微粒污垢的特性分析

利用微粒污垢的一个预测模型，结合实验结果，研究了管壳式换热顺管内微粒污垢的积聚特性，考察了颗粒质量分数，颗粒直径和流体速度对渐近污垢热阻的影响，提出了大、中、小粒子及未知尺寸粒子的尺寸界限的参考值。     

不同工况下冲孔矩形翼涡流发生器的纳米氧化镁颗粒污垢特性

为了研究不同工况情况下冲孔矩形翼涡流发生器的纳米氧化镁颗粒的污垢特性,通过实验对比了冲孔矩形翼涡流发生器和未冲孔矩形翼涡流发生器的污垢特性,探讨了水浴温度、工质质量浓度及工质流速对颗粒污垢的影响。实验结果表明:相同工况下,冲孔矩形翼涡流发生器较未冲孔涡流发生器具有更优的抑垢效果;随着水浴温度的升高,污垢热阻渐近值增加,而且结垢速率也增大;污垢热阻渐近值随着工质质量浓度的增加而增大,结垢速率有略微提升;随着工质流速的增大,污垢热阻渐近值和结垢速率均降低。     

Particulate fouling in waste incinerators as influenced by the critical sticking velocity and layer porosity

Fouling of heat transfer surfaces introduces a major uncertainty into the design and operation of heat exchange equipment. Fouling layers as observed on the tube bundles of the economizer in a Dutch waste incinerator were thin and powdery. The fouling layer showed an asymptotic growth rate with a levelling off increase of the thickness. In this study, the influence of the critical sticking velocity on the growth rate of particulate fouling layers is described. The critical sticking velocity of an incident particle hitting a powdery layer is defined as the maximum impact speed at which the particle will stick to the layer. Since the critical sticking velocity is a key parameter in the deposition mechanism, a well-defined experimental set-up has been built to assign it. Experimental results showed that the critical sticking velocity increases with the porosity of the fouling layer. Literature shows that the porosity of a thin sintered powdery layer changes with the layer thickness. Based on the experimental results and the variation of porosity with thickness for a thin sintered powdery fouling layers, a correlation is developed which shows that the sticking velocity decreases exponentially as the fouling layer thickness increases. Therefore, fewer particles are likely to stick as the fouling layer builds up and consequently the deposition rate decreases. The change in the critical sticking velocity as the fouling layer builds up contributes to the explanation of the asymptotic growth of particulate fouling layers on the tube bundle of waste incinerators.     

Influence of gas velocity on particulate fouling of exhaust gas recirculation coolers

The objective of this research is to study the influence of gas flow velocity on particulate fouling of exhaust gas recirculation (EGR) coolers. An experimental setup has been designed and constructed to simulate particulate fouling in EGR coolers in diesel engines. The setup consists of soot generator, gas/particle flow heater, testing section for EGR coolers and finally an exhaust system. Two sets of fouling experiments have been performed with and without water injection, and the gas velocity in each set has varied between 30, 70 and 120 m/s. The concentration of soot particles in the gas flow is 100 mg/m³, and the average diameter of the particles is 130 nm with a standard deviation of 55 nm. It has been found that the thermal resistance and thickness of the fouling layer and the fouling rate decrease as the gas velocity in the EGR cooler increases. If EGR coolers are operated with a gas velocity, which is just lower than the critical flow velocity for the largest particle in the flow, quick deterioration of the thermal performance of the heat exchanger will nevertheless occur. This strongly indicates that the gas velocity should exceed a certain critical flow velocity in order to prevent particulate fouling. In addition, the presence of water vapour in the gas flow improves the thermal performance of the cooler and decreases the fouling rate, and its influence decreases as the gas velocity increases.     

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。     

蓄热元件波纹参数对流动传热性能影响分析

为研究搪瓷蓄热元件波纹倾角和宽度对流动传热性能的影响,采用Fluent软件对某搪瓷蓄热元件进行数值模拟,得到蓄热元件不同波纹倾角和宽度时内部流场速度、温度分布及努塞尔数和阻力系数随雷诺数的变化曲线。模拟结果表明:波纹倾角由20°增大到60°时,蓄热元件阻力系数增大,努塞尔数增加,且阻力系数增幅大于努塞尔数增幅;波纹宽度由6 mm增加到10 mm时,阻力系数增幅较小,努塞尔数增幅较大。     

Nucleate pool boiling heat transfer characteristics of dilute Al2O3–ethyleneglycol nanofluids

Pool boiling heat transfer coefficients of dilute stabilized Al₂O₃–ethyleneglycol nanofluids as possible coolant fluid are experimentally quantified. The influence of different parameters such as heat flux, heating surface nano-roughness, concentration of nanofluids and fouling resistance on the pool boiling heat transfer coefficient of alumina nanofluids has experimentally been investigated and briefly discussed. Results demonstrated that there are two heat transfer regions with different mechanisms namely free convection and nucleate boiling heat transfer. Studies on the influence of parameter demonstrated that with increasing the heat flux, the pool boiling heat transfer coefficient of nanofluids significantly increases. In contrast, with increasing the concentration of nanofluid, due to the deposition of nanoparticles on the surface, the average roughness of the surface and the heat transfer coefficient dramatically deteriorate, while a significant increase in fouling resistance is reported. Also, studies reveal asymptotic and rectilinear behaviors of fouling resistance parameter in nucleate boiling and free convective domains.     

基于污水换热器污垢不同生长阶段的除垢试验研究

为了提高污水换热器的除垢效果,文章以管壳式污水换热器为研究对象,以沙粒作为除垢粒子,以污垢热阻变化率表征除垢效果,在利用烘干灼烧失重法、能谱分析法和微观结构分析法分析污垢成分的基础上,运用污垢热阻法进行除垢试验研究。通过试验研究了在污垢生长的诱导期、生长期和渐近期3个阶段除垢对污垢热阻值的影响。试验结果表明:管壳式污水换热器管内污垢的主要成分为含水量较高的有机物;结垢工况下,污垢热阻渐近值为0.74×10^-3(m^2·K)/W;在污垢生长的诱导期、生长期和渐近期进行除垢后,污垢热阻渐近值分别为0.4×10^-3,0.42×10^-3,0.6×10^-3(m^2·K)/W,与结垢工况相比,污垢热阻渐近值分别降低了45.9%,43.2%,18.9%,除垢工况下污垢热阻增长速率较结垢工况明显减缓。     

Analysis of Fouling in Six-Start Spirally Corrugated Tubes

Abstract

Fouling on heat exchanger surfaces can weaken the heat-transfer capability, increase the energy consumption, and even cause the failure of the whole system. In coaxial heat exchangers, spirally corrugated tubes perform better than smooth ones concerning heat transfer and antifouling. In this article, a parametric study on the antifouling performance of a six-start spirally corrugated tube is carried out with a solid–liquid two-phase model. First, comparisons between a smooth tube and a specific sample six-start spirally corrugated tube on the solid particle volume fraction distributions are carried out. Then, the effects of solid particle diameter, the main geometric parameters, including pitch and the corrugation depth, are investigated. Analyzing the solid particle volume fraction with different geometries, solid particle diameters and Reynolds number, the roles played by the centrifugal force, drag force, and gravity of solid particles on fouling performance in six-start spirally corrugated tubes are obtained. In addition, the corrugation depth affects the volume fraction range more while the pitch affects more on the steady range of particle volume fraction. This work is of significance for further design of spirally corrugated tubes and analysis of fouling problems in heat exchangers.     

Experimental Study of Fouling Resistance in Twisted Tube Heat Exchanger

This paper discusses fouling of a twisted tube heat exchanger under different conditions of fluid velocity and heat input. The fluid velocity was varied from 0.5 to 2.0 m/s, whereas the heat input to the heat exchanger was varied from 200 to 800 W. The experimental results show that for low fluid velocity of 0.5 m/s, the fouling resistance showed noticeable variation with respect to heat input, whereas for high velocity ranges, that is, 1.0–2 m/s, the variation in fouling resistance is less. The fouling in twisted tube steadily increases with time for different values of heat input from 1000 min onward for fluid velocity in the range from 1.0 to 2.0 m/s. It is also observed that fouling resistance curves overlap for various values of heat input. During the initial 1000 min of the test duration, the maximum fouling in a twisted tube heat exchanger decreases with increase in fluid velocity from 1.0 to 2.0 m/s. This behavior of the fouling rate can be attributed to the fact that at higher fluid velocity, flow becomes turbulent, and this in turn flushes the fouling particles. The time-series correlations for the fouling resistance are found to be logarithmic in nature.     

影响换热面结垢诱导期的表面特性研究

换热面结垢是一个普遍存在的问题,而结垢诱导期的长短对污垢形成过程具有重要的影响,即使在相同实验条件下,不同材料换热面的结垢诱导期仍相差较大.因此本文通过对附着在换热面上的半球形污垢晶核进行受力分析,发现污垢晶核与换热面之间的附着力对其结垢诱导期长短起决定性作用,然后根据颗粒与平板间附着力模型,计算了污垢晶核与具有不同表面能的换热面间附着力,并与相应的结垢诱导期进行对比.结果表明:结垢诱导期的长短与换热面的表面能、污垢晶核与换热面间的附着力及表面粗糙度尺度有关.     

Investigating the Impact of Boiling Conditions on the Fouling of a Crude Oil

ABSTRACT

The impacts of nucleate boiling and pressure on crude oil fouling are factors that have not been heavily investigated in previous research. Variables such as wall temperature and fluid velocity/shear are often a main focus, as they are key variables for predictive fouling models, which provide insight to fouling thresholds. Research detailed in this report shows that nucleate boiling and pressure greatly impact the measured fouling rate of a crude oil tested using the Heat Transfer Research, Inc., rotating fouling unit. When nucleate boiling is occurring, the use of fouling resistance plots to measure fouling rates is not a reliable method due to the impact boiling has on the heat transfer coefficient. Visual inspection of fouling deposits to validate fouling resistance data has also been found to be critical. Images of fouling deposits are included. Fouling under nonboiling conditions was shown to increase with increasing pressure.     

Functional Correlation of Surface Temperature and Flow Velocity on Fouling of Cooling-Tower Water

Experimental fouling data have been analyzed on the basis of the change in overall heal transfer coefficient of the fouling test section. It is assumed that thermal hydraulic conditions in the test section remain reasonably constant for the duration of a fouling test. The model ofTaborek et al.|I| is used, and two parameters,/8,. and Rf?, that can be determined for each fouling test are derived by regression analysis. The parameter Rf? contains all the factors that influence fouling, while 1/ ?_c, contains shear stress, deposit thickness, and bonding strength of the deposit. The parameter R?is the asymptotic fouling resistance and ? is the lime constant of the fouling resistance-time curve. These parameters were determined as a function of surface temperature. Limited data were available to indicate the effect of velocity on the parameters. The parameters 1/8, and R? may be used to predict the history of fouling in a heat exchanger. Until more.data are obtained and analyzed in this fashion, the values of 1/ ?_c. and R? obtained in this paper should be applied at conditions for which the fouling data were obtained. A numerical example is presented.     

Mitigation of ice crystallization fouling in stationary and circulating liquid–solid fluidized bed heat exchangers

Liquid–solid fluidized bed heat exchangers are attractive ice crystallizers since they are able to mitigate ice crystallization fouling and exhibit high heat transfer coefficients. Experiments show that the fouling removal ability of stationary fluidized beds increases with decreasing bed voidage (95–80%) and increasing particle size (2–4 mm). The removal of ice crystallization fouling appears to be more effective in circulating fluidized beds, especially at high circulation rates. Fouling removal is realized by both particle–wall collisions and pressure fronts induced by particle–particle collisions. A comparison between ice crystallization experiments and impact characteristics shows that the removal rate is proportional to the impulse exerted on the wall. A model based on these phenomena is discussed and predicts the transition temperature difference for ice crystallization fouling in both stationary and circulating fluidized beds with an average absolute error of 9.2%.     

Numerical analysis of heat transfer and flow field around cross-flow heat exchanger tube with fouling

Fouling is one of the main problems of heat transfer which can be described as the accumulation on the heat exchanger tubes, i.e.; ash deposits on the heat exchanger unit of the boiler. A decrease in heat transfer rate by this deposition causes loss in system efficiency and leads to increasing in operating and maintenance costs. This problem concerns with the coupling among conduction heat transfer mode between solid of different types, conjugate heat transfer at the interface of solid and fluid, and the conduction/convection heat transfer mode in the fluid which can not be solved analytically. In this paper, fouling effect on heat transfer around a cylinder in cross flow has been studied numerically by using conjugate heat transfer approach. Unlike other numerical techniques in existing literatures, an unstructured control volume finite element method (CVFEM) has been developed in this present work. The study deals with laminar flow where the Reynolds number is limited in the range that the flow field over the cylinder is laminar and steady. We concern the fouling shape as an eccentric annulus with constant thermal properties. The local heat transfer coefficient, temperature distribution and mean heat transfer coefficient along the fouling surface are given for concentric and eccentric cases. From the results, we have found that the heat transfer rate of cross-flow heat exchanger depends on the eccentricity and thermal conductivity ratio between the fouling material and fluid. The effect of eccentric is dominant in the region near the front stagnation point due to high temperature and velocity gradients. The mean Nusselt number varies in asymptotic fashion with the thermal conductivity ratio. Fluid Prandtl number has a prominent effect on the distribution of local Nusselt number and the temperature along the fouling surface.     

三维变形管管内积灰特性数值模拟研究

烟气中的高灰分、高黏度、高腐蚀性成分不可避免地造成换热器烟气侧换热面积灰结垢的问题,如何有效解决这些问题一直是烟气换热器研究的焦点之一.三维变形管内的螺旋流增强了管内流体的湍动程度从而实现强化传热,基于其内部灰尘受气流携带而处于湍动便不易沉积的特点,本文通过数值模拟的方法探讨了三维变形管几何参数、粒径、气体流速对灰尘颗...     

Performance of the herringbone wavy fin under dehumidifying conditions

An experimental study reporting the airside performance of the herringbone wavy fin geometry in wet conditions is conducted. In the visualization of the condensate flow pattern, a very special “locally dry” spot of the corrugation wavy channel having a corrugation angle of 15° and a fin spacing of 8.4 mm is seen. This phenomenon is related to the recirculation of the airflow across the apex. Conversely, this phenomenon is not so clearly seen either for a fin pitch of 2.6 mm with a corrugation angle of 15° or a corrugation angle of 25°. Flow visualization of the non-uniform distribution of the condensation in the facets results in a dependence between axial length and friction factor. Based on the present test results, airside performance in terms of Nusselt number and Fanning friction factor for the present herringbone wavy fin geometry in wet conditions are developed. The mean deviations of the proposed correlations are 2.52% and 4.81%, respectively.     

Conjugate fluid flow and kinetics modeling for heat exchanger fouling simulation

Thermal treatment of fluid foods represents a major unit operation in the food industry, to ensure the product's safety and quality features. But during the thermal treatments of such sensible fluids in common plate heat exchangers, food constituents such as proteins can be thermally damaged and precipitated to form fouling that greatly affect the treatment efficiency and alter the product's desired features.Computational Fluid Dynamics simulations can then be successfully exploited, bringing forth temperature and velocity information that yield for deposit distributions when coupled to biochemical notations for thermal denaturation of fluid constituents.The present work exploits such modeling for a single-channel heat exchanger during pasteurization of milk. The model enforces a conjugate system of differential equations to a heat exchanger's corrugated plate to combine flow, heat transfer and local transport of β-lactoglobulin. A preliminary computation has been performed that could be applied to geometry optimization (different corrugation shape and orientation) and for a variety of biochemically evolutive products.     

Particulate Fouling and On-Line Cleaning of Ferric Oxide Particles in Internally Enhanced Tubes

This paper describes the results of accelerated particulate fouling tests performed on three enhanced tubes and a plain tube. The tests were performed using ferric oxide as the foulant material. Three enhanced tubes included 25 start, 10 start helically ribbed tube and a ripple tube. Effect of the water velocity (1.2–1.7 m/s) on fouling resistance was investigated. The maximum fouling resistance occurred in the 25 start helically ribbed tube (about 8.0 × 10^?5 m²K/W after 100 hours). For the 10 start helically ribbed tube, the fouling resistance was relatively small (less than 1.8 × 10^?5 m²K/W). The rippled and plain tubes show almost negligible fouling resistance. High velocity flushing was effective for all the tubes except for the 25 start helically ribbed tube. On-line brush cleaning maintained the fouling resistance below 1.8 × 10^?5 m²K/W for all tubes. The fouling concentrations used in the tests were significantly higher than would be expected in commercial heat exchangers. Also, the velocity range investigated was lower than would be expected in heat exchanger operation.     

温度对纳米氧化镁颗粒污垢特性的影响

为了探讨温度对纳米氧化镁颗粒污垢结垢特性的影响,通过改变循环工质入口温度和水浴温度实验研究了温度对纳米颗粒污垢在交叉缩放椭圆管中的结垢特性,并通过静置沉降实验验证了入口温度对循环工质聚沉情况的影响。结果表明,循环工质入口温度和水浴温度对纳米颗粒污垢特性都有显著的影响。随着循环工质入口温度的升高,污垢热阻渐近值明显减小,并且污垢热阻达到渐近值的时间缩短。随着水浴温度的升高,污垢热阻渐近值也随之减小,但是达到渐近值的时间略有增加。