Plain Plate Fin Research Articles

Numerical optimization of obstructed high temperature heat exchanger for recovery from the flue gases by considering ash fouling characteristics

PurposeThe purpose of this paper is to recommend a validated numerical model for simulation the flue gases heat recovery recuperators. Due to fulfill of this demand, the influences of ash fouling characteristics during the transient/steady-state simulation and optimization of a 3D complex heat exchanger equipped with inner plain fins and side plate fins are studied.Design/methodology/approachFor the particle dispersion modeling, the discrete phase model is applied and the flow field has been solved using SIMPLE algorithm.FindingsAccording to obtained results, for the recuperator equipped with combine inner plain and side plate fins, determination of ash fouling characteristics is really important, effective and determinative. It is clear that by underestimating the ash fouling characteristics, the achieved results are wrong and different with reality.Originality/valueFinally, the configuration with inner plain fins with characteristics of: di =5 mm, do = 6 mm, dg = 2 mm, dk = 3 mm and NIPFT = 9 and side plate fins with characteristics of: TF = 3 mm, PF = 19 mm, NSPF = 17·2 = 34, WF = 10 mm, HF = 25 mm, LF = 24 mm and ß = 0° is introduced as the optimum model with the best performance among all studied configurations.

Experimental and numerical study and comparison of performance for wavy fin and a plain fin with radiantly arranged winglets around each tube in fin-and-tube heat exchangers

A new kind of plain plate fin with twelve vortex generators of delta winglets around each tube in fin-and-tube heat exchanger proposed by the present authors is experimentally studied in this paper. Experiments of four heat exchanger surfaces of real size are conducted to compare the comprehensive characteristics of the proposed fin with a circular wavy fin: two wavy fin-and-tube heat exchangers with 6 rows of tubes at the fin pitch of 2.54 mm and 2.117 mm (10-wavy and 12-wavy for short) respectively; and two proposed fin-and-tube heat exchanger surfaces with five rows of tubes at the same fin pitches (10-LVG and 12-LVG) correspondingly. The inlet velocity of the air side varies from 1.5 m/s to 7.5 m/s, and the water side flow rate is fixed at a certain value at each air inlet velocity. Experimental results indicate that the heat transfer rate and pressure penalty of heat exchanger surfaces using proposed fin with five-row tubes are almost the same with the six-row tubes wavy heat exchanger surfaces. Correlation of the Nusselt number Nu and the friction factor f on the air side are achieved. Entransy analysis is conducted to reveal the heat enhancement mechanism.

Experimental characterization of an additively manufactured heat exchanger for dry cooling of power plants

Air-cooled heat exchangers for power plant cooling are receiving much attention lately, as they require little or no water for cooling when compared to water-cooled systems. This paper focuses on the design, fabrication, and experimental characterization of a novel additively manufactured air–water heat exchanger for dry cooling of power plants. The heat exchanger consists of manifold-microchannels on the air side and rectangular channels on the water side in a cross-flow configuration. By using additive manufacturing, the manifold-microchannel heat exchanger can be fabricated as a single component, which eliminates the assembly process. Three prototype heat exchangers were fabricated using direct metal laser sintering (DMLS) out of stainless-steel (SS17-4), titanium alloy (Ti64), and aluminum alloy (AlSi10Mg). Air-side heat transfer coefficients in the range of 100–450 W/m2 K at pressure drops of 50–2000 Pa were recorded for the titanium alloy heat exchanger for air flow rate ranging from 1.89 L/s to 18.9 L/s. Based on our analysis and compared to conventional heat exchangers, the performance of this manifold-microchannel heat exchanger was superior. Compared to wavy fin and plain plate fin heat exchangers, up to 30% and 40% improvement, respectively, in gravimetric heat transfer density was recorded for the entire range of experimental data. Compared to state-of-the-art dry cooling, nearly 27% improvement in gravimetric heat transfer density was noted at air-side coefficient of performance (COPair) of 172.

Study on the consistency between field synergy principle and entransy dissipation extremum principle

This paper is aiming at numerically demonstrating the interrelationship and consistency between field synergy principle (FSP) via the field synergy number (Fc) and the entransy dissipation extremum principle (EDEP). Numerical simulation is conducted by using the FLUENT software and the user defined function programs (UDF) for fin-and-tube surfaces (plain plate and slotted fins) and composite porous materials. The thermal boundary conditions include given heat flux and given surface temperature. The flow includes laminar and turbulent. The air properties may be constant or vary with temperature. Based on the numerical data the analyzed results from the FSP via Fc are totally consistent with the results analyzed by the EDEP for all the cases studied. Such consistency between the FSP and the entransy theory can be regarded as a kind of demonstration of the reliability and correctness of both the FSP and the entransy theory.

A comparison of thermal-hydraulic performance of various fin patterns using 3D CFD simulations

The objective of the present numerical study is to investigate the thermal-hydraulic characteristics of various circular (plain circular fin, crimped spiral fin, serrated fin) and plate fins [plain plate fin, wavy fin and fin with punched delta winglet pair (DWP)]. The geometrical parameters are, tube outer diameter (OD)=7mm, fin spacing (S)=3–7mm, fin height (hf)=5mm, number of tube rows (Nr)=2, transverse tube pitch (Pt)=23mm and longitudinal tube pitch (Pl)=18mm. The Reynolds number based on hydraulic diameter has been considered in the range of 2500⩽Reh⩽4000. The circular fins outdo the plate fin in terms of the heat transfer coefficient and the heat transfer per unit pumping power for the circular fins has been found to be 140–170% higher as compared to the plate fins for a fixed Reynolds number. Local Nusselt number around the periphery of the tubes showed that fin patterns affect the wake region formation and flow separation on the tube surface. It has been observed that heat transfer coefficient decreases with an increase in the fin spacing due to increase in bypass flow rate through the heat exchanger at a constant Reynolds number. The heat transfer per unit pumping power showed an increase of more than 100% with an increase in the fin spacing from 3 to 6mm. The flow patterns have been studied in detail for serrated fin, crimped fin and fin with DWP. The vortices generated by the fins merge with the horseshoe vortices and enhances the heat transfer in the domain. The fin efficiency has been found to be in the range of 77–83% for the circular fins and 55–66% for the plate fins for a range of 2500⩽Reh⩽4000. Only one circular fin design i.e., crimped fin provides higher volume goodness factor as compared to the plate fins. In general it has been observed that more compact heat exchanger can be designed using plate fins but at a higher pumping power cost.

Experimental characterization of heat transfer in an additively manufactured polymer heat exchanger

In addition to their low cost and weight, polymer heat exchangers offer good anticorrosion and antifouling properties. In this work, a cost effective air-water polymer heat exchanger made of thin polymer sheets using layer-by-layer line welding with a laser through an additive manufacturing process was fabricated and experimentally tested. The flow channels were made of 150μm-thick high density polyethylene sheets, which were 15.5cm wide and 29cm long. The experimental results show that the overall heat transfer coefficient of 35–120W/m2K is achievable for an air-water fluid combination for air-side flow rate of 3–24L/s and water-side flow rate of 12.5mL/s. In addition, by fabricating a very thin wall heat exchanger (150μm), the wall thermal resistance, which usually becomes the limiting factor on polymer heat exchangers, was calculated to account for only 3% of the total thermal resistance. A comparison of the air-side heat transfer coefficient of the present polymer heat exchanger with some of the commercially available plain plate fin heat exchanger surfaces suggests that its performance in general is superior to that of common plain plate fin surfaces.

A review on the thermal hydraulic characteristics of the air-cooled heat exchangers in forced convection

In this paper, a review is presented on the experimental investigations and the numerical simulations performed to analyze the thermal-hydraulic performance of the air-cooled heat exchangers. The air-cooled heat exchangers mostly consist of the finned-tube bundles. The primary role of the extended surfaces (fins) is to provide more heat transfer area to enhance the rate of heat transfer on the air side. The secondary role of the fins is to generate vortices, which help in enhancing the mixing and the heat transfer coefficient. In this study, the annular and plate fins are considered, the annular fins are further divided into four categories: (1) plane annular fins, (2) serrated fins, (3) crimped spiral fins, (4) perforated fins, and similarly for the plate fins, the fin types are: (1) plain plate fins, (2) wavy plate fins, (3) plate fins with DWP, and (4) slit and strip fins. In Section 4, the performance of the various types of fins is presented with respect to the parameters: (1) Reynolds number, (2) fin pitch, (3) fin height, (4) fin thickness, (5) tube diameter, (6) tube pitch, (7) tube type, (8) number of tube rows, and (9) effect of dehumidifying conditions. In Section 5, the conclusions and the recommendations for the future work have been given.

Performance Evaluation of Heat Transfer Enhancement in Plate-fin Heat Exchangers with Offset Strip Fins

Generally, the Offset Strip Fin (OSF) in a plate-fin heat exchanger provides a greater heat transfer coefficient than plain plate-fin, but it also leads to an increase in flow friction. A new parameter, called relative entropy generation distribution factor, Ψ*, is proposed to evaluate the thermodynamic advantages of OSFs. This parameter presents a ratio of relative changes of entropy generation. The relative effects of the geometrical parameters α, γ and δ are discussed. The results show that there exist the optimum values of α and γ at a certain flow condition, which obviously maximize the degree of the heat transfer enhancement of OSFs.

Fluid Flow and Heat Transfer in Plate-Fin and Tube Heat Exchangers in a Transitional Flow Regime

The unsteady behaviors of fluid flow and heat transfer in plain plate-fin and tube heat exchangers with a wide range of fin spacings from 2.06 mm to 16.48 mm and tube diameter 8.28 mm are studied by a large eddy simulation technique (LES). Velocity fluctuations and vortex sheddings induced by the tubes in the channel are modeled. The results found that the flow in passages of large spacings is quite different from that of small spacings. The flow is co-determined by two effects: the duct effect and the tube bank effect. The tube bank effect is more dominant with increasing fin spacings.

A new performance evaluation method and its application in fin-tube surface design of small diameter tube

In this paper, a simple yet efficient performance comparison method is proposed based on the assumptions of constant properties and identical frontal area. For this method, no correlations are required, and a small number of discrete data are sufficient. To illustrate the feasibility of the proposed approach, a new slotted fin with 4 mm tubes is designed to replace the original louvered fin with tubes of 7 mm. The orthogonal design method is adopted in the fin design to reduce the number of computational cases significantly, and yet a nearly optimum combination of major geometric factors can still be obtained. The reasonable parametric combination of 3 global parameters is obtained by analyzing the numerical results of 16 plain plate fins. Based on this result, 3 new slotted fins with different fin pitches are studied. The slotted fin with a fin pitch of 1.4 mm is recommended after considering the heat transfer, comprehensive performance, and cost of material and operation. The result shows that compared with the original louvered fin, the recommended fin not only increases the heat transfer rate by 2.2%, 22.5%, and 13.7% under an identical flow rate, identical pressure drop, and identical pumping power constraint, respectively, but also saves approximately 36% of the copper tube materials.

Thermal Optimal Design for Plain Plate-Fin Heat Sinks by Using Neuro-Genetic Method

An effective artificial neural network together with a genetic algorithm have been demonstrated for predicting the optimal thermal performance of plain plate-fin heat sinks in a ducted flow under multi-constraints such as pressure drop, mass, and space limitations. A series of constrained optimal designs can be efficiently performed. Comparisons of the optimal results between the artificial neural network with genetic algorithm (ANN-GA) and the response surface methodology with sequential quadratic programming (RSM-SQP) methods are made. Although more training patterns are needed for the ANN-GA method as compared to that for the RSM-SQP method, the ANN-GA method which has randomly uniform-distributed training patterns in the whole solving domain can be applied to the global region of interest, not just in the region of operability. Consequently, a globally precise optimal solution can be achieved with the ANN-GA method; while the solution obtained with the RSM-SQP method may cause a significant error if the optimal values of the design variables happen to be located beyond the region of operability.

3D numerical simulation on fluid flow and heat transfer characteristics in multistage heat exchanger with slit fins

In this paper, a numerical investigation is performed for three-stage heat exchangers with plain plate fins and slit fins respectively, with a three-dimensional laminar conjugated model. The tubes are arranged in a staggered way, and heat conduction in fins is considered. In order to save the computer resource and speed up the numerical simulation, the numerical modeling is carried out stage by stage. In order to avoid the large pressure drop penalty in enhancing heat transfer, a slit fin is presented with the strip arrangement of “front coarse and rear dense” along the flow direction. The numerical simulation shows that, compared to the plain plate fin heat exchanger, the increase in the heat transfer in the slit fin heat exchanger is higher than that of the pressure drop, which proves the excellent performance of this slit fin. The fluid flow and heat transfer performance along the stages is also provided.

Numerical study of local heat transfer coefficient and fin efficiency of wavy fin-and-tube heat exchangers

Three-dimensional numerical simulations were performed for laminar flow of wavy fin-and-tube heat exchangers by using body-fitted coordinates (BFC) method with fin efficiency effect accounted. The prediction results of average Nusselt number, friction factor and fin efficiency were compared with the related experimental correlations [R.C. Xin, H.Z. Li, H.J. Kang, W. Li, W.Q. Tao, An experimental investigation on heat transfer and pressure drop characteristics of triangular wavy fin-and-tube heat exchanger surfaces, J. Xi'an Jiaotong Univ. 28 (2) (1994) 77–83] and Schmidt approximation [T.E. Schmidt, Heat transfer calculations for extended surfaces, Refrigerating Engineering (April 1949) 351–357]. For Reynolds numbers based on the tube outside diameter ranging from 500 to 4000, the mean deviation is 3.3% for Nusselt number, 1.9% for friction factor and 3.6% for fin efficiency. The distributions of local Nusselt number and fin efficiency on fin surface were studied at wavy angle equal to 0° (plain plate fin), 10° and 20° respectively. The local Nusselt number decreases along the air flow direction, but fin efficiency increases in general. The wavy angle can greatly affect the distributions of local Nusselt number and fin efficiency, and make the distributions present fluctuation along the flow direction. The result also shows that the fin efficiency at the inlet region of wavy fin is larger than that of plain plate fin at the same region. With the increase of Reynolds number, the effects of wavy angle on the distributions of local Nusselt number and fin efficiency are more and more significant.

Optimum Design of Two-Row Slotted Fin Surface with X-Shape Strip Arrangement Positioned by “Front Coarse and Rear Dense” Principle, Part II: Results and Discussion

In Part I of this article, design considerations for 15 slotted fin surfaces are introduced, and the physical/mathematical model and numerical methods are described. The strips in all slotted fin surface are designed according to two general guidelines: “front coarse and rear dense” and “X-shape arrangement.” In this article, the specific flow and heat transfer characteristics of the 15 slotted fin surfaces and their comparisons with the plain plate surface are presented. The major findings are as follows. For all 15 slotted fin surfaces, their j/f versus Re curves cross with the curve of the plain plate fin surface at some turning Reynolds number, beyond which the j/f ratio of slotted fin surfaces is higher than that of plain plate fin surfaces, and vice versa. The variation character of heat transfer rate versus Reynolds number under identical pumping power condition has the same feature, with its turning Reynolds number being appreciably lower than the former one. However, at the identical flow rate the heat transfer rates of all the slotted fin surfaces are higher than that of the plain plate fin, and the larger the velocity, the more significant the enhancement. Among the four techniques adopted—changing the number of strips, shortening the strip length, changing the location of the strips, and splitting one strip into two—the effect of strip location is the most and that of splitting one into two is the least. The field synergy principle analysis shows that the higher heat transfer rate of the slotted fin surfaces comes from their better synergy between velocity and temperature gradient. Two patterns of slotted fin are recommended for the two-row plate fin-and-tube surface.

Three dimensional numerical simulation and analysis of the airside performance of slotted fin surfaces with radial strips

PurposeTo provide some heat transfer and friction factor results for fin‐and‐tube heat transfer surfaces which may be used in air conditioning industry.Design/methodology/approachNumerical simulation approach was adopted to compare the plain plate fin and three types of radial slotted fin surfaces.FindingsIt is found that at the same frontal velocity (1.0‐3 m/s) the plain plate fin has the lowest heat transfer rate with the smallest pressure drop. The full slotted fin surface has the highest heat transfer rate with the largest pressure drop penalty. The partially slotted fin (where the strips are mainly located in the rear part of the fin) and the back slotted fin are some what in between. Under the identical pumping power constraint, the partially slotted fin surface behaves the best.Research limitations/implicationsThe results are only valid the two‐row fin surface.Practical implicationsThe results are very useful for the design of two‐row tube fin surfaces with high efficiency.Originality/valueThis paper provides original information of slotted fin surface with radial strips from the field synergy principle.

Three-Dimensional Numerical Simulation on Laminar Heat Transfer and Fluid Flow Characteristics of Strip Fin Surface With X-Arrangement of Strips

In this paper, a numerical investigation of air side performance of strip fin surface is presented. Three-dimensional numerical computation was made for a model of a two-row finned tube heat exchanger. The tube configuration is simulated with step-wise approximation, and the fin efficiency is also calculated with conjugated computation. Four types of fin surfaces were studied: A-the whole plain plate fin; B-the strip fin with strips located in the upstream part of the fin; C-the strip fin with strips located in the downstream part of the fin; and D-the strip fin with strips covering the whole fin surface. It is found that the strip fin with strips located in the downstream part of the fin surface (fin C) has higher heat transfer rate than that with strips in the upstream part (fin B) at the same conditions, while the pressure drop of fin C is a bit lower than that of fin B. A comprehensive performance comparison was conducted by using the goodness factor and the pumping power consumption per unit surface area. It is revealed that between the two strip fins the performance of fin C is better than fin B with same strip number. Detailed discussion is provided from the view point of synergy between velocity and temperature gradient. It is shown that the synergy between velocity and temperature field becomes worse in the downstream part of the fin surface, and it is this place that enhancement technique is highly needed. The strip location of fin C just fits this situation. The present numerical work provides useful information on where the enhancement element should be positioned.

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.

Calculation of Fin Efficiency for Wet and Dry Fins

This paper presents a quantitative evaluation of methods used to calculate fin efficiency for dry and wet fins. The exact solution for circular fins uses Bessel functions, which are tedious to evaluate. The authors provide an empirical modification to the Schmidt equation (Hong and Webb equation), which provides improved accuracy for circular fins. Existing methods to evaluate the fin efficiency of rectangular, plain plate fins are evaluated. The plate-fin methods are compared against the fin efficiency calculated by the sector method using Bessel functions. Use of the sector method with the Hong and Webb equation is recommended. Cases for which the simple “equivalent circular fin” method is a very good approximation are defined. Methods to calculate the fin efficiency of enhanced fin geometries are discussed. The authors show that the wet surface fin efficiency for any fin geometry may be calculated using a simple modification of the dry surface fin efficiency equation. Example calculations are presented, which show that the wet surface fin efficiency can be as much as 35% below the dry surface fin efficiency for high humidity conditions.

Experimental study on heat transfer and pressure drop characteristics of four types of plate fin-and-tube heat exchanger surfaces

In this paper, air side heat transfer and pressure drop characteristics of twelve three-row plate fin-and-tube heat exchanger cores of four types of fin configurations have been experimentally investigated. The heat transfer and friction factor correlations for the twelve cores are provided in a wide range of Reynolds number. It is found that in the range of Reynolds number tested, the Nusselt number of the slotted fin surface is the largest and that of the plain plate fin is the lowest while the Nusselt numbers of two types of wavy fins are somewhere in between.

Performance of Perforated Heat Exchanger Surfaces

Heat transfer and flow friction behavior have been measured for plain and perforated plate-fin and 10-deg herringbone surfaces. For the range of geometries and Reynolds numbers studied, the perforations produced no significant improvement in heat transfer for the same pressure drop. Heat transfer behavior was determined with the “single blow” transient technique. The “initial rise” evaluation procedure was used for low Ntu test packs. Entrance length effects on both heat transfer and friction behavior were correlated for L/4rh as low as 45.