Time-resolved spatial patterns and interactions of soot, PAH and OH in a turbulent diffusion flame

International audienceSoot control raises important fundamental issues and industrial challenges, which require a comprehensive under- standing of processes governing its formation, interactions and destruction in turbulent flames. A physical insight of the soot space-time evolution in a turbulent diffusion flame is reported in this article by combining three simultaneous high sampling rate imaging diagnostics operating at a frame rate of 10 kHz: light scattering from soot particles, planar laser induced fluorescence (PLIF) of the OH radical, a marker of the flame region, and planar laser induced fluorescence of Polycyclic Aromatic Hydrocarbons (PAHs), classically identified as soot precursors. Images issued from these diagnostics provide a spatially resolved information on the production of soot, its interaction with the turbulent flow and its link with the flame surface and with soot precursors regions. It is shown in particular that soot pockets are highly distorted by turbulent eddies forming a characteristic layered pattern. A statistical analysis is also proposed to analyze such high-speed imaging results. Information on soot-OH-PAH correlation deduced from the high speed imaging could be employed to verify the adequacy of models devised to represent soot dynamics in direct or large eddy simulations of turbulent flames

Experimental investigation of high-frequency combustion instabilities in liquid rocket engine.

International audienceHigh-frequency instabilities in liquid propellant rocket engines are experimentally investigated in a model scale research facility. Liquid oxygen and gaseous methane are injected in the combustion chamber at 0.9 MPa through three coaxial injectors vertically aligned. High-amplitude transverse pressure fluctuations are generated in the chamber at frequencies above 1 kHz by a rotating toothed wheel actuator which periodically blocks an auxiliary lateral nozzle. The chamber eigenmodes are identified in a first stage by examining the response of the system to a linear frequency sweep. In a second stage the chamber is excited at the frequency corresponding to the first transverse (1T) mode. The effect of the pressure mode on combustion is observed with intensified and high-speed cameras. Photo-multipliers and pressure sensors are also used to characterize the system behavior and examine phase relations between the corresponding signals. Flame structure modifications observed for specific injection conditions correspond to a strong coupling between acoustics and combustion which notably modifies the flow dynamics, augments the flame expansion rate and enhances heat transfer to the wall

Combustion performance of plasma-stabilized lean flames in a gas turbine model combustor

This work presents an experimental study of the stabilization of lean methane-air flames by nanosecond repetitively pulsed (NRP) discharges. The experimental facility consists of a gas turbine model combustor with a Lean-Premixed-Prevaporized injector. The working pressure is 1 atm. The fuel is injected through two stages, each stabilized by swirl. The main stage consists of a multipoint annular injection. This facility is representative of a single sector of a gas turbine combustion chamber. The NRP discharges significantly extend the lean blow-off limit for a wide range of operating conditions, down to an equivalence ratio of 0.16. Lean flame stabilization is demonstrated for flame thermal powers up to 100 kW, with an electric power of less than 0.2% of the flame thermal power. We also observe plasma-assisted lean flames emiting less NOX than the leanest stable flames without plasma. Finally, by exploring the application of various pulse patterns instead of applying the discharges continuously at a constant repetition frequency, the plasma-to-flame power ratio required to stabilize a lean flame is decreased to 0.06% and the pollutant emissions can be further decreased

Structure and dynamics of cryogenic flames at supercritical pressure

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)(O-2 = 5.04 MPa)) but the temperature of the injected liquid oxygen is below its critical value (T-O2 < T-c(O-2) = 154K. Data obtained from imaging of OH* radicals emission, CH* radicals emission in the case of LOx/GCH(4) 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

Structure of cryogenic flames at elevated pressures

This paper presents new experimental results on cryogenic jet flames formed by 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 Damköhler 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

Reduced Order Modeling Approach to Combustion Instabilities of Liquid Rocket Engines

International audienceThis article describes the numerical framework for a reduced order tool which aims at simulating combustion instabilities in liquid rocket engines. The numerical framework relies on the projection of the pressure fluctuations on the eigenmodes of the system. Pressure fluctuations are solutions of the wave equation of the system. After projection on the eigenmodes, the wave equation takes the form of series of second order harmonic equations with source terms that drive combustion instabilities and damping terms that attenuate them. A test rig was developed to study cavity interactions, injector impedance as well as damping effects. Damping rates measured on the test rig show a trend which is consistent with what is observed in liquid rocket engines. On a whole, the test rig can be used to validate simplified models of combustion instabilities. The global framework of the reduced order tool developed to predict combustion instabilities in liquid rocket engines was first validated by comparing the data from simulations against experimental results from the test rig in a series of non-reacting experiments. The tool was then used in a case study of a full-scale rocket engine. This engine, under certain operating conditions, exhibits instabilities. Stable and unstable behaviors have been revealed by the temporal evolution of pressure amplitudes. Nomenclature p = pressure [Pa] x = position vector [m,m,m] t = time [s] * PhD, Ingénieur ISAE-ENSMA, m