Franck Richecoeur

High-Frequency Transverse Acoustic Coupling in a Multiple-Injector Cryogenic Combustor

High-frequency combustion oscillations are investigated experimentally. The combustor fed by cryogenic propellants operates under elevated pressure conditions (p c = 0.9 MPa) and is equipped with three coaxial injectors fed by liquid oxygen and gaseous methane. Injection parameters are in the typical range used in rocket engines. This experiment simulates on a model scale conditions prevailing in such systems, but full similarity is not achieved. The chamber exhibits a set of resonant modes with eigenfrequencies above 1 kHz. The study focuses on high-frequency dynamics resulting from a strong coupling between one of the transverse modes and combustion. The combustor is forced with an external actuator. The eigenmodes are identified with a linear frequency sweep, and then the system is modulated at the first transverse resonant frequency. The flame motion and response are observed with a high speed and two intensified charge-coupled-device cameras recording phase-conditioned images. In a set of experiments carried out on the multiple-injector combustor, operating conditions were changed systematically to determine parameter ranges leading to combustion sensitivity to transverse excitation. Strong coupling is observed in this way with a spectacular modification of the flame spread. Emission from the three flames is notably intensified when this coupling occurs, whereas thermocouples placed on the lateral walls detect a rapid increase in temperature. The OH* emission intensity that can be linked to the heat-release rate is increased. A phase analysis indicates that the pressure and OH* emission oscillate transversally and in phase at the modulation frequency. This behavior is also observed with the high-speed camera, which also features enhanced reactive vortices convected in the downstream direction at a lower frequency.

A Methodology for On the Fly Acoustic Characterization of the Feeding Line Impedances in a Turbulent Swirled Combustor

A methodology is proposed to determine on the fly the acoustic impedances at the boundaries of a combustor under operation and without the need for an external forcing device. The methodology is applied hereby to obtain the reflection coefficients of the air and fuel feeding lines supplying a swirled combustor operating in lean conditions and featuring an unstable regime. It is based on a three-microphone technique and uses the combustion roar noise inside the chamber to get estimates of the boundary impedances in the low freguency limit. Conditions under which this method yields reliable results for the reflection coefficient are examined, and a criterion based on the coherence between microphone signals is proposed to determine the frequency bandwidth for impedance reconstruction. The method also takes into account the flow Mach number in the supplying pipes, which can be non-negligible because of their reduced diameter. The technique is validated against data obtained in a dedicated impedance measurement setup equipped with a high-efficiency loudspeaker.

Interactions between propellant jets and acoustic modes in liquid rocket engines: experiments and simulations

The interaction between transverse acoustic waves and non-reactive coaxial jets are experimentally and numerically investigated. Multi coaxial element injector is used to inject liquid oxygen and gaseous methane in a high pressure (3 MPa) chamber. A transverse secondary nozzle is located on the top of the chamber and is periodically blocked by a rotating toothed wheel to generate high amplitude transverse pressure waves in the chamber. The coupling between the acoustic wave and the cryogenic is observed by backlighting the five jets. Jets structure is compared with LES numerical simulation performed with coaxial gaseous oxygen and methane injection at ambient. Similar transverse pressure wave is numerically generated and the specie evolution is reported here. Comparative results are obtained, namely a reduction of the central oxygen core and an oscillatory motion of the flow. Further phenomena are pointed out by comparing the frequency spectrum of the modulated and non-modulated cases.

Thermoacoustic Shape Optimization of a Subsonic Nozzle

Indirect combustion noise originates from the acceleration of nonuniform temperature or high vorticity regions when convected through a nozzle or a turbine. In a recent contribution (Giauque et al., 2012, “Analytical Analysis of Indirect Combustion Noise in Subcritical Nozzles,” ASME J. Eng. Gas Turbies Power, 134(11), p. 111202) the authors have presented an analytical thermoacoustic model providing the indirect combustion noise generated by a subcritical nozzle when forced with entropy waves. This model explicitly takes into account the effect of the local changes in the cross-section area along the configuration of interest. In this article, the authors introduce this model into an optimization procedure in order to minimize or maximize the thermoacoustic noise emitted by arbitrarily shaped nozzles operating under subsonic conditions. Each component of the complete algorithm is described in detail. The evolution of the cross-section changes are introduced using Beziers splines, which provide the necessary freedom to actually achieve arbitrary shapes. Beziers polar coordinates constitute the parameters defining the geometry of a given individual nozzle. Starting from a population of nozzles of random shapes, it is shown that a specifically designed genetic optimization algorithm coupled with the analytical model converges at will toward a quieter or noisier population. As already described by Bloy (Bloy, 1979, “The Pressure Waves Produced by the Convection of Temperature Disturbances in High Subsonic Nozzle Flows,” J. Fluid Mech., 94(3), pp. 465–475), the results therefore confirm the significant dependence of the indirect combustion noise with respect to the shape of the nozzle, even when the operating regime is kept constant. It appears that the quietest nozzle profile evolves almost linearly along its converging and diverging sections, leading to a square evolution of the cross-section area. Providing insight into the underlying physical reason leading to the difference in the noise emission between two extreme individuals, the integral value of the source term of the equation describing the behavior of the acoustic pressure of the nozzle is considered. It is shown that its evolution with the frequency can be related to the global acoustic emission. Strong evidence suggest that the noise emission increases as the source term in the converging and diverging parts less compensate each other. The main result of this article is the definition and proposition of an acoustic emission factor, which can be used as a surrogate to the complex determination of the exact acoustic levels in the nozzle for the thermoacoustic shape optimization of nozzle flows. This acoustic emission factor, which is much faster to compute, only involves the knowledge of the evolution of the cross-section area and the inlet thermodynamic and velocity characteristics to be computed.

Experimental Analysis of Simultaneous Non-Harmonically Related Unstable Modes in a Swirled Combustor

The present study investigates combustion instabilities generated in a turbulent swirled combustor featuring two non-harmonically related unstable modes. Sound pressure and chemiluminescence spectra show the presence of two peaks located around 180 Hz and 280 Hz during unstable operation. The low frequency acoustic response of the test-rig is then analyzed using a two-coupled-cavity model including a realistic impedance of the system at the premixer inlet. This analytical approach is used to link the two observed frequencies to the first chamber and premixer modes respectively. Analytical predictions are compared with acoustic pressure measurements to determine the structure of these modes. The Rayleigh source term in the energy balance is also computed and shows that the two modes feed acoustic energy simultaneously in the system. High-speed PIV data gathered under unstable operation are filtered around these two frequencies to obtain phase conditioned images. Results show that the unsteady flow in the flame region features distinct dynamics associated to a bulk longitudinal oscillation of the flow in the flame arms at 180 Hz and large wrinkles in the radial direction at 280 Hz.© 2011 ASME

Experimental and Analytical Study of the Acoustic Properties of a Gas Turbine Model Combustor With a Choked Nozzle

Combustion is now considered as a non-negligible contributor to gas turbine noise. Combustion noise can be divided into two types: direct combustion noise directly caused by flame surface fluctuations and indirect combustion noise caused by non-homogeneities in the burnt gases, which radiate sound when interacting with the first turbine stages. The aim of the present project is to obtain an extensive experimental database as well as a better understanding of the physical phenomena inside a pressurized combustion chamber with a choked exhaust nozzle. To do so, a pressurized model scale combustor has been developed, containing a tangential admission injector creating a swirling premixed flow. Satisfactory premixing is obtained in the injection device by a porous media. The combustion chamber shows large optical accesses and various ports for pressure and temperature sensors. On the upstream side, an impedance control device is installed while, downstream, the exhaust nozzle can be easily varied to study its influence on noise generation. A mean chamber pressure higher than 2 bar can be reached for the targeted operating points.The present analysis of the flame behaviour is a first step towards the study of combustion noise. The flame dynamics are characterized by spectral analysis of the dynamic pressure in the combustion chamber. The aim of this work is to determine the dominating acoustic modes during combustion operation. With the help of analytical calculations, the test bench is first modelled as a two cavity system, and later as a five cavity system, taking into account the feeding lines. The nozzle can be assumed as choked due to the pressurization of the chamber. With this method, the majority of the acoustic modes can be identified and explained. The study shows that these modes are linked to the geometry of the whole combustor including the injection tube, the combustion chamber and the feeding lines.Copyright

Impact of acoustic boundary conditions on confined combustion noise

The noise produced by aircraft engines at take‐off and landing can be very harmful for airport neighbors, an issue that became recently particularly crucial. While combustion instabilities, where a strong coupling between flames and acoustic modes of the combustion chamber cause very high levels of harmonic sound, the wide bandwidth noise emitted by confined flames in stable situations is less well‐documented. It has been recently shown by Tran et al. (2007) that varying the burner upstream acoustic boundary condition was a very efficient method for damping combustion instabilities and creating ideal conditions for the study of combustion noise. In the present study, the noise level induced by stable combustion is investigated for various upstream conditions. The test bench consists in a swirl stabilized burner, connected to a rectangular combustion chamber, equipped with microphone ports for acoustic measurements. The 40 kW burner is composed of two identical stages fed with air and propane. A photomulti...