Flame-acoustic Interaction Research Articles

Flame-acoustic interactions in high-pressure H2/air diffusion flames

Understanding flame-acoustics interaction (FAI) that can potentially trigger thermoacoustic instabilities is of critical importance to mitigate the serious side effects of pressure oscillations. While FAI has been studied extensively in premixed flames, comparatively less analysis has been conducted to explore FAI mechanisms in non-premixed flames, especially under high-pressure conditions. This study aims to numerically investigate FAI in high-pressure hydrogen/air counterflow diffusion flames, with special attention on the impact of pressure on FAI. To this end, a fully compressible unsteady counterflow solver combined with Navier-Stokes Characteristic Boundary Conditions (NSCBC) is employed to capture the reflection of the acoustic waves at the boundaries. The results show that for all ambient pressures ( 1 ≤ P a ≤ 60 bar) considered here, real-gas effects on flame structures are negligible and that increasing P a would lead to a power-law dependence of density gradient on P a with an exponent of 1.5. Furthermore, all tested cases show acoustic growth with time under fully reflecting boundary conditions, which is found to be pressure dependent. The flames become less resilient to FAI when the pressure (or density gradient) is increased. The results from the spectral analysis show that for all ambient pressures, there is only a single dominant frequency at the low-density fuel side and two dominant frequencies at the high-density oxidiser side. This indicates that the pressure oscillation is more sinusoidal at the H 2 side than that at the air side. Moreover, the phase space portraits of pressure signals indicate that the dynamics of the system are becoming periodic and approaching a limit cycle. It is found that the integrated heat release rate and the pressure fluctuations are partially in phase, which is responsible for the growth of high-frequency acoustic waves at both fuel and oxidiser sides. On the other hand, the significant density gradient contributes to the growth of low-frequency acoustic waves at the oxidiser side due to the high degree of acoustic reflectivity at the density boundary.

Experimental study on curvature effects and preferential diffusion for perturbed laminar premixed ammonia-air flames

Ammonia-air flames are known for low reactivity and have been posing as a huge hindrance in employing the chemical as a sustainable fuel of tomorrow. Curvature is a parameter that could influence the flame structure and so the position of the maximum heat release rate. Flame-acoustic interactions on a Bunsen burner are performed to study the local flame response to highly perturbed flows. NH2* chemiluminescence is used to study the reactivity of these flames. Non-perturbed flames are used as a reference to understand the inherent behaviour of Bunsen ammonia flames. A case study has been chosen for an equivalence ratio ranging between 1.0 and 1.4 at atmospheric conditions to study perturbed flames. The objective is to study the effect of curvature induced by the perturbations on the reactivity of the flame. It was seen that this given case study was quite complex as the flame response was to multiple factors like the effect of Lewis number, convective-diffusion velocities, decomposition of ammonia into hydrogen, thereby, promoting preferential diffusion of hydrogen in both large-scale and locally for certain cases apart from the generated acoustic perturbation which itself dictates the flow regime of the fresh gases, etc. Since the Damköhler number was around 1, the perturbation time scales and the reactivity time scales were comparable and so none of the effects could be ignored. It was concluded that for richer flames where Le>1, the negative curvature promoted the production of hydrogen leading to local enhancement in reactivity. A change in the local thickness due to the induced curvature was seen for all conditions.

Modal dynamics of self-excited thermoacoustic instabilities in even and odd numbered networks of lean-premixed combustors

Can-to-can acoustic interactions in a circumferential network of lean-premixed combustors have a prominent role in the development of complicated modal dynamics and large-scale pattern formations in can-annular gas turbine combustion systems. Despite recent progress in understanding these instabilities, however, previous relevant studies have been limited to an even number of constituent combustors; a fundamental characteristic of the modal dynamics in a can-annular arrangement consisting of an odd number of combustors remains unknown. To address this question, we compare key features of thermoacoustic instabilities in four-coupled (N = 4) and five-coupled (N = 5) can-annular combustor configurations obtained over a broad range of operating conditions. In conjunction with FEM-based Helmholtz simulations, our experimental data show that the dynamic behavior of the even-N case is characterized by the well-known interaction pattern, out-of-phase alternating modulations among the networked combustors. For the odd-N case, however, the can-annular system features complex phase dynamics originating from simultaneous excitation of the degenerate mode pair. We highlight that the development of self-excited azimuthal instabilities in the annular cross-talk section is associated with the modal interaction of the degenerate pair in the odd-N configuration. The initially degenerate pair can be split into two closely-spaced non-degenerate modes, and in consequence their modal interaction gives rise to a periodic transition between spinning and standing azimuthal modes in the annulus. Despite the formation of remarkably different inter-combustor interactions in the two can-annular configurations, however, the entire set of instability data collapses into very similar frequency regimes, suggesting the importance of flame-acoustic interactions occurring within each combustor, namely intra-combustor interactions.

Numerical Investigation of the Effects of Chemical Species and Chemical Kinetic Mechanisms on Laminar Premixed Flame-Acoustic Wave Interactions

Numerical simulation of interactions between acoustic waves and flames is of utmost importance in thermo-acoustic instability research. In this study, interactions between a one-dimensional Methane-Air laminar premixed flame and acoustic waves with a frequency of 50 to 50000 Hz are simulated by simultaneously solving the equations for energy conservation, chemical species transport, state and continuity in one-dimensional space. By assuming that the flame thickness is smaller than the acoustic wavelength, the spatial pressure fluctuations can be neglected and the flame experiences only a time-varying acoustic pressure. The GRI mechanisms, as well as their reduced mechanisms, are considered to obtain results for steady flames without acoustic waves, and the interaction of unsteady flames with acoustic waves. Results show that the total heat-release-rate fluctuations for the flame is affected by increasing the frequency of the acoustic wave. An increase in frequency first increases the total heat released, and then decreases it. The obtained results are in good agreement with those of other researchers. Furthermore, at the presence of acoustic waves, various chemical species can affect the total heat-release-rate fluctuations. With Rayleigh's instability criterion, it can be shown that H2O, CO2 and O2 are the main species to the fluctuations of the total heat release rate and lead to flame instability. Results show that heat-release-rate of H2O specie is the most important on the total heat-release-rate. Therefore, for the flame-acoustic waves interaction problem, the best mechanism is the one that could predict the concentration of H2O more precisely.

Pressure dependence of combustion instability for premixed syngas/air in a closed channel

Syngas is a promising alternative energy carrier with low carbon and pollutants emissions, which has huge application potential in internal combustion engines and gas turbines. The combustion characteristics of stoichiometric syngas/air mixtures with varied initial pressure (0.5–2.5 atm) and hydrogen content (10%–90% v/v) were examined through experiments in a rectangular closed channel. The flame images and overpressure dynamics were captured by high-speed schlieren photography and pressure sensors. The flame instabilities and the flame-acoustic wave interaction were explored. The results showed that the flame morphology and dynamics were enhanced with increasing hydrogen content and initial pressure. The Darrius-Landau instability has an impact on flame deformation, which was strengthened with growing initial pressure, while the thermo-diffusive instability could be neglected during combustion. At later stages after tulip inversion, the flame-acoustic wave interaction was relatively outstanding inducing wrinkled flame front and cellular structures on the flame surface.

A Novel Decomposition Approach Preventing Spurious Entropy Generation in Hybrid Thermoacoustic Stability Computations

Abstract Prominent approaches for the computation of thermoacoustic stability are hybrid methods like the linearized Navier–Stokes equations (LNSE) or the linearized Euler equations (LEE). The transient fluctuations around a precomputed steady-state mean flow field solved with these sets of equations naturally include the energy transition between acoustic, vertical, and entropic modes. It is common practice to account for flame-acoustic interactions by applying measured or computed flame transfer functions (FTF) as a volumetric source term proportional to the mean heat release rate in the energy equation. However, the underlying assumption of a static flame is the root cause of spurious entropy production, which may ultimately falsify the thermoacoustic stability predictions. In the present paper, a methodology to include arbitrary flame movement in the governing set of equations is presented. The procedure makes use of an arbitrary Lagrangian-Eulerian (ALE) description of conservation equations and is demonstrated for the Euler equations. The resulting set of linear perturbation equations is then applied to two test cases. First, the frequency response of a one-dimensional premixed air-methane flame is evaluated. Secondly, the frequency response of the first longitudinal eigenmode of an experimental premixed, swirl-stabilized combustor is computed. To demonstrate the reduction of spurious entropy waves, the results are compared to those of the classic LEE.

Experimental Studies on the Explosion Behaviors of Premixed Hydrogen-Air Mixtures in a Narrow Channel.

Premixed hydrogen–air explosion experiments were carried out in a 1000 mm × 50 mm × 10 mm half-open narrow channel, concerning with the influences of equivalence ratio and ignition position on explosion behaviors. Experimental phenomena were different from explosion in large space. The results indicated that when ignited at the closed end of the channel, three overpressure peaks appeared, caused by the rupture of the film, Helmholtz Oscillation, and the flame-acoustic interaction, respectively. As the equivalence ratio of the hydrogen–air mixtures varied from 0.6 to 1.6, the peak overpressure first increased and then decreased. The maximum peak overpressure occurred at ϕ = 1.2. The hydrogen flame would develop into the plane tulip structure without the influence of the end wall. With the ignition position moved to the open end, overpressure wave and flame oscillated significantly. Compared with other ignition positions, the minimum value of Pmax was obtained at IP950. Based on the explosion behaviors in the narrow channel, it was concluded that the closer the ignition was to the open end, the easier the oscillation was to be formed, the smaller the explosion hazard was.

Experimental study on the premixed syngas-air explosion in duct with both ends open

Premixed flame of stoichiometric syngas-air mixture with various hydrogen volume fractions, 10% ≤ X (H2) ≤ 90%, propagating in a duct with both ends open is experimentally investigated in this study. Two representative ignition locations, i.e., Ig-1, locating at the center of the duct, and Ig-2, locating at the right open end, are considered. Results show that the tulip flame is first attained in the duct with both ends open at 10% ≤ X (H2) ≤ 50% as the flame is ignited at Ig-1. However, the flame maintains the convex shape with the cellular structure on the flame surface as the flame is ignited at Ig-2. The cellular structure results from Darrieus-Landau instability, but the Darrieus-Landau instability cannot invert the convex flame front. The flame tip and pressure dynamics have been examined. When the flame is ignited at Ig-1, the flame oscillates violently, and the overpressure profiles oscillate as a Helmholtz-type. When the flame is ignited at Ig-2, the left flame front propagates in an atmospheric pressure with a nearly constant speed. The prominent flame acceleration and oscillation are not observed at Ig-2 because of lacking flame acoustic interaction. What's more, the characteristic time of flame propagation has been compared. The time tw is shorter while the time tp is longer than the calculated value, and the time te has been delayed by both open ends. The flame propagation process is moderated as the flame propagates in the duct with both ends open.

Effect of concentration and ignition position on vented methane–air explosions

Experiments were conducted in a 1 m3 vessel with a top vent to investigate the effect of methane concentration and ignition position on pressure buildup and flame behavior. Three pressure peaks (p1, p2, and Pext) and two types of pressure oscillations (Helmholtz and acoustic oscillations) were observed. The rupture of vent cover results in p1 that is insensitive to methane concentration and ignition position. Owing to the interaction between acoustic wave and the flame, p2 forms in the central and top ignition explosions when the methane–air mixture is near–stoichiometric. When the methane–air mixture is centrally ignited, p2 first increases and then decreases with an increase in the methane concentration. The external explosion-induced Pext is observed only in the bottom ignition explosions with an amplitude of several kilopascals. Under the current experimental conditions, flame–acoustic interaction leads to the most serious explosions in central ignition tests. Methane concentration and ignition position have little effect on the frequency of Helmholtz and acoustic oscillations; however, the Helmholtz oscillation lasts longer and first decreases and then increases as the methane concentration increases for top ignition cases. The ignition position significantly affects the Taylor instability of the flame front resulting from the Helmholtz oscillation.

Flame dynamics during intermittency and secondary bifurcation to longitudinal thermoacoustic instability in a swirl-stabilized annular combustor

In this experimental study on a laboratory-scale turbulent annular combustor with sixteen swirl-stabilized burners, we study the flame-flame and flame-acoustic interactions during different dynamical states associated with the longitudinal mode of the combustor. We simultaneously measure the acoustic pressure and CH* chemiluminescence emission of the flame using a high-speed camera. Upon increasing the equivalence ratio, the combustor undergoes the following sequence of transition: combustion noise (CN) to low-amplitude longitudinal thermoacoustic instability (TAI) through the state of intermittency, and from low-amplitude to high-amplitude longitudinal TAI through a secondary bifurcation. We report the first evidence of secondary bifurcation from low-amplitude TAI to high-amplitude TAI for a turbulent thermoacoustic system which allows us to test the flame response at two different amplitude of perturbation in a natural setting. TAI arises at the first longitudinal mode of the combined burner tube and combustion chamber. The frequency of oscillations remain the same during the above transition. We find a significant difference in the dynamics of the flame interactions during the periodic part of intermittency and low- and high-amplitude TAI. Specifically, during the periodic part of intermittency, the phase difference between the local heat release rate (HRR) measured from various burners show significant phase slips in time. During low-amplitude TAI, there are fewer phase slips among the HRR response of the burners, which result in a state of weak synchronization among the flames. During high-amplitude TAI, we find that the flames are in perfect synchrony amongst themselves and with the pressure fluctuations. We then quantify the degree of temporal and spatial synchronization between different flames, and flames and pressure fluctuations using the Kuramoto order parameter and the phase-locking value. We show that synchronization theory can be conveniently used to characterize and quantify flame-acoustic interactions in an annular combustor.

Experimental study on the effects of ignition location and vent burst pressure on vented hydrogen-air deflagrations in a cubic vessel

A series of experiments was conducted in a cubic vessel of 0.5×0.5×0.5m to investigate the effects of ignition location and vent burst pressurePvon vented stoichiometric hydrogen-air deflagrations inside and outside the vessel. The results show that two overpressure peaks resulting from the vent rupture and the flame-acoustic interaction respectively, were always found for front ignition. Another overpressure peak generated by the external explosion was observed for center ignition andPv<69.8kPa. For the back ignition, three overpressure peaks successively produced by the vent rupture, the flame venting and the external explosion were recorded forPv<57.3kPa. The maximum peak overpressure inside the vessel always increases withPv, and the maximum peak overpressure for back ignition is significantly larger than those for front and center ignitions, and the latter two cases almost have the same values. The external overpressure-time curves present a single peak for front ignition and two peaks for back ignition. The external overpressure increases withPvfor front and back ignitions. However, the external overpressure increases forPv<57.3kPa, but it is insensitive toPvasPv>57.3kPa for center ignition. The external flame speed increases withPvfor front ignition while it is independent of the latter for center and back ignitions. The maximum length of the external flame for back ignition is much larger than that for front ignition and slightly larger than that for center ignition.

Experimental study of end-vented hydrogen deflagrations in a 40-foot container

Experiments were conducted in an enclosure with the same overall dimensions as a 40-foot ISO container to study the vented hydrogen-air deflagrations. This work focuses on the effects of hydrogen concentration, ignition location and obstacles on the overpressure and the structural response of the container wall. For center ignition, three overpressure peaks, which resulted from the vent opening, Helmholtz oscillation and acoustic oscillation, respectively, were recorded inside the container without obstacles. However, with the increase of hydrogen concentration, the third overpressure peak disappears when the obstacles are added in the container. Unlike center ignition, only two overpressure peaks were observed for back ignition. Due to the difference in reactivity of hydrogen-air mixture, the first overpressure peak is generated by the vent burst for low hydrogen concentration, or the venting of flame for high hydrogen concentration. The overpressure induced by the flame-acoustic interaction was not monitored with the increase of the hydrogen concentration and the installation of obstacles for back ignition. The overpressure for back ignition is more influenced by the obstacles than that for center ignition, when hydrogen concentration is larger than 12%. The displacement-time curves share similar trends with the pressure-time curves. The first peak displacement changes linearly with the corresponding first peak overpressure. However, the displacement caused by the second overpressure peak is significantly increased, especially for high hydrogen concentration and back ignition in the case with two obstacles.

Time-Domain Bloch Boundary Conditions for Efficient Simulation of Thermoacoustic Limit Cycles in (Can-)Annular Combustors

Abstract Thermo-acoustic eigenmodes of annular or can-annular combustion chambers, which typically feature a discrete rotational symmetry, may be computed in an efficient manner by utilizing the Bloch-wave theory. Unfortunately, the application of the Bloch-wave theory to combustion dynamics has hitherto been limited to the frequency domain. In this study, we present a time-domain formulation of Bloch boundary conditions (BBC), which allows to employ them in time domain simulations, e.g., computational fluid dynamics (CFD) simulations. The BBCs are expressed as acoustic scattering matrices and translated to complex-valued state-space systems. In a hybrid approach an unsteady, compressible CFD simulation of the burner-flame zone is coupled via characteristic-based state-space boundary conditions to a reduced order model of the combustor acoustics that includes BBCs. The acoustic model with BBC accounts for cross-can acoustic coupling and the discrete rotational symmetry of the configuration, while the CFD simulation accounts for the nonlinear flow–flame acoustic interactions. This approach makes it possible to model limit cycle oscillations of (can-)annular combustors at drastically reduced computational cost compared to CFD simulations of the full configuration and without the limitations of weakly nonlinear approaches that utilize a flame describing function. In this study, the suggested approach is applied to a generic multican combustor. Results agree well with a fully compressible CFD simulation of the complete configuration.

Effect of ignition position on vented hydrogen–air deflagration in a 1 m3 vessel

This study investigates the effect of the ignition position on vented hydrogen-air deflagration in a 1 m3 vessel and evaluates the performance of the commercial computational fluid dynamics (CFD) code FLACS in simulating the vented explosion of hydrogen-air mixtures. First, the differences in the measured pressure-time histories for various ignition locations are presented, and the mechanisms responsible for the generation of different pressure peaks are explained, along with the flame behavior. Secondly, the CFD software FLACS is assessed against the experimental data. The characteristic phenomena of vented explosion are observed for hydrogen-air mixtures ignited at different ignition positions, such as Helmholtz oscillation for front ignition, the interaction between external explosion and combustion inside the vessel for central ignition, and the wall effect for back-wall ignition. Flame-acoustic interaction are observed in all cases, particularly in those of front ignition and very lean hydrogen-air mixtures. The predicted flame behavior agree well with the experimental data in general while the simulated maximum overpressures are larger than the experimental values by a factor of 1.5–2, which is conservative then would lead to a safe design of explosion panels for instance. Not only the flame development during the deflagration was well-simulated for the different ignition locations, but also the correspondence between the pressure transients and flame behavior was also accurately calculated. The comparison of the predicted results with the experimental data shows the performance of FLACS to model vented mixtures of hydrogen with air ignited in a lab scale vessel. However, the experimental scale is often smaller than that used in practical scenarios, such as hydrogen refueling installations. Thus, future large-scale experiments are necessary to assess the performance of FLACS in practical use.

Non-dimensional groups for similarity analysis of thermoacoustic instabilities

An approach for similarity analysis of thermoacoustic combustion instabilities is proposed. A set of non-dimensional Π-groups is deduced from a solution based on modal expansion of the quasi-1D Helmholtz equation with a time-lagged heat source. Although thermoacoustic stability depends on a considerable number of parameters, which describe the geometry and thermodynamic characteristics of a combustor, the acoustic boundary conditions, the strength of flow-flame-acoustic coupling, etc., it is found that the respective non-dimensional frequencies and growth rates of thermoacoustic modes are dominated by only two Π-groups: Firstly, flame potency, which is governed by the intensity of mean and fluctuating rate of heat release and a parameter that quantifies the strength of flame-acoustic interaction for a given mode. The second Π-group is the non-dimensional time lag of the flame. Eigenfrequency locus plots for three combustor configurations are generated by varying interaction index and time lag of the flame model. It is found that similarity analysis explicates universal features of thermoacoustic instability and helps to classify thermoacoustic modes. Moreover, similarity analysis establishes not only the stability of eigenmodes, but also the sensitivity of eigenfrequency or growth rate to changes of any parameter contained in any of the two Π groups. In closing, the relevance of flame potency for the design of stable combustors is discussed.

The effect of venting process on the progress of a vented deflagration

Vented deflagrations are one of the most challenging phenomenon to be replicated numerically in order to predict its resulting pressure time history. As a matter of fact a number of different phenomena can contribute to modify the burning velocity of a gas mixture undergoing a deflagration, especially when the flame velocity is considerably lower than the speed of sound. In these conditions acceleration generated by both the flow field induced by the expanding flame and from discontinuities, as the vent opening and the venting of the combustion products, affect the burning velocity and the burning behavior of the flame. In particular the phenomena affecting the pressure time history of a deflagration after the flame front reaches the vent area, such as flame acoustic interaction and local pressure peaks, seem to be closely related to a change in the burning behavior induced by the venting process. Flame acoustic interaction and local pressure peaks arise as a consequence of the change in the burning behavior of the flame. This paper discuss the analysis of the video recording of the flame front produced during the TP experimental campaign, performed by UNIPI in the project HySEA, to describe qualitatively the contribution of the generated flow field in a vented deflagration and its influence in the peaks of the pressure-time history.

Numerical Study of a Methane Jet Diffusion Flame in a Longitudinal Tube with a Standing Wave

Abstract Flame stability plays an important role in a combustion/propulsion system, which involves the interaction between flow, acoustics, and flame. Although intensive studies have been carried out to investigate the flame-acoustic interactions, the jet flame excited by standing waves formed in a longitudinal tube has not received much attention. In this work, a methane-burnt (CH 4 ) jet diffusion flame in a longitudinal tube with a standing wave produced from a loudspeaker is studied numerically. 2-D unsteady RANS simulations are performed by using ANSYS FLUENT with the standard k − ɛ turbulence model and a one-step Eddy-Dissipation combustion model. The acoustic fluctuations are generated by using User Defined Functions. The numerical results show that a longitudinal standing wave can be successfully obtained in the tube by the numerical method, and both the acoustic velocity node and antinode can be observed. It is also found that the jet flame characteristics are highly sensitive to its axial location in the tube when the standing wave is present. The flame is unsteady when it is located at the velocity antinode where large velocity fluctuation exists. When the jet is placed at acoustic velocity nodes, however, the flame is relatively steady. Although the minimum velocity fluctuation at the two velocity nodes is almost at the same level, different flame temperatures are observed, due to the different turbulence kinetic energy.

Combustion Instability Suppression in Gaseous Oxygen/Hydrogen Combustors Using Methane Dilution

Combustion stability characteristics of a turbulent diffusion flame established between a center jet of gaseous oxygen and coflowing jets of gaseous hydrogen blended with different amounts of gaseous methane are studied in a rectangular combustor operating under atmospheric pressure conditions. A compression driver, mounted near the injector, is used to acoustically excite the flame from a transverse direction. Resulting flame perturbations are studied using chemiluminescence imaging, dynamic pressure measurements, and high-speed flow visualizations. Both steady-state perturbations and perturbations as the acoustically forced flames transition from one fuel blend to another are studied. Simultaneous measurements of pressure oscillations and heat release oscillations are used to obtain local Rayleigh indices showing locations that drive or dampen the instability. Transient measurements associated with real-time in situ methane blending are used to obtain timescales associated with the suppression process. Interesting intermittencies in heat release oscillations driven by acoustic forcing and local hydrodynamics are explored. Heat release oscillations, which drive combustion instability, are substantially reduced when gaseous methane is blended with gaseous hydrogen while holding ignition characteristics relatively constant. The reduction appears to result from a lowering of the density difference between the propellant streams upon methane dilution fuel–oxidizer density ratio. The approach could potentially be used in shear-coaxial combustors where instability from similar flame–acoustic interactions are common.

An Acoustic Time-of-Flight Approach for Unsteady Temperature Measurements: Characterization of Entropy Waves in a Model Gas Turbine Combustor

Lean premixed combustion promotes the occurrence of thermoacoustic phenomena in gas turbine combustors. One mechanism that contributes to the flame–acoustic interaction is entropy noise. Fluctuations of the equivalence ratio in the mixing section cause the generation of hot spots in the flame. These so-called entropy waves are convectively transported to the first stage of the turbine and generate acoustic waves that travel back to the flame; a thermoacoustic loop is closed. However, due to the lack of experimental tools, a detailed investigation of entropy waves in gas turbine combustion systems has not been possible up to now. This work presents an acoustic time-of-flight based temperature measurement method which allows the measurement of temperature fluctuations in the relevant frequency range. A narrow acoustic pulse is generated with an electric spark discharge close to the combustor wall. The acoustic response is measured at the same axial location with an array of microphones circumferentially distributed around the combustion chamber. The delay in the pulse arrival times corresponds to the line-integrated inverse speed of sound. For the measurement of entropy waves in an atmospheric combustion test rig, fuel is periodically injected into the mixing tube of a premixed combustor. The subsequently generated entropy waves are measured for different forcing frequencies of the fuel injection and for different mean flow velocities in the combustor. The amplitude decay and phase lag of the entropy waves adhere well to a Strouhal number scaling for different mean flow velocities.

Analysis of acoustic pressure oscillation during vented deflagration and proposed model for the interaction with the flame front

Explosion relief panels or doors are often used in industrial buildings to reduce damages caused by a gas explosion. Decades of research produced a significant contribution to the understanding of the phenomena involved. Among the aspects that need further research, interactions between acoustic oscillations and the flame front is one of the most important. Interactions between the flame front and acoustic oscillations has raised technical problems in lots of combustion applications as well and has been studied theoretically and experimentally in such cases. Pressure oscillations have been observed in vented deflagrations and in certain cases they are responsible for the highest pressure peak generated during the event. At Scalbatraio laboratory of Pisa University a test facility (CVE) was built in order to investigate vented hydrogen deflagrations. This paper presents an overview of the results obtained during several experimental campaigns. The tests are analysed with the focus on the investigation of flame acoustic interaction phenomena. Qualitative and quantitative analysis is presented and the generated possible physical phenomena are investigated.