共查询到20条相似文献,搜索用时 15 毫秒
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ABSTRACTIn recent years, open-cell metal foam has gained attention for utilization for exhaust gas recirculation coolers due to its large surface area and porous structure. Theoretically, the porous foam structure would have better transfer heat through conduction and convection processes. However, the exhaust gases that enter the cooler would carry particulate matter, which may deposit within the foam structure. The existing fouling studies cannot explain the underlying mechanisms of particulate deposition thoroughly within the foam structure. This study reviews the particulate fouling of heat exchangers, particularly in the exhaust gas recirculation system. Some past approaches to investigate fouling, particle transport, and deposition in the metal foam heat exchangers for many different applications are also included. In addition, this study also includes the challenges that lie ahead in implementing the metal foam heat exchangers in the industries. 相似文献
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The influence of flow direction with respect to gravity on particulate fouling of heat exchangers is investigated experimentally to determine the optimal flow direction to minimize fouling. Four orientations of flow have been investigated: horizontal flow, upward flow, downward flow, and a flow under an angle of 45°. It is observed that fouling starts at the point of stagnation irrespective of the flow direction, and also at the top of the heat exchanger tubes. Particulate fouling grows from these two points till they meet and the fouling layer covers the whole surface of the heat exchanger tube. Fouling at the upper half of the tubes is much faster than the lower half of the tubes, and the fouling rate is faster at the bottom tubes of the heat exchanger section than at the upper tubes. The best orientation for lingering particulate fouling is the downward flow, where the flow stagnation point coincides with the top point of the heat exchanger tubes and the growth of the fouling layer only starts from one point. 相似文献
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This article describes particulate fouling experiments performed on small-scale and full-scale plate heat exchangers for three different corrugation angles (30 deg, 45 deg and 60 deg). The velocity effect has been studied as well as the particle type and concentration effects. The test duration ranges between 20 and 1,500 h in order to reach asymptotic behavior. The results clearly indicate that the corrugation angle has a major influence on the asymptotic fouling resistance. Increasing the corrugation angle leads to lower values for the fouling resistance. Furthermore, for a given corrugation angle, the asymptotic fouling resistance is inversely proportional to the velocity squared. Finally, the asymptotic fouling resistance is proportional to the particle concentration. Fouling mitigation can be obtained by taking into account at the design stage the heat exchanger geometry and fluid velocity. 相似文献
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Christopher A. Bennett 《传热工程》2013,34(5):468-474
This paper expounds a simple, fundamental theory for predicting sedimentation particulate fouling thresholds for horizontal flows inside heat exchanger tubes. The velocities and shear stresses at the tube wall predicted by this theory for keeping particulate matter suspended compare favorably with industrial experience and proprietary Chevron data. This theory is also not inconsistent with the literature in that the shear stress required for sedimentation fouling mitigation is roughly 4–6 Pa. However, the situation where small particles become encapsulated in the viscous sublayer cannot be resolved with a simple sedimentation particulate fouling threshold theory at this time, necessitating future research. 相似文献
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Due to hardness of cold water supply in many countries, there is a risk of fouling in domestic hot water (DHW) counterflow plate heat exchangers. The scaling will result in increased resistance to heat transfer, which has negative effects on the economics of the district heating network. A common approach is to clean or change the heat exchanger periodically, which can be expensive if only limited fouling has occurred (unnecessary) or if a higher than expected scaling layer has formed (inefficiency). A better approach is to monitor the state of the heat exchangers and clean them when actually required. This would result in more energy-efficient operation and provide an optimum schedule for heat exchanger cleaning. This can be simple if the heat exchangers are operating under steady-state conditions; however, if large variations in the inlets are experienced, as is the case with the mass flows in DHW heat exchangers, it quickly becomes impossible with standard methods. In this paper it is proposed to monitor the state of the heat exchanger online by using measurements that are easily obtainable under normal operation and applying fast mathematical models to estimate the overall heat transfer coefficient of the heat exchanger. The results show that the methods proposed can be used to detect fouling in DHW heat exchangers. 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
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Fouling is a challenging, longstanding, and costly problem affecting a variety of heat transfer applications in industry. Mathematical models that aim at capturing and predicting fouling trends in shell-and-tube heat exchangers typically focus on fouling inside the tubes, while fouling on the shell side has generally been neglected. However, fouling deposition on the shell side may be significant in practice, impairing heat transfer, increasing pressure drops, and modifying flow paths. In this paper, a new model formulation is presented that enables capturing fouling on the shell side of shell-and-tube heat exchangers including the effect of occlusion of the shell-side clearances. It is demonstrated by means of an industrial case study in a crude oil refinery application. The model, implemented in an advanced simulation environment, is fitted to plant data. It is shown to capture the complex thermal and hydraulic interactions between fouling growth inside and outside of the tubes, the effect of fouling on the occlusion of the shell-side construction clearances, and to unveil the impact on shell-side flow patterns, heat transfer coefficient, pressure drops, and overall exchanger performance. The model is shown to predict the fouling behavior in a seamless dynamic simulation of both deposition and cleaning operations, with excellent results. 相似文献
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This paper reports the influence of the spiral angle on the heat transfer performance during condensation inside spiraled micro-fin tubes having constant geometric parameters such as fin height, pitch, shape (apex angle), and fin number, as various papers previously published in this field had not clearly established this influence. Tests were conducted for condensation of R-22, R-134a, and R-407C inside a smooth tube (9.52 mm outer diameter) and three micro-fin tubes with approximately the same diameter and spiral angles of 10°, 18°, and 37°, respectively. Experimental results indicated a heat transfer augmentation with spiral angle increase. A new semi-empirical predictive correlation was developed for the practical design of spiraled micro-fin tubes. The proposed new correlation predicted the majority of experimental results of the present study within a deviation zone of ±20 percent. 相似文献
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污水换热器污垢生长特性试验研究 总被引:3,自引:1,他引:2
污水换热器污垢是影响污水冷热源应用的主要因素.了解污垢的形成规律是污水换热器应用与研究的基本要求.基于工程现场实验,通过热阻法测定污水换热器管程内污垢的热阻变化模型Rf(θ)=8×10-4[1-exP(-θ/188)].提出了污垢形成过程有效热阻与有效总传热系数的概念.并通过热阻模型提出了渐进型污垢形成过程的压降模型△(△p)=A-Bexp[-t/t0).为进一步的研究与应用提供了理论基础. 相似文献
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The industrial problem of the air-side fouling of compact heat exchangers has been studied in a laboratory wind tunnel for particles in the μm to mm range. The measurements of pressure drop across the exchanger as well as the quantification and classification of the particles blocked by and passing through it were taken for discrete ranges of particle sizes. Observation showed that the blocked particles either drop to the wind tunnel floor, remain on the outside surface of the exchanger (falling to the floor once the wind velocity drops), or penetrate a short distance into it according to the relative values of fin spacing and particle size. These last are the most detrimental to exchanger performance. There is a critical particle size for which this penetration is maximal. A brief non-dimensional geometrical analysis allows one to predict this critical size range for any finned exchanger. It has been found to be between 0.5–0.7 times the diameter of the largest sphere that can be inscribed between the fins. Confirmation of this was found with a second exchanger. The addition of humid conditions within the tunnel or on the exchanger itself did not modify these values. Subtracting the pressure drop due to the clean exchanger from the total measured value confirms that the foulant acts like an extra mechanical filter in series with the exchanger. This is quite understandable given the short penetration length of the particles (up to 3 mm). Finally, the effects of a closed wind tunnel test section on the measurements for non-isothermal conditions in the exchanger are discussed. 相似文献
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Nae-Hyun Kim 《传热工程》2018,39(1):40-50
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. 相似文献
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Christopher A. Bennett 《传热工程》2013,34(15):1246-1250
A theory for mathematically modeling asphaltene adhesion fouling in heat exchanger tubes was derived and its agreement with experiment suggested feasibility. The premise of this theory is that asphaltene adhesion fouling requires the formation of a chemical bond—modeled here as a sulfur–sulfur bond—which is strong enough to resist fluid dynamic forces. This theory suggests that once an asphaltene monolayer is adsorbed onto a heat transfer surface, shear stress alone may be insufficient for preventing further fouling unless the asphaltene flocculate size distribution can be manipulated, the chemically labile heteroatoms can be deactivated, or the asphaltenes can be kept in solution. A method for calculating fouling threshold shear stresses and flocculate diameters is expounded. 相似文献
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Abstract This paper addresses two-phase flow distribution phenomena in multiple header–tube junctions used in heat exchangers. Because of phase separation, it is very difficult to obtain uniform two-phase flow distribution to the branch tubes. The flow distribution is strongly influenced by the header orientation (horizontal or vertical) and the number of branch tubes. Other factors that influence the flow distribution are the flow direction in the header (upflow or downflow), the header shape and tube end projection into the header, and the location and orientation of the inlet and exit connections. The source of maldistribution is the flow in the dividing headers. Work performed by the authors and others (including patents) are discussed. The possibilities for eliminating two-phase flow maldistribution are identified and discussed. This investigation shows that solutions, which provide uniform flow distribution, are very design-specific. Change of the geometry or operating parameters will require modification of the design. 相似文献
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Fouling in shell-and-tube heat exchangers was modeled by combining Hasson's ionic diffusion model for scaling from CaCO3 solutions with a model for predicting the temperature distribution developed by Gaddis and Schlünder. Using the computed results, clean heat exchanger design rules were tested for fouling conditions. The effects of fouling on the efficiency of heat exchanger configurations were determined. 相似文献
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JERRY TABOREK 《传热工程》2013,34(2):34-45
This article attempts to update the rather sketchy methods for double pipe exchangers in the present literature, especially those for longitudinal finned tubes. Areas and conditions for the most useful application of double and multitube exchangers are outlined. Calculation methods are presented for plain double pipe units, as well as finned tube units, with a new development in the important transition region with “cut-and-twist” turbulency promoters. Equations for the mean temperature difference for units with flow in series-parallel are also given. 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