下降管反应器内颗粒流动升温过程数值研究

采用数值模拟的手段研究了下降管反应器内包含不同尺寸及密度冷热颗粒混合物的流动传热特性。双流体模型及离散单元法分别被用于描述颗粒混合物的流动过程并与实验结果进行了对比。反应器内气固相间传热,颗粒混合物间碰撞传热,以及壁面与气/固两相间的热量传递采用计算流体力学和离散单元法相耦合的方式进行了模拟,对颗粒到达反应器出口前影响温度变化趋势的因素展开了分析研究。模拟结果表明:在V型下降管反应器内,粒度较小的颗粒以沿壁面向下滑动为主;较大尺寸颗粒向下流动过程中在反应器截面上分布区域较广;当反应器壁面热边界条件发生变化时,颗粒升温过程变化明显,采用恒温壁面冷颗粒升温速率明显提高;同时热载体颗粒数目越多,冷颗粒在下降管反应器内升温越快。     

300MW循环流化床锅炉稀相区的传热研究

本文针对燃烧煤矸石的循环流化床锅炉的传热情况展开研究,以山西平朔电厂1台300 MW的循环流化床锅炉为实例,采用环核模型和颗粒团更新模型,对稀相区的传热系数分布进行建模计算,本文所建模型考虑炉内床温实际分布特征,根据现场温度实测数据对模型进行修正,研究了对流和辐射换热系数在不同负荷下沿炉膛高度的变化情况。锅炉在较高负荷下运行时,负荷的波动对颗粒团壁面覆盖时均份额影响较小,继而对炉内对流换热影响较小。炉内环形区温度沿床高的偏差随负荷升高略有减小,且对辐射换热影响比对流换热大。随着负荷升高,对流换热系数沿炉高下降增大,而辐射换热系数沿炉高下降减小,高负荷时炉内总换热系数沿炉高下降25%左右,低负荷时沿炉高下降28%左右,高负荷下炉内沿高度温差更小,传热更稳定。     

连续液滴撞击热壁面传热特性的研究

为了研究不同碰撞速度以及不同换热方式对连续液滴撞击热壁面传热特性的影响,以8滴去离子水和铜板为试验材料,设置了速度分别为0.63 m/s、0.77 m/s、0.89 m/s、0.99 m/s的8滴去离子水液滴在膜态蒸发(铜板温度60℃)和核态沸腾(铜板温度110℃)两种换热方式下的液滴撞击热铜板试验,探究铜板热流密度值的变化。结果表明:膜态蒸发换热方式下,液滴撞击热壁面速度越大,铜板的最大热流密度值越大,液滴与壁面之间的换热效果越好。核态沸腾换热方式下,以0.89 m/s的速度值为转折点,当连续液滴撞击热壁面速度小于0.89 m/s时,液滴撞击速度越大,铜板热流密度值越大;当连续液滴撞击热壁面速度大于0.89 m/s时,随着液滴撞击速度的增大,铜板的热流密度值减小。     

超临界碳氢燃料中结焦固体颗粒的沉积规律研究

为了研究碳氢燃料中的结焦颗粒在冷却通道内的沉积规律,对超临界碳氢燃料中结焦固体颗粒在冷却管路中的积聚情况进行了数值模拟和实验研究。采用欧拉-拉格朗日离散颗粒模型,sst k-w模型对液固两相耦合下结焦颗粒的沉积进行数值模拟。模拟结果表明:结焦颗粒在圆柱形孔板附近更容易沉积;壁面越粗糙,燃料质量流量越大,燃料温度越高,结焦颗粒的积聚现象越弱;结焦颗粒直径越大,在壁面处的沉积率越大。实验结果表明结焦颗粒在圆柱形孔板中的沉积比在渐变形孔板中的沉积严重。采用渐变型孔板,减小结焦颗粒的直径有助于改善结焦颗粒的沉积现象。     

A numerical investigation into the charge behaviour of a spherical phase change material particle for high temperature thermal energy storage in packed beds

基于高温相变材料,对填充床储热系统中储热单元球体的储热性能进行了模拟研究.研究了不同传热流体温度和球体直径对球体储热性能的影响规律,对导热为主的相变储热过程与导热和自然对流共同作用的相变储热过程进行了比较分析,同时还探讨了高温辐射换热的影响.结果表明,相变时间随球体直径的增大而增大,随传热流体温度的增大而减小.当考虑相变区域自然对流时,总的相变时间显著减少,和单纯导热相比,完全相变时间缩短了近16%.在导热和自然对流的基础上加上辐射传热后可以看出,辐射换热强化了球体内的传热过程,加快了相变材料的熔化速度,强化了自然对流的作用.     

竖直降落式反应器中颗粒流流动传热的CFD-DEM模拟

掌握竖直降落式热解反应器中污泥颗粒的流动与传热规律是设计反应器的关键．采用计算流体力学与离散元(CFD-DEM)耦合的方法模拟了竖直降落式反应器中污泥颗粒的流动与传热过程．首先,利用文献中的实验结果与传热模拟结果比对,验证了传热模型的合理性．进而考虑了热气流输入、反应器尺寸和污泥颗粒变化等对传热特性的影响．其中颗粒的质量随温度的变化规律通过热重实验测得,进而通过拟合得到颗粒的粒径与温度的关系．计算结果表明,颗粒在反应器内的流动接近于“活塞流”,气相压降较大(1329 Pa/m);颗粒与气相之间的对流传热占据主导地位,污泥颗粒热解传热过程颗粒形变的影响较大、不可忽略,反应器管径超过60 mm以后,管径增大会导致壁面传热占比急剧下降．此计算为污泥竖直降落式热解反应器的设计提供了依据．     

三维填充床内换热及热回流机理研究

利用计算流体力学（CFD）对顺序排列多孔介质小球的三维填充床进行数值模拟。研究填充床内位置及空气流速变化对温度分布、努塞尔数影响,并对多孔介质小球的热回流特性进行分析,揭示填充床内传热机理。结果表明：相比于气-固两相交替存在处,与小球相切处的热的非平衡性更强。最高温度上游的换热强度与下游相比更强烈;当流速增加时,上游的对流换热作用增强,下游变化不大。在热回流过程中,在入口区域对流换热占主导地位,导热和辐射换热作用较弱;在主流场区域,导热占主导地位,其次是辐射换热,对流换热作用最弱。     

泡沫金属蓄能相变材料的传热数值模拟

采用双温度模型对泡沫金属基复合相变材料的传热过程进行数值模拟。通过孔隙努赛尔数描述金属骨架与相变材料之间的传热,定义壁面努赛尔数描述整体传热性能。将方程无量纲化分析斯蒂芬数以及粘度对熔化传热过程的影响。结果表明,自然对流使上部熔体熔化更快;增大斯蒂芬数时,熔化界面推进速度加快,壁面努赛尔数减小;粘度主要影响格拉晓夫数,粘度减小,对流换热增强,熔化界面出现明显倾斜,进一步加强上部区域的熔化界面推进;在熔化后期,粘度越小,壁面努赛尔数越大。     

双玻璃窗空气间层的最佳厚度

在直接受益式太阳房中,为减少直接受益窗的热损失,往往要使用双层玻璃窗。如果我们不考虑窗户冷风渗透,仪考虑双层窗的传热热损失,那么,双层玻璃窗空气间层多厚时其传热热损失最小呢?从传热学角度分析,我们可以把双层玻璃窗空气间层看成是二维封闭空间的传热问题,其外层玻璃是冷壁面,内层玻璃是热壁面,因此成为一个有限封闭空间内两壁面之间对流、辐射和导热的综合传热过程。可以进一步认为,当内外层玻璃温度一定时,两玻璃之间的辐射换热量不随它们之间空气间层厚度而变化,也就是说此时可以认为辐射换热量是一个常数。因此,确定最佳厚度归结为寻找使对流和导流传热量达到最小     

循环流化床颗粒团更新传热模型的修正

对颗粒团更新传热模型进行了修正,引入新的床内流动特性的研究结果。改进了颗粒团覆盖壁面的百分比和颗粒团-壁面之间的气膜厚度的表达式,使之与床的宏观运行参数以及床体,床料的参数相联系,从而避免了前人模型中凭经验来确定一个比例系数的缺陷,对于弥散相和壁面间的辐射传热,考虑到了它们的直接辐射和弥散相辐射到颗粒团,再经颗粒团反射到壁面的传递过程,模型计算结果和有关实验数据的对比吻合较好,图4表1参10     

Numerical modeling of axial bed-to-wall heat transfer in a circulating fluidized bed combustor

The water-wall surfaces located above the secondary air inlet within the circulating fluidized bed (CFB) combustor are exposed to the axial bed-to-wall heat transfer process. In the current work, the axial bed-to-wall heat transfer coefficients are estimated for three different axial voidage profiles (covering three widely occurring average particle concentrations) in order to investigate the effect of voidage, time, initial and fixed temperature of the bed and annulus, and gas gap between wall and solid particles; on the axial heat transfer process. A 2D thermal energy balance model is developed to estimate the axial heat transfer values for the gas–solid suspension along the height of the riser column with horizontally changing mass distribution. The gas–solid mass distribution is fixed with time thus providing a spectrum of changes in axial bed-to-wall heat transfer profile with time. The current work provides an opportunity to understand the axial heat transfer relationship with particle concentration and instantaneous behaviour. The results from the work show that: (i) first few seconds of the suspension temperature near the wall has maximum energy thus providing a small time frame to transfer more heat to the surface (CFB wall); (ii) both axial and horizontal particle concentrations (influenced by the operating conditions) affect the axial heat transfer locally; (iii) initial temperature of the bed between average and maximum values provide end limits for the axial heat transfer; (iv) annulus region has higher thermal energy than the core due to increased particle presence; and (v) a particle-free zone near the wall (gas gap) having a maximum thickness of 1 mm, tends to reduce up to 25% of axial heat transfer value. The model trends have close agreement with experimental trends from published literature; but the model values differ when correlating with real values due to inconsistencies in riser diameter and nature of variation in parameters.     

Thermal performance investigation of a single medium temperature phase change microcapsule used for wind power absorption and heat storage system

Phase change microcapsules have a wide application in the heat storage system. The medium temperature heat storage systems such as medium temperature solar thermal plants, waste heat recovery systems and wind power absorption systems. In order to analyse the effects of configuration parameters and materials on phase change heat transfer process in a single medium temperature microcapsule, an enthalpy-transforming model was applied to trace the location of the solid-liquid interface and obtain the liquid fraction at different time in the melting process. Based on this model, the effects of particle size, the effects of wall thickness, the effects of wall materials and different medium temperature phase change materials were analyzed. The numerical results show that the larger particle size has a longer melting time, the melting time of 50 μm particle size and 250 μm particle size is 0.036 s and 2.48 s, respectively. In addition, the melting time of microcapsules with different wall thicknesses from the 1μm to 9μm is the same i.e., 0.14 s. Therefore, the wall thickness has little effect on the melting time of microcapsules. Besides, the microcapsule with the erythritol as inner material and the polystyrene as wall material has the longest melting time. Furthermore, the thermal conductivity of the wall materials is the main factor affecting the melting time. Moreover, the product of latent heat and density of phase change material is the main factor of the melting time.     

Effect of pressure and temperature on cluster and particle heat transfer in a pressurized circulating fluidized bed

The present work reports the influence of pressure and bed temperature on particle‐to‐wall heat transfer in a pressurized circulating fluidized bed (PCFB). The particle convection heat transfer plays a dominant role in determining the bed‐to‐wall heat transfer coefficient. So far, no information is reported on the effect of pressure and bed temperature on particle‐to‐wall heat transfer in a PCFB in the published literature. The present investigation reports some information in this direction. The effect of system pressure and bed temperature are investigated to study their influence on cluster and particle heat transfer. The particle convection heat transfer coefficient increases with system pressure and bed temperature due to higher cluster thermal conductivity. The increase in particle concentration (suspension density) results in greater cluster solid fraction and also the particle concentration near the wall is enhanced. This results in higher cluster and particle convection heat transfer between the bed and the wall. Higher particle convection heat transfer coefficient results in enhanced heat transfer between the bed and the wall. The results will also help to understand the bed‐to‐wall heat transfer mechanism in a better way in a PCFB. Copyright © 2001 John Wiley & Sons, Ltd.     

Numerical simulation of the devolatilization of a moving coal particle

The devolatilization of an isolated coal particle moving relative to the surrounding gas is numerically simulated using a competing reaction model of the pyrolysis and assuming that the released volatiles burn in an infinitely thin diffusion flame around the particle or not at all. The temperature of the particle is assumed to be uniform and the effects of the heat of pyrolysis, the intraparticle mass transfer resistance, and the variation of the particle radius are neglected. The effects of the size and velocity of the particle and of the temperature and oxygen mass fraction of the gas on the particle and flame temperature histories, the devolatilization time and the yield of light and heavy volatiles are investigated. The motion of the particle may have an important effect on the shape and position of the flame of volatiles, but it has only a mild effect on the devolatilization process for the particle sizes typical of pulverized coal combustion. This effect increases for large particles or in the absence of radiation. The relative motion enhances the heat transfer between the particle and the gas, causing the devolatilization time to decrease at high gas temperatures and to increase at low gas temperatures. The numerical results are compared with a blowing-corrected Nusselt number correlation often used in heat transfer models of the process.     

Stability analysis of MHD radiative mixed convective flow in vertical cylindrical annulus: Thermal nonequilibrium approach

In the present study, the influence of the induced magnetic field on the MHD mixed convective electrically conducting fluid flow inside the vertical cylindrical annulus is analyzed numerically. The heat transfer is presumed to be due to a combination of mixed convection and radiation. The stability of the flow is examined when the solid and fluid phases are not in local thermal equilibrium. The governing equations are solved numerically by both finite difference and finite element methods. To control the flow formation rate more accurately the induced magnetic field is also considered in this study. As the magnetic Prandtl number (Pm) and Hartmann number (M) get enhanced, the velocity and induced magnetic fields get retarded in the annulus due to the presence of drag-like force, namely, the Lorentz force. When there is an increase in the mixed convection parameter the induced magnetic field gets enhanced. An increase in radiation parameter tends to decline the fluid temperature and reverse the behavior of the solid temperature. Increment in Pm decreases the wall shear stress near the conducting cylinder. Increasing values of porous, magnetic, and radiation parameters lead to an unstable system with smaller heat transfer coefficient values but the system gets stabilized for larger values of heat transfer coefficient. The results could be used as first-hand information for comprehending and developing the thermal flow phenomenon in porous media. The obtained numerical results are in good accordance with the existing results. Using an artificial neural network, heat transfer characteristics are analyzed through mean square error and regression analysis.     

Fundamental heat transfer mechanism between bed‐to‐membrane water‐walls in circulating fluidized bed combustors

In the present work, the fundamental mechanism between bed‐to‐membrane water‐walls in the riser column of a circulating fluidized bed (CFB) combustor is presented. The bed‐to‐membrane water‐wall heat transfer depends on the contributions of particle heat transfer, dispersed phase heat transfer and radiation heat transfer. The fundamental mechanism of particle heat transfer and the effect of fraction of wall exposed to clusters and gas gap thickness between cluster and wall on particle heat transfer coefficient and bed‐to‐wall heat transfer coefficient are investigated. The influence of operating parameters like cross‐sectional average volumetric solids concentration and bed temperature on particle and bed‐to‐wall heat transfer are also reported. The present work contributes some fundamental information on particle heat transfer mechanism, which is responsible for increasing the bed‐to‐wall heat transfer coefficient (apart from dispersed phase convection and radiation heat transfer). The details on particle heat transfer mechanism will enable to understand the basic heat transfer phenomena between bed‐to‐membrane water‐walls in circulating fluidized bed combustors in a detailed way, which in turn will aid for better design of CFB combustor units. The particle heat transfer mechanism is significantly influenced by the fraction of wall exposed to clusters and gas gap thickness between clusters and wall. Copyright © 2003 John Wiley & Sons, Ltd.     

Effects of primary soot particle size distribution on the temperature of soot particles heated by a nanosecond pulsed laser in an atmospheric laminar diffusion flame

Temperature histories of nanosecond pulsed laser heated soot particles of different primary particle size distributions were calculated using a single primary particle based heat and mass transfer model under conditions of a typical atmospheric laminar diffusion flame. The critical peak soot particle temperatures beyond which soot particle sublimation cannot be neglected were identified to be about 3300–3400 K. Knowledge of this critical soot particle temperature is required to conduct low-fluence laser-induced incandescence experiments in which soot sublimation is avoided. After the laser pulse, the temperature of smaller primary soot particles decreases faster than that of larger ones as a result of larger surface area-to-volume ratio. Unlike the common belief that the peak soot particle temperature is independent of the primary particle diameter, the numerical results indicate that this assumption is valid only when soot sublimation is negligible and for primary soot particle diameters greater than about 20 nm. The effective temperature of a soot particle ensemble having different primary particle diameters in the laser probe volume was calculated based on the ratio of the total thermal radiation intensities of soot particles at 400 and 780 nm to simulate the experimentally measured soot particle temperature using two-color optical pyrometry. In the non-sublimation regime, the initial effective temperature decay rate after the peak soot temperature is related to the Sauter mean diameter of the primary soot particle diameter distribution. At longer times, the effective temperatures of soot particle ensembles start to display different decay rates for different soot primary particle diameter distributions. A simple approach was proposed in this study to infer the two parameters of lognormal distributed primary soot particle diameter. Application of this approach was demonstrated in an atmospheric laminar ethylene diffusion flame with the inferred primary soot particle diameter distribution compared with independent ex situ measurement.     

Heat transfer in a directly irradiated solar receiver/reactor for solid–gas reactions

Particle laden solar receivers can be used at high temperatures for efficient heat transfer and fuel generation via chemical reactions. A theoretical analysis of a directly irradiated, particle laden, solar receiver is presented here and compared with experiments. The radiation characteristics of the particles are approximated using a method, which adapts Mie theory to certain cases where a solar receiver is used with seeded particles of variable sizes and shapes. Based on this model carbon black particles whose effective radius, r_p, is less than 100 nm are inefficient in absorbing solar energy and the most suitable particle sizes is in the same range as the wavelengths of the radiation (100 nm < r_p < 1000 nm). The heat transfer coefficient between the particles and the gas was calculated using a refined limiting sphere model developed for the transition regime between molecular and continuum transfer. Previous models assume that there are no gas molecule collisions in the energy transfer layer and the mean free path of the gas molecules is equal to the thickness of this layer. The present model accounts for molecule collisions in the energy transfer layer and therefore enables the thickness of this layer to be larger than one mean free path length. The model was extended to estimate the Nusselt number for gases with several atoms as well as for monatomic gas. A code to simulate the flow and heat transfer in the receiver was developed, utilizing the models for heat transfer from sunlight to the particles and from the particles to the gas. The receiver simulations show good agreement with the wall temperature distribution measured in experiments, but the gas exit temperature in the model was significantly lower than the measured value. This discrepancy could be due to limitations of the simulation code and the particle heat transfer models. The simulation suggests that changing the Nusselt number and particle radius have a small influence on the receiver wall and gas temperatures. Increasing the particle cloud concentration improves the receiver heat transfer up to a threshold value; further increase of the particles concentration has only a marginal influence on the receiver’s heat transfer. This result from the receiver modeling was in a good agreement with solar experiments.     

ANALYSIS OF TWO-PHASE RADIATION IN THERMALLY DEVELOPING POISEUILLE FLOW

Combined conductive and radiative heat transfer in a thermally developing two-phase Poiseuille flow in a cylindrical duct is studied here. A two-phase radiative transfer equation (RTE) considering radiation by both gas and particles is taken into account. A complexform of nonlinear integrodifferential RTE is solved by the discrete ordinates method (DOM, or so called SN method) in axisymmetric geometry. After such validation, namely, the solution in a two-dimensional channel flow between two flat plates is compared with that solved by the zone method, the program is then applied to fully developed gas-particle two-phase flow in a cylindrical duct. A parametric study is performed for gas and particle absorption coefficients, particle number density, particle emissivity, and wall emissivity. The results show a significant effect of two-phase radiation on the thermal characteristics. However, in all cases, it was found that conduction is predominant near the wall.     

NUMERICAL SIMULATION OF THE GAS FLOW AND MASS TRANSFER BETWEEN TWO COAXIALLY ROTATING DISKS

A problem of combined conductive and two-phase radiative heat transfer in a two-dimensional rectangular enclosure with two-phase (gas-particles) media is analyzed. A two-phase radiative transfer equation (RTE) considering radiation by both gas and particles is studied. Its nonlinear integrodifferential RTE is solved using the discrete ordinates method (DOM, or so-called S N method). To validate the program, we compare the solution in a two-dimensional rectangular black enclosure with others. The DOM is then applied to the unsteady thermal development in two-phase media contained in a rectangular enclosure. A parametric study is performed by changing the gas and particle absorption coefficients, particle number density, particle emissivity, wall emissivity, and aspect ratio of the enclosure. The results confirm a significant effect of the two-phase radiation on the thermal development in the geometry. However, it is found that the conduction is predominant near the hot wall.