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Spray combustion was studied by injecting monodisperse 125 or 200 μm ethanol droplets in a premixed natural gas fuel flame flowing against an opposed heated air jet. Phase Doppler anemometry and chemiluminescence measurements allowed to characterise both droplet parameters and the local reacting mixture. Both types of droplets crossed the flow stagnation plane and entered the opposite jet. However, 125 μm droplets reversed their motion and oscillated around the stagnation plane, leading to increased droplet residence time in hot regions. The fuel vapour released by droplets close to the stagnation plane, mixed with the surrounding air and led to the ignition of a second fuel vapour flame below the natural gas flame. For 125 μm droplets, mean local equivalence ratio of the fuel vapour flame was about 0.8, suggesting lean-premixed combustion; whereas 200 μm droplets led to stoichiometric combustion. “Group Combustion” number was estimated from measurements and suggested that 125 μm and 200 μm droplets burned in different “Group Combustion” regimes.

We have studied and developed two compact nanosecond lasers delivering up to 11 mJ, TEM00 and linearly polarized nanosecond pulses centered at 1064 nm. Their use in ignition of a prototype of aeronautic combustion engine0 is presented.

Local, time-dependent measurements of mixture fraction of the reacting mixture were obtained in a swirl-stabilised natural gas-fuelled, nominally non-premixed burner using the intensity of chemiluminescence from OH∗ and CH∗ radicals. The measurements quantified the mean, rms of fluctuations and probability density functions of local mixture fraction at the stabilisation region of the flame. In addition, the probability of flame presence and the degree of lean or rich versus stoichiometric reaction is reported. The burner was operated for three air flow Reynolds numbers (Re=18970, 29100 and 57600), at an overall equivalence ratio of 0.32, without and with imposed oscillations to the air flow of the burner at the resonance frequency of 350 Hz. Results show that combustion occurred in a partially premixed mode for all flow conditions, although fuel and air were injected separately in the reaction zone. The mean local mixture fraction was nearly stoichiometric at the base of the flame without imposed air oscillations, but with large fluctuations leading to around 80% of lean or rich reaction. The degree of non-stoichiometric reaction increased with axial distance from the burner exit and Reynolds number and lean reaction dominated. Imposed air oscillations led to lifted flames and increased the degree of non-stoichiometric reaction for Re=18970 and 29100, whereas the flame remained attached onto the injector for Re=57600 and little modification of the mixture fraction was observed.

Local temporally-resolved measurements of chemiluminescent intensity from OH ∗ , CH ∗ and C ∗ 2 radicals were obtained in premixed counterflow flames operating with propane and prevaporised fuels (isooctane, ethanol and methanol), for different equivalence ratios and strain rates. The results quantified independently the effects of fuel type, strain rate and equivalence ratio on chemiluminescent emissions from flames. The ability of chemiluminescent intensity from OH ∗ , CH ∗ and C ∗ 2 radicals to indicate heat release rate depends strongly on fuel type. The intensity ratio OH ∗ /CH ∗ has a monotonic decrease with equivalence ratio for all fuels and can be used to measure equivalence ratio of the reacting mixture. For propane and isooctane, the OH ∗ /CH ∗ ratio remains independent of flame strain rate, whereas some dependence is observed for ethanol and methanol.

Simultaneous measurements of PLIF-kerosene and PLIF-OH have been successfully performed in a multipoint injection system for various overall equivalence ratio, air inlet temperature between 480 and 730 K and pressure up to 2.2 MPa. Single shot 2D-maps of the spatial distribution of kerosene vapour and OH radical in the combustor have been recorded with good signal-to-noise ratio. Results show that depending on the split between the pilot and the main injectors, the flame front exhibits a single or a double structure. Good spatial correlation between the repartition of the kerosene vapour and the position of the flame front was observed; in particular, no “dark zone” is observed between the fuel and the flame front. As temperature and pressure increase, fuel evaporation improves and the spatial distribution of OH radical becomes more homogeneous in the combustor, suggesting a partially-distributed combustion. To cite this article: M. Orain et al., C. R. Mecanique 337 (2009).

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 pre...

Assessing the feasibility of combustion in miniaturised combustors (volume less than 1 cm3 ) is a key point for the development of micro gas turbines. This paper presents the results obtained in a combustion chamber operating with a hydrogen-air mixture. A stable combustion was obtained with an output power between 100 and 1200 W, for air mass flow rate from 0.1 to 0.5 g/s, and equivalence ratio between 0.3 and 0.7. Experimental results were obtained using thermocouples to measure temperature of the burnt gases at the outlet of the combustor, and information on combustion efficiency and output power was derived. In addition, laser-based measurements were performed using spontaneous Raman spectroscopy and Rayleigh scattering to determine radial profiles of temperature and main species concentrations at the outlet of the combustor.

Experimental investigation of an isolated droplet burning in a convective flow is reported. Acetone droplets were injected in a steady laminar diffusion counterflow flame operating with methane. Planar laser-induced fluorescence measurements applied to OH radical and acetone was used to measure the spatial distribution of fuel vapour and the structure of the flame front around the droplet. High-magnification optics was used in order to image flow areas with a ratio of 1:1.2. The different combustion regimes of an isolated droplet could be observed from the configuration of the envelope flame to that of the boundary-layer flame, and occurrence of these regimes was found to depend on the droplet Reynolds number. Experimental results were compared with 1D numerical simulations using detailed chemistry for the configuration of the envelope flame. Good agreement was obtained for the radial profile of both OH radical and fuel vapour. Influence of droplet dynamics on the counterflow flame front was also investigated. Results show that the flame front could be strongly distorted by the droplet crossing. In particular, droplets with high velocity led to local extinction of the flame front whereas droplets with low velocity could ignite within the flame front and burn on the oxidiser side.

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.