Method of Measurement of Admittance of Composite Solid Propellants Using Impedance Tube Technique

• Published in: Combustion science and technology 2024-12, Vol.196 (16), p.3877-3900
• Main Authors: Ganesan, S., Chakravarthy, S. R., Subhash Chandran, B. S.
• Format: Article
• Language: English
• Subjects: Acoustics; admittance; Chambers; Combustion; combustion response; Combustion stability; Composite propellants; Composite solid propellant; Electrical impedance; Energy conservation; governing equations; HTPB propellants; Impedance; Impedance tube; Measurement methods; Propellant combustion; Propellants; Rocket engines; Room temperature; Solid propellants; solution procedure; Temperature gradients
• ISSN: 0010-2202; 1563-521X
• DOI: 10.1080/00102202.2023.2201377

Mitigation of combustion instability of solid propellant rocket motors is ever being attempted by several researchers as the problem is very complicated in nature. The acoustic admittance of composite solid propellants (AP/HTPB/RDX/Al) is experimentally investigated using Impedance tube technique. It is one of the indirect experimental techniques to determine the combustion response of the solid propellants besides T-burner. The governing equations based on the conservation of mass, momentum and energy (reactive flow is simplified to have only the axial temperature gradient) are linearized and solved to determine the acoustic admittance of the burning propellant. Different methods of analysis were explored to validate/compare the results obtained and are matching well. Some of the unreported details of the analysis are brought out. A novel way of utilizing the outer tube besides the inner tube as against the literature (wherein the inner tube is kept inside the outer tube) to maintain the chamber pressure is explored. A test conducted at fundamental acoustic mode (110 Hz) at 2 MPa at room temperature (303 K) is considered for the analysis. A rotary valve capable of operating up to a mean chamber pressure of 12 MPa is developed in this work to act as an acoustic driver for the experiments. The advantage of the impedance tube technique over T-burner is that the former is capable of measuring both the real and imaginary part of combustion response whereas the latter can only measure the real part. The imaginary part of the combustion response besides the real part is useful to predict the stability characteristics of the solid propellant rocket motor.