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Upon review of past experimental results and theoretical efforts it is apparent that the mechanism by which combustion noise is generated is not well understood. A theory of combustion noise is developed in this paper which follows rigorously from the principles of fluid mechanics. Lighthill's approach, used in his studies of aerodynamic noise, is closely followed in the present work. The sound radiated from open, turbulent flames is found to depend strongly upon the structure of such flames; at present their structure is not well known. However, meaningful bounds and scaling rules for the sound power output and spectral content are derived based upon the present limited knowledge. A framework is developed which explains past experimental work and the origin of combustion noise.

Tn order to estimate the scaling rules for combustion noise, a tu -rb lence autocorrelation function of the reaction rate must be estimated. This depends in part upon the active reaction volume in a turbulent flame. This volume may be measured by direct flame photography centering on the radiation of an active radical in the reaction zone. The paper describes experiments on fuel-lean propane, propylene, and ethylene-air turbulent open premixed flames with burners of 0.4" -0.96" diameter and velocities to 600 fps. The flame volume is measured from photographs of the flame centered on the CH emission. The volume is correlated to the flow velocity, burner diameter, laminar flame speed and fuel mass fraction in the form of a power law by a regression technique. The scaling rule obtained from experiments is shown to follow the law V cc UD 3 f 3 /s L . The scaling laws generated are compared with estimates previously drawn from Strahle's theory of combustion noise. Implications concerning the turbulence structure in the reaction zone are drawn from the correlations. Refraction, Convection and Diffusion Flame Effects in Combustion Generated Noise

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This paper deals with noise sources which are centra! to the problem of core engine noise in turbopropulsion systems. The sources dealt with are entropy noise and direct combustion noise, as well as a nonpropagating psuedosound which is hydrodynaniic noise. It is shown analytically and experimentally that a transition can occur from a combustion noise-dominant situation to an entropy noise-dominant case if the contraction of a terminating nozzle to the combustor is high enough. In the conibustor tested, entropy noise is the dominant source for propagationai noise if the combustor is choked at the exit. It is also speculated that there might be another unexplored noise source interior of the combustor. Analysis techniques include spectral, cross spectral, correlation, and ordinary and partial coherence analysis. Measurements include exterior and interior fluctuating and mean pressures and temperatures.

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