Turbulent combustion processes are inherently unsteady and, thus, a source of acoustic radiation. While prior studies have extensively characterized their total sound power, their spectral characteristics are not well understood. This work investigates these acoustic spectral features, including the flame's low- and high-frequency characteristics and the scaling of the frequency of peak acoustic emissions. The spatiotemporal characteristics of the flame's chemiluminescence emissions, used as a marker of heat release fluctuations, were measured and used to determine the heat release spectrum, spatial distribution and spatial coherence characteristics. These heat release characteristics were then used as inputs to an integral solution of the wave equation and compared to measured acoustic spectra obtained over a range of conditions and burners and at several spatial locations. The spectral characteristics of the flame's acoustic emissions are controlled by two processes: the underlying spectrum of heat release fluctuations that are ultimately the combustion noise source, and the transfer function relating these heat release and acoustic fluctuations. An important result from this work is the clarification of the relative roles of these two processes in controlling the shape of the acoustic spectrum. This transfer function is primarily controlled by the spatiotemporal coherence characteristics of the heat release fluctuations which are, in turn, strongly influenced by burner configuration/geometry and operating conditions. Low-frequency acoustic emissions are controlled by the heat release spectrum essentially independent of flame geometry. Both the heat release spectrum and heat release-acoustic transfer function are important at intermediate and high frequencies. An important feature of the investigated geometry that controls the heat release phase dynamics is the high-velocity flow relative to the flame speed and anchored character of the flame. This leads to convection of flame sheet disturbances (i.e. heat release fluctuations) along the front that dominates the high frequency and peak frequency scaling of the flame's acoustic emissions.

Acoustic radiation from turbulent premixed flames

Turbulent premixed combustion processes are inherently unsteady and, thus, a source of acoustic radiation. While prior studies have extensively characterized the total sound power radiated by turbulent flames, few measurements of their spectral characteristics exist. This article presents systematic measurements of the acoustic spectra over a 100 Hz-30 kHz frequency range from a turbulent, natural gas-fueled flame. Measurements were made in an anechoic environment at several burner diameters, flow velocities, and equivalence ratios. The results show that, for a fixed burner diameter, the turbulent flame's acoustic spectrum has a nearly universal shape whose total power is a function of equivalence ratio and flow velocity. The functional dependence of the power spectra on frequency does vary with burner diameter, however, as the frequency of peak acoustic emissions generally shifts to lower frequencies with increases in burner diameter. These trends are quite similar to those exhibited by the cold flow vel...

Parametric studies of acoustic radiation from premixed flames

The metamaterial under investigation here consists of a periodic arrangement of unit plates in a grid-like frame such that there is a contrast in the local areal mass between cell interior and cell wall. In the low frequency range and under normal incidence this metamaterial panel exhibits a sound transmission loss significantly larger than the transmission loss of an unstructured panel with the same homogeneous mass per unit area. However, when the incident sound field is diffuse, the relative advantage of the metamaterial barrier is reduced or eliminated. A sequence of experiments is documented to demonstrate that the relative advantage of the metamaterial barrier can be realized even in a diffuse sound field by creating a hybrid barrier system which embeds the metamaterial layer between a normalizing waveguide layer on the incident side and an absorbing layer on the transmitted side. The sound normalizing waveguide layer is a lattice structure, and the absorbing layer is high performance glass fiber mat. By using measurements of the transmission loss of a 1.2 m square panel system the role of each of these components is demonstrated.

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Experiments on the low frequency barrier characteristics of cellular metamaterial panels in a diffuse sound field.

Turbulent premixed combustion processes are inherently unsteady and, thus, a source of acoustic radiation. While prior studies have extensively characterized the total sound power radiated by turbulent flames, few measurements exist of their spectral characteristics. This paper reports the results of measurements obtained in an anechoic environment with varying approach flow characteristics (i.e., turbulence intensity and mean flow profile), flow velocity and equivalence ratio. This and a prior study show that for a fixed burner diameter, the acoustic spectra of a turbulent premixed flame has a nearly universal shape whose total power is a function of equivalence ratio, approach flow turbulence intensity, or flow velocity. The functional dependence of the power spectra upon frequency does vary with burner diameter, however, as the frequency of peak acoustic emissions shifts to lower frequencies with increases in burner diameter. This behavior was previously attributed to the analogous change in spectra in the velocity fluctuations, and it was suggested that the spectrum of acoustic temporal fluctuations, P(f), is related through some frequency-localized transfer function to that of the velocity fluctuations, U(f); i.e., P(f)/U(f)=H(f). The present study modifies this conclusion by showing that the acoustic spectra does not change with corresponding changes in the temporal velocity spectra (for a fixed burner diameter).

http://www.researchgate.net/profile/Tim_Lieuwen/publication/228718000_Effect_of_approach_flow_turbulence_characteristics_on_sound_generation_from_premixed_flames/links/53ea10ff0cf28f342f417af8.pdf

Effect of Approach Flow Turbulence Characteristics on Sound Generation from Premixed Flames

Turbulent premixed combustion processes are inherently unsteady and, thus, a source of acoustic radiation. The objective of this paper is to examine the basic scaling of the flame’s acoustic emission spectra and establish its dependence upon geometric and flow variables using a piloted Bunsen burner. The results show that the flame's acoustic spectrum has a nearly universal shape that can be character ized by four parameters: the total acoustic power, OASPL, the frequency of peak emissions, fpeak , and the power law scaling of the spectra in the low ( f fpeak ) frequency regime. The frequency of peak emissions scales with Strouhal number , St = fpeak . L f/U ave , where Lf and Uave denote the flame length and axial flow velocity, respectively. The OASPL of combustion noise scales as fpeak . Q, where Q denotes the average heat release from the flame. The high frequency acoustic emissions sca les as f

H.G. Leventhall

Papers

Noise sources in turbulent gaseous premixed flames

Household appliance noise

The acoustics of domestic rooms

Calculation of the perceived level of noise (PLdB) using Stevens' method (Mark VII)

The noise of natural gas burners