Combustion noise

In the combustor, the only two parameters that can be measured are fuel pressure and evenness of combustion noise. Turbine inlet temperatures are not usually measured due to very high temperatures and limited probe life. Table 19-6 shows the effect of various parameters on important functions of the combustor. 

Flue gas ductwork will be insulated, which helps to reduce any transmitted combustion noise. Where large rectangular flue ductwork is used, it must be adequately stiffened and insulated to reduce any risk of reverberation being set up from the combustion system. 

Flame dynamics is intimately related to combustion instability and noise radiation. In this chapter, relationships between these different processes are described by making use of systematic experiments in which laminar flames respond to incident perturbations. The response to incoming disturbances is examined and expressions of the radiated pressure are compared with the measurements of heat release rate in the flame. The data indicate that flame dynamics determines the radiation of sound from flames. Links between combustion noise and combustion instabilities are drawn on this basis. These two aspects, usually treated separately, appear as manifestations of the same dynamical process. 

These data indicate that thermal losses during unsteady flame-wall interactions constitute an intense source of combustion noise. This is exemplified in other cases where extinctions result from large coherent structures impacting on solid boundaries, or when a turbulent flame is stabilized close to a wall and impinges on the boundary. However, in many cases, the flame is stabilized away from the boundaries and this mechanism may not be operational. 

W.C. Strahle. Combustion noise. Prog. Energ. Combust. Sci., 4 157-176,1978. 

B.N. Shivashankara, W.C. Strahle, and J.C. Handley. Evaluation of combustion noise scaling laws by an optical technique. AIAA., 13 623-627,1975. 

In Chapter 5.2, S. Candel, D. Durox, and T. Schuller consider certain aspects of perturbed flame dynamics. The relation between combustion instability and noise generation is described by reference to systematic experiments. The data indicate that acoustic emission is determined by flame dynamics. On this basis, combustion noise can be linked with combustion instability. 

A. A. Putnam and D. J. Brown, Combustion Noise Problems and Potentials, in Combustion Technology Some Modern Developments, H. B. Palmer and J. M. Beer, eds.. New York Academic Press, 1974, 127-162. 

Song, X.-R. C., Experimental study of combustion noise generated by oxygen-fuel burners, in Proceedings of the 1993 National Conference on Noise Control Engineering Noise Control in Aeroacoustics, Williamsburg, VA, 1993, 97-102. 

The shorter the ignition delay period, the higher the cetane number of the fuel and the smaller the amount of fuel in the combustion chamber when the fuel ignites. Consequently, high-cetane-number fuels generally cause lower rates of pressure rise and lower peak pressures, both of which tend to lessen combustion noise and to permit improved control of combustion, resulting in increased engine efficiency and power output. 

The time between the beginning of fuel injection and the start of combustion is called ignition delay . As stated earlier, higher cetane number fuels result in shorter ignition delays, providing improved combustion, lower combustion noise, easier cold starting, faster warm-up, less smoke, and in many engines, reduction of... 

Cetane number improvers or diesel ignition improvers reduce combustion noise and smoke. The magnitude of the benefit varies among engine designs and operating modes, ranging from no effect to readily perceptible improvement. 

The destabilization (mixing) of a jet flow is desirable in many applications, including combustion, noise suppression at jet engine exhaust, infrared signature reduction of engine exhaust for platform stealth, and employment of a lighter and cheaper material for the lift flap such as that used in C-17 aircraft. 

Additional results showing radial variation and Re number dependence allow one to conclude that the combustion noise source is distributed over a wider spatial extent than for the air jet and the difference between reacting and nonreacting SPL decreases with increasing Re number. Further spectral analysis of these results is underway, together with similar measurements in swirling and partially premixed flames and combustors. 

There are several strategies that can be used to reduce combustion noise (see Bussman and Jayakaran [26] and Baukal [27] for more details). One strategy is either to move the source of the undesirable sound away from the people or to move the people away from the sound. However, this may not be practical for many industrial applications. Another strategy is to put some type of... 

Calculation of the Typical Combustion Noise Spectrum of a Stable Burning Flare from the Overall Sound Pressure Level OASPL... 

Another problem with using the TAB concept to predict combustion noise is that it does not take the frequency characteristics of the noise emissions into account. Frequency characteristics are very important for the design of adequate noise control measures and will also determine the human perception of the noise emissions since the ear s sensitivity for noise is frequency-dependent (see Section 8.1). 

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