Modelling of unsteady combustion in low emission systems

Deliverables

The main results are: - The development of innovative non-intrusive diagnostics in two-phase flow combustion. - The combination of adavanced optical diagnostics to investigate simultaneously the gaseous and liquid phases -- For LEMTA, this development concerns mainly the implementation of two colors laser-induced fluorescence (applied to droplet temperature measurement) in combusting monodisperse droplet streams, the extension of this technique to the measurement of temperature distribution within combusting droplets (for stationary and periodic phenomena), which was the first measurement of this kind. The experimental techniques applied to monodisperse droplet streams are highly useful laboratory tools, which can provide very accurate data (droplet temperature) on single droplets behaviour (even in combustion). Finally, the extension of two colors laser-induced fluorescence to polydisperse spray has been carried out: it constitutes a more flexible tool able to operate closest to real spray conditions and also closest to real aero-engines configurations. For CORIA and ONERA, the development concerns mainly the rainbow technique. The theoretical work carried out at CORIA on the description of the rainbow in the Nussenzveig framework gives the possibility of a more accurate process of the ONERA experimental records by taking into account the ripple structure. Nowadays, we focus our efforts on the effect of radial gradient of temperature in rainbow and LIF measurements. -- Two kinds of diagnostics have been combined - Two colors laser induced fluorescence applied to droplet temperature measurement has been combined with planar laser induced fluorescence (PLIF)applied to concentration measurement of fuel vapor in the close vicinity of the droplet. The relationship between the droplet temperature and the surrounding fuel vapour concentration has been demonstrated and quantified. - The temperature measurement of the liquid droplets by two colors laser-induced fluorescence has been combined with the size measurement by PDA. With the use of these two measurements, the heating of the droplet and its evaporation can be directly monitored. Two kinds of benefits of these results can be distinguished: - The development of these new measurement techniques allows collecting new experimental data on selected test cases. It will promote the development of new physical models and validation tools for the computer codes. It should improve the efficiency and predictive capabilities of the calculation tools for the engine disigners. - The development of the measurement techniques has already promoted cooperations with instrumentation manufacturers (especially DANTEC Dynamics) and manufacturing agreement will be expected shortly for two colors laser-induced fluorescence applied droplet temperature measurements. Furthermore, the priciples of the techniques will be published in archival scientific literature. Further applications of two/three colors LIF on real fuels (like kerozene or diesel or automotive petrol) can be forseen in the next future. The technique is operational on simple single component fuels (ethanol, acetone). Further application on real fuel requires extension of the calibration principles for the real fuel. Then the techniques may be then used in industrial sprays and combustors. Extension of the PLIF technique for fuel vapour concentration measurements in sprays operating with more realistic aeronautical fuels such as kerosene or surrogates is currently under study at ONERA. New molecules will be probed in the future to determine their ability to provide information on equivalence ratio and temperature of the gas phase. Application of the technique to gas turbines operating at real conditions (typically air preheated at 800 K and 3.0 MPa) is foreseen in the short term. LEMTA, ONERA and CORIA participates to a new french program that aims to improve the measurements techniques and especially to address their extension to multicomponent fuels. One of the objectives of this collaboration is the development of the Global rainbow technique to measure the droplet temperature/ Composition in real complex sprays.

The objective of this experimental programme is to evaluate and quantify the influence of radiation on the vaporization process of fuel liquid droplets. The radiation transfer from combustion gases, and the interaction between droplet vaporization and radiation is still ignored in most existent models, although there is evidence that such effects are important and can not be neglected in some applications. The main purpose of the experimental work is to provide data for the validation of modelling work to be carried out. It comprises measurements of droplet vaporisation when exposed to different radiative heat loads. A droplet of liquid fuel (different liquid fuels are to be used) will be exposed to symmetrically arranged radiative intensities. One of the main results will be the construction of a database for different radiation/convection conditions for fuel droplets (different diameters and fuel are to be studied). The structure and contents of this database is to be performed with a close cooperation with an Industrial partner (Snecma). A considerable part of the wall heat load of gas turbine stems from radiation caused by high flame temperatures and high pressures. Therefore, detailed knowledge of the radiative heat load is required to determine the amount of cooling air and to predict wall temperatures. Although the radiative heat transfer is not expected to be as important in gas turbine combustion chambers, as it is in, e.g., large furnaces in steel, glass and cement industries and power station boilers, it is still expected to play an important role, when accurate determination of fuel vaporization rates are concerned, in gas turbine combustion chambers, due to the presence of soot, that cannot be overlooked in certain engine regimes. The radiation influences the gas temperature distribution, and the formation of soot and other pollutants, as well as the flame structure. Hence, there is a need to establish and quantify the contribution of the radiation term in the heat transfer process from the gas to the liquid phase and assess how adequate some vaporisation models are when applied to real sprays. The ultimate objective is to gather the fundamental understanding of the vaporisation/radiation interaction phenomena, so that a comprehensive analytical model can be produced. This would allow designers to verify the adequate performance of a combustion model before committing to an expensive validation programme.

Models that have been developed the in the past to explain important mechanisms that control extinction and stability concentrated on simple 1-D flame configurations. Additionally effects of turbulence usually were of minor interest. Since most practical combustion devices burn liquid fuels like kerosene and exhibit complex turbulent flow patterns, the results of those idealised systems cannot be extrapolated to turbulent reacting two-phase flow systems. On the other hand, due to computational limitations, it is not possible to include all mechanisms describing accurately extinction phenomena into a turbulent combustion model. Therefore, it is important to identify the key parameters that are responsible for the extinction of turbulent aerodynamically stabilised flames. The stability performance of practical systems is strongly influenced by several hardware specific parameters like swirl number, air distribution, fuel placement and spray characteristics. In a first step the investigation will concentrate on gaseous flames in order to reduce the complexity of the system. The presumably most important parameters are the turbulent length scales and the fluctuation intensities of both velocity and species mass fractions as well as an appropriate chemical time scale. Turbulent combustion models, that incorporate these parameters mentioned above while not making excessive computational demands are models based on a presumed PDF approach (PDF = Probability Density Function). Such approaches that are based on a mixture fraction and a single reaction progress variable have proven to be suitable for describing important characteristics of swirling flames such as the lean blow off limit. In any case the success of a PDF-based model is crucially dependent on the ability to describe marginal PDFs and a reliable reaction model that is able to capture the main characteristics of the combustion chemistry near extinction. The aim of this task is the extension and validation of an existing JPDF model, already used and verified within the scope of the CFD4C project. The knowledge of the JPDFs of mixture fraction and reaction progress near extinction is crucial in order to assess the applicability of the turbulent reaction model. Existing detailed experimental data for turbulent flow and mixing will serve as a database for the validation and evaluation of the presumed PDF model. For the selected test cases, numerical simulations will be performed and the calculated PDFs will be compared with the experimental data. Based on this evaluation it will be possible to assess the necessity for the more sophisticated but computationally more expensive transported PDF models in order to describe the physics of flames near extinction. If the key parameter is instead the proper representation of the chemical kinetics, the reaction model will be altered in order to describe lean blowout with satisfactory accuracy. Finally the model will be applied to the same combustor but for different operating conditions and its applicability will be shown. The results of the project will deepen the understanding of physical sub processes that lead to extinction of turbulent aerodynamically stabilised diffusion flames and will shed a light on the necessity to utilise more complex turbulent combustion models. The resulting model will improve the capability of predicting lean stability limits of aero engine combustors and might guide the design and development procedure for combustors that meet stringent demands on increasing combustion performance.

Using the innovative non-intrusive diagnostics in combusting two phase flow developped in the framework of the MUSCLES program (two colors laser induced fluorescence combined to PDA or planar laser-induced fluorescence on vapor of acetone, rainbow thermometry or IR thermometry), a database on several basic test cases in both evaporating/combusting monodissperse droplet streams and evaporating polydisperse sprays has been constructed. The database contains a wide rang of test cases has been constructed: - Combusting ethanol droplet stream: simultaneous droplet diameter and temperature measurements, for various inter droplet distances - Evaporating acetone droplet stream (pre-heated fuel or droplets injected in heated air): simultaneous droplet diameter and temperature measurement, with acetone vapour concentration characterisation - Ethanol polydisperse spray: droplet diameter distribution with measurement of local droplet mean temperature. The main benefits of these results are: - The data collected on the different experiments will be used according to two directions The test cases on laboratory experiments will promote the development of more accurate physical models. The measurements and identification of the heating and evaporation fluxes on streaming and combusting droplets with interaction phenomena has been carried out: new correlations with more physical phenomena will be established and could be valuably inserted in computational codes of industrial partners, for the design of combustion chambers. The measurement of the temperature distribution within droplets is a great step in the understanding of the heat transfer within droplets, especially during the heating phase of the droplets in the combustion chamber. New heat transfer models, taking into account the fluid circulation inside the droplets will be developed in the light of these experimental data. All these experimental data and new physical models will be published in both archival literature and International Conferences. The test case on polydisperse evaporating spray is closest to real sprays in combustion chambers and will help the industrial partners to validate and assess the existing RANS/LES computational codes.

Low-order model that predicts frequency and amplitude of oscillations induced by combustion-acoustic interactions in gas turbine combustors. The results can be exploited to better understand non-linearity and to provide design tools for gas turbine manufacturers.

The comparisons between the different competitive techniques will help in a better understanding of the range of use and potentialities of the tested techniques and provide a gain in the understanding of their sensitivity to the different experimental parameters. The following tools are used to build the comparisons: - An experimental bench generating a mono-disperse droplet stream in different configurations such as heating, cooling, evaporating and burning, isolated droplet or dense streams, with the following measurement techniques: -- LIF mean temperature measurement technique, developed at LEMTA; -- IR surface temperature measurement technique, developed at ONERA; -- Rainow refractometry mean temperature and droplet size measurement technique, currently under development at ONERA ; - Evaporation models of monocomponent droplet streams ; - Droplet-laser interaction models, including Generalised Lorenz-Mie Theory, to compute Rainbow signals. The definitions, assumptions and sensitivity are different among the above listed. The comparisons that are made give way to improvements and, as the whole process is iterated, individual convergence and global agreements are expected. This process will lead to a better understanding of the phenomena under study

This aim of this experiment is getting information about the unsteady characteristics of the two-phase flow inside a LPP duct. The rate of fuel vaporisation, the mixing process and the fuel placement at the combustor inlet are investigated that all play a primary role in the inception of combustion instabilities phenomena. The experiments are carried out on a large-scale model of an industrial swirl-flow LPP burner designed by Avio at operating conditions scaled at atmospheric conditions. Two configurations are investigated (different geometries of the radial inflow swirlers with same swirl number). With the help of advanced measurement techniques (LDV, PDA, PIV) the time-varying characteristics of the continuous as well as the dispersed phases have been investigated. The mutual effects of the unsteady air flow and liquid fuel spray are analysed. The data give an idea of the complex interaction of the vortex breakdown, and flow recirculation within swirling premixing devices and their possible influence on combustion oscillations. As an outcome of this task, information will be obtained for advanced swirl-premixing LPP devices in order to prevent dangerous working conditions, such as flash-back and auto-ignition. The experimental study is accompanied by numerical simulations that give a deeper insight into the complex physical interactions contributing to the analysis and comprehension of the experimental results. The CFD code developed by University of Genova, NastComb, based on Lagrangian time-derivatives, is used as a support for the interpretation of the experimental data and in the same time it was validated on the database produced by experiments. This code will be extensively used in the next future as a numerical tool for the design of fuel preparation systems.

The efficiency and pollutant formation as well as the combustion stability in a gas turbine engine operated in lean premix conditions, is strongly dependent on the atomization and mixing processes. An efficient atomization process, producing a spray with a small mean drop size, will achieve a greater degree of vaporization in a given premix time than a spray of larger mean drop size and, therefore, achieve a higher degree of premix in a given fuel injector. Similarly, the degree of uniformity of the spray, depending on the liquid droplets dispersion in the air-stream, may have an impact on temperature uniformity. While both of these features influence pollutant formation, reaction temperature uniformity has also an impact on flame stability that could result in, or suppress, resonant pressure fluctuations via Rayleigh coupling. Liquid jets in cross flows are often used as a means of introducing fuel into premix ducts. The dynamic behaviour of a liquid jet yields stripping/shedding mechanisms, which may be the inception of sub-harmonic fluctuation in downstream premixed combustion. A more detailed understanding of the phenomena occurring can clarify the role of different mechanisms in promoting or suppressing the oscillating behaviour of combustion and is a mandatory step to further evolve gas-turbine engines. This task aimed to study the injection of a round liquid jet of fuel in a premixing channel where a high pressure, high temperature airflow flows across the injection direction. A better understanding of the unsteady phenomena associated with jet break-up and droplet formation will also improve the accuracy of CFD calculations. It will help to improve the specification of boundary conditions through a better quantification of initial droplet conditions and will also lead to improved modelling capabilities through a better understanding of the underlying physics. In this framework the main achievements of this part of the project were - The region close to the nozzle tip presents a dramatic reduction of the instability with respect to the surrounding, allowing identifying an extremely stable (virtual) core of the liquid jet. - Liquid properties play a significant role in the onset of oscillating behaviour of the jet. The computation of a fluctuation index showed that the lower surface tension of jet A-1 is responsible of a more extensive destabilization of the spray plume. This is an important result to be taken in account when using experimental results obtained on the round of experiments made on jet of other liquids (typically water). - The orientation of the jet axis with respect to the airflow direction affects the liquid atomization and dispersion in the airflow. Experiments suggested that the inclination of the spray axis in a partially opposed direction with respect to the airflow is beneficial to the liquid dispersion in the air and can be useful in achieving a faster and more uniform mixing. Further studies are required to identify the optimum geometry. - Atomization and deformation do not significantly affect the penetration of the kerosene jet along the liquid injection direction; the distance of the breakdown position depends mainly on the momentum ratio and only weakly on the Weber number. Only a weak dependence on the air viscosity has been found. - A significant correction on the extension of the liquid entrainment along gas streamwise direction has to be performed in respect to the low temperature condition. More specifically the breakdown position along this direction is dependent on aerodynamic Weber number calculated on the liquid jet velocity. This affects the deformation and consequently the drag entrainment of the kerosene column as well as of liquid fragments of the primary atomization. - The generalized profile equation can be successfully scaled with only two parameters, which are the coordinates of the breakdown position. - The breakdown process occurs downstream of the break-up and contact lengths. Its average position can be determined reliably in non-reacting environment with automatic and objective procedures. - The spray evaporation does not affect significantly these atomization aspects, even though it affects other characteristics.

Numerical part Although gas-phase turbulent combustion modeling have been widely studied with a wide range of suggested closures, relatively few studies have been dedicated to turbulent spray combustion and acoustic because of modeling difficulties. When undergoing turbulent fluctuations or acoustic waves, droplets tend to form clusters leading to local high level of droplet density. The present work points out evaporating droplet influence on mixture fraction and proposes how to introduce these effects in turbulent combustion models. Month 12 (D 4.2): Development of a DNS Database This work considers a geometry involving one non-homogeneous direction: the spatially decaying turbulence (SDT). It simulates a grid-turbulence with a high kinetic energy at the inlet that decays along the streamwise direction. Thus the droplets undergo a natural polydispersion process. Even if the injected spray is monodisperse, polydispersion occurs because of joint effect of droplet evaporation and their mixing by turbulence structures. To evaluate the spray impact on the mixing, the injected turbulence properties remain the same in all the configurations. Test cases are thus performed with the same injected turbulence and the observed differences occur because of two joint effect: the turbulence modification induced by droplet momentum and the local evaporation rate resulting from the droplet dispersion. Considering data storage, memory and time consumption, we assure a compromise with a configuration that computes a million of droplets tracked individually in a Lagrangian frame that evolves onto a 129x65x65 Eulerian mesh. Then, to respect DNS restriction, the size of the domain is (respectively) 16 l_t x 8 l_t x 8 l_t for an injected spectral turbulence characterized by an acoustic Reynolds number Re = 5000 and a 50% turbulent rate. COntact for the database: Julien.Reveillon@coria.fr Month 24 (D 4.6 & 4.7): Analysis of subgrid models for spray evaporation In the context of either Reynolds averaged Navier-Stokes calculations (RANS) or Large Eddy Simulation, non-premixed (or partially premixed) turbulent combustion usually adopts the mixture fraction concept.This conserved scalar indicates the mixing between reactives: $Z = 0$ in the oxidizer stream and $Z=1$ in the gaseous fuel stream. To provide information on reactant mixing through the computational domain, one strategy is to presume the form of the probability density function (PDF). The beta-PDF is the most commonly used shape. It requires two input parameters: the large scale level of the mixture fraction, and its subgrid fluctuations Z_v. All the models that have been found in the litterature to close the mixture fraction evolution and specific models developped in the framework of the European project have been tested thanks to the database developped during year 1. Details may be found in our 24th month or final report. Month 36 - extended Month 42 (D4.15): Combustion instabilities are observed in numerous industrial systems and more particularly in aeronautical engines. They create many undesirable effects as, for example, an increase of wall heat fluxes, flames extinction and flashback, or strong vibrations of the mechanical structure, which can lead to its destruction. In spite of many research tasks based on this topic, these instabilities are difficult, if not impossible, to predict. To begin with, it is necessary to identify the phenomena responsible for the presence of combustion instabilities and their consequences on various processes such as injection, atomization, spray evaporation, reactants mixing, chemical reactions, interactions between flames and walls, etc. For that reason, multiple theoretical, experimental and numerical works are dedicated to the understanding and the analysis of the fundamental physical mechanisms of the couplings between these various processes and acoustic phenomena in the chamber. To our knowledge, there is no numerical simulation dedicated to the analysis of the interactions between spray combustion and acoustic instabilities. The objective of this task is two-fold: first to demonstrate the capability of a DNS solver to capture the complex spray/flame/acoustic interactions and then, to focus on the impacts of velocity modulations on reaction rate through an analysis of the transfer function. A classical Bunsen configuration has been selected and experimental comparisons were made possible thanks to the data of the EM2C laboratory, Ecole Centrale Paris obtained in the framework of this project. The final report shows a complete validation of the simulations with the experimental data of EM2C laboratory. The validation has been made for both gaseous and two-phase flows and all the reduced frequencies up to a value of 30 through the transfer function amplitude and phase.

The main objective of this experimental programme is to identify aerodynamic features within a gas turbine combustion system that may be sensitive to pressure fluctuations generated by the presence of heat release. Such features could provide a feedback mechanism and thereby magnify oscillations, associated with the unsteady heat release process, at certain frequencies. Measurements have therefore been undertaken at nominally atmospheric pressure to examine various parts of the combustor aerodynamic flow field and assess its sensitivity to acoustic excitation over a range of frequencies (50Hz ~ 1500Hz). The acoustic excitation is provided by loudspeakers to enable the non-reacting flow to be studied, thereby allowing decoupling of the combustion and airflow instabilities. The work has been undertaken in 2 phases namely; (i) Phase 1 in which both axial and radial low emission fuel injectors are studied in isolation and (ii) Phase 2 where a sector rig is used to study the effect of excitation on the overall combustor flow field. The phase 1 measurements were initially with single phase flow (i.e. airflow only) and utilised an axial and radial fuel injectors of comparable effective areas. Analysis of the experimental data enabled the response of each injector flow field to the acoustic excitation to be characterised over the range of excitation. At low frequencies the injectors behave in a quasi-steady way in which the acoustically generated change in static pressure, at the injector exit plane, alters the injector pressure drop and hence the mass flow passing through it. As the excitation frequency is increased, though, the response of the injector flow field generally decreases relative to that of the quasi-steady case. However, it should be noted that superimposed on this distribution are (i) localised maximum and minima and (ii) different parts of the injector flow field appear to respond differently to the excitation. Further two phase flow measurements were undertaken in which liquid fuel was simulated by the introduction of water into the fuel injector. Laser light sheet imaging techniques were then used to visualise the fuel sheet and subsequent break up downstream of the injector. Phase averaging of the images enabled the effect of excitation to be identified relative to the random fluctuations in the fuel spray. The observed periodic fluctuations of the fuel spray at various frequencies appears to correlate with the flow field response measured in the single phase measurements. However, further measurements would be required to confirm this. In stage 2 measurements have been made within an aero-style gas turbine combustion system incoporating a pre-diffuser, dump cavity and flame tube. In certain regions of the combustion system naturally occurring flow field instabilities were observed. When the acoustic pressure fluctuations are applied at the frequency of the naturally occurring instabilities a large flow field response is observed for modest excitation levels. Measurements also indicated how these coherent structures can be convected downstream and influence the flame tube and internal flow field along with other regions of the combustor The experimental programme has characterised the response of various combustor flow field features to acoustic excitation. This characterisation, along with an understanding of the mechanisms responsible for the observed characteristics, will be of significant benefit to the designer of modern low emission gas turbine systems. These are particularly prone to combustion instabilities due to the desire to operate at lean conditions in order to maximise the emissions performance. In addition to improved modelling of these instabilities components can be designed such that, where possible, there response characteristics do not match that of the preferred frequencies at which instabilities are observed in a specific combustor geometry. In this way the potential feedback between fluctuations in the combustor flow field and the unsteady heat release process can be minimised. Hence low emission systems can be designed with both good emissions performance and acceptable levels of fluctuating pressures within the combustion system. It is the intention to submit the above results in various publications with the consent of the MUSCLES partners and European Union.

This task is mainly experimental and is focused on the fluctuations analysis of swirl stabilised kerosene flames. It is clear that coherent and quasi-periodic oscillations characterise isothermal swirl flows but only few of these features are effective in burning conditions for spray flames. The scope of this experimental work is trying to clarify this lack of knowledge that affects whatever kind of enhancement in terms of combustion efficiency and pollutant abatement. This is very outstanding taking into account that stabilisation mechanisms are mainly based on this type of configuration for diffusion controlled combustion. This work will provide detailed data of kerosene fired, reactive flows with strongly three-dimensional and fluctuating velocity and temperature distributions. The extended database will provide an advanced understanding of this type of flames and will be used as a benchmark for the validation of numerical calculations. In particular, PDPA measurements with high level of resolution coupled with a high-speed camera will follow the evolution of dimensions and trajectories of particles and pollutant species. The different classes of particles will be distinguished by Time Resolved Laser Light Scattering. The experimental data and the correlations developed will be used as input database for CFD simulations or test cases taking into account the local flame oscillations close to extinction. Data analysis will be discussed with the industrial partner for the improvement of in-house CFD codes and to implement optimised design rules for combustors.

Goal of the research project was to investigate how pressure oscillations inside an aero engine combustor may affect the spray generated by an airblast nozzle. The interaction of the pressure oscillations inside the combustor and spray formation of air-blast nozzle was studied experimentally. Diagnostics included phase-locked (with respect to the pressure oscilaltions) Mie scattering from the droplets and phase locked PDA measurements of the droplets. In addition, the flow field was investigated by LDA. The most important result was that at constant mass flow of fuel: - The size of the droplets emerging form the nozzle is almost anaffected by the pressure oscillations. - The droplet rate oscillates strongly with the same frequency as the pressure oscillations. - There is a phase shift between pressure oscillation and the fluctuations of the droplet rate which depends on the frequency of the pressure oscillations.

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