Shorter Flame Research Articles

F161 低NOx拡散燃焼バーナの開発(一般セッション 燃焼)

This study focuses on fundamental characteristics of DME combustion, aiming at the development of low-NOx multi-ports burner. Multi-ports burner consists of a fuel-port and surrounding multi-air-ports. The strong re-circulation flow is formed by the small air jets, thus the short flames form a cluster on every burner unit so that the thermal NOx generation is significantly suppressed to a very low level. The NOx emission of the DME the newly advanced multi-ports burner was reduced to 40ppm at 0% O_2. With the help of the low-NOx combustion system, referred to as the tube-nested combustion, NOx emission of DME was further reduced to 20ppm at 0% O_2.

CO/UNBURNED HYDROCARBON EMISSIONS OF STRONGLY-PULSED TURBULENT DIFFUSION FLAMES

The CO and unburned hydrocarbon (UHC) emissions of pulsed turbulent diffusion flames were examined by injecting unheated ethylene into a combustor with an air coflow at atmospheric pressure. In all cases the flames were fully modulated (fuel flow fully shut off between injection intervals). Video imaging was performed and time-averaged emissions were measured on the combustor centerline. For short injection times ( ≤ 46 ms), compact, puff-like structures were generated. Longer injection times produced elongated flame structures with flame lengths closer to that of steady flames. The highest emission indices of CO and UHC were found for compact, isolated puffs. The emissions for all flames approached the steady-flame levels for a duty cycle (jet-on fraction) of approximately 0.4. This suggests that there are combinations of injection time and duty cycle for fully modulated, turbulent diffusion flames that produce emissions comparable to the steady flame but with a significantly shorter flame length.

D104 UPDATED LOW NOx COMBUSTION TECHNOLOGIES FOR BOILERS, 2003

Hitachi Group has been dedicating a great effort to research and development of low NOx combustion technologies in response to global environment protection. For pulverized coal combustion, we firstly developed a unique low-NOx burner, HT-NR, based on the theory of “In-Flame NOx reduction”. For this sophisticated and proven design, and our continuous contribution to low-NOx combustion technology, we were awarded the JSME (Japan Society of Mechanical Engineers) Medal as well as the highest prize from Director-General of the Environment Agency. Our further effort has led to the development of the second-generation low-NOx burner, HT-NR2, which enhances NOx decomposition capability. The HR-NR2 burner system has been applied to both domestic and foreign boilers, and has received high reputation from all users. Complying with the increasing need of higher efficiency, easier maintenance, and lower cost, the third generation low-NOx burner, HT-NR3 with more simplified structure, was developed to realize the concept of a wider and shorter flame for extremely low NOx combustion. Its excellent performance was confirmed in an actual plant

Combustion Air Jet Influence on Primary Zone Characteristics for Gas-Turbine Combustors

Results are presented of emissions and lean blowout measurements in a generic combustor primary zone of an aircraft gas-turbine engine for the effects of opposed, circular combustion air jets of a range of sizes, positioned half a dome height downstream from the dome. The primary zone was operated at simulated engine high-power conditions and representative dome and liner pressure drops. Most of the NO x is generated in the jets. Although an optimum jet size is identified, the lowest overall emissions resulted from a no-combustion air-jet configuration; air jets are necessary, however, to confer lean blowout stability and short flame length.

Optimising the Combustion of Low CalorificValue Gases by Utilising Transient Flow Phenomena in Swirl Burners

The aerodynamics caused by the PVC are highly complex and understanding the three dimensional phenomenon it is therefore of paramount importance. The information gained from the novel PIV system contributed substantially to the understanding of the complex flow pattern associated with the PVC. Secondary vortices have been identified reacting as closed loop feedback system to recycle and burn active combustion species. They also provides re-ignition for those combustion species and hence aids the overall flame stabilisation. The 100kW model swirl burner/furnace system was used to carry out measurements under rich operation conditions due to safety consideration of the exhaust system of the full scale rig. The CO emissions under piloted premixed conditions in this case were substantially lower than under non premixed conditions. Under premixed conditions the CO emissions were extraordinary low when compared to non premixed conditions. The NOx emissions confirm the advantages of piloted premixed or premixed operation conditions. NOx were 30% lower in case of piloted premixed and 60% lower in case of premixed operation conditions. The higher exhaust gas temperatures under non premixed and piloted premixed operation conditions compared to the premixed case were reflected in the NOx emissions. Intense turbulence caused by the PVC reduced the formation of thermal NOx. Lower outlet temperatures and lower CO emissions at the same power indicate higher e ciency due to premixed combustion conditions and subsequently utilising the PVC and its associated phenomena. Investigations on the industrial scaled 2MW swirl burner/furnace system were only carried out under lean operation conditions. Non premixed and piloted premixed fuel entry modes were compared. Under non premixed conditions a long flame extending down to the furnace exit occurred. Piloted premixed conditions required an axial pilot flame to ensure flame stability for fuel simulated calorific values less than 1.5MJ/m3. All the results clearly show that operating swirl burner/furnace systems under piloted premixed or premixed (where possible due to flame stability limits) conditions has clear advantages over the axial fuel entry mode. The resulting short and intense flame provides more e cient combustion and wider flame stability limits (under very lean operation conditions).

Flame patterns of acoustic wave and flame interaction in a cylindrical tube

In this short paper, flame and acoustic wave interactions under laboratory conditions have been reported. The test rig consists of a cylindrical Perspex tube and a fuel tube (burner) positioned along the central axis of the tube. The acoustic characteristics of the rig are measured. The flame instability with and without acoustic excitation has been investigated. A high shutter speed colour camera has been applied to capture many interesting unstable flame patterns. It has been found that strong flame instability only occurs at particular frequencies. The position of the burner inside the cylindrical tube also has an effect on flame instability.

Structure and Flame Length of Fully-Modulated, Turbulent Diffusion Flames

The turbulent flame structure and flame length of fully-modulated diffusion flames was examined over a range of pulsing frequencies, injection flow rates, and duty-cycles. An injection system employing an electronically-controlled solenoid value was used to discharge puffs of unhealed natural gas and ethylene fuel into still air at one atmosphere pressure. Video imaging of the luminosity from the sooting regions of the flame revealed two distinct types of flame structure. For small injected volumes and short injection times, compact, puff-like structures with a short flame length were generated. More elongated “cigar-shaped” structures, with a longer flame length closer to that of steady-state flames, resulted from longer injection times and larger injected volumes. An injection parameter which characterizes the transition from puff-like to cigar-shaped flame behavior is presented. For puffs, an increase in duty-cycle generally led to an increase in flame length. This increase was less for cigar-shaped flames. The downstream location of the puff-like flame structures increased roughly with time to the 1/2 power, in agreement with buoyant thermals and starting jets.

Synthesis of fullerenes and fullerenic nanostructures in a low-pressure benzene/oxygen diffusion flame

Samples of condensable material from laminar low-pressure benzene/argon/oxygen diffusion flames were collected and analyzed by high-performance liquid chromatography to determine the yields of fullerenes and by high-resolution transmission electron microscopy (HRTEM) to characterize the fullerenic material (i.e., curved-layer nanostructures) on and within the soot particles. The highest concentration of fullerenes was always detected just above the visible stoichiometric surface of a flame. The percentage of fullerenes in the condensable material increases with decreasing pressure. The overall highest amount of fullerenes was found for a surprisingly high dilution of fuel with argon. A comparison of the flames with the same cold gas velocity of fuel and oxygen showed a strong dependence of fullerene content on flame length. A shorter flame, resulting from higher dilution or lower pressure, favors the formation of fullerenes rather than soot, and the amount of soot and precursors of both soot and fullerenes is less at lower pressure and higher dilution. This behavior indicates a stronger correlation of fullerene consumption to the total amount of soot than of fullerene formation to precursor concentration. The maximum flame temperature seems to be of minor importance in fullerence formation. The HRTEM analysis of the soot showed an increase of the curvature of the carbon layers, and hence increased fullerenic character, with increasing distance from the burner up to the point of maximum fullerence concentration. After this maximum, where soot and fullerences are consumed by oxidation, the curvature decreases. In addition to the soot, the samples included fullerenic nanostructures such as tubes and spheroids including highly ordered multilayered or onionlike structures. The soot itself shows highly ordered regions that appear to have been cells of ongoing fullerenic nanostructure formation.

Experimental Studies on Combustion and Heat Transfer Characteristics in a Furnace.

Two types of fuel nozzle are used in a cylindrical furnace which is 1 m in inner diameter and 4.8 m in height. The one is axial injection type with single hole (nozzle 1) where fuel and air flow in the same direction coaxially upward. The other is radial injection type with multi holes (nozzle 2) where fuel and air flow in the crosswise direction. Two types of fuel nozzle form extremely different types of turbulent diffusion flames. Nozzle 1 forms a long flame, while nozzle 2 forms a short flame with recirculating zone at the center of the flame just above the nozzle tip. Measurements of profiles of temperature, species concentration, together with the heat flux to the furnace wall are carried out in a turbulent diffusion flame of propane. Their interconnections with combustion and heat transfer characteristics are investigated and discussed under various furnace operating condition of air swirling, inlet air temperature, recirculating exhaust gas, air ratio, fuel flow rate and type of nozzle.

Aspects of oscillating flames

Photography and chemieluminescence from CH radicals have been used to identify the reaction zones and quantify the areas and shapes of kerosene-fuelled flames with swirl numbers of 0.7 and 0.8 and an overall equivalence ratio of 0.25. The air flow was oscillated at a frequency of 350 Hz and the results suggest that the oscillations caused a sequence of vortex rings at the burner exit and that these distorted the reaction zone and increased its area in the near burner region leading to an overall shorter flame. For the swirl number of 0.7, the flame was lifted and the oscillations led to an increase in the average lift off length whereas the higher swirl number caused an attached flame with and without oscillations. The stretch rate, evaluated from the variation of the flame area in time, was higher for the lifted flame suggesting that lift off was caused by local extinction.

Studies of Lean Blowout in a Step Swirl Combustor

The design requirements of a modern gas turbine combustor are increasingly dictated by wide stability limits, short flame length, and uniform mixing. To achieve the best trade-off between these three factors, flame characteristics (length, shape, mixedness), lean blowout (LBO), and optimum combustor configuration should be investigated over a wide range of inner and outer air velocities, inner and outer vane angles, and co- versus counterswirl arrangements. Such an investigation was performed in a step swirl combustor (SSC) designed to simulate the fuel–air mixing pattern in a gas turbine combustor dome fitted with an airblast atomizer. It was found that an increase in the outer vane angle and a decrease in inner air velocity decreased the flame length. LBO was improved when outer flow swirl intensity was increased. An optimum hardware and velocity configuration for the SSC was found for inner swirl = 45 deg, outer swirl = 60 deg, coswirl direction, and inner air velocity = outer air velocity = 16 m/s. This optimum SSC configuration yielded: (i) low values of LBO, (ii) short flame length, (iii) uniformly mixed stable flame, and (iv) little or no variation in these characteristics over the range of operation of SSC. Finally, the co- versus counterswirl arrangements and the operation of the optimized combustor configuration are discussed.

Solid Propellant Flame Structure

AbstractPlanar Laser Induced Fluorescence (PLIF), UV/Vis Absorption, and thermocouple measurements were done for HNF, RDX, HMX, and XM39 deflagration with and without CO2 laser-support. RDX and especially HNF have very short self-deflagration flame length scales. HMX and XM39 have taller self-deflagration flames. XM39 has a marked dark zone with plateau temperature about 1400K. RDX's dark zone, present under laser supported deflagration, collapses when the external laser flux is removed. PLIF was used to measure the 2D NH, OH, and CN species profiles for these materials and OH temperature profile for RDX and HNF under non-laser supported conditions. The best spatial resolution for the RDX PLIF was about 4μm. Sandwiches of HNF and various binders were studied with PLIF and while obvious diffusion flames were present at low pressure, they are weak and are not expected to be burn rate controlling.

On the Physics of Jet Diffusion Flames

Abstract A physical model for the laminar jet diffusion flame including chemical kinetics effects is provided. We produce the appropriate set of dimensionless groups needed to discuss the role of buoyancy and jet outlet velocity U o on such flames. We are particularly interested in flame lengths the behaviour of the flame is shown to split in two regimes. The regime of short flames is buoyancy controlled and the flame length scales with the intensity of the gravity g as L ∼ g−1/31’3 in the reaction sheet limit. The regime of long flames is governed by the outlet velocity as L ∼ (U o. The predictions compare well with published experimental data and the extension of our model to the candle flame problem reveals that the flux of consumpiion of the candle is decreasing with increasing gravity. A possible mechanism leading to this result is proposed.

High-Intensity Burners with Low Nox Emissions

Experimental combustion and NOx emissions results are summarized for a range of jet shear layer combustion systems that have rapid fuel and air mixing, short intense flames, a high turn-down ratio and low NOx characteristics. Two burner sizes of 76 and 140 mm are investigated for propane and natural gas. Three jet shear layer burners are compared with axial and radial swirlers. The combustion techniques were developed for application to low NOx combustion systems for industrial gas turbines, where NOx emissions as low as 10 ppm at 15 per cent oxygen have been demonstrated. It is shown that at one bar pressure, gas turbine combustors and high-intensity burners operate at similar air flow, blockage and pressure loss conditions.

Prediction and assessment of flammability hazards associated with metered-dose inhalers containing flammable propellants.

Several potential replacements for chlorofluorocarbons (CFCs) in metered-dose inhalers (MDIs) are flammable. The flammability hazard associated with their use was assessed using a range of MDIs containing 0-100% (w/w) n-butane (flammable) in HFC-134a (non-flammable) fitted with either 25-, 63-, or 100-microliters metering valves or continuous valves. In flame projection tests each MDI was fired horizontally into a flame, and the ignited flume length emitted from the MDI was measured. Flame projections of greater than or equal to 60 cm were produced by all formulations fitted with continuous valves which contained greater than or equal to 40% (w/w) n-butane in HFC-134a. Using metering valves the maximum flame projection obtained was 30 cm. This was observed with a formulation containing 90% (w/w) n-butane in HFC-134a and a 100-microliters valve. For a particular formulation, smaller metering valves produced shorter flame projections. Because many MDIs are used in conjunction with extension devices, the likelihood of accidental propellant vapor ignition was determined in Nebuhaler and Inspirease reservoirs and a Breathancer spacer. Ignition was predictable based on propellant composition, metered volume, number of actuations, and spacer capacity. Calculated n-butane concentrations in excess of the lower flammability limit [LFL; 1.9% (v/v)] but below the upper flammability limit [UFL; 8.5% (v/v)] were usually predictive of flammability following ignition by a glowing nichrome wire mounted inside the extension device. No ignition was predicted or observed following one or two 25-microliters actuations of 100% n-butane into large volume Nebuhaler (750 ml) or Inspirease (660 ml) devices.(ABSTRACT TRUNCATED AT 250 WORDS)

Coal burners for the entrained flow gasifier.

Gasification characteristics were studied in a single-burner gasifier and a four-burner gasifier to determine the upper and lower burner structures for two-stage circling gasifiers. In order to achieve high efficiencies under stable gasification conditions, it is necessary to obtain long residence times of reactive chars injected from the upper burners and to obtain short flames for the lower burners. It was found that an internal mixing-type burner could achieve a better coal/oxygen mixture than an external type, and favorable mixing was easily attained by adding swirling action to both burner types. The flame length of the internal mixing burner with swirl was shorter than that of the other burner without swirl. Carbon conversion of the internal mixing burners with strong angular momentum was higher than that of the other type of burner.

Combustion characteristics of hydrogen in a test swirl combustor for a micro gas turbine.

The combustion characteristics of hydrogen in a test swirl combustor for a micro gas turbine have been studied by varying the following parameters; the swirl number of a vane-type swirler, the throat diameter of an air nozzle and the mean air jet velocity through the nozzle throat. It was found that, with high degree of swirl, a stable hydrogen flame attached to a hydrogen injector were formed even in high velocity air streams because of the formation of a reverse flow zone in the downstream region of the injector. The configurations of the stable flames and the temperature distributions in the combustor were scarcely sensitive to the swirl number and the air jet velocity, but were sensitive to the air nozzle throat diameter. At small throat diameters of the nozzle, diffusion type short hydrogen flames were formed and the temperature distributions in the combustor became uniform just downstream of the flames.

Visible structure of buoyant diffusion flames

Natural gas diffusion flames stabilized on 0.10, 0.19 and 0.50 m. diameter porous bed burners have been studied for heat release rates ranging from 10 to 200 kW. Flame heights were measured from video tape recordings and by eye averaged techniques. The dependence of flame height on a dimensionless heat addition parameter shows a transition for values of the parameter around unity. For flames taller than three burner diameters, the initial diameter of the fire does not affect the length of the flame whereas for short flames, initial geometry becomes important. Another prominent feature of these flames is the presence of large scale axisymmetric structures which are formed close to the burner surface with more or less regular frequency and which rise through the flame region. These structures are responsible for the fluctuations of the flame top and strongly influence the geometry of the flame.

Variation of the length of gaseous fuel diffusion flames in an accompanying air flow

These authors study the effect of the relative area of the edges of the fuel nozzles or pylons on the variation of the flame length in an accompanying oxidizer flow, and seek to establish the conditions of reliable flame propagation to all the fuel jets. A diagram illustrates the experimental set-up and charts present the data gathered. Analysis of these data leads the authors to conclude: that as the relative area of the edges is increased, raising relative accompanying air velocity leads to an intensified mixing and a shorter flame; that the effect of relative area of the edges on flame length for various fuels can be represented by a single curve; and that these results are valid for both individual jets and groups of jets provided they are characterized by autonomous mixing or total fusion.

Heat transfer from flames in a rotary kiln

Heat flow in the flame zone of a direct-fired rotary kiln has been modeled mathematically. The flame has been assumed to be cylindrical in shape, backmixed radially, and moving axially in plug flow. The length of the flame and the rate of entrainment of secondary air have been characterized by empirical equations reported in the literature. It has been shown that the axial component of radiation can be reasonably neglected since it is relatively small compared to the radial component. The resulting one-dimensional model is capable of predicting the axial temperature profiles of the flame and wall and the axial profiles of heat flux to the solids bed and refractory wall. The model has been employed to study the influence on heat flow to the bed of the following variables: fuel type (fuel oil, natural gas, producer gas), firing rate, temperature of secondary air, pct primary air, and oxygen enrichment. Of the three fuels, combustion of fuel oil gives the longest flame and the greatest heat input to the solids in the flame zone. Raising the secondary-air temperature increases the flame length significantly but has a small effect on the maximum flame temperature and heat flux to the solids. Increasing percent primary air decreases the flame length and increases the peak values of flame temperature and solids heat flux but reduces the quantity of heat received by the solids in the flame zone. Oxygen enrichment results in a shorter flame, higher maximum flame temperature, and increase in the heat transferred to the solids in the flame zone.