Envelope Flame Research Articles

Flame heights and fraction of stoichiometric air entrained for rectangular turbulent jet fires in a sub-atmospheric pressure

This paper investigates the flame height and the fraction of stoichiometric air entrained (n, the basic parameter in flame heights correlation) for rectangular turbulent jet fires at sub-atmospheric pressure, for which no data are available in the literature. Comprehensive experiments are carried out in a naturally sub-atmospheric pressure (Lhasa city, 64kPa) by rectangular nozzles whose aspect ratio varied from 1:1 to 1:71. The flame heights are measured and correlated by the unified formula suggested by Quintiere and Grove which was obtained at standard atmospheric pressure, meanwhile the change of the important parameter, n or Cf (reflecting air entrainment into flame and thus being basic in flame height correlation) in the sub-atmospheric pressure is addressed. It is found that: (a) the flame height in the sub-atmospheric pressure can be well approached by the unified formula suggested by Quintiere and Grove; however (b) the correlation parameter Cf is larger (1.6 times that in standard pressure) or the value of n is smaller (73% of that in standard pressure) in the sub-atmospheric pressure than that in the standard pressure. The relative change of n at the sub-atmospheric pressure is then theoretically deduced from the change of entrainment strength (C1) and flame envelope along with pressure drop. The theoretically deduced ratio of n (or Cf) between these two pressures agrees with the experimentally correlated results. Then, by accounting for the changes of the correlation parameters, a proposed explicit model can generally predict the flame heights in both pressures.

Application of blue laser triangulation sensors for displacement measurement through fire

This paper explores the use of blue laser triangulation sensors to measure displacement of a target located behind or in the close proximity of natural gas diffusion flames. This measurement is critical for providing high-quality data in structural fire tests. The position of the laser relative to the flame envelope can significantly affect the measurement scatter, but has little influence on the mean values. We observe that the measurement scatter is normally distributed and increases linearly with the distance of the target from the flame along the beam path. Based on these observations, we demonstrate how time-averaging can be used to achieve a standard uncertainty associated with the displacement error of less than 0.1 mm, which is typically sufficient for structural fire testing applications. Measurements with the investigated blue laser sensors were not impeded by the thermal radiation emitted from the flame or the soot generated from the relatively clean-burning natural gas.

Experimental study of the effect of turbulence on the structure and dynamics of a bluff-body stabilized lean premixed flame

The structure of an unconfined, lean propane–air flame stabilized on an axisymmetric bluff body is studied for different levels of turbulence intensity ranging from 4 to 30% in the approach flow. Simultaneous planar laser induced fluorescence imaging of hydroxyl radical (OH) and formaldehyde (CH2O) was performed to determine the effects of turbulence on local flame structure. In an accompanying experiment high speed particle image velocimetry at 5kHz was used to obtain the time resolved velocity field and evaluate flame surface density, brush thickness and probability density functions of curvature and strain rates conditioned on the flame front. The low turbulence intensity flame featured wrinkling and mostly symmetric flame structures with thin regions of heat release along the flame front. For moderate turbulence intensity (14%), pronounced formation of cusps and unburnt mixture fingers was observed with continuous heat release regions. However, the local flame structure was observed to be strongly altered for the 30% turbulence intensity. Localized extinction occurred along the flame surface creating isolated pockets of OH with heat release occurring along the boundary. Pockets of preheated reactants along with fresh reactants were observed inside the flame envelope. The overall flame appearance was observed to intermittently switch from varicose (symmetric) to sinuous (asymmetric) type (vortex shedding mode). The curvature and strain rates pdfs broadened with increasing levels of turbulence intensity. The flame brush thickness showed an initial increase with increasing turbulence levels but saturated beyond 24% turbulence intensity. Measurements showed reduction in flame surface density with increasing levels of turbulence intensity.

Resolved flow simulation of pulverized coal particle devolatilization and ignition in air- and O2/CO2-atmospheres

A resolved laminar flow simulation approach is used to investigate the effect of enhanced oxygen levels on single coal particle ignition, comparing the numerical results against experimental data for well-defined conditions (Molina and Shaddix, 2007). Devolatilization is described by a generic boundary condition at the particle surface that accounts for both convective and diffusive phenomena during pyrolysis. The heating rate history of the particle is obtained by solving for intra-particle heat transfer and heat exchange between the particle and its surroundings. The time evolution of volatile release is captured by using the particle mean temperature to calculate the devolatilization rate from a single kinetic rate law with CPD-fitted parameters. The assumed volatile composition includes both light gases and larger hydrocarbons to represent tars. A skeletal kinetic mechanism for pyrolysis and oxidation of hydrocarbon and oxygenated fuels containing 52 species and 452 reactions is used to accurately describe homogeneous chemistry. Particle heat-up, pyrolysis, ignition and envelope flame stabilization are characterized in four gas atmospheres differing in oxygen content and the use of either N2 or CO2 as balance gas. In agreement with the experimental evidence, enhanced oxygen levels shorten ignition delay time τign and result in a higher intensity of the combustion process according to temperature and radical production peaks for all studied mixtures. For the studied oxy-mixtures the presence of CO2 in substitution of N2 delays ignition. The observed behavior is coherent with the different thermo-physical properties of the gas mixtures. The sensitivity of predicted ignition delay to a set of uncertainties is also discussed. It is found that while the absolute values of predicted ignition delay time are functions of potential particle preheating, particle Reynolds number and the chosen criterion to extract ignition delay, the relative trends among the gas mixtures remain in line with the experimental evidence.

Experimental Studies on Burning Characteristics of Methanol, Diesel, and Sunflower Biodiesel Fuels

ABSTRACTThe combustion of liquid fuel droplets provides fundamental information that is relevant to spray combustion. A porous sphere setup has been used to investigate non-premixed combustion experimentally with three different liquid fuels, viz. methanol, diesel, and biodiesel (sunflower oil methyl ester). Experiments have been performed at atmospheric pressure with spheres of different diameters and over a range of free stream air velocities, resulting in different types of flames from envelope flame to wake flame. The measured mass burning rates are compared with the predictions from a theoretical model developed for envelope flames considering mixed convective transport. The model predictions are first validated for methanol and subsequently the model is applied for the other two fuels to evaluate their transfer numbers from the experimental mass burning rate data. A power law variation of nondimensional mass burning rate with effective Reynolds number (with contributions of forced and natural convection) is proposed for each of the fuels irrespective of sphere size and air velocity. The transition air velocities (from envelope to wake flames) for different fuels and the change in mass burning rate on transition are measured.

Experimental study on spontaneous ignition and flame propagation of high-pressure hydrogen release via a tube into air

Spontaneous ignition and subsequent flame propagation of high-pressure hydrogen release via a tube into air are experimentally investigated using pressure records, flame detection and direct high-speed photographs. The study shows that as the burst pressure increases the likelihood of spontaneous ignition increases and the initial ignition is closer to the burst disk. With the increase of tube length, the possibility of spontaneous ignition increases, while the critical release pressure for spontaneous ignition decreases. It is also found that a strong shock wave generated to trigger the ignition and a long tube to promote the growth of the flame are two key factors for the transition from spontaneous ignition inside the tube to jet flame in the air. After the flame exits from the tube, a flame envelope is formed in the front of the hydrogen jet, which gradually splits into upstream and downstream combustion regions. The upstream flame region propagates forward. However, the downstream flame region moves back toward the tube exit. The flame is then stabilized at the tube exit and gradually grows. Noticeable deflagration events were observed to occur successively in the semi-enclosed space. The deflagration leads to a significant increase of pressure in the chamber. And the overpressure of the deflagration is higher than that of the leading shock wave. Both the overpressures of the leading shock wave and the deflagration increase with the release pressure. A stable jet flame is formed outside the tube subsequent to the deflagration. And different jet flame configurations are observed at different controlling mechanisms of flow.

Effect of Particle Concentration on Shape Deformation and Secondary Atomization Characteristics of a Burning Nanotitania Dispersion Droplet

Secondary atomization characteristics of burning bicomponent (ethanol–water) droplets containing titania nanoparticles (NPs) in dilute (0.5% and 1 wt.%) and dense concentrations (5% and 7.5 wt.%) are studied experimentally at atmospheric pressure under normal gravity. It is observed that both types of nanofuel droplets undergo distinct modes of secondary breakup, which are primarily responsible for transporting particles from the droplet domain to the flame zone. For dilute nanosuspensions, disruptive response is characterized by low intensity atomization modes that cause small-scale localized flame distortion. In contrast, the disruption behavior at dense concentrations is governed by high intensity bubble ejections, which result in severe disruption of the flame envelope.

Experimental study on combustion of a methane hydrate sphere

The combustion behavior of a methane hydrate sphere under normal gravity is experimentally investigated. The initial diameter of the sphere is 20 mm. Variation in temperature at the center of the sphere (T c) is measured with a K-type thermocouple at ignition temperatures (T c,i) from 193 to 253 K at 20 K intervals. Variation in the near-surface temperature of the sphere (T s) is measured at ignition temperatures (T s,i) from 233 to 263 K at 10 K intervals. Two combustion phases are observed. When the hydrate is ignited, a stable flame envelope is formed around the sphere (phase 1). In phase 1, the surface of the sphere is dry. After a few seconds, water formed by dissociation of the methane hydrate appears on the surface and methane bubbles are formed by methane ejected from inside the sphere (phase 2), thus destabilizing the flame and causing local extinction. Methane bubbles move down along the surface and merge into a large methane bubble at the bottom of the sphere. This bubble bursts, releasing methane to form a temporary flame, and the water drops from the hydrate sphere. Water on the surface is cooled by the hydrate inside, and an ice shell confines the methane gas that dissociated inside the sphere. Because the dissociation occurs continuously inside the hydrate, the inner pressure gradually increases and at some instant, the ice cracks and methane gas is ejected from the cracks, which results in a micro-explosion with a flame. In phase 1, the surface temperature is below the freezing point of water, and so the surface remains dry and a stable flame envelope is formed; in phase 2, the surface temperature is above the freezing point, and so water appears on the surface. When the temperature at the center of the sphere is lower (193, 213, or 233 K), some methane hydrate remains even after flame extinction because heat transfer from the flame decreases in phase 2 as a result of local extinction. The diameter of the sphere decreases during combustion in accordance with the d-square law, which indicates that the heat of dissociation is supplied by the flame and methane is supplied by the dissociation of methane hydrate in the sphere, as in single-droplet combustion.

Interaction of Flame Flashback Mechanisms in Premixed Hydrogen–Air Swirl Flames

An experimental study is presented on the interaction of flashback originating from flame propagation in the boundary layer (1), from combustion driven vortex breakdown (2) and from low bulk flow velocity (3). In the investigations, an aerodynamically stabilized swirl burner operated with hydrogen–air mixtures at ambient pressure and with air preheat was employed, which previously had been optimized regarding its aerodynamics and its flashback limit. The focus of the present paper is the detailed characterization of the observed flashback phenomena with simultaneous high speed (HS) particle image velocimetry (PIV)/Mie imaging, delivering the velocity field and the propagation of the flame front in the mid plane, in combination with line-of-sight integrated OH*-chemiluminescence detection revealing the flame envelope and with ionization probes which provide quantitative information on the flame motion near the mixing tube wall during flashback. The results are used to improve the operational safety of the system beyond the previously reached limits. This is achieved by tailoring the radial velocity and fuel profiles near the burner exit. With these measures, the resistance against flashback in the center as well as in the near wall region is becoming high enough to make turbulent flame propagation the prevailing flashback mechanism. Even at stoichiometric and preheated conditions this allows safe operation of the burner down to very low velocities of approximately 1/3 of the typical flow velocities in gas turbine burners. In that range, the high turbulent burning velocity of hydrogen approaches the low bulk flow speed and, finally, the flame begins to propagate upstream once turbulent flame propagation becomes faster than the annular core flow. This leads to the conclusions that finally the ultimate limit for the flashback safety was reached with a configuration, which has a swirl number of approximately 0.45 and delivers NOx emissions near the theoretical limit for infinite mixing quality, and that high fuel reactivity does not necessarily rule out large burners with aerodynamic flame stabilization by swirling flows.

Wind-blown pool fire, Part I: Experimental characterization of the thermal field

Experiments were conducted to investigate the macroscopic thermal behaviour of 2m diameter Jet A fires in crosswinds of 3–10m/s. This scenario simulated an outdoor transportation accident with the fire representing a burning pool of aviation fuel. To date, the limited number of experiments that have been conducted to examine wind effects on fire behaviour have generally been performed at small scale, which does not fully simulate the physics of large fires, or in outdoor facilities, with poorly controlled wind conditions. This paper presents the first systematic characterization of the thermal environment in a large, turbulent, pool fire under controlled wind conditions. Three parameters describing flame geometry – flame tilt, flame length and flame drag – were measured using temperature contour plots and video images. The temperature-based method of estimating flame geometry provided considerable improvement over visually based methods, particularly when significant smoke blockage of the luminous flame envelope occurred. At low wind speeds, significant plume curvature was observed, hindering description of flame tilt by a single angle. As the wind speed increased up to 10m/s, extremely high levels of flame tilt, flame drag and flame length occurred.

Soot loading, temperature and size of single coal particle envelope flames in conventional- and oxy-combustion conditions (O2/N2 and O2/CO2)

A fundamental laboratory study on the volatile-phase combustion of pulverized coal was conducted at the particle level. A primary goal has been to simultaneously assess soot volume fraction, fv, and soot temperature, T, in the diffusion flame (soot mantle) forming around a single burning bituminous coal particle, upon ignition of its volatile matter. This assessment was conducted with emission-based pyrometric methods. A secondary goal of this study has been to compare these radiative parameters, fv and T, in conventional air- and in simulated dry oxy-fuel combustion. Both fv, and T measurements were spatially averaged in the flame but temporally resolved throughout the combustion history of single particles. In addition, the size of the volatile envelope flames was assessed both pyrometrically and cinematographically. Combustion of three different bituminous coals took place with various oxygen partial pressures in nitrogen and in carbon dioxide background gases. Single particles, 75–90μm, were injected and burned in a transparent drop-tube furnace (DTF) at laminar-flow atmospheric-pressure and a wall temperature of 1400K. The free-falling bituminous coal particles heated up and devolatilized, their volatile matter ignited and formed bright envelope flames, often with distinctive soot contrails in the wakes of the flames. The radiative parameters fv, T and flame size, of these luminous envelope flames were assessed using different emission-based models for the analysis of the three-color pyrometric intensities. The particle envelope flames of all three coals were found to contain comparable to each other soot volume fractions, in the range of 20–90ppm. At identical furnace gas temperatures and identical O2 mole fractions, when the background N2 gas was replaced with CO2, the particle envelope flames of the bituminous coals were characterized by lower soot volume fractions, lower temperatures and bigger sizes. As the O2 mole fraction increased in either N2 or CO2 background gases, soot volume fractions increased to a maximum and then decreased, temperatures increased monotonically and flame sizes decreased. In CO2-based combustion, an oxygen mole fraction in the neighborhood of 35% was necessary to elevate the measured flame temperature to match that of conventional air-based combustion; however, the soot volume fraction was lower than that in air-based combustion regardless of the oxygen mole fraction.

Flame size and volumetric heat release rate of turbulent buoyant jet diffusion flames in normal- and a sub-atmospheric pressure

This paper investigates the flame size (i.e. envelop surface area and flame volume) and the volumetric heat release rate of turbulent jet diffusion flames, in both normal and in a sub-atmospheric pressure. Experiments on turbulent jet diffusion flames, produced with nozzles of 4, 5, 6 and 8mm in diameter and using propane as fuel, have been carried out at two different altitudes: Hefei, 50m and 100kPa and Lhasa, 3650m and 64kPa. Results have shown both the flame envelope surface area, Af, and the flame volume, Vf, to be much larger in the sub-atmospheric pressure than in the normal pressure (i.e. Af∼p−4/5; Vf∼p−7/5). The flame envelope surface area has been found to scale with the heat release rate, Q̇, by the power of 4/5, Af∼Q̇4/5. The flame volume, Vf, has also been found to scale with the heat release rate by the power of 9/10, Vf∼Q̇9/10. The volumetric heat release rate, Q̇‴, has been found to be a function of both the heat release rate, Q̇, and the ambient pressure, p (Q̇‴∼Q̇0.1;Q̇‴∼p7/5). General non-dimensional correlations for all the present data, obtained for the different nozzle diameters and the two ambient pressures, have also been proposed for the flame envelope surface area and the flame volume, respectively.

Autoignition and combustion characteristics of kerosene droplets with dilute concentrations of aluminum nanoparticles at elevated temperatures

In this experimental study, we investigated the effects of high ambient temperatures and dilute concentrations of nanoparticles (NPs) on the autoignition and combustion characteristics of kerosene-based nanofluid droplets. An isolated kerosene droplet containing 0.1%, 0.5% or 1.0% by weight of aluminum (Al) NPs suspended on a silicon carbide (SiC) fiber was suddenly exposed to an elevated temperature (in range 400–800°C) at atmospheric pressure (0.1MPa) under normal gravity, and the autoignition and combustion characteristics were examined. The ignition delay time, burning rate constant and combustion characteristics of pure and stabilized kerosene droplets were also observed for comparison. The results indicate that, similar to pure kerosene droplets, the ignition delay time of NP-laden kerosene (n-Al/kerosene) droplets also followed the Arrhenius expression and decreased exponentially with increasing temperature. However, the addition of dilute concentrations of Al NPs to kerosene reduced the ignition delay and lowered the minimum ignition temperature to 600°C, at which pure kerosene droplets of the same initial diameter were not ignited. In contrast to the combustion of pure and stabilized kerosene droplets, the combustion of n-Al/kerosene droplets exhibited disruptive behavior characterized by sudden reductions in the droplet diameter without any prior expansions caused by multiple-time bubble formation and their subsequent rupture at or near the droplet’s surface. This bubble pop-up resulted in droplet trembling and fragmentation and ultimately led to enhancement in gasification, vapor accumulation and envelope flame disturbance. The NPs were also brought out of the droplets through these disruptions. Consequently, the burning time and total combustion time of the droplets were reduced, and almost no residue remained on the fiber following combustion. Thus, the combustion rate of n-Al/kerosene droplets was substantially enhanced compared with pure kerosene droplets at all tested temperatures.

Numerical and experimental study of soot formation in laminar diffusion flames burning simulated biogas fuels at elevated pressures

The effects of pressure and composition on the sooting characteristics and flame structure of laminar diffusion flames were investigated. Flames with pure methane and two different methane-based, biogas-like fuels were examined using both experimental and numerical techniques over pressures ranging from 1 to 20atm. The two simulated biogases were mixtures of methane and carbon dioxide with either 20% or 40% carbon dioxide by volume. In all cases, the methane flow rate was held constant at 0.55mg/s to enable a fair comparison of sooting characteristics. Measurements for the soot volume fraction and temperature within the flame envelope were obtained using the spectral soot emission technique. Computations were performed by solving the unmodified and fully-coupled equations governing reactive, compressible flows, which included complex chemistry, detailed radiation heat transfer and soot formation/oxidation. Overall, the numerical simulations correctly predicted many of the observed trends with pressure and fuel composition. For all of the fuels, increasing pressure caused the flames to narrow and soot concentrations to increase while flame height remained unaltered. All fuels exhibited a similar power-law dependence of the maximum carbon conversion on pressure that weakened as pressure was increased. Adding carbon dioxide to the methane fuel stream did not significantly effect the shape of the flame at any pressure; although, dilution decreased the diameter slightly at 1atm. Dilution suppressed soot formation at all pressures considered, and this suppression effect varied linearly with CO2 concentration. The suppression effect was also larger at lower pressures. This observed linear relationship between soot suppression and the amount of CO2 dilution was largely attributed to the effects of dilution on chemical reaction rates, since the predicted maximum magnitudes of soot production and oxidation also varied linearly with dilution.

Updated jet flame radiation modeling with buoyancy corrections

Radiative heat fluxes from small to medium-scale hydrogen jet flames (<10 m) compare favorably to theoretical predictions provided the product species thermal emittance and optical flame thickness are corrected for. However, recent heat flux measurements from two large-scale horizontally orientated hydrogen flames (17.4 and 45.9 m respectively) revealed that current methods underpredicted the flame radiant fraction by 40% or more. Newly developed weighted source flame radiation models have demonstrated substantial improvement in the heat flux predictions, particularly in the near-field, and allow for a sensible way to correct potential ground surface reflective irradiance. These updated methods are still constrained by the fact that the flame is assumed to have a linear trajectory despite buoyancy effects that can result in significant flame deformation. The current paper discusses a method to predict flame centerline trajectories via a one-dimensional flame integral model, which enables optimized placement of source emitters for weighted multi-source heat flux prediction methods. Flame shape prediction from choked releases was evaluated against flame envelope imaging and found to depend heavily on the notional nozzle model formulation used to compute the density weighted effective nozzle diameter. Nonetheless, substantial improvement in the prediction of downstream radiative heat flux values occurred when emitter placement was corrected by the flame integral model, regardless of the notional nozzle model formulation used.

Pulverized coal combustion with opposing/cross-flow methane/air mixtures

Pulverized coal co-firing with opposing premixed methane/air flames in a cross-flow arrangement was found to extend the coal burning limits and reduce its NOx emissions by creating a high temperature NOx-reducing zone between the resulting multiple flame envelopes. By using a primary air stream, the stability limit reached a peak value of 25.1 m/s while the exhaust NOx mole fraction got values as low as 1.2 × 10–4 due to HCN reaction with NOx generated from both the coal primary flame envelope and the premixed flames. The NOx exhaust emissions were reduced to values below 60 ppm as the gas/coal heat input ratio increased to 1.5, where a nonunity Lewis number predicted the peak NOx concentrations across the annular region surrounding the flame. The effectiveness of NOx reduction increased by employing triple flames in place of the primary air since each NOx peak generated from methane combustion was followed by a region of NOx reduction upon consuming a significant amount of the HCN released from coal. Extending the coal flame stability limit to 32.3 m/s contributed to NOx reduction via decreasing the residence time for NOx formation across the combustor exit section. Decreasing the triple flame mixture fraction gradient and keeping a higher overall equivalence ratio reduced the NOx exhaust concentrations to 50 ppm and increased the percent of heat transfer by radiation to reach a peak of 48.8%. Decreasing the particle size to 135 µm and increasing the number of opposing jets to 16 pronounced favorable aerodynamic effects by minimizing the HC and NOx emissions, respectively, to values below 0.16% and 65 ppm. Increasing the opposing jets’ diameter to separation reduced the flame length by 26.7%, while maximizing the staging height kept the NOx emissions below 40 ppm at a strain rate of 2050 s–1.

Combustion of single biomass particles in air and in oxy-fuel conditions

The combustion behaviors of four different pulverized biomasses were evaluated in the laboratory. Single particles of sugarcane bagasse, pine sawdust, torrefied pine sawdust and olive residue were burned in a drop-tube furnace, set at 1400 K, in both air and O2/CO2 atmospheres containing 21, 30, 35, and 50% oxygen mole fractions. High-speed and high-resolution images of single particles were recorded cinematographically and temperature–time histories were obtained pyrometrically. Combustion of these particles took place in two phases. Initially, volatiles evolved and burned in spherical envelope flames of low-luminosity; then, upon extinction of these flames, char residues ignited and burned in brief periods of time. This behavior was shared by all four biomasses of this study, and only small differences among them were evident based on their origin, type and pre-treatment. Volatile flames of biomass particles were much less sooty than those of previously burned coal particles of analogous size and char combustion durations were briefer. Replacing the background N2 gas with CO2, i.e., changing from air to an oxy-fuel atmosphere, at 21% O2 impaired the intensity of combustion; reduced the combustion temperatures and lengthened the burnout times of the biomass particles. Increasing the oxygen mole fraction in CO2 to 28–35% restored the combustion intensity of the single biomass particles to that in air.

A comparative study of the port geometrical effects on sharp corners’ jet triple flames

The performance of sharp corners’ jet triple flames was modulated by compromising the increase in both the number of sharp corners between 3 and 16 and the overall perimeter up to 218% with the reduction in vertex angle to 30°. While the turbulent kinetic energy peak value correlated with the vertex angle and its average value depended on the number of corners, the entrainment rate gain was pronounced in accordance with the perimeter. By having each port stabilizing the inner flame wing such that the sharp corners lay on the diffusion flame envelope at its base, the flow strain due to corners increased the triple flame stability limit to a maximum of 9.2m/s and minimized the NOx emissions to 14ppm. Upon incorporating the swirl impact, the highest firing rate was reached at a jet velocity of 26.3m/s at which excessive entrainment of reactants into corners provided a compact flame with a firing intensity of 50.3MJ/m3. While the flame length was related to the extent of jet deformation from the circular shape, a flame shortening by 67% was verified by an enhancement of 192% over the circular port entrainment rate. Improved combustion efficiencies with respective decrease in CO and HC emissions to 61ppm and 0.04% were reported by maximizing the effect of sharp edged appendices. The swirl introduced a favorable advection transport, whereby the turbulence peaks at corners merged to provide a relative increase of 11.8 times in the average turbulence energy. By effectively combining the active and passive control techniques via providing swirl and modulating the port geometry, the lean operation limit extended by 19% from that of the circular port whereby NOx emissions for triple flames case II had a minimum of 4ppm.

Experimental studies the burning process of gelled unsymmetrical dimethylhydrazine droplets under oxidant convective conditions

Gelled hypergolic propellants offer potential safety and performance improvements over conventional liquid and solid propellants. Understanding the combustion process of single gelled droplets is the first basic step to predict their behavior in the future combustion chambers. Experimental studies were performed to investigate the burning behavior of gelled unsymmetrical dimethylhydrazine (UDMH) droplets under oxidant convective conditions. The effects of oxidant convective conditions including velocity and temperature on the burning behavior were analyzed. The burning process was broken down into four stages: heating and swelling period, initial combustion period, violent combustion period, and stable and extinguished combustion period. Sometimes the droplet inside seemed to be porous or botryoidal. The microexplosion period lasted for a long time sometimes exceeding about 70% of the burning lifetime, and the phenomenon of gas jet combustion due to the burst steam from microexplosions was founded. The conversion of burning flame from a layered and enveloped flame structure to an escaped flame structure with increase in the convective velocity was observed. When the enveloped flame appeared, it was more helpful to the burning process, and the intensity of microexplosions and the burning rate constant were found to increase with the convective temperature. When the escaped flame appeared, it was disadvantageous to the burning process, and the intensity of microexplosions decreases with rise in convective velocity. Compared with the escaped flame, the intensity of microexplosions of the enveloped flame was lower, but the frequency was higher.

Sooting behaviour of n-heptane laminar diffusion flames at high pressures

The effect of pressure on sooting behaviour of n-heptane is studied in co-flow n-heptane/air laminar diffusion flames at pressures above atmospheric in a high pressure combustion chamber. The fuel is diluted with either nitrogen or helium to keep a non-smoking flame at elevated pressures, and the selected fuel mass flow rate of n-heptane provided diffusion flames in which the soot was completely oxidized within the visible flame envelope. The flame stability proved to be a challenge and stable flames were possible only at certain pressures for a sufficiently long duration to permit measurements. The soot volume fractions and temperatures were measured by spectral soot emission as a function of pressure for nitrogen-diluted n-heptane flames at 2, 5 and 7atm. For helium-diluted n-heptane flames, line of sight soot emission data at 3, 4, and 5atm are presented at two heights above the burner exit. Comparison of limited nitrogen-diluted n-heptane data to previous measurements of soot yields indicate that soot formation in diffusion flames of n-heptane seems to be slightly more sensitive to pressure than that in aliphatic gaseous fuel diffusion flames within the pressure range considered in this work.