Acoustic radiation from turbulent premixed flames

Abstract

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.

Spatial correlation of heat release rate and sound emission from turbulent premixed flames

“…They found that the two-point correlations of heat release rate, 33 Ω, and the rate of change of fluctuating heat release rate, Ω 1 , can be 34 well represented by Gaussian-type functions commonly used in classical 35 turbulence, and that the length scale of the correlation volume, v cor , is 36 0.5δ L . This model was then shown to give a good agreement with recent 37 experimental measurements [20] of the far-field overall sound pressure level The construction of Ω 1 requires the rate of change of fluctuating heat 40 release rate, which was calculated in Refs. [5,6] indirectly by using a balance equation for a progress variable and taking the instantaneous reaction rate 42 to be a function of the progress variable.…”

“…This model was then shown to give a good agreement with recent 37 experimental measurements [20] of the far-field overall sound pressure level The construction of Ω 1 requires the rate of change of fluctuating heat 40 release rate, which was calculated in Refs. [5,6] indirectly by using a balance equation for a progress variable and taking the instantaneous reaction rate 42 to be a function of the progress variable. In this way the time derivative 43 is obtained using the spatial derivative of the progress variable field at one 44 single time step from the DNS data [16][17][18].…”

“…Refs. [5,6] Case I Case II Case III Figure 9: Comparison of OASPL between the experimental data [20], previous calculations [5,6] and current predictions from Cases I, II and III.…”

“…It has been recently noted [5,6] that the two-point 17 spatial correlation of the rate of change of fluctuating heat release rate, 18 and hence the associated correlation volume v cor and its length scale, is 19 crucial to predicting combustion noise. In previous studies, different length 20 scales have been suggested to estimate v cor empirically, ranging from the 21 laminar flame thickness δ L [12], turbulence integral length scale Λ [13] to the 22 turbulent flame-brush thickness [14,15] or a combination of them. However, 23 the two-point correlation of heat release rate has not received sufficient 24 attention primarily owing to the lack of availability of reliable numerical 25 or experimental data for the fluctuating heat release rate required to directly 26 investigate the correlation length scale.…”

“…Recently, Rajaram [20] and Rajaram & Lieuwen [42] reported combustion Table 4 will be used for the OASPL prediction To predict the far-field OASPL for the thirteen flames in Table 4, RANS 427 data of the mean reaction rate field,ẇ(y, t), is required for the volume 428 integral in Eq. (21).…”