Lower Flame Temperature Research Articles

INTERACTION BETWEEN COUNTERFLOW FLAMES OVER BINARY TSUJI BURNERS ARRANGED IN SIDE-BY-SIDE

ABSTRACT This numerical study modifies Tsa and Chen's preliminary combustion model (2003) by using a multi-block grid to investigate further the interaction of flames over binary Tsuji burners. The effects of inter-cylinder spacing (L) and inflow velocity (Uin) are investigated. A wider inter-cylinder spacing generally corresponds to lower flame transition velocity, associated with the transformation of the envelope flame into the wake flame. However, the combustion efficiency increases with L. The twin envelope diffusion flames merge into a larger envelope diffusion flame completely when L is equal to or less than 1.5D. Only one vortex is present behind each burner when L = 1.5D or 2D. However, no vortex is present when L = 1.2D. When L is equal to or greater than 3.5D, no interference occurs between the two flames. The mechanism of control of the interaction between twin counterflow diffusion flames involves oxygen deficiency between the dual flames. In the case of varying Uin at fixed L = 3D, the dual envelope diffusion flames transform into dual wake flames as Uin increases to 0.79 m/sec. The dual wake flames are extinguished as Uin increases further to 1.96 m/sec. Increasing the inflow velocity can enhance the interaction between dual envelope flames. A larger inflow velocity yields a lower flame temperature due to the flame stretch effect. Three vortices are present behind each cylinder when the flames are at the near-extinction velocity. For a fixed inter-cylinder spacing, the dual flames tend to attract each other normally. However, the dual flames repel each other as the inflow velocity increases to the near-extinction limit.

The effect of oxygen and carbon dioxide concentration on soot formation in non-premixed flames

The influence of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation was studied by Time Resolved Laser Induced Incandescence (TIRE-LII) and TEM photography in non-premixed co-flowing flames. TIRE-LII method was used to measure the distribution of two-dimensional soot volume fraction and primary particle size. The soot was directly sampled by the thermophoretic method, and its diameter was examined by TEM photography. Two suitable delay times of the TIRE-LII method affecting measurable range and sensitivity were determined by comparing TEM photographs with the TIRE-LII signal. The effects of oxygen concentration and carbon dioxide as diluents in the oxidizer side on soot formation were investigated with these calibrated techniques. An O 2+(CO 2, N 2, and [Ar+CO 2]) mixtures in co-flow were used to isolate carbon dioxide effects systematically. The primary particle number concentration and soot volume fraction were abruptly decreased by the addition of carbon dioxide to co-flow. This suppression was resulted from the short residence time in inception region because of the late nucleation and the decrease of surface growth distance by the low flame temperature due to the higher thermal capacity and the chemical change of carbon dioxide. The increase of oxygen concentration in the co-flow caused an enhancement of soot nucleation and thus the residence time increase, but the specific growth rate showed almost the same value regardless of the co-flow mixture in the growth region. This result suggests that the specific growth rate has a weak dependence on the relative change of co-flow conditions in non-premixed co-flowing flames.

Numerical study for the combustion characteristics of Orimulsion fuel in a small-scale combustor

A numerical and experimental study has been made in order to figure out the combustion characteristics of Orimulsion fuel for the possible substitution of traditional fuels in existing boilers. To this end, a comprehensive computer model has been developed using Patankar’s SIMPLE algorithm for the complex process associated with the turbulent reaction of this fuel. The program includes several phenomenological models such as behavior of fuel droplet, multi-phase reaction and formation of pollutant species such as NO and SO 2 in radiation-participating, turbulent flows. The calculation results of species and temperature profiles have been successfully compared with the experimental data measured in a small-scale combustor. Comparing the combustion characteristics of Orimulsion with that of heavy fuel oil, one of the noticeable features of Orimulsion combustion is relatively low flame temperature together with the delayed appearance of the peak flame. This is considered to be attributed to the effect of moisture content and the increased flow rate of Orimulsion fuel on the basis of an equivalent calorific value. Further this feature of Orimulsion flame observed in calculation and experiment are consistent with other experimental findings reported in open literature. It is found that the flame characteristics of Orimulsion fuel can be changed to some extent by the variation of operation conditions such as the injection velocity of Orimulsion fuel and the swirl strength of air stream. Further, other parametric study has been made in terms of the type of atomizing fluid, equivalence ratio, radiation model and grid size. In general the results are physically acceptable and consistent. Therefore the computer program developed is considered as a viable tool for the advanced design and determination of optimal operating condition for the large scale of Orimulsion combustor.

Particle size distribution function of incipient soot in laminar premixed ethylene flames: effect of flame temperature

Particle size distribution functions (PSDFs) of incipient soot formed in laminar premixed 24.2% ethylene–37.9% oxygen-diluent (nitrogen and/or argon) flames with an equivalence ratio of 1.92 were studied by online sampling and scanning mobility particle sizer. Two series of flames were studied to quantify the effect of flame temperature on the characteristics of PSDFs. In the first series, the variation of the flame temperature was accomplished by varying the cold gas velocity. Temperature in the second series of flames was manipulated by the diluent composition from argon to nitrogen. The results show that for flames with the maximum temperature ( T max) around 1800 K the soot PSDFs were distinctively bimodal. As the flame temperature was increased to ∼1850 K, bimodality faded away. The distribution was unimodal for T max > 1900 K. The variation of the characteristics of the PSDF as a function of the flame temperature is consistent with the theoretical explanation that bimodality is the result of competition between persistent particle nucleation and particle–particle coagulation in low-temperature flames.

Low flame temperature limits for mixing-controlled Diesel combustion

The low flame temperature limits for mixing-controlled Diesel combustion were investigated in a constant-volume combustion chamber at well-defined ambient conditions. Flame temperatures were controlled by varying ambient oxygen concentration or by using fuel-lean mixing-controlled combustion. Pressure rise measurements show that combustion efficiency remains high for flame temperatures as low as 1500–1600 K for conditions where the ambient gas temperature was greater than 1000 K. This low flame temperature limit is less than those of propagating flame processes in engines but close to that of HCCI combustion. Chemiluminescence imaging shows that a cool flame exists prior to the quasi-steady lift-off length, suggesting that ignition processes are continuously occurring within the Diesel fuel jet as air and fuel mix upstream of the high-temperature reaction zone at the lift-off length.

05/00382 Regulatory and voluntary approaches for enhancing building energy efficiency: Lee, W. L. and Yik, F. W. H. Progress in Energy and Combustion Science, 2004, 30, (5), 477–499

Heat input, thermal efficiency, and emissions behaviour are the key performance characteristics of domestic gas appliances, which considerably depend on altitude. Because of the low atmospheric pressure and oxygen contents on the Tibetan Plateau, atmospheric induction stoves designed for plain areas tend to exhibit poor combustion performances when operated on a plateau, where poor energy utilization and indoor-air pollution are specific issues. In this study, a comparative study was conducted in a plateau and plain region of China to investigate the influence of altitude on a natural gas stove’s combustion characteristics. The experimental results indicated that: (1) enhancing the gas supply pressure can eliminate the effect of altitude on the heat input; (2) there is a larger heat loss and higher thermal efficiency during incomplete combustion in Lhasa; and (3) oxygen-lacking conditions promote the formation of carbon monoxide and the lower flame temperatures reduce the emission of nitrogen oxides.

1124 平たんバーナ低温度炎におけるベンゼン添加の影響(GS-6,7 噴流火炎の混合と燃焼生成物の特性(1),研究発表講演)

The effects of benzene addition on PAHs formation are studied in a stabilized laminar low-temperature flame under an atmospheric pressure, to understand the processes of soot formation in the internal combustion engines. The burner was operated with a diethyl-ether/air mixture at an equivalence ratio of 3.0, containing 1.6-ppm benzene. Single-ring aromatics and PAH ranging from two- to four-fused aromatic rings were identified by microprobe sampling and a liquid chromatograph with a fluorescence detector. Both the cool- and blue-flame appearance were advanced with the benzene addition. The benzene addition suppressed the growth of PAH, contrary to the more production of single-ring aromatic hydrocarbons, such as, phenylacetylene and styrene. The phenol formation was detected at the time when a cool flame appeared. It is presumed that the benzene-origin HO_2 and OH formed during the low-temperature oxidation have enhanced the cool-flame reaction, even if the amount of benzene addition was not high.

Mathematical modelling of NO emissions from high-temperature air combustion with nitrous oxide mechanism

A study of the mathematical modelling of NO formation and emissions in a gas-fired regenerative furnace with high-preheated air was performed. The model of NO formation via N 2O-intermediate mechanism was proposed because of the lower flame temperature in this case. The reaction rates of this new model were calculated basing on the eddy-dissipation-concept. This model accompanied with thermal-NO, prompt-NO and NO reburning models were used to predict NO emissions and formations. The sensitivity of the furnace temperature and the oxygen availability on NO generation rate has been investigated. The predicted results were compared with experimental values. The results show that NO emission formed by N 2O-intermediate mechanism is of outstanding importance during the high-temperature air combustion (HiTAC) condition. Furthermore, it shows that NO models with N 2O-route model can give more reasonable profile of NO formation. Additionally, increasing excess air ratio leads to increasing of NO emission in the regenerative furnace.

Numerical and experimental study on silica generating counterflow diffusion flames

Temperatures and concentrations of OH radicals in silica generating counterflow oxy-hydrogen diffusion flames are measured using a broadband coherent anti-Stokes Raman spectroscopy (CARS) and a planar laser induced fluorescence (PLIF) techniques to study thermo-chemical effects of SiCl 4 addition to flames. Numerical analysis considering detailed chemical reactions including silica generating reactions is also conducted. The experimental results demonstrate that temperatures decrease in preheated zone due to the increase in specific heat of the gas mixture while the decrease is mitigated in particle formation zone due to the heat release through hydrolysis and oxidation reactions of SiCl 4. Also, OH concentrations significantly decrease in silica formation flame, which can be attributed to the consumption of oxidative radicals during the silica generating reactions of SiCl 4 and depletion of OH by HCl. The numerical simulation agrees well for flames having relatively low flame temperatures of 1750 K but underestimates the decrease in OH concentration for high temperature flame over 2700 K. The disagreement for the high temperature flames would imply possible OH consumption via direct reactions between OH radicals and silicon chlorides, which is expected to be highly sensitive to temperature.

Experimental investigation of combustion in porous heating burners

The combustion characteristics of liquefied petroleum gas inside porous heating burners have been investigated experimentally under steady-state and transient conditions. Cooling tubes were embedded in the postflame region of the packed bed of a porous heating burner. The flame speed, temperature profile, and [NO x ] and [CO] in the product gases were monitored during an experiment. Due to the heat removal by the cooling tubes, a phenomenon termed metastable combustion was observed; this is that only one flame speed exists at a particular equivalence ratio for maintaining stable combustion within the porous bed of the porous heating burner. This behavior is quite different from that of porous burners without cooling tubes, in which an extended range of flame speeds usually is found for maintaining stable combustion. After metastable combustion has been established in a porous heating burner, a change in the equivalence ratio will stop the metastable combustion and drive the flame out of the packed bed. From the steady-state results, the porous heating burner was shown to maintain stable combustion under fuel-lean conditions with an equivalence ratio lower than the flammability limit of a normal free-burning system. The flame speed in a porous heating burner was found to decrease with an increase in the length of the porous bed. Combustion within a porous heating burner has the features of low flame temperature, extended reaction zone, high preheating temperature and low emissions of NO x and CO. The flame temperature ranged from 1050 to 1250 °C, which is ∼ 200 °C lower than the adiabatic flame temperature at the corresponding equivalence ratio. The length of the reaction zone could be more than 70 mm and the preheating temperature ranged from 950 to 1000 °C. Both [NO x ] and [CO] were low, typically below 10 ppm.

고온공기를 이용한 제트확산화염의 연소특성에 관한 실험

For the development of high efficiency and low emission combustion systems, high temperature air combustion technology has been tested by utilizing preheated air over 1100 K and exhaust gas recirculation. In this system, combustion air is diluted with large amount of recirculated exhaust gases, such that the oxygen concentration is relatively low in the reaction zone, leading to low flame temperature. Since, the temperature fluctuations and sound emissions from the flame are small and flame luminosity is low, the combustion mode is expected to be flameless or mild combustion. Experiment was performed to investigate the turbulent flame structure and NO emission characteristics in the high temperature air combustion focused on coflowing jet diffusion flames which has a fundamental structure of many practical combustion systems. The effect of turbulence has also been evaluated by installing perforated plate in the oxidizer inlet nozzle. LPG was used as a fuel. Results showed that even though NO emission is sensitive to the combustion air temperature, the present high temperature air combustion system produce low NO emission because it is operated in low oxygen concentration condition by the high exhaust gas recirculation.

平たんバーナ低温度炎におけるすす前駆物質の生成(燃焼反応とPM問題,内燃機関の燃焼化学と排出物制御)

The formation of polycyclic aromatic hydrocarbons (PAHs) and ion has been studied on the laminar, diethyl-ether/air premixed low-temperature flames under atmospheric pressure and the equivalence ratio ranging from 3.0 to 6.0, to obtain an advanced knowledge of soot formation processes. A flat-burner is enclosed by a concentric quatz tube to prevent infiltration of surrounding air. Ion formation is recognized to be related to soot precursor formation. The ion current has been measured using an ionization gap. A liquid chromatograph with a fluorescence detector was applied to separate and determine PAHs contained in specimens sampled from low-temperature flames. PAHs was obtained by microwave extraction of soluble organic fraction (SOF) on silica glass filters. It is indicated a possibility that the ion is generated from the exited oxygen molecules and acetylene generation due to the interrelation between ion current and light emission. It has also been found that the PAHs first appears during the cool flame period.

低温度希薄水素予混合火炎の火炎構造(エンジンと新燃料(2),エンジンと新燃料)

In this study, combustion characteristics and flame structures of low temperature flames of lean hydrogen mixtures were investigated by chemical kinetics computations. Mixtures having nearly the same laminar burning velocity were prepared by adding nitrogen to hydrogen-oxygen mixtures where the equivalence ratio was varied. The initial pressure was varied from 1 to 4 atm. As a result, it was clarified that low temperature reactions (R05-R06-R04) were dominative in these flames regardless of the initial pressure, however, there were differences in flame structures and combustion characteristics depending on the equivalence ratio.

Triaminoguanidine Dinitramide, TAGDN: Synthesis and Characterization

AbstractTwo dinitramide salts with very high heat of formation have been identified, i.e. triaminoguanidine dinitramide (TAGDN) and guanylazide dinitramide (GADN). Thermochemical calculations showed that TAGDN‐propellants have a higher impetus than the corresponding GADN‐propellants at a given flame temperature. Due to the fact that a low flame temperature has to be prefered in a gun, TAGDN seems better suited as a component in gun propellants compared to GADN. TAGDN was thus synthesized, analyzed and characterized to determine some of its properties.

Studies on Different Types of Nitrocellulose in Triple Base Gun Propellant Formulations

Currently triple base propellants are used for tank gun ammunition. Nitroguanidine (NQ) propellant is the most promising among them due to various advantages like low flame temperature, flashlessness, low barrel erosion, long shelf life, etc. Therefore, it will continue to be used in the field of gun propulsion systems for several years to come. However, there is scope for enhancing the performance of triple base propellant with respect to energy level and mechanical properties. Nitrocellulose (NC) is the energetic binder cum fuel used in the triple base composition propellant and constitutes a sizeable percentage of the compound. Hence, one of the promising triple base compositions was selected, and a systematic study was carried out by using different types of NC (having varying percentages of nitrogen content) to study the variation in mechanical properties, energy content, linear burning rate coefficient, and pressure exponent. The results are discussed in this paper.

Emulsion Combustion and Flame Spray Synthesis of Zinc Oxide/Silica Particles

Two flame spray methods, emulsion combustion method (ECM) and flame spray pyrolysis (FSP), were compared for synthesis of pure and mixed SiO2 and ZnO nanoparticles. The effect of silicon precursor was investigated using liquid hexamethyldisiloxane (HMDSO) or SiO2 sol, while for ZnO zinc acetate (ZA) was used. Gas phase reaction took place when using HMDSO as Si precursor, forming nanoparticles, whereas the SiO2 sol used as Si source was not evaporated in the flame, creating large aggregates of the sol particles (e.g. 1 μm). The FSP of ZA produced ZnO homogeneous nanoparticles. Lower flame temperatures in ECM than in FSP resulted in mixed gas and liquid phase reaction, forming ZnO particles with inhomogeneous sizes. The FSP of HMDSO and ZA led to intimate gas-phase mixing of Zn and Si, suppressing each other's particle growth, forming nanoparticles of 19 nm in BET-equivalent average primary particle diameter. Nucleation of ZnO and SiO2 occurred independently by ECM of HMDSO and ZA as well as by FSP of the SiO2 sol and ZA, creating a ZnO and SiO2 mixture. The reaction of ZnO with SiO2 was likely to be enhanced by ECM of the SiO2 sol and ZA where both Zn and Si species were not evaporated completely, resulting in ZnO, β-willemite and Zn1.7SiO4 mixed phase.

Formation of ZnO, MgO and NiO Nanoparticles from Aqueous Droplets in Flame Reactor

Nanoparticles of ZnO, MgO and NiO were produced from droplets of aqueous salt solution in the flame spray pyrolysis reactor. Conventional spray pyrolysis, in which electrical furnace reactor is used, is reported to produce nanoparticles only from acetate precursor. If the reactor pressure is low (∼60torr), nitrate salt precursor is also known to produce nanoparticles. In this paper, we report that nanoparticles are produced from nitrate as well as acetate salt precursor solution when propane–oxygen diffusion flame is used to decompose aqueous aerosol droplets. At low flame temperature, however, nanoparticles are not formed and the particle morphology is similar to the morphology produced by the conventional spray pyrolysis. At high flame temperature, nanoparticles are formed, regardless of the salt type. Nanoparticles are formed at lower flame temperature from acetate salts than from nitrate salts. All nanoparticle prepared in this work were fully crystallized and the size measured from transmission electron microscopy images was 30nm. This size agreed well with the particle size calculated from X-ray diffraction and specific surface area data.

An experimental analysis of low-temperature and premixed combustion for simultaneous reduction of NOx and particulate emissions in direct injection diesel engines

A new combustion concept, named MK (modulated kinetics) combustion, has been developed, which reduces NOx and smoke simultaneously through high exhaust gas recirculation (EGR) and retarded injection timing. High-speed photography was employed to investigate the physical and chemical processes of MK combustion, and the results revealed that the combustion features premixed combustion and the low-temperature flames were accompanied by transparent appearances. Heat flux measurements and KIVA calculations were also made to investigate the effects of swirl, which serves to improve thermal efficiency in MK combustion. It was apparent that the swirl effectively governs the fuel distribution in the combustion chamber, suppressing HC formation and improving thermal efficiency by preventing the flames from contacting the cavity walls. Throughout these experiments, ignition delay and fuel injection duration were found to be the two key parameters that control MK combustion. Accordingly, ignition delay was prolonged by cooled EGR and fuel injection duration was shortened by high injection pressure to allow the MK combustion operation in a high load range.

Ignition timing control of homogeneous charge compression ignition engines by pulsed flame jets

In diesel engines, most of the fuel is burned in a diffusion combustion phase, which inherently leads to the formation of excessive amounts of soot and NOx emissions. In order to decrease soot and NOx emissions simultaneously, the concept of a lean homogeneous charge compression ignition engine has been proposed. The onset of the combustion of the engine depends on the autoignition of the fuel, so it is quite difficult to control the ignition timing. On the other hand, it has been revealed that Pulsed Flame Jet (PFJ) has a great potential to enhance ignition reliability and burning rate in lean mixtures. In this article, autoignition characteristics of n -butane/air mixtures in a rapid compression machine (RCM) were shown first. The appearance of low temperature flames was observed in autoignition of n -butane in the RCM used here, and it was realized that the final compression conditions in the RCM correspond to the upper end of the low-temperature range of the positive temperature dependence region of ignition delay. Then the combustion tests with PFJ were carried out and it was demonstrated that the onset of combustion can be controlled by PFJ, and it was revealed that PFJ has a potential for the ignition timing control of the homogeneous charge compression ignition engine.

Dynamics and flammability limit of stretched premixed flames stabilized by a hot wall

Dynamics and flammability limit of stretched, ultralean methane/air premixed flames stabilized by a hot wall were investigated theoretically and numerically using the large activation energy asymptotic approach and the detailed chemistry approach, respectively. An analytical expression describing the flame temperature and location was obtained. Several new findings on the flame regimes, bifurcation, and flammability limit are demonstrated. The analytical results showed that the appearance of hot wall results in new flame bifurcations. Except for the high-temperature flame, there exists a hot-wall-stabilized low-temperature flame which dramatically extends the flammable region of stretched flame to low stretch. Sublimit flame can be stabilized by a hot wall and has a non-zero flame speed even with a wall temperature less than its adiabatic flame temperature. Furthermore, the results also showed that the effects of radiation heat loss and the Lewis number effect are suppressed by the hot wall, and that stretched flames at a Lewis number below unity can extinguish at a finite distance from the wall. Numerical calculations with detailed chemistry well reproduced the predictions of the analytical results. In addition, numerical results showed that except for the low-temperature flame on the low-stretch-rate side, there exists another low-temperature flame regime on the high-stretch-rate side, which dramatically extends the flammable region to the high stretch rate and yields multiple flame extinctions. Theoretical analysis qualitatively agreed well with the numerical calculations, but failed in the prediction of the low-temperature flame at a high stretch rate. The present results provided useful guidance for the application of ultralean combustion in new combustion technology.