Characteristics Of Diffusion Flames Research Articles

Impact of Oxygen Content on Flame Dynamics in a Non-Premixed Gas Turbine Model Combustor

In this study, large eddy simulation (LES) was used to investigate the dynamic characteristics of diffusion flames in a swirl combustion chamber at an oxygen content of 11–23 wt% and temperature of 770 K. The proper orthogonal decomposition (POD) method was employed to obtain flame dynamic modes. The results indicate that oxygen content has a significant impact on the downstream flow and flame combustion characteristics of the swirl combustion chamber. With oxygen content increasing, the size of the recirculation zone is reduced, and the flame field fluctuations are intensified. The pressure and heat release fluctuations under different oxygen contents were analyzed using frequency spectrum analysis. Finally, the flame modes were analyzed using the POD method, and it was found that the coherent structures are asymmetric relative to the local coordinate system. At an oxygen content of 11 wt%, they exhibit larger coherent structures, while at an oxygen content of 23 wt%, they exhibit numerous turbulent structures.

Experimental study on the buoyant turbulent diffusion flame height of various intermittent levels

This work studies the buoyant turbulent diffusion flame characteristics considering various flame intermittent levels and mean flame projection area. Although many investigations have been carried to reveal the mean flame height evolution of the buoyant turbulent diffusion flame, the turbulent diffusion flame height of various intermittent levels, occurs in the un-controllable (unplanned) fire accident (such as the gas fuel pipeline/tank leakage). The underlying evolutions of the buoyant turbulent diffusion flame with the heat release rate and intermittent levels have not been fully elucidated. Experiments are conducted by various burner dimensions under varied heat release rates. The flame geometries/heights at various intermittent levels are measured and quantified comprehensively by two cameras from the front- and side- view. It shows that the mean flame height declines as burner area/dimension increases. All experimental data for mean flame height in the present work and previous work are well correlated by the modified normalized heat release rate derived from the new characteristic length. Results also show that the flame height at various intermittent levels decreases with the intermittent level. A model of the flame heights at various intermittent levels is proposed considering the intermittent level and mean flame height to predict the experimental data in this work and previous work well. Finally, a flame projection area model is proposed based on the assumption of the isosceles triangle flame projection geometry. This work provides huge amount experimental data and prediction models to describe buoyant turbulent diffusion flame behaviours, which is meaningful for evaluating thermal failure probability and laying of gaseous fuel transportation pipelines.

Characteristic studies of diffusion flame tilt angle and length in a branched tunnel fire under the effect of airflow

ABSTRACT The reduced scale experiments were conducted to investigate the LPG flame geometry in branched tunnel fire under different airflow velocities to better comprehend the diffusion flame characteristics in narrow – long space with multiple entrance. Five different heat release rates varied from 1.72 kW to 5.18 kW were considered. The flame tilt angle and flame length were measured and correlated. Results show that the flame tilts downward to multiple entrance direction (ramp and mainline) under natural ventilation, which is mainly related to the asymmetry tunnel cross-section dimensions between fire source two sides. The experiments presented three different flame tilting conditions under different heat release rates and airflow velocities due to the dominance of buoyancy force or inertial force. The flame tilt angle is correlated using the nondimensional heat release rate and Froude number, and the predictions by proposed correlation agree reasonably well with the experimental data. The flame length increases with the heat release rate and elongated by airflow. The empirical model for dimensionless flame length are developed both under natural and longitudinal ventilation.

Investigating the oxidative reactivity and nanostructural characteristics of diffusion flame generated soot using methyl crotonate and methyl butyrate blended diesel fuels

Combustion-generated particulate matter has a consequential fallout on climate, environment, and human health. Thus, comprehension of soot formation and mitigation processes using biodiesel additives is a focal point of modern combustion research. This study presents an intricate investigation of enhanced oxidative reactivity and its dependence on the soot nanostructural properties induced by distinctive blending of a saturated and an unsaturated biodiesel surrogates namely, methyl butyrate (MB) and methyl crotonate (MC) respectively with pure diesel. 25% MB-75% diesel fuel resulted in the lowest sooting propensity and lowest activation energy (155.4 ± 2 kJ/mol) for soot oxidation in comparison to pure diesel fuel and diesel soot (175.1 ± 2 kJ/mol). The soot was collected using a smoke point apparatus involving a wick-fed laminar diffusion flame at atmospheric pressure after meticulous observation of the smoke points. The detailed nanostructural characterization of the flame generated soots were performed using high-resolution transmission electron microscopy, X-ray diffraction, elemental analysis and Raman spectroscopy. BET surface area analysis, and thermogravimetric analysis were carried out for surface area distinction and activation energy calculations respectively. It revealed that, addition of MB to diesel resulted in improved fuel combustion with reduction in primary soot particle diameter, greater inter-planar separation, increased fringe tortuosity and greater crystal structure disorder resulting in its enhanced reactivity with O2. The 25% MC-75% diesel blend resulted in relatively greater sooting propensity while its soot exhibited lower crystal structure disorder and greater activation energy (165.0 ± 2 kJ/mol) for O2 induced oxidation compared to 25% MB-75% diesel, possibly due to the resonance stabilized radical (RSRs) formation that altered the fuel combustion chemistry. Therefore, this study successfully depicts that the structural differences in the surrogate fuels do influence the soot formation and oxidation kinetics. These structural effects therefore need to be considered when formulating the global multiphase kinetic models for biodiesel-diesel fuel combustion.11Dedicated to Prof. Purnakala Samant on the occasion of her 50th birthday celebration.

Experimental study on flame characteristics of double fires in a naturally ventilated tunnel: Flame merging, flame tilt angle and flame height

A series of fire tests was carried out to investigate the diffusion flame characteristics of double fires generated from separated burners in a naturally ventilated tunnel, considering different heat release rates and fire separation distances. The results show that the flame tilt angle, as well as the horizontal projected flame length, first increases with fire separation distance and then remains constant, but the vertical flame length first decreases and then remains constant, which is different from two fires in free spaces where flames do not tilt when the separation distance is relatively long. This difference is caused by the non-dimensional fire induced air flow velocity in the tunnel, which is mainly related to the tunnel cross-section dimensions and burner radius. Three regions can be identified, i.e. flame vertical merging, plume vertical merging and non-merging with flame tilted. The critical flame merging separation distance, estimated by the flame merging probability, is greater than that of two fires in an open environment due to the fire-induced air flows. The merging flame height is lower than that of a single fire with a same heat release rate. A correlation was proposed to estimate flame height of two fires in a tunnel by the modified non-dimensional heat release rate using an air entrainment perimeter as the characteristic length. This work enhances the understanding of diffusion flame behaviors of double fires in naturally ventilated tunnels.

Effects of wind flow and sidewall restriction on the geometric characteristics of propane diffusion flames in tunnels

Diffusion flame characteristics are important for predicting and controlling the energy transfer to fuel surface and surrounding targets during an undesirable energy source releasing in a tunnel. To explore the effects of wind flow and sidewall restriction on the geometric characteristics of turbulent diffusion flames in tunnels, experiments of propane burner fires were performed in a bench-scale wind tunnel. Two different fire source positions, i.e., centerline fire and sidewall fire, were considered. The energy (heat) release rate of the fire source and the ventilation velocity were varied in the range of 15–105 kW and 0–3 m/s, respectively. The flame shape, flame tilt angle and flame drag length were investigated. The results suggest that the flame tilt angles of two fire source positions both increase faster first and then increase slowly with increasing ventilation velocity, and the flame tilt angle of the sidewall fire is smaller than that of the centerline fire due to the inhibition of the frictional drag force. With the increase in ventilation velocity, the flame drag length of the centerline fire is first smaller than, and then approaches, and eventually exceeds that of the sidewall fire owing to the varying force mechanism. Finally, correlations of flame drag length were proposed and compared with previous experimental results.

Comparison of Jet Diffusion Flame Characteristics and Flame Temperature of Dimethyl Ether (DME) and Liquefied Petroleum Gas (LPG)

Dimethyl Ether (DME) is currently considered as alternative to be used as energy source. Its similar physical properties with LPG made it possible to be used in household gas stove using same apparatus. Concern comes on its combustion properties regarding its lower calorific value than LPG. This research aimed to investigate the jet diffusion flame characteristics of DME compare to LPG in terms of flame height and lift-off flame, with correlation being made with fuel flow rate and burner hole diameter. The flame temperature is also measured on the same experiments. The results show that DME produce lower flame height than LPG. The flame height produced by both fuels were independent to burner hole diameter. The transitional regime in term of Reynold number is differ significantly between DME and LPG where DME reach the transitional regime at lower Re. The flame temperature of DME at nozzle tip and dark zone is higher than LPG.

Flames of Swirling Double-Concentric Jets Subject to Acoustic Excitation at Resonance

The effects of acoustic excitation at resonance on the flame appearances, flame lengths, flame temperatures, and combustion product concentrations of combusting swirling dual-disk double-concentric jets were studied. The Reynolds number of the annular swirling air jet was varied, while it was fixed at 2500 for the central propane jet. The central fuel jet was acoustically forced by a loudspeaker, which was installed using downstream longitudinal irradiation. The central jet pulsation intensities were measured by a calibrated, one-component hot-wire anemometer. The instantaneous full-length and close-up flame images were captured to identify the characteristic flame modes. Long-exposure flame images were taken to measure the flame lengths. The axial and radial temperature distributions of flames were measured using a homemade, fine-wire R-type thermocouple. The concentrations of combustion products were measured by a gas analyzer. Four characteristic flame modes, blue-base wrinkled flame, yellow-base anchored flame, blue-base anchored flame, and lifted flame, were observed in the domain of central jet pulsation intensity and annular swirling jet Reynolds number. The lifted flame, which was formed at large central jet pulsation intensities, presented characteristics of a premixed flame due to significant mixing induced by violent, turbulent flow motions. It was short and stable, with high combustion efficiency and low toxic emissions, when compared with the unexcited flame and other excited characteristic flame modes, which presented characteristics of diffusion flame.

Flame behavior and thermal structure of combusting plane jets with and without self-excited transverse oscillations

The flame behavior and thermal structure of combusting plane jets with and without self-excited transverse oscillations were investigated experimentally. The transversely-oscillating plane jet was generated by a specially designed fluidic oscillator. Isothermal flow patterns were observed using the laser-assisted smoke flow visualization method. Meanwhile, the flame behaviour was studied using instantaneous and long-exposure photography techniques. Temperature distributions and combustion-product concentrations were measured using a fine-wire type R thermocouple and a gas analyzer, respectively. The results showed that the combusting transversely-oscillating plane jets had distributed turbulent blue flames with plaited-like edges, while the corresponding combusting non-oscillating plane jet had laminar blue-edged flames in the near field. At a high Reynolds number, the transversely-oscillating jet flames were significantly shorter and wider with shorter reaction-dominated zones than those of the non-oscillating plane jet flames. In addition, the transversely-oscillating combusting jets presented larger carbon dioxide and smaller unburned hydrocarbon concentrations, as well as portrayed characteristics of partially premixed flames. The non-oscillating combusting jets presented characteristics of diffusion flames, and the transversely-oscillating jet flame had a combustion performance superior to its non-oscillating plane jet flame counterpart. The high combustion performance of the transversely-oscillating jets was due to the enhanced entrainment, mixing, and lateral spreading of the jet flow, which were induced by the vortical flow structure generated by lateral periodic jet oscillations, as well as the high turbulence created by the breakup of the vortices.

Numerical study of the swirl direction effect at the turbulent diffusion flame characteristics

Numerical study of the swirl direction effect at the turbulent diffusion flame characteristics

Effects of electric field intensity and frequency of AC electric field on the small-scale ethanol diffusion flame behaviors

The Alternating Current (AC) electric field was applied on the small-scale flame in order to promote the characteristics or control the combustion. The effect of AC electric field on the small-scale ethanol diffusion flame behaviors was studied experimentally and numerically. The inner diameter of the burner nozzle was 0.9mm and the flame was established at room temperature and atmospheric pressure. Different electric field intensities and frequencies of AC electric field were generated by two parallel plate electrodes which were respectively placed above and below the flame. Numerical results combined with experimental observations were used to analyze the small ethanol diffusion flame characteristics under the electric field. The results show that flame behaviors are significantly influenced by the applied AC electric field. The body forces acting on charged particles in the electric field are considered under the AC electric field. The charged particles are accelerated in the applied electric field and transfer the momentum to the neutral particles. At a certain flow rate of ethanol, the flame temperature and flow velocity increase with the increasing of electric field intensity and AC frequency. The diffusion effect is strengthened by the electric field and the mass fraction distribution of the component has a tendency of inward contraction, which is consistent with the tendency of flame becoming shorter. It also can be concluded that the applied AC electric field promotes the mixing of fuel and oxidizer, thus enhances the combustion process. The reaction rate in the flame becomes faster and reaction position is closer to the fuel side. The combined effects of diffusion and reaction enhancements affect the flame behaviors.

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.

Characteristics of diffusion flames with accelerated motion

The aim of this work is to present an experiment to study the characteristics of a laminar diffusion flame under acceleration. A Bunsen burner (nozzle diameter 8 mm), using liquefied petroleum gas as its fuel, was ignited under acceleration. The temperature field and the diffusion flame angle of inclination were visualised with the assistance of the visual display technology incorporated in MATLAB?. Results show that the 2-d temperature field under different accelerations matched the variation in average temperatures: they both experience three variations at different time and velocity stages. The greater acceleration has a faster change in average temperature with time, due to the accumulation of combustion heat: the smaller acceleration has a higher average temperature at the same speed. No matter what acceleration was used, in time, the flame angle of inclination increased, but the growth rate decreased until an angle of 90?: this could be explained by analysis of the force distribution within the flame. It is also found that, initially, the growth rate of angle with velocity under the greater acceleration was always smaller than that at lower accelerations; it was also different in flames with uniform velocity fire conditions.

Performance enhancement of inverse diffusion flame burners with distributed ports

An experimental study regarding the influence of the air port design on the inverse diffusion flame characteristics was performed for two different burner configurations, wherein the combustion/heat transfer performance was optimized. In the first configuration that comprised a single air port and distributed fuel ports, decreasing the air jet diameter enhanced the combustion efficiency by 3.6% as testified by recording a maximum decrease of 81.3% in the CO emissions. As the central air jet Reynolds number thus reached 18038, the stability limit was extended by 181% in terms of the air jet velocity; while the flame length was reduced by 38.6%. Inasmuch as the flow residence time across the high-temperature zone thus decreased, NO x emissions as low as 6 ppm were obtained. In the other configuration that had multiple air ports, there was a decrease in the CO, HC, and NO x emissions respectively by 69%, 58%, and 31% in comparison to those of the single port having the same port area. It was found that the smallest jet diameter was accompanied by the highest peak temperature as well as the maximum heat flux to a plate on which the flame impinged, respectively, having the values of 1761℃ and 11.4 kW/m2. In this regard, increasing the air jet Reynolds number by 30% increased the peak flame temperature by 8.3%. In the primary equivalence ratio range between 0.8 and 2.0, the impingement plate optimum spacing for the maximum heat flux was found to correlate well with Reynolds number. Combining the nonunity Lewis number and the Gas Research Institute-3 (GRI-3) kinetics with the k − ɛ model verified the fuel jet deflection and soot development, but overpredicted the NO x emissions by 11.5%. Increasing the center-to-center distance between the central air jet and the distributed fuel ports reduced the flame length by 19%.

The Effect of Percentage of CO<sub>2</sub> on the Biogas Counterflow Diffusion Flame Stability

This study aims to determine the effect of the percentage of CO2 in the biogas to the characteristics of biogas diffusion flame. Counterflow configuration was used in this study to investigate detail structure of diffusion flame and the flame stability of biogas. The concentration of CO2 in the biogas was varied from 0% to 50% while the mass flow rate of the reactants was varied from 4 to 14 L / min. The results showed that the CO2 in the biogas fuel affect the diffusion flame characteristics, especially the area of luminous yellow flame formed in the fuel flow. In the low oxygen concentration, percentage of CO2 did not affect the diffusion flame stability. However, the flame stability was more influenced by the rate of diffusion between fuel and oxygen. Therefore, the combustion of biogas is more recommended to be done in the low oxygen concentration.

Diffusion Combustion of Liquid Heptane in a Small Tube with and without Heat Recirculating

In order to understand diffusion flame characteristics in a small tube, combustion of liquid n-heptane and air was experimentally and numerically studied. A tube of ID 4 mm and OD 6 mm made of quartz was used as the burner. Liquid n-heptane was delivered into a capillary from a syringe pump. Stable flames were established inside the burner with and without heat recirculating. Additionally, numerical simulations were conducted, and effects of equivalence ratio and external heat loss coefficient on diffusion flame were studied. Results show that, for a diffusion flame of liquid n-heptane in a small tube, as fuel flow rate increases, the flammable limits increase. The diffusion flame position moves downstream with increasing air flow, eventually stabilizing at the bottom of the outer tube until extinction. When the flame passes in the tube, the peak temperature would occur on the wall. If there is heat recirculating, the wall temperature of the inner tube is higher than the boiling point of liquid n-heptane. It is conducive to the pre-evaporation of liquid n-heptane. In contrast, if there is no heat recirculating, liquid fuel will be accumulated in the tube. The heat loss coefficient has a great influence on flammable limits of the tube burner without heat recirculating.

The effect of acoustic oscillations on diffusion combustion of methane

Aspects of ecological safety related to the combustion of hydrocarbons—in particular, the demand for reducing the content of toxic compounds in combustion products—are extremely topical. In addition, there is still important task of increasing the efficiency of using all kinds of fuel, including hydroarbons. One possible mechanism of controlling combustion processes—namely, the effect of acoustic perturbations on the diffusion combustion of methane—has been considered. The influence of acoustic oscillations on the characteristics of diffusion flame has been studied, and the dependence of the concentration of nitrogen oxides in the combustion products on the acoustic signal frequency has been determined.

The Effect of Upstream Annular Cavities in Circular Jet Burners on Stability Characteristics of Jet Diffusion Flames

The characteristics of diffusion flames from circular jet burners in the presence of upstream annular cavities are investigated experimentally. The influences of cavity dimensions such as cavity diameter and cavity length, and postcavity length, on methane flame characteristics are investigated. Experimental investigation employing 18 circular jet burners with an upstream cavity of different dimensions and a burner without cavity has been carried out. For burners with cavity, smaller flame lengths are obtained in the laminar region when compared to the burner without cavity due to enhanced mixing. The transition from laminar to turbulent regime occurs at higher flow rates for burners with cavity. Liftoff velocity, the fuel velocity at which the flame lifts off from the burner exit, and blowoff velocity, at which the flame blows off, are higher with the inclusion of cavity. This suggests that improved flame stability and increased operating range can be attained in burners with cavity as compared to that without cavity. Within the burners with cavity, there is an optimum cavity diameter and postcavity length, which provides maximum flame stability. The cavity length to diameter ratio is also a critical parameter; when the ratio is less than unity, increased stability is achieved.

Influence of propellant components flowrate on the emission characteristics of diffusion flames of coaxial jets

In this paper, the results are reported for an experimental investigation of distributing temperature and electrical potentials in the laminar diffusion flames of coaxial jets of propane and air depending on the components flowrate.

The effect of CO 2 dissolved in a diesel fuel on the jet flame characteristics

This paper is concerned with an experimental study of the jet diffusion flame characteristics of fuel containing CO 2. Using diesel fuel containing dissolved CO 2 gas, experiments were performed under atmospheric conditions with a diesel hole-type nozzle of 0.19 mm orifice diameter at constant injection pressure. In this study, four different CO 2 mass fraction in diesel fuel such as 3.13%, 7.18%, 12.33% and 17.82% were used to study the effect of CO 2 concentration on the jet flame characteristics. Jet flame characteristics were measured by direct photography, meanwhile the image colorimetry is used to assess the qualitative features of jet flame temperature. Experimental results show that the CO 2 gas dilution effect and the atomization effect have a great influence on the flame structure and average temperature. When the injection pressure of diesel fuel increased from 4 MPa to 6 MPa, the low temperature flame length increased from 18.4 cm to 21.7 cm and the full temperature flame length decreased from 147.6 cm to 134.7 cm. With the increase of CO 2 gas dissolved in the diesel fuel, the jet flame full length decreased for the jet atomization being improved greatly meanwhile the low temperature flame length increased for the CO 2 gas dilution effect; with the increase of CO 2 gas dissolved in the diesel fuel, the average temperature of flame increases firstly and then falls. Experimental results validate that higher injection pressure will improve jet atomization and then increased the flame average temperature.