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To minimize the negative effects of scale formation in heat exchangers, new anti-fouling strategies are focusing on the modification of heat transfer surfaces. These modifications should lead to tailor-made surfaces for different technical applications. The aim of this surface modification is the extension of the induction period to minimize the negative effects of fouling and maximize the endurance of the heat exchanger. To achieve this, different surface coatings on stainless steel were investigated with respect to fouling tendency. The effects of flow velocity with respect to Reynolds number on the induction time of CaSO4 crystallization fouling were tested in different test units. Diamond-like carbon (DLC) coatings extend the induction time at every measured flow velocity. At higher Reynolds numbers, the effect of different surface crystallization due to energetic modification is reduced because of the dominating effect of the low adhesive surface. Thus the induction time can be extended by the factor of 2 for low fluid velocities (DLC or SICON®) and by more than 14 for higher Reynolds numbers (DLC and SICON®). The combination of limited nucleation spots due to electro-chemical treatment of the substrate before coating can give a tailor-made surface with maximum induction time for crystallization fouling. 相似文献
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Fouling of heat exchangers is a prevalent operating drawback in many process industries. Efficient chemical inhibitors have predominantly been used for many years to combat deposit formation. Nevertheless, new stringent environmental legislations limit their utilization. On-line mitigation techniques, such as propulsion of projectiles at different injection rates, can also be utilized, which can, in turn, minimize the need for chemical inhibitors. Nevertheless, the experimental data are scarce and nonconclusive. In this experimental study, two spherical-type projectiles of different sizes and hardness have been used to clean the inner surface of a single heated tube that was subjected to the deposition of calcium sulfate. Projectiles were then introduced at different injection rate of every 2, 5, 10, 15, and 30 min. The experimental results show that (i) the projectiles would expedite initial nucleation of crystals even if they are soft and easy to propel inside the tube; (ii) fouling can only be mitigated if the projectiles exert a shear force such that the corresponding removal rate is greater than the net rate of the deposition; and finally (iii) harder projectiles with larger surface contact area are more efficient in cleaning the surface compared to those that are softer. 相似文献
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ABSTRACTThe fouling layers on heat exchanger surfaces exhibit complicated structures, which essentially affect flow hydrodynamics, fouling kinetics, and hence the heat transfer performance. Numerical models developed so far for the fouling process, however, are based exclusively on the assumption of an impermeable fouling layer with a uniform porous structure. In order to quantitatively evaluate the effect of fouling layer structure on fouling dynamics, this work systematically investigated four representative schemes for fouling layer characterization: a homogeneous porous medium that is impermeable to water (HoIm), a heterogeneous porous medium that is impermeable to water (HeIm), a homogeneous porous medium that is permeable to water (HoPe), and a heterogeneous porous medium that is permeable to water (HePe). Under the same operational conditions, four models offer significantly different prediction results on the fluid velocity, temperature distribution, and fouling resistance. It is concluded that numerical model development should take the fouling layer structure into account, and the scheme of HePe that best resembles a real fouling layer structure should be a promising option. 相似文献
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针对现有的污垢析晶沉积模型不能有效模拟真实污垢生长的问题,建立了一种引入析晶沉积动力学模型的多物理场耦合数值模型。模型基于格子Boltzmann方法和有限差分方法,模拟了微通道非等温热表面上近壁面处的沉积物溶质质量浓度分布和污垢生长过程,研究了流速、壁温和沉积物溶质质量浓度对微通道热表面污垢析晶沉积的影响。结果表明:沉积初始时刻流速和壁温对近壁面沉积物溶质质量浓度分布具有不同程度的影响,随着污垢不断生长,污垢-流体界面处的析晶沉积速率减小;相比于流速,沉积物溶质质量浓度对污垢热阻的影响更为显著。 相似文献
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为研究翼型涡流发生器的结构对析晶污垢沉积的影响,利用数值模拟的方法研究在改变其间距、攻角和翼型的情况下,污垢沉积的变化情况。通过引入抑垢率的计算,来表征涡流发生器的抑垢特性。根据实验得出的光片单位面积污垢沉积量与模拟光片得出结果对比,说明验证模拟方法的可行性。通过模拟研究发现在两种攻角下,间距与抑垢率之间的变化关系相同。当间距为10至60mm时,抑垢率随间距的增加而减小。当间距为60至80mm时,则是反向变化。在相同攻角下,流动方向投影面积与抑垢率之间呈现同相变化。投影面积的影响效果与间距相关。 相似文献
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This paper analyzes the effects of injecting additional signals in induction machines for the purpose of speed control. A new saturation model able to correctly model the interaction between the added signal and saturation of the motor core due to the main torque-producing flux is presented. The introduction of a variable saturation factor is used to model the variation of the saturation level due to the additional signal. A third harmonic rotor circuit is also introduced to take account of the third harmonic component of the air-gap flux, due to saturation. An additional balanced voltage set is added to the normal supply to analyze the effects of such signals on a saturated induction motor. Simulation results of such a model both at no load and full load are presented together with experimental measurements. 相似文献
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The beneficial aspects of enhanced or extended heat transfer surfaces may be offset if operated under fouling conditions. In this article, preliminary experimental results for crystallization fouling of CaSO4 solutions onto surfaces with different structures are reported. Flat stainless steel plates (50 mm × 59 mm) with “V”-shaped grooves on the side of fluid flow were used as heat transfer surfaces. Experiments were carried out under both clean and fouling conditions to discern how the same surface structures perform under such circumstances. In addition, the impact of both the direction of grooves with respect to fluid flow (crossed, longitudinal, and mixed flow grooves) and the groove dimensions has also been investigated. Fouling trends are discussed in terms of induction time and fouling rate. Significant differences have been found for the various flow conditions. 相似文献
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ABSTRACTCrystallization fouling on heat transfer surfaces is a severe problem and a complex phenomenon in multiple-effect distillation plants with horizontal tube falling film evaporators for seawater desalination. The choice of tube material affects the wettability, the adhesion forces between surface and deposit, and the induction time of crystallization fouling. The effects of surface properties on crystallization fouling from seawater have been investigated in a horizontal tube falling film evaporator in pilot plant scale. Experiments were performed with artificial seawater and various tube materials. The tube surfaces were characterized by measuring surface roughness and contact angles and by determining surface free energies. The tube materials show qualitative and quantitative differences with respect to scale formation. The interfacial defect model was applied to the system. Spreading coefficients of CaCO3 scale on the aluminum alloys 5052 and 6060 and stainless steel grade 1.4565 were calculated to be higher than those on copper–nickel 90/10 and aluminum brass, but the quantities of CaCO3 scale measured on the tube surfaces were much lower compared to CuNi 90/10 and aluminum brass. The application of advanced approaches such as the interfacial defect model depends on the precise knowledge of interfacial free energies, which are very difficult to find. However, results suggest that more similar values of the interfacial free energies of heat transfer surface and deposit lead to increased scale formation. 相似文献
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Fouling formation on heat exchanger surfaces due to crystallization of inverse solubility salts is one of the fundamental problems in process industries. Despite numerous studies carried out in recent years, comprehensive understanding of crystallization fouling mechanism remains a challenge to chemical engineers. In this review, we first focus on the basic crystallography during deposition of calcium salts, paying attention to crystal structures and crystal forms, as well as nucleation and the subsequent crystal growth process. We then endeavor to relate a number of factors to fouling rate, which may be classified into three categories: solution composition, operating parameters, and heat exchanger surface characteristics. Each aspect is discussed from the crystallization viewpoint (science) and in terms of possible industrial applications (practice). Combining the basic knowledge of crystallography with the information from experimental investigations, several fouling mitigation methods have also been described that may reduce fouling. It is hoped that some of the ideas discussed here will provide possible economic and environmental benefits. Finally, we also try to throw some light on the future direction for research. 相似文献
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Pruthvik A. Raghupathi 《传热工程》2017,38(14-15):1296-1304
ABSTRACTAs fresh water becomes increasingly scarce desalination has become an important technique to meet the portable water requirements around the world. Thermal distillation continues to be one of the most important and widely used methods of desalination currently used. Scale formation, corrosion of the heater surface, and the subsequent degradation of the heat exchanger is one of the biggest challenges in thermal desalination. In this paper, pool boiling of seawater is characterized using standard artificial sea water. Various boiling characteristics such as critical heat flux and heat transfer coefficient are analyzed. The nature of the scales formed on the heater surface and their effect on the heat transfer efficiency are studied. A passive method to reduce the thermal resistance due to scale buildup from crystallization fouling using stainless steel beads is examined. 相似文献
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A two-dimensional (2D) cone shape has been added to the normal circular tubes of heat exchangers to minimize the area of stagnation and to streamline the air flow around the heat exchanger tubes. An experimental setup has been developed to study the influence of the apex angle of the cone-shaped tubes on particulate fouling of heat exchangers. Fouling experiments have been performed in which calcium carbonate particles are injected during the experiments and the deposition of particles on the tubes of the heat exchanger is monitored. Four sets of experiments have been performed, in which normal cylindrical tubes and coned tubes with an apex angle of 60°, 90°, and 120° are examined. It was found that particulate fouling ceased if the apex angle of the cone-shaped tubes is smaller than 90°. The attached cones enhance the flow around the tubes of the heat exchanger, by minimizing the stagnation area and keeping the flow attached to the tubes starting from the tip of the attached cone until separation, such that particles that deposit on the top of the tubes of the heat exchanger can be removed by the air flow. 相似文献
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To avoid the negative effects caused by fouling in heat exchanger equipment, the heat exchanger surface can be modified energetically or mechanically. Thus, mechanical, chemical, and thermal stability of the coatings with respect to the fouling and cleaning conditions is crucial. The surface is typically characterized by the measurement of the contact angles of different wetting fluids to calculate surface energy and tactile roughness measurements. The influence of several cleaning and fouling cycles on surface energy and the composition of the coatings has been investigated. The experimental investigation of different cleaning methods from acid to base solution displays the influence of the interface reactions on the surface energy. Structural analysis of the plasma-activated chemical vapor deposition (PACVD) coatings show a build-in of oxygen inside the a-C:H matrix with time, resulting in higher surface energies and an increase of polar interactions. Also, structural defects of the coatings have been analyzed by a defined disturbance of the coating process or mechanical treatment of the already coated material. These defects act as a starting point for crystallization fouling due to reduced activation energy of nucleation. Depending on the interface and process conditions, defects can enhance fouling if the crystals are able to adhere on the coated surface. The results should lead to a better understanding of interface reactions, stability of coatings, and the aging of surfaces. 相似文献
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针对超超临界锅炉水冷壁节流孔板结垢问题,建立了节流孔板的数值计算模型,分析了结垢的主要原因以及孔板结构参数对结垢程度的影响.在大量数值计算的基础上,采用BP人工神经网络建立了孔板结构参数与结垢程度之间的关系模型并采用遗传算法进行结构优化,得到节流孔板的最佳结构参数.结果表明:节流孔板处压力突降造成磁性氧化铁的析出并沉淀是节流孔板结垢的主要原因;节流孔板的入口倒角、出口倒角、孔径和厚度等结构参数对孔板壁面附近的最大压降幅度和孔板节流效果的影响不同,孔板的厚度越小越好,出口倒角对结垢几乎没有影响,而孔板的孔径和入口倒角对结垢程度影响较大;得到的最佳孔板尺寸参数为孔径9.54mm,入口倒角35°。 相似文献
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吴筱敏 《小型内燃机与摩托车》1999,(5)
在火花塞电极上加一偏置电压,当混合气燃烧时,混合气与氧气发生反应并产生大量离子,离子通过火花塞电极间隙形成离子电流。本文对该离子电流的检测原理,离子电流的基本特性进行了详细的分析和研究,提出了直接利用火花塞电极作为传感器检测汽车发动机燃烧室内工作过程的新办法,作者对离子电流与信号电压的关系,离子电流与偏置电压的关系,积碳对离子电流的影响进行了试验研究,并成功地检测出发动机气缸内工作过程中的离子电流信号,为将火花塞作为检测传感器进行深入细致的研究奠定了基础 相似文献
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Studies of iron oxide deposition on Alloy-800 heat exchanger tubes have been part of a continuing research program at the University of New Brunswick (UNB); the present work formulates mechanisms for the effect of bubbles on deposition in water under boiling conditions. To supplement results from earlier deposition experiments in a fouling loop at UNB, measurements of bubble frequency and departure diameter as a function of heat flux were performed. High-speed movies of bubbling air/water systems indicated that a pumping action moved particles from adjacent areas at the surface to bubble nucleation sites. To explain the observations, the model considers deposition and concomitant removal. Deposition includes microlayer evaporation and filtration through the porous deposit. The deposit is sparse in the first stage, when the dominant process is microlayer evaporation including particle trapping and pumping, creating spots of deposit. Filtration becomes more important as the deposit thickens to a stage when microlayer evaporation becomes negligible. Chimney effects then control. Turbulence due to detaching and collapsing bubbles affects removal. In subcooled boiling, collapsing bubbles generate enough turbulence to maintain much of the deposit labile, while in bulk boiling bubble detachment from the nucleation site is dominant and a smaller portion of the deposit is labile and subject to removal. Model predictions are presented and shown to agree quite well with experimental data. 相似文献