Philippe Scouflaire

STRUCTURE AND DYNAMICS OF CRYOGENIC FLAMES AT SUPERCRITICAL PRESSURE

ABSTRACT A detailed understanding of liquid propellant combustion is necessary for the development of improved and more reliable propulsion systems. This article describes experimental investigations aimed at providing such a fundamental basis for design and engineering of combustion components. It reports recent applications of imaging techniques to cryogenic combustion at high pressure. The flame structure is investigated in the transcritical range where the pressure exceeds the critical pressure of oxygen (p > p c (O2 = 5.04MPa)) but the temperature of the injected liquid oxygen is below its critical value . Data obtained from imaging of OH* radicals emission, CH* radicals emission in the case of LOx/GCH4 flames and backlighting provide a detailed view of the flame structure for a set of injection conditions. The data may be used to guide numerical modelling of transcritical flames and the theoretical and numerical analysis of the stabilization process. Calculations of the flame edge are used to illustrate this aspect. Results obtained may also be employed to devise engineering modelling tools and methodologies for component development aimed at improved efficiency and augmented reliability.

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.

Structure of cryogenic flames at elevated pressures

Abstract: This paper presents new experimental results on cryogenic jet flames formed bb a coaxial injector at a pressure of 70 bar, which approaches the pressures found in rocket engines. This element, fed with liquid oxygen and gaseous hydrogen, is placed in a square combustion chamber equipped with quartz windows. The flame is examined via spectroscopy, OH* emission, and backlighting, the aim being to provide basic information on the flame structure. It is found that some of the OH* emission is absorbed by the OH radicals present in the flame. A detailed examination of this effect is presented, in which it is shown that, for this turbulent flame, the Abel transform gives the position of the intense reaction region, whether or not absorption is signficant. The flame is attached to the oxygen injector as at low pressure. At high pressure, flame expansion is reduced compared with low pressure and is also less dependent on the momentum flux ratio between the hydrogen and the oxygen streams. An analysis of the relevant Damkohler numbers suggests that this is because the rate of combustion is mainly controlled by large-scale turbulent mixing at high pressure, and it is dominated by jet break-up, atomization, and vaporization at low pressures. Jet break-up is particularly dependent on the momentum flux ratio. Finally; the mean volumetric heat release rates and flame surface density in the experimental facility are estimated.

Flame stabilization in cryogenic propellant combustion

Cryogenic combustion is of considerable technological interest in propulsion applications. Cryogenic propellants used in rocket engines provide the high performance needed for spacecraft launching and have operated safely for a number of years, but the processes that control combustion in such devices are still not well understood. Among the many important issues, flame stabilization constitutes one basic problem. This question is investigated in this article by imaging the flame originating from a single, coaxial injector fed with liquid oxygen and gaseous hydrogen. Results of experiments carried out on a facility for cryogenic propellant combustion research operated by ONERA are used to characterize the mechanisms that control the flame-holding process at atmospheric pressure, 5, and 10 bars. Data acquired correspond to elastic scattering by the spray, emission of OH radicals, and planar laser-induced fluorescence of these radicals. Fluorescence is obtained by pumping the X2II (v″=0)→A2Σ(v′=1) band of OH, and off-resonance light radiation is observed. This database provides the general structure of the flame in the injector near-field, and may be used to determine the position of the flame stabilization region. Simultaneous acquisition of laser-induced fluorescence and elastic scattering was used to locate the flame in respect to the liquid. It is shown that in all cases investigated the flame is initiated at a close distance from the injector exhaust plane.

OH Planar Laser-Induced Fluorescence and Emission Imaging in High Pressure LOx/Methane Flames

The application of planar laser-induced fluorescence of OH to high-pressure liquid oxygen/gaseous methane flames is investigated in this article. As pressure is increased, the maximum level of OH fluorescence decreases while an interfering light intensity increases. It is shown that suitable data can only be obtained by properly tuning the detection scheme. Narrowband filtering of OH fluorescence is required to reduce the level of interfering signals. An analysis of the interfering light indicates that it is associated with polycyclic aromatic hydrocarbon fluorescence originating from a region surrounding the flame. OH and polycyclic aromatic hydrocarbon fluorescence signal amplitudes become comparable at a pressure of 2.5 MPa which constitutes an upper bound for standard imaging. Below that limit the flame is well characterized and features thin, wrinkled OH layers developing in the vicinity of the liquid oxygen jet. The initial flame sheet is continuous but it becomes highly corrugated further downstream when the liquid oxygen jet breaks down. The flame edge standoff distance is greater than a few LOx post lip sizes indicating that stabilization is less well achieved than in the case of liquid oxygen/hydrogen flames where combustion typically begins at less than one lip size from the injector.

Influence of nanosecond repetitively pulsed discharges on the stability of a swirled propane/air burner representative of an aeronautical combustor

This paper reports on an experimental study of the influence of a nanosecond repetitively pulsed spark discharge on the stability domain of a propane/air flame. This flame is produced in a lean premixed swirled combustor representative of an aeronautical combustion chamber. The lean extinction limits of the flame produced without and with plasma are determined and compared. It appears that only a low mean discharge power is necessary to increase the flame stability domain. Lastly, the effects of several parameters (pulse repetition frequency, global flowrate, electrode location) are studied.

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.

A new experimental setup for high-throughput controlled non-photochemical laser-induced nucleation: application to glycine crystallization

Non-photochemical laser-induced nucleation (NPLIN) has been a growing field of study since 1996, and more than 40 compounds including organics, inorganics and proteins have now been probed under various conditions (solvents, laser types, laser beams etc.). The potential advantages of using this technique are significant, in particular polymorphic control. To realize these benefits, the objective is a carefully designed experimental setup and highly controlled parameters, for example temperature and energy density, in order to reduce the uncertainty regarding the origin of nucleation. In this paper, a new experimental setup designed to study NPLIN is reported. After a full technical description of the present setup, the different functionalities of this device will be illustrated through results on glycine. Glycine crystals obtained through NPLIN nucleate at the meniscus and exhibit different morphologies. The nucleation efficiency, as a function of the supersaturation of the solution used and the laser beam energy density, has also been established for a large number of samples, with all other parameters held constant.

Characterization of the Acoustic Interactions in a Two-Stage Multi-Injection Combustor Fed With Liquid Fuel

Burners operating in lean premixed prevaporized (LPP) regimes are considered as good candidates to reduce pollutant emissions from gas turbines. Lean combustion regimes result in lower burnt gas temperatures and therefore a reduction on the NOx emissions, one of the main pollutant species. However, these burners usually show strong flame dynamics, making them prone to various stabilization problems (combustion instabilities, flashback, flame extinction). To face this issue, multi-injection staged combustion can be envisaged. Staging procedures enable fuel distribution control, while multipoint injections can lead to a fast and efficient mixing. A laboratory-scale staged multipoint combustor is developed in the present study, in the framework of LPP combustion, with an injection device close to the industrial one. Using a staging procedure between the primary pilot stage and the secondary multipoint one, droplet and velocity field distributions can be varied in the spray that is formed at the entrance of the combustion chamber. The resulting spray and the flame are characterized using OH-Planar Laser Induced Fluorescence (OH-PLIF), High Speed Particle Image Velocimetry (HSPIV) and Phase Doppler Anemometry (PDA) measurements. Three staging values, corresponding to three different flame stabilization processes, are analyzed, while power is kept constant. It is shown that mean values are strongly influenced by the fuel distribution and the flame position. Using adequate post-processing, the interaction between the acoustic field and the droplet behavior is characterized. Spectral analysis reveals a strong acoustic-flame coupling leading to a low frequency oscillation of both the velocity field and the spray droplet distribution. In addition, acoustic measurements in the feeding line have shown that a strong oscillation of the acoustic field leads to a change in fuel injection, and hence droplet behavior.

Effect of Fuel Distribution on Spray Dynamics in a Two-Staged Multi-Injection Burner

Burners operating in lean premixed prevaporized (LPP) regimes are considered as good candidates to reduce pollutant emissions from gas turbines. Lean combustion regimes result in lower burnt gas temperatures and therefore a reduction on the NOx emissions, one of the main pollutant species. However, these burners usually show strong flame dynamics, making them prone to various stabilization problems (combustion instabilities, flashback, flame extinction). To face this issue, multi-injection staged combustion can be envisaged. Staging procedures enable fuel distribution control, while multipoint injections can lead to a fast and efficient mixing. A laboratory-scale staged multipoint combustor is developed in the present study, in the framework of LPP combustion, with an injection device close to the industrial one. Using a staging procedure between the primary pilot stage and the secondary multipoint one, droplet and velocity field distributions can be varied in the spray that is formed at the entrance of the combustion chamber. Non-reactive and reactive flows are characterized through an extensive Phase Doppler Anemometry (PDA) campaign. Three staging values, corresponding to three different flame stabilization processes, are analyzed, while power is kept constant. It is shown that mean values and droplet distributions are affected by the staging procedure in the non-reactive as in the reactive situations. Using adequate post-processing, it is also possible to study non-reactive and reactive flow/flame dynamics. Spectral analysis shows that the non-reactive flow is strongly structured by a high frequency rotating structure that can clearly be associated with a precessing vortex core (PVC), while the reactive situation encounters a strong acoustic-flame coupling leading to a low frequency oscillation of both the velocity field and the spray droplet distribution. In this last situation, high frequency phenomena, which may be due to PVC, are still visible.Copyright