Hydrogen Diffusion Flame Research Articles

Morphological Characteristics of Propane–Hydrogen Diffusion Flame: Experiments and Correlations

ABSTRACT This study investigated the flame height and liftoff height of turbulent diffusion jet flames of the Propane–Hydrogen mixed gas. A series of experiments have been conducted to examine their flame height and liftoff height with different concentrations of propane and hydrogen. The experimental results show that the flame height decreases with the increase in hydrogen concentration. The accuracy of the Froude number for the prediction of flame height has been verified. The liftoff height is proportional to the propane flow rate, while it was inversely proportional to the hydrogen concentration due to the increased laminar flame speed. A theoretical model incorporating laminar flame speed, heat release rate, and fuel exit velocity has been obtained. These investigations are crucial for enhancing safety in hydrogen-enriched pipeline systems, which is helpful for the use of hydrogen in energy systems and global sustainability efforts.

Practical approaches for evaluating radiative heat from high pressure hydrogen flame

As the construction of hydrogen stations and other hydrogen-related facilities progresses, it becomes increasingly crucial for safety designs to account for scenarios where leaked hydrogen might ignite. Since a hydrogen flame is usually invisible, detection must rely on radiative heat or temperature. While it is possible to predict accurate radiative heat flux from a high pressure hydrogen jet flame through detailed numerical calculations of hydrogen combustion with its radiation, the objective of this study is to provide a simpler and more accurate prediction method. Previous experimental data of radiative heat flux, Qr from high pressure hydrogen jet flames were compiled, and a simple experimental equation Qr=1463.3(Lo/Lf)−2.32 was derived, where Lf and Lo represent the flame length and the distance perpendicular to the flame axis from the midpoint of the flame length to the observer, respectively. Radiative heat fluxes were computed through detailed numerical simulations using three types of water vapor radiation models: Goody's narrow random band model, the exponential broad band model by Edward and Elsasser's narrow regular band model. The temperature and water vapor partial pressure distributions obtained from numerical simulations were used as input conditions. The results were found to be in good agreement with the experimental equation. Moreover, a simple diagram representing the radiative heat flux from a hydrogen flame was derived using the method of cylinder-like approximation. The results of this study show that if the hydrogen pressure and leakage aperture diameter are given, the radiative heat flux at a certain distance can be predicted with high accuracy. Additionally, the average value of emissivity for the hydrogen diffusion flame was found to be on the order of 0.03.

Influence of buoyancy on the mixing, flame structure, and production of NO in hydrogen diffusion flames

This study investigates the impact of buoyancy on the mixing and flame structure of hydrogen jet diffusion flames. A validated Computational Fluid Dynamics (CFD) model for hydrogen diffusion flames is developed, employing the k−ω turbulence model and a steady flamelet combustion model. By accounting for the buoyancy term, the model accurately predicts the stoichiometric hydrogen flame length within a mere 0.2% deviation from the experimental value. In contrast, excluding the buoyancy term leads to a prediction divergence of around 5%.Deeper analysis of the mixture fraction field indicates that buoyancy enhances mixing in the flame’s far-field region, facilitating a more rapid dilution of the fuel stream within the air. This, in turn, results in a shorter flame. The scope of the modelling is expanded to encompass buoyancy effects onhydrogen flames with jet Reynolds numbers (ReJ) up to 15,000. While the buoyancy’s influence on flame control weakens as ReJ increases, a noticeable ∼4% variation in stoichiometric flame length persists between scenarios with and without buoyancy.

Influence of distributed hydrogen injection and combustion on supersonic boundary layer stability and transition

This theoretical study recorded the influence of the distributed injection of hydrogen through a permeable surface and its combustion on the stability and transition of a supersonic boundary layer (BL). The laminar base flow for a multicomponent flat-plate BL was computed using the computational fluid dynamics solver with finite-rate chemistry for free-stream Mach number (M) = 2. Linear stability theory (LST) equations for a reactive gas mixture were derived in the local parallel base flow approximation. Stability calculations based on the developed theory revealed the possibility of a decrease in the local spatial amplification rates of unstable perturbations. A double reversal in the maximal amplification rate magnitudes of perturbations was obtained, wherein at first, they increased, then decreased to zero, and then rose again. The influence of the distributed injection and combustion of hydrogen in the supersonic BL on the position of the laminar-turbulent transition was estimated using the LST-based eN-method. Calculated and experimentally obtained dependencies of the relative transition Reynolds number on the relative mass flow rate of injected hydrogen were compared. It was found that depending upon the flow conditions, the hydrogen diffusion flame could accelerate or decelerate the transition in the supersonic M = 2 BL, when compared to absence of hydrogen burning. Two counteracting effects compete: heat supply by combustion exerts stabilizing influence, while low-molecular-weight gas blowing from the surface exerts destabilizing influence. Depending on the interplay of these two factors, it would be possible to obtain acceleration or deceleration of transition.

Direct numerical simulations of the Taylor–Green vortex interacting with a hydrogen diffusion flame: Reynolds number and non-unity-Lewis number effects

Understanding the interactions between hydrogen flame and turbulent vortices is important for developing the next-generation carbon neutral combustion systems. In the present work, we perform several direct numerical simulation cases to study the dynamics of a hydrogen diffusion flame embedded in the Taylor–Green Vortex (TGV). The evolution of flame and vortex is investigated for a range of initial Reynolds numbers up to 3200 with different mass diffusion models. We show that the vortices dissipate rapidly in cases at low Reynolds numbers, while the consistent stretching, splitting, and twisting of vortex tubes are observed in cases with evident turbulence transition at high Reynolds numbers. Regarding the interactions between the flame and vortex, it is demonstrated that the heat release generated by the flame has suppression effects on the turbulence intensity and its development of the TGV. Meanwhile, the intense turbulence provides abundant kinetic energy, accelerating the mixing of the diffusion flame with a contribution to a higher strain rate and larger curvatures of the flame. Considering the effects of the non-unity-Lewis number, it is revealed that the flame strength is more intense in the cases with the mixture-averaged model. However, this effect is relatively suppressed under the impacts of the intense turbulence.

Flame synthesis of tungsten‐doped titanium‐dioxide nanoparticles using novel precursor combination of liquid titanium tetra‐isopropoxide and solid tungsten mesh

AbstractTungsten‐doped titanium‐dioxide (W‐TiO2) nanoparticles are successfully synthesized using a multiple‐diffusion‐flame burner with a separate center tube. Vaporized titanium tetra‐isopropoxide (TTIP) precursor issues from a center tube to produce TiO2 nanoparticles, while a tungsten mesh, suspended above the surrounding multiple over‐ventilated hydrogen diffusion flames, serves as the solid‐phase metal doping source. At a lower tungsten loading rate, W‐TiO2 nanoparticles are generated, as indicated by an obvious angle shift of 0.15° for the entire x‐ray diffraction spectrum. However, at a higher tungsten loading rate, homogenous nucleation of WOx occurs before or concurrently with TiO2 nucleation, producing mixed nanopowders, permitting fewer tungsten ions to be doped into TiO2. Ultraviolet–visible spectroscopic characterization reveals that the as‐synthesized W‐TiO2 nanoparticles possess augmented absorbing ability in the visible‐light wavelength range, where the band gap is reduced from 3.20 to 3.05 eV, compared with that for the nondoped TiO2 nanoparticles.

Effect of Burner Wall Material on Microjet Hydrogen Diffusion Flames near Extinction: A Numerical Study

Characteristics of microjet hydrogen diffusion flames stabilized near extinction are investigated numerically. Two-dimensional simulations are carried out using a detailed reaction mechanism. The effect of burner wall material, thickness, and thermal radiation on flame characteristics such as flame height and maximum flame temperature are studied. Results show that the flame stabilizes at lower fuel jet velocities for quartz burner than steel or aluminum. Higher flame temperatures are observed for low conductive burners, whereas the flame length increases with an increase in thermal conductivity of the burner. Even though thermal radiation has a minor effect on flame characteristics like flame temperature and flame height, it significantly influences the flame structure for low conductive burner materials. The burner tip and its vicinity are substantially heated for low conductive burners. The effect of burner wall thickness on flame height is significant, whereas it has a more negligible effect on maximum flame temperature. Variation in wall thickness also affects the distribution of H and HO2 radicals in the flame region. Although the variation in wall thickness has the least effect on the overall flame shape and temperature distribution, the structure near the burner port differs.

Femtosecond laser activation and sensing of hydroxyl for velocimetry in reacting flows.

A molecular tagging method for velocity measurements in reacting environments such as propulsion devices and high-temperature combustion-assisted wind tunnels is described. The method employs a femtosecond (write) laser to photodissociate H2O, a common combustion product, into a locally high concentration of OH radicals. These radicals are tracked by planar laser-induced fluorescence (PLIF) from the A2Σ-X2Π (1-0) vibrational band excited by a time-delayed 284 nm (read) laser sheet. As a variant of hydroxyl tagging velocimetry, the source laser can also be used to dissociate nitrogen for femtosecond laser electronic excitation tagging velocimetry to mark the time-zero location of the write laser for velocimetry in non-reacting regions using the same imaging system without OH PLIF. The OH tracer lifetime is studied in a hydrogen-air Hencken burner operating at Φ=0.5-1.8 to evaluate the tracking capability for velocimetry over a range of conditions. Effects of changing read laser wavelength, excitation energy, and influence of background flame emission are also studied. The data processing methodology and results are described for tracking displacements with 9-25 µm uncertainty in a hydrogen diffusion flame. This method presents several advantages in operational convenience and availability of laser sources, and it provides an avenue for improvements in the repetition rate, precision, and applicability over previously demonstrated hydroxyl tagging schemes.

Buoyancy effects on hydrogen diffusion flames confined in a small tube

In this work, buoyancy effects on hydrogen jet flames confined in a small tube without air co-flow were numerically investigated. The results show that the extinction limit of fuel velocity under buoyant condition is much lower than that without buoyancy. Moreover, hydrogen flames under buoyant condition attatch the nozzle exit for all fuel velocities investigated; however, the flames without buoyancy surround the lower wall at low fuel velocity. In addition, combustion is nearly complete in the presence of buoyancy, whereas the combustion efficiency under non-buoyant condition is below 45%. Furthermore, flame temperature under buoyant condition is much higher compared to the counterpart under non-buoyant condition at low and moderate fuel velocities. Analysis reveals that in the case without buoyancy, the negative gauge pressure in the annular space is unable to entrain sufficient air from the ambient. Hence, hydrogen has to diffuse downwards to sustain the flame and complete combustion is unrealizable.

Numerical study of a laminar hydrogen diffusion flame based on the non-premixed finite-rate chemistry model; thermal NOx assessment

The present study examines thermal NOx formation on a laminar hydrogen diffusion flame based on a patented multi-flame diffusion burner installed in a domestic boiler. The analysis is performed through CFD modeling by means of ANSYS-FLUENT 19.0. Detailed chemical, thermal, and transport parameters are implemented, in order to obtain accurate results using finite-rate chemistry together with laminar and low-Reynolds turbulence models. The Damköhler number is calculated using a Matlab code developed in-house. The chemical kinetics of the thermal NOx route are analyzed for several inlet flame power levels, so as to demonstrate the effectiveness of the flame-splitting method at reducing the formation of thermal NOx. The numerical results are contrasted through measurements from literature for validation purposes. In the present work, the flame power is varied from 0.05 kW up to 0.8 kW. In contrast to turbulent non-premixed hydrogen flames, the numerical results provide a quasi-constant trend in thermal NOx formation at higher power levels (0.4–0.8 kW). The heat exchange rate between the flame and the combustion chamber and its influence on thermal NOx formation are all carefully analyzed.

On the possibility of simultaneous temperature, species, and electric field measurements by coupled hybrid fs/ps CARS and EFISHG.

Optical signals of gas temperature, species concentration, and electric field are monitored in simple mixtures at room temperature and in the fuel-rich region of a hydrogen diffusion flame. A two-beam pure-rotational coherent anti-Stokes Raman scattering (CARS) approach was utilized for the temperature and species detection, where the combined pump/Stokes pulse doubled as the electric field induced second harmonic generation (EFISHG) pump for the electric field detection. Time-averaged EFISHG signals in environments with argon, nitrogen, oxygen, hydrogen, and air were found to match the relative hyperpolarizabilities of the molecules tabulated in literature. Measurements in a dynamic H2-air environment represented the ability to monitor the signal dependence of species on a single-shot basis. Time-averaged EFISHG signals in different thermal environments showed the expected ∝1T2 EFISHG signal dependence when also correcting for relative H2/N2 concentrations. Finally, measurements in a flame showed the ability to monitor the EFISHG signal dependence on the gas temperature on a single-shot basis in a plasma discharge environment.

Coherent Anti-Stokes Raman Spectroscopy of a Hydrogen Diffusion Flame in a Ramjet

Dual-pump coherent anti-Stokes Raman spectroscopy (CARS) was used to measure the mole fractions of major species as well as the rotational and vibrational temperatures of molecular nitrogen in a hydrogen-fueled dual-mode scramjet flowpath operated in the “ram” mode. Developments in CARS methods and uncertainties are described, including a detailed analysis of the effects of spatial averaging. The mean and standard deviation of the turbulent fluctuations of the temperature and mole fractions at multiple planes in the flowpath and scatter plots of vibrational and rotational temperature are presented. The flame is stabilized downstream of the ramp and grows under the influence of turbulence and rollup of the counterrotating vortex pair formed at the ramp; combustion continues in accelerating flow approaching the end of the measurement domain. Thermal nonequilibrium is observed in the mixing of air with the molecular hydrogen jet close to the injector. Spatial averaging errors are significant at some locations; new methods to correct these errors are described.

Electric field in Ns pulse and AC electric discharges in a hydrogen diffusion flame

Time-resolved electric field is measured in ns pulse and AC sine wave dielectric barrier discharges sustained in an atmospheric pressure hydrogen diffusion flame, using picosecond second harmonic generation. Individual electric field vector components are isolated by measuring the second harmonic signals with different polarizations. Electric field measurements in a ns pulse discharge are self-calibrating, since the field follows the applied voltage until breakdown. Electric field is measured in a ns pulse discharge sustained both in the hydrogen flow below the flame and in the reaction zone of the flame. Peak electric field in the reaction zone is lower compared to that in the near-room temperature hydrogen flow, due to a significantly lower number density. In hydrogen, most of the energy is coupled to the plasma at the reduced electric field of E/N ≈ 50–100 Td. In both cases, the electric field decreases to near zero after breakdown, due to plasma self-shielding. The time scale for the electric field reduction in the plasma is relatively long, several tens of ns, indicating that it may be controlled by a relatively slow propagation of the ionization wave over the dielectric surfaces. In the AC discharge, the electric field is put on the absolute scale by measuring a Laplacian electric field between two parallel cylinder electrodes. The measurement results demonstrate that a strong electric field in the plasma-enhanced flame is produced during the entire AC voltage period, without correlation with the random micro-discharges detected in the plasma images. The measurement results indicate consistently higher peak electric field during the negative AC half-period, as well as a significant electric field offset. Both the asymmetry and the offset of the electric field are likely responsible for the ion wind resulting in the flame distortion. The results suggest that at the present conditions the ion wind is dominated by the transport of negative ions generated in the ambient air plasma near the flame. The results demonstrate a significant potential of ps second harmonic generation diagnostics for non-intrusive measurements of the electric field in atmospheric pressure flames enhanced by electric discharge plasmas.

Experimental and numerical study of OH* chemiluminescence in hydrogen diffusion flames

A co-flow burner was designed to generate axisymmetric diffusion flames for the application of line-of-sight optical diagnostics to hydrogen flames. Chemiluminescence images of OH* from laminar hydrogen diffusion flames, with and without co-flowing air, were recorded using an intensified camera system with a narrow-band filter at approximately 310 nm. The spectra of OH* chemiluminescence was acquired by a separate optical system. Local concentrations of the radiating radical OH* were determined using the inverse Abel transformation and calibration against a light source of known radiance. The uncertainty of the OH* concentration measurements is analysed to be −22% to +12% in the current experimental configuration. Numerical reconstruction of the physical flames was performed using a two dimensional axisymmetric flow model coupled with a detailed H2/O2 oxidation chemistry mechanism and an OH* chemiluminescent sub-scheme which includes options to use 6 different rate coefficients recommended in the literature for the OH* formation reaction H+O+M⇌OH*+M (R1). The numerical simulations using the rate coefficient of 1.5 × 1013exp(−5.98kcalmol−1/RT)cm6mol−2s−1 for R1 demonstrate the best agreement with the measured OH* chemiluminescence.

Ps four-wave mixing measurements of electric field in a ns pulse discharge in a hydrogen diffusion flame

Time-resolved electric field in ns pulse discharge plasmas generated in room air and in an atmospheric pressure hydrogen diffusion flame has been measured by ps four-wave mixing, for plane-to-plane electrode geometry. Electric field is put on the absolute scale using the Laplacian field measured before breakdown. The results show that peak electric field during breakdown in the flame, approximately 40 kV/cm, is significantly lower compared to that in room air, 75 kV/cm, due to higher temperature of combustion products. In both cases, peak electric field is higher compared to DC breakdown field. Both in air and in the flame, the electric field follows the applied voltage before breakdown and decreases rapidly after breakdown, due to charge separation and plasma self-shielding. The electric field in air is compared with the predictions of an analytic model of ns pulse breakdown, showing good agreement between the predicted and the measured breakdown field. The model also predicts earlier breakdown as well as breakdown voltage reduction as the temperature is increased, in qualitative agreement with the experimental data. The use of the present ps four-wave mixing diagnostics for measurements of electric fields below ∼20 kV/cm in atmospheric pressure flames is challenging, due to low signal-to-noise. The sensitivity of the present diagnostics is controlled by the high temperature and low N2 fraction in the combustion product mixture, as well as by the limited bandwidth of the Stokes beam generated by the stimulated Raman cell, which provides access only to several rotational levels of nitrogen molecules. The present diagnostics will have much better sensitivity in high-pressure flames, since the four-wave mixing signal scales as the squared number density of nitrogen.

Zonal Hybrid Reynolds-Averaged Navier–Stokes/Large-Eddy Simulation of a Hydrogen-Fueled Scramjet Combustor

A zonal hybrid Reynolds-averaged Navier–Stokes/large-eddy simulation framework is implemented to investigate the diffusion combustion of hydrogen–air in a model scramjet combustor. The computational domain is segregated into Reynolds-averaged Navier–Stokes and large-eddy simulation regions. An upwind convective flux discretization method and shear-stress transport turbulence models are applied to the Reynolds-averaged Navier–Stokes region, and the low-dissipative convective flux discretization method and improved delayed detached-eddy simulation are applied to the large-eddy simulation region. To generate turbulence at the Reynolds-averaged Navier–Stokes/large-eddy simulation interface, a synthetic-eddy method is employed. The geometries and boundary conditions of the model scramjet combustor are based on an experiment conducted at German Aerospace Center. The turbulent hydrogen diffusion flame is modeled using the steady flamelet approach. A total of nine computation cases are examined to analyze the influence of the numerical methods. As a result, the zonal hybrid method using the low-dissipative convective flux discretization method and synthetic-eddy method shows the best agreement with the experimental data in terms of quantitative temperature and rms velocity fluctuation profiles.

Dual-pump CARS measurements in a hydrogen diffusion flame in cross-flow with AC dielectric barrier discharge

This paper presents dual-pump coherent anti-Stokes Raman scattering (CARS) measurements for simultaneous detection of flow temperature and relative concentration, applied to the characterization of a discharge-coupled reacting jet in a cross flow. The diagnostic is hydrogen Q-branch based, providing a much wider dynamic range compared to detection in the S-branch. For a previously developed dielectric barrier discharge, aligned co-axially with the fuel jet, OH planar laser induced fluorescence measurements show that the disturbance in the flame boundary leads to mixing enhancement. The H2-N2 dual-pump CARS measurement was used to map two-dimensional temperature distributions. The increase of the maximum temperature was up to 300 K, with 50% more H2 consumption, providing the reason for the decrease in the flame length by 25%. The increase of the relative H2O-H2 fraction was accompanied with a temperature increase, which indicates local equivalence ratios of below 1. The H2-O2 dual-pump measurements confirmed that the fuel-oxidizer ratios remain in the fuel-lean side at most of the probed locations.

Dielectric-barrier-discharge plasma-assisted hydrogen diffusion flame. Part 1: Temperature, oxygen, and fuel measurements by one-dimensional fs/ps rotational CARS imaging

One-dimensional hybrid fs/ps CARS imaging provides single-laser-shot measurements of temperature, oxygen, and hydrogen in a plasma-assisted hydrogen diffusion flame. The coaxial dielectric-barrier-discharge burner collapses the Re ∼50 hydrogen diffusion flame to within ∼5 mm of the burner surface at an applied AC potential of 8.75 kV at 18 kHz, coinciding nicely with the full spatial extent of the 1D CARS measurements. Translating the burner through the measurement volume allowed for measurements at numerous radial locations in increments of 1 mm with a resolution of 140 µm × 30 µm × 600 µm, sufficient to resolve spatial gradients in this unsteady flame. Longer probe delays, required for improved dynamic range in regions of high temperature fluctuations, proved difficult to model as a result of a nontrivial decay in the O2 Raman coherence arising from complexities associated with the triplet ground electronic state of the O2 molecule. Oxygen linewidths were treated empirically using the observed O2 coherence decay in spectra acquired from the product gases of lean, near-adiabatic H2/air flames stabilized on a Hencken flat-flame burner. While still leading to errors up to 10% at worst, the empirically determined Raman linewidth factors eliminated any systematic error in the O2/N2 measurements with probe delay. Temperature measurements in the Hencken Burner flames proved to be insensitive to probe pulse delay, providing robust thermometry. Demonstration of this technique in both the canonical Hencken burner flames and a new DBD burner validates its effectiveness in producing multiple spatially resolved measurements in combustion environments. Measurements in the DBD burner revealed an unsteady, counterflow flattened flame structure near the fuel orifice which became unsteady as the reaction zone curves towards the surface for larger radial positions. Fluctuations in the fuel concentration were largest at the source, as the large, plasma-generated, unsteady external toroidal vortex that dominates the transport in this flame provides enhanced ventilation of the flame surface in close proximity to the fuel tube.

Extinguishment of hydrogen laminar diffusion flames by water vapor in a cup burner apparatus

Transient computations with full hydrogen chemistry were performed to reveal the flame structure and extinguishment process of co-flow, hydrogen diffusion flame suppressed by water vapor. As the concentration of water vapor was increased, the flame detached away from the burner brim and formed an edge flame at the flame base. Water vapor showed larger chemical inhibition effect than nitrogen when extinguishing hydrogen flame, which was attributed to its enhanced third body effect in the reaction H + O2 + M = HO2 + M. The minimum extinguishing concentration (MEC) of water vapor and nitrogen was predicted by Senecal formula and perfectly stirred reactor (PSR) model respectively. The MECs predicted by PSR model agree with the MECs calculated by Fluent, which shows that 1) the flame extinction is controlled by the flame base, and 2) radiation absorption is negligible. The measured MECs are in a reasonable agreement with the values calculated by Fluent, which demonstrates the accuracy of the CFD model. A simple model was used to investigate the relative importance of extinguishing mechanisms of water vapor. The results show that in a co-flow configuration the thermal cooling and chemical inhibition effect are the main extinguishing mechanisms in suppressing hydrogen diffusion cup burner flame.

Extinguishment of hydrogen diffusion flames by ultrafine water mist in a cup burner apparatus – A numerical study

Transient simulations with full hydrogen chemistry were performed to reveal the flame structure and extinguishment process of co-flow hydrogen diffusion flame suppressed by ultrafine water mist (UFM). As UFM was added incrementally to the oxidizer stream, the flame experienced a series of destabilization process, i.e., detachment, drifting and blowoff. The simulations predicted that the critical mass flow rate of 10-μm UFM was 6 g/min, which is in agreement with the value calculated by a perfectly stirred reactor model and the value measured by the experiments. The critical mass flow rate exhibited a plateau region as the diameter increased from 5 μm to 20 μm. The optimal diameter for UFM was ≈ 10 μm. A scrutiny on the extinguishing mechanisms reveals that both the chemical kinetic effect and latent heat play important roles in determining the optimal diameter in this configuration. For the chemical kinetic effect, water molecule inhibits the flame through 1) enhancing the chain-terminating reaction H + O2 (+M) = HO2 (+M) and 2) subsequently scavenging free radicals in the flame. An energy equation was used to investigate the relative importance of extinguishing mechanisms for UFM. It shows that the thermal cooling outweighs the chemical kinetic effect in terms of contributions to flame inhibition although the chemical kinetic effect is obviously enhanced compared with N2.