Air-Side Fouling of Compact Heat Exchangers for Discrete Particle Size Ranges

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.     

A Comparison of Metal-Foam Heat Exchangers to Compact Multilouver Designs for Air-Side Heat Transfer Applications

High-porosity metal foams, with novel thermal, mechanical, electrical, and acoustic properties, are being more widely used in various industrial applications. In this paper, open-cell aluminum foam is considered as a highly compact replacement for conventional louver fins in brazed aluminum heat exchangers. A model based on the ?-N_TU method is developed to compare the flat-tube, serpentine louver-fin heat exchanger to the flat-tube metal-foam heat exchanger. The two heat exchangers are subjected to identical thermal-hydraulic requirements, and volume, mass, and cost of the metal-foam and louver-fin designs are compared. The results show that the same performance is achieved using the metal-foam heat exchanger but a lighter and smaller heat exchanger is required. However, the cost of the metal-foam heat exchanger is currently much higher than that of the louver-fin heat exchanger, because of the high price of metal foams. If the price of metal foam falls to equal that of louver-fin stock (per unit mass), then the metal-foam heat exchanger will be less expensive, smaller, and lighter than the louver-fin heat exchanger, with identical thermal performance.     

Air-Side Flow and Heat Transfer in Compact Heat Exchangers: A Discussion of Enhancement Mechanisms

Abstract

The behavior of air flows in complex Heat exchanger passages is reviewed with a focus on the heat transfer effects of boundary-layer development, turbulence, spanwise and streamwise vortices, and wake management. Each of these flow features is discussed for the plain, wavy, and interrupted passages found in contemporary compact Heat exchanger designs. Results from the literature are used to help explain the role of these mechanisms in Heat transfer enhancement strategies.     

Heat Transfer and Pressure Drop Under Dry and Humid Conditions in Flat-Tube Heat Exchangers With Plain Fins

Flat-tube heat exchangers could be an interesting alternative to make indirect cooling of display cabinets more energy-efficient. This application involves low air velocities in combination with condensation of water vapor on the air side, so plain fins could be suitable. Two different heat exchangers having flat tubes and plain fins on the air side were evaluated experimentally. One of the heat exchangers had continuous plate fins, and the other had serpentine fins. The performances during dry and wet test conditions were compared and related to theoretical predictions for different assumptions. The influence of air velocity, air humidity, and inclination angle was investigated. The results show that, in most cases, the heat transfer performance is somewhat reduced under wet conditions in comparison with dry test conditions, and that wet heat transfer surfaces lead to an increased pressure drop. At the lower air velocity range that was investigated, the heat exchanger having continuous plate fins drained better than the one with serpentine fins.     

Numerical Investigation of the Air-Side Thermal Hydraulic Performance of a Louvered-Fin and Flat-Tube Heat Exchanger at Low Reynolds Numbers

In this study, the air-side heat transfer and fluid flow characteristics of a louvered-fin and flat-tube type heat exchanger used in the household refrigerators were investigated numerically. Louver angles of 20°, 24°, 28°, and 32° and fin pitches of 1.50, 2.00, and 2.50 mm were tested. To represent domestic refrigeration systems, the simulations were conducted for low Reynolds numbers, between 223 and 573. The results were evaluated using the volume goodness factor for the air side. The best performance was obtained with louver angle of 20° and fin pitch of 1.50 mm at Reynolds number of 229 over the investigated cases. It was demonstrated that the Colburn j-factor, friction factor, Stanton number, and volume goodness factor did not change linearly with respect to the parameters considered.     

Effect of Waffle Height on the Air-Side Performance of Wavy Fin-and-Tube Heat Exchangers under Dehumidifying Conditions

An experimental study was carried out to examine the effect of the waffle height on air-side performance of typical fin-and-tube heat exchangers under dehumidifying conditions. The waffle heights examined in the present study are 1.18 and 1.58 mm, respectively. A total of 12 samples of heat exchangers, including eight having wavy fin and four having plain fin configurations were tested. The test results indicated that the effect of waffle height on the heat transfer enhancement ratio, compared to the plain-fin counterpart, is pronounced only for smaller fin pitch and larger waffle height, while its effect on the pressure drop is very pronounced throughout the test range, and is almost two times higher than in dry conditions.     

一种多股流换热器综合性能优化设计方法

在综合考虑了体积、重量和阻力等因素的基础上,对多股流换热器进行通道排列和优化设计,并运用无量纲分析法,自定义了syn因子、syn线等用以评价换热器综合性能的指标。详细分析了应用syn因子综合优化翅片结构的过程,与实际的设计结果比较表明：该方法适用于多股流换热器的综合性能优化设计。     

Fin Pattern Effects on Air-Side Heat Transfer and Friction Characteristics of Fin-and-Tube Heat Exchangers with Large Number of Large-Diameter Tube Rows

Air-side heat transfer and friction characteristics of nine kinds of fin-and-tube heat exchangers, with a large number of tube rows (6, 9, and 12, respectively) and large diameter of tubes (18 mm), are experimentally investigated. The test samples consist of three types of fin configurations: plain fin, slit fin, and fin with delta-wing longitudinal vortex generators. The working fluid in the tube is steam. Results show that when the number of tube is larger than 6, the heat transfer and friction performance for three kinds of fins is independent of the number of tube rows, and slit fin provides higher heat transfer and pressure drop than the other two fins. The heat transfer and friction factor correlations for all the heat exchangers were acquired with Reynolds numbers ranging from 4000 to 10000. The air-side performance of heat exchangers with plain fin, slit fin, and longitudinal vortex-generator fin were evaluated under three sets of criteria, and the results showed that the heat exchanger with slit fin has better performance than that with vortex-generator fin, especially at high Reynolds numbers.     

A Calculation Method for the Optimized Thermal-Fluid Dynamic Sizing of Heat Exchangers

A thermal-fluid dynamic calculation model has been developed for sizing some compact heat exchangers. In particular, the exhaust heat recovery systems are considered, as the internal regeneration considerably improves the turbogas power cycles efficiency. The calculation model is designed for selecting the best heat transfer surfaces and optimizing some objective-functions. For almost 60 plate-fin heat transfer surfaces, analytical correlations are derived in a database-like form, to calculate the Colburn j factor and Fanning f factor as functions of Reynolds number. A model is developed also for innovative PCHEs, using a simplified approach. The sizing procedure, based on the core mass velocity equation, is implemented on an electronic worksheet. If the minimum heat exchanger effectiveness and the maximum pressure drops are input, the model gives the heat exchanger core sizing for the possible combinations of the heat transfer surfaces. For each combination, the calculation method minimizes (or maximizes) the selected objective-function by means of an optimization procedure, performed by a solver through the Newton or the conjugate gradients algorithm. The thermal-fluid dynamic calculation method is applied to a high-temperature recuperator with a high-streams pressure ratio. The results show the difficulty of arranging streams with highly different volumetric flows (i.e., same massive flow rates but very different pressures) on the two sides of the heat exchanger, so that very unbalanced aspect ratios arise for the cores, due to the limits imposed to the maximum allowable pressure drops.     

A New Approach to Correlate the Frictional Performance of Fin-and-Tube Heat Exchangers in Wet Conditions

This study presents a new approach to correlate the frictional performance of fin-and-tube heat exchangers in "completely wet" and "partially wet" conditions by using "completely dry" data. A total of 22 samples of uncoated plain fin-and-tube heat exchangers are used to develop the associated frictional correlation. The developed correlation is capable of predicting the frictional performance of "completely wet" and "partially wet" conditions with a mean deviation of 7.2%.     

效率分析在热交换器中的应用

人们在设计热交换器和分析热交换器的热性能时通常采用能量平衡的方法.该文利用效率分析热交换器的热性能,并在此基础上给出了热交换器的效率与其热负荷、换热管直径以及冷、热流体流速的关系.图5参8     

Second Law Efficiency Analysis of Heat Exchangers

Analytical analysis of unbalanced heat exchangers is carried out to study the second law thermodynamic performance parameter through second law efficiency by varying length‐to‐diameter ratio for counter flow and parallel flow configurations. In a single closed form expression, three important irreversibilities occurring in the heat exchangers—namely, due to heat transfer, pressure drop, and imbalance between the mass flow streams—are considered, which is not possible in first law thermodynamic analysis. The study is carried out by giving special influence to geometric characteristics like tube length‐to‐diameter dimensions; working conditions like changing heat capacity ratio, changing the value of maximum heat capacity rate on the hot stream and cold stream separately and fluid flow type, i.e., laminar and turbulent flows for a fully developed condition. Further, second law efficiency analysis is carried out for condenser and evaporator heat exchangers by varying the effectiveness and number of heat transfer units for different values of inlet temperature to reference the temperature ratio by considering heat transfer irreversibility. Optimum heat exchanger geometrical dimensions, namely length‐to‐diameter ratio can be obtained from the second law analysis corresponding to lower total entropy generation and higher second law efficiency. Second law analysis incorporates all the heat exchanger irreversibilities. © 2013 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.21109     

预测多股流板翅式换热器动态特性的网络法

提出了一种基于两股流换热器网络动态特性预测多股流换器动态性能的网络法。经与实验对比，表明这是一种简便，正确描述多股流换热器动态性能的方法。     

一种板壳式换热器传热准则关系式的实验分析

基于板式换热器的研究成果,对板壳式换热器进行实验研究,利用等流速法求解传热准则关系式.通过控制冷流体、热流体流速相同,对1/1流程组合的板壳式换热器进行实验,并对比和分析实验数据.结果表明:利用板式换热器的经验值拟合得到的传热准则关系式在不同定性温度下并不相同;1/1流程组合板壳式换热器冷流体、热流体的传热准则关系式的参数均为定值,即系数为0.182 5,雷诺数指数为0.75,普朗特数指数为0.34;板壳式换热器的传热准则关系式与板程流程数无关,且与壳程1流程和2流程也无关,但与3流程和4流程则呈现出系数相关的结论,3流程和4流程的系数分别为1流程的0.82倍和0.68倍.     

竖直U型管地热换热器的准三维传热模型

在地热换热器原有研究工作的基础上，进一步考虑流体工质在钻孔深度方向上的温度分布，分析竖直埋管地热换热器钻孔内的传热过程，导出了竖直埋管地热换热器效能的解析式。由此，可以抛弃以往简化模型当中的不合理假设，为地热换热器的设计和模拟提供了更加精确合理的理论基础和计算方法。     

Dynamic Response Analysis of Tube Sheet for U-tube Heat Exchangers

以U型管式换热器管板为例,利用Ansys有限元软件的瞬态分析模块对其动应力进行了有限元计算,通过有限元分析对模型进行了简化和当量处理,并对换热器管板进行了强度和疲劳评定.结果表明：计算和分析结果更符合U型管式换热器管板真实的动应力分布,其最大动应力在管板与壳程筒体连接应力槽处,适当增大过渡圆角半径可使管板两侧应力趋于相等;由温度产生的热应力对管板总应力影响很大,在计算中不能忽略温度载荷的影响;评定结果表明,该换热器符合强度和疲劳要求.     

湿法烟气脱硫工艺的旁路再热计算

通过对工业锅炉湿法烟气脱硫的工艺分析 ,确定了烟气在脱硫过程中烟气特性变化情况 ,给出了旁路烟气再热的计算方法     

折流板位置对换热器性能的影响

采用FLUENT数值模拟方法,研究了简化模型下弓形折流板和螺旋折流板换热器,对应于不同间距/螺距时,流动参量的变化对换热器整体流动与传热性能的影响,进而研究非等距换热器.结果表明,两种结构对应的壳程压力损失和换热系数均随壳程流量的增加而增大,而螺旋折流板结构单位压降下换热系数大于弓形折流板,并且其性能受折流板螺距变化的影响较小,体现了螺旋折流板结构的优越性.为进一步研究非等距型换热器提供了依据.