Modeling of aluminum particle combustion with emphasis on the oxide effects and variable transport properties

A simplified analytical modeling of single aluminum particle combustion was conducted. Ignition and quasi-steady combustion (QSC) were separately formulated and integrated. Both the heat transfer from the hot ambient gas and the enthalpy of heterogeneous surface reaction (HSR) served to cause the particle ignition. Conservation equations were solved for QSC parameters in conjunction with conserved scalar formulation and Shvab-Zeldovich function. Limit temperature postulate was formulated by a sink term pertinent to the dissociation of the aluminum oxide near the flame zone. Effective latent heat of vaporization was modified for the thermal radiation. Ignition and QSC of the aluminum particle were predicted and discussed with emphasis on the effect of the aluminum oxide and variable properties. The model was validated with the experiments regarding ignition delay time, burning rate, residue particle size, flame temperature, QSC duration, and stand-off distance of the envelop flame. Agreement was satisfactory and the prediction errors were limited within 10%.     

Observation of coal fragmentation in early stages of combustion

Measurements of coal fragmentation in the early stages of combustion were undertaken in the size range of crushed coals for Chinese Dongjin and Indonesian Roto coals. A flat flame burner fed with a premixed mixture of methane, air and partly hydrogen was used for the burning of a single coal particles. A high speed video camera system was used for the observation of coal fragmentation during approximately 80 msec. Fragmentation is consistently observed in the controlled combustion environment over a gas flame temperature range of 1220K–1320K. The data indicate that a single coal particle often disintegrates into two, three, and sometimes more fragments. The dominant mechanisim of fragmentation is that producing two fragments in primary fragmentation. The Dongjin coal breaks up more extensively than the Roto coal with the frequency of fragmentation exhibiting a strong particle size dependence and a weaker gas flame temperature dependence. The mean time of primary fragmentation for the Dongjin coal falls to between 10 msec and 20 msec and does not remarkably vary with particle size and gas flame temperature. The mean time of primary fragmentation for the Roto coal is strongly dependent on the particle size, whilst shows less gas flame temperature dependence.     

Numerical simulation of dump combustor with auxiliary fuel injection

Solutions of the steady, two-dimensional Navier-Stokes, thermal energy, radiation flux, and species conservation equations have been computed for a dump combustor geometry with downstream constriction. The equations are solved in the conservative finite-difference form on a nonuniform rectilinear grid of sufficient resolution to capture the momentum/thermal boundary layers accurately. The numerical procedure, SIMPLEST, which is a variation of SIMPLE, is used to ensure more rapid convergence. A κ−ε model is incorporated for the enclosure of turbulence with a wall function. The combustion model, ESCRS (extended simple chemically-reacting system) with eddy break-up model for the reaction rates of species conservation equation or energy equation is also used. For the good destruction of hazardous waste, it is effective that the waste is injected in the recirculation zone of high temperature with the condition not disturbing the combustion cavity. Excess oxygen from a lean core flame can be effectively utilized for waste destruction with the hydrocarbon waste injection as fuel-rich condition. The core flame has a significant impact on the structure of recirculation cell, in some cases completely changing the nature of the flow within the cavity. The optimum velocity of waste injection exists for the highest temperature at the recirculation zone. A premixed flame type for the auxiliary fuel is good for high temperature and long residence time to be large cavity in the recirculation zone.     

Measurements of Non-reacting and Reacting Flow Fields of a Liquid Swirl Flame Burner

The understanding of the liquid fuel spray and flow field characteristics inside a combustor is crucial for designing a fuel efficient and low emission device.Characterisation of the flow field of a model gas turbine liquid swirl burner is performed by using a2-D particle imaging velocimetry(PIV)system.The flow field pattern of an axial flow burner with a fixed swirl intensity is compared under confined and unconfined conditions,i.e.,with and without the combustor wall.The effect of temperature on the main swirling air flow is investigated under open and non-reacting conditions.The result shows that axial and radial velocities increase as a result of decreased flow density and increased flow volume.The flow field of the main swirling flow with liquid fuel spray injection is compared to non-spray swirling flow.Introduction of liquid fuel spray changes the swirl air flow field at the burner outlet,where the radial velocity components increase for both open and confined environment.Under reacting condition,the enclosure generates a corner recirculation zone that intensifies the strength of radial velocity.The reverse flow and corner recirculation zone assists in stabilizing the flame by preheating the reactants.The flow field data can be used as validation target for swirl combustion modelling.     

Effects of propane pyrolysis on basic flame structures of non-premixed jet flame

Recently, high temperature combustion techniques have been extensively adopted to improve thermal efficiency of combustion devices. When the temperature is sufficiently high, fuel pyrolysis may occur and then the flame structure can be affected. Thus, the understanding flame structure at high temperature coupled with fuel pyrolysis is essential to design burners operated in high temperature environments. In this study, propane was heated up to 1100 K using an electric heater, and the characteristics of fuel pyrolysis and its effects on non-premixed jet flames were experimentally investigated at room temperature. Normalized volume flow rates and components were measured regarding the fuel pyrolysis. The normalized volume flow rates of the propane were increased significantly through the pyrolysis process, and they agreed with numerical results based on an ordinary chemical mechanism. Variations of flame height and soot region were compared for the pyrolysis temperature. Conclusively, in spite of the significant growth in the volume flow rate by the propane pyrolysis, flame height was not significantly affected. On the contrary, the soot region was slightly extended by the fuel pyrolysis. This study will help to extend understanding on jet flame characteristics especially at high temperature conditions.     

四角切圆锅炉炉内煤粉燃烧过程数值模拟

利用计算流体动力学软件PHOENICS 3.5对一台200 MW四角切圆水平浓淡燃烧煤粉炉进行数值模拟研究,采用多流体两相流动模型及煤粉燃烧综合模型,计算得出在垂直方向不同二次风风量分布的工况下,炉内各截面处的烟气温度、燃料浓度、燃烧产物组分浓度以及炉内辐射热流的分布。结果表明,在燃烧器出口处出现了高煤粉浓度和烟气高温区,并出现气固两相分离的现象,使得煤粉着火及时,燃烧器区域维持较高温度,并防止水冷壁结渣,炉内温度、炉膛出口氧量和飞灰可燃物的计算结果和试验结果相比,吻合较好。二次风分级配风工况下,下部燃烧器区烟气温度升高,但氧气推迟混入,相应位置飞灰可燃物有所增加。计算模型能够合理地模拟水平浓淡煤粉气流在大型锅炉炉膛内的燃烧过程,适用于运行工况的优化和炉内污染物的控制。     

An experiment analysis of MILD combustion with liquid fuel spray in a combustion vessel

Mild combustion is characterized by its distinguished features, such as suppressed pollutant emission, homogeneous temperature distribution, reduced noise, and thermal stress. Recently, many studies have revealed the potential of MILD combustion in various power systems but most studies have been focused on gas phase fuel MILD combustion. Therefore, further study on MILD combustion using liquid fuel is needed for the application to a liquid-fueled gas turbine especially. In this work, we studied experimentally on the formation of liquid fuel MILD combustion under the condition of high dilution by burnt gas generated from a first premixed flame in two stages combustor which consists of the first premixed burner and secondary combustor. In particular, the effects of burnt gas velocity and oxygen level of burnt gas on the formation of liquid fuel MILD combustion were investigated. The results show that as the burnt gas velocity through the nozzle becomes higher, the color of flames was changed from yellow to pale blue and flames became very short. The OH radical measured by ICCD camera was uniformly distributed on the pale blue flame surface and its intensity was very low compared to conventional liquid diffusion flame. As burnt gas velocity is increased, local high-temperature region appeared to be diminished and the flame temperature became spatially uniform. And CO emission was sampled around 1 ppm and NOx emission was measured around 10 ppm under the overall equivalence ratio of 0.8 to 0.98 for 40 mm or less diameter of velocity control nozzle. This low NOx emission seems to be attributed to maintaining the average temperature in secondary combustor below the threshold temperature of thermal NOx formation. In view of the uniform temperature distribution, low OH radical intensity and low NOx emission data in the secondary combustor, formation of stable MILD combustion using kerosene liquid fuel could be verified at high burnt gas velocity.

     

中心给粉旋流燃烧器气固两相流动的数值模拟

电厂采用的煤粉燃烧技术应达到稳燃、低污染、防结渣及防高温腐蚀的要求。中国电厂燃用煤的煤质偏差,煤种多变。在燃用这些煤的时候,锅炉的稳燃能力较低。针对这些问题,提出中心给粉旋流煤粉燃烧技术。由于燃烧器的气固流动特性对燃烧器的性能有很大的影响,利用可实现的k-ε和Lagrangian随机轨道模型对中心给粉旋流燃烧器的气固两相流动进行数值模拟,并将计算结果和三维相位多普勒测速技术(Phase-Doppler anemometry,PDA)试验结果进行详细比较,计算值和试验值速度分布的趋势基本相同。计算和试验结果表明,在轴向方向产生了回流区,切向速度分布出现典型的Rankine涡结构,中心线附近区域的径向速度小。当颗粒的轴向速度衰减为0之后,颗粒的运动方向发生偏转,开始向后上方运动。颗粒迂回型运动轨迹延长了煤粉在回流区中的停留时间。     

Relationship Between Physical Structure and Tribology of Single Soot Particles Generated by Burning Ethylene

Ethylene gas is burnt and the soot generated is sampled thermophoretically at different heights along the flame axis starting from a region close to the root of the flame. The morphology and crystallinity of the particle are recorded using high resolution transmission electron microscopes. The hardness of a single particle is measured using a nanoindenter. The frictional resistance and material removal of a particle are measured using an atomic force microscope. The particles present in the mid-flame region are found to have a crystalline shell. The ones at the flame root are found to be highly disordered and the ones at the flame tip and above have randomly distributed pockets of short range order. The physical state of a particle is found to relate, but not very strongly, with the mechanical and tribological properties of the particles.     

Application of dfb diode laser sensor to reacting flow (II) —liquid-gas 2-phase reacting flow—

Diode laser sensor is conducted to measure the gas temperature in the liquid-gas 2-phase counter flow flame. C₁₀H₂₂ and city gas were used as liquid fuel and gas fuel, respectively. Two vibrational overtones of H₂O were selected and measurements were carried out in the spray flame region stabilized the above gaseous premixed flame. The path-averaged temperature measurement using diode laser absorption method succeeded in the liquid fuel combustion environment regardless of droplets of wide range diameter. The path-averaged temperature measured in the post flame of liquid-gas 2-phase counter flow flame showed qualitative reliable results. The successful demonstration of time series temperature measurement in the liquid-gas 2-phase counter flow flame gave us motivation of trying to establish the effective control system in practical combustion system. These results demonstrated the ability of real-time feedback from combustor inside using the non-intrusive measurement as well as the possibility of application to practical combustion system. Failure case due to influence of spray flame was also discussed.     

Thermophoretic transport and deposition of particles in vertical tube flow with variable wall temperature and thermal radiation

A particulate two phase flow with variable wall temperature has been studied for examining the deposition of particles in the thermal radiation and mixed convection flow associated with the manufacture of optical fiber preforms. The two-dimensional governing equations of continuity, momentum and energy have been solved numerically including the effects of thermal radiation and buoyancy (upward or downward flow) in the vertical tube flow. A particle trajectory model has been adopted to predict the particle transport, and P-1 approximation has been used to evaluate the radiation heat transfer. In the upward flow case, a high deposition efficiency is obtained and the deposition zone of the downward flow is broader than that of the upward flow. Thermal radiation makes the deposition zone broader and the deposition efficiency smaller.     

DYNAMIC ANALYSIS OF PARTICLE FLYING VELOCITY IN HIGH VELOCITY OXYGEN FUEL SPRAY

0INTRODUCTIONIn thermal spraying process, the flying velocity of the particles is one of the most important factors that control the quality of thermal sprayed coatings［1，2］. The coating deposited by the particles with higher flying velocity is dense and well bonded. Many researchers［1,2］ had identified that the flying velocity of the particles for depositing depends on the velocity and mass rate of flow of a HVOF gun, and the time or length of the particles being accelerated. Based…     

Computational modeling of dilute gas-particle flows in an ultrasonic gas flowmeter

A computational procedure is presented to predict particle-laden turbulent gas flows in an ultrasonic flowmeter using curvilinear coordinates. The Eulerian-Lagrangian hybrid model is used for the two-phase flow predictions. The turbulent gas phase is formulated using the Eulerian governing equations whereas the particle phase is formulated using the Lagrangian governing equations. The effect of the gas turbulence on discrete particle dispersion is accounted for using a particle-eddy interaction model. An efficient numerical algorithm is described to locate particles with an optimized search path in three-dimensional Eulerian control volumes. Numerical results are compared with experimental measurements for both the gas and particle phases. In addition, the sensitivity of particle deposition onto the meter-duct wall is investigated by using different particle diameters and densities.     

适用于燃用贫煤1 025 t/h锅炉的中心给粉旋流燃烧器

提出了中心给粉旋流煤粉燃烧器,并针对某厂采用EI-DRB型燃烧器设计燃用贫煤的1 025 t/h锅炉稳燃能力差,不能燃用设计煤质的问题,进行了实验室冷态试验及锅炉冷、热态试验,得出了新型的燃烧器结构,并将下层8只燃烧器改造为新型燃烧器.试验表明,中心给粉燃烧器的回流区最大直径、长度与燃烧器最外层直径之比分别为1.40和1.89,可卷吸足够的高温烟气及时点燃煤粉,得出了外二次风叶片角度、一次风量、二次风量及三次风对燃烧器出口射流的影响规律;在实际运行参数下,EI-DRB型燃烧器没有回流区,不利于稳定燃烧.得出了二次风挡板开度和给粉机转数对燃烧器出口处温度场的影响规律.采用新型燃烧器后,锅炉效率提高,当电负荷降至140 MW时,锅炉可以不投油稳定运行,在燃用贫煤、无烟煤和贫煤的混煤时(混合比为11),锅炉在高负荷和低负荷下均可稳定运行.锅炉NOx排放下降.     

Effects of additive and preheat on the partially premixed CH4-air counter flow flames considering non-gray gas radiation

Detailed structures of the counterflow flames formed for different inlet fluid temperatures and different amount of additives are studied numerically. The detailed chemical reactions are modeled by using the CHEMKIN-II code. The discrete ordinates method and the narrow band based WSGGM with a gray gas regrouping technique (WSGGM-RG) are applied for modeling the radiative transfer through non-homogeneous and non-isothermal combustion gas mixtures generated by the counterflow flames. The results compared with those obtained by using the SNB model show that the WSGGM-RG is very successful in modeling the counterflow flames with non-gray gas mixture. The numerical results also show that the addition of CO₂ or H₂O to the oxidant lowers the peak temperature and the NO concentration in flame. But preheat of fuel or oxidant raises the flame temperature and the NO production rates. O₂ enrichment also causes to raise the temperature distribution and the NO production in flame. And it is found that the O₂ enrichment and the fuel preheat were the major parameters in affecting the flame width.     

The characteristics of pulverized coal combustion in the two stage cyclone combustor

Numerical investigations on air staging and fuel staging were carried out with a newly designed coaxial cyclone combustor, which uses the method of two stage coal combustion composed of pre-combustor and main combustor. The pre-combustor with a high air/fuel ratio is designed to supply gas at high temperature to the main combustor. To avoid local high temperature region in this process, secondary air is injected in the downstream. Together with the burned gas supplied from the pre-combustor and the preheated air directly injected into main combustor, coals supplied through the main burner react rapidly at a low air/fuel ratio. Strong swirling motion of cyclone combustor keeps the wall temperature high, which makes slagging combustion possible. Alaska, US coal is used for calculations. Predictions were made for various coal flow rates in the main combustor for fuel staging and for the various flow rate of secondary air in the pre-combustor for air staging. In-scattering angles are also chosen as a variable to increase residence times of coal particles. Temperature fields and particle trajectories for various conditions are described. Predicted temperature variations at the wall of the combustor are compared with corresponding experimental data and show a similar trend. The in-scattering angle of 20° is recommended to increase the combustion efficiency in the main chamber.     

A comparative study of the ignition and burning characteristics of afterburning aluminum and magnesium particles

Ignition and the burning of air-born single aluminum and magnesium particles are experimentally investigated. Particles of 30 to 106 μm-diameters were electrodynamically levitated, ignited, and burnt in atmospheric air. The particle combustion evolution was recorded by high-speed cinematography. Instant temperature and thermal radiation intensity were measured using two-wavelength pyrometry and photomultiplier tube methods. Ignition of the magnesium particle is prompt and substantially advances the aluminum particle by 10 ms. Burning time of the aluminum particles is extended 3 to 5 times longer than the magnesium particles. Exponents of a power-law fit of the burning rates are 1.55 and 1.24 for aluminum and magnesium particles, respectively. Flame temperature is slightly lower than the oxide melting temperature. For the aluminum, dimensionless flame diameter is inert to the initial particle size, but for the magnesium inversely proportional to the initial diameter.     

Investigation of a flame holder geometry effect on flame structure in non-premixed combustion

In this paper the effect of flame holder geometry on flame structure is studied. The obtained numerical results using realizable k-? and β-PDF models show a good agreement with experimental data. The results show that increasing in flame holder length decreases flame length and increases flame temperature. Additionally, it is observed that flame lengths decrease by increasing in flame holder radius and increase for larger radii. Furthermore in various radii, the flame temperature is higher for smaller flame lengths. It was found that behavior of flame structure is mainly affected by the mass flow rate of hot gases that come near the reactant by the recirculation zone.     

Analysis of combustion and flame propagation characteristics of LPG and Gasoline fuels by laser deflection method

This work is to investigate the combustion characteristics and flame propagation of the LPG (liquified petroleum gas) and gasoline fuel. In order to characterize the combustion processes of the fuels, the flame propagation and combustion characteristics were investigated by using a constant volume combustion chamber. The flame propagation of both LPG and gasoline fuels was investigated by the laser deflection method and the high-speed Schlieren photography. The result of laser deflection method show that the error of measured flame propagation speed by laser method is less than5% compared with the result of high-speed camera. The flame propagation speed of the fuel is increased with the decrease of initial pressure and the increase of initial temperature in the constant volume chamber. The results also show that the equivalence ratio has a great effect on the flame speed, combustion pressure and the combustion duration of the fuel-air mixture.     

Swirl effect on the spray characteristics of a twin-fluid jet

In the liquid fuel combustion chamber, employed fuel must be atomized before being injected into the combustion zone. Therefore, the complete fuel atomization is the most important condition for the combustion efficiency. The atomization quality strongly affects the combustion performance, exhaust pollutant emissions, and flame stability. Therefore, the whole process of spray atomization is of fundamental significance. During past decades many experimental and theoretical studies in this field have been carried out and some improved results have been obtained. Two-phase atomizers, having a variety of advantages such as spray uniformity, appreciable atomization, and smaller SMD with an increase of ambient gas, are considered to be applied in various industrial processes. The purpose of present study is to investigate the mean velocity, turbulence shear stress, turbulence intensity, mean drop size distribution, and droplet data rate in a two-phase swirling jet using PDPA systems.