Experimental study on chemiluminescence characteristics in Methane-Oxygen laminar flame affected by DC electric field

Puzach S., Liubov L., Кamchatova E., Nosova L., Degtyareva V., Tarasova V., Komarova L.

When reconstructing cultural heritage sites, significant changes to the original design planning are not allowed. More rational methods are needed to increase the fire resistance of historical buildings, which will ensure their fire safety and preserve their architectural value. Nowadays, most heritage sites do not meet the safety requirements of modern buildings. The purpose of the study is to develop a methodology for increasing the fire resistance of cast iron structures. The key tasks are increasing the fire resistance of buildings during reconstruction and ensuring their fire safety during operation. The tasks have been achieved by developing a new methodology for increasing the fire resistance of cast iron. It includes an integrated approach to assessing the risk of a fire, a predictive model for the occurrence of fire danger, as well as various scenarios for the fire development caused by cast iron heating. The results’ analysis has allowed us to determine the fire resistance limits of cast iron structures. The scientific novelty lies in the study of the fire resistance of cast iron structures using a three-dimensional mathematical model. The resulting values have been obtained via differential equations of the laws of mass conservation, momentum, gaseous energy, and the optical density of smoke. Doi: 10.28991/CEJ-2024-010-02-015 Full Text: PDF

Kim Y., Hong J., Jeon M., Park D.G., Yoon S.H.

We investigated the impact of AC electric fields on a nonpremixed coflow-jet flame. To eliminate the azimuthal component of the ionic wind, we created a radially applied electric field using an axisymmetric electrode structure consisting of a nozzle and a cylindrical mesh surrounding the co-flow burner. By applying various voltages and frequencies, we identified four distinct flame behaviors: oscillating flame with applied frequency, fluctuating flame, instability-induced extinction, and blowout. Each behavior was analyzed and discussed using phase analysis, flame length variation over time synchronized with the polarity change of the applied voltage, and FFT analysis. When an AC voltage (VAC) of 2 kV was applied, the oscillating flame with applied frequency was observed regardless of the AC frequency (fAC). Notably, a double-peak oscillation was observed due to the asymmetric bidirectional ionic wind resulting from the polarity change. The fluctuating flame was exclusively observed at VDC = 5 kV with fAC = 1 Hz. In addition to the double-peak oscillation, an overlapping counter-rotating vortex-induced oscillation was observed, which was confirmed to be caused by the positive ions directed towards the nozzle in the negative voltage section. At VAC = 5 and 7 kV with fAC = 10 Hz, the oscillation frequency of the counter-rotating vortex induced oscillation (≈ 10 Hz) was almost identical to fAC, resulting in constructive interference, ultimately leading to the instability-induced extinction. Blowout occurred at VAC = 7 kV with fAC = 1 Hz, where the strong bi-directional ionic wind caused necking of the fuel stream and resulted in flame extinction. Finally, we derived a critical frequency for observing a stable flame regardless of the AC voltage by comparing and analyzing the collision reaction time with the applied frequency.

Huang Y., Hossain M.M., Cao X., Zhang B., Li J., Xu C.

• A novel iterative approach is proposed to retrieve the soot temperature and volume fraction of sooting flames. • A self-absorption correction strategy is established with an aid of hyperspectral imaging. • Numerical simulations are carried out to evaluate the proposed approach. • A hybrid hyperspectral video imaging technique is designed and implemented. • The performance of the proposed technique is evaluated through ethylene/air diffusion flames. Due to the complex optical properties of sooting flames, the accuracy of flame emission-based measurement techniques depends on an appropriate self-absorption correction strategy. Thus, this paper presents a novel iterative procedure to retrieve the soot temperature and volume fraction of sooting flames along with a self-absorption correction strategy through hyperspectral imaging. Numerical simulations were carried out on suppositional flames to investigate the performance of the proposed technique. Relative errors obtained from the simulations are below 1.5%, indicating a better accuracy can be achieved by the proposed reconstruction technique. A hybrid camera hyperspectral video imaging technique is designed and implemented with the concept of compressed sensing. A significant improvement in flame spectrum acquisition has been observed. Experiments were carried out under ethylene-air diffusion flames to validate the technique. The reconstructed flame soot temperature and volume fraction distributions demonstrated that the proposed hyperspectral video imaging technique improves the soot temperature and volume fraction reconstruction accuracy.

Chien Y., Stocker D.P., Hegde U.G., Dunn-Rankin D.

This work describes the preparatory and initial measurements of diffusion flames under the influence of an electric field aboard the International Space Station (ISS), as part of the Advanced Combustion via Microgravity Experiments (ACME) project. Intended as the foundation publication for the experimental effort, the work comprehensively includes the space experiment methods, the capabilities of the ACME insert, experiment procedures, data, and limitations and constraints. The measurements presented include images and ion currents of small diffusion flames of methane (in air), subjected to a ramping electric field in microgravity. While there have been prior microgravity studies of Electric Field Effects on Laminar Diffusion Flames (E-FIELD Flames) using a drop tower, the current measurements represent the first mapping of the effects of an ion-driven wind on flame behavior under an electric field in the absence of the confounding influences of buoyancy. The paper describes the challenges of remote measurement and manipulation of flames on the ISS and presents preliminary results from the first set of coflow flames. The on-orbit tests began in March 2018 using the modular ACME hardware within the ISS’ Combustion Integrated Rack (CIR) and were completed in the same year November. The results show that the flame is most compact at saturation while the measured voltage-current characteristic (VCC) curve demonstrates parabolic behavior after saturation which differs from observations in 1 g on Earth.

Cheng L., Barleon N., Cuenot B., Vermorel O., Bourdon A.

For several years now plasma assisted combustion has been the subject of intense research due to stabilization effects a plasma can have on flames. Particularly, experiments have shown the promising impact of Nanosecond Repetitively Pulsed discharges on combustion while not exceeding an energy consumption of a few percent of the flame power. In this work, an incremental methodology with a step-by-step approach has been used to build a single plasma mechanism upon which combustion is added using the GRI 3.0 and Konnov v0.6. The methodology focuses on three key aspects of plasma assisted combustion: fast gas heating, slow gas heating and radical production. Selected experiments focusing on one or more of these aspects allow to validate the mechanism in large ranges of temperature (300-1500 K) and pressure (0.1-1 bar) in air, methane-air and argon diluted mixtures using glow and spark discharges. These experiments include a plasma assisted ignition case on which the ignition delay time is well captured by the mechanism. Slow gas heating has been modeled using a vibrational relaxation model validated against a detailed vibrational description. Discussions on ambiguous rates for critical reactions of excited nitrogen quenching are made in the light of their impact on the results on the chosen experiments. Finally, the resulting 100-species GRI 3.0-based and 264-species Konnov v0.6-based plasma mechanisms are reduced to make them suitable for multi-dimensional simulations. The DRGEP reduction method, based on plasma experiments and canonical combustion cases, is applied allowing to reduce the number of species by a factor larger than two. For the GRI-3.0 plasma mechanism, the reduced mechanism contains 47 species and 429 reactions. Hence significant performance is gained, opening the way to multi-dimensional simulations of plasma assisted combustion.

Liu A., Luo K.H., Rigopoulos S., Jones W.

In this article, a coupled PBE-CFD framework has been proposed to study counterflow non-premixed flames and soot formation under an external electric field . This framework integrates the population balance equation (PBE) for nanoparticle dynamics into an in-house CFD solver for the multicomponent reactive flows. Different electric properties have been considered in this model. An ion mechanism used in both fuel-rich and fuel-lean combustion is combined with a detailed chemistry for neutral gaseous species and small-size aromatics to retain the full chemistry. In order to model soot particles carrying charges and the movement of the reacting fluid medium in the electric field, a second PBE for the production and transport of charges on soot particles is introduced for the first time and incorporated into the original PBE for the number density of particles. Also, the electric force for the gas mixture is included in the momentum equations. The electric drift velocities for ions and soot particles are also considered in the transport equations of ions and the PBE of soot particles, respectively. The simulations have shown that the presence of the electric field modifies the stagnation plane of the counterflow flames and reduces the soot formation in both rich-fuel and lean-fuel conditions in agreement with experimental observations. The application of the soot particle charging model, accompanied by a proper electric correction factor on the nanoparticle processes of nucleation and surface growth, significantly improves the stability of the flame structure. The introduction of the electric correction factor reveals that the suppression of soot formation in an electric field is mainly caused by the inhibited chemical reactions of the PAH nucleation and particle surface growth, which is more important than the electric drift of the charged particles. Reducing the critical size of the particle charging process enhances the electric drift of nascent soot, thus lessening its subsequent evolution.

Xie F., Wu R., Wei J., Song X., Li J., Lv P., Wang J., Yu G.

Here, the OH* and CH* chemiluminescence characteristics of a single coal particle flame combustion in a visual drop-tube furnace (VDTF) for different oxygen fraction, X i,O2 , in O 2 /CO 2 atmosphere were investigated using an optical diagnostic technique. The results indicate that the combustion of single coal particles is composed of a volatile reaction stage followed by a volatile-char reaction stage. During volatile reaction stage, the single coal particle burns in an enveloped flame. The flame becomes brighter and the flame length increases gradually. In addition, the OH* and CH* intensity shows peak values with axial distance along the furnace, with that of OH* appearing earlier as the oxygen fraction increases. In the volatile-char reaction stage, the flame length fluctuates slightly, and OH* and CH* peak position are spread over a smaller range of distance. The value of X i,O2 affects the reaction rate as it promotes the diffusion of oxygen to the interior of coal particle, as the intensity of OH* and CH* increasing rapidly for X i,O2 > 0.4. This indicates that the rate of reaction is controlled by X i,O2 . Furthermore, the peak intensity ratio of OH* and CH* increases linearly with X i,O2 . • O 2 /CO 2 atmosphere of single coal particle was studied in visual drop-tube furnace (VDTF). • Single coal particle combustion characteristic was studied using optical diagnostic technique. • OH* and CH* chemiluminescece distributions at different X i,O2 were obtained. • Effects of X i,O2 on distribution characteristics of OH* and CH* were further analyzed. • Quantitative relation among the OH* and CH* peak intensity, OH*/CH* and X i,O2 were studied.

Kang H., Kim K.T.

The crux of the technical issues facing the development of hydrogen gas turbine engines is the problem of how to enable low NOx, low dynamics combustor operations without detrimental flashback events in extremely fast lean-premixed hydrogen flames. While flashback mechanisms and nitrogen oxides emissions have been investigated extensively for high hydrogen content flames, the self-excited dynamics of lean-premixed pure hydrogen flame ensembles remain unclear. Here we use phase-resolved OH* chemiluminescence and OH PLIF imaging in a series of experiments in a multi-element injector configuration consisting of 293 small-scale injectors with an inner diameter of 3.0 mm. We collect a large experimental dataset to explore a variety of collective phenomena of the lean-premixed hydrogen-air flame ensemble, and perform systematic investigations of self-induced combustion instabilities. Our observations demonstrate that ultra-compact pure hydrogen flames generate high-amplitude pressure perturbations over a broad range of characteristic frequencies between 400 and 1800 Hz, corresponding to the third to tenth order eigenmodes . Low-frequency flame dynamics developed under relatively low equivalence ratio conditions involve a complex balance among several coexisting phenomena, including strong vortex interactions and periodic extinction-reignition processes, giving rise to large-scale asymmetric oscillations of the entire reaction zone. By contrast, the flame surface dynamics at an intermediate frequency of ~600 Hz exhibit prominent symmetric oscillations accompanied by merging and pinch-off of the constituent flames. Unexpectedly, high-frequency instabilities at approximately 1720 Hz (screech tones) are not influenced by such structurally complex flame dynamics, but by exceptionally simple, seemingly linear, flame surface motion without sudden flame area annihilation events like those observed for lower frequency cases. Despite the extremely low level of heat release rate fluctuations, on the order of less than 1%, the clustered premixed hydrogen flames are capable of producing disproportionately large pressure perturbations in excess of 12 kPa, originating from the synchronous phase dynamics of acoustic pressure and clustered flames’ heat release rate oscillations. These findings provide new insight into the driving mechanisms underlying high-frequency combustion dynamics of densely arranged pure hydrogen-air flames.

Xia H., Wang J., Ju R., Li Y., Mu H., Huang Z.

Effect of DC electric field on lean premixed biogas turbulent flame structure at low and medium turbulence intensity was studied experimentally using OH-PLIF. OH signal distribution, turbulent burn...

Sayed-Kassem A., Elorf A., Gillon P., Idir M., Sarh B., Gilard V.

An applied DC electric field was experimentally demonstrated to modify the flame structure and gas dynamic in an ethylene diffusion flame. The aim of this paper is to investigate the influence of the electric field on the flow field and its impacts on the flame behavior. A numerical study has been performed to elucidate the experimental observations and to monitor the effect of electric body force on the flame. The numerical model was validated by comparing the computed results to experimental measurements from the literature. The resulting computed flame shape was compared to a visible image taken during the experiment. The simulated OH mole fraction, the burning rate and the computed velocity and temperature are presented. The developed model proved the ability to reproduce qualitatively the experimental flame behavior when submitted to the electric field. The electric field is shown to modify the flame shape (flame tip, flame shortness and flame deformation), to promote the burning process and to improve the ion production. Results show that the modifications are due to an air entrainment acting specifically near the burner zone enhancing the mixture and changing the fluid dynamic in this region. The ionic wind is demonstrated to increase the maximum burning rate and promoting ions' formation mostly near the burner. A more detailed model (detailed ions' chemistry and soot model with charged particles, detailed electric diffusion) is necessary to gain a better understanding of the influence of electric field on diffusion combustion and soot formation. • A numerical model was developed to simulate ethylene diffusion combustion and validated with experimental measurements. • The developed model was extended to cover the effect of a DC electric field on ethylene non-premixed combustion. • An applied electric field deforms the flame structure, promotes the combustion rate and modifies flame dynamic. • Ionic wind is responsible for the observed modifications acting essentially near the burner exit.

Turner M.A., Paschal T.T., Parajuli P., Kulatilaka W.D., Petersen E.L.

The coupling of CFD simulations with detailed chemical kinetics presents great progress in predicting the complex behavior of reacting flows, but also requires validated input parameters in the form of experimental data. The spatial profile of a combustion wave represents one such parameter, which can be directly measured using chemiluminescence imaging of a spherically expanding flame. In this work, emission signals from electronically excited methylidyne (CH*) and hydroxyl (OH*) radicals near 434 nm and 315 nm, respectively, from spherically expanding methane–air flames at 1 atm and 298 K were recorded for equivalence ratios of 0.8, 1.0, and 1.2. Spatial profiles of normalized intensity were compared to predicted profiles from AramcoMech2.0. The effect of image resolution was investigated by repeating experiments for three levels of image pixel density. An Abel inversion was employed to extract intensity profiles of CH* and OH* at flame radii up to 6.5 cm. Measured flame thickness increased as flames grew in size, but this behavior diminished as image resolution increased. A linear stretch correlation was used to extrapolate measured thicknesses to an unstretched thickness for each experimental condition. Radical-based flame thicknesses and corresponding spatial profiles were found to be highly dependent on image resolution, and at high resolution, measured flame thickness appeared to approach the kinetically predicted radical-based thicknesses. This paper lays the foundation for future, comprehensive measurements of spherical, laminar flames that can resolve the flame zone details to a level of precision not typically seen in the literature, providing benchmark data for both kinetics model validation and CFD model inputs. As a result, the measurements thus far indicate that the measured flame zone thickness based on electronically excited species is much closer to the length scale typically predicted by kinetics models than what has been seen in most experiments to date.

Sayed-Kassem A., Gillon P., Idir M., Gilard V.

Electric fields were proved to affect combustion stability, flame form, and pollutant emissions. The influence of the electric field on soot formation and growth is still an open topic. In this con...

Tang Y., Yao Q., Cui W., Zhuo J., Li S.

ABSTRACTThe response of the laminar premixed flame to a counterflowing non-thermal plasma jet has been studied experimentally and numerically. The plasma is driven by alternating current and sustai...

Wang J., Li Y., Xia H., Ju R., Zhang M., Mu H., Huang Z.

Electric assisted combustion for hydrogen enriched hydrocarbons may even extend the lean burn limit and provide the further improvement on combustion stability. This study investigates the effect of hydrogen enrichment and DC electric field on lean CH4/air flame propagation. Electric field inside the chamber was generated by mesh and needle electrodes. Effect of hydrogen enrichment on the ion mole fraction in the flame was discussed based on reaction mechanism included neutral and ion reactions. The flame propagation images, flame displacement speed were used to evaluate the combined influences of hydrogen enrichment and electric field on propagating flame. Results showed that the hydrogen addition would increase positive ions mole fraction and the peak value is mainly determined by H3O+. This would be due to that CH increases with hydrogen fraction, which is the main species in the initial reaction for the ion reactions. Electric field effect about flame propagation was suppressed with hydrogen addition due to the competition between the increment in ion mole fraction and the decrement in flame time. Electric assisted combustion is more evident at leaner conditions and elevated pressure. The ratio of ionic wind velocity to flow velocity may be the determined factor to predict the electric field effect about propagating flame. The tendency based on this ratio is in accordance with the experimental results for various hydrogen fraction and equivalence ratio at elevated pressure.

Zhu H., Hu C., Guo Q., Gong Y., Yu G.

The chemiluminescence characteristics of the excited state radicals are significant for flame emission spectroscopy. In this paper, the spatial profiles of OH* and CH* radicals in laminar and turbulent CH4/O2 diffusion flames were obtained by the UV imaging system and the high-spatial-resolution line-scan hyperspectral camera. The OH* and CH* molar concentration were calculated through CFD numerical simulation. The results show the two-dimensional and radial OH* and CH* distribution features for methane diffusion flames, and OH* generates near the oxygen side whereas CH* generates near the fuel side of the flame front. In turbulent flames, with the increase of the equivalence ratio (denoted as [O/C]e), the OH* emission intensity decreases whereas the OH* molar concentration increases instead. Both CH* emission intensity and molar concentration increase with increasing [O/C]e. Moreover, the OH* emission intensity and flame structure under different velocities present different trends, and the trend of OH* peak emission intensity with [O/C]e can be used to characterize the flame flow state.