Visible Flame Height Research Articles

Soot formation characteristics of diffusion flames of methane doped with toluene and n-heptane at elevated pressures

Laminar co-flow diffusion flames of methane doped with n-heptane and toluene were studied experimentally to assess the sooting characteristics of two liquid fuels with increasing pressure. Experiments were conducted in the high-pressure combustion chamber that had been used previously for high-pressure soot formation studies in laminar diffusion flames. Either toluene or n-heptane was added to the methane such that 7.5% of the total carbon would be from the liquid fuel so that the results could be used to infer the pressure dependence of sooting propensities of the two liquid fuels. Pressure range was from atmospheric to 8 atm for methane and methane+n-heptane flames, whereas for methane+toluene mixture it was from atmospheric to 6 atm. A constant carbon mass flow rate of 0.41 mg/s for the three fuels was maintained at all pressures to have tractable measurements. Visible flame heights, as marked by the luminous soot radiation, were constant at all pressures except for methane at 1 atm. Variation of the maximum soot volume fractions, maximum soot yields, and the line-of-sight averaged soot temperatures of the three flames, pure methane, toluene-doped methane, and n-heptane-doped methane, with pressure were evaluated from soot spectral emission measurements which were collected line-of-sight but converted to radially-resolved values by using an Abel type inversion algorithm assuming axisymmetry of the laminar diffusion flames. Maximum soot volume fractions and maximum soot yields in n-heptane- and toluene-doped flames showed the higher sooting propensity of toluene in comparison to n-heptane at elevated pressures. Sooting propensity, in terms of both maximum soot yield and maximum soot volume fraction, of the methane+toulene flame displayed a relatively weaker dependence on pressure as compared to those of methane and methane+n-heptane mixture.

A new fire performance test for cavity wall insulation

Cavity walls containing combustible insulation present an increased risk for fire propagation in a confined, concealed space. Damage to the building resulting from ignition of combustible insulation can be extensive; especially so, in the absence of horizontal and vertical fire-stops. Current codes and standards assess fire performance of exterior wall assemblies subjected to external ignition sources. However, test methods do not address the potential fire hazard resulting from ignition of combustible insulation within the wall cavity. A fire performance test has been developed that evaluates the fire propagation behavior of combustible insulation in a configuration that is representative of the actual installation. The test utilizes a full scale cavity wall assembly and offers fire performance evaluation of insulation of any thickness for either a 51 mm or 102 mm wide air space. A propane sand burner was selected as the ignition source; in addition to being reliable and repeatable, its heat output was carefully calibrated to be representative of potential ignition scenarios that may occur within a cavity wall. During the 15 minute fire performance test, the sample is continuously subjected to the propane sand burner exposure fire. An acceptable sample will produce a peak chemical heat release rate less than 100 kW and a maximum visible flame height less than 1.8 m. This fire performance test method is being incorporated into FM Approvals Standard for Cavity Walls and Rainscreens, Class Number 4411 [1] and is suitable for incorporation into other codes and standards. © Owned by the authors, published by EDP Sciences, 2016 Language: en

Experimental Characterization of Gelled Jet A1 Spray Flames

Gelled propellants provide energetic performance similar to conventional liquid propellants and safety during storage and handling like a solid propellant. Experiments on unconfined gelled Jet A1 spray flames and the comparison with ungelled spray flames are reported for the first time in this paper in terms of the global features, burning regimes, stability limits, visible flame height, emission spectra, natural luminosity, and CH ∗ chemiluminescence. Propellants were atomized by an internally impinging two-fluid atomizer, developed specifically for efficient atomization of non-Newtonian gels. Swirling and non-swirling spray flames were successfully stabilized on a burner incorporating bluff body and annular jet of combustion air over a wide range of operating parameters. Structural features of the atomizer impart high momentum to the (central) spray jet, such that the recirculation zone could be penetrated under all conditions. Long-exposure smoke and high-speed visualizations were employed to study cold flow structures and droplet-vortex interactions. Short-exposure direct and backlit imaging were used to observe global features of spray flames. Stability limits and visible flame heights were mapped for different thermal inputs, swirl numbers, and flow rates of atomizing and combustion air jets. Non-swirling stable anchored, partially blown off, and neck-blown off flames were observed. Lifted, and a transition regime, in which the flame could burn in stable and lifted mode repetitively, were observed for the swirling flames. Interactions between central and annular jets are important in these regimes, determining flame shape, symmetry, and flame height. Jet-like propagation zone determines the flame height through its dependence on momentum of spray jet. The length of this zone is affected by variations in thermal input, gas-liquid ratio, and air-fuel ratio. The gelled Jet A1 flames are remarkably shorter despite having a larger average droplet size than ungelled Jet A1. This experimental observation directly supports theoretical predictions reported in literature. These flames are more luminous than ungelled Jet A1, especially at the base and the neck regions. While, majority of the heat is released in the jet-like propagation zone for both the flames, significant heat is released in the neck zone of ungelled Jet A1 spray flame in comparison to ungelled Jet A1 spray flame due to intense turbulence and smaller droplet size.

Characterization of swirling CNG inverse jet flame in recessed coaxial burner

This paper is concerned with the experimental investigation of swirling inverse jet flame (IJF) characteristics such as visible flame appearance, visible flameheight, temperature distribution, post flame emissions and flame noise in a recessed coaxial burner. Furthermore, variation in lean blowout limits of swirling IJF with aerodynamic swirl number is also reported. Based on the observation of visible appearance of swirling IJF with aerodynamic swirl number, two flame regimes are identified. Besides this, semi-empirical correlations for flame height and lean blowout limits are attempted in this work which can be used for the design and development of future burner systems employing swirling inverse jet flame. Apart from this, an attempt has been made to understand the flow features and flame structure of swirling IJF through numerical simulations. The swirling inverse jet flame configuration studied in this work is capable of establishing compact blue flame with better thermal and emission characteristics and hence can be employed in thermal applications.

Dependence of sooting characteristics and temperature field of co-flow laminar pure and nitrogen-diluted ethylene–air diffusion flames on pressure

Pressure dependence of sooting characteristics and the flame temperature field of pure ethylene and ethylene diluted with nitrogen in co-flow laminar diffusion flames was investigated experimentally. The pressure range for ethylene was from atmospheric to 7atm and to 20atm for nitrogen-diluted ethylene flames. Spectrally-resolved line-of-sight soot radiation emission measurements were used to obtain radially resolved temperatures and soot volume fractions by using an Abel type inversion algorithm. A constant mass flow rate of ethylene was maintained at 0.48mg/s at all pressures to match the carbon flow rates of gaseous alkane fuels experiments reported previously. Visible flame heights, as marked by the luminous soot radiation, initially increased with pressure, but changed little above 5atm. Maximum local soot volume fraction of ethylene flames seems to scale with pressure raised to the third power (about 2.8). This is argued to be a relatively stronger pressure dependence of maximum soot volume fraction as compared to other gaseous fuels. A similarly higher pressure dependence was observed when the maximum soot yields of ethylene and other gaseous fuels were compared. It was shown that the soot yield dependence of ethylene flames does not conform to the unified dependence on pressure which was demonstrated for gaseous alkane fuels recently. The sooting propensity of nitrogen-diluted ethylene flames was shown to be less than that of n-heptane flames diluted with similar amount of nitrogen. Flame temperature profiles and averaged temperatures of ethylene flames showed similar characteristics as the other gaseous fuels, however radial temperature gradients in ethylene flames were much higher than those in gaseous alkane fuel flames.

The influence of gravity levels on soot formation for the combustion of ethylene-air mixture

The reduced mechanism coupled with 2D flame code using CHEMKIN II to investigate the effect of gravity on flame structure and soot formation in diffusion flames. The results show that the gravity has a rather significant effect on flame structure and soot formation. The visible flame height and peak soot volume fraction in general increases with the gravity from 1g decreased to 0g. The peak flame temperature decreases with decreasing gravity level. Comparing the calculated results from 1g to 0g, the flame shape becomes wider, the high temperature zone becomes shorter, the mixture velocity has a sharp decrease, the soot volume fraction has a sharp increase and CO and unprovided species distribution becomes wider along radial direction. At normal and half gravity, the flame is buoyancy controlled and the axial velocity is largely independent of the coflow air velocity. At microgravity (0g), the flame is momentum controlled.

Effects of recessed air jet on turbulent compressed natural gas inverse diffusion flame shape and luminosity

Effects of the recession of the central air jet on the visible flame height, necking zone, and luminosity of a turbulent compressed natural gas-air inverse diffusion flame in a coaxial burner are investigated in this experimental study. The inner circular tube of the coaxial burner is recessed by 0.25da, 0.5da, and 1.0da, where da is the central tube inner diameter. From the visual observation, the flame height and the necking zone height are observed to decrease exponentially with the air jet Reynolds number with no recession of the central air jet. However, only a marginal reduction in the visible flame height is observed with an increase in the recession height of the air jet as compared to the necking zone height. Interestingly, the necking zone at the flame base disappears beyond the critical recession height of the central jet. Moreover, the recession is found to be effective in eradicating the fuel rich zone and soot ring at the flame base of turbulent compressed natural gas inverse diffusion flame at lower air jet Reynolds numbers.

Formation of Liquid Methane–Water Mixture during Combustion of a Laminar Methane Jet at Supercritical Pressures

During the study of soot formation in laminar jet flames of methane at elevated pressures in a high-pressure combustion chamber, we have observed an anomalous occurrence at the chamber pressures of 6 MPa and higher. At pressures between 6 and 9 MPa, after the laminar methane jet flame had been stabilized on a co-flow circular nozzle-type burner, one of the following occurrences was observed: (a) the jet flame is completely extinguished without any external interference, and immediately after, a liquid material started to flow out of the fuel nozzle, and (b) the jet flame started flickering or changing its shape, and at the same time, a liquid stream started trickling out of the nozzle. This outflowing liquid influenced the jet flame by widening the flame near its base and decreasing the visible flame height. In consideration of the temperatures and the pressures involved, the liquid is believed to be a mixture of methane and water formed by the water vapor in the combustion products of the flame mixing with supercritical methane inside the nozzle.

Vortex dynamics and structures of methane/air jet diffusion flames with air coflow

In this paper, high speed direct/schlieren imaging system together with Particle Image Velocimetry (PIV) system is applied to investigate the vortex dynamics and structures of methane–air coflow diffusion flames. The schlieren and PIV images show that the dynamics of the vortices outside the visible flame are strongly affected by the coflow air velocity. The coflow air is observed to push the initiation point of toroidal vortices from the nozzle exit to downstream. When the vortices are pushed beyond the visible flame height, the typical flame flickering behavior disappears and the flame becomes stable. The critical air flow rate at which the flame stops oscillating is observed to increase with the fuel flow rate. The visible flame shows no obvious change when the coflow flow rate exceeds a critical value. However, the flow pattern and vortices outside the visible flame keep changing at different air flow rates. The velocity vectors and vorticity contours at different air flow rates are presented and analyzed. The shedding frequency of the toroidal vortex is in good agreement with the flame flickering frequency obtained from a photomultiplier.

On the behavior of flame expansion in pool fire extinguishment with steam jet

Theoretical analysis and quantitative methodology are proposed to predict the behavior of flame expansion in pool fire extinguishment with an impinging steam jet. The theoretical analysis contains the following three standpoints: (1) a fuel vapor-rich core exists in the base of the pool fire while oxygen is lacking. The existence of a fuel vapor-rich core is the prerequisite for flame expansion phenomenon. (2) The root cause responsible for the flame expansion is the fuel vapor convection by the impinging steam jet. An expanded fire ball is formed due to the burning of the convected fuel vapor, resulting in flame expansion phenomenon. (3) The flame expansion phenomenon could be divided to three typical stages: the flame height-decreasing stage, the fire-expanding stage, and the burn-out stage. The flame height-decreasing stage is the stage in which the steam jet impacts the flame and reaches the fuel vapor-rich core. The fire-expanding stage is the stage in which the fuel vapor is convected and burnt as a fire ball after the steam impacts the fuel vapor-rich core. The fire is then suppressed in the burn-out stage. Three quantitative features of the flame expansion phenomenon, the expansion ratio of the flame width ϕl, the expansion ratio of the flame volume ϕv, and the expansion ratio of the flame radiation ϕr, are defined to characterize the level of the expansion. A series of extinguishment tests using medium-size pool fires with an impinging steam jet were conducted to validate the theoretical analysis. The experimental results reveal that the theoretical analysis describes the flame expansion phenomenon quite well. Based on the experimental data, the height of the fuel vapor-rich core was calculated for each test case, and it was found that the height of the fuel vapor-rich core was about 0.2 times the visible flame height. The durations of the three stages and the flame shapes calculated using the proposed theoretical analysis compare favorably with the experimental measurements. The calculation shows that the expansion ratio of the flame width is about 2.0 in each test case of the medium size pool fire; the expansion ratio of the flame volume approximately matches the calculated value, and the calculated values of the expansion ratio of the flame radiation are close to the experimental values.

Soot formation in laminar ethane diffusion flames at pressures from 0.2 to 3.3 MPa

The effects of pressure on soot formation and the structure of the temperature field were studied in co-flow ethane-air laminar diffusion flames over the pressure range of 0.1–3.34MPa in a high pressure combustion chamber. The selected fuel mass flow rate provided diffusion flames in which the soot was completely oxidized within the visible flame envelope and the flame was stable at all pressures considered. The spatially resolved soot volume fraction and soot temperature were measured by spectral soot emission as a function of pressure. The visible (luminous) flame height remained almost unchanged from 1.52 to 3.34MPa, whereas it increased considerably from atmospheric to 1.52MPa. Flame cross-sectional area, measured at the flame height of 5mm either bounded by maximum flame temperature or maximum soot volume fraction contours, showed an inverse dependence on pressure. Peak carbon conversion to soot, defined as the percentage of fuel’s carbon content converted to soot, showed a strong dependence on pressure at lower pressures; but this dependence grew weaker as the pressure was increased. This dependence can be expressed as a pressure scaling in the form of a power law. However, the exponent of pressure was not constant: it was about 2.2 for pressures between 0.2 and 0.51MPa, about 1.1 for pressures between 0.51 and 1.52MPa, and about 0.4 for pressures between 1.52 and 3.34MPa. Averaged flame temperatures decreased with increasing pressure as a result of enhanced heat loss from the flame by soot radiation. The maximum temperature gradients increased with pressure at lower flame heights; at higher locations in the flame, after an initial increase at the lower pressure range, gradients reached a plateau at about 1.5–2.0MPa.

Flame Height of a Turbulent Diffusion Jet Flame with a Comparatively High Initial Discharging Velocity

A series of experiments was carried out in order to investigate the visible flame height of a jet flame with a high initial discharging velocity where the fuel-release time and mass flow rate of fuel were varied. The values of non-dimensional heat release rate (Q*) in present experiment were in the range of 107 Q* 8. Even if the value of Q* was larger than 106, when the non-dimensional number (RM) employed by Heskestad which implies the ratio of gas release momentum to the momentum made by a purely buoyant diffusion flame was less than 0.1, the flame height with a comparatively high initial discharging velocity increased with the heat release rate. In other words, whether flame height depends on the Q*2/5 or not can be decided by the range of momentum ratio (RM) regardless of the range of non-dimensional heat release rate (Q*). This can be also decided by the fire Froude number employed by Delichatsios.

Numerical study of the effects of gravity on soot formation in laminar coflow methane/air diffusion flames under different air stream velocities

Numerical simulations of laminar coflow methane/air diffusion flames at atmospheric pressure and different gravity levels were conducted to gain a better understanding of the effects of gravity on soot formation by using relatively detailed gas-phase chemistry and complex thermal and transport properties coupled with a semi-empirical two-equation soot model. Thermal radiation was calculated using the discrete-ordinates method coupled with a non-grey model for the radiative properties of CO, CO2, H2O, and soot. Calculations were conducted for three coflow air velocities of 77.6, 30, and 5 cm/s to investigate how the coflowing air velocity affects the flame structure and soot formation at different levels of gravity. The coflow air velocity has a rather significant effect on the streamwise velocity and the fluid parcel residence time, especially at reduced gravity levels. The flame height and the visible flame height in general increase with decreasing the gravity level. The peak flame temperature decreases with decreasing either the coflow air stream velocity or the gravity level. The peak soot volume fraction of the flame at microgravity can either be greater or less than that of its normal gravity counterpart, depending on the coflow air velocity. At sufficiently high coflow air velocity, the peak soot volume fraction increases with decreasing the gravity level. When the coflow air velocity is low enough, soot formation is greatly suppressed at microgravity and extinguishment occurs in the upper portion of the flame with soot emission from the tip of the flame owing to incomplete oxidation. The numerical results provide further insights into the intimate coupling between flame size, residence time, thermal radiation, and soot formation at reduced gravity level. The importance of thermal radiation heat transfer and coflow air velocity to the flame structure and soot formation at microgravity is demonstrated for the first time.

NO x emission characteristics of partially premixed laminar flames of H 2/CO/CO 2 mixtures

NO x emission indices were experimentally measured for partially premixed laminar flames of five different H 2/CO/CO 2 fuels over a wide range of equivalence ratios. Through those fuels, the effects of H 2/CO ratio and CO 2 concentration on NO x emissions, flame appearance, visible flame height and flame temperature are presented. EINO x values increase when 1.0 ≤ Φ ≤ 1.6, then remain near the highest value, before decreasing slowly when 3.85 ≤ Φ ≤ ∞. The increase of the CO 2 concentration reduces the EINO x for the whole range of equivalence ratios, while the increase in the H 2/CO ratio reduces the EINO x when Φ ≤ 2.0 and is inconsequential for richer mixtures. The variation in flame temperatures approximates EINO x trends. The variation of flame color from blue to orange when the H 2/CO ratio is increased might be explained by higher CO levels in by-product combustion.

Conclusions Drawn from the Buncefield and Naples Incidents Regarding the Utilization of Consequence Models

AbstractIn both incidents, as a result of overfilling, a very large mass of winter gasoline (mf = 300 t in Buncefield, mf = 700 t in Naples) was released, which in both cases led to a major fire of several days duration and involved 22 of a total of 41 tanks (Buncefield) and 24 of a total of 31 tanks (Naples). Whilst the Buncefield site was unconfined or only slightly confined, the Naples site had a high degree of confinement due to walls, buildings as well as a high embankment. The most important, current consequence models as well as new consequence models for the prediction of the extraordinarily high flame heights, extraordinarily high thermal radiation as well as the extraordinarily high maximum peak overpressure are described and discussed. The analysis of the Buncefield incident so far has shown that the maximum visible relative flame height lies in the region of 2.5 < (H/d)max,Bunc < 6.5 and the predicted value lies in the region 1.8 < (H/d)max,calc < 1.9. For large, black smoky fires the estimation of the critical thermal separation distance is not dependent on the total fire, but on the height of a hot, clear burning zone. In addition, for multiple tank fires there is a considerable increase in the mass burning rate, the flame height, the surface emission power as well as the thermal separation distance. With the new consequence models maximum peak overpressures for an unconfined vapor cloud explosion (UVCE) of ≈ 1.3 bar (Buncefield) and of ≈ 1.5 bar (Naples) are predicted. For a partially confined vapor cloud explosion (VCE) a maximum peak overpressure of ≈ 3 bar for a jet ignition into a corridor is predicted.

Soot formation and temperature field structure in co-flow laminar methane–air diffusion flames at pressures from 10 to 60 atm

The effects of pressure on soot formation and the structure of the temperature field were studied in co-flow methane–air laminar diffusion flames over a wide pressure range, from 10 to 60atm in a high-pressure combustion chamber. The selected fuel mass flow rate provided diffusion flames in which the soot was completely oxidized within the visible flame envelope and the flame was stable at all pressures considered. The spatially resolved soot volume fraction and soot temperature were measured by spectral soot emission as a function of pressure. The visible (luminous) flame height remained almost unchanged from 10 to 100atm. Peak soot concentrations showed a strong dependence on pressure at relatively lower pressures; but this dependence got weaker as the pressure is increased. The maximum conversion of the fuel’s carbon to soot, 12.6%, was observed at 60atm at approximately the mid-height of the flame. Radial temperature gradients within the flame increased with pressure and decreased with flame height above the burner rim. Higher radial temperature gradients near the burner exit at higher pressures mean that the thermal diffusion from the hot regions of the flame towards the flame centerline is enhanced. This leads to higher fuel pyrolysis rates causing accelerated soot nucleation and growth as the pressure increases.

Effect of elliptic burner geometry and air equivalence ratio on the nitric oxide emissions from turbulent hydrogen flames

An experimental study on turbulent hydrogen flames from circular and elliptic burners with varying degrees of premixedness (diffusion, fuel-rich, stoichiometric, and fuel-lean) is presented. Flame stability, visible flame height, flame radiation, global nitric oxide (NO) concentration, and inflame temperature and NO concentration profiles were measured. We found that the elliptic burner flames had lower liftoff velocity, were shorter, and radiated less heat to the surrounding as compared to circular burner flames. Global NO concentration decreased with an increase in air equivalence ratio for both circular and elliptic burner flames. Peak in-flame NO concentration along the flame centerline increased with a decrease in air equivalence ratio. Elliptic burner flames produced higher peak in-flame temperatures. Overall, the elliptic burner flames produced less peak NO as compared to circular burner flames at all air equivalence ratios except zero (diffusion flames) in accordance with the global emission measurements.

Effects of partial premixing on pollutant emissions in swirling methane jet flames

Experimental measurements of visible flame heights, temperatures, and pollutant emission indices in partially premixed swirling flames with a broad range of central fuel tube equivalence ratios (Φ F ) are reported. Two cases of partially premixed swirling flames are studied; one with a constant fuel tube exit velocity and the other with an increased exit velocity. With increasing partial premixing, the visible flame height decreases and the overall flame color changes from yellow to blue. Temperature measurements indicate that the flame structures become thinner and temperatures increase continuously with increasing partial premixing. Emission index, EINO x and EICO, values decrease with increasing levels of partial premixing and reach a minimum value at Φ F ≈ 3, followed by an increase as Φ F approaches the blowoff limits. The reduction in EINO x and EICO at optimum partial premixing (Φ F ≈ 3), as compared with that for the non-premixed swirling flame is at least 23% and 77%, respectively. The emission index for NO x scales very well with the fuel mass fraction and the fuel-air momentum flux ratio. Good agreement is achieved between the predictions and measured results.

Oxygen-enhanced/natural gas flame radiation

Total radiation measurements for oxygen-enhanced/natural gas diffusion flames are reported here. The parameters that were studied included the oxidizer composition ( Ω = 0.25−1.00), the burner firing rate ( q f = 3−28 kW), the equivalence ratio ( φ = 0.55−1.45), and the axial ( L r = 0.5−6.5) and radial ( D r = 0.4−1.0) position of the radiometer. The measured radiation ranged from 10 to 65 kW m −2. The nozzle Reynolds numbers ranged from 480 to 10 300. The location of the peak flame radiation ranged from 9 to 15% of the visible flame length. The stability limits and visible flame heights are also reported for a burner with a wide operating range.

NO x emission and major species concentrations in partially premixed laminar methane/air co-flow jet flames

Measurements of visible flame heights, global radiative heat loss fractions, distributions of mole fractions of stable gas species, and pollutant emission indices in laminar partially premixed flames burning various fuel-rich mixtures of CH 4 and air in an overventilating co-flow of air are reported. Mole fractions of CO 2, CO, H 2, O 2, N 2, CH 4, C 2H 4 and C 2H 2 were measured, using sampling and gas chromatography, at several radial locations at three different heights above the fuel tube for a fixed fuel flow rate and six different fuel tube equivalence ratios. Mole fractions of H 2O were inferred from the dry based measurements. With increasing levels of partial premixing following effects are observed: (1) the visible flame height decreases and the overall flame color changes from yellow to blue; (2) the radiative heat loss fraction first decreases and then reaches a constant value; (3) the mole fractions of CO decrease and those of CO 2 and H 2O increase in the lean parts of the flame; (4) mole fractions of C 2H 2 decrease and those of C 2H 4 first increase and then decrease in the rich parts of the flame; (5) mole fractions of CO and H 2 first decrease slightly and then increase in the rich parts of the flame; and (6) the O 2 mole fractions at the point of negligible CH 4 mole fraction decrease. Measurements of emission indices for NO, NO x , CO and HC show that, for a fixed fuel flow rate and overall equivalence ratio, an optimum level of partial premixing exists.