Heat transfer and friction characteristics of crimped spiral finned heat exchangers with dehumidification

This study experimentally examines the air-side performance of a total of 10 cross flow heat exchangers having crimped spiral configurations under the dehumidification. The effect of tube diameter, fin spacing, fin height, transverse tube pitch, and tube arrangements are examined. The results indicate that the heat transfer coefficient of wet surface is slightly lower than that of dry surface. The effect of tube diameter on the air-side performance is significant. Larger tube diameter not only gives rise to lower heat transfer coefficient but also contributes significantly to the increase of pressure drops. This phenomenon is applicable in both dry and wet condition. For wet surface, the influence of fin height is negligible and the effect of fin spacing on the heat transfer performance is rather small. However, increasing of the fin spacing tends to have a lower heat transfer coefficient. The tube arrangement plays an importance role on the heat transfer coefficient, narrower transverse pitch gives higher heat transfer coefficient. The proposed correlations can predict 75% and 95% of experimental data within 15%.     

Three-dimensional performance analysis of plain fin tube heat exchangers in transitional regime

Three-dimensional CFD simulations are carried out to investigate heat transfer and fluid flow characteristics of a four-row plain fin-and-tube heat exchanger using the Commercial Computational Fluid Dynamics Code ANSYS CFX 12.0. Heat transfer and pressure drop characteristics of the heat exchanger are investigated for Reynolds numbers ranging from 400 to 2000. Fluid flow and heat transfer are simulated and results compared using both laminar and turbulent flow models (k-ω) with steady and incompressible fluid flow. Model validation is carried out by comparing the simulated case friction factor (f) and Colburn factor (j) with the experimental data of Wang et al. [1]. Reasonable agreement is found between the simulations and experimental data. In this study the effect of geometrical parameters such as fin pitch, longitudinal pitch and transverse pitch of tube spacing are studied. Results are presented in the form of friction factor (f) and Colburn factor (j). For both laminar and transitional flow conditions heat transfer and friction factor decrease with the increase of longitudinal and transverse pitches of tube spacing whereas they increase with fin pitches for both in-line and staggered configurations. Efficiency index increases with the increase of longitudinal and transverse pitches of tube spacing but decreases with increase of fin pitches. For a particular Reynolds number, the efficiency index is higher in in-line arrangement than the staggered case.     

Numerical evaluation of a heat exchanger with inline tubes of different size for reduced fouling rates

This paper describes the numerical evaluation of a novel cross flow tube bundle heat exchanger that combines tubes of different diameter in an inline arrangement for the purpose of reducing gas side particulate fouling rates while preserving acceptable levels of heat transfer and pressure drop performance. Three arrangements are compared: a common inline tube bundle heat exchanger with cylinders of equal diameter and two other arrangements that consist of alternately placed cylinders with a diameter ratio of d/D = 0.5, at two different transverse spacings. Numerical calculations are performed in order to study heat transfer, pressure drop and fouling rates from flue gases with suspended ash particles. The alternating tube sizes achieve a suppression of the vortex shedding mechanism that has previously been shown to enhance downstream particle deposition. Results show that, compared to the standard arrangement, the tube bundle with unequal cylinders placed at the largest transverse spacing achieves a significant (～30%) reduction in particle deposition rate without sacrificing acceptable values of heat transfer per unit volume and low pressure drop.     

A comparative study of the airside performance of heat sinks having pin fin configurations

This study performs an experimental study of pin fin heat sinks having circular, elliptic, and square cross-section. A total of twelve pin fin heat sinks with inline and staggered arrangements were made and tested. The effect of fin density on the heat transfer performance is examined. For an inline arrangement, the circular pin fin shows an appreciable influence of fin density whereas no effect of fin density is seen for square fin geometry. This is associated with the unique deflection flow pattern accompanied with the inline circular fin configuration. For the staggered arrangement, the heat transfer coefficient increases with the rise of fin density for all the three configurations. The elliptic pin fin shows the lowest pressure drops. For the same surface area at a fixed pumping power, the elliptic pin fin possesses the smallest thermal resistance for the staggered arrangement.     

Convective Heat Transfer and Pressure Drop over Elliptical and Flattened Tube

A numerical study on the effect of the effect of elliptical and flattened tube bundle geometry on the convective heat transfer and pressure drop is presented in this article. The analysis has been carried out to evaluate the performance of these bundle geometries in the design of a compact and effective single phase shell and tube heat exchanger. The temperature, velocity, and pressure drop profiles are obtained from solving the mass, momentum, and energy conservation equations. The comparison is made for inline and staggered bundle with different pitch to diameter ratio and inlet velocity for elliptical and flattened tubes. The pitch to diameter ratio is varied from 1.25 to 2.5 for Reynolds number ranging from 200 to 2000 which is in the laminar flow region. The heat transfer coefficient over the staggered and inline tube bundle decrease with an increase in pitch. The same kind of variation is also observed for the pressure drop in the case of both elliptical and flattened tube bundle. The study shows that the transverse pitch with respect to cross flow affects more than the longitudinal pitch.     

Experimental investigation of heat transfer and pressure drop in serrated-fin tube bundles with staggered tube layouts

An experimental investigation of the heat transfer and pressure drop performance of ten finned tube bundles using serrated fins is presented. All tube bundles had staggered layouts, and the influence on varying tube bundle layout, tube and fin parameters are presented. The heat transfer coefficient experienced a maximum when the flow areas in the transversal and diagonal planes were equal. An increase in the fin pitch increased the heat transfer coefficient; the same was observed with an increase in fin height. The pressure drop coefficient showed no influence of the tube bundle layout for small pitch ratios, but dropped significantly for higher ratios. Increasing fin pitch reduced the pressure drop, whereas varying fin height had insignificant effect. None of the literature correlations were able to reproduce the experiments for the entire range of tested conditions. A set of correlations were developed, reproducing the experimental data to within ±5% at a confidence interval of 95%.     

Effect of fins on forced convection heat transfer around a tube in an aligned‐arranged tube bundle

The effect of fins on heat transfer around a tube in an aligned‐arranged tube bundle was investigated experimentally, and the obtained results were compared for three arrangements, i.e., single tube, single tube row, and staggered‐arrangement. It was found from the experiment that the effect of fins begins to appear in an aligned‐arrangement with larger fin spacing than in a staggered‐arrangement. The degradation in the local heat transfer coefficient due to fins can be recognized not only on the rear region of the tube, as observed in other arrangements, but also on the frontal region. As a result of this phenomenon, the degradation in the average heat transfer coefficient in an aligned‐arrangement becomes larger than in other arrangements with the same fin spacing. © 2005 Wiley Periodicals, Inc. Heat Trans Asian Res, 34(8): 555–563, 2005; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20091     

Effects of Different Fin Spacings on the Nusselt Number and Reynolds Number in Perforated Finned Heat Exchangers

In this study, the effect of holes placed on perforated finned heat exchangers on convective heat transfer was experimentally investigated. Six-millimeter-diameter holes were opened on each circular fin on a heating tube in order to increase convective heat transfer. These holes were placed on the circular fins in such a way as to follow each other at the same chosen angle. The holes created turbulence in a region near the heating tube surface on the bottom of the fin. Experiments were then performed to analyze the effect of this turbulence on heat transfer and pressure drop. These experiments were carried out at five different fin spacings at the angular locations of 30° and 60° in order to determine the optimum fin spacing. Moreover, further experiments were carried out for counterflow and parallel-flow arrangements to determine the effects of the flow directions of the heating fluid and heated fluid. Results show an increase in Nusselt number with increasing modified Reynolds number. In addition, when different fin spacing to heating tube external diameter ratios were examined, at a ratio of 0.414 and angular locations of 30° and 60°, 11% and 8.6% increase in heat transfer were obtained, respectively, for parallel-flow arrangement compared to counterflow. For parallel flow, pressure drop values were 3.5% and 3.8% lower at 30° and 60°, respectively.     

螺旋翅片管束换热过程的熵产分析

对不同翅片间距Sf、管束横向节距St和管束纵向节距Sl的9组螺旋翅片管束的换热和流动过程进行了试验研究.分析了换热过程的熵产,研究了雷诺数(RP)、翅片间距、管束横向节距和管束纵向节距对管束换热熵产数NsH、流动熵产数NsF和总熵产数Ns的影响.结果表明:对不同布置方式的管束,随着Re的增加,NsH迅速减小,NsF逐渐增加,Ns先减小后增加;翅片间距对NsH影响较小,在高Re下,翅片间距增大时,NsF和Ns均明显降低;横向节距对NsH几乎没影响,但随着横向节距的增加,NsF和Ns均明显降低;管束纵向节距对NsH、NsF和Ns的影响都很小.     

螺旋翅片管束传热和阻力特性的试验研究

对13个不同翅片间距、翅片高度、横向管间距、纵向管间距的螺旋翅片管束换热器在不同雷诺数条件下的传热和阻力特性进行了试验研究,得出了翅片间距、翅片高度、横向管间距、纵向管间距及雷诺数与换热特性Nu和阻力特性Eu的准则关系式,并对准则关系式进行了分析.结果表明:随着横向管间距和翅片间距的增大,螺旋翅片管的传热得到强化,但随着纵向管间距和翅片高度的增加,螺旋翅片管的传热有所减弱;随着横向管间距、纵向管间距和翅片间距的增大,螺旋翅片管的阻力减少,但随着翅片高度的增加,螺旋翅片管的阻力增加.     

Experimental Study of Heat Transfer and Pressure Drop for H-type Finned Oval Tube with Longitudinal Vortex Generators and Dimples under Flue Gas

To enhance heat transfer and reduce fouling of the finned-tube surface in economizers of coal-fired power plants, heat transfer and pressure drop characteristics for H-type finned oval tube with longitudinal vortex generators (LVG) and dimples, both in-line and staggered arrangements, are studied experimentally under flue dust condition. In addition, the ash samples and heat exchanger surfaces after the test are analyzed to help understanding the ash fouling and tube wear mechanisms. Compared to the original H-type finned oval tube, the Nusselt number of H-type finned oval tube bank with longitudinal vortex generators and dimples is improved by 34.5–41.7% (in-line arrangement) and 28.1–31.7% (staggered arrangement) within the studied Reynolds numbers, while the Euler number is increased by 21.9–28.3% (in-line arrangement) and 19% (staggered arrangement) in the clean finned-tube surface state. In the stable fouling state, the Nusselt number is improved by 37.7–42.3% (in-line arrangement) and 27.8–45.1% (staggered arrangement), while the Euler number is increased by 22.9–25.2% (in-line arrangement) and 33.3–42% (staggered arrangement). The results show that the novel fin structures can both inhibit fouling and enhance heat transfer effectively.     

Convection heat transfer from tube banks in crossflow: Analytical approach

The main objective of this analytical study is to investigate heat transfer from tube banks in crossflow under isothermal boundary condition. For this purpose, a control volume is selected from the fourth row of a tube as a typical cell to study the heat transfer from an in-line or staggered arrangement. An integral method of boundary layer analysis is employed to derive closed form expressions for the calculation of average heat transfer from the tubes of a bank, that can be used for a wide range of parameters including longitudinal pitch, transverse pitch, Reynolds and Prandtl numbers. The models for in-line and staggered arrangements are applicable for use over a wide range of parameters when determining heat transfer from tube banks.     

Design of a novel, intensified heat exchanger for reduced fouling rates

This paper describes an integrated approach into the design and evaluation of a novel tube bundle heat exchanger that achieves higher heat transfer levels at lower levels of pressure drop, while remaining less susceptible to gas-side fouling. The approach combines laboratory scale experiments with industrial observations and numerical simulations of full-scale heat exchangers to study the thermal, hydraulic and fouling characteristics of tube bundle heat exchangers. Three arrangements are compared and the advantages of the proposed novel arrangement are demonstrated. Enhanced heat transfer rates are combined with reduced pressure drop and gas-side fouling rates through careful design of the shape of the tube cross-section and reduced transverse spacing.     

Conjugate heat transfer in channels with heat-conducting inclined fins

Conjugate heat transfer in a finned channel with equally spaced fins placed transversely to the flow direction following in-line and staggered arrangements is evaluated. The fins and channel walls are heat-conducting and are fully coupled to a turbulent fluid flow problem. The hydrodynamic and thermal effects of the fin blockage ratio, fin angle, and flow velocity were investigated. The simulations show that the fin arrangement is of paramount importance on the performance of the heat exchanger: the staggered fin configuration provided lower pressure drop and higher heat transfer rate than the in-line fin arrangement for different flow conditions.     

Enhanced thermal performance analysis of buried heat transfer tubes thermal storage system filled with microcapsule particles

The heat transfer enhancement performance of a phase change buried tubes thermal storage system is influenced by major parameters such as arrangement of heat transfer tubes, fin structure and fin geometry size. We developed a three-dimensional numerical model with two different arrangements and five different enhanced heat transfer structures respectively. For the sake of analysis the effects of arrangement of heat transfer tubes, fin structure and fin geometry size. In addition, we applied the enthalpy-transforming model to obtain the liquid fraction and location of the solid-liquid interface at different time in the phase change process. The numerical results show that the melting time of the thermal storage system model with a triangle arrangement is about 6.1% longer than that of the model with a square arrangement. Besides, the melting time of the model with 55 mm tube pitch is about 16.7% shorter than that of tube pitch with 60 mm. Moreover, the buried tube thermal storage system models with circle fins have the shortest melting time, which is 18 seconds. Melting time of the model with circle fins is about 40% shorter than that of the model with smooth tube. In addition, the melting time of the model with 3 mm fin thickness is 10 seconds, which is the shortest. The model with thicker fins means the shorter time of melting process. Moreover, the melting time of the model with 10.5 mm fin spacing is about 23.5% shorter than that of the model with 12.5 mm fin spacing, which is 13 seconds. In conclusion, the main factor of the melting time is the heat transfer area. It provides a guidance for the design and reconstruction of the type of heat storage structure.     

Effect of fin pitches on the optimum heat transfer performance of crimped spiral fin-and-tube heat exchangers

This study conducted experiments on the optimized fin pitch for crimped spiral fin-and-tube heat exchangers. The experiments covered a size range of 2.4–6.5 mm, which is the manufacturing limitation for this kind of fin. The water-flow arrangement used in this experiment combined the parallel cross-flow and the counter cross-flow in a two-row configuration. Ambient air was used as the working fluid on the air-side, and hot water was used on the tube-side. The effects of fin pitches on the heat transfer coefficient and pressure drop characteristics were studied. The results clearly showed that the convective heat transfer coefficient (h_o) for a fin pitch of 2.4 mm is relatively low compared with that of other fin pitches with the same air frontal velocity. Using larger fin pitches (i.e., 4.2, 6.2, and 6.5 mm) resulted in negligible differences in the pressure drop. Moreover, this work introduces the parameter of three performances indexes, which can be expressed as the ratio of the desired output to the required input, for optimization purposes. Due to the difference in optimum fin pitch obtained by these performance indexes, an intersection analysis was conducted. The results indicated that the optimum fin pitch is 4.2 mm for this work, which could be valuable for the effective design for industrial thermal-system applications.     

Experimental Investigation of the Effect of Tube-to-Tube Porous Medium Interconnectors on the Thermohydraulics of Confined Tube Banks

This experimental study investigates the effect of tube-to-tube copper porous interconnectors on the thermohydraulic performance of an in-line and staggered confined tube bank. The porous medium, having a transverse thickness equal to that of the diameter of the tube (9 mm), connects longitudinally six successive tubes kept as in-line and staggered arrangements with a square pitch of 2.0. The tubes are subjected to a constant and uniform heat flux and are cooled by forced convection under laminar-transition flow range (200 < Reynolds number < 1500) using air with a Prandtl number of 0.71 as cooling fluid. Experimental data presented here establish that by introducing tube-to-tube porous medium interconnectors for the maximum Reynolds number tested here, a reduction in the pressure drop by 18% is observed in the in-line configuration while the heat transfer is enhanced by 100% in the staggered configuration, when compared to their respective configurations without the porous medium. Defining an overall energy gain as the ratio of the heat transfer enhancement due to the presence of the porous inserts to the pressure drop incurred, it is seen that fixing the porous inserts in the in-line configuration is advantageous.     

Performance analysis of finned tube and unbaffled shell-and-tube heat exchangers

This work considers an optimum design problem for the different constraints involved in the designing of a shell-and-tube heat exchanger consisting of longitudinally finned tubes. A Matlab simulation has been employed using the Kern's method of design of extended surface heat exchanger to determine the behavior on varying the values of the constraints and studying the overall behavior of the heat exchanger with their variation for both cases of triangular and square pitch arrangements, along with the values of pressure drop. It was found out that an optimum fin height existed for particular values of shell and tube diameters when the heat transfer rate was the maximum. Moreover it was found out that the optimum fin height increased linearly with the increase in tube outer diameter. Further studies were also performed with the variation of other important heat exchanger design features and their effects were studied on the behavior of overall performance of the shell-and-tube heat exchanger. The results were thereby summarized which would proclaim to the best performance of the heat exchanger and therefore capable of giving a good idea to the designer about the dimensional characteristics to be used for designing of a particular shell and tube heat exchanger.     

氦气冲刷螺旋管束传热的数值研究

采用CFD软件对氦气冲刷螺旋管束的传热特性进行了数值模拟。计算时采用了轴对称简化模型;湍流模拟采用低Re k-ε模型。通过与实验数据对比,发现低Re模型比壁面函数法更适合计算冲刷管束类型的流动。计算结果表明,顺排管束前几层平均Nu高于叉排管束,而深层管平均Nu低于叉排管束;管列距离较大时排列方式对深层管的传热影响很小;管束与边界距离约为管束中心部分氦气流道宽度的一半时,各列传热管传热和氦气出口温度都较为均匀;管束横向位置发生偏移将导致管束内流动、传热出现不均匀。结果对于螺旋管蒸汽发生器设计具有参考意义。     

鳍片管代替光管省煤器及结构优化改造综述

在分析鳍片管束结构与传热特点的基础上,介绍了鳍片管代替光管省煤器的改造设计和应用情况。分析了鳍片管的直径、鳍片高度、厚度、管间距等参数及管束布置方式对省煤器传热特性、阻力特性、积灰和磨损等方面的影响。提出了鳍片管省煤器受热面优化设计的方法和步骤。