在火箭推進系統中，若燃燒室中持續產生壓力擾動，則稱為燃燒不穩定(combustion instability)現象，其可能導致火箭熄火甚至造成推進系統損壞。為使液態火箭具穩定推進性能，本論文針對液態火箭燃燒室之燃燒不穩定現象之抑制機構進行分析，並利用聲學模擬實驗對燃燒室的幾何構型（直徑為5cm及6.4cm兩種，長度則分別為6.5cm、8cm及11.5cm）進行聲學特性探討。實驗改變燃燒室內部壓力、流體速度以及裝置工程抑制裝置等方式，分析其對於燃燒室共振頻率之影響，並利用聲學理論分析溫度之影響。實驗觀察顯示，燃燒室幾何構型及溫度為影響共振頻率之主要因素，而裝置噴嘴後所產生之共振頻率較未裝置噴嘴時複雜。實驗設定條件中燃燒室內部壓力(14.7psi~150psi)及流體速度(1.5m/s~4.5m/s)對於共振頻率影響較不明顯。在抑制裝置之研究方面，結果顯示裝置隔板可有效抑制切向共振頻率，且其高度越高共振頻率抑制效果越佳。赫姆霍茲共振器抑制效果較四分之一波長共振器佳，而赫姆霍茲共振器所設計的實驗條件中，為避免引發新的共振頻率，較佳之設計應以入口端面積約佔噴注盤面積0.2%~0.5%、總共振器入口端面積不超過3%之噴注盤面積為基準；並應根據噴注盤體積限制，以共振腔長徑比為1時，設計最大之共振腔體積。然而，因實際運作時的燃燒室內部之聲速無法準確預測，故必須實際量測所產生的燃燒不穩定頻率及近噴注盤面之溫度，以估算聲速並設計赫姆霍茲共振器。

In the liquid rocket propulsion, the combustion instability may severely damage the system. This research focused on the design analysis of the suppression mechanism for combustion instabilities using acoustic simulations. The model combustors with variable inside diameters (5.0cm and 6.4cm) and lengths (6.5cm, 8cm and 11.5cm) were used to simulate the acoustic characteristic in liquid rocket engines. By utilizing a speaker to transmit sound waves into the model combustors, microphone was set to measure their acoustic properties.
The experimental results showed that the length of the combustor affected mainly the frequencies of the longitudinal resonance of sound waves, and, the diameter of combustor affected the frequencies of the tangential sound-wave resonance. In the experiments, the installation of a convergence nozzle complicated the resonant frequencies in the chamber, while the chamber pressure showed little effects on the resonating frequency up to 150psi.
In the acoustic suppression simulation, quarter-wave resonator, Helmholtz resonator, and baffle devices were tested. The results showed that baffles on injector plate suppressed tangential sound-wave resonance, and the height of the baffle’s significantly increase the effectiveness of suppression. Compare to quarter-wave resonator, Helmholtz resonator showed a better suppression effect. To avoid inducing new resonating frequencies, the inlet area of the resonator should be 0.2%~0.5% of the injection surface area, while the total inlet area of the installed resonators should be less than 3% of the injection surface area. By maximizing the cavity’s volume and keeping the cavity’s length-to-diameter ratio closed to unity, the Helmholtz resonator showed the best suppression effects on the resonating wave in the chamber. In practical, the speed of sound and the instability in the combustion chamber were too complicate to be predicted. The design of an effective Helmholtz resonator for instability suppression shall require the information of the measured frequency spectrum of instabilities as well as the measured temperatures near the position of the resonator.

1.Li. Jun, Sheng. Hong-Zhi, Wei. Xiao-Lin, Tian. Wen-Dong,and Wu. Dong-Yin, “Characteristics of Combustion Instability in a Narrow Combustion Cylinder” , Combustion Science and Technology, Vol. 9, No. 2, pp.161~164, 2003
2.L.Renea Ellison and Marlow D.Moser, “Combustion Instability Analysis and the Effects of Drop Size on Acoustic Drining Rocket Flow”, NASA NCC8-200, 2004
3.張寶炯，“中小推力可儲存推進劑火箭發動機的燃燒不穩定性問題”，上海航天，Vol. 434, No. 11, pp.11~15, 1994
4.Dieter K. Huzel and David H. Huang, “Design Of Liquid Propellant Rocket Engines 2nd Edition”, Rocketdyne Division, North American Rockwell, Inc, 1971
5.Sutton,George Paul, “Rocket Propulsion Elements 7th edition”, John Wiley & Sons, INC, 2000.
6.A. Osherov and B. Natan, “Combustion Instability in a small Liquid Rocket Motor”, The Aeronautical Journal, Vol. 103, No. 23, pp.245~252, May 1999
7.E. Christensen and T.E. Nesman, “Fastrac Rocket Engine Combustion Chamber Acoustic Cavities”, Propulsion Enegineering Research Center, pp.18~23, October 1998
8.E. Laudien, R. pongratz,R. Pierro D. Prelick, “Experimental Procedures Aiding the Design of Acoustic Cavities”, Liquid Rocket Engine Combustion Instability, AIAA Vol.169, pp.377~399, 1995
9.Zhang Meng-Zheng, “Review of Combustion Instability Testing Research on Liquid Propellant Rocket Engine”, Journal of Rocket Propulsion, Vol. 31, No. 6, pp.12~16, 2005
10.YAN Pu, WANG Min-qing, SHENG Mei-ping, “FEM Numerical Analysis for Instability Combustion in a Firebox”, Noise and Vibrztion Conrrol, Vol.25 No.5, 2005
11.A.Santana Jr., M.S. silva, P.T. Lacava and L.C.S. Goes, “Acoustic Cavities Design Procedures”, Thermal Engineering, Vol.6 No.2, pp.27-33, December 2007
12.A. Nicole,M. Habiballah, “Numerical Simulation of a Quarter-wave Resonator Operation”, European Conference for Aerospace Sciences
13.Nie Wan sheng,Zhuang Feng chen, Zhang Zhong guang, “Acoustic analysis of resonator for combustion instability suppression in liquid rocket engines”, Applied Acoustics, Vol.20 No.4, 2001
14.張寶炯，孫敬良，“第一屆液體火箭燃燒不穩定會議綜述”，上海航天, Vol.434 No.13, pp.25~35, 1993
15.Vigor Yang, Josef M. Wieker and Myong Won Yoon, “Acoustic Waves in Combustion Chambers”, AIAA Vol.169, pp.357~375, 1995
16.Zhang Meng-zheng, Zhang Zhi-tao, Yang Guo-hua, Wang liang, “Simulation test and application of chamber acoustic characteristics”, Experimental Technology and Management, Vol.24 No.8, Aug.2007