A comparison of the heat transfer behaviors of biogas–H2 diffusion and premixed flames

An experimental study was conducted to investigate the influence of hydrogen addition on the heat transfer characteristics of a biogas (60%CH₄–40%CO₂) flame. Results show improved flame stability and higher flame temperature in the premixed flame upon hydrogen addition. Both temperature and burning speed are increased in 1.0 ≤ Ф ≤ 1.5. Comparison of the premixed and diffusion flames reveals that the former yields higher heat transfer than the latter, due to higher flame temperature and larger volume of hot gas in the premixed flame. The total heat transfer rates of the two flames show opposite trends with increasing level of hydrogen addition, which is explained by the different structures. In the premixed flame, the contact of large cool core with target plate configures the high-temperature flame zone to a radial location with larger distance from the stagnation point than that of the diffusion flame, contributing to its higher heat transfer rate.     

An experimental comparative study of the stabilization mechanism of biogas-hydrogen diffusion flame

This work describes an experimental study of the effect of hydrogen addition on the stabilization characteristics of laminar biogas diffusion flame. The focus is to identify and compare various factors influencing the blowoff process. Three compositions of biogas, BG₄₀, BG₅₀ and BG₆₀ were considered and the amount of hydrogen added was varied from 5% to 25% of the biogas by volume.With increasing hydrogen addition, the critical flow velocity beyond which the flame blows off increases faster than the laminar burning velocity (LBV) does, indicating that flame stabilization is not solely dependent on laminar burning velocity. An exponential relationship is observed between LBV and flame propagation speed. Therefore, both flame propagation speed and LBV, together with other factors, contribute to flame stabilization. The reason for no stable lift for either biogas or H₂-biogas flame is analyze by Schmidt number calculation, and the results agree with the literature. Also found is that hydrogen added to biogas accelerates the fuel mass diffusion, which may play an important role for stabilization of the nozzle-attached flame.The CO₂-C₃H₈ and BG₆₀ flames were compared to exclude the possible dominant role played by insufficient heat release and/or excessive heat loss due to CO₂ present in biogas. Tested on varied-size burners show that flame stabilization depends on burner pore size, where larger diameter allows better flame stability. The universal equation for predicting blowout/blowoff velocity in the literature was found to be invalid for H₂-enriched biogas flame and a new scaling law was put forwards.     

Experimental study on flame stability of biogas / hydrogen combustion

The flame stability of biogas blended with hydrogen combustion was experimentally studied in the constant volume combustion bomb. The variations of characteristic parameters of flame instability and effect of pressure and fuel component proportion on flame shape were analyzed. The experimental results show that the flame instability increases with the decrease of equivalence ratio, and the global flame stability decreases with increase of CO₂ fractions. With increase of initial pressure of biogas and hydrogen mixture, Markstein length decreases, hydrodynamic instability decreases, but the thermal mass diffusion instability has no effect. The effect of increase of the hydrogen ratio on flame stability is more obvious, with the increase of initial pressure and hydrogen ratio together, both hydrodynamic instability and thermal mass diffusion instability increase. This research can provide experimental basis for the design and development of biogas blended with hydrogen engines.     

The effect of hydrogen addition on biogas non-premixed jet flame stability in a co-flowing air stream

An investigation of the stability limits of biogas jet non-premixed (diffusion) flames in a co-flowing air stream was conducted. The stability limits were determined experimentally for two different methane–carbon dioxide mixtures that represent the typical biogas composition. Moreover, the effect of jet nozzle diameter was also investigated. It was found that with the presence of a significant amount of CO₂ in the fuel, the stability limits were very low and the flames can only be stabilized over a very small range of co-flowing air velocities. As expected, an increase in carbon dioxide concentration resulted in the narrowing of the region for stable flames. However, it was shown that the flame stability of such mixtures can be enhanced very significantly over a much wider range of co-flowing air velocities by introducing a small amount of hydrogen into the fuel. Results obtained in the current experimental setup indicate that an increase in the stability limits by approximately four-fold when 10% (by vol.) of hydrogen is added under the same operating conditions. The effect of the addition of hydrogen on the enhancement of biogas stability is most significant with a 10% initial addition. The degree of enhancement diminishes with further increases in hydrogen addition from 10% to 30%.     

Experimental study of flame zones variations of biogas enriched with hydrogen

Enriching biogas with hydrogen could enable conventional natural gas systems to be used for clean energy. This technique has generally been evaluated using laboratory devices, so this study addresses a conventional combustion system, consisting of a 100 kW burner fed with biogas-hydrogen mixtures instead of natural gas. Flame behavior and ignition behavior were investigated. The flame structure was analyzed by infrared thermography. The tests were performed with three different mixtures of CH₄–CO₂ recreating an energetically rich biogas, 30% CO₂ (BG70), standard biogas 40% CO₂ (BG60) and poor biogas 50% CO₂ (BG60). Then, each biogas type was enriched with hydrogen up to 20%. Major improvements were obtained between 5% and 10% hydrogen composition since the flame stability increases considerably. Flame structure closest to natural gas flame was achieved for BG60 and BG70 at 10% H₂. However, the flame temperature remained lower than that of natural gas in all cases.     

Characterization of biogas-hydrogen premixed flames using Bunsen burner

Experimental study is conducted to clarify the effects of hydrogen addition to biogas and hydrogen fraction in the biogas-H₂ mixture on the stability, thermal and emission characteristics of biogas-H₂-air premixed flames using a 9 mm-ID-tube Bunsen burner. Variation in biogas composition is allowed to range from BG₆₀ (60%CH₄–40%CO₂), down to BG₅₀ (50%CH₄–50%CO₂) and to BG₄₀ (40%CH₄–60%CO₂). For each biogas, the fraction of hydrogen in the biogas-H₂ mixture is varied from 10% to 50%. The results show that upon hydrogen addition and increasing hydrogen fraction in the fuel mixture, there are corresponding changes in flame stability, laminar burning velocity, flame tip temperature and CO pollutant emission.     

A comparison of the emission and impingement heat transfer of LPG–H2 and CH4–H2 premixed flames

Experiments were performed to add hydrogen to liquefied petroleum gas (LPG) and methane (CH₄) to compare the emission and impingement heat transfer behaviors of the resultant LPG–H₂–air and CH₄–H₂–air flames. Results show that as the mole fraction of hydrogen in the fuel mixture was increased from 0% to 50% at equivalence ratio of 1 and Reynolds number of 1500 for both flames, there is an increase in the laminar burning speed, flame temperature and NO_x emission as well as a decrease in the CO emission. Also, as a result of the hydrogen addition and increased flame temperature, impingement heat transfer is enhanced. Comparison shows a more significant change in the laminar burning speed, temperature and CO/NO_x emissions in the CH₄ flames, indicating a stronger effect of hydrogen addition on a lighter hydrocarbon fuel. Comparison also shows that the CH₄ flame at α = 0% has even better heat transfer than the LPG flame at α = 50%, because the longer CH₄ flame configures a wider wall jet layer, which significantly increases the integrated heat transfer rate.     

Effect of hydrogen addition on the structure and stabilization of a micro-jet methane diffusion flame

The micro-jet diffusion flame can act as the heat source for the micro power generation systems due to some advantages. The present work investigates the effect of hydrogen addition on the structure and stabilization of micro-jet methane diffusion flame by numerical simulation. The results show that the oval flame becomes more and more circular with the increase of hydrogen addition fraction. The addition of hydrogen remarkably suppresses the increase of the flame height with the inlet velocity. The methane sharply decreases around the outlet of the micro-jet tube due to the high fresh fuel temperature. The intermediate species (e.g., H₂ and CO) increase sharply before the flame front, and they are consumed sharply within the flame front. With the increase of hydrogen addition fraction, the concentration gradients of reactive species increase before the flame front, while the flame temperature decreases. In addition, with the increase of hydrogen addition fraction, the micro-jet flame root shifts toward the tube-wall and downstream direction at the radial and axial directions, respectively, and the addition of hydrogen decreases the anchoring temperature of the micro-jet flame root, which is conductive to improve the flame stabilization. Meanwhile, a large hydrogen addition fraction is detrimental for the flame stabilization in terms of the thermal interaction between the micro-jet flame and tube-wall. However, the positive effects brought by a large hydrogen addition fraction are noticeably larger than the adjunctive negative effects. This study not only provides the guideline for further expanding the operating range of the micro-jet methane diffusion flame but also helps us to gain insights into the mechanism of hydrogen addition on improving the flame stabilization.     

Flame characteristics of methane/air with hydrogen addition in the micro confined combustion space

This paper investigated methane/air flame characteristics with hydrogen addition in micro confined combustion space experimentally and computationally. The focus is on the effect of hydrogen addition on the methane/air flame stabilization, the onset of flame with repetitive extinction and ignition (FREI), and the global flame quenching in decreasing continuously combustion space. Furthermore, the effects of hydrogen addition on the flame temperature and the local equivalence ratio distribution were analyzed systematically using numerical simulations. In addition, the effects of hydrogen addition on the concentrations of OH and H radicals, and the critical scalar dissipation rate of local flame extinction were discussed. With a higher hydrogen ratio, the mixing is faster, and the flame is smaller. When the micro confined space is narrower, the heat loss to the combustor walls has a higher impact on the flames. The flames with higher hydrogen ratios have therefore lower peak flame temperatures and lower concentrations of H and OH radicals. The results show that hydrogen addition can effectively widen the stable combustion range of methane/air flames in the micro confined space by about 20% when the hydrogen addition ratio reaches 50%. The frequency and the maximum propagation velocity of FREI flames can be increased as well. The quenching distance of methane/hydrogen/air flames decreases nearly linearly with the increase of hydrogen ratio. This is attributed to the higher critical scalar dissipation rate of local flame extinction in flames with a higher hydrogen ratio.     

沼气掺氢发动机燃烧稳定性试验

在一台单缸火花点火发动机上开展了燃用不同组分配比的沼气模拟气体的掺氢混合气的燃烧稳定性试验研究.研究结果表明:在15%～35%的掺氢比范嗣内,随着混合气中掺氢比的增加,发动机循环变动变小,燃烧稳定性提高.掺氢导致平均指示压力的循环变动系数减小,燃烧放热率加快,火焰发展期缩短.其中35%掺氢比的混合气比15%掺氢比的混合...     

Effects of hydrogen concentration on the emission and heat transfer of a premixed LPG-hydrogen flame

This paper describes an experimental study of the effect of hydrogen concentration on the emission and heat transfer characteristics of a laminar premixed LPG-hydrogen flame. The mole fraction of hydrogen in the fuel mixture was varied from 0% to 50%. The equivalence ratio of the fuel/air mixture was kept at stoichiometry and the mixture jet Reynolds number was fixed at Re = 1500 for most of the tests. The results show that upon varying hydrogen content in the fuel mixture, there is a corresponding change in the appearance, pollutant emissions and heat transfer characteristics of the flame.     

Experimental study of the effects of hydrogen addition on the thermoacoustic instability in a variable-length combustor

The current study examined the self-excited thermoacoustic instability of hydrogen/methane premixed flames using a variable-length combustor (300–1100 mm). The global dynamic pressure, heat release rate oscillation, together with the flame dynamics were studied. Results showed that both the hydrogen concentration and the chamber length were critical in determining the acoustic oscillation mode and instability trend. Low-frequency primary acoustic modes (<200 Hz) were mainly excited when the hydrogen concentration was low, whereas primary acoustic modes with relatively higher frequencies (~400 Hz) tended to occur in cases with a high hydrogen proportion (>40%). For primary acoustic modes lower than 200 Hz, the primary oscillation frequency tended to increase linearly with a rising hydrogen proportion. Heat release oscillation and flame dynamics analyses demonstrated that for the flame with large-scale shape deformation, the initial addition of hydrogen would intensify the heat release oscillation. Nevertheless, a further increase in the hydrogen level tended to inhibit the heat release oscillation by weakening the flame shape deformation. Eventually, a sufficient high-level of hydrogen addition would weaken the primary acoustic modes that have similar frequencies.     

Experimental study on self-acceleration characteristics of unstable flame of low calorific value gas blended with hydrogen

The self-acceleration characteristics of cellular flame of low calorific value (LCV) gas in a constant volume combustion bomb were studied, the propagation process of spherical flame with different hydrogen (H₂) addition and initial pressure was analyzed, and the flame radius versus time was also discussed. The experimental results show that the self-acceleration of the cellular flame of LCV gas blended with hydrogen appears at high pressure, high hydrogen ratio and lean burn. The acceleration index increases with the increase of hydrogen addition and the reduction of equivalence ratio, and increases with the increase of initial pressure, but the acceleration index does not infinitely increase with the increase of initial pressure. With the increase of the hydrogen addition and the reduction of the equivalence ratio, the critical radius of the cellular flame decreases, which is shown that increase of the hydrogen addition and the lean burn will make appearance of cellular flame in advance. When the ratio of hydrogen is less than 60%, the critical Peclet number decreases with increase of hydrogen addition, when the ratio of hydrogen continues to increase to 80%, it increases slightly. The research in this paper provides an experimental basis for the in-depth study of engine combustion of LCV gas blended with hydrogen.     

Investigating the effect of local addition of hydrogen to acoustically excited ethylene and methane flames

While lean combustion in gas turbines is known to reduce NOx, it makes combustors more prone to thermo-acoustic instabilities, which can lead to deterioration in engine performance. The work presented in this study investigates the effectiveness of secondary injection of hydrogen to imperfectly premixed methane and ethylene flames in reducing heat release oscillations. Both acoustically forced and unforced flames were studied, and simultaneous OH and H atom PLIF (planar laser induced fluorescence) was conducted. The tests were carried out on a laboratory scale bluff-body combustor with a central V-shaped bluff body. Two-microphone method was used to estimate velocity perturbations from pressure measurements, flame boundary images were captured using high speed Mie scattering, while global heat release fluctuations were determined from OH* chemiluminescence.The results showed that hydrogen addition considerably reduced heat release oscillations for both methane and ethylene flames at all the forcing frequencies tested, with the exception of methane flames forced at 315 Hz, where oscillations increased with hydrogen addition. The addition of hydrogen reduced the extent of flame roll-up for both methane and ethylene flames, however, this reduction was larger for methane flames. NOx exhaust emissions were observed to increase with hydrogen addition for both methane and ethylene flames, with absolute NOx concentrations higher for ethylene flames, due to higher flame temperatures.     

Effects of hydrogen addition on the forced response of H2/CH4 flames in a dual-nozzle swirl-stabilized combustor

The effects of hydrogen addition on the forced response of H₂/CH₄ flames are analyzed in a dual-nozzle swirl-stabilized combustor. The hydrogen volumetric content in the fuel is varied from 0% to 40%. Flame transfer function (FTF) is used to compare the forced response of the flames. The FTF gain featuring the local maximum and minimum values, which occurred commonly in the FTFs under all hydrogen contents, is determined by two different mechanisms: the change in the flame angle and the flame roll-up phenomenon. Among two mechanisms, the flame roll-up phenomenon has a more important role in determining the FTF characteristics. In addition, hydrogen addition attenuates the local maximum gains and decreases the FTF phase slope. The change in the flame roll-up behavior, which is induced by a short and compact flame distribution at high hydrogen contents, is the primary reason of these differences in the FTF.     

Characteristic of cryogenic hydrogen flames from high-aspect ratio nozzles

Unintentional leaks at hydrogen fueling stations have the potential to form hydrogen jet flames, which pose a risk to people and infrastructure. The heat flux from these jet flames are often used to develop separation distances between hydrogen components and buildings, lot-lines, etc. The heat flux and visible flame length is well understood for releases from round nozzles, but real unintended leaks would be expected to be from higher aspect-ratio cracks. In this work, we measured the visible flame length and heat-flux characteristics of cryogenic hydrogen flames from high-aspect ratio nozzles. Heat flux measurements from 5 radiometers were used to assess the single-point vs the multi-point methods for interpretation of heat flux sensor data, finding the axial distance of the sensor for a single-point heat flux measurement to be important. We compare the flame length and heat flux data to flames of both cryogenic and compressed hydrogen from round nozzles. The aspect ratio of the release does not affect the flame length or heat flux significantly, for a given mass flow under the range of conditions studied. The engineering correlations presented in this work enable the prediction of flame length and heat flux which can be used to assess risk at hydrogen fueling stations with liquid hydrogen and develop science-based separation distances for these stations.     

Updated jet flame radiation modeling with buoyancy corrections

Radiative heat fluxes from small to medium-scale hydrogen jet flames (<10 m) compare favorably to theoretical predictions provided the product species thermal emittance and optical flame thickness are corrected for. However, recent heat flux measurements from two large-scale horizontally orientated hydrogen flames (17.4 and 45.9 m respectively) revealed that current methods underpredicted the flame radiant fraction by 40% or more. Newly developed weighted source flame radiation models have demonstrated substantial improvement in the heat flux predictions, particularly in the near-field, and allow for a sensible way to correct potential ground surface reflective irradiance. These updated methods are still constrained by the fact that the flame is assumed to have a linear trajectory despite buoyancy effects that can result in significant flame deformation. The current paper discusses a method to predict flame centerline trajectories via a one-dimensional flame integral model, which enables optimized placement of source emitters for weighted multi-source heat flux prediction methods. Flame shape prediction from choked releases was evaluated against flame envelope imaging and found to depend heavily on the notional nozzle model formulation used to compute the density weighted effective nozzle diameter. Nonetheless, substantial improvement in the prediction of downstream radiative heat flux values occurred when emitter placement was corrected by the flame integral model, regardless of the notional nozzle model formulation used.     

Flame structure changes resulting from hydrogen-enrichment and pressurization for low-swirl premixed methane–air flames

With the current focus on alternate and renewable fuels, fuel flexibility has become a driving factor in the design of new turbines. Flame stability is heavily impacted by the presence of hydrogen in the fuel stream (as is common in many alternative fuels). This study examines how the flame dynamics change in response to the systematic addition of hydrogen in a low-swirl lean premixed methane–air burner. Stability maps for these test cases show that adding hydrogen broadens the blow-off limits, with 20% hydrogen resulting in a 7% change while 40% hydrogen results in a 35% larger stable region. The most dramatic manifestation of hydrogen addition is the greatly decreased radius of curvature of the local flame surface, which is visible from the increased wrinkling of the flame front. Increases in both pressure and hydrogen enrichment result in higher means and variances of flame front curvatures. The flame surface density is in agreement with the aforementioned flame front curvature PDFs in that increasing the pressure and hydrogen concentration leads to an increase in the maximum flame surface density.     

Numerical simulation of the laminar hydrogen flame in the presence of a quenching mesh

Recent studies of J.H. Song et al. [1], and S.Y. Yang et al. [2] (see also references therein) have been concentrated on mitigation measures against hydrogen risk. The authors have proposed installation of quenching meshes between compartments or around the essential equipment in order to contain hydrogen flames. Preliminary tests were conducted which demonstrated the possibility of flame extinction using metallic meshes of specific size.Considerable amount of numerical and theoretical work on flame quenching phenomenon has been performed in the second half of the last century and several techniques and models have been proposed to predict the quenching phenomenon of the laminar flame system (see for example [3] and references therein). Most of these models appreciated the importance of heat loss to the surroundings as a primary cause of extinguishment, in particular, the heat transfer by conduction to the containing wall. The supporting simulations predict flame-quenching structure either between parallel plates (quenching distance) or inside a tube of a certain diameter (quenching diameter).In the present study the flame quenching is investigated assuming the laminar hydrogen flame propagating towards a quenching mesh using two-dimensional configuration and the earlier developed models. It is shown that due to a heat loss to a metallic grid the flame can be quenched numerically.     

On the stability of a turbulent non-premixed biogas flame: Effect of low swirl strength

Biogas like other low calorific value fuels has a very narrow stable region when operating in diffusion flame mode owing to their low burning velocity in conjunction with the unburned flow high velocity. This paper presents an experimental study on the effect of the burner geometry on the stability limits of a turbulent non-premixed biogas flame. The main focus of the study is on the role of the low swirl strength of the co-airflow, and the fuel nozzle diameter. The results revealed that the swirl plays a dominant role on the flame mode (attached or lifted) as well as on its operating/stability limits. However, the results revealed that the swirl effect prevails only at relatively moderate to high co-airflow velocity. That is, the swirl does not have an apparent effect at weak co-airflow when the flame is attached. Whereas, it becomes dominant at relatively high co-airflow velocity where the attached flame lifts off and stabilizes at a distance above the burner. Correlations were proposed to describe the lifted biogas flame blowout limits.