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1.
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Hydrogen (H2) is considered as a carbon-free alternative fuel. The heat release characteristics of H2 flame as a key parameter in its combustion process are unclear. In this study, the combustion and heat release characteristics of H2/air diffusion flame on a micro-jet array burner were experimentally and numerically investigated. It is shown that the OH distribution and flame length based on Bilger mechanism are in good agreement with the experimental results. Furthermore, the intensity and distribution of OH and heat release rate can be adjusted by the thermal power and equivalence ratio. A uniform flame with intensive heat release rate can be achieved at a thermal power of 0.1 kW. R41: H + O2 = OH + O and R43: H + O2 + M = HO2 + M are the main reactions with oxidizer consumption to form reactive radicals. R40: OH + H2 = H2O + H and R47: OH + OH = O + H2O with OH consumption are the main heat release reactions at the upstream and downstream of the flame.  相似文献   

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The paper presents a numerical investigation of the critical roles played by the chemical compositions of syngas on laminar diffusion flame instabilities. Three different flame phenomena – stable, flickering and tip-cutting – are formulated by varying the syngas fuel rate from 0.2 to 1.4 SLPM. Following the satisfactory validation of numerical results with Darabkhani et al. [1], the study explored the consequence of each species (H2, CO, CH4, CO2, N2) in the syngas composition. It is found that low H2:CO has a higher level of instability, which however does not rise any further when the ratio is less than 1. Interestingly, CO encourages the heat generation with less fluctuation while H2 plays another significant role in the increase of flame temperature and its fluctuation. Diluting CH4 into syngas further increases the instability level as well as the fluctuation of heat generation significantly. However, an opposite effect is found from the same action with either CO2 or N2. Finally, considering the heat generation and flame stability, the highest performance is obtained from 25%H2+75%CO (81 W), followed by EQ+20%CO2, and EQ+20%N2 (78 W).  相似文献   

5.
Flamelet models for turbulent combustion typically employ the assumption of unity Lewis number, i.e., equal thermal and species diffusivities. These models have been employed to predict ignition delay times and ignition location in combusting sprays. However, there is the interesting question: what would be the effects of including multicomponent species diffusion on the ignition predictions? In this work, a one-dimensional n-heptane-air diffusion flame is chosen to study the effects of multicomponent diffusion on predicted ignition characteristics. The ambient conditions selected include typical in-cylinder conditions of a medium-duty diesel engine: pressure 10-40 bar and air temperature 850-1000 K. The ignition and oxidation of n-heptane are predicted using a reaction mechanism consisting of 34 species and 56 steps. The mixture fraction is computed separately as a passive species, the diffusion coefficient, of which is equal to the local thermal diffusion coefficient. From these computations, the transient structure of the flamelet, including ignition, is obtained. The results are compared with those obtained with the unity Lewis number assumption. The implications of the unity Lewis number assumption on the predicted ignition characteristics are discussed.  相似文献   

6.
Simultaneous point measurements of temperature, mixture fraction, major species, and OH concentrations in a lifted turbulent hydrogen jet flame are reprocessed to obtain the Favre average and conditional mean profiles. Large discrepancies between the Favre average and the ensemble average temperature, H2O, and OH mole fractions are found at the lifted flame base, due to density weighting of fairly large samples of unreacted mixtures. Conditional statistics are used to reveal the reaction zone structure in mixture fraction coordinates. The cross-stream dependence of conditional reactive scalars, which is most notable at the lifted flame base and decreases to negligible levels with increasing streamwise positions, could be attributed to radial differences in both the Damköhler number and the level of partial premixing. Conditional results indicate that the lifted flame is stabilized at the outer region of the jet characterized by low strain rates and lean mixtures. Comparison of the measured conditional mean OH vs H2O with a series of stretched laminar partially premixed flame and diffusion flame calculations reveals that strong partial premixing takes place at the lifted flame base and the strain rates vary from a=14,000 to 100 s−1. The level of partial premixing and the strain rate decrease with increasing downstream locations. The range of estimated scalar dissipation rates (χ≈1–0.13 s−1) at a further downstream location (x/D=33.3) is in agreement with reported values and the flame composition reaches an equilibrium condition at x/D=194.4. These results combined with previously reported data provide a benchmark data set for evaluation and refinement of turbulent combustion models for lifted hydrogen jet flame predictions.  相似文献   

7.
This paper reveals lift-off behavior of jet diffusion flames in sub-atmospheric pressures less than 100 kPa, in view of that the current knowledge on this topic is limited for normal pressure conditions. Physically, the variation of ambient pressure may have significant influence on the lift-off behavior of jet diffusion flames due to the change of some critical parameters such as laminar flame speed. In this work, experiments are conducted in a large pressure-controllable chamber of 3 m (width) × 2 m (length) × 2 m (height) at different sub-atmospheric pressures of 60 kPa, 70 kPa, 80 kPa, 90 kPa as well as at normal pressure of 100 kPa. Axisymmetric turbulent jet diffusion flames are produced by nozzles with diameters of 4 mm, 5 mm and 6 mm using propane as fuel. It is revealed that the lift-off height increases as the pressure decreases and being much higher than that in normal pressure condition. The laminar flame speed with its dependency on pressure is introduced to interpret such behavior based on classic Kalghatgi model. It is found theoretically that the lift-off height has a power law dependency on pressure by P1−n, where n is overall reaction order of the fuel which is usually larger than 1 indicating a negative power law function with pressure (for example p−0.75 for propane as n = 1.75) as well verified by the experimental correlation. Finally, a global model is proposed by including such pressure dependency function into the Kalghatgi model, which is shown to well collapse the experimental results of lift-off heights of different sub-atmospheric pressures.  相似文献   

8.
Soot formation in laminar diffusion flames   总被引:2,自引:0,他引:2  
Laminar, sooting, coflow diffusion flames at atmospheric pressure have been studied experimentally and theoretically as a function of fuel dilution by inert nitrogen. The flames have been investigated with laser diagnostics. Laser extinction has been used to calibrate the experimental soot volume fractions and an improved gating method has been implemented in the laser-induced incandescence (LII) measurements resulting in differences to the soot distributions reported previously. Numerical simulations have been based on a fully coupled solution of the flow conservation equations, gas-phase species conservation equations with complex chemistry, and the dynamical equations for soot spheroid growth. The model also includes the effects of radiation reabsorption through an iterative procedure. An investigation of the computed rates of particle inception, surface growth, and oxidation, along with a residence time analysis, helps to explain the shift in the peak soot volume fraction from the centerline to the wings of the flame as the fuel fraction increases. The shift arises from changes in the relative importance of inception and surface growth combined with a significant increase in the residence time within the annular soot formation field leading to higher soot volume fractions, as the fuel fraction increases.  相似文献   

9.
A numerical study of an axisymmetric coflow laminar ethylene-air diffusion flame at atmospheric pressure was conducted using detailed chemistry and complex thermal and transport properties and two different methodologies: (1) the direct simulation method of solving the two-dimensional axisymmetric elliptic governing equations, and (2) the steady-state stretched diffusion flamelet model. Soot formation and radiative heat transfer were not taken into account in these calculations, both for simplicity and to avoid the complications associated with the issues of how to incorporate these chemical and physical processes into the flamelet model. The same reaction mechanism and thermal and transport properties were used in the 2D direct simulation and the generation of the flamelet library. The flamelet library was generated from the solutions of counterflow ethylene-air diffusion flames at a series of stretch rates. Results of the 2D direct simulation and the flamelet model are compared in physical space. Although the overall results of the flamelet model are qualitatively similar to those of the direct simulation, significant differences exist between the results of the two methods even for temperature and major species. The direct simulation method predicts that the peak concentrations of CO2 and H2O occur in different regions in the flame, while the flamelet model results show that the peak concentrations of CO2 and H2O occur in the same region. The flamelet model predicts an overly rapid approach to the equilibrium structure in the downstream region, leading to significantly higher flame temperatures. The main reason for the failure of the flamelet model in the downstream region is due to the neglect of the effects of multidimensional convection and diffusion and the fundamental difference in the chemical structure between a coflow diffusion flame and a counterflow diffusion flame. The findings of this paper are highly relevant to understanding the flamelet model results in the calculations of multidimensional turbulent diffusion flames.  相似文献   

10.
High-resolution transmission electron microscopy (HRTEM) has been performed on soot samples collected from two smoking laminar ethylene diffusion flames (one steady and one unsteady) and from the active-flaming region of a 5-m diameter JP-8 pool fire. The motivation for this study is to improve the understanding of the influence of soot microstructure on its optical properties. The soot sampling positions in the steady ethylene flame correspond to locations of maximum soot mass growth, partial soot oxidation, and quenched oxidation along a common streamline. Visual examination of the HRTEM images suggests that the graphitic crystalline layers of soot undergo increased densification along the sampled streamline in the steady laminar flame. Quantitative image analysis reveals a small decrease in the mean graphitic interlayer spacing (d002) with increasing residence time in the high-temperature region. However, the differences in the mean interlayer spacing are far smaller than the spread of interlayer spacings measured for any given soot sample. Post-flame samples from the unsteady ethylene flame show interlayer spacing distributions similar to the lower region of the steady flame. The soot samples from the pool fire show little evidence of oxidized soot and have interlayer spacings similar to the unsteady ethylene flame. Previous research in the carbon black field has demonstrated a direct relation between the graphitic interlayer spacing and the optical absorptivity of the carbon. Consequently, the current HRTEM results offer support to recent measurements of the dimensionless extinction coefficient of soot that suggest that the optical absorptivity of agglomerating soot shows only minor variations for different fuels and flame types.  相似文献   

11.
A statistical approach to the dynamics of diffusion-flame holes is presented. The dynamics of the holes are assumed to be controlled by the edge-flame velocity that is determined by the mixture fraction rate of dissipation, a random variable in a turbulent flow. The formulation is then specialized to the case of small circular holes and a stochastic model is used to investigate the dynamics of the joint probability density function of flame-hole radius and scalar dissipation. The associated Fokker-Planck transport equation for the joint pdf is solved and the hole area evolution with time is computed. Furthermore, the one-dimensional marginal probability density function transport equation for the hole radius is derived and the conditional edge-flame velocity is studied for both expanding and collapsing holes.  相似文献   

12.
Buoyant jet diffusion flames are frequently used to investigate phenomena associated with flares or fires, such as the formation and emission of soot, polycyclic aromatic hydrocarbons (PAH), and carbon monoxide (CO). To systematically investigate the influence of transient vortex-flame interactions on these processes, laminar jet flames may be periodically forced. Previous work has demonstrated that forcing the fuel stream at a (low) frequency close to the natural buoyant instability frequency will trigger the production of vortices on the air side of the high-temperature reaction zone, coupling the overall flame response to the forcing frequency. In the work reported here, measurements in methane/air and ethylene/air slot flames show that over a substantial range of forcing frequencies and amplitudes, the dominant, air-side vortex production is locked at precisely one-half the excitation frequency of the fuel stream. This phenomenon is examined in detail through the utilization of several laser diagnostic techniques, yielding measurements of both the frequency response of the flames and phase-locked images of the internal flame structure. Under some conditions the subharmonic response of the flame leads to transient separation of the PAH and soot layers from the surrounding high-temperature flame zone, potentially affecting the soot formation and radiation processes. This data should provide useful information for comparison with detailed modeling aimed to improve the understanding of the complex nature of the buoyant instability in jet flames.  相似文献   

13.
By using OH-PLIF technique, experiments were conducted for laminar Bunsen flame of premixed CO/H2/air mixtures with equivalence ratio ranging from 0.5 to 1.8. Reynolds number was varied from 800 to 2200, XH2 = H2/(H2+CO) in the mixture was varied from 20% to 100% to study the effects of both preferential diffusion and flame curvature on flame structures and laminar flame burning velocities. Results showed that the combined effects of preferential diffusion and curvature gave an interesting phenomenon of the flame OH radical distributions on high hydrogen content flames. Furthermore, with the increase of H2 fraction in fuel mixture, the effects of both preferential diffusion and flame curvature were increased. Interpretation of flame stretch effect on laminar burning velocity is also provided in this paper.  相似文献   

14.
The effects of adding water vapor to the air stream on flame properties and soot volume fraction were investigated numerically in a laminar coflow ethylene/air diffusion flame at atmospheric pressure by solving the fully elliptic conservation equations and using a detailed C2 reaction mechanism including PAH up to pyrene and detailed thermal and transport properties. Thermal radiation was calculated using the discrete-ordinates method and a statistical narrow-band correlated-k based wide band model for the absorption coefficients of CO2 and H2O. Soot formation was modeled using a PAH based inception model and the HACA mechanism for surface growth and oxidation. Addition of water vapor significantly reduces radiation heat loss from the flame primarily through reduced soot loading and flame temperature. The added water vapor affects soot formation and flame properties through not only dilution and thermal effects, but also through chemical effect. The chemical effect is as significant as the dilution and thermal effects. The primary pathway for the chemical effect of water vapor is the reverse reaction of OH + H2 ↔ H + H2O. Our numerical results confirm that the reduced H radical concentration leads to lower PAH concentrations and consequently lower soot inception rates. In contrast, the radiation effect due to the added water vapor was found to have a minor influence on both flame structure and soot formation in the laminar diffusion flame investigated.  相似文献   

15.
OH planar laser-induced fluorescence (PLIF) and particle image velocimetry have been used to study the frequency response of laminar C3H8-air counterflow diffusion flames to assess the adequacy of the steady-flamelet models. Particle image velocimetry was used to determine the flame strain rate, while OH PLIF was used both to measure temperature at the flame front, using the two-line PLIF technique, and the reaction-zone width. Both measurements demonstrate that the frequency response of flames subjected to a time-varying flow field is diffusion-limited. At the 30-Hz and 50-Hz forcing frequencies, the maximum reaction-zone temperature and width were found to respond quasi-steadily. However, at higher forcing frequencies-i.e., 100 and 200 Hz-transient behavior is evident from the phase relationship between the imposed sinusoidal strain rate and the resulting peak temperature and reaction-zone width. The measured values of the OH-field widths were well fit by an offset sine function. In all cases when the oscillation amplitude is normalized by the cycle mean strain rate and plotted against the non-dimensional flow field frequency, the results collapse onto a single line having a steep negative slope.  相似文献   

16.
The heat release response of a two-dimensional (2-D) co-flow diffusion flame is theoretically investigated for the mass fraction fluctuations of reactant concentration at the inlet boundary and for time-varying spatially uniform flow velocity field in the domain. The present work is an extension of the Burke-Schumann steady flame model to an oscillatory situation, but in a 2-D framework. The governing equation of the problem is the scalar advection equation for the Schwab-Zel'dovich variable. An exact solution is found in terms of an infinite series for the case of mixture fraction fluctuations at the inlet boundary. The cases of time-varying uniform flow velocity field and a combination of velocity and mixture fraction fluctuations are investigated numerically. The temperature and heat release rate of the flame are thermodynamically calculated utilizing the mixed-is-burnt approach. The main results of the paper are the response functions. The nonlinearity in the calculation of the heat release rate and the convective nonlinearity due to velocity fluctuations result in the generation of higher harmonics in the response function for a given sinusoidal excitation. Therefore, the response function is decomposed and obtained for each of the significant harmonics. The results show that, in general, the response function decreases with increase in the excitation frequency as reported with premixed flames in the literature. However, in the present case, the decrease occurs when the excitation time-scale is less than the diffusion time-scale. Interesting flame shape variations such as flame clip-off, flipping between overexpanded and underexpanded conditions, and flame wrinkling are observed in the case of mixture fraction oscillations.  相似文献   

17.
The interaction of non-unity Lewis number (due to preferential diffusion and/or unequal rates of heat and mass transfer) with the coupled effect of radiation, chemistry and unsteadiness alters several characteristics of a flame. The present study numerically investigates this interaction with a particular emphasis on the effect of unequal and non-unity fuel and oxidizer Lewis numbers in a transient diffusion flame. The unsteadiness is simulated by considering the flame subjected to modulations in reactant concentration. Flames with different Lewis numbers (ranging from 0.5 to 2) and subjected to different modulating frequencies are considered. The results show that the coupled effect of Lewis number and unsteadiness strongly influences the flame dynamics. The impact is stronger at high modulating frequencies and strain rates, particularly for large values of Lewis numbers. Compared to the oxidizer side Lewis number, the fuel side Lewis number has greater influence on flame dynamics.  相似文献   

18.
Effect of content of hydrogen (H2) in fuel stream, mole fraction of H2(XH2) in fuel composition, and velocity of fuel and co-flow air (Vavg) on the flame characteristics of a co-flow H2/N2 laminar diffusion flame is investigated in this paper. Co-flow burner of Toro et al. [1] is used as a model geometry in which the governing conservation transport equations for mass, momentum, energy, and species are numerically solved in a segregated manner with finite rate chemistry. GRI3 reaction mechanisms are selected along with the weight sum of grey gas radiation (WSGG) and Warnatz thermo-diffusion models. Reliability of the newly generated CFD (computational fluid dynamics) model is initially examined and validated with the experimental results of Toro et al. [1]. Then, the method of investigation is focused on a total of 12 flames with XH2 varying between 0.25 and 1, and Vavg between 0.25 and 1 ms?1. Increase of flame size, flame temperature, chemistry heat release, and NOx emission formation resulted are affected by the escalation of either XH2 or Vavg. Significant effect on the flame temperature and NOx emission are obtained from a higher XH2 in fuel whereas the flame size and heat release are the result of increasing Vavg. Along with this finding, the role of N2 and its higher content reducing the flame temperature and NOx emission are presented.  相似文献   

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20.
The structure and extinction characteristics of counterflow diffusion flames with flame radiation and nonunity Lewis numbers of the fuel and oxidant are examined using multiscale asymptotic theory, and a model expressed in terms of the jump relations and reactant leakages with the proper consideration of the excess enthalpy overlooked in previous analyses is developed. The existence of the dual extinction limits in the presence of radiative heat loss, namely the kinetic limit at small Damköhler number (high stretch rate) and the radiative limit at large Damköhler number (low stretch rate), are identified. It is found that the former is minimally affected by radiative loss, while a substantial amount of heat loss is associated with the radiative limit. Reactant leakage, however, is the root cause for both limits. The influence of radiative loss on the extinction Damköhler numbers is found to be through its effects on the flame temperature, the excess enthalpy, and the reduced extinction Damköhler number. At both extinction limits, the contribution from the flame temperature is always important and dominant. The contributions from the other two, however, could be important in some special cases. At small LeF, the contribution from the reduced extinction Damköhler number is large and even dominant under small radiative loss. The contribution from the excess enthalpy is important for small LeO and it may be comparable to the contribution from the flame temperature when radiative loss is small. Thus, overlooking the excess enthalpy in previous analyses may have resulted in rather large error in the predicted extinction Damköhler numbers, especially the kinetic one.  相似文献   

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