椭圆形换热单管积灰特性的数值模拟研究

椭圆形换热管作为一种强化换热元件,在抗积灰性能方面具有一定优势。本文基于ANSYS FLUENT软件平台建立了一套模拟程序,针对椭圆形换热单管的积灰特性展开了数值模拟研究。重点研究了换热管的椭圆度、烟气流速以及飞灰颗粒粒径对飞灰沉积特性的影响。研究表明,当换热管的椭圆度在1 ~ 2之间变化时,5 ~ 100 μm粒径的颗粒沉积率最小值大体分布在1.2 ~ 1.6之间;烟气流速在1.14 ~ 9 m/s变化时,粒径30 μm以上颗粒的撞击率增大,但黏附率下降,最终沉积率呈下降趋势;随着颗粒直径的增大,颗粒的撞击率增大,但其黏附率下降,而其沉积率先增大后降低,烟气流速在3 ~ 9 m/s变化时,颗粒沉积率的峰值出现在粒径为20 ~ 30 μm之间。     

燃准东煤锅炉辐射受热面的灰沉积特性

建立了与气固两相流动燃烧模型相耦合的飞灰沉积模型,对一台660 MW超临界锅炉燃用准东五彩湾煤时的飞灰沉积特性进行了数值模拟,并且讨论了烟气流速和灰颗粒粒径对飞灰沉积速率的影响规律.结果表明:在BMCR工况下,炉膛辐射受热面上的飞灰颗粒沉积分布主要集中在高度为30～45 m处的区域,且各墙的沉积分布规律呈现出相似性;烟气流速为9.5～12.5 m/s时,辐射受热面上的飞灰沉积速率与烟气流速成正比;烟气流速大于12.5 m/s时,辐射受热面上的飞灰沉积速率与烟气流速成反比;辐射受热面上飞灰颗粒的沉积速率的整体变化趋势是随灰颗粒粒径的增大而增大的.     

含尘烟气在波纹板换热面上流动积灰换热数值模拟研究

含尘烟气在换热器表面积灰,会降低换热器传热性能,增加流动阻力,并会造成换热器表面腐蚀。通过对含尘烟气流过波纹板换热面研究,建立数学模型,采用数值模拟方法,分析了不同流速、不同含尘粒径及浓度的烟气流过波纹板换热表面的沉积、换热与阻力特性。研究结果表明:含尘烟气在波纹板换热面上流动时,不同粒径尘粒的沉积速率不同,尘粒的沉积速率在圆凸表面的迎风侧比背风侧高,尘粒沉积率随流速的增加而降低。为含尘烟气换热设备研发设计与烟气余热利用提供参考。     

板式换热器颗粒污垢特性的实验研究

以纳米氧化镁颗粒溶液为实验工质,进行了板式换热器颗粒污垢特性的实验研究,分析了颗粒质量浓度、颗粒粒径、流速和低温介质温度对颗粒污垢热阻的影响.结果表明:板式换热器颗粒污垢无明显诱导期存在,结垢速率和污垢热阻渐进值均随颗粒质量浓度的增大而增大,且增大幅度逐渐减小;颗粒粒径对污垢热阻的影响较明显,在相同质量浓度下,颗粒粒径越小,结垢速率越快,且污垢热阻越大;流速对污垢热阻的影响较为复杂,高流速下的结垢速率略大于低流速下,且高流速下达到稳定时的污垢热阻渐进值小于低流速下;低温介质温度对颗粒污垢热阻的影响不明显.     

三种埋管换热器换热性能的模拟与实验研究

本文采用数值模拟与实验相结合的方法,研究了单U型、双U型及套管式地埋管换热器在不同管内流速下的换热情况以及埋管周围土壤温度场的变化情况。研究表明:实验值与模拟值相吻合;地埋管换热器换热能力随着管内流速的增大而增强;三种形式地埋管换热器换热能力以套管式最优,双U型及单U型地埋管次之。     

硅灰在对流管束表面沉积特性的试验研究

硅冶炼炉余热烟气中的硅灰在换热器表面沉积影响换热效率,研究了硅灰的基本粘结特性,及其在对流管束表面的沉积特性和清除方式。试验结果表明：硅灰的粘结性不强,主要在管壁迎风侧形成单侧楔形干松灰,虽然温度影响灰样的粒径分布,但管壁温度对硅灰的沉积和清除特性影响不大;越往烟气下游的受热面积灰越严重;存在临界烟气流速12m／s,当烟气流速超过该值,增大流速不再有利于减轻积灰;钢珠清灰可高效地清除管壁积灰,而机械振打效果不佳。     

内螺纹管在低温烟气中的传热特性研究

为了研究内螺纹管对低温烟气传热强化的效果,通过对内螺纹管换热器和光管换热器在低温烟气中的传热试验,比较分析内螺纹管和光管两种换热器在不同工况下的传热系数,根据试验数据计算拟合出试验传热关联式。结果表明:烟气流速对总的换热系数有较大的影响,工质水流速不变,烟气流速从2.0 m/s增加到3.0 m/s时,内螺纹管换热器的换热系数增长率为17.1%;管内工质水无相变时,工质水流速对总的换热系数影响不大;低温换热的热阻主要集中在烟气侧;内螺纹管可以强化低温烟气的换热,但强化效果不明显;内螺纹管工质水侧的传热关联式Nu=0.009Re~(0.985)Pr~(0.4)(1.1×10~4Re2.3×10~4)。     

翅片管换热器积灰对空气侧传热和压降影响的实验研究

文章选取室外实际运行的翅片管换热器,在采样分析其表面沉积粉尘粒度的基础上,搭建换热器积灰实验台,以平直翅片管换热器为测试样件,研究积灰对换热器空气侧传热和压降的影响。结果表明,换热器表面不同位置沉积粉尘的粒径分布规律相近,沉积粉尘的粒径集中分布在1～100μm;换热器空气侧积灰对压降的影响大于对换热的影响;高风速能够减小积灰对换热器空气侧传热和压降的影响,当风速从1.5 m/s增大到3.0m/s,空气侧污垢热阻减小54%,压降增幅减小18.3%;换热器稳定后的空气侧污垢热阻及压降增幅,在低风速时随粉尘粒径的增大而减小,高风速时随粉尘粒径的增大而增大。     

燃气中颗粒在换热表面沉积特性

研究Stokes数对颗粒在换热器壁面捕获及沉积的影响,采用欧拉-拉格朗日方法及El-Batsh和Haselbacher提出的粒子沉积模型模拟稀疏气固两相流,讨论了颗粒横掠受热表面的沉积分布。结果表明:不管是改变粒径还是改变流速导致的St数变化,对颗粒沉积特性都有影响;St数减小,圆管周向的颗粒沉积量增加,沉积区域增大且分布均匀;St数减小到一定程度,顺叉排管束对沉积影响的区别减小。     

准东褐煤燃烧形成的颗粒在圆管上的沉积数值模拟

准东煤中的碱性化合物在燃烧时会形成熔点低的灰烬,在锅炉的运行中容易引起传热表面的结垢,烟气通道的堵塞,传热速率的降低以及过热器管的腐蚀等问题。文中提出了一个模型用以描述和预测高碱金属成分烟气飞灰沉积的形成。在目前的工作中,建立的数学模型包括了从燃料床中释放出飞灰颗粒和碱蒸气向换热表面运输过程以及飞灰颗粒和碱蒸气的沉积行为。结果表明,烟气速度主要通过影响飞灰颗粒的碰撞效率对沉积产生影响,而在高烟气速度条件下,炉温通过改变积灰表面的熔融组分来影响飞灰颗粒的黏附效率。惯性碰撞和热泳引起的颗粒沉积决定了高炉温下灰分的形成,碱蒸气直接冷凝引起的沉积量主要存在于前期。     

Unsteady CFD simulation on ash particle deposition and removal characteristics in tube banks: Focusing on particle diameter,flow velocity,and temperature

This study establishes a numerical deposition and removal model coupled with unsteady gas–solid turbulent flow to predict transport, impaction, sticking, and removal (or rebounding) characteristics of ash particles with high alkali metals based on Ansys Fluent software extended by user–defined functions. Dynamic mesh analytical strategy is employed to adjust the grid on the interface of flue gas and deposition layer to illustrate ash deposit growth characteristics. In this study, the effects of particle size, flow velocity, and inlet gas temperature on formation and distribution of ash deposits are studied. The results indicate that ash deposit distribution has significant particle size dependence. Larger particles tend to deposit on the windward side of first–row tubes, and account for the greatest share of total deposited mass. Smaller particles may deposit on the leeward side of the tube surfaces. Variation in impaction rates with time are influenced by the thermophoretic force and enlarged deposition area. With the increase in flow velocity, total deposited mass increases gradually. The influence of flow temperature on impacting rates is mainly embodied in the effect of flow temperature on thermophoretic force. However, the increase in inlet gas temperature has little effect on the sticking efficiency and ash deposit distribution. Nevertheless, the total deposited mass shows an increasing trend with inlet flue gas temperature.     

Numerical evaluation of a heat exchanger with inline tubes of different size for reduced fouling rates

This paper describes the numerical evaluation of a novel cross flow tube bundle heat exchanger that combines tubes of different diameter in an inline arrangement for the purpose of reducing gas side particulate fouling rates while preserving acceptable levels of heat transfer and pressure drop performance. Three arrangements are compared: a common inline tube bundle heat exchanger with cylinders of equal diameter and two other arrangements that consist of alternately placed cylinders with a diameter ratio of d/D = 0.5, at two different transverse spacings. Numerical calculations are performed in order to study heat transfer, pressure drop and fouling rates from flue gases with suspended ash particles. The alternating tube sizes achieve a suppression of the vortex shedding mechanism that has previously been shown to enhance downstream particle deposition. Results show that, compared to the standard arrangement, the tube bundle with unequal cylinders placed at the largest transverse spacing achieves a significant (～30%) reduction in particle deposition rate without sacrificing acceptable values of heat transfer per unit volume and low pressure drop.     

Simulation of particle deposition on the tube in ash-laden flow using the lattice Boltzmann method

The LBM-Lagrange tracking method with multiple relaxation time (MRT) model has been developed to predict the flow field and particle deposition a circular or elliptical tube in ash-laden gas turbulent flow with Re of 10,229. The model can be used for predict particle deposition effect on thermal resistance or fouling factor of heat exchangers mostly operating in turbulent flow.Particle deposition morphology on the circular and the elliptical tubes were obtained with the lattice Boltzmann method (LBM). The particle deposition mechanism has been investigated. The dominating mechanism of particle deposition on the circular tube is Brownian diffusion for the Stokes number of 0.002, whereas the dominating mechanism of particle deposition is drag inertia for the Stokes number larger than 0.031. When the long axis of the elliptical tube is parallel to the flow, both the collision efficiency and the deposition efficiency for the elliptical tube are fewer than those of the circular tube which means less particle deposition. It also can be concluded that both ratios of the collision efficiency and the deposition efficiency decrease with increasing axial length ratio of the elliptical tube. The elliptical tube is better than the circular tube as heat transfer surface in the aspect of preventing ash particle deposition.     

Fouling Reduction Characteristics of a No-Distributor-Fluidized-Bed Heat Exchanger for Flue Gas Heat Recovery

Heat exchangers and heat exchanger networks are extensively used for the purpose of recovering energy. In conventional flue gas heat recovery systems, the fouling by fly ashes and the related problems such as corrosion and cleaning are known to be major drawbacks. To overcome these problems, a single-riser no-distributor-fluidized-bed heat exchanger is devised and studied. Fouling and cleaning tests are performed for a uniquely designed fluidized bed-type heat exchanger to demonstrate the effect of particles on the fouling reduction and heat transfer enhancement. The tested heat exchanger model (1 m high and 54 mm internal diameter) is a gas-to-water type and composed of a main vertical tube and four auxiliary tubes through which particles circulate and transfer heat. Through the present study, the fouling on the heat transfer surface could successfully be simulated by controlling air-to-fuel ratios rather than introducing particles through an external feeder, which produced soft deposit layers with 1 to 1.5 mm thickness on the inside pipe wall. Flue gas temperature at the inlet of heat exchanger was maintained at 450°C at the gas volume rate of 0.738 to 0.768 CMM (0.0123 to 0.0128 m³/sec). From the analyses of the measured data, heat transfer performances of the heat exchanger before and after fouling and with and without particles were evaluated. Results showed that soft deposits were easily removed by introducing glass bead particles, and also heat transfer performance increased two times by the particle circulation. In addition, it was found that this type of heat exchanger had high potential to recover heat of waste gases from furnaces, boilers, and incinerators effectively and to reduce fouling related problems.     

Effect of Condensable Species on Particulate Fouling

The flue gases emanating from the combustion of fuels or gasification process invariably comprise particulate matter and many chemical species in vapor form. The temperature of the flue gases gradually reduces when passing through different sections of heat exchanger, such as the superheater, evaporator, and so on. If the temperatures of the heat exchanger tube surface and the gas phase are favorable for condensation, the chemical species in the vapor form will condense on the particles and on the tube surface. The particle deposition behavior under these conditions is drastically different from the one observed in dry particulate fouling. In order to model the particle deposition under such circumstances, it is important to evaluate the criteria for particle adhesion to the surface. Impaction experiments of particles impacting a surface coated with a thin liquid film and particles that are coated with a liquid film impacting over a dry surface are performed to evaluate the limiting parameters under which a particle sticks to the surface without rebounding. The effects of liquid viscosity, liquid film thickness, and interacting material properties are evaluated. The experimental results are compared to the results of existing models and a suitable model for fouling is proposed. Controlled fouling experiments are performed for varying liquid films coated over a deposition tube under various process conditions to mimic the condensation effects on fouling. The results are compared with detailed impaction experiments.     

玻璃窑余热利用关键技术问题的探讨

结合玻璃窑的特点,提出影响玻璃窑烟气余热参数的主要因素,给出玻璃窑在燃用各种燃料下的典型烟气成分、粉尘特性和酸露点,对玻璃窑烟气粉尘在锅炉受热面表面的积灰机理和腐蚀过程进行了探讨,并提出余热锅炉减少积灰、预防低温腐蚀的主动措施和被动措施,针对玻璃窑窑压调整平稳的要求,提出了余热锅炉投运后的窑压调节和控制方式.     

Computer-controlled scanning electron microscopy of minerals in coal-Implications for ash deposition

The nature of mineral matter in coal determines its transformation into ash during combustion and the nature of resulting ash (e.g. chemical composition and particle size distribution), and subsequently influences the ash deposition behaviour. The behaviour of mineral matter is primarily influenced by two parameters: the mineral grain size, and whether the mineral grains are within the coal matrix or not. Computer-controlled scanning electron microscopy (CCSEM) of coal provides such information on mineral matter in coal. CCSEM data are, therefore, processed to predict the fouling and slagging characteristics of several coals. The fraction of basic oxides in each mineral grain may be considered as an indicator of stickiness of the corresponding ash particle due to formation of low melting compounds. The cumulative mass fraction of mineral grains with certain basic oxides or viscosity of resulting ash particles from included and excluded minerals are proposed as alternative indices for ash deposition.

The excluded mineral matter is in equilibrium with the combustion flue gases at the gas temperatures, whereas the included minerals are in equilibrium with the atmosphere within char at the burning char particle temperature. It is predicted from thermodynamic calculations based on this understanding that almost all the evaporation is either from the included mineral matter or from the atomically dispersed minerals in coal. This is due to the high temperature and reducing atmosphere inside the char particle. The release of the evaporated species is controlled by diffusion through the burning char particle and, therefore, may be estimated theoretically. The amount of mineral matter that is vaporized may then be related to fouling, whereas the melt phase present on the surface of large ash particles may be related to slagging. The theoretical speculations on the physical character of ash derived from these indices are compared with the experimental data obtained from combustion of coals in a drop-tube furnace.     

A novel method used to study growth of ash deposition and in situ measurement of effective thermal conductivity of ash deposit

Ash deposition always brings boilers some trouble due to fouling or slagging. In this paper, a completely controlled system was developed to study the growth of ash deposit. A novel sampling probe was designed to online measure the heat flux through ash deposit. Additionally, the thickness of ash deposit can be obtained by an online figure collecting system. The results of this research showed that as the thickness of ash deposit increased, the heat flux decreased. It was also found that at the initial stage of ash deposition when the thickness of ash deposit is approximately 1 mm, the heat flux through ash deposit had a sharp reduction. An effective method was attempted to situ measure the effective thermal conductivity of the ash deposit in the simulated combustion flue gas. It was found that temperature of the ash deposit layer had no obvious effect on its value. It was concluded that the structure of ash deposit had no obvious change in a short deposition time of 30 min with varied surface temperatures of the probe head between 400 °C and 600 °C.     

Influence of phosphorus on ash fouling deposition of hydrothermal carbonization sewage sludge fuel via drop tube furnace combustion experiments

Phosphorus effect on ash fouling deposition produced during combustion process of sewage sludge solid fuel is a very important factor. Previous studies have only focused on decrease of the ash melting temperature and increase of slagging and sintering by phosphorus content. Therefore, research regarding combustion fouling formation and its effect on temperature reduction of deposit surface by phosphorus content is insufficient. Ash fouling is an important factor, because ash in the combustion boiler process deposits on the surface of heat exchanger and interferes with heat exchange efficiency. In particular, temperature reduction of heat exchanger surface via fouling should be considered together with fouling deposition, because this is related to the heat exchanger efficiency. Synthetic ash, phosphorus vaporization, and drop tube furnace experiments were performed to investigate effect of phosphorus on ash fouling formation and temperature reduction of deposit surface under combustion condition. Phosphorus was highly reactive and reacted with ash minerals to produce mineral phosphate, which promoted ash fouling deposition during the combustion experiments. In contrast, the occurrence of sintering on deposited fouling resulted in formation of a large hollow structure, which alleviated the temperature reduction on the deposit surface. Phosphorus content had a substantial correlation with fouling deposition behavior and influenced reduction in the surface temperature of the heat exchanger, because it led to generating low temperature mineral phases.