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Project Problem To understand the fundamental flame-acoustic coupling phenomena of a single burning droplet, a test facility is needed to study the effects in a prescribed acoustic field. Acoustic waveguides are commonly used devices to generate these acoustic wave conditions. These devices are sized based upon the desired frequency range in mind, which is 500 – 2000 Hz for this application. With this information, the expected acoustic field for both standing and traveling wave forcing can be determined. Approach The geometry of the waveguide is rectangular with an internal length of Lx = 915 mm and a volume of V = .016 m3, which has two sets of speakers on each end to create the desired acoustic conditions, as well as an optical viewing port for high-speed combustion diagnostics of the burning droplet. Under this summer RCEU program, these parts were manufactured using UAH’s CNC machine out of ¼” stainless steel plates. The end plates have threaded holes for the speakers and a series of pressure ports to measure the oscillatory pressure local to the test section. The waveguide’s modular design allows it to be scaled, enabling other forcing conditions to be explored in future studies. For example, the end plates may have a variation to add another speaker, or alternative length sections can be installed to alter the resonant characteristics of the waveguide. Additionally, a signal conditioning system is needed to process the pressure readings from a pair of Kulite XCE-IC-093-5G pressure transducers. The pressure transducers produce an output signal in the range of millivolts. However, the data acquisition unit requires the input to be in volts. Therefore, an amplifier circuit was designed and prototyped using an AD620ANZ operational amplifier accompanied by a voltage inverter. Open-source software for the development and design of printed circuit boards (PCB’s) was utilized for circuit design and integrated-circuit footprint layout. Results Using acoustic theory to determine the resonant condition for a container closed at both ends (i.e., f = nc/2Lx), the waveguide was designed to produce the resonant conditions withing the desired range for the experiments (i.e., f = 866 Hz for n = 1, f = 1732 Hz for n = 2, etc.). Subsequently, the acoustic field for both standing and travelling wave forcing was characterized at the fundamental resonant frequency of the device using acoustic theory. Conclusion With the newly built acoustic waveguide, experiments for acoustically-coupled droplet combustion can be conducted. The next steps are to modify the waveguide to include the needed features/hardware for this experiment. This includes a syringe-pump fuel injection system and implementing high-speed combustion diagnostics (e.g., OH* chemiluminescence). The results will be analyzed using advanced image processing at frame-rating on the order of ~100 kfps. Through these methods, further understanding of condensed phase combustion will be achieved.
Research and Creative Experience for Undergraduates (RCEU)
Mechanical and Aerospace Engineering
College of Engineering
Acoustics, Waveguide, flame-acoustic coupling, acoustic waves, acoustic field, standing wave, traveling wave, standing wave forcing, traveling wave forcing
Wilson, Hunter, "Development of a Waveguide to Investigate Acoustically-Forced Droplet Combustion" (2023). Summer Community of Scholars Posters (RCEU and HCR Combined Programs). 442.