Fuel Exit Velocity 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.

Experimental Study on the Evolution of Flame Trajectory Line Length for Two Buoyancy-controlled Linear Jet Flames at Different Inclined Angles

This paper presents an experimental study of the flame trajectory line length for a buoyancy-controlled dual linear jet flame at different inclined angles (0° ~ 90°). Two nozzles of the same size (80mm*1mm) were used for the experiments, propane was used as the fuel, the range of fuel exit velocity was 0.63 ~ 4.17m/s, the range of burner spacing was 0-0.6m, and a digital camera was used to record the flame morphology. In this paper, a critical spacing Dc is introduced to indicate the change of flame height, and a prediction model of Dc and fuel exit velocity Uf is established to divide the flame trajectory length with spacing into two stages, namely: when S<Dc, the flame trajectory length increases significantly with the decrease of nozzle spacing; when S>Dc, the flame trajectory length does not change significantly with the increase of spacing, and finally approaches to a single free flame trajectory length. When S>Dc, the flame trajectory length did not change significantly with the increase of burner spacing, and finally approached to a single free flame trajectory length, so the flame trajectory length showed a complex non-monotonic evolution. Based on the analysis of the air entrainment, two global models are developed to predict the trajectory lengths of the buoyancy-controlled dual-jet flame system in two states with different initial inclined angles. the model was applied to correlate all the data in this study as well as the data in previous studies.

Evaluation of flame geometry of horizontal turbulent jet fires in reduced pressures: A numerical approach

Accidental releases of hydrocarbons can result in jet fires, which have the potential to escalate accidental events to catastrophes. The geometry and size of a jet flame are vital parameters deciding the potential of the flames to trigger such a series of accidental events called domino effects. In this study, Computational Fluid Dynamics (CFD) simulations on horizontal turbulent jet fires are carried out using Fire Dynamics Simulator (FDS®). The results are validated with previous experimental results. Eight different fuel exit velocities ranging between 27.5 and 125 m/s, and 5 different ambient pressures: 0.6, 0.7, 0.8, 0.9 and 1 atm, are used for studying two main flame geometrical parameters: jet flame length and flame area. The simulation results obtained for the flame lengths are found to be higher in reduced pressure cases. In the present study, the classic correlation proposed by Quintiere and Grove is extended to reduced pressure conditions by refining the ‘air entrainment strength (C)’ values. These air entrainment strength constants were found to have a linear relationship with the ambient pressures. In addition, it is revealed that low ambient pressure conditions can also result in higher jet flame areas. The previously established relationship between the flame area and the non-dimensional heat release rate (i.e.AF∼Q*4/5) is also verified with the present obtained results, with an introduced proportionality constant (k) showing linear variations with the ambient pressure.

Effect of sub‐atmospheric pressures on buoyancy‐dominated hydrocarbon fuel fires: An experimental analysis on thermal flickering frequency

SummaryThermal flickering of a fire is caused by the buoyancy‐induced Kelvin‐Helmholtz‐type instability in the buoyant shear layer between burned gases and ambient air. The study of flickering frequency and inherent buoyancy‐induced instability are of great importance for fire essential parameters and their applications like fire detection, burner, and furnace designs. The flickering frequency of hydrocarbon fuel fires involved methane, ethylene, and propane was studied at low pressures ranging from 0.03 to 0.1 MPa. Three different fuel flow rates were configured for each fuel type of diffusion flame. Generally, the flickering behaviors were observed falling into three regimes: tip flickering, intermittent flickering, and continuous flickering. The continuous flickering appeared when the fuel flow rate or pressure increased above a particular level. The observed thermal flickering frequency varied from about 8 Hz at 0.03 MPa to 12 Hz at 0.1 MPa and was found insensitive to fuel type and flow rate. Thermal flickering frequency increased with the increasing of pressure with a power of 0.27. Buoyancy force dominates the flame flicking behavior under a wide range of sub‐atmospheric pressures and fuel exit velocities. In addition, multiple frequencies phenomenon was observed in the tests.

A global model for flame pulsation frequency of buoyancy-controlled rectangular gas fuel fire with different boundaries

Pulsation frequency is an important characteristic parameter for buoyancy-controlled fuel diffusion flames. Fire experiments of a rectangular source with different aspect ratios were conducted in an open space and against sidewalls made from a calcium silicate board. Due to the blocking effect to restrict air entrainment to fire plumes, sidewall significantly reduced the flame pulsation frequency. Furthermore, the effect of the fuel exit velocity on the pulsation frequency became intense as the aspect ratio of the rectangle was increased to 7.45. Based on the modified hydraulic diameter for a rectangular fire source with a sidewall and corner, a global model was developed for predicting the flame pulsation frequency of the rectangular fire source with free, sidewall, and corner boundaries. The coefficient of determination of this improved model is 0.9991, and the local errors of this model are less than 15% considering all of the experimental data in the present work and available in the literature. This work provides a method for predicting flame pulsation frequency, accounting for sidewall effect and aspect ratio.

Computational investigation of flame extension height for horizontally oriented rectangular source jet fires impinging on a vertical plate

SummaryThis paper investigates the flame extension height induced by a horizontally oriented rectangular source fire impinging on a vertical plate, which has rarely been reported in literature. Focusing on the scenarios with nonimpinging flame lengths that are comparable with or larger than the plate‐source distance, a series of numerical experiments were conducted, in which the exit velocities varied from 27.5 to 125 m/s and the orifice aspect ratios ranged from 1 to 4. Results show that the flame extension height increases with increasing exit velocity and becomes larger with increasing orifice aspect ratio at a given fuel exit velocity. New correlations are proposed to predict the flame extension height based on the unburnt fuel after impinging, assuming a kite flame shape of circular and elliptical cross section. Moreover, the new proposed correlations provide a general and practical basis for estimating the flame extension height of horizontally oriented rectangular source impinging jet fires.

Size, effective density, morphology, and nano-structure of soot particles generated from buoyant turbulent diffusion flames

With a global gas flaring volume of ∼140 billion cubic meters, flares are an important source of particulate emissions; however, very little is known about the physical and morphological properties of these particle emissions. To study these properties, a laboratory pipe flare producing a buoyant turbulent diffusion flame was used which allowed controlled experiments on flames up to ∼3 m tall. Three flare diameters (38.1, 50.8, and 76.2 mm) were used in this study with fuel exit velocities of 0.5, 0.9, and 1.5 m/s. ‘Light’, ‘medium’, and ‘heavy’ fuel compositions (consisting of C1 to C4 alkanes, carbon dioxide, and nitrogen in concentration representative of flares in the Alberta, Canada upstream oil and gas sector) were used, where heavier compositions refer to a greater concentration of higher order alkanes. Size distributions of soot particles were measured using a scanning mobility particle sizer. Mass-mobility relationship and effective density of particles were determined using a tandem arrangement of a differential mobility analyzer, a centrifugal particle mass analyzer and a condensation particle counter. Morphology and nano-structure of the particles were studied using transmission electron microscopy and Raman spectroscopy, respectively. Results showed that the particle median diameter and concentration increased as the fuel composition was changed from light to medium to heavy. On the other hand, particle morphology, measured by the relationships between particle mass vs. mobility (or effective density) and primary particle size vs. particle aggregate size, was independent of fuel composition, flow rate, or flare size and was in good agreement with previously reported values for that of soot particles from different internal combustion engines. Previously developed relations between effective density and primary particle size work well for the soot particles of this study. Raman spectroscopy indicated slightly lower D1/G ratios (more graphitic content) for the heavier fuels.

The effect of exit Reynolds number on soot volume fraction in turbulent non-premixed jet flames

Soot volume fraction (SVF) was measured in five attached turbulent non-premixed jet flames of a C2H4H2N2 fuel mixture using the Laser-Induced Incandescence (LII) technique. The five flames comprise two sets with exit strain rates of 4100 and 7500 s−1, respectively. Within each set, the exit Reynolds number was changed both by varying the jet diameter and the fuel exit velocity of the flames. Measurements of the mean, instantaneous and integrated SVF reveal a weak inverse dependence on the exit Reynolds number. A minor dependence of the axial and radial profiles of soot intermittency on the exit Reynolds number is also observed. The total soot yield is found to scale linearly with both the jet exit diameter and the fuel flow rate for the two flame sets of different exit strain. The total soot yield is also found to be a strong function of both the exit strain and the flame volume, but to be almost independent of the exit Reynolds number. A non-negligible effect of buoyancy on SVF is also deduced from the global correlations.

Experimental study and analysis on the interaction between two slot-burner buoyant turbulent diffusion flames at various burner pitches

This paper investigated the interaction of two slot-burner buoyant turbulent diffusion flames at various burner pitches. Experiments were conducted by employing two identical slot burners (length: 142.5 mm; width: 2 mm) using propane as fuel at various fuel exit velocities. Results showed that with an increase in burner pitch, the interaction of the two flames made a transition from merging to non-merging, resulting in a complex non-monotonic evolution of flame height. An analytical model was developed to characterize the critical burner pitch for flame merging, which was found to be proportional to the free flame height, or have a 2/3 power law dependence on the fuel exit velocity. As a result of their interaction, the flame merging point height increased till the two flames separated with the increase in burner pitch. Meanwhile, the flame height was shown to first decrease, then increase and finally approach the free flame height with the increase in burner pitch. A scaling non-dimensional formula was finally proposed, based on the analysis of the change in air entrainment into the flame from the space between the two burners with burner pitch variation. This proposed formula was shown to well correlate the above transitions based on a newly defined non-dimensional heat release rate using an “effective” entrainment perimeter as a characteristic length, which includes the burner width and length as well as the additional flame base “extension” due to the change in burner pitch.

Flame heights of line-source buoyant turbulent non-premixed jets with air entrainment constraint by two parallel side walls

This paper investigates the flame height of line-source buoyant turbulent non-premixed jets with air entrainment constraint by two parallel side walls at various separation distances. Laboratory-scale experiments are carried out by employing two line-source nozzles (2mm×142.5mm and 2mm×217mm) using propane as fuel. The nozzles are arranged with two kinds of orientations relative to the side walls, i.e. with its longer or shorter side perpendicular to the side wall. The flame heights are measured for various fuel supply exit velocities and side walls separation distances. Results show that the flame height changes little with side walls separation distance when the longer side of the nozzle is perpendicular to the side walls; meanwhile decreases with increase in side walls separation distance and finally approaches the value of a free jet after the separation distance beyond a critical value when the shorter side of the nozzle is perpendicular to the side walls. This confirms the physics that the air entrainment into the buoyant turbulent non-premixed jets for such line-source nozzle is mostly from the longer side; meanwhile rarely from the shorter side. The air entrainment per height is reduced hence the flame height increases with decrease in side walls separation distance when the shorter side of the nozzle is perpendicular to the side walls. The critical side walls separation distance normalized by the nozzle width is found to be linear function of the non-dimensional fuel exit velocity normalized by the buoyancy induced air entrainment velocity using the flame height of the free jet as characteristic length. Finally, a non-dimensional correlation is proposed to characterize the flame heights for various side walls separation distances when the shorter side of the line-source nozzle is perpendicular to the side walls.

The effect of exit strain rate on soot volume fraction in turbulent non-premixed jet flames

Laser-Induced Incandescence (LII) measurements from soot were used to determine the soot volume fraction (SVF) in three attached turbulent non-premixed jet flames, of a similar C2H4H2N2 fuel mixture, with different exit strain rate. For all three cases, the jet exit Reynolds number was maintained at 15,000 while varying both the jet diameter and the fuel exit velocity. Measurements are reported of the mean, instantaneous, and radially integrated SVF, together with soot intermittency, for all three flames. They reveal an inverse relationship between the exit strain rate and all of the aforementioned quantities. This observation is consistent with previous trends reported both in laminar flames and turbulent flames, although this is the first systematic assessment for flames that are also well-suited to model validation. The probability density functions of maximum instantaneous SVF exhibit similar trends with regards to exit strain rate, although the most probable SVF increases with decreasing exit strain. The volume integrated SVF reveals an inverse relationship with the exit strain rate, which can be characterized by an exponential fit. A linear correlation was also found between the exit strain rate and the maximum instantaneous SVF, the maximum mean SVF, and the total soot volume per flow rate of fuel.

Magnetically induced flame flickering

This paper addresses a still unknown effect of a high downward gradient of the square of the magnetic flux density on a laminar methane diffusion flame established over an axisymmetric co-flow burner. Because the magnetic force is expected to act mainly on paramagnetic oxygen, a parametric study varying the co-flowing stream oxygen content, the oxidizer velocity and the magnitude of the gradient of the square of the magnetic flux density was performed. CH∗ radicals spontaneous chemiluminescence was captured to track the flame sheets. Despite a low Reynolds number at the fuel exit and oxidizer flow velocities slightly larger than the fuel exit velocity, the flame flickering was triggered by downward gradients of the square of the magnetic flux density of magnitudes from 0.4 and 1.4T2/m for O2 contents of 0.75 and 0.55, respectively. This flickering is shown to result from an axisymmetric mode of a convective instability. Qualitative speculations based on previous works suggest that the magnetic body force may alternately strengthen buoyancy and shear stress, therefore hastening the instability onset. Indeed, at relatively low Grashof numbers, a region of flickering conditions shows up and grows with a magnetic Reynolds number. The sensitivities of the flickering frequency and amplitude are finally addressed, namely showing that over the investigated range of parameters the frequency spreads from 10.5 to 14Hz.

Numerical Study of Emission Characteristics of a Jet Flame in Cross-Flow

The authors numerically investigated the effect of changes in the fuel exit velocity and the cross-flow on the length, radiant fraction, and emission indices of pollutant species (NOx and CO), as well as the ratio NO2/NOx, of a high momentum jet flame in cross-flow. This flow configuration is of generic interest, and also practically relevant to flaring operations. The flow field is computed based on the Reynolds-Averaged Navier–Stokes equations incorporating the realizable k−ϵ turbulence closure. The combustion process is modeled using the unsteady Eulerian particle flamelet model based on the mixture fraction approach, and the heat loss by radiation is accounted for using the discrete ordinates method. Comparison of the predicted flame length and the trend of the emission indices with experimental measurements reveal good agreement for the range of jet-to-cross-flow momentum flux ratios investigated, namely 100–800.

Experimental studies of bluff-body stabilized LPG diffusion flames

Bluff-body stabilized turbulent jet diffusion flame has received renewed attention in recent years due to its practical applications. An experimental study is carried out to investigate the effect of coaxial air velocity, U a, and lip-thickness, δ of the bluff-body on the flame stability limits and emission levels. The stability limits of a typical diffusion flame can be characterized in terms of two parameters namely flame lift-off height and blow-off velocity. It is experimentally observed that lift-off height is not linearly dependent on the fuel exit velocity, U f, as compared to the simple jet. The flame stability is found to be improved for larger lip-thickness bluff-body because of the presence of lower pressure in the wake region behind the bluff-body. Flame length is observed to be dominated by buoyancy and momentum regimes. The transition from buoyancy to momentum regime is found to be extended with increase in lip-thickness. It is also observed that the blow-off limit is also extended further by 10% as compared to simple jet diffusion flames under similar conditions. The emissions data are reported in terms of mass based emission index, EINO x (g [NO x ]/kg [fuel]) for a wide range of flow conditions. It is concluded that the addition of coaxial air in the larger lip-thickness bluff-body flames causes a marginal reduction in emission levels relative to smaller lip-thickness bluff-body.

Effect of Dilution, Pressure, and Velocity on Smoke Point in Laminar Jet Flames

Smoke point measurements of diluted methane and ethylene flames were made in a co-flowing laminar jet diffusion flame at pressures up to 8 atm. The smoke point corresponds to the fuel flow rate where the soot production is exactly offset by the soot oxidation, and as such is sensitive to changes in rates of production or oxidation. Flame height in these flames was measured as a function of pressure, diluent, and dilution level as well as both fuel exit velocity profile (i.e., plug or parabolic) and fuel/air velocity ratio. As pressure increases, the smoke point became less sensitive to diluent or dilution level. In addition to heat capacity and thermal diffusivity differences between CO2 and He for example, the large differences in kinematic viscosity was shown to play an important role in the diluent's ability to suppress the fuel's propensity to form soot.

Experimental studies of flame stability and emission characteristics of simple LPG jet diffusion flame

In the present study, experiments were carried out to measure the lift-off height, HL; flame length, Lf and blow-off velocity for a simple LPG (liquefied petroleum gas) jet diffusion flames. It is observed that lift-off height is proportional to the fuel exit velocity, Uf. A semi-empirical correlation between lift-off height and global strain rate, Uf/Df is proposed. Two regimes identified either as buoyancy or momentum dominated were characterized by Froude number, Fr. For momentum dominated jet diffusion flames, Lf/Df remains almost constant and therefore is independent of the Froude number. The NOx emissions, expressed in terms of emission index, EINOx is found to decrease with Uf. This decreasing trend is consistent with the concept that increasing jet velocity reduces the residence time as reported in the literature. The present data is also compared with the available data of propane gas and found to be in good agreement well particularly in trend wise. Besides these data, EINOx scaling law is also reported in the present study.

Combined experimental and computational study of laminar, axisymmetric hydrogen–air diffusion flames

We investigate the structure of two-dimensional, axisymmetric, laminar hydrogen–air flames in which a cylindrical fuel stream is surrounded by coflowing air, using laser-diagnostic and computational methods. Spontaneous Raman scattering and coherent anti-Stokes Raman scattering (CARS) are used to measure the distributions of major species and temperature. Computationally, we solve the governing conservation equations for mass, momentum, energy, and species, using detailed chemistry and transport. The fuel is diluted with nitrogen (1:1) to reduce heat transfer to the burner, to match the zero temperature gradient at the fuel exit. Three average fuel exit velocities are studied: 18, 27, and 50 cm/s. Comparisons of the measured and computed results are performed for radial profiles at a number of axial positions, and along the axial centerline. Peak major species mole fractions and temperatures are quantitatively predicted by the computations, and the axial species profiles are predicted to within the experimental uncertainty. In the radial profiles studied, base-case computations excluding thermal diffusion of light species were in excellent agreement with the measurements. While the addition of thermal diffusion led to some discrepancy with the measured results, the magnitude of the differences was no more than 25%. The computations predicted the axial centerline profiles from the burner exit to the maximum temperature well, though the experimental temperatures in the downstream mixing region decreased somewhat faster than the computed profiles. Radiative losses are seen to be negligible in these flames, and changes in transport properties and variations in initial flow velocities generally led to only modest changes in the axial profiles. The results also show that the detailed axial profiles of major species and temperature at different fuel jet velocities scale quantitatively with the jet velocity.

Numerical Prediction of Diluted Hydrogen Turbulent Jet Diffusion Flame Using a Scalar Probability Density Function Method Considering Detailed Chemical Kinetics

A scalar probability density function (PDF) approach considering a detailed chemical kinetics is applied to a diluted hydrogen jet diffusion flame to evaluate the performance. The flame is formed on the nozzle of an inner diameter of 6 mm, with a fuel exit velocity of 30 m/s, surrounded by two annular pipes issuing airs of higher and lower velocities of 30 m/s and 3 m/s, respectively. The flow field has been solved on the basis of the k-ε two equation model. The modeled PDF transport equation has been solved by an Eulerian Monte-Carlo method. The results are compared with those of two flamelet calculations, which are based on the conventional laminar flamelet model method and a scalar PDF method based on the conserved scalar approach. The present PDF method predicts the maximum temperature on the center axis close to the experimental one than the other methods. The temperature downstream of the maximum temperature position has however been overestimated due to the underestimate of the mixing. The present PDF method has reproduced the extinction and reignition phenomena, which cannot be easily predicted by the conventional flamelet calculation. The variation in the diffusion flame structure due to the flame stretch, which was experimentally confirmed by Barlow et al., has been also reproduced. Moreover, the PDF calculation has revealed that intermediate radical species are created largely under the flame stretch effect.

산소부화와 희석제에 따른 비예혼합 화염의 안정성

CO₂ is well known greenhouse gas which is the major source of global wanning. Reducing CO₂ emission in combustion process can be achieved by increasing combustion efficiency, oxygen enriched combustion and recirculation of the emitted CO₂ gas. Stability of non-premixed flame in oxygen enriched environment will be affected by the amount of oxygen, kind of diluents and fuel exit velocity. The effects of these parameters on flame liftoff and blowout are studied experimentally in oxidizer co flowing burner. Experiments were divided into three cases according as where CO₂ gas was supplied. - 1) to coflowing air, 2) to fuel with O₂-N₂ co flow, 3) to co flowing oxygen. Flame in air coflowing case was lifted in turbulent region. Flame lift and blowout in laminar region with the increase in CO₂ volume fraction in CO₂-Air mixture makes flame lift and blowout in laminar region. Increase in oxygen volume fraction makes flame stable-i.e. flame liftoff and blowout occur at higher fuel flowrates. Liftoff height was non-linear function of nozzle exit velocity and affected by the O₂ volume fraction. It was found that the flame in O₂-N₂ coflow case was more stable than O₂-CO₂ case. Liftoff heights vs (nozzle exit velocity/laminar burning velocity)^3.8 has a good correlation in O₂-CO₂ oxidizer case.br/

Strain rates measured along the wrinkled flame contour within turbulent non-premixed jet flames

The thin, wrinkled CH reaction layers within moderate- (Re = 9,100) and high- (18,600) Reynolds-number turbulent non-premixed jet flames were identified by using planar laser-induced fluorescence, and the in-plane strain rates on these reaction layers were measured using simultaneous Particle Imaging Velocimetry (PIV). The PIV diagnostics resolved the Taylor scale; the strain-limited diffusion length scale was fully resolved for half the cases studied and nearly resolved for the others. In the high-Reynolds-number jet, instantaneous strain rates on the flame surface are highly intermittent, with peak values exceeding 10,000 s −1. Mean strain rates, conditioned on the CH-peak contour, are relatively constant (150 s −1) in the Re = 9100 flame and increase (650–1700 s −1) with axial location in the Re = 18,600 flame, resulting from the flame wrinkling process. The CH-layer thickness does not appear to respond in amplitude or in phase with the strain field, indicating that quasi-steady conditions do not occur. The strain field apparently oscillates at frequencies as high as 5–10 kHz—which is the inverse of the crossing time of integral-scale eddies—perhaps because only the low-frequency component of strain effectively acts on the flame. Mean axial velocities, conditioned on the CH-peak contour, were found to remain constant from the flame base to tip and to approximately equal the product of the stoichiometric mixture fraction and the fuel-exit velocity, in agreement with prediction.