A Lean Premixed injection system (LP) was experimentally investigated at elevated pressure and air inlet temperature, corresponding to engine conditions, i.e. with high swirl number and elevated fresh gases velocities. OH-PLIF, NO-PLIF and kerosene-PLIF laser diagnostics were used to study the flame structure and the NO formation within the primary zone. These experimental studies were complemented with PIV measurements. The acquired data allows the evaluation of the coupling of aerodynamics with the flame structure. Starting from there, the combustion process governing the formation of NO pollutant into the flame was analyzed with high spatial resolution. The Zeldovich pathway has been found to control the NO formation in the inner recirculation zone while the nitrous oxide pathway is found to be important especially in the regions in which the residence time of burnt gases is small. Effect of pressure and FAR also produced significant changes in the NO production. It does appear, however that no universal behavior can be found for the pressure dependence of NO.

This study presents a new experimental methodology to report quantitative measurements of kerosene mole fraction and temperature by combining kerosene-PLIF and OH-PLIF diagnostics. These optical diagnostics are applied simultaneously in a gas turbine model combustion chamber equipped with an Lean Premixed (LP) aero-engine injection system developed by SAFRAN Helicopter Engines (SAFRAN HE), in real operating conditions with pressure up to 1.8 MPa. The method is based on the analysis of kerosene-PLIF images acquired simultaneously on two spectral broadband of collection of the kerosene fluorescence under a 266 nm excitation. To obtain quantitative data, the knowledge of the kerosene fluorescence spectra under a large range of pressure, temperature and oxygen concentration is first determined in a high-pressure cell. An in-situ calibration of the fluorescence signal is then performed in the combustion chamber with operating conditions representative of the kerosene fluorescence signal measured during the experiment. This calibration allows the determination of the kerosene mole fraction accuracy, which is estimated to be better than 12%. Two optical arrangements (axial and radial planes) are finally used to give complementary and previously unseen results. The results highlight the strong correlation between the fuel distribution, the flame topology and the reaction zone intensity. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

A gas turbine model combustor was equipped with an industrial Lean Premixed fuel injection system operating with liquid commercial kerosene (Jet-A1) at atmospheric pressure. Large optical accesses enable joint Particle Image Velocimetry (PIV) and OH planar laser-induced fluorescence (PLIF) measurements at repetition rates up to 5 kHz and 10 kHz, respectively. Using these diagnostics, flame topologies and non-stationary events were investigated in operating conditions representative of the ones encountered in real aeronautic propulsion systems. The flame shape was analyzed in terms of interactions between the different flame zones responsible for flame stabilization in confined swirled flames. Data processing of the strain rate and vorticity fields highlighted the existence of two shear layers that interfere differently with the inlet air/fuel mixing jet. An inner shear layer (ISL) between the Inner Recirculation Zone (IRZ) and the fresh inlet flow is located at the upper base of the fuel spray. An Outer Shear Layer (OSL) is also identified between the Outer Recirculation Zone (ORZ) and the fresh incoming flow. Spanwise-oriented vortices are produced from this latter, with a growth rate function of the free stream speed ratio (fresh incoming reactants and the flow circulating inside ORZ). The detailed analysis of the shear layers gives new insight on the flame structures obtained from OH-PLIF data. Finally, high-speed simultaneous measurements of flow velocity and OH distributions highlighted unusual flame pinching mechanisms leading to the release of subsequent unburned pockets propagating in the burned gases. (C) 2019 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

A Lean-Premixed (LP) aero-engine injection system was experimentally studied using optically-based measurements. Experiments were conducted under relevant operating conditions up to 1.38 MPa and using commercial kerosene as fuel.First of all, the structure of the reaction zone and the flame length into the combustion chamber have been studied with CH* chemiluminescence. It is observed from the data measurements that combustion can produce two types of flames, a V-shaped flame in which combustion is stabilized a few mm downstream from the injector and a tulip flame in which combustion is developing inside the injection system. The flame is found to be shorter and more confined when increasing the pressure. To complement this study, experiments were also performed using the OH-PLIF measurement technique. Data processing of the absorption of OH fluorescence signals along the laser propagation allowed the determination of the absolute distribution of OH concentration without any calibration of the OH fluorescence signals. The obtained values are in agreement with estimated premixed adiabatic chemical equilibrium results. Furthermore, the flame front location and its structure were captured from gradient based filtering operations on OH-PLIF signals. Finally, pollutant emissions were also measured with an exhaust gas sampling probe positioned downstream from the combustor outlet. It has been found that NOx emission increases with Fuel Air Ratio (FAR) and pressure whereas CO exhibits an inverse trend.

Dynamical processes in gas turbine combustors play a key role in flame stabilization. Those phenomena are investigated in a gas turbine model combustor for a partially premixed kerosene/air flame at atmospheric pressure (Fig. 1). The large optical accesses of the combustion chamber enable the application of laser diagnostics. Stereo-PIV, Planar laser induced fluorescence (PLIF) of OH at 10 kHz and simultaneous particle image velocimetry (PIV) at 5 kHz have been implemented. The flow field is characterized by PIV in front view and by stereo-PIV in a horizontal plane while the flame structure is visualized by OH-PLIF. Simultaneous OH-PLIF/PIV measurements are used to investigate the interactions between the flow field and the flame (Fig. 2, left). POD and image processing tools are developed to analyze images (Fig. 2, right). Temporal analysis of the results demonstrates the development of a local flame extinction mechanism. Fig. 1: kerosene/air gas turbine model combustor Fig. 2: (Left) Simulatanous PIV/OH-PLIF measurement, (Right) associated POD reconstruction colored by vorticity