Hydrogen Diffusion Flame Research Articles

A Ternary Flame System for Soot Oxidation Studies

A ternary flame system is presented that allows observations of soot oxidation in a hydrogen diffusion flame in the absence of hydrocarbons. A propylene/air laminar jet diffusion flame emits a soot column with a diameter of 3 mm that passes through a ring burner supporting a hydrogen diffusion flame. The soot oxidizes in a region that is 60 mm long, laminar, steady, axisymmetric, and accessible for optical and sampling diagnostics. Temperatures and soot loading can be controlled nearly independently in this region and a broad range of mixture fraction is encountered without interference from soot formation.

Characterization of a Nitrogen Diluted Hydrogen Diffusion Flame for Model Validation

Dilute hydrogen diffusion flames have been considered as a gas turbine combustion strategy that provides relatively low levels of NOx emissions for application in integrated gasification combined cycle power generation. These flames also represent a challenging environment for computational modeling efforts due to the complexity of molecular transport effects, turbulence-chemistry interaction, and near extinction flame conditions. In order to provide data for validation of computational modeling efforts, measurements of major species concentration and flame temperature were made in such a flame using spontaneous Raman scattering. Experimental results demonstrate the importance of differential species diffusion, which occurs due to the disparity between diffusion characteristics of hydrogen and nitrogen. Additionally, the flame temperatures observed were quite low relative to the equilibrium flame temperature, due to flame strain. This confirms the fact that suppression of the thermal mechanism of NOx formation plays a significant role in reducing NOx emissions from this type of flame.

Modeling analysis on the silica glass synthesis in a hydrogen diffusion flame

Silica glass ingot synthesis by flame hydrolysis deposition (FHD) is an important approach to obtain high-purity synthetic silica glass with high refractive index homogeneity. Using the precursor of silicon tetrachloride (SiCl4), the silica ingot (SiO2) can be synthesized in the oxy-hydrogen diffusion flame through a set of kinetic reactions and subsequent cooling. The homogeneity of the synthetic silica glass is influenced by the temperature/diameter equality of silica droplets and the residue of radicals (e.g. OH) in the synthetic silica. In this study, the mixing system of oxy-hydrogen diffusion flame and silica droplets in a furnace were modeled by a Euler–Lagrange multi-phase model, where the interphase exchange of mass, momentum and energy between the gas phase molecules and liquid phase silica droplets during the formation and transportation of silica droplets were included. The silica droplets were tracked by a Lagrangian formulation that includes the discrete phase inertia, hydrodynamic drag, the force of gravity and the dispersion due to turbulent eddies. The molecular gas phase reactions are described by a simple kinetic mechanism involving the major species during the formation of molecular SiO2, with kinetic and thermodynamic parameters taken from the literature when available.The effect of initial equivalence ratio on the flame structure was analyzed in the study, where the high-temperature region is uniform over the glass ingot and the OH residue is limited at unity equivalence ratio. The probability distributions of silica droplet diameter and temperature were analyzed by employing a developed droplet growth model, where the condensation rate is controlled by the in situ SiO2 vapor concentration and the local flow-condition-determined mass transfer rate. The maximum diameter of silica droplets in the furnace is 1.39×10−4m, and the percentage of silica droplets decreases with the increasing of diameter. Two high probability regions were observed for the droplet temperature distribution, the temperature below 1500K accounts for 40% of the total droplet number, and the temperature range between 3500 and 4500K accounts for 58%. The droplet diameter on the ingot cap distributes in the range from 4×10−5 to 9×10−5m with approximately probability of 85%. The distribution of droplet temperature on the ingot cap is much uniform, with more than half (67%) of the droplets has the temperature between 3100 and 3200K.

Blowout of turbulent jet diffusion flames

Experiments have been performed to determine the blowout of jet diffusion flames with pure fuels, oxygenated fuels, mixed fuels, and diluted fuels. Stability tests were conducted with pure hydrocarbons at the C2 level to determine the effects of structural differences in the fuels. Diffusion flame blowout models were also used to correlate and interpret the data. Ethylene is more stable than ethane because of the additional heat release from the double-carbon bond and ethane is more stable than dimethyl ether. The blowout pressures of mixtures of ethylene and ethane are not linear contributions of component blowout pressures. Stability tests were conducted with ethylene diluted with air and nitrogen. Since both diluents have similar properties and also have a similar density to ethylene, factors in the blowout process, such as the laminar flame speed and air–fuel mass ratio, were isolated and measured. Stability tests with hydrogen diluted with helium, nitrogen, carbon dioxide, and sulfur hexafluoride were also conducted. The diluted hydrogen diffusion flames become less stable as the complexity of the diluent increases.

Testing of a Hydrogen Diffusion Flame Array Injector at Gas Turbine Conditions

High-hydrogen gas turbines enable integration of carbon sequestration into coal-gasifying power plants, though NOx emissions are often high. This work explores nitrogen dilution of hydrogen diffusion flames to reduce thermal NOx emissions and avoid problems with premixing hydrogen at gas turbine pressures and temperatures. The burner design includes an array of high-velocity coaxial fuel and air injectors, which balances stability and ignition performance, combustor pressure drop, and flame residence time. Testing of this array injector at representative gas turbine conditions (16 atm and 1750 K firing temperature) yields 4.4 ppmv NOx at 15% O2 equivalent. NOx emissions are proportional to flame residence times, though these deviate from expected scaling due to active combustor cooling and merged flame behavior. The results demonstrate that nitrogen dilution in combination with high velocities can provide low NOx hydrogen combustion at gas turbine conditions, with significant potential for further NOx reductions via suggested design changes.

Effect of turbulence and radiation models on combustion characteristics in propane–hydrogen diffusion flames

This paper presents numerical simulation results of propane, propane–hydrogen blending diffusion flames in a combustion chamber. The numerical simulations using Fluent CFD code were carried out by changing fuel blending from pure propane (100% C3H8) to propane–hydrogen blending including 90% C3H8–10% H2, 80% C3H8–20% H2, 10% C3H8–90% H2, 20% C3H8–80% H2 by volume. A two-dimensional axis-symmetric numerical model was solved to investigate the effects of the turbulence and radiation models on the combustion characteristics such as temperature, and gas concentration distributions. The combustion reaction scheme in the flame region was modeled using eddy dissipation model with global reaction scheme. The effects of two turbulence models including RNG k–ε, and Reynolds Stress Model, RSM, and two different radiation models including P–I and discrete transfer model were examined on combustion characteristics. The predictions are validated and compared with the published experimental and simulation results. Numerical results show that the velocity profiles, temperature gradients, CO2 and O2 concentrations profiles are overall agreement with published measurement and simulation results in the literature.

Modeled quenching limits of spherical hydrogen diffusion flames

Hydrogen–air diffusion flames were modeled with an emphasis on kinetic extinction. The flames were one-dimensional spherical laminar diffusion flames supported by adiabatic porous burners of various diameters. Behavior of normal (H2 flowing into quiescent air) and inverse (air flowing into quiescent H2) configurations were considered using detailed H2/O2 chemistry and transport properties with updated light component diffusivities. For the same heat release rate, inverse flames were found to be smaller and 290K hotter than normal flames. The weakest normal flame that could be achieved before quenching has an overall heat release rate of 0.25W, compared to 1.4W for the weakest inverse flame. There is extensive leakage of the ambient reactant for both normal and inverse flames near extinction, which results in a premixed flame regime for diffusion flames except for the smallest burners with radii on the order of 1μm. At high flow rates H+OH(+M)→H2O(+M) contributes nearly 50% of the net heat release. However at flow rates approaching quenching limits, H+O2(+M)→HO2(+M) is the elementary reaction with the largest heat release rate.

Experimental observations of complex flame propagations initiated at different locations of an impingement configuration

The effect of ignition location on the propagation of premixed and diffusion flames of hydrogen and mixtures of hydrogen and carbon dioxide towards and away from an impingement plate was studied using distance-time relations and velocity–time curves for the edge of the flame boundary at the centre line. It is shown that ignition location, fuel composition and jet velocity have large effects on the flame propagation speed, and non-linear acceleration was observed for flames ignited at the plate. The effect of the angle of mixing of the unburned fuel with the surrounding air was found to have an effect on the downwards propagation, and also, delay of downwards propagation after ignition was observed for some cases. It was found that air/fuel premixing hinders the initial downwards propagation of the flame when ignited at the plate. Propagation velocities of up to 25ms−1 were observed when the flame was ignited at the nozzle, aided by the velocity of the fuel jet. However, accelerations much higher than the gravitational acceleration were observed, indicating that buoyancy may not be a significant factor in the high velocities of the flames. It was also found that jet velocity had a higher effect on the propagation velocity for the premixed and H2/CO2 flames than the diffusion flames.

The role of turbulent fluctuations on radiative emission in hydrogen and hydrogen-enriched methane flames

A theoretical analysis is reported in the present work to quantify the increase of radiative emission due to turbulence for hydrogen and hydrogen-enriched methane diffusion flames burning in air. The instantaneous thermochemical state of the reactive mixture is described by a flamelet model along with a detailed chemical mechanism. The shape of the probability density function (pdf) of mixture fraction is assumed. The results show that turbulent fluctuations generally contribute to reduce the Planck mean absorption coefficient of the medium, in contrast with the blackbody emissive power, which is significantly increased by turbulence. If the turbulence level is relatively small, the influence of turbulence on the absorption coefficient is marginal. Otherwise, fluctuations of the absorption coefficient of the medium should be taken into account. The scalar dissipation rate and the fraction of radiative heat loss have a much lower importance than the turbulence intensity on the mean radiative emission.

화염법으로 제조된 감마-Al2O3나노입자의 화염조건에 따른 입자특성 연구

본 논문은 수소를 연료로 하는 확산화염을 이용하여 알루미나 나노입자를 합성할 때, 합성되는 알루미나 나노입자의 특성에 미치는 화염온도의 영향을 조사하였다. 합성된 나노입자의 특성을 전자현미경 이미지, 결정 구조 분석, 비표면적과 기공의 크기 분석, 화염온도 측정 등의 여러 특성분석 방법으로 조사하였다. 사용된 화염의 중심축 최고온도는 산화제의 산소농도가 19, 21, 30, 47%인 각각의 실험조건에서 1507.8K, 1593.8K, 1753.1K, 1998.7K으로 측정되었다. SEM 이미지 분석 및 BET 비표면적 측정을 통해서는 47% 산소농도인 경우 에는 50 nm 수준의 독립적인 구형입자가 생성되었음을 확인할 수 있었으며, 19%와 21%의 경우에는 응집된 상태의 20-30 nm 수준의 입자를 볼 수 있었다. XRD 결과에서는 감마(γ)-알루미나가 주를 이루는 것으로 판단되었다. 이상의 결과를 바탕으로 촉매 담체로 사용하기 위한 알루미나 나노입자를 연소합성 하기 위한 가장 적절한 조건으로 실험했던 네 경우 중에서는 산화제의 산소농도가 21%인 두 번째 경우를 선택할 수 있었다.

Development of a micro-flame ionization detector using a diffusion flame

A micro-flame ionization detector (micro-FID) design is presented that is targeted for use in a portable gas sensor. Our micro-FID is based on a diffusion flame and features a folded flame structure that is more sensitive than a counter-flow flame designs. Unlike conventional FIDs that use a premixed or open diffusion flame, an air–hydrogen diffusion flame is employed and tested in an encapsulated structure of Quartz–Macor–Quartz layers. Diffusion flames are generally known to be more controllable and stable than premixed flames, where the stability of the micro-FID plays an important role for portable gas sensors. Various channel designs for oxidant and fuel flows meeting with different angles at the burner cavity are tested to obtain a stable flame and high output sensitivity over methane test samples. To verify the empirically designed microchannel, we simulate the temperature distribution in the microchannel by using computational fluid dynamics (CFD) software. To gauge the sensitivity of the device, the collected electric charges per mole (C/mol) is calculated and taken as a reference value of ionization efficiency. The result of the folded flame design is 1.959×10−2C/mol for methane that is about 34 times higher than the result using a counter-flow flame, which is 5.73×10−4C/mol for methane, while one of the commercial macro FIDs’ is 10−1C/mol. This result shows that the micro-FID using the folded flame structure has higher ionization efficiency with less leakage of the analytes than of the classical counter-flow flame design.

Development of reduced reaction mechanisms for hydrogen and methanol diffusion flames

Biofuels, such as biohydrogen, biomethanol, bioethanol and biodiesel may be the future of energy for transportation. The methanol can be used directly for powering Otto engines or fuel cells; it is commonly used in biodiesel production for its reactivity. Biohydrogen is a replacement for fossil and biorenewable liquid fuels. Based on a mechanism composed by 20 reversible elementary reactions among 8 reactive species, we propose a reduction process to obtain the mechanism of hydrogen, and from 129 reversible elementary reactions among 23 reactive species, we obtain a reduced mechanism for the methanol. Therefore, the aim of this work is the development of an algorithm consisting of 7 steps, which eliminates the fastest reactions of the reaction mechanism since the slowest reactions are rate-determining. The main advantage of the strategy is the decrease of the work needed to solve the system of chemical equations.

Effect of silica addition on thermal stability of heat treated flame synthesised TiO2 nanoparticles

The effect of silica addition on the thermal stability of flame synthesised TiO2 nanoparticles was investigated. TiO2 nanoparticles with varying silica content were prepared using hydrogen diffusion flames. Characterisation and measurement of photocatalytic ability of formed nanoparticles before and after the heat treatment were carried out. On the basis of transmission electron microscope images and particle diameters that were calculated from specific surface area data and X-ray diffraction patterns, it is believed that the addition of silica might interrupt the growth of smaller particles to larger ones in the flame, but could enhance the thermal stability of the formed nanoparticles. Meanwhile, based on the decomposition of methylene blue, the anatase fractions and the calculated particle diameters, the TiO2 nanoparticles that were synthesised with the addition of silica corresponding to 0·03 L min−1 of the silica precursor carrier gas were the most thermally stable among those tested with heat treatment at 1000°C.

Non-gray modeling of radiative heat transfer in hydrogen combustion scenarios

SUMMARY New weighted-sum-of-gray-gases model (WSGGM) parameters for H2O vapor are derived from emissivity correlations in the open literature and presented for use in hydrogen combustion simulations. Predictions employing the new WSGGM are seen to compare favorably against the spectral-line-based WSGGM on benchmark problems as well as in media conditions representative of turbulent hydrogen diffusion flames (Sandia Flame A and a model hydrogen gas-turbine combustor). The Sandia Flame A calculations were performed in a decoupled manner employing experimental measurements of temperature and gas compositions as inputs. The measured temperature variance data were employed to model the turbulence–radiation interactions. A coupled computational fluid dynamic (CFD) calculation was performed to obtain the conditions within the gas-turbine combustor geometry. The observed accuracies of the proposed set of WSGGM parameters in conditions encompassing a wide range of H2O vapor concentrations and temperature non-homogeneities encountered in combustion media make them amenable to implementation in CFD codes. Copyright © 2011 John Wiley & Sons, Ltd.

To duckweeds ( Landoltia punctata), nanoparticulate copper oxide is more inhibitory than the soluble copper in the bulk solution

CuO nanoparticles (CuO-NP) were synthesized in a hydrogen diffusion flame. Particle size and morphology were characterized using scanning mobility particle sizing, Brunauer–Emmett–Teller analysis, dynamic light scattering, and transmission electron microscopy. The solubility of CuO-NP varied with both pH and presence of other ions. CuO-NP and comparable doses of soluble Cu were applied to duckweeds, Landoltia punctata. Growth was inhibited 50% by either 0.6 mg L −1 soluble copper or by 1.0 mg L −1 CuO-NP that released only 0.16 mg L −1 soluble Cu into growth medium. A significant decrease of chlorophyll was observed in plants stressed by 1.0 mg L −1 CuO-NP, but not in the comparable 0.2 mg L −1 soluble Cu treatment. The Cu content of fronds exposed to CuO-NP is four times higher than in fronds exposed to an equivalent dose of soluble copper, and this is enough to explain the inhibitory effects on growth and chlorophyll content.

Rotational CARS thermometry in diffusion flames: On the influence of nitrogen spectral line-broadening by CH4 and H2

Rotational CARS N2 thermometry has been developed for improved accuracy in spatial flame regions with high concentrations of either H2 or CH4, which are typical conditions on the fuel side of diffusion flames. In initial validation experiments up to 700K, rotational CARS measurements were performed in binary mixtures of H2 and N2. Subsequent spectral evaluation with commonly used N2–N2 Raman line widths clearly underestimated the real temperatures. Inclusion of recently calculated N2–H2 spectral line-broadening coefficients weighted by the concentration of hydrogen resulted in CARS temperatures in much better agreement with the true temperatures. Based on this validation work, improved temperature accuracy was achieved in flame measurements on the fuel side of a hydrogen diffusion flame by probing nitrogen simultaneously with the detection of the hydrogen S0-transition at 354cm−1, and weighting the species specific Raman line widths with the information of relative concentration. The validation experiments were repeated for binary mixtures of nitrogen and methane up to 500K. It was shown that N2–N2 Raman line widths gave reproducible temperatures independent of methane concentration in binary mixtures with nitrogen, thus no explicit knowledge on N2–CH4 Raman line widths is needed for this temperature region. In general, the impact of Raman line widths for accurate rotational CARS thermometry is demonstrated.

Candoluminescence of cave gypsum

A selection of gypsum specimens from a variety of caves as well as CaSO4 synthesized in the laboratory emit both a green and yellow candoluminescence when excited by a hydrogen diffusion flame. The green emission is attributed to dehydration of gypsum to bassanite and the yellow emission appears upon further dehydration to anhydrite. The source of the luminescence is ascribed to minor concentrations of Mn2+ in the gypsum.

Effects of ultrasonic processing on phase transition of flame-synthesized anatase TiO 2 nanoparticles

The effect of ultrasonic processing on the phase transformation of flame-synthesized anatase TiO 2 nanoparticles heated to the rutile phase was investigated. TiO 2 nanoparticles of various sizes were prepared using a coflow hydrogen diffusion flame and an ultrasonic processor. Smaller nanoparticles having a similar portion of anatase phase using the ultrasonic processor were produced. On the basis of scanning electron microscopy images and specific surface areas, we observed that smaller nanoparticles tended to be sintered more easily than larger nanoparticles. From X-ray diffraction analysis, we demonstrated that when heated, TiO 2 nanoparticles synthesized using the ultrasonic processor at 60% of its maximum amplitude were transformed from the anatase phase to the rutile phase more easily than those formed without or with the ultrasonic processor operated at 20% of its maximum amplitude.

Simultaneous detection of selenium by atomic fluorescence and sulfur by molecular emission by flow-injection hydride generation with on-line reduction for the determination of selenate, sulfate and sulfite

An inductively coupled plasma atomic fluorescence spectrometry (ICP-AFS) instrument, was modified so that it was capable of monitoring transient chromatographic or flow-injection profiles and that sulfur molecular emission and selenium atomic fluorescence could be monitored simultaneously in an argon–hydrogen diffusion flame on a glass burner. The analytes were introduced as hydrogen selenide and hydrogen sulfide, generated on a flow-injection manifold. Selenate was reduced to hydride-forming selenite by microwave-assisted on-line reaction with hydrochloric acid, and sulfate, or sulfite, was reduced to hydride-forming sulfide by a mixture of hydriodic acid, acetic acid and sodium hypophosphite. The effects of the nature of reducing agent, flow rate, microwave power and coil length were studied. The limit of detection (3 s) for selenium was 10 μg L −1, and for sulfide was 70 μg L −1 (200-μL injection volume). The calibration was linear for selenium up to 2 mg L −1 and to 10 mg L −1 for sulfide. The throughput was 180 h −1. The three sulfur species could be differentiated on the basis of reactivity at various microwave powers.

Limits for hydrogen leaks that can support stable flames

Quenching and blowoff limits of hydrogen diffusion flames on small burners were observed. Four burner types, with diameters as small as 8 μm, were considered: pinhole burners, curved-wall burners, tube burners, and leaky fittings. In terms of mass flow rate, hydrogen had a lower quenching limit and a higher blowoff limit than either methane or propane. Hydrogen flames at their quenching limits were the weakest flames recorded to date, with mass flow rates and heat release rates as low as 3.9 μg/s and 0.46 W. The quenching limit for a hydrogen flame at a 6 mm leaky compression fitting was found to be 28 μg/s. This limit was independent of supply pressure (up to 131 bar) and about an order of magnitude lower than the corresponding limits for methane and propane.