# Patent Analysis of the Development of Technologies Applied to the Combustion Process
## 3 Results and Discussion
If uncontrolled, these instabilities can cause undesirable consequences, such as component vibrations, increased heat-transfer rates, flame blow-off, and flashback. In the worst-case scenarios, these oscillations can result in system deterioration of the engine, leading to structural damage [27]. Over the years, different approaches  
Figure 1: Temporal analysis of patent documents. (**a**) Earliest priority year; (**b**) estimated expiration year. have been studied in order to reduce and/or control thermoacoustic instability during the combustion process [28; 29; 30; 31]. Modifying the geometry and configuration of combustion burners, for example, adjusting the fuel-supplying system, and adding acoustic damper devices, such as Helmholtz resonators, are widely used strategies in research [32; 33; 34]. Acoustic excitation is a passive control strategy employed to stabilize combustion-engine systems; it uses external excitations, such as a loudspeaker device, for acoustic forcing and fuel modulation to promote energy flow to the system and attenuate combustion oscillations, thus reducing instabilities [29; 33; 35; 36]. The first report on a flame undergoing an acoustic excitation was created by [PERSON] and [PERSON]. Their group investigated acoustic excitation in a turbulent jet flame for frequency ranging from 2 to 1300 Hz and an oscillating velocity amplitude of 0.13-0.89 relative to the bulk velocity provided by the burner. A notable change in flame structure was observed at a non-dimensional frequency of approximately 0.2 under resonant conditions. Thereafter, the flame was viewed under acoustic excitation, and a decrease in flame height and an increase in flow mixing were noticed upon resonant acoustic excitation of the burner pipe. These findings showed the possible enhancement of combustion performance when the acoustic excitation is used [35]. [PERSON] et al. [28] evaluated the effects of different control strategies on combustion instability by maintaining a loudspeaker interface for acoustic excitation and found that the non-linear controller could effectively suppress thermoacoustic instability. [PERSON] et al. [30] proposed a new approach to control the operation of a thermoacoustic Stirling electricity generator using an additional acoustic source connected through a feedback loop and demonstrated that the external forcing of thermoacoustic self-sustained oscillations impacted the performance of the engine and improved the efficiency of thermoelectric transduction. [PERSON] et al. experimentally evaluated the non-linear response of the low-swirl flame to acoustic excitation with large amplitude. They used an acoustic excitation system that includes an arbitrary function/wave generator, an amplifier and a loudspeaker set at the base of the test rig. The results demonstrated that vortices were formed and convected downstream along the flame approximately at bulk flow velocity, inducing flame rollup and shear layer instabilities, and also as the perturbation level increases, the high anti symmetric and helical modes are inhibited [36]. [PERSON] et al. investigated the behavior of a methane diffusion flame under acoustic excitation using simultaneous observations of Mie scattering and schlieren/shadowgraph imaging. They also evaluated the velocity field and temperature distributions in the flame were measured by particle image velocimetry (PIV). The acoustic excitation was provided by a loudspeaker with a diameter of 160 mm driven by a sinewave generator combined with an AC amplifier. The excitation frequency used was 100, 150, 200, and 300 Hz. The results indicated that acoustic excitation promoted changes in flame behavior, where the flame height, oscillation amplitude, and luminosity decreased as the acoustic excitation amplitude increased [37]. The use of acoustics to improve the combustion process is considered a recent development, and possibly for this reason, related patent filings are rare. However, the application of the technology is growing in the literature.