Thermodynamics-Acoustics Coupling Studies on Self-Excited Combustion Oscillations Maximum Growth Rate

• Published in: Journal of thermal science Vol. 31; no. 5; pp. 1591 - 1603
• Main Author: Zhao, Dan
• Format: Journal Article
• Language: English
• Published: Berlin/Heidelberg Springer Berlin Heidelberg 01.09.2022; Springer Nature B.V
• Subjects: Acoustic coupling; Acoustic resonance; Acoustic waves; Acoustics; Boundary layers; Chemical reactions; Classical and Continuum Physics; Combustion chambers; Coupling; Damping; Electric vehicles; Engineering Fluid Dynamics; Engineering Thermodynamics; Heat; Heat and Mass Transfer; Heat release rate; Hydrocarbons; Kinetic energy; Limit cycle oscillations; Physics; Physics and Astronomy; Specific heat; Thermodynamics; Viscous drag; Waves; thermodynamics; chemical reaction; energy conversion; thermo-acoustics; hydrocarbon; acoustics
• ISSN: 1003-2169; 1993-033X
• DOI: 10.1007/s11630-020-1361-8

Unlike electric vehicles and electric aircrafts, hydrocarbon-fuelled (fossil) engine systems are much noisier. By conducting one-step chemical reaction-thermodynamics-acoustics coupling studies and experimental measurements, we explore the universal physics of how hydrocarbon-fuelled combustion is a noise maker. We also explain that how combustion-sustained noise at a particular frequency ω is intrinsically selected. These frequencies correspond to the acoustic resonance nature of the combustor. We find that a reacting gas in which the rate of chemical reacting increases with temperature is intrinsically and naturally unstable with respect to acoustic wave motion, since its modal growth rate α is greater than 0. Acoustic disturbances tend to exponentially i.e. exp( αt ) increase first and then are limited by nonlinear effects and finally grow into limit cycle oscillations. The growth rate α is found to increase first and then decrease with the gradient of the heat release rate with respect to the temperature change, i.e. heat capacity. The maximum ( α / ω ) max depends on the specific heat ratio γ , which is related to the speed of sound. The unstable nature could be changed by introducing some acoustic dissipative/damping mechanisms, such as the boundary layer viscous drag and boundary losses. It is shown that such losses could lead to increased critical heat capacity, below which stable combustors can be designed. Finally, the acoustical energy consisting of both potential and kinetic energy is found to grow exponentially faster by 100% than the acoustic disturbance amplitude.