Numerical and Experimental Investigations of Combustion Instabilities of Swirled Premixed Methane-Air Flames With Hydrogen Addition

In this work, hydrogen assisted (hydrogen enrichment and piloting) swirl stabilized flames are studied experimentally via MIRADAS experiment. First of all, static stability characteristics, such as flame lengths and flame attachment characteristics are studied via CH* chemiluminescence flame images and cases with hydrogen piloting, methane piloting and hydrogen enrichment are compared to the reference case of perfectly premixed methane-air combustion for a wide range of equivalence ratios and bulk velocities. It is found out that hydrogen piloting is the most efficient method to attach the flames and extend operating ranges of the combustion chamber. Next the dynamic stability characteristics of the setup is studied experimentally via stability maps and it is shown that injection of a very small portion of the thermal power worth of hydrogen results in a more stable system and an extension in the stable operating points in the stability maps, meaning safer overall operation. Hydrogen enrichment and methane piloting are also explored, and it is demonstrated that these methods are not effective in changing stability maps, stability maps are not effected. Subsequently, the forced flame responses are studied experimentally and it is shown that hydrogen piloting and hydrogen enrichment causes a drop in the global time delay of the flame transfer function. With hydrogen piloting, there is a global drop in the flame transfer function gain, however for hydrogen enriched cases, the gain is increased. For methane piloted cases, there is a global reduction in the flame transfer function gain, however the time delay is not affected. Consequently, to explore why and how the global flame transfer function is changed with different injection strategies, forced flame images are studied. It is shown that the changes in flame transfer function is caused by the competition behavior between the local heat release responses for hydrogen piloted cases. Simply put, there is a phase difference between the local responses near the injection tube and the flame edges, causing a "pull-back" effect, which in turn causes a drop in the flame transfer function gain. Next the effect of different injection strategies on the pollutant emissions are investigated. It is demonstrated that adding hydrogen, in pilot injection or hydrogen enrichment configuration, causes a drop in CO_2 emissions for the same thermal power. Piloting strategies cause a slight increase in NOx emissions, however results show that an optimization is possible to obtain flames that are stable, low CO_2and low NOx. Finally, LES calculations and their comparisons with experimental results are presented. The capability of LES calculations in predicting flame responses is demonstrated and it is shown that the flame responses originate from the interactions of the vortices that are formed as a result of acoustic pulsations and the flames. Flames are wrapped around these vortices which increase the flame surface area. Further down the forcing cycle, the rolled up portions of the flames start touching the combustion chamber walls and gets quenched which causes a loss of flame surface area. These changes in flame surface area result in a fluctuating heat release rate, consisting the flame response

A generalized non-reflecting inlet boundary condition for steady and forced compressible flows with injection of vortical and acoustic waves

This paper describes a new boundary condition for subsonic inlets in compressible flow solvers. The method uses characteristic analysis based on wave decomposition and the paper discusses how to specify the amplitude of incoming waves to inject simultaneously three-dimensional turbulence and one-dimensional acoustic waves while still being non-reflecting for outgoing acoustic waves. The non-reflecting property is ensured by using developments proposed by Polifke et al. [1, 2]. They are combined with a novel formulation to inject turbulence and acoustic waves simultaneously at an inlet. The paper discusses the compromise which must be sought by the boundary condition formulation between conflicting objectives: respecting target unsteady inlet velocities (for turbulence and acoustics), avoiding a drift of the mean inlet velocities and ensuring non-reflecting performances for waves reaching the inlet from the computational domain. This well-known limit of classical formulations is improved by the new approach which ensures that the mean inlet velocities do not drift, that the unsteady components of velocity (turbulence and acoustics) are correctly introduced into the domain and that the inlet remains non-reflecting. These properties are crucial for forced unsteady flows but the same formulation is also useful for unforced cases where it allows to reach convergence faster. The method is presented by focusing on the expression of the ingoing waves and comparing it with the classical NSCBC approach [3]. Four tests are then described: (1) the injection of acoustic waves through a non reflecting inlet, (2) the compressible flow establishment in a nozzle, (3) the simultaneous injection of turbulence and ingoing acoustic waves into a duct terminated by a reflecting outlet and (4) a turbulent, acoustically forced Bunsen-type premixed flame

Impact of symmetry breaking on the Flame Transfer Function of a laminar premixed flame

This work presents a numerical study of the acoustic response of a laminar flame with tunable asymmetry. A V-shaped premixed flame is stabilised in the wake of a cylindrical flame holder that can be rotated. The configuration is symmetric when the flame holder is fixed but increasing its rotation rate breaks the symmetry of the flow. This configuration is submitted to acoustic forcing to measure the effect of rotation of the flame holder on the Flame Transfer Functions. It appears that the asymmetry of the two flame branches changes their respective time delays, resulting in interference in the global unsteady heat release rate fluctuations. Consequently, the Flame Transfer Function exhibits dips and bumps, which are studied via laminar Direct Numerical Simulation. Potential applications for the control of combustion instabilities are discussed

Impact of symmetry breaking on the Flame Transfer Function of a laminar premixed flame

This work presents a numerical study of the acoustic response of a laminar flame with tunable asymmetry. A V-shaped premixed flame is stabilised in the wake of a cylindrical flame holder that can be rotated. The configuration is symmetric when the flame holder is fixed but increasing its rotation rate breaks the symmetry of the flow. This configuration is submitted to acoustic forcing to measure the effect of rotation of the flame holder on the Flame Transfer Functions. It appears that the asymmetry of the two flame branches changes their respective time delays, resulting in interference in the global unsteady heat release rate fluctuations. Consequently, the Flame Transfer Function exhibits dips and bumps, which are studied via laminar Direct Numerical Simulation. Potential applications for the control of combustion instabilities are discussed

Suppression of instabilities of swirled premixed flames with minimal secondary hydrogen injection

The impact of hydrogen addition on the dynamics of a methane-air premixed flame is explored for different injection strategies. The configuration is a swirled injector with a central tube for pilot fuel injection. Keeping the air flow rate and thermal power of the burner constant, it is shown that even for very small flow rates of hydrogen, as low as one percent of the thermal power, flame stabilization and combus- tor stability are greatly altered when pure hydrogen is injected through the central tube as a pilot jet. It is also shown that fully premixing the same quantity of hydrogen with methane or use of methane for the pilot jet has no significant effects compared to hydrogen pilot injection strategy. The flame re- sponse to forced flow perturbations is use to interpret the observed features. It is shown that hydrogen piloting drastically changes the gain of the flame transfer function at low frequencies and its phase lag at high frequencies, while other injection strategies barely change the flame response for these minute flowrates. CO and NOx emissions are finally examined for the different injection strategies. NOx emissions are found to drastically increase with hydrogen piloting compared to other injection strategies. These ex- periments indicate that pure hydrogen injected in minute fractions may be used as an efficient passive control means to mitigate combustion instabilities, but a compromise needs to be made with emissions

Phosphor thermometry on a rotating flame holder for combustion applications

This study presents a method to measure wall temperatures of a rotating flame holder, which could be used as a combustion control device. Laser induced phosphorescence (LIP) is found to be a reliable technique to gather such experimental data. The paper first investigates how the coating (thickness, emissivity and lifetime) influence the flame stabilization. While the low thermal conductivity of the coating is found to induce a temperature dif- ference of 3 − 5 K, the emissivity increases by 40%. Nevertheless, the transient and steady-state flame location are not affected. Second, because temperature measurements on the rotating cylinder are likely to fail due the long phosphor lifetimes, we modify the classical point-wise arrangement. We propose to illu- minate a larger area, and to correct the signal with a distortion function that accounts for the displacement of the target. An analytical distortion function is derived and compared to measured ones. It shows that the range of mea- surements is limited by the signal extinction and the rapid distortion function decay. A diagram summarizes the range of operating conditions where mea- surements are valid. Eventually, these experimental data are used to validate direct numerical simulations. A sensitivity analysis shows that the precision of the technique still gives a good agreement for the flame location

Influence of hydrogen content and injection scheme on the describing function of swirled flames

The dynamics of V-shape swirled lean premixed methane/air flames enriched with hydrogen is examined for two injection schemes. The response of the flames submitted to harmonic flowrate modulations is compared when hy- drogen is premixed with the main methane/air flow and when it is injected pure as a pilot jet, directly at the flame base. Experiments are carried out at constant thermal power. Results obtained for the flame describing function (FDF) show that for a given hydrogen content, the premixed and pilot injection strategies lead to drastically different responses, although the shape of these flames are close. In the fully premixed strategy, the frequency bandwidth over which the flame is very responsive widens as the flame shortens due to the higher reactivity of the hydrogen enriched combustible mixture. Increasing the hydrogen content leads to an increased receptivity of the premixed flame sheet to incident flow perturbations. The opposite effect is seen for the pilot injection strategy due to a rebalancing of the heat release rate distribution along the flame brush with higher reaction rates close to the flame base compared to the reference methane/air case and the fully premixed hydrogen injection strategies. With hydrogen pilot injection, heat release rate fluctuations at the flame base interfere with those further downstream along the reaction layer, leading to an overall reduction of the FDF gain. This mechanism is evidenced with a set of experiments and confirmed by a low order model that considers a non uniform distribution of the heat release along a wrinkled flame sheet. It is also shown to persist when the forcing level is varied. These experiments indicate that the FDF of swirling V-flames can be lowered over a broad frequency range with hydrogen piloting due to a higher reactivity at the flame base

Detection of precursors of combustion instability using convolutional recurrent neural networks

Many combustors are prone to Thermoacoustic Instabilities (TAI). Being able to avoid TAI is mandatory to efficiently operate a system without sacrificing neither performance nor safety. Based on Deep Learning techniques, and more specifically Convolutional Recurrent Neural Networks (CRNN)1, this study presents a tool able to detect and translate precursors of TAI in a swirled combustor for different fuel injection strategies. The tool is trained to use only time-series recorded by a few sensors in stable conditions to predict the proximity of unstable operating points on a mass flow rate / equivalence ratio operating map, offering a real-time information on the margin of the system versus TAI. This allows to change operating conditions, and detect the directions to avoid in order to remain in the stable domain

Stabilization mechanisms of CH4 premixed swirled flame enriched with a non-premixed hydrogen injection

High-fidelity Large Eddy Simulations (LES) are performed to study the effect of hydrogen injection on a lean turbulent CH4 /Air premixed flame. An Analytically Reduced Chemistry (ARC) mechanism is used to achieve a detailed description of CH4/Air-H2 chemistry. First, a validation of this kinetic scheme against the detailed GRI-Mech 3.0 mechanism is presented considering both simplified and complex transport properties. When hydrogen is added to the mixture, large variations of the mixture Prandtl and of the N2 Schmidt numbers are observed depending on the local species concentrations, features that are missed by simplified models. LES is then applied to study the structure and stabilization mechanisms of a lean (φ = 0.8) premixed CH4/Air swirled flame enriched with hydrogen by using different transport modeling strategies. First, the fully pre- mixed CH4/Air case is considered and results are found to validate the LES approach. In agreement with experiments, a classical V-shape flame is stabilized in the low-velocity region near the flame holder created by a central recirculation zone (CRZ). Then, hydrogen enrichment is achieved injecting 2% of the CH4 thermal power with a central fuel injection lance. Both premixed and diffusion flame branches are present in this case, impacting flame stabilization and flame angle. The flame root of the main premixed flame is stabilized by a diffusion flame kernel created by the injected hydrogen reacting with the oxygen in excess of the premixed stream. Moreover, the H2 consumed with the remaining oxygen in burnt gases leads to the formation of a second flame branch inside the CRZ which is responsible of an increase of the flame angle. Given the high concentration of hydrogen, an impact of the molecular transport models is observed on the flame lift-off height highlighting the importance of using complex transport properties in any LES involving hydrogen combustion

Mesure de la température d'un accroche-flamme tournant par phosphorescence induite par laser

International audienceLa connaissance précise des températures de parois de chambres de combustion aéronautiques est primordiale avec le développement de foyers compacts efficaces et peu polluants. Cette étude décrit l’utilisation de la phosphorescence induite par laser (LIP en anglais) appliquée à un accroche-flamme cylindrique mis en rotation, ce dernier pouvant servir à effectuer du contrôle de la combustion. Afin de tenir compte de la durée de décroissance des luminophores et du temps effectif de collection du signal, le dispositif classique ponctuel est modifié afin d’illuminer une plus grande surface de l’objet. Une fonction de correction analytique est dérivée afin de tenir compte de la convolution systématique du signal avec le mouvement du cylindre. La principale limitation de ce type de traitement est liée la perte rapide de signal. Les mesures obtenues valident des simulations numériques isothermes. Les résultats indiquent que les variations de température liées à la précision de la technique sont négligeables alors qu’un écart plus important altère la position de stabilisation de la flamme