C. Rolon

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.

Experimental and numerical study of chemiluminescence in methane/air high-pressure flames for active control applications

Chemiluminescence of excited OH * , CH * , and C 2 * radicals was investigated as a tool for combustion control. A parametric study in premixed methane/air flames is presented regarding the effects of pressure (1 to 10 bar) and equivalence ratio (0.6 to 1.1). The experimental geometry corresponds to a Bunsen-type burner, with pilot flames to achieve steady combustion at very lean conditions. The burner was set in a pressurized vessel to control ambient pressure. The chemiluminescence was spatially measured using an intensified CCD camera with interference filters centered on the three radical emission bands. A monochromator and a low-resolution spectrometer were used to obtain spectrally resolved data. The three diagnostic techniques show good agreement. The experimental results show that the chemiluminescence of the radicals investigated has different dynamics for given pressure and equivalence ratio conditions. The OH * radical seems more suitable for lean flames, while CH * and C 2 * have a more monotonic behavior and stronger dynamics for richer flames. A numerical simulation with complex chemistry and transport modeling based on the PREMIX code was performed for two different kinetic schemes including OH * and CH * . A comparison is presented for integrated chemiluminescence (both spectrally and spatially), as well as for local excited radical concentration trends within the range of experimental conditions. Good qualitative agreement is found with the experimental results except for rich flames, where disagreements due to kinetic schemes are observed. As a conclusion, a new strategy for flame sensing using chemiluminescence over several wavelengths is proposed.

Investigation of Cryogenic Propellant Flames Using Computerized Tomography of Emission Images

Cryogenic propellant combustion is investigated in this paper. It is shown that the mean e ame structure may be obtained by applying computerized tomography principles to oxygen‐ hydrogen (OH) emission images obtained from experiments on a shear coaxial injector. The data correspond to injection conditions typical of those found in rocket motors, but to lower operating pressures of 1, 5, and 10 bar. The transformed emission images yield the mean volumetric OH emission distribution. This quantity may be roughly interpreted as the mean volumetric rate of reaction. The data provide the location of the mean e ame zone and cone rm that stabilization takes place in the immediate vicinity of the injection plane. I. Introduction L IQUID oxygen ‐ gaseous hydrogen rocket engines have been used for a number of years because they yield the high specie c-impulse values needed in space propulsion applications. Cryogenic propellants thus diminish the cost per mass of payload in orbit, but pose specie c storage, handling, and operating problems. Current rocket motor design relies on extensive experience and technological expertise. The detailed processes involved in cyrogenic combustion are, however, not yet fully documented. An improved understanding of the mode of e ame stabilization and of the e ame structure in the near e eld of the injector head would be quite valuable. This information could be used to improve design methodologies and enhance reliability of operation. Such information would be useful for more accurate predictions of heat transfer rates to the engine walls. In this context, knowledge about whether the e ame is stabilized right on the injector lip or at a distance as a lifted e ame is of considerable interest. The stabilization region is specie cally investigated in this paper on the basis of experiments carried out on a cryogenic model scale combustor designated as Mascotte. This facility, operated by ONERA, is dedicated to basic research and technological studies. Data gathered at this facility include planar laser-induced e uorescence (LIF), planar laser light scattering, and emission imaging. Simultaneous recording of light elastic scattering and hydroxyl radical (OH) e uorescence images has allowed identie cation of the e ame stabilization. When the liquid oxygen (LOX) is injected by a central tube and is surrounded by an annulus of high-speed gaseous hydrogen, it is shown 1 that the e ame is established in the outer boundary of the LOX jet, where the hydrogen stream velocity is low. It is also found that the laser-induced OH ‐ e uorescence signal level remains in the same range over the zone visualized, with little change in the signal amplitude as a function of the axial distance. However, the emission images of the excited OH radical appear to yield a different picture of the e ame stabilization region. The emission amplitude is low close to the injector and increases rapidly at a distance. From these specie c features

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.