Numerical study of the effect of imaginary circle diameter and initial flow field on the aerodynamic field in a tangentially fired furnace

In this paper,the effect of the imaginary circle diameter φi and the initial flow field on the aerodynamic field in a tangentially fired furnace was studied by numerical simulation and experments in the cold model.Results show that merely reducing the imaginary circle diameter φi can not significantly reduce the rotational diameter φ in the range considered.The flow still rotates counter-clockwise stably and does not change rotation direction when the direction of all jet axes are deflected suddenly to the opposite rotation direction by up to 5.4° in a counter-clockwise flow filed.It is the first time that the numerical simulation results were obtained which agreed quite well with this experimental phenomena qualitatively.The experimental data,ie.e,the rotational diameter φ and the maximum velocity on the symmetric central line of furnace Vm,are only a bit larger than the simulation resuts.It is shown that the initial flow field has an important influence on the aerodynamic field in the funace.Other measures have to be taken as well in order to reduce φ to resist slagging and high temperature corrosion of furnace tubes.Moreover,a new kind of grid arrangement was proposed in this paper,which can reduce effectively the false diffusion at the exit zone of burner.     

Thermal and emission characteristics in a tangentially fired boiler model furnace

Tangentially fired furnaces are vortex‐combustion units and are intended for use as low NO formation furnaces. NO characteristics inside these furnaces depend on many parameters. The present study investigates numerically the problem of pollution in a real furnace of a 1699 MW tangentially fired boiler having 32 burners. NO formation contour maps in a tangentially fired furnace are presented. The study covered different combustion air temperature values, different fuel–air ratios and different cases of tripped burners. Available experimental measurements were used for validating the calculation procedure. The details of the temperature and NO fields were obtained from the solution of the conservation equations of mass, momentum and energy and transport equations for scalar variables in addition to the equations of the turbulence model. The equations governing the NO formation were solved to calculate the NO distribution. The simulation provided more insight on the correlation between the total NO concentration and the maximum furnace temperature and furnace average temperature. The results have shown that the furnace average temperature and NO concentration decrease as the excess air factor increases for a given air mass flow rate. As the combustion air temperature increases, furnace temperature increases and the thermal NO concentration increases sharply. The results show that the temperature distributions are significantly distorted by tripping any of the burners. The results show that tripping one or two burners either adjacent or opposite or tripping four results in regions of high temperature gases close to the walls. Heat absorptions in super‐heater and economizer are greatly influenced by combustion air temperature and excess air factor and are slightly influenced by burner tripping. Copyright © 2009 John Wiley & Sons, Ltd.     

Control of ash related problems in a large scale tangentially fired boiler using CFD modelling

The objective of this study is to develop a three-dimensional combustor model for predicting the performance of full-scale tangential fired (TF) boiler and to determine the flow patterns of the gas and coal particles, with an emphasis on formation mechanism of gas flow deviations and uneven temperature on the super-heaters, re-heaters and divisional super-heater sections of the furnace. The importance of these simulations is to identify the locations of optimum additive injection ports to achieve maximum impact of additives in the combustion process to minimize the temperature deviation and reduce ash-related issues. This study is a classic example of numerical investigation into the problem of turbulent reacting flows in large scale furnaces employed in thermal power plants for the remediation of ash deposition problems. Present work also provides an investigation of the influence of number of tripped burners on the characteristics of the flow and thermal fields. Excellent agreement between the simulation results and key boiler design values and available site operation records following full-scale trials indicate that the calculations are reliable. The results obtained from the present work are directly relevant to coal-fired utilities for not only demonstrating the effectiveness of computational fluid dynamics (CFD) based tools in combating operational issues but also provides an alternative to conventional remediation strategies.     

Study on the Effect of the Side Secondary Air Velocity on the Aerodynamic Field in a Tangentially Fired Furnace with HBC—SSA Burner

INTroDUCTIONThe Horizontal Bias Combustioll Pulverized Coal(HBC) burner for tangential firing system has advantages of improving flame stabilization, slaggingresistance, pollution control and high combustionefficiency[']. However, in the thin and tall furnace firedwith coal with strong slagging telldency) it is possiblethat slagging occurred on water-cooled wall, especiallywhen the cross-sectional and volumetric heat releaserate are very high in the burner domain.The HBC burner with Si…     

生物质成型燃料双层炉排锅炉温度场试验研究

生物质能是极具开发潜力的可再生能源。生物质直接燃烧技术的深入研究及锅炉技术的成熟和锅炉类型的多样化为更好开发生物质能源创造了条件。生物质成型燃料与原生物质及煤相比燃烧性质有较大差别,依据该燃料的特点采用双层炉排层燃锅炉。根据有限元分割成型原理布置测点,用热电偶温度计和红外测温仪测出各点温度,得出各个方向温度分布图,并对其进行分析比较,为水冷壁的合理布置、进风量选择及锅炉稳定燃烧、经济燃烧提供理论依据。     

煤粉锅炉旋流燃烧器冷态流场数值模拟

旋流煤粉燃烧器因具备形成中心回流区,卷吸高温烟气,促进煤粉着火等显著优势和特点,在电站锅炉中的应用日益广泛.而旋流煤粉燃烧器的流动情况对锅炉的稳定燃烧和安全运行都具有重要影响.通过建立物理模型和数学模型,对旋流燃烧器进行了三维数值计算得到燃烧器中钝体形状、二次风速及外二次风叶片角度等因素对回流区产生的具体影响.计算结果...     

Experimental and numerical study on the flow characteristics in curved rectangular microchannels

In this paper, the flow characteristics in curved rectangular microchannels with different aspect ratios and curvature ratios for Re numbers ranging from 80 to 876 are investigated. The obtained experimental results are compared with simulated values based on classical Navier–Stokes equations and available correlation in the literature. An empirical equation based on experimental data is proposed to provide a better prediction of the frictional pressure drop in the curved rectangular microchannels.     

A numerical study on thermal analysis and cooling flow fields effect on PEMFC performance

Thermal management and water management are two important interconnected topics in the design and increase the efficiency of PEM fuel cells. Suitable cooling flow field design with proper performance is an important factor in increasing the lifetime of PEM fuel cell, because non-uniformity of temperature reduces the stability and durability of PEM fuel cell. Different cooling strategies are considered for removing of heat generation by PEM fuel cell, because the fuel cell temperature remains in a tolerable range and homogeneous as possible. In the first step, determine the value and location of heat sources in fuel cell, is important and appropriate cooling strategy can be applied. In this study, a PEM fuel cell with serpentine gas flow field is simulated with six different cooling flow fields simultaneously, e.g. conventional serpentine (Model 1), typical of MPSFFs (model 2), typical of serpentine (Model 3), parallel-serpentine (Model 4), conventional spiral (Model 5) and conventional parallel (Model 6). This simulation showes a correct perception of temperature distribution in PEMFC. The results indicate that Model 5 has a good temperature and performance based on the minimum and maximum temperature gradient, Index of uniform temperature (IUT), however has a more pressure drop. For second choice when the pressure drop is important, the Model 3 has a better performance than other models. Also, thermal analysis in these cases shows that ohmic, entropic and reaction heating included 20%, 35%, 45% of the total heat generation by PEMFC, approximately.     

Experimental and numerical study of downward bubbly flow in a pipe

An experimental and numerical study of the local structure of downward gas–liquid flow in a vertical pipe with 20-mm inner diameter is reported. In the experiment, the electrodiffusion technique was used in combination with electrical conductivity measurements. To examine the effect of gas-phase dispersion on flow characteristics, two different gas–liquid mixers were used capable of producing large-diameter (>1 mm) and small-diameter (<1 mm) gas bubbles at identical rate characteristics of the flow. The unified heterogeneous-medium mechanics approach was used to develop, in the Eulerian two-velocity approximation, a calculation model for downward turbulent liquid/air bubble flows. It is shown that, as the volumetric gas flow rate of the mixture at the inlet to the pipe increases, local maxima of continuous phase velocity and bubble concentration emerge in the near-wall zone of the flow, with liquid turbulence suppressed in the wall zone and enhanced in the core of the flow.     

电站锅炉SCR系统流场的冷态试验与数值模拟的研究

采用冷态试验和数值模拟相结合对某电厂660 MW燃煤锅炉SCR脱硝系统中整流格栅、多孔板、导流板的设计方案进行了速度场和浓度场分布及压降的研究。结果表明数值模拟结果和冷态试验结果较为吻合。其中AIG上游速度分布CV值为15.4%,催化剂入口速度分布CV值为8.1%,浓度场CV值为5.7%,满足系统设计要求。催化剂入口处安装整流格栅可以优化进入催化剂层的气流方向,使烟气垂直进入催化剂层。     

Parametric effect on the flow and mixing properties of transverse gaseous injection flow fields with streamwise slot: A numerical study

Mixing process between the injectant and air in supersonic crossflow depends on the injector configuration and the jet-to-jet spacing heavily. In the current study, the three-dimensional Reynolds-averaged Navier–Stokes (RANS) equations coupled with the two equation SST k-ω turbulence model were employed to simulate the mixing process induced by an array of three spanwise-aligned small-scale rectangular portholes, and the influences of the jet-to-jet spacing, the jet-to-crossflow pressure ratio and the aspect ratio of the injector on the flow field properties were evaluated. Two quantitative objectives were considered in this article, namely the fuel penetration depth and the mixing efficiency. The obtained results show that the flow field induced by the array of three spanwise-aligned small-scale rectangular portholes is a multiobjective design optimization problem, and the large aspect ratio is beneficial for the mixing enhancement in supersonic crossflow. However, it is not beneficial for the flame holding. The interaction between the adjacent injectors has a great impact on the fuel penetration depth in the far-field, especially for the larger jet-to-crossflow pressure ratio, and this is due to its wider fuel plume.     

Investigation on the effects of fly ash particles on the thermal radiation in biomass fired boilers

Ash is produced in combustion of biomass. Some part of this matter is called fly ash and is carried by the flow and causes not only air pollution and erosion, but also can affect the thermal radiation. The effects of fly ash particles on the thermal radiation are considered in this investigation. By analyzing sampled data in an electrostatic filter, a realistic particle size distribution is found. Although the optical data on biomass fly ash are not available, however, similarity between coal and biomass ash compositions showed that the optical constants of the low Fe coal fly ash can be applied for the biomass fly ash. The Mie theory is used to predict scattering and absorption coefficients and phase function. The mean Planck scattering and absorption coefficients and phase function are predicted by averaging over the particle size distribution and Planck function, respectively. The effects of fly ash particles on thermal radiation are evaluated by a three-dimensional test case. It is assumed that the medium is a mixture of non-grey gases and different level of particle loading. Predicted results from the test case showed that the fly ash can be influential on the thermal radiation. In addition, in selected fly ash volume fractions, the effect of scattering by particles is not so important on the radiative heat source and radiative heat flux to the wall whereas their absorption effect is important and can increase the radiative heat source and wall heat fluxes.     

A numerical study of steady flow and temperature fields within a melt spinning puddle

A numerical solution of the momentum and energy equations yields steady flow and temperature fields within a melt spinning puddle. The model accounts for inertial, viscous, surface tension, and wetting effects, and relies on a temperature-dependent viscosity to solidify ribbon to an amorphous state. A reference simulation is presented, followed by results that examine the effect of varying individual parameters. Results demonstrate a strong influence of nozzle wetting on puddle size; significant recirculation upstream of the nozzle slot; and the presence of both recirculation and an unrelated bump downstream of the slot.     

地铁车站模型内流场的实验测定及数值模拟

CFD技术及其商业软件的发展使人们可以用数值模拟的方法预测地铁车站内环境,评价通风效果,改进通风排烟系统的设计,在提供舒适环境的同时,进一步降低能耗.为了对数值计算结果进行检验,对某地铁车站模型内的气流组织进行了实验测定,并采用商业软件Fluent对地铁车站模型内的速度场进行了数值模拟.在数值计算中采用κ-ε方程作为紊流模型,以现场实测数据作为边界条件,计算结果与实测数据吻合较好.结果表明,采用商业软件对地铁车站内气流组织的数值模拟可以获得较准确的车站内流场的详细数据,从而可以对整个通风排烟系统进行全面评价.     

新型半导体制冷空调伞温度场流场的实验研究

针对传统制冷空调伞设备庞大且不节能环保等问题,提出一种小型便携式半导体制冷空调伞,自行搭建用于测试操作参数对新型半导体制冷空调伞温度场和流场影响的实验台,在定环境温度、空气流量和距地高度的实验条件下进行实验研究。实验结果表明,与传统制冷空调伞相比,新型半导体空调伞不仅可营造局域舒适性环境,而且还扩展了制冷系统的应用范围。     

A numerical study of flow and thermal fields in tilted Rayleigh–Bénard convection

Convection in enclosures heated from below can affect crystals grown from melts. Experiments designed to study such convection can be influenced by small tilts of the experimental system with respect to gravity. Because of the additional body force, tilting can mask flow transition points, making comparisons with stability studies difficult. In this study, the classic Rayleigh–Bénard problem is re-examined numerically with the addition of various tilt angles in cubical enclosures of liquid tin (Pr = 0.008). The results presented are applied to experiments which measure both molecular diffusivities as well as convection in the melt.     

Experimental and numerical study of sidewall profile effects on flow and heat transfer inside microchannels

The laminar flow characteristics inside slightly tapered silicon microchannels in the hydraulic diameter range of 53–112 μm are investigated. Velocity profiles for planes located at different channel depth are measured using micro resolution particle image velocimetry (micro-PIV). It is revealed that the location of the maximum velocity deviates from the mid-plane along the depth direction due to the wall taper. Numerical simulations are also carried out to examine the effects of the sidewall angle on flow and heat transfer. Large deviation in the velocity profile and consequent significant degradation in heat transfer are observed for considerably tapered microchannels.     

Three-dimensional numerical study on cell performance and transport phenomena of PEM fuel cells with conventional flow fields

In this paper, a three-dimensional numerical model of the proton exchange membrane fuel cells (PEMFCs) with conventional flow field designs (parallel flow field, Z-type flow field, and serpentine flow field) has been established to investigate the performance and transport phenomena in the PEMFCs. The influences of the flow field designs on the fuel utilization, the water removal, and the cell performance of the PEMFC are studied. The distributions of velocity, oxygen mass fraction, current density, liquid water, and pressure with the convention flow fields are presented. For the conventional flow fields, the cell performance can be enhanced by adding the corner number, increasing the flow channel length, and decreasing the flow channel number. The cell performance of the serpentine flow field is the best, followed by the Z-type flow field and then the parallel flow field.     

制粉系统防堵煤综合治理改造技术研究及工程实践

本文通过分析某电厂制粉系统雨季堵煤的原因,提出使用降低给煤机出口煤流的速度的方法,并加大给煤机到磨煤机之间落煤管的距离,同时对磨煤机内部流动过程进行了三维数值模拟,获得了磨煤机内部风粉混合物运动的规律,达到了煤流顺利通过落煤管而不与内壁接触的目的,取得了良好的工程改造效果。     

Experimental study of effects of one kind of tangentially non-uniform tip clearance on the flow field at an exit of a compressor cascade passage

This paper presents an experimental investigation of effects of one kind of tangentially non-uniform tip clearance on the flow field at an exit of a compressor cascade passage.The tests were performed in a low-speed large-scale cascade with the uniform tip clearance and the non-uniform clearance.The three-dimensional flow field was measured at the exit at three incidence angles of 0°,5°,and 8° using a mini five-hole pressure probe.The measurement results show that the non-uniform tip clearance can moderate the leakage flow and blow down more low-energy fluids at the tip corner and decrease the accumulation of low-energy fluids which cause the flow blockage in the blade passage.In the meantime,the non-uniform clearance can weaken the tangential migration of the low-energy fluids in the endwall boundary layer and reduce the secondary loss and the flow blockage in the tip region.