Coaxial Jet Diffusion Flame Research Articles

Experimental and numerical investigations of soot formation in propane laminar coflow flames in O2/N2 and O2/CO2 atmospheres

Oxygen-rich combustion is a new type of clean combustion technology with important application prospects. At present, there are few detailed analyses on the mechanism changes of soot formation under oxygen enrichment of propane. In this paper, the effects of oxygen-rich (O2/N2, O2/CO2) combustion on soot formation in the propane laminar flow coaxial jet diffusion flame were investigated by using the experiment and numerical simulation. The flame image was taken by a color (CCD) camera in the visible band, and the two-dimensional distribution of temperature and soot volume fraction in the flame was reconstructed. In numerical simulation, the soot production model considers a detailed description of nucleation via collisions among heavy polycyclic aromatic hydrocarbons, particle aggregation, polycyclic aromatic hydrocarbons condensation, surface growth and oxidation through the hydrogen abstraction acetylene addition mechanism. The results show that the experimental results were compared with the numerical simulation results, and the numerical simulation results predict the temperature and soot change of oxygen-rich flame well. With the increase in oxygen concentration, the peaks of soot and temperature in the two oxygen-rich atmospheres gradually increased. With the increase in oxygen concentration, the peaks of soot and temperature in the two oxygen-rich atmospheres gradually increased. Comparing the combustion mechanism changes of the two oxygen-rich combustion systems, it was found that the hydrogen abstraction acetylene addition mechanism was the main cause of soot generation. The OH oxidation is dominated near the fuel nozzle side, and O2 oxidation is dominant downstream. With the substitution of CO2 for N2, the chemical effect of CO2 reduces the flame temperature and inhibits the formation mechanism and oxidation mechanism of soot to some extent. This in turn leaded to a reduction in the amount of soot produced.

Effects of hydrogen addition on soot emission of methane and propane coaxial jet diffusion flames

Effects of hydrogen addition on soot emission of methane and propane coaxial jet diffusion flames

Confinement Effects on Coaxial Jet Diffusion Flame

ABSTRACT A combined experimental and numerical study has been conducted to investigate the characteristics of confined coaxial jet diffusion flames in air coflow, using methane as fuel. The main concerns were focused on the unequal fuel velocity (0.1–72 m/s)/air velocities (0.045–0.53 m/s) and confinement ratio (δ*, defined as the ratio of the confinement tube diameter and the jet diameter), and most of the flow remains laminar while the ranges of δ* and gas velocities vary widely. The flame height linearly increases with the central fuel jet velocity, until the flame tip-opening or liftoff occurs, which is similar to that of the free jet flame. The critical fuel jet velocity for flame liftoff decreases with the ratio of air velocities to δ * . The critical fuel jet velocity of flame tip-opening increases with δ*, and the critical volumetric flow rate of air increases to a maximum, then decreases, indicating a combined effect of the oxygen supplement and the mixing with the fuel. Under smaller δ* with high jet flow velocity, a circular plane flame is observed before flame extinction. The formation of a recirculation zone in a confinement with combustion reaction has a significant influence on flame behaviors. A critical nondimensional Craya-Curtet number is used to predict the onset of flow recirculation with combustion reaction. At a certain coflow air velocity, the critical number initially increases and then remains constant with δ*, while at a certain δ*, the critical number decreases with air velocity. When the Craya-Curtet number is higher than the critical point, recirculation cannot be established, resulting in the formation of a tip-closed flame. With the decrease of the Craya-Curter number, the recirculation evolves to the upstream of the flame, then, the flame may be lifted off, along with the recirculation zone moving downstream. With the effect of recirculation, the stoichiometric mixture fraction contour shrinks toward the central axis where there is a higher local velocity. Therefore, the flame will be lifted off under a lower jet velocity in a smaller δ * . With the increase of the jet velocity, the Craya-Curtet number further decreases, and the recirculation becomes much stronger than the upstream inflow, resulting in the flame being compressed into a circular plane flame at a small δ* with a relatively high jet flow velocity or blowout directly at a relatively large δ * .

Multiple-scale thermo-acoustic stability analysis of a coaxial jet combustor

In this paper, asymptotic multiple-scale methods are used to formulate a mathematically consistent set of thermo-acoustic equations in the low-Mach number limit for linear stability analysis. The resulting sets of nonlinear equations for hydrodynamics and acoustics are two-way coupled. The coupling strength depends on which multiple scales are used. The double-time-double-space (2T-2S), double-time-single-space (2T-1S) and single-time-double-space (1T-2S) limits are revisited, derived and linearized. It is shown that only the 1T-2S limit produces a two-way coupled linearized system. Therefore this limit is adopted and implemented in a finite-element solver. The methodology is applied to a coaxial jet combustor. By using an adjoint method and introducing the intrinsic sensitivity, (i) the interaction between the acoustic and hydrodynamic subsystems is calculated and (ii) the role of the global acceleration term, which is the coupling term from the acoustics to the hydrodynamics, is analyzed. For the confined coaxial jet diffusion flame studied here, (i) the growth rate of the thermo-acoustic oscillations is found to be more sensitive to small changes in the hydrodynamic field around the flame and (ii) increasing the global acceleration term is found to be stabilizing in agreement with the Rayleigh Criterion.

Correlation of optical emission and turbulent length scale in a coaxial jet diffusion flame

This article investigates the correlation between optical emission and turbulent length scale in a coaxial jet diffusion flame. To simulate the H2O emission from an H2/O2 diffusion flame, radiative transfer is calculated on flame data obtained by numerical simulation. H2O emission characteristics are examined for a one-dimensional opposed-flow diffusion flame. The results indicate that H2O emission intensity is linearly dependent on flame thickness. The simulation of H2O emission is then extended to an H2/O2 turbulent coaxial jet diffusion flame. Time series data for a turbulent diffusion flame are obtained by Large Eddy Simulation, and radiative transfer calculations are conducted on the LES results to simulate H2O emission optical images. The length scales of visible structures in the simulated emission images are determined by the procedure proposed by Ivancic and Mayer (2002) [8]. The length scales of emission intensity are compared with the integral length scales of velocity and temperature evaluated from LES flowfield data. The results clearly indicate that the length scale of emission intensity agrees well with the integral length scale of temperature, and is also close to that of the radial velocity component. Finally, the explanation as to why the integral length scale of temperature can be extracted from emission intensity distributions is stated.

An Experimental Study of Flame Characteristics of Jet Diffusion Flames in Cylindrical Furnaces (1st Report, Effect of Inner Diameter of Furnace on NOx Emission Properties)

The combination of a burner and a combustion chamber is an important factor controlling flame characteristics. However, to our knowledge, this factor has yet to be investigated systematically. In the present study, coaxial jet diffusion flames in cylindrical combustion chambers have been studied in terms of inner diameters of the combustion chambers, global equivalence ratios, and turbulence in airflow. A fuel nozzle is composed of a stainless steel tube having an inner diameter (i.d.) of 2 mm with a coaxial pilot burner of 3.19 mm i.d., surrounded by two air coaxial tubes of 12 mm i.d. and 30 mm i.d., respectively. The inner and outer air tubes are for higher and lower airflows, respectively, and the turbulence in the airflow is changed by the velocity difference. The main fuel is propane. Hydrogen is used for the pilot flame, with a volumetric fuel ratio of 0.3. Each wall of the combustion chamber is made of a heat-resistant glass Pyrex tube so that each flame can be visualized. The inner diameter of the furnace is varied in order to investigate the effect of furnace size on the flame characteristics. The increase in the diameter of the combustion chamber has been found to enhance the exhaust gas self-recirculation, because the NOx emission decreases. The increase in turbulence in the airflow strengthens the entrainment of the exhaust gas transported upstream by the recirculation vortex. The increase in the global equivalence ratio from 0.2 to 0.8 in the present study decreases the oxygen concentration of the exhaust gas and leads to diluted combustion through the exhaust gas self-recirculation. A proper combination of these factors has been found to yield a low NOx combustion.

An Experimental Study of Flame Characteristics of Jet Diffusion Flames in Cylindrical Furnaces (1st Report, The Effect of Inner Diameter of Furnace on NOx Emission Property)

The combination of burner and combustion chamber is one of important factors controlling flame characteristics, which has never been to our knowledge investigated systematically. In the present study, coaxial jet diffusion flames in cylindrical combustion chambers have been studied in terms of inner diameter of the combustion chamber, global equivalence ratio and turbulence in air flow. A fuel nozzle is composed of a stainless steel tube of an inner diameter of 2mm with a coaxial pilot burner of 3.19mm i.d., surrounded by two air coaxial tubes of 12mm i.d. and 30 mm i.d. The inner and outer air tubes are for higher and lower air flows, respectively, and the turbulence in air flow is changed by the velocity difference. The main fuel is propane. Hydrogen is used for the pilot flame, with a volumetric fuel ratio of 0.3. The wall of combustion chamber is made of heat-resistant glass to visualize the flames and the inner diameter is changed in a range between 95 mm and 182 mm to investigate the effect of size of furnace on flame characteristics. It has been found that the increase in the diameter of combustion chamber enhances the exhaust gas self-recirculation, as the result decreases the NOx emission. The increase in turbulence in air flow strengthens the entrainment of the exhaust gas transported by the recirculation vortex to the flame. The increase in the global equivalence ratio from 0.2 to 0.8 in the present study decreases the oxygen concentration of the exhaust gas and leads to the diluted combustion through the exhaust gas self-recirculation. It has been found that a proper combination of these factors can yield the low NOx combustion.

B111 円筒型燃焼炉内の噴流拡散火炎の燃焼特性に関する研究 : 水素火炎の場合(OS-2 環境負荷低減のための燃焼研究と技術開発I)

The combination of burner and combustion chamber is one of important factors controlling flame characteristics, which has never been to our knowledge investigated systematically. In the present study, coaxial jet diffusion flames in cylindrical combustion chambers have been studied in terms of inner diameter of the combustion chamber, global equivalence ratio and turbulence in air flow. It has been found that the increase in the diameter of combustion chamber enhances the exhaust gas self-recirculation, as the result decreases the NOx emission.

Analysis of Combustion Characteristics of Coaxial Jet Diffusion Flame with Steam Addition

A numerical simulation of a coaxial jet diffusion e ame with steam addition was performed by using an overall one-step chemical reaction. The effect of steam addition on the combustion characteristics, especially the effects of steam addition method on the decrease in the e ame temperature, was examined. The results assist in better understanding theeffectsofsteam additiononNO x reductionincoaxialjetdiffusione ame.Thee amestructureand the effects of the steam addition are found to be similar to the results of the countere ow diffusion e ame, simulated by meansof a detailed elementary chemical kinetics. The applicability of thelaminare ameletmodel forprediction of the local and occasional e ame structure in the turbulent jet e ame with steam addition has been considered.

Simulation of Dynamic Methane Jet Diffusion Flames Using Finite Rate Chemistry Models

Detailed calculations for methane jet diffusion flames under laminar and transitional conditions are made using an axisymmetric, time-dependent computational fluid dynamics code and different chemical-kinetics models. Comparisons are made with experimental data for a steady-state flame and for two dynamic flames that are dominated by buoyancy-driven instabilities. The ability of the three chemistry models-namely, 1) the modified Peters mechanism without C 2 chemistry, 2) the modified Peters mechanism with C 2 chemistry, and 3) the Gas Research Institute's Version 1.2 mechanism-in predicting the structure of coaxial jet diffusion flames under different operating conditions is investigated. It is found that the modified Peters mechanisms with and without C 2 chemistry are sufficient for the simulation of jet diffusion flames for a wide range of fuel-jet velocities. Detailed images of the vortical structures associated with the low- and transitional-speed methane jet flames are obtained using the reactive-Mie-scattering technique. These images suggest that a counter-rotating vortex is established upstream of the buoyancy-induced toroidal vortex in the low-speed-flame case and that the shear-layer vortices that develop in the transitional-speed flame are dissipated as they are convected downstream. The time-dependent calculations made using the modified Peters chemistry model have captured these unique features of the buoyancy-influenced jet flames. Finally, the unsteady flame structures obtained at a given height are compared with the steady-state flame structures.

Experimental Study of Turbulent Diffusion Flames Stabilized on a Bluff Body. (Flame Structure).

An experimental investigation was conducted on the structure of turbulent diffusion flames of hydrogen stabilized on a bluff body. A coaxial jet diffusion flame was formed using a cylindrical nozzle with an extremely thick rim, which works as a bluff body. In the present study, special attention was paid to the effect of combustion on the aerodynamic processes. The following results were obtained. (1) The combustion markedly enhances the penetration of the central fuel jet. (2) The laminarization phenomenon, reported in simple jet diffusion flames, was observed more conspicuously in this combustion field as well, and it was assumed to exert an important influence on the flame structure

Experimental and Numerical Studies of Triple Jet Diffusion Flames.

A coaxial jet diffusion flame is often used to experimentally analyze the phenomena in turbulent diffusion combustion and to examine the modeling technique for them. In this flame, however, a low turbulence region is inevitably formed in the periphery, which becomes a disadvantage in its use as a research object. In the present study, another coaxial air jet of high velocity was added to usual coaxial jet diffusion flames between the central fuel jet and the surrounding air flow. In this flame, called a triple jet diffusion flame, large turbulence is generated in the peripheral region of the flame by controlling the high-velocity air. In this report, the characteristics and usefulness of the flame are clarified by experiments and numerical simulations.

Experimental Study of Turbulent Diffusion Flames Stabilized on a Bluff Body. 1st Report. Flame Structure.

An experimental investigation was conducted on the structure of hydrogen turbulent diffusion flames stabilized on a bluff body. A coaxial jet diffusion flame was formed using a cylindrical nozzle with an extremely thick rim, which works as a bluff body. In the present study, special attention was paid to the effect of combustion on the aerodynamic processes. The following results were obtained. (1) The combustion remarkably enhances the penetration of the central fuel jet. (2) The laminarization phenomenon, reported in simple jet diffusion flames, was observed more conspicuously also in this combustion field, and it was supposed to exert an important influence on the flame structure. (3) An accelerating flow field often exists in complicated flow fields with a reverse flow, where turbulence generation terms containing Reynolds normal stress become negative. This phenomenon sometimes exerts considerable effects on the turbulence behavior.

An approach to the theoretical description of turbulent jet diffusion flames—Part I: Combustion model and calculation

The microscopic structure of the reaction zone in turbulent diffusion flames may be considered to be composed of numerous flame sheets, each of which behaves like a laminar diffusion flame. Wohlenberg [1] has proposed a combustion model for turbulent diffusion flames with the basic premise being that a large part of the macroscopic effects of turbulence on the reaction rate are accounted for by the increase in reaction interface extension due to the turbulence. Starting from this model, the authors proposed a model by attaching importance to the stretching of the reaction interface and the molecular diffusion of reactants, and introduced an equation giving the local reaction rate for turbulent diffusion flames. A function B involved in this equation expresses the effect of turbulence upon the reaction rate and is correlated with the local turbulent Reynolds number. The equation of local reaction rate was used as a source term of the conservation equations on turbulent diffusion flames. Using the GEMIX 4 program developed for boundary layers by Patankar and Spalding [2], numerical calculations were conducted on two coaxial jet diffusion flames of hydrogen and propane. A reasonably good agreement was found with the experimental results, e.g., flame shape, temperature distribution and coexistence of reactants.