Laminar Jet Flames Research Articles

FLOW VISUALIZATION OF EXTINGUISHING GAS RELEASED FROM BURSTING SOAP BUBBLES

In order to clarify the process of flame extinguishment by an inert gas capsule, the bursting phenomena of the soap bubble, and the blowout phenomena of a methane–air laminar jet diffusion flame by a soap bubble capsule filled with nitrogen gas have been visualized using a laser sheet method. As a result, it is found that when the soap bubble bursts, two different kinds of nitrogen gas flow are formed. The first flow is generated by the pressure difference between the inside and outside of the soap bubble. The second flow is generated by the liquid soap film dynamics in the soap bubble bursting. Moreover, it is seen that the maximum velocities of the first flow and the second flow depend linearly on the curvature of the soap bubble, and the value of the maximum velocity of the second flow is always about two and half times larger than that of the first flow. Combined, these two flows of nitrogen gas extinguish the laminar jet flame. The first flow extinguishes the flame base locally, and the second flow leads to blowout of the entire flame.

Autoignited laminar lifted flames of methane/hydrogen mixtures in heated coflow air

Autoignited lifted flame behavior in laminar jets of methane/hydrogen mixture fuels has been investigated experimentally in heated coflow air. Three regimes of autoignited lifted flames were identified depending on initial temperature and hydrogen to methane ratio. At relatively high initial temperature, addition of a small amount of hydrogen to methane improved ignition appreciably such that the liftoff height decreased significantly. In this hydrogen-assisted autoignition regime, the liftoff height increased with jet velocity, and the characteristic flow time – defined as the ratio of liftoff height to jet velocity – correlated well with the square of the adiabatic ignition delay time. At lower temperature, the autoignited lifted flame demonstrated a unique feature in that the liftoff height decreased with increasing jet velocity. Such behavior has never been observed in lifted laminar and turbulent jet flames. A transition regime existed between these two regimes at intermediate temperature.

Raman/Rayleigh scattering and CO-LIF measurements in laminar and turbulent jet flames of dimethyl ether

To reduce the impact of combustion of fossil fuels on air quality and climate change, dimethyl ether (DME) is a promising alternative diesel fuel candidate. Technical combustion processes, including formation of pollutants, are influenced by turbulence–chemistry interaction. Therefore, accurate prediction by computational combustion models of combustion systems burning DME must account for multiple scalars and scalar gradients. The testing of such models requires detailed experiments. Here a study is presented on the feasibility of simultaneous species and temperature measurements in turbulent dimethyl ether flames, using line-imaged Raman/Rayleigh scattering of the major species H2, O2, N2, CO, CO2, H2O, C2H6O and laser induced fluorescence of CO. The measurement system and data evaluation methods developed to investigate methane–air flames are extended to address dimethyl ether flames. The Raman signal intensity and spectral shape of the Raman scattering from dimethyl ether over a range of temperatures are presented, based on measurements in electrically heated flows and laminar jet flames. These data are used to develop an iterative method for data evaluation that allows determination of indispensable crosstalk correction terms for the concentration measurements of O2 and CO2. Issues of fluorescence interferences, mainly from C2 radicals on the fuel-rich side of the reaction zone, and their corrections are discussed. Laminar flame calculations are used to investigate the role of the intermediate species (CH4, CH2O, C2H4, C2H2, C2H6, CH3) in the reaction zone. In particular, their effect on the mixture fraction calculation and its relationship to the experimentally determined mixture fraction is examined. The impact of the intermediate species on deviations in concentration and temperature profiles due to species-specific Raman- and Rayleigh scattering cross-sections is demonstrated. Finally, species concentrations and temperature profiles from measurements in a turbulent piloted jet flame of dimethyl ether are shown.

Temperature measurement of a premixed radially symmetric methane flame jet using the Mach–Zehnder Interferometry

The temperature field of a premixed methane symmetric laminar flame jet is visualized by studying the interferograms of the flame, using the Mach–Zehnder Interferometry. Two kinds of oxidizers are chosen for combustion: industrially pure oxygen and oxygen-enriched air. The flame is chosen to be both lean, and rich. For the lean oxygen-enriched combustion (OEC), the equivalence ratio was held constant at 0.5, and the oxygen enrichment was adjusted to 0.5 and 0.6, and for rich OEC, equivalence ratio is chosen to be 1.2 while the oxygen enrichment was 0.7 and 0.8. For methane/oxygen combustion, the equivalence ratio varied from 0.35 to 0.55 for the lean flame, and 1.3 and 1.7 for the rich flame. Attempt was made to keep the Reynolds number unchanged at 500, for OEC, and 1000, for methane/oxygen flame. In the present study a non-contact method is successfully developed to measure the temperature field of a premixed radially symmetric laminar methane flame jet. The effect of oxygen enrichment and equivalence ratio on temperature field is also investigated and depicted.

Extinguishment of a Laminar Jet Diffusion Flame Using a Soap Bubble Filled with Nitrogen Gas

Inert gas, such as nitrogen, argon ord carbon dioxide, is able to extinguish the flame more cleanly than water and dry chemical extinguishing agent. However, to extinguish fire completely, large amount of inert gas is needed to be released at the vicinity of flame or into confined fire space. If a capsule is filled with an extinguishing inert gas, and ruptures due to contact with the flame zone, the high concentration extinguishing gas can be directly supplied to the flame. By using the capsule, it will be possible to increase the effectiveness and decrease the amount of use of the inert extinguishing gas in fire-fighting. In the present study, in order to clarify the fundamental characteristics of flame extinguishment by the inert gas capsule, extinguishment experiments of a methane-air laminar jet diffusion flame by using a soap bubble filled with nitrogen gas have been performed. Soap bubble is used as the nitrogen gas capsule. As a result, when the soap bubble bursts, the two different kinds of the filling gas flow are formed. The first flow is generated by the pressure difference between the inside and outside of the soap bubble. The second flow is generated by the soap liquid film dynamics. In the soap bubble extinguishment, these two flows of nitrogen gas extinguish the laminar jet flame. The first flow extinguishes the flame base locally, and the second flow leads to blow out the whole flame.

Extinguishment of a Laminar Jet Diffusion Flame Using a Soap Bubble Filled with Nitrogen Gas

Inert gases, such as nitrogen, argon and carbon dioxide, are able to extinguish a fire more cleanly than water and dry chemical extinguishing agents. However, to extinguish the fire completely, a large amount of inert gas is needed to be released at the vicinity of the fire or into a confined space containing the fire. If a capsule is filled with an extinguishing inert gas, and ruptures due to contact with the flame zone, the high concentration extinguishing gas can be directly supplied to the fire. By using this capsule, it may be possible to increase the effectiveness and decrease the amount of the extinguishing inert gas needed in firefighting. In the present study, in order to clarify the fundamental characteristics of flame extinguishment by an inert gas capsule, extinguishment experiments of a methane-air laminar jet diffusion flame by a soap bubble capsule filled with nitrogen gas have been performed. Visualization of flow released from the bursting bubble has also been carried out by using laser tomography and schlieren techniques. Results show that, when the soap bubble bursts, two different kinds of nitrogen gas flow are formed. The first flow is generated by the pressure difference between the inside and outside of the soap bubble. The second flow is generated by the soap liquid film dynamics. Combined, these two flows of nitrogen gas extinguish the laminar jet flame. The first flow extinguishes the flame base locally, and the second flow leads to blow out of the whole flame.

Computed Tomography of Chemiluminescence (CTC): High resolution and instantaneous 3-D measurements of a Matrix burner

Computed Tomography of Chemiluminescence (CTC) is a diagnostic technique that provides instantaneous 3-D information on flame geometry and excited species concentrations. The technique reconstructs the 3-D chemiluminescence intensity field using Computed Tomography (CT) from integral measurements of chemiluminescence in the form of camera images. The CTC sensor has been demonstrated for a methane–oxygen Matrix burner consisting of 21 laminar diffusion jet flames using affordable commodity cameras. High resolution 3-D reconstructions obtained from 48 views show good agreement with the observed flame shape and resolve wavelengths of approximately 220μm. This is sufficient to capture the multiple flame fronts, showing the suitability of CTC for wrinkled turbulent flames. Instantaneous 3-D reconstructions using 10 simultaneous camera measurements also show good agreement with the observed flame shape and are seen to be tolerant of error in the camera location. Measurements with exposure times as short as 62μs were found to achieve more than sufficient signal-to-noise ratios for successful tomographic reconstructions. Obvious applications for CTC are the measurement of (instantaneous) flame-surface density, wrinkling factor, flame normal direction, and possibly heat release, as well as the observation of transient developments.

Effects of plate temperature on heat transfer and emissions of impinging flames

Experiments were conducted to investigate the heat transfer and CO/NO X emissions of a premixed LPG/air circular flame jet impinging upwards normally to a flat rectangular plate. Temperatures of the impingement plate were controlled by cooling water at 38 °C, 58 °C and 78 °C which was circulating at its back in order to create different plate temperatures. Under each plate temperature, the effects of Reynolds number ( Re), equivalence ratio ( Ф) and nozzle-to-plate distance ( H) on the heat transfer and CO/NO X emissions were examined. The Re was selected to be 500, 1000 and 1500 to ensure laminar flame jets. The values of Ф were chosen to cover fuel-lean, stoichiometric and fuel-rich conditions. The H varied from 3 d to 7 d with an interval of 1 d. The flame-side temperature of the impingement plate is enhanced when the cooling water temperature increases, but the temperature difference across the impingement plate is reduced. Heat transfer from the flame to the plate is suppressed at higher cooling water temperature. The heat transfer rate is the highest when the cooling water temperature is at 38 °C and the lowest heat flux is obtained at 78 °C. At the highest cooling water temperature of 78 °C, the CO emission is reduced whereas the NO X emission is enhanced. However, this trend is reversed at the lowest cooling water temperature of 38 °C.

Electric fields effect on liftoff and blowoff of nonpremixed laminar jet flames in a coflow

The stabilization characteristics of liftoff and blowoff in nonpremixed laminar jet flames in a coflow have been investigated experimentally for propane fuel by applying AC and DC electric fields to the fuel nozzle with a single-electrode configuration. The liftoff and blowoff velocities have been measured by varying the applied voltage and frequency of AC and the voltage and the polarity of DC. The result showed that the AC electric fields extended the stabilization regime of nozzle-attached flame in terms of jet velocity. As the applied AC voltage increased, the nozzle-attached flame was maintained even over the blowout velocity without having electric fields. In such a case, a blowoff occurred directly without experiencing a lifted flame. While for the DC cases, the influence on liftoff was minimal. There existed three different regimes depending on the applied AC voltage. In the low voltage regime, the nozzle-detachment velocity of either liftoff or blowoff increased linearly with the applied voltage, while nonlinearly with the AC frequency. In the intermediate voltage regime, the detachment velocity decreased with the applied voltage and reasonably independent of the AC frequency. At the high voltage regime, the detachment was significantly influenced by the generation of discharges.

Fundamental Studies of NOx Emission Characteristics in Dimethyl Ether (DME)/Air Non-premixed Flames

The NOx emission characteristics of dimethyl ether (DME) in laminar co-axial jet and counterflow non-premixed flames were investigated using experimental and numerical approaches, respectively. The flame structure and NOx emissions of DME were compared to those of C2H6, which has equivalent methyl structures but lacks an oxygen atom. Experimental results showed that, in the co-axial jet case, the combustion of DME had the characteristics of a partial premixed flame. Additionally, it had a shorter flame and lower NOx emissions compared to the C2H6 flame. It is thus concluded that the major cause of low NOx emissions from DME co-axial jet flames may be the short flame length because of the lower stoichiometric air/fuel ratio. The activation of reburning NO chemistry because of the characteristics of the partially premixed flame may also play a role. The computational results of the DME counterflow non-premixed flame revealed that the EINO decreased by approximately 50% relative to that of the C2H6 flame. Although the overall NOx reaction path of the DME flame is similar to that of the C2H6 flame, it is concluded that the DME non-premixed flame has a distinct NO reduction mechanism. This is associated with reburning NO chemistry in the fuel-rich region because of fast pyrolysis and oxidation reactions in comparison to that of the C2H6 flame.

Heat Transfer and Emission Characteristics of Impinging Rich Methane and Ethylene Jet Flames

Flame impingement heat transfer has widespread industrial and domestic applications and attaining high heat flux as well as low emission of pollutants is the important prerequisite for all such applications. In this article, the heat transfer and emission characteristics of a laminar flame jet impinging on a flat target plate have been investigated experimentally. The effect of reactant jet Reynolds number, equivalence ratio and burner to plate separation distance on the average heat flux, and emissions of CO and NOx are studied using methane and ethylene fuels. Results indicate that the heat flux is maximized under certain operating conditions of jet Re, equivalence ratio, and separation distance between the burner and the target. Fuel type is found to have an effect on the heat transfer rate because of the varying luminosity of the flame with different fuels. Operating regimes that produce lower emission of pollutants are also identified. Findings of this article have direct industrial relevance to flame impingement heat transfer applications that have small target plate-to-burner port diameter ratios.

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.

Heat-transfer distribution for an impinging laminar flame jet to a flat plate

Impinging flame jets are widely used in applications where high heat-transfer rates are needed, for instance in the glass industry. During the heating process of glass products, internal thermal stresses develop in the material due to temperature gradients. In order to avoid excessive thermal gradients as well as overheating of the hot spots, it is important to know and control the temperature distribution inside a heated glass product. Therefore, it is advantageous to know the relation describing the convective heat–flux distribution at the heated side of a glass product. In a previous work, we presented a heat–flux relation applicable for the hot spot of the target [M.J. Remie, G. Särner, M.F.G. Cremers, A. Omrane, K.R.A.M. Schreel, M. Aldén, L.P.H. de Goey, Extended heat-transfer relation for an impinging laminar flame jet to a flat plate, Int. J. Heat Mass Transfer, in press]. In this paper, we present an extension of this relation, which is applicable for larger radial distances from the hot spot.

Temperature field measurement of a premixed butane/air slot laminar flame jet with Mach–Zehnder Interferometry

The temperature field of a premixed butane/air slot laminar flame jet is visualized by studying the interferogram of the flame using Mach–Zehnder Interferometry. The introduction of a slot burner with a large aspect ratio enables the three-dimensional temperature field of the flame to be observed directly from the fringe patterns of the obtained two-dimensional interferogram. Subsequently, the temperature field of an open flame jet at a specified Reynolds number or equivalence ratio is studied by varying one parameter while keeping the other unchanged. At the Reynolds number of 400, temperature fields of the flame jet at different equivalence ratios ranging from 1.0 to 2.0 are studied and compared. Similarly, at an equivalence ratio of 2.0, temperature fields of the flame jet at different Reynolds numbers ranging from 400 to 900 are investigated. In the present study, a non-contact method is successfully developed to measure the temperature field of a premixed butane/air slot laminar open flame jet.

DME/Air 비예혼합화염의 NOx 생성특성

The NOx emission characteristics of DME in laminar coaxial jet and counterflow nonpremixed flames were investigated using experimental and numerical approaches, respectively. The flame structure and NOx emission of DME were compared with those of C₂H? and C₃H?. The DME flame was calculated using the Kaiser’s mechanism, while the C₂H? and C₃H? flames were calculated using the C₃ mechanism. These mechanisms were combined with the modified Miller-Bowman mechanism for the analysis of NOx. Experimental results show in coaxial jet flame that DME flame has the characteristics of partial premixed flame and the flame length decreases up to 1/3 than that of C₃H? in the same condition of fuel mass flowrate. Then, the NOx emission of DME decreases to 40% approximately, comparing with that of C₃H?. In the calculated results of counterflow nonpremixed flame, DME flame shows the EINO decreases up to 50% approximately than those of C₂H? and C₃H? flames when the equivalent fuels are consumed per unit mass and time. Although the overall NOx reaction path of DME is similar with other hydrocarbon fuels, it can be identified that DME flame has a distinct NO reduction mechanism due to the reburning NO chemistry in fuel rich region. From these results, we can conclude that the different NOx emission characteristics of DME flame with other hydrocarbon fuels are attributed to not the temperature increase and the activation of NO reactions due to O atom in DME fuel but the rapid processes of pyrolysis/oxidation.

The response of buoyant laminar diffusion flames to low-frequency forcing

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.

Extended heat-transfer relation for an impinging laminar flame jet to a flat plate

Many industrial applications use flame impingement to obtain high heat-transfer rates. An analytical expression for the convective part of the heat transfer of a flame jet to a plate is derived. Therefore, the flame jet is approximated by a hot inert jet. In contradiction with existing convective heat-transfer relations, our analytical solution is applicable not only for large distances between the jet and the plate, but also for close spacings. Multiplying the convective heat transfer by a factor which takes chemical recombination in the cold boundary layer into account, results in an expression for the heat flux from a flame jet to the hot spot of a heated plate. Numerical and experimental validation show good agreement.

Analysis of the heat transfer of an impinging laminar flame jet

Flame jet impingement is used in many industrial processes. In this paper an analytical expression is derived for the heat flux of a laminar flame impinging on a flat plate, where the flame jet is approximated by a hot inert jet with the position of the tip of the flame taken equal to the nozzle position. The heal flux in this expression is dependent on the nozzle-to-plate spacing, in contradiction to existing (semi-analytical) relations. The geometry is divided in a region far from the plate and a region dose to the plate. For both regions the velocity profiles are calculated using only the dominant terms of the balance equations. Subsequently these profiles are linked to each other at the boundary between the two zones. Implementing the resulting velocity profile for the complete geometry in the energy equation and integrating over the whole domain results in an expression for the heat flux from the flame to the plate at the hot spot. Numerical calculations show very good agreement with the results of the analytical derivation.

Mixing characteristics in strongly forced non-premixed methane jet flames

Previous and current work show that flame length and soot luminosity of laminar and turbulent non-premixed jet flames can be significantly reduced by high-frequency, high-amplitude forcing of the fuel flow rate. The current work focuses on understanding the physical mechanisms responsible for these changes to the flow/flame structure for the case where the forcing is sufficiently strong that significant reverse flow exists at the exit of the fuel delivery tube. The high-amplitude forcing is achieved by pulsing the flow at the organ-pipe resonant frequency of the fuel delivery tube. Quantitative mixture fraction imaging in non-reacting jets indicates that the strongly pulsed jets exhibit dramatically enhanced mixing as compared to unpulsed jets. In the pulsed jets, the mean mixture fraction falls below 0.08 in as little as 3 D downstream of the nozzle exit and no pure jet fluid exists outside of the nozzle due to in-tube mixing. Simultaneous OH and acetone PLIF performed in methane jet flames shows that the OH zones are much broader and more wrinkled in the resonantly pulsed jet as compared to the unpulsed jet, and are greatly contorted by the vortical structures. A unique feature of the pulsed flames is that the reaction zones appear to close downstream of a vortical structure, just a few diameters downstream of the nozzle exit, in a region where the mixture fraction imaging (for non-reacting flows) shows reduced mixture fraction. Furthermore, the flame anchors on the outer-upstream edge of the vortical structures where the fuel mixture fraction is reduced due to enhanced entrainment. The significant in-tube premixing and the enhanced entrainment appear to be the predominant mechanisms that cause the reduction in length and luminosity of high-amplitude, high-frequency pulsed non-premixed jet flames.

Molecular species adsorbed on soot particles issued from low sooting methane and acetylene laminar flames: A laser-based experiment

Recent advances in the field of laser desorption/laser ionization mass spectrometry (LD/LI/MS) have renewed interest in these separation methods for fast analysis of chemical species adsorbed on soot particles. These techniques provide mass-separation of the desorbed phase with high selectivity and sensitivity and require very small soot samples. Combining LD/LI/MS with in situ measurements of soot and gaseous species is very promising for a better understanding of the early stage of soot growth in flames. In this work, three lightly sooting laminar jet flames (a methane diffusion flame and two premixed acetylene flames of equivalence ratio ( ϕ) = 2.9 and 3.5) were investigated by combining prompt and 50 ns-delayed laser-induced incandescence (LII) for spatially resolved measurements of soot volume fraction ( f v) and laser-induced fluorescence (LIF) of polycyclic aromatic hydrocarbons (PAH). Soot and PAH calibration is performed by two-colour cavity ring-down spectroscopy (CRDS) at 1064 and 532 nm. Soot particles were sampled in the flames and analysed by LD/LI/Time-of-flight- MS. Soot samples are cooled to −170 °C to avoid adsorbed phase sublimation (under high vacuum in the TOF-MS). Our set-up is novel because of its ability to measure very low concentration of soot and PAH together with the ability to identify a large mass range of PAHs adsorbed on soot, especially volatile two-rings and three-rings PAHs. Studied flames exhibited a peak f v ranging from 15 ppb (acetylene, ϕ = 2.9) to 470 ppb (acetylene, ϕ = 3.5). Different mass spectra were found in the three flames, each exhibiting one predominant PAH mass; 202 amu (4-rings) in methane, 178 amu (3-rings) in acetylene, ϕ = 2.9 and 128 amu (2-rings) in acetylene, ϕ = 3.5. These variations with flame condition contrasts with other recent studies and is discussed. The other PAH masses ranged from 102 (C 8H 6) to 424 amu (C 34H 16) and are well predicted by the stabilomer grid of Stein and Farr.