Numerical study of flow patterns of compact plate-fin heat exchangers and generation of design data for offset and wavy fins

Thermo-hydraulic design of compact heat exchangers (CHEs) is strongly dependent upon the predicted/measured dimensionless performance (Colburn factor j and Fanning friction factor f vs. Reynolds number Re) of heat transfer surfaces. Also, air (gas) flow maldistribution in the headers, caused by the orientation of inlet and outlet nozzles in the heat exchanger, affects the exchanger performance. Three typical compact plate-fin heat exchangers have been analyzed using Fluent software for quantification of flow maldistribution effects with ideal and real cases. The headers have modified by providing suitable baffle plates for improvement in flow distribution. Three offset strip fin and 16 wavy fin geometries used in the compact plate-fin heat exchangers have also been analyzed numerically. The j and f vs. Re design data are generated using CFD analysis only for turbulent flow region. For the validation of the numerical analysis conducted in the present study, a rectangular fin geometry having same dimensions as that of the wavy fin has been analyzed. The results of the wavy fin have been compared with the analytical results of a rectangular fin and found good agreement. Similarly, the numerical results of offset strip fin are compared with the correlations available in the open literature and found good agreement with most of the earlier findings.     

Natural Convection in a Square Cavity with Discrete Heating at the Bottom with Different Fin Shapes

Numerical study is carried out to investigate the effect of different fin shapes on heating a square cavity by small heating strip located at the bottom wall. The natural convection of air is considered with constant heat flux from heat source which is located at the center of the bottom wall. The width of the heating strip is assumed to be 20% of the total width of the bottom wall. The remaining (non-heated) part of the bottom wall and the top wall are adiabatic and the side walls are maintained at constant temperature. The investigation considered four shapes of aluminum fins with equal area and equal base width. The easy to fabricate fin shapes are considered as: rectangular, one triangular, two opposite triangular and two isosceles triangular shape. Other parameters considered are the total area of the fin (or the height of the fin) and the Grashof number in the laminar flow range. It is found that the heat transfer can be enhanced by either increasing the Grashof number or the height of the fins. In most of the investigated cases the heat transfer in the case of the two opposite triangular fins shape is found to be higher than that of the other shapes under the same conditions.     

The Influence of Strip Location on the Pressure Drop and Heat Transfer Performance of a Slotted Fin

In this article, a numerical study is conducted to predict the air-side heat transfer and pressure drop characteristics of slit fin-and-tube heat transfer surfaces. A three-dimensional steady laminar model is applied, and the heat conduction in the fins is also considered. Five types of slit fins, named slit 1, slit 2, slit 3, slit 4, and slit 5, are investigated, which have the same global geometry dimensions and the same numbers of strips on the fin surfaces. The only difference among the five slit fins lies in the strip arrangement. Slit 1 has all the strips located in the front part of the fin surface, then, following the order from slit 1 to slit 5, the strip number in the front part decreases and, correspondingly, the strip number in the rear part increases, so that all the strips of slit 5 are located in the rear part. Furthermore, slit 1 and slit 5, slit 2 and slit 4, have a symmetrical strip arrangement along the flow direction. The numerical results show that, following the order from slit 1 and slit 5, the heat transfer rate increases at first, reaching a maximum value at slit 3, which has the strip arrangement of “front coarse and rear dense”; after that, it begins to decrease, as does the fin efficiency. Although they have the symmetrical strip arrangement along the flow direction, slit 5 has 7% more Nusselt number than slit 1, and slit 4 also has 7% more Nusselt number than slit 2, which shows that strip arrangement in the rear part is more effective than that in the front part. Then the difference of heat transfer performance among five slit fins is analyzed from the viewpoint of thermal resistance, which shows that when the thermal resistances in the front and rear parts are nearly identical, the optimum enhanced heat transfer fin can be obtained. This quantitative rule, in conjunction with the previously published qualitative principle of “front sparse and rear dense,” can give both quantitative and qualitative guides to the design of efficient slotted fin surfaces. Finally, the influence of fin material on the performance of enhanced-heat-transfer fins is discussed.     

空气横掠矩形翅片椭圆管束换热规律的数值研究

采用Fluent软件对矩形翅片椭圆管束空气侧的对流换热情况进行了三维数值模拟,获得了不同流速下翅片表面温度分布,分析了迎面风速与换热系数之间的关系,随着速度的增大,空气侧的换热系数增加,并拟合了换热计算公式。同时分析了不同翅片间距对换热的影响因素,随着翅片间距的增大,空气侧换热系数增加,而且随着Rg数的增加,换热的强化更加明显。     

Laminar Free Convection Around a Horizontal Cylinder with External Longitudinal Fins

The numerical solution of the laminar free convection of air around a horizontal cylinder with external longitudinal fins has been reported in this paper. The cylinder surface as well as the surfaces of each fin were assumed to be at a uniform temperature. The fluid drawn over a large angular domain moves out through a narrow, almost vertical strip known as plume, the thickness of which reduces with increasing buoyancy. The heat transfer increases with an increase in Grashof number, the number of fins, and fin length. For a constant fin surface, more fins of lower length result in a better heat transfer for Gr beyond about 10 ³ .     

Development of Colburn j Factor and Fanning Friction Factor Correlations for Compact Surfaces of the Triangular Perforated Fins Using CFD

The necessity of increased heat transfer surface area has resulted in the development of compact heat exchangers, which are widely used in the aerospace and automobile industries. Hence perforations are made on triangular plain fins to study the effects on the heat transfer coefficient. A numerical model has been developed for the perforated fin of a triangular plate fin heat exchanger. Perforated fin performance has been analyzed with the help of computational fluid dynamics (CFD) by changing the various parameters of the fin. The Colburn j factor and the Fanning friction factor are calculated for different Reynolds numbers. The values of the Colburn j factor and the Fanning friction factor are validated for known geometric fins with available data in the literature and extended to triangular perforated fins. The correlations have been developed between Reynolds number, Colburn j factor, and Fanning friction factor by taking into account fin height, fin thickness, and fin spacing. The present numerical analysis is carried out for air media.     

Heat transfer enhancement in hairy fin systems

Heat transfer inside fin systems composed of primary rectangular fins with large number of slender rods attached on their surfaces is modeled and analyzed analytically in this work. The terminology “hairy fin systems” is used to refer to this kind of fin systems. One and two dimensional analyses are employed in the analysis and appropriate performance indicators are evaluated in order to measure the superiority of hairy fin systems over rectangular fins. It is found that hairy fin systems can transfer more heat than rectangular fins under specific conditions. The enhancement in heat transfer through hairy fin systems is found to increase as the rods thermal conductivity increases or as both the rods diameter and main convection coefficient decrease. Moreover, decreasing the rods diameter is found to decrease the sensitivity of the heat flow within the hairy fin systems to the rods thermal conductivity. Finally, the results of this work demonstrate that the increase in heat flow through hairy fin systems is significant enough to allow them to be utilized in the design of thermal systems.     

Experimental Study on Heat Transfer and Pressure Drop Characteristics of Four Types of Plate Fin－and－TUbe Heat Exchanger Surfaces

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A numerical study of flow behavior in the shell and helical finned-tube heat exchanger

Extended surfaces mostly aim to improve the heat transfer upon increasing the area of heat transfer. In this paper, the influence of using fins on flow behaviors and the heat transfer of the shell and tube heat exchanger has been investigated. In this regard, the present results are verified with available experimental data in the literature for a helical tube without fins. The effects of fin density (fin per inch), its height, and material have been studied on the heat transfer rate. In addition, the effects of radial pitch and the number of coil loops are studied. The results indicate that implementing extended surfaces significantly increases the heat transfer rate. The increase of fin density from 8 to 12 and the height from 11.5 to 13.5 mm enhances heat transfer up to 48% and 43% depending on Dean number, respectively. The rise of coil pitch augments the overall heat transfer, and it is more efficient at lower Dean numbers. The predicted results also show that the fin material does not have any significant effect on heat transfer.     

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In this study, fully developed laminar flow and convective heat transfer in an internally finned tube heat exchanger are investigated numerically. The flow is assumed to be both hydrodynamically and thermally developed with uniform outside wall temperature. Parameters of the thickness, length, and number of fins and thermal conductivity ratio between fin and working fluid are varied to obtain the friction factor as well as Nusselt number. The results show that the heat transfer improves significantly if more fins are used; however, the pressure drop turns out to be large in this heat exchanger. In addition, it is found that the emergence of closed-loop isotherms between the areas of two neighboring fins leads to heat transfer enhancement in the internally finned tube. When the fin number is smaller than 14, there appears a maximum Nusselt number at about 0.8 of the dimensionless fin length. Finally, an experiment is conducted to verify the numerical results.     

Comparative design study of a diesel exhaust gas heat exchanger for truck applications with conventional and state of the art heat transfer enhancements

The exhaust gas of heavy duty diesel engines can provide an important heat source that may be used in a number of ways to provide additional power and improve overall engine efficiency. The sizing of a heat exchanger that can manage the heat load and still be of reasonable size and weight without excessive pressure drop is of significant importance especially for truck applications. This is the subject of the present work. To approach the problem, a total of five different configurations are investigated and a comparison of conventional and state of the art heat transfer enhancement technologies is included. Two groups of configurations are examined: (a) a classical shell and tube heat exchanger using staggered cross-flow tube bundles with smooth circular tubes, finned tubes and tubes with dimpled surfaces and (b) a cross-flow plate heat exchanger, initially with finned surfaces on the exhaust gas side and then with 10 ppi and 40 ppi metal foam material substituting for the fins. Calculations were performed, using established heat exchanger design methodologies and recently published data from the literature to size the aforementioned configurations. The solutions provided reduce the overall heat exchanger size, with the plate and fin type consisting of plain fins presenting the minimum pressure drop (up to 98% reduction compared to the other configurations), and the 40 ppi metal foam being the most compact in terms of size and weight. Durability of the solutions is another issue which will be examined in a future investigation. However, coupling of the exhaust heat exchanger after a particulate trap appears to be the most promising solution to avoid clogging from soot accumulation.     

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.     

Improvement of heat transfer through fins: A brief review of recent developments

Fins or extended surfaces are generally used in heat exchangers to enhance heat transfer between the main surface and ambient fluid. Various types of simple‐shaped fins, namely, rectangular, square, annular, cylindrical, and tapered, have been used with different geometrical combinations. To satisfy industrial demand, different trials have also been carried out for designing optimized fins. The optimization of fins can be performed either by enhancing heat dissipation at an exact fin weight or by diminishing the weight of the fin by precise heat dissipation. Recently a notable amount of work on some typical fins, like, porous fins and perforated fins, has also been carried out. This paper presents a brief review on heat transfer enhancement using fins of different types considering variable thermophysical and geometric parameters, which will also be useful for future use of geometrical modifications of extended surfaces, based on the cost and availability of space.     

Optimization of short micro pin fins in minichannels

Finned minichannels are modeled in order to optimize microstructure geometry and maximize heat transfer dissipation through convection from a heated surface. Six pin fin shapes – circle, square, triangle, ellipse, diamond and hexagon – are used in a staggered array and attached to the bottom heated surface of a rectangular minichannel and analyzed. Also, using square pin fins, different channel clearance over fins are investigated to optimize the fin height of the fins with respect to that of the channel. Fin width and spacing are investigated using a ratio of fin width area to the channel width. Fin material is then varied to investigate the heat dissipation effects. Triangular fins with larger fin height, smaller fin width, and spacing double the fin width maximizes the number of fins in each row and yields better performance. Correlations describing the Nusselt number and the Darcy friction factor are obtained and compared to previous ones from recent studies. These correlations only apply to short fins in the laminar regime. Completely understanding the effects of micro pin fins in a minichannel is essential to maximizing the performance in small scale cooling apparatuses to keep up with future electronic advancements.     

Numerical investigation of fluid flow and heat transfer over louvered fins in compact heat exchanger

Numerical investigation of fluid flow and heat transfer characteristics over louvered fins and flat tube in compact heat exchangers is presented in this study. Three-dimensional simulations of single and double row tubes with louvered fins have been conducted. Simulations are performed for different geometries with varying louver pitch, louver angle, fin pitch and tube pitch and for different Reynolds number. Conjugate heat transfer and conduction through the fins are considered. The air-side performance of heat exchanger is evaluated by calculating Stanton number and friction factor. The results are compared with experiment and a good agreement is observed. The local Nusselt number variation along the top surface of the louver is calculated and effects of geometrical parameters on the average heat transfer coefficient is computed. Design curves are obtained which can used to predict the heat transfer and the pressure drop for a given louver geometry.     

Heat transfer enhancement by using a rectangular wing vortex generator on the triangular shaped fins of a plate‐fin heat exchanger

The present numerical analysis pertains to the heat transfer enhancement in a plate‐fin heat exchanger employing triangular shaped fins with a rectangular wing vortex generator on its slant surfaces. The study has been carried out for three different angles of attack of the wing, i.e., 15°, 20° and 26°. The aspect ratio of the wing is not varied with its angle of attack. The flow considered herein is laminar, incompressible, and viscous with the Reynolds number not exceeding 200. The pressure and the velocity components are obtained by solving the continuity and the Navier– Stokes equations by the Marker and Cell method. The present analysis reveals that the use of a rectangular wing vortex generator at an attack angle of 26° results in about a 35% increase in the combined spanwise average Nusselt number as compared to the plate‐triangular fin heat exchanger without any vortex generator. © 2010 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20285     

Parametric study on the performance of a heat exchanger with corrugated louvered fins

The Taguchi method is a well-known parametric study tool in engineering quality and experimental design. This study analyzes five experimental factors (flow depth, ratio of fin pitch and fin thickness, tube pitch, number of louvers and angle of louver) affecting the heat transfer and pressure drop of a heat exchanger with corrugated louvered fins using the Taguchi method. Fifteen samples are selected from experimental database and the heat transfer and flow friction characteristics are analyzed. The results show that flow depth, ratio of fin pitch and fin thickness and the number of the louvers are the main factors that influence significantly the thermal hydraulic performance of the heat exchanger with corrugated louvered fins. Therefore, these three factors are considered as the main factors for an optimum design of a heat exchanger.     

Enhancement of Heat Transfer for Plate Fin Heat Exchangers Considering the Effects of Fin Arrangements

In the present study, the potential of rectangular fins with different fin types of inner zigzag-flat-outer zigzag (B-type) and outer zigzag-flat-outer zigzag (C-type) and with different fin angles of 30° and 90° for 2 mm fin height and 10 mm offset from the horizontal direction for heat transfer enhancement with the use of a conjugated heat transfer approach and for pressure drop in a plate fin heat exchanger is numerically evaluated. The rectangular fins are located on a flat plate channel (A-type). The numerical computations are performed by solving a steady, three-dimensional Navier–Stokes equation and an energy equation by using FLUENT software program. Air is taken as working fluid. The study is carried out at Reynolds number of 400 and inlet temperatures, velocities of cold and hot air are fixed as 300 K, 600 K and 1.338 m.s^?1, 0.69 m.s^?1, respectively. This study presents new fin geometries which have not been researched in the literature for plate fin heat exchangers. The results show that while the heat transfer is increased by about 10% at the exit of a channel with the fin type of C, it is increased up to 8% for the fin angle of 90° when compared to a channel with A-type under the counter flow. The heat transfer enhancements for different values of Reynolds number and for varying fin heights, fin intervals and also temperature distributions of fluids are investigated for parallel and counter flow.     

NUMERICAL DESIGN OF EFFICIENT SLOTTED FIN SURFACE BASED ON THE FIELD SYNERGY PRINCIPLE

In this article, a numerical investigation of the flow and heat transfer in a three-row finned-tube heat exchanger is conducted with a three-dimensional laminar conjugated model. Four types of fin surfaces are studied; one is the whole plain plate fin, and the other three are of slotted type, called slit 1, slit 2, and slit 3. All four fin surfaces have the same global geometry dimensions. The three slotted fin surfaces have the same numbers of strips, which protrude upward and downward alternatively and are positioned along the flow direction according to the rule of “front coarse and rear dense.” The difference in the three slotted fins is in the degree of “coarse” and “dense” along the flow direction. Numerical results show that, compared to the plain plate fin, the three types of slotted fin all have very good heat transfer performance in that the percentage increase in heat transfer is higher than that in the friction factor. Among the three slotted fin surfaces, slit 1 behaves the best, followed by slit 2 and slit 3 in order. Within the Reynolds number range compared ( from 2,100 to 13,500), the Nusselt number of slit 1 is about 112–48% higher than that of the plain plate fin surface under the identical pumping constraint. An analysis of the essence of heat transfer enhancement is conducted from the field synergy principle, which says that the reduction of the intersection angle between the velocity and the temperature gradient is the basic mechanism for enhancing convective heat transfer. It is found that for the three comparison constraints the domain-average synergy angle of slit 1 is always the smallest, while that of the plain plate fin is the largest, with slit 2 and slit 3 being somewhat in between. The results of the present study once again show the feasibility of the field synergy principle and are helpful to the development of new types of enhanced heat transfer surfaces.     

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.