Frédéric Grisch

Measurements and modeling of acetone laser-induced fluorescence with implications for temperature-imaging diagnostics

Recent determinations of the temperature dependence of acetone fluorescence have permitted the application of acetone planar laser-induced fluorescence imaging, which was already popular for mapping concentration, to the measurement of temperature. With a view toward developing temperature-imaging diagnostics, we present atmospheric-pressure fluorescence and absorption results acquired with excitation at eight wavelengths across the absorption feature of acetone and at temperatures from 300 to 1000 K. Modeling of the fluorescence yield of acetone is shown to be useful in explaining both these results and the variation of acetone fluorescence with pressure and composition that was observed in several studies. The model results in conjunction with the photophysics data provide guidance for the application of temperature diagnostics over a range of conditions while also suggesting useful multiparameter imaging approaches.

Temperature imaging with single- and dual-wavelength acetone planar laser-induced fluorescence.

Two sensitive techniques for temperature imaging by use of acetone planar laser-induced fluorescence, applicable at temperatures up to 1000 K, are introduced and demonstrated. Photophysics data on the wavelength-dependent temperature variation of acetone fluorescence permit the implementation of a single-wavelength technique in environments with constant pressure and constant acetone mole fraction, and a dual-wavelength method can be applied in flows with mixing and (or) chemical reaction. Preliminary imaging results are presented for acetone-air flow over a heated cylinder (single-wavelength strategy) and for a heated laminar jet (dual-wavelength strategy).

Experimental studies of high-pressure cryogenic flames on the mascotte facility

ABSTRACT Combustion processes involved in cryogenic propellant LOX/H2 systems are very complex. In order to improve understanding and modeling of such processes, a test facility, called Mascotte, was developed by ONERA in the early 1990s. A research program involving teams from universities and ONERA has run for several years on the Mascotte bench and the purpose of this article is to summarize the results of systematic research obtained at ONERA. The first part of the article briefly describes the test facility and the related low and high-pressure combustors. In the second part, findings of both qualitative and quantitative measurements using high-speed photography, shadowgraphy, backlighting, and Coherent Anti-Stokes Raman Scattering are presented and discussed for different operating conditions. Differences of flame structure in sub- and supercritical regimes are emphasized. In particular, imaging techniques show evidence of the absence of ligaments and droplets around the LOX jet at supercritical pressures. Differences of flame front behavior with pressure is also demonstrated with both shadowgraphy and CARS diagnostics.

Experimental Study of Strut Injectors in a Supersonic Combustor Using OH-PLIF

The present experimental study, carried out in the framework of ONERA-JAXA cooperation, deals with the ignition and hydrogen flame development in a supersonic air flow at Mach 2.5, with stagnation conditions of 0.6 MPa and 1620 K. Different types of struts for hydrogen injection (equivalence ratio of 0.45) have been investigated. Some (Alternating-Wedge struts) are specially designed to enhance fuel-air mixing by the creation of streamwise vortices. The principal aim of this study is to obtain information on the flow structures in the supersonic combusting mixing layer, and the location of ignition and combustion zones. Instantaneous maps of the OH radical by means of the OH-PLIF technique in two planes downstream of the strut injector and conventional observations of the flame by video camera have been obtained. These visualizations are completed by global measurements of the ignition delays and heat release by means of wall pressure measurements. The efficiency of the different strategies of fuel injections are then compared and discussed.

CARS thermometry in high pressure rocket combustors

Abstract Coherent anti-Stokes Raman spectroscopy (CARS) has been used to investigate high pressure cryogenic liquid oxygen/gaseous hydrogen (LOX/GH 2 ) flames at pressures up to 6.5 MPa. The method employs two synchronized broadband CARS systems to probe simultaneously H 2 and H 2 O. Temperature is deduced from the H 2 and H 2 O CARS profiles while the H 2 O concentration is deduced from the modulation from the resonant the H 2 O resonant signal superimposed on the nonresonant background signal. Details of the modeling of the CARS spectra at high pressure and the data reduction are discussed. Problems issued from the presence of liquid phase are also investigated. The results demonstrate the applicability and usefulness of CARS for investigating cryogenic combustion under representative rocket model combustor regimes where the pressure range is above the supercritical condition ( P >5.04 MPa).

Fluorescence Spectroscopy of Kerosene Vapour: Application to Gas Turbines

Progress to develop Planar Laser-induced Fluorescence on kerosene vapour is reported. Experiments were carried out in a heated test cell operating between 400 K and 700 K, at pressures between 0.1 MPa and 0.7 MPa. For a wavelength excitation of 266 nm, the fluorescence spectrum of kerosene vapour presents two separated fluorescence bands respectively in the 270-310 nm and in the 310-420 nm spectral regions which have different temperature, pressure and oxygen molar fraction dependence. Attribution of these spectral bands was achieved by comparison with reference spectra of several pure species which were analysed separately. Four species were identified, belonging to singlering aromatics and two-ring aromatics, as being primarily responsible for kerosene fluorescence. Temperature, pressure and mixture composition dependence of kerosene vapour absorption and fluorescence provides useful guidance for kerosene PLIF allowing quantitative imaging of local equivalence ratio and temperature. Illustrative results of visualisation of mixing layer between a jet and the surrounding air and measurement of local equivalence ratio are examined for a subsonic turbulent preheated kerosene/air jet exiting in air.

Advanced optical diagnostics applied to dynamic flames and turbulent jets

Experimental studies were conducted to obtain single shot measurements of temperature and concentration in a variety of dynamic flames. The structure of a low speed hydrogen-air buoyant flame is investigated to characterize vortex-flame interactions. In this flame, the temperature and the molar fraction of nitric oxide (NO), atomic oxygen (O), atomic hydrogen (H) and hydroxyl (OH) have been obtained using a combination of coherent anti-Stokes Raman spectroscopy (CARS), laser-induced fluorescence (LIF) and degenerate four-wave mixing (DFWM). Good agreement was found between the simulated and the measured data. A monodispersed droplet stream flame was investigated to study the diphasic combustion of ethanol. In this case, the temperature, nitric oxide (NO) and fuel concentration have been measured. Finally, two-dimensional maps of OH and acetone are recorded using planar LIF (PLIF). Application of this technique to a supersonic H2/air flame and to a subsonic ethylene/air reacting jet mainly illustrates the turbulent character of combustion and provides a deeper insight into the chemical mechanisms of these flows. The present paper emphasizes the importance of applying quantitative and instantaneous optical techniques to different combustion and turbulent media.

Simultaneous temperature, concentration, and velocity fields measurements in a turbulent heated jet using combined laser-induced fluorescence and PIV

Mixing process of a passive scalar in a heated turbulent jt at Reynolds numbers between 13,000 and 21,000 is studied experientaly using combined two-color Planar Laser-Induced Fluorescence (2λ-PLIF) and Particle-Image Velocimetry (PIV). The PLIF system is based on acetone fluorescence for temperature and concentration measurement. The aim of the present paper is to obtain a reliable reference data set for the validation of numerical simulation of turbulent fluxes. Experiment was carried out on a heated turbulent jet of acetone-seeded air emanating from the 10 mm-diameter nozzle exit of an electric air heater with exit temperature Ti = 500 K. The jet is seeded to approximately 3% acetone by bubbling the air stream through liquid acetone. The mean and fluctuating dynamic and thermal fields are investigated and determined. This tool will allow to determine the temperature-velocity as well as the concentration-velocity cross-correlations in order to characterize the turbulent characteristics of the flow such as the turbulent diffusivity and the turbulent Prandtl number.

Temperature and mixture-fraction imaging of gaseous flows using acetone PLIF

Progress in developing planar laser-induced fluorescence (PLIF) in acetone-seeded flows is reported. Wavelength and temperature dependencies of acetone absorption and fluorescence are presented which allow optimization of acetone PLIF for quantitative imaging of density, mixture fraction, and temperature. Singleand dual-wavelength excitation strategies are characterized for imaging of temperature. Illustrative results for mixture-fraction imaging using pulsed excimer laser (XeCl at 308 nm) excitation are presented for a subsonic jet in crossflow; images are detected using a 512 x 512 element unintensified CCD camera. Example results for temperature imaging are presented using single-wavelength (248 nm) excitation in a uniformly seeded flow over a heated object and using dual-wavelength (248 nm and 308 nm) excitation of a heated jet mixing with ambient air. In both cases, the sensitivity of the fluorescence signal (or ratio of signals) to temperature is high, indicating good potential for acetone-based PLIF temperature imaging in flows up to 1000 K. (Author)

Analysis of the Flame Structure in a Trapped Vortex Combustor Using Low and High-Speed OH-PLIF

Operating with lean combustion has led to more efficient “Low-NOx” burners but has also brought several technological issues. The burner design geometry is among the most important element as it controls, in a general way, the whole combustion process, the pollutant emissions and the flame stability. Investigation of new geometry concepts associating lean combustion is still under development, and new solutions have to meet the future pollutant regulations. This paper reports the experimental investigation of an innovative staged lean premixed burner. The retained annular geometry follows the Trapped Vortex Combustor concept (TVC) which operates with a two stage combustion chamber: a main lean flame (1) is stabilized by passing past a vortex shape rich-pilot flame (2) located within a cavity. This concept, presented in GT2012-68451 and GT2013-94704, seems to be promising but exhibits combustion instabilities in certain cases, then leading to undesirable level of pollutant emissions and could possibly conduct to serious material damages. No precise information have been reported in the literature about the chain of reasons leading to such an operation. The aim of this paper is to have insights about the main parameters controlling the combustion in this geometry. The flame structure dynamics is examined and compared for two specific operating conditions, producing an acoustically self-excited and a stable burner. Low and high-speed OH-PLIF laser diagnostics (up to 10 kHz) are used to have access to the flame curvature and to time-resolved events. Results show that the cavity jets location can lead to flow-field oscillations and a non-constant flame’s heat release. The associated flame structure, naturally influenced by turbulence is also affected by hot gases thermal expansion. Achieving a good and rapid mixing at the interface between the cavity and the main channel leads to a stable flame.Copyright