Visible Flame Length Research Articles

Stoichiometric Methane-Air Flame Propagation in a Diverging Hele-Shaw Channel

ABSTRACT Design and optimization of various small-scale combustors requires knowledge on the features of flame propagation in narrow spaces between solid walls. In the present work, experiments on laminar propagation of stoichiometric premixed methane-air flames in a planar channel formed by two parallel closely spaced (3–6 mm) glass plates are presented. The channel of 1.2 m length has straight side walls diverging at an angle between 5 and 25 degrees; ignition is performed near the closed narrow end of the channel. Video recordings of the propagating flames are taken and processed to obtain the flame coordinate, visible velocity, flame front shape and length, and cell width. For each channel geometry, visible flame propagation speed is obtained and compared with the normal flame speed. Positions of the angular points (cusps) separating the cells are obtained and analyzed in order to establish the statistical features of cellular flames. It is shown that flame propagation in the channels with the gap width exceeding 4.5 mm exhibits significant oscillations of the visible speed and flame length. These oscillations are of non-acoustic nature, their frequency is 4–8 Hz, and their amplitude increases with the gap width. For the developed cellular flames, the ratio of cell size to gap width is bounded to the range of 4–10. The cell size probability density functions are shown to obey the gamma distribution, except for the widest gap of 6 mm. The main novelty of the approach and its significance for practical applications is that flame propagation in narrow gaps is studied, with the effects of channel geometry and gas flow in the channel taken into account explicitly.

Experimental investigation on hydrogen jet flame behavior and temperature distribution with different shapes of outlet

Hydrogen's wide flammability range causes its high-pressure jet flames to exhibit large-scale, high-temperature characteristics, posing risks of casualties and property damage. This study investigates the impact of different nozzle shapes and release pressures on flame behavior and temperature distribution through high-pressure hydrogen jet flame experiments. Circular outlets (0.5–4 mm diameter) and rectangular outlets (aspect ratios 1 to 8) are used, covering release pressures from 20 to 220 bar. High-pressure hydrogen is released through a nozzle into ambient air, igniting to form a flame. Flame length, width, and axial temperature are measured. Findings indicate that visible flame size increases with release pressure. The models for dimensionless visible flame length and width, considering the dimensionless heat release rate, are proposed for different nozzle shapes. Additionally, the dimensionless temperature rise along the flame axis is observed to be a function of the dimensionless distance and dimensionless heat release rate.

A comparative study of gaseous fuel flame characteristics for different bluff body geometries

Flame stabilization by a bluff-body is commonly used in many combustion applications to improve mixture characteristics, enhance flame stability and assist in combustion control. The main objectives of the present experimental work are concerned with studying the flame stabilization and combustion characteristics of Liquefied Petroleum Gas (LPG) with different bluff body shapes and sizes. An experimental test rig that consists of a gaseous fuel line, compressed air line, mixing chamber and burner is designed and constructed. Different bluff body shapes such as disc, cone, V-gutter, horizontal and vertical cylinders are used. The effects of different Blockage Ratios (B.R.) of 0.25, 0.35, 0.45, and 0.55 are studied at a constant equivalence ratio (Փ) of 1.85 for different bluff body shapes. The blowoff limits, temperature distributions, flame shapes, exhaust species concentrations and visible flame length are experimentally measured and discussed. The results indicated that increasing the blockage ratio leads to increasing the maximum flame temperature, increasing the flame diameter, and decreasing the flame length for all bluff body shapes. The centerline axial NO concentration has its peak values coincide with the maximum centerline axial temperatures. The flame using bluff body cone is more stable than using other bluff body shapes i.e., it has wide flammability limits or a wide range of operating conditions.

Temperature profile and visible flame length of blended pool fires at quiescent air conditions

Temperature profile and visible flame length of blended pool fires at quiescent air conditions

Combustion and emission characteristics of premixed biogas mixtures: An experimental study

In this study, effects of fuel composition, swirl number and hydrogen addition on combustion and emission characteristics of various biogas mixtures were experimentally investigated. To this end, a laboratory scale combustor and a swirl stabilized premixed burner were designed and manufactured. Later on, this combusting apparatus was equipped with flow, control, safety and measurement tools, hence entire test system was constituted. Combustion and emission characteristics of tested biogas mixtures were determined by measuring temperature and species (CO 2 , CO, O 2 and NO) distributions throughout the combustion chamber. Additionally, flame structures of tested biogas mixtures were evaluated by examining flame luminosity, visible flame length and flame thickness from instantaneous flame images. Results of this study showed that both radial and axial temperature distribution variations of tested biogas mixtures differently alter with hydrogen addition based on the gas composition. Although flame temperature increases with swirl number at burner outlet, it presents a non-monotonous dependence on swirl number outside the flame region because of the modified flow characteristics. This is also the case for emissions of CO 2 . • Premixed combustion characteristics of various biogas mixtures were investigated. • Gas composition, swirl number and hydrogen addition effects were examined. • Hydrogen addition differently altered temperature distribution based on the gas composition. • Flame temperature presented a non-monotonous dependence on swirl number.

The flame mitigation effect of vertical barrier wall in hydrogen refueling stations

Reliability and safety of hydrogen refueling stations directly affect the public acceptance of hydrogen energy. Due to high storage pressure (up to 90 MPa), the firefighting measures in hydrogen refueling stations are different from those in traditional gas stations. The mitigation effect of barrier walls against jet flame, the reduction ratio of flame length and flame deflection angle were studied by an in-house version of FireFOAM solver. The code employed a modified eddy-dissipation concept combustion model within the framework of large eddy simulation. A real-gas state equation of hydrogen in a simplified form was adopted to describe the complex behavior of high-pressure hydrogen. The notional nozzle approach was applied to replace the actual hydrogen jet exit based on mass and momentum conservation. The simulation results show that the visible flame length decreases with the growth of height difference between the barrier wall and hydrogen jet exit, which can be fit using the power function. The flame deflection angle is proportional to the height difference, which means the barrier wall can greatly reduce the dangerous area behind the wall. However, the increase of the wall height can also cause a significant increase in the flame radiation fraction, because the surface area of the flame grows after impinging on the barrier wall. When the height difference is larger than 1 m, further increase in the height of barrier wall cannot significantly promote the fire resistance effect.

Experimental Investigation of Stability and Structure of Vertical LPG Inverse Diffusion Flames Issuing from an Elliptic Burner

The present paper experimentally examines the influence of the progressive variations of the central air jet velocity in a concentric circular-elliptical inverse diffusion flame (IDF) on the visual and thermal structure and stability of the developed flames. All experiments are conducted at a fixed fuel flow rate (liquefied petroleum gas) throughput that emerges from the annular elliptic passage having an aspect ratio of 2:1. The visual images are aided via a digital camera and shadowgraphs, while the thermal structure (axial and radial temperature profiles) is acquired using a bare fine wire (125 μm) thermocouple; rendering radiation loss insignificant. The visual images and shadowgraphs clearly indicate the existence of four regimes: (i) an annular partially premixed region at the burner rim, (ii) an inner central premixed blue flame, (iii) an outer luminous yellowish post combustion zone and (iv) the flame tip buoyant zone. The progressive increases of the central average air velocity (𝑉𝑎 = 7.4 m/s up to 31.3 m/s) result in shortening the visible flame length and narrowing the flame width. These are coupled with changing the flame appearance from a yellowish diffusion flame with a sooty core regime to an intense central premixed flame surrounded by a soot ring to blue flames exhibiting intense central radiation regime associated with soot oxidation. At extremely high air velocity > 21 m/s, locals flame extinction and re-ignition occurs at the boundaries of the ellipse minor axis and further increase of 𝑉𝑎 causes complete extinction of the main flame. These findings are very much supported by the mean gas temperature measurements that indicate steep temperature rise associated with the formation of a central premixed combustion within the flame core which is followed by the diffusion mode of combustion. A plateau of the axial temperature profiles is observed in the transition zone between the two regions whereby the rise in temperature due to soot oxidation is balanced by the radiation loss.

Characteristic of cryogenic hydrogen flames from high-aspect ratio nozzles

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

Effects of Air Temperature on Combustion Characteristics of LPG Diffusion Flame

In the present study, the combustion characteristics of LPG gaseous fuel diffusion flame at elevated air temperatures were experimentally investigated. An experimental test rig was manufactured to examine a wide range of operating conditions. The investigated parameters are the air temperatures of 300, 350, 400, 450, and 500 K with constant percentage of nitrogen addition in combustion air stream of 5 % to give low oxygen concentration of 18.3 % by mass at constant air swirl number, air to fuel mass ratio, and thermal load of 1.5, 30, and 23 kW, respectively. The gaseous combustion characteristics were represented as axial and radial temperatures distributions, temperatures gradient, visible flame length and species concentrations. The results indicated that as the air temperature increased, the chemical reaction rate increased and flame volume decreased, the combustion time reduced leading to a reduction in flame length. The NO concentration reaches its maximum values near the location of the maximum centerline axial temperature. Increasing the combustion air temperature by 200 K, the NO consequently O2 concentrations are increased by about % 355 and 20 % respectively, while CO2 and CO concentrations are decreased by about % 21 and 99 % respectively, at the combustor end.

Experimental study of radiative heat transfer from laminar non-premixed methane flames diluted with CO2 and N2

This paper presents an experimental study about the effect of fuel dilution with inert gases on the radiative heat transfer characteristics of laminar non-premixed coflow methane flames. The visible flame lengths were determined and flickering is characterized when necessary. Radiative heat flux distributions were measured and used to determine the radiative fraction of each flame. In total, 55 flames were studied with powers up to 0.554 kW and with CO2 and N2 dilutions up to 60%. The flames were grouped in series of constant power and constant flow rate. It is shown that the addition of CO2 may decrease or increase the radiative fraction depending on the dilution level and on the characteristic residence time of the flame. Dilution with N2 always leads to a decrease of the radiative fraction. The results are relevant for practical applications as in biogas combustion and also proved to be useful for validation of radiative heat transfer models.

Experimental study of spray combustion characteristics of air-blast atomizer

In the present work, the characteristics of spray combustion an airblast atomizer with an innovative design were experimentally investigated. The flame was stabilized in a water-cooled combustor using air swirler. A test rig was manufactured to investigate the different operating conditions — which include the atomizing Air to Liquid fuel mass Ratio (ALR) of 5, 6, 7 and 8 and the elevated Atomizing Air Temperature (AAT) of 300, 350, 400, and 450 K. The spray combustion characteristics are represented in the form of axial and radial temperatures distributions, temperature gradients, visible flame length, and species concentrations. The axial and radial temperature distributions of flame were measured using the type R thermocouple. The species concentrations were measured by an infrared AO2000 series gas analyzer. The results indicate that the high-temperature combustion region is shifted upstream and outward and the flame length becomes shorter by increasing the value of both ALR and AAT. In addition, the NO concentration reaches its maximum values near the location of the maximum centerline axial temperature. Moreover, it was found that increasing the AAT leads to, on the one hand, decreasing the gas temperature, the O2 and the CO concentrations at the combustor end, on the other hand, it leads to increasing both the NO and the CO2 concentrations.

An Investigation of Soot Volume Fraction and Temperature for Natural Gas Laminar Diffusion Flame Established From a Honeycomb Gaseous Burner

The present work is an experimental investigation that aims at studying the effects of different fuel additives on the soot volume fraction and temperature in a well-defined vertical laminar diffusion flame configuration, and these additives include a diluent (argon) that suppresses the formation of soot and a soot promoter (acetylene) that accelerates and intensifies the soot formation. Three different measuring techniques are employed throughout the whole experimental program, namely, a high-resolution digital camera (up to 3.7 fps) for flame visualization, a bare wire Pt/Pt-13% rhodium fine thermocouple of 15 µm wire diameter for measuring the mean gas temperature inside the flame region and a laser system for measuring the in-flame soot volume fraction. The results indicated that the soot inception zone (deep dark parabolic shape) occurs at the immediate vicinity of the burner. The soot oxidation zone is characterized by high luminosity, and it begins after the fuel is largely consumed. The increased percentages of acetylene in the fuel mixture would lead to extending the length of this zone to ultimately occupy the whole visible flame length, where the luminosity becomes independent of the amount of soot. The temperature within the soot surface growth zone (orange color) continues increasing but at a lower rate that reflects the domination of diffusion combustion mode. Limited partial oxidation may be anticipated within this zone due to the relatively high temperature, which is not high enough to cause luminosity of the soot particles.

The effect of inverse diffusion flame burner-diameter on flame characteristics and emissions

An experimental investigation was performed to study the effect of change of the IDF burner diameters on the flame characteristics (flame length, axial temperature distribution, and flame appearance) and flame emissions. The study was performed using a coaxial (CoA) circular inverse diffusion flame burners of different diameters. Burners' air diameters (da) were varied from 6 mm to 22.5 mm. The burner fuel diameters were varied according to the change of air diameter to preserve constant equivalence ratio and constant aspect ratio (0.5). The study is carried out at constant equivalence ratio (Φ = 3) and constant air Reynolds number (Rea = 2500). The measured parameters are the flame axial temperature, flame appearance, and emission along the flame center line. The results show significant differences in the flame appearance. The smaller diameters produce shorter flame lengths. The visible flame lengths were varied from 170 to 300 mm. Also, an early formation of CO & CO2and early depletion of O2 were obtained by reducing the burner's sizes. For all the experiments conducted in the present study, the centerline temperature distribution produced by the smaller nozzles diameters show higher flame temperature. The Peak temperatures were varied according to burner size, from 1600 to 950 °C.

Combustion Characteristics of a Two-Dimensional Twin Cavity Trapped Vortex Combustor

Trapped vortex combustor (TVC) is a relatively new concept, having potential application in gas turbine engines. In this work, an attempt has been made to characterize the 2D twin cavity TVC experimentally in terms of its visible flame length, pollutant emission level, and exit temperature profile. Besides this, numerical results are also discussed to explain certain intricacies in flow and flame characteristics. Experimental results reveal that visible flame length value is sensitive to mainstream Reynolds number (Rems), primary (cavity) air velocity (Vp), and cavity equivalence ratio (Φc). For a particular Rems and Φc, an increase in Vp results in longer flame length; whereas, flame length gets shortened at higher mainstream Reynolds number cases. Numerical studies indicate that shortening of flame length at higher Rems cases is caused due to quenching of flame at the shear layer by the incoming flow. An attempt has been made to correlate flame length data with the operating parameters and Damkohler number (Da); Da takes care of flame quenching effects. Moreover, it is also brought out that the emission profile at the combustor exit is dependent on primary air velocity, mainstream Reynolds number, and cavity equivalence ratio. Emission studies indicate that higher primary air velocity cases make the carbon monoxide (CO) and unburned hydrocarbon (UHC) emission levels to lower values. Reduction in emission level is caused mainly due to the flame merging effects. Besides this, the influence of cavity flame merging on the exit temperature profile uniformity is also brought out. This study reveals that merging of cavity flames is essential for the optimized operation of a 2D trapped vortex combustor.

Developement of porous media burner operating on waste vegetable oil

A newly designed cooking stove using Wasted Vegetable Oil (WVO) as fuel was introduced. Porous media, containing 2cm diameter of spherical ceramic balls, was used as a flame stabilizer. Steam was successfully applied in a burner at this scale to atomize WVO droplet and entrain air into the combustion zone as well as to reduce soot and CO emission. DIN EN 203-1 testing standard was adopted and the experiment was conducted at various firing rate with the water flow rate at 0.16, 0.20 and 0.22kg/min. Temperature, emissions, visible flame length, thermal efficiency as well as combustion efficiency were evaluated. Under the current WVOB design, it was suitable to operate the burner at the range of nominal firing rate between 325 and 548kW/m2 with water flow rate of 0.16kg/min, at burner height to diameter ratio of 0.75, giving CO and NOx emissions up to 171 and 40ppm, respectively (at 6% O2). Thermal efficiency was at around 28% where the combustion efficiency was approximately at 99.5%. The performance of WVO burner could be improved further if increasing the H/D ratio to 1.5, yielding thermal efficiency up to 42%.

Experimental study on propane jet fire hazards: Comparison of main geometrical features with empirical models

An experimental study of jet fires is performed to understand the flame geometry of jet fires. Horizontal jet fire experiments are performed by varying the mass flow rate of fuel through a 19 mm nozzle. The exit velocities varied from 25 to 210 m/s, flame lengths from 1 to 6 m and Froude Numbers from 2 × 103 to 2 × 105. The experimental measurement includes the flame detection using standard CCD camera to capture the details of flame morphology. The frames obtained from CCD camera are reconstructed using image visualization technique to obtain the flame morphology like flame length and lift-off length. The modeling includes comparison of experimentally determined jet flames with empirical correlations. The flame length is well validated with three empirical correlations. The lift-off length is found to be under-predicted by all the models. With extended analysis on image processing, it is observed that lift-off length is sensitive to the threshold intensity value chosen for image processing, whereas, flame length parameter is independent of the threshold intensity value. The deviation between experimental and predicted values is also attributed to soot formation which affects the lift-off lengths and the heat radiated by jet fires. The flame area obtained from image visualization methods are compared to flame area obtained from empirical models. The effect of air entrainment is also studied on flame geometry. It is observed that the horizontal extent of visible flame length is affected by the wind flowing in the downwind direction of jet fire. The vertical extent of the jet fires is found to reduce with wind flowing in the crosswind direction.

A 3D numerical study of LO2/GH2 supercritical combustion in the ONERA-Mascotte Test-rig configuration

Cryogenic propellants LOx/H2 are used at very high pressure in rocket engine combustion. The description of the combustion process in such application is very complex due essentially to the supercritical regime. Ideal gas law becomes invalid. In order to try to capture the average characteristics of this combustion process, numerical computations are performed using a model based on a one-phase multi-component approach. Such work requires fluid properties and a correct definition of the mixture behavior generally described by cubic equations of state with appropriated thermodynamic relations validated against the NIST data. In this study we consider an alternative way to get the effect of real gas by testing the volume-weighted-mixing-law with association of the component transport properties using directly the NIST library data fitting including the supercritical regime range. The numerical simulations are carried out using 3D RANS approach associated with two tested turbulence models, the standard k-Epsilon model and the realizable k-Epsilon one. The combustion model is also associated with two chemical reaction mechanisms. The first one is a one-step generic chemical reaction and the second one is a two-step chemical reaction. The obtained results like temperature profiles, recirculation zones, visible flame lengths and distributions of OH species are discussed.

Influence of the hydrogen-rich on the furnace thermal efficiency

In this research a full-scale furnace is used to recover the hydrogen-rich tail gas as fuel. Adding hydrogen gas to hydrocarbon fuel will reduce the ignition delay of methane, increase the flame velocity and speed up the relatively slow reaction rate of methane to improve the flame stability. The results show that the flame length and orange-yellowish brightness decrease as the amount of tail gas fuel added to the natural gas increases, because of the lower C/H ratio in the flame. Moreover, at a fixed flow rate of hydrocarbon fuel, the moving length of the burning flame is reduced as the amount of hydrogen increases, and thus the visible flame length becomes shorter. Additionally, burning the mixture of tail gas reduces the pressure and increases the gas rising velocity in the furnace radiation and convective zones compared to burning pure tail gas, and thus the gas temperatures in the convective zone and in the flue are raised. The furnace convective zone temperature and the flue gas temperature are 793.6 °C and 350.7 °C, respectively, for burning the mixture fuel (45 vol. % tail gas + 55 vol. % natural gas) vs. 648.5 °C and 346.3 °C for burning the pure tail gas.

Diffusive burning of blended peroxy-fuels: Some experimental results

Dampening of energetic properties and the effects of blending proportions of isododecane on the diffusive burning behaviour of peroxy-fuels are experimentally studied. Blended peorxy-fuels are obtained by adding isododecane in the proportions of 25wt.%, 50wt.% and 75wt.%, respectively, in technical pure peroxy-fuels. The fuels were burned in form of pool fires with diameters 0.02m≤d≤1m. The mass burning rates and relative flame lengths are found to be weakened with increasing diluent proportions. By measuring the mass burning rates and visible flame lengths of pool fires of different samples of fuel blends an optimum blending criterion is developed. Furthermore, it is shown that the dilution proportions and flame characteristics can be correlated by empirical equations.

Phenomenological model of soot production inside a non-buoyant laminar diffusion flame

An original phenomenological model for soot production inside a laminar, flat plate boundary layer diffusion flame is presented. The model is compared with experimental measurements conducted in microgravity. For the experiments, the fuel, ethylene, is injected through a flat porous burner into an oxidizer stream flowing parallel to the burner surface. The oxidizer is a mixture of 35% oxygen and 65% nitrogen. The fuel and oxidizer velocities are systematically varied. The analysis of the data shows that the streamwise location of the maximum flame height can be considered an unambiguous characteristic length of the flame as opposed to the maximum visible flame length. Analysis of the streamwise location of the maximum flame height enables to establish the transition between “open-tip” and “closed-tip” behavior as well as scaling laws for the soot volume fraction. A scaled soot volume fraction is found to follow a linear relationship with the streamwise coordinate normalized by the burner length. This correlation appears to be valid for the whole range of conditions investigated, knowing that this range does not cover the blow-off regime.