. This suggests that the compositional indirect noise requires potential consideration with the implementation of low-emission combustors.Comment: 11 pages, 3 figure

On indirect noise in multicomponent nozzle flows

“…First, the thermodynamic properties are evaluated from a one-dimensional counterflow-diffusion flame calculation, which is the basis of flamelet models in turbulent combustion (e.g., Williams 1985). This is to characterize the compositional inhomogeneity exiting the combustion chamber upstream of the nozzle (Magri et al 2016). Secondly, the strengths of the acoustic sources Ψ ‫א,‬ (2.8) andΦ (2.11) are evaluated.…”

“…The unsteady interaction between the specific entropy perturbation, s ′ , the compositional perturbation Z ′ and the mean-flow gradient, dũ/dη, is the source of indirect noise. Physically, not only do density variations create noise through entropy mechanisms, but also differences in species generate noise through the chemical potential (Magri et al 2016) and heat-capacity variation. Such a sound generation is caused by the tendency of the compositional blob to contract/expand with a different rate than the surrounding fluid.…”

“…which generalizes the compact-nozzle choking condition of Magri et al (2016) including changes in the specific-heat capacities.…”

“…This was theoretically shown in compact subsonic nozzles (Ihme 2017) and at higher Mach-number regimes with/without shock waves (Magri et al 2016). By evaluating the nozzle-transfer functions with algebraic expressions (Magri et al 2016) and numerical integration of the differential equations (Magri et al 2017) of a kerosene mixture, it was shown that compositional noise can be a contributor to indirect noise in lean mixtures and supersonic regimes. Compositional noise was experimentally shown by Rolland et al (2017) as applied to air-helium compositional blobs accelerated through compact-choked nozzles.…”

Combustion‐Generated Noise: An Environment‐Related Issue for Future Combustion Systems

“…[3,5,6] Thec urrent work is focused on predicting the direct combustion noise emitted from unconfined,p remixed flames.I nl ow-Mach number (Ma) combustion flows,t he fluctuation in heat release,c aused by the interaction of the chemical reactions and the turbulent flow, represents the main noise-generating source. [7,8] As ac onsequence,c ombustion noise correlates strongly with turbulent fluctuations and is broadband in the spectral domain.…”