运行工况对圆形楞涡流发生器CaSO4污垢特性的影响

为了研究运行工况对圆形楞涡流发生器CaSO4污垢特性的影响。采用数值模拟方法研究了布置圆形楞涡流发生器矩形通道内壁面CaSO4析晶污垢的沉积过程。主要分析了CaSO4溶液的浓度、壁面温度、入口速度和入口温度对污垢沉积率、剥蚀率和污垢热阻的影响。结果表明,随入口速度的增大沉积率和剥蚀率均增大,而污垢热阻值降低。随着壁面温度的增大沉积率、剥蚀率和污垢热阻均增大。随工质浓度的增大沉积率、剥蚀率和污垢热阻也是均增大。随入口温度的增大沉积率、剥蚀率和污垢热阻却基本不变。     

矩形通道中不同楞型涡流发生器的CaSO4污垢特性

为研究不同楞型涡流发生器的污垢特性,对安装有相同长宽高的圆形楞、矩形楞和三角楞三种涡流发生器的矩形通道进行了模拟研究。在入口温度不变的情况下,分别考察了工质流速、质量浓度以及壁面温度对三种涡流发生器污垢特性的影响。结果表明:三种涡流发生器的污垢热阻都具有相同的变化趋势。随着速度、质量浓度和壁温的增大,污垢热阻达到平衡的时间越来越短。污垢热阻随着流体速度的增大而减小,随着工质质量浓度的增加而变大,随着壁面温度的升高而增大。在相同的工况条件下通过对比三种涡流发生器可知,装有圆形楞涡流发生器的通道内污垢热阻渐近值最大,矩形楞次之,三角楞最小。     

浮点型涡流发生器CaSO_4析晶污垢沉积的模拟研究

采用污垢析晶污垢模型,对布置有浮点的涡流发生器的矩形通道进行了污垢沉积数值模拟。研究表明:单位面积污垢沉积量随着浮点尺寸的增大而减小,随着浮点排列间距的增加而增大。但当半径一定且浮点间距值小于两倍的浮点直径值时,间距变小不会减少污垢沉积量。将所得模拟结果与相关实验对比,验证了所采用模型的准确性。     

两种柱形涡流发生器CaSO_4析晶污垢特性实验研究

为得到涡流发生器污垢规律,采用质量浓度为2 100mg/L的硫酸钙过饱和溶液进行了传热过程中的污垢生成实验,并通过离线称重法得到了装有涡流发生器试片表面单位面积污垢沉积量生长曲线.结果表明：涡流发生器直径和排列间距对壁面污垢沉积量有显著影响.当涡流发生器直径（4mm）一定时,试片表面单位面积污垢沉积量均随着涡流发生器排列间距的增大而增加;在排列间距（10mm）一定时,污垢沉积量随着涡流发生器尺寸的增大而减少.     

微液滴在窄矩形通道内热泳沉积特性的研究

利用Fluent软件对微液滴随空气流在窄矩形通道内流动时的沉积情况进行了模拟,得到微液滴在窄矩形通道内的质量浓度分布和速度分布.结果表明:在存在温度梯度的情况下,微液滴会在窄矩形通道的管壁处发生沉积,且随着温度梯度的增大,微液滴在管壁处的沉积效应增强;通过增大温度梯度,可以进行微液滴的脱除.     

涡流发生器表面参量与析晶污垢热阻间的关联分析

为研究换热表面上涡流发生器的流动参数和结构参量对析晶污垢的影响,并比较各类参数的关联程度。通过数值计算的方式,得出在不同入口速度、壁面温度和工质质量浓度下污垢热阻的变化情况。并得出在不同楞长、排列间距和半径下污垢热阻的变化情况。并根据灰色关联原理分析三个流动参数和三个结构参数与污垢热阻的关联程度。经分析其中两种流动参数对污垢热阻的影响高于所研究的结构参数的影响,但污垢热阻与楞长的关联度高于工质质量浓度的关联度。     

三角形粗糙元的微通道内流动换热的模拟分析

应用蒙特卡洛直接模拟(direct simulation Mont Carlo,DSMC)方法数值分析具有三角粗糙元表面平行平板微通道内气体二维流动与换热.模拟表明:微通道内粗糙元对流动与换热有明显的扰动;粗糙微通道内的壁面速度滑移小于光滑微通道,并随粗糙元变大,速度出现更为严重的跳跃,甚至出现漩涡,增加了通道内的压力损失;随粗糙元变大,气体在壁面处滞留时间变长,增加了单位质量气体与壁面之间的换热.     

流场剪切力对微生物污垢影响的CFD模拟

再生水水质复杂,在再生水源热泵板式换热器中极易形成微生物污垢,严重影响换热性能和系统安全。在微生物污垢的研究中,微生物污垢所处流场与微生物污垢的受力和生长是密不可分的。利用CFD方法,借助FLUENT软件,对微生物污垢所处流场进行模拟,从改变流场和强化剪切力的角度出发,主要探究了在矩形流道的基础上,加入主动脉冲流、含有微刻痕、含有颗粒相的流场剪切力对微生物污垢生长的影响。模拟结果显示:方波形式的脉冲流以及微刻痕可以有效增加壁面剪切力,且脉冲周期越小、微刻痕尺寸越小,壁面剪切力增加越多;含有颗粒相的流场,随着颗粒粒径的增加,颗粒个体碰撞概率增加,单位质量碰撞概率减小。     

半球型涡流发生器结构对污垢沉积影响

通过数值模拟的方法,研究半球型涡流发生器的结构参数变化对析晶污垢沉积过程的影响。考察了球窝半径、球凸半径、球窝与球凸圆心距以及布局对污垢热阻的影响。结果表明,经过模拟得出球凸半径变化对污垢热阻的影响高于球窝变化产生的影响。球窝前置时对污垢热阻的影响大于球凸前置时的影响。圆心距对污垢热阻的影响呈现波动变化的趋势。     

半球型涡流发生器结构对污垢沉积的影响

通过数值模拟的方法,研究半球型涡流发生器的结构参数变化对析晶污垢沉积过程的影响。考察了球窝半径、球凸半径、球窝与球凸圆心距以及布局对污垢热阻的影响。结果表明,经过模拟得出球凸半径变化对污垢热阻的影响高于球窝变化产生的影响。球窝前置时对污垢热阻的影响大于球凸前置时的影响。圆心距对污垢热阻的影响呈现波动变化的趋势。     

Crystallization fouling of CaCO3 – Analysis of experimental thermal resistance and its uncertainty

Crystallization fouling occurs when dissolved salts precipitate from an aqueous solution. In the case of inversely soluble salts, like calcium carbonate (CaCO₃), this may lead to crystal growth on heated walls. Crystallization may also take place in the bulk solution either via homogeneous nucleation or heterogeneous nucleation on suspended material.In this paper, surface crystallization of CaCO₃ and crystallization in the bulk fluid and its effect on the fouling rate on a heated wall are studied. The fouling experiments are done in a laboratory scale set-up of a flat plate heat exchanger. Accuracy of the results is analyzed by uncertainty analysis. SEM and XRD are used to determine the morphology and the composition of the deposited material.The uncertainty analysis shows that the bias and precision uncertainties in the measured wall temperature are the largest source of uncertainty in the experiments. The total uncertainty in the fouling resistance in the studied case was found to be ±13.5% at the 95% confidence level, which is considered to be acceptable.Surface crystallization rate is found to be controlled by the wall temperature indicating that the surface integration dominates the fouling process. The flow velocity affects the fouling rate especially at high wall temperature by decreasing the fouling rate with increasing flow velocity. Crystallization to the bulk fluid is found to enhance significantly the fouling rate on the surface when compared to a case in which fouling is due to crystal growth on the surface.     

Fouling of a Heated Rod in a Stirred Tank System

A batch stirred tank device has been developed for measuring fouling from oil samples. The unit consists of a baffled tank equipped with a centrally mounted long blade stirrer, and an electrically heated rod located at 40% of the radius of the tank. Heat transfer from the rod was first characterized. The velocity field was measured, from which the approach velocity to the probe was determined, which allowed the wall shear on the heating probe to be calculated from a literature equation. Fouling of a heavy oil fraction was studied in 1- to 2-day experiments with bulk oil temperatures typically at 320°C, initial probe surface temperatures to 536°C, and stirrer speeds of 100–900 rpm. Micrometer-sized iron oxide particles were added to the oil, such that fouling was due to a combination of particle deposition and coke formation. Deposition rates were measured thermally from the change in heat transfer coefficient when fouling was relatively heavy, and by thickness and mass accumulation when fouling was light. Effects of oil type, film temperature, stirrer rotation speed (or probe wall shear stress), and concentration of suspended particles on deposition rate and deposit composition are presented.     

Electrokinetically modulated flow of couple stress magneto‐nanofluids in a microfluidic channel

In the present article, the theoretical investigation is presented for the mixed electrokinetic and pressure‐driven transport of couple stress nanoliquids in a microchannel with the effect of magnetic field and porous medium. This topic has gained a remarkable scope in nanoscale electro‐osmotic devices. The formulation of the present mathematical problem is simplified using the Debye‐Hückel linearization assumption. The merging model has important features such as the thermal Grashof number, solutal Grashof number, Joule heating, Helmholtz‐Smoluchowski velocity. The analytical solutions are presented for the axial velocity, temperature, and solute concentration. The expressions for the heat transfer rate, solute mass transfer rate, and surface shear stress function at the walls are also presented. The results display that, the velocity of the couple stress nanofluid is less in the case of pure electro‐osmotic flow as compared to that of combined electro‐osmotic and pressure‐driven flow. When the Joule heating parameter vanishes, the temperature and solute concentration profiles are linear, otherwise nonlinear. The shear stress function is larger in the case of pure electro‐osmotic flow and it is smaller for the combined effects of electro‐osmotic and pressure gradient. The present analysis places a significant observation that the various zeta potential plays an influential role in heartening fluid velocity. The analysis is relevant to electrokinetic hemodynamics and microfluidics.     

Fouling thresholds in bare tubes and tubes fitted with inserts

Maya crude oil fouling reveals a straightforward dependency of initial fouling rate on surface temperature but a rather complex dependency on velocity in bare tubes, the initial fouling rate showing a maximum and then decreasing significantly towards zero as the velocity is increased. Surface shear stress clearly is an important parameter. CFD simulation of fluid flow in a tube fitted with a hiTRAN® insert reveals a complex distribution of surface shear stress. To compare the insert situation with the bare tube, an equivalent velocity concept is introduced on the basis that at a given average velocity the fluid flow results in the same average wall shear stress regardless of whether the tube is bare or is fitted with an insert. Using the equivalent velocity concept, the fouling data obtained using both a bare tube and a tube fitted with inserts can be correlated using a single model. Moreover, the fouling threshold conditions below which fouling is negligible, can be predicted for both situations.     

LBM modeling and analysis on microchannel slip flow and heat transfer under different heating conditions

Abstract

This article aims to explore the effects of buoyancy force and thermal boundary condition on the mixed convection heat transfer performance of air in a horizontal microchannel. Three different heat flux models, including bottom wall heated, top wall heated (single wall heating – a novel heating approach compared to recent studies) and both walls heated, are analyzed at four different values of the Grashof number (Gr?=?0, 100, 300, 600) using a lattice Boltzmann method (LBM). The slip velocity boundary condition is also applied to the bottom and top walls. It can be found that the buoyancy force changes the velocity distribution structure near the bottom wall and top wall, particularly at the inlet regions in all models, and a negative slip velocity is generated due to the backflow formed at a relatively large Grashof number and it strictly determines the local wall friction coefficient. Either the bottom wall or the top wall is heated. A vortex is found close to the top wall because the mixing position of the hot and cold fluids is in the vicinity of the top wall. This feature facilitates the heat transfer near the top wall and core flow zone. The thermal performance is most positive for the case when the top wall is heated due to the generation of an induced vortex and no influence of the vortex near the bottom wall.     

运行工况对多向扰流强化管CaSO4污垢特性的影响

为更深入并准确研究运行工况条件对多向扰流强化管CaSO_4污垢特性的影响,基于FLUENT软件的UDF功能构建了恒壁温条件下结垢传质过程与温度场的耦合作用关系,进一步采用田口法对运行工况致垢的贡献率进行了模拟比较,分析了贡献率较大的运行工况对污垢特性的影响。结果表明:溶液溶度致垢的贡献率占53.2%,而壁面温度、进口流速和进口温度的贡献率分别为22.2%、19.3%和5.3%;溶液溶度在4.0～2.5 kg/m~3,污垢热阻降低达90.47%,并且随溶度降低其相邻溶度间降低比例基本不变;壁面温度在340.0～315.0 K时,污垢热阻降低了65.22%,在前一阶段相邻温度间降低比例基本上不变,当达到320.0 K后降低明显;流速在1.0～2.5 m/s时,随流速的增加,污垢热阻降低68.65%,且随流速的增加,相邻流速间降低的速度明显减缓。     

Turbulent convection mass transfer of water with internal mass sources

Convective turbulent mass transfer in heated tubes is modeled with internal mass sources resulting from crystallization. The analysis considers the influence of internal mass sources on the concentration distribution, average concentration of colloidal particles and dissolved impurities, and the mass flux at the wall. It was found that if the mass transfer coefficient in the case which considers internal mass sources is defined properly, the Sherwood number and the mass transfer coefficient with internal mass sources are equal to those without internal mass sources. The mass flux and the increase in the wall temperature beneath the iron oxide deposit layer were predicted using two crystallization models. The model predicting crystallization at the wall only is recommended based on predictions of the maximum increase in the wall temperature beneath the deposit layer. © 2000 Scripta Technica, Heat Trans Asian Res, 29(3): 166–180, 2000     

Numerical investigation of the bubble growth in horizontal rectangular microchannels

To explore the mechanism of flow boiling in microchannels, the processes of a single-vapor bubble evaporating and two lateral bubbles merging in a 2D microchannel are investigated. The temperature recovery model based on volume of fluid method is adopted to perform the flow boiling phenomena. The effects of wall superheat, Reynolds number, contact angle, surface tension, and two-bubble merger on heat transfer are discussed. Wall superheat dominates the bubble growth and is roughly proportional to wall heat flux. The update of velocity and temperature fields’ distribution in the channel increases with increasing inflow Reynolds number, which improves the wall heat flux markedly. Besides, the area of thin liquid film between the wall and the bubble is enlarged by reducing the contact angle, thus, expanding the wall heat flux several times compared with the single-phase cross section. However, variation of surface tension (0.0589, 0.1?N/m) is found to be insignificant.     

Fouling of Some Canadian Crude Oils

A thermal fouling study was undertaken using three sour Canadian crude oils. Experiments were carried out in a re-circulation fouling loop equipped with an annular (HTRI) electrically heated probe. Fluids at pressures of about 1000–1340 kPa under a nitrogen atmosphere were re-circulated at a velocity of 0.75 m/s for periods of 48 hours, and the decline in heat transfer coefficient followed under conditions of constant heat flux. Bulk temperatures were varied over a range of 200–285^?C, and initial surface temperatures ranged from 300–380^?C. Heat fluxes were in a range of 265–485 kW/m².

Surface temperature effects on fouling of the three oils were compared, and fouling activation energies were estimated. For the lightest oil, a more detailed study of velocity and bulk and surface temperature effects was carried out. The fouling rate decreased slightly with increasing velocity but increased with both surface and bulk temperatures; a rough correlation was developed using a modified film temperature weighted more heavily on the surface temperature. Deposits showed high concentrations of sulfur and minerals, indicating the importance of iron sulfide deposition.