Finite circular fin method for wavy fin-and-tube heat exchangers under fully and partially wet surface conditions

The present study proposes the finite circular fin method for analyzing the heat and mass transfer characteristics of wavy fin-and-tube heat exchangers under fully and partially wet surface conditions. The analysis is carried out by dividing the wavy fin-and-tube heat exchanger into many tiny segments. The tiny segments can be analyzed based on surface conditions, i.e. fully wet, fully dry or partially wet surface condition. From the experimental results, it is found that the heat and mass transfer characteristics are insensitive to the inlet relative humidity but the effect of relative humidity on mass transfer characteristic become more pronounced when the partially wet surface condition takes place. The heat transfer characteristic is independent of the fin spacing. Effect of fin spacing on mass transfer characteristic is small when fin spacing is larger than 2.5 mm. However, at smaller fin spacing, the mass transfer characteristic slightly decreases when the relative humidity increases. The ratios of h_c,o/h_d,oC_p,a are in the range of 0.6–1.2. Correlations are proposed to describe the heat and mass transfer characteristics. These correlations can describe 95.63% of the heat transfer characteristic within 15% and 95.14% of the mass transfer characteristic within 20%. Correspondingly, 94.68% of the ratios of h_c,o/h_d,oC_p,a are predicted by the proposed correlation within 20%.     

Air side heat transfer coefficients of discrete plate finned-tube heat exchangers with large fin pitch

The objective of this study is to investigate the heat transfer characteristics of discrete plate finned-tube heat exchangers with large fin pitches. Thirty-four heat exchangers were tested with variations of fin pitches, the number of tube rows, fin alignment, and vertical fin space. The j-factor of the discrete plate finned-tube exchanger was analyzed as a function of coil geometry and then compared with that of the continuous plate finned-tube heat exchanger. For fin pitches of 7.5–15 mm, the j-factors of the discrete plate finned-tube heat exchangers were 6.0–11.6% higher than those of the continuous plate finned-tube heat exchangers. Two separate correlations for the j-factor were developed for the inline and the staggered fin alignment in the discrete plate finned-tube heat exchangers to predict the measured data within a relative deviation of 2.9%.     

An optimization study on the finned tube heat exchanger used in hydride hydrogen storage system – analytical method and numerical simulation

Metal hydrides show great potential for hydrogen storage. However, for efficient hydrogen storage, thermal management is the technical barrier. Among the different heat exchangers proposed in the literature, finned tube heat exchangers are of great technological interest due to their adaptability to wide range of practical applications, high compactness and high heat transfer efficiency. In the present paper, the optimization of finned heat exchanger considering both enhanced heat transfer and vessel volume efficiency is conducted. A semi-analytical expression of heat transfer rate from a single fin is derived. The effects of fin dimension (fin thickness and radius) on the heat exchanger performance are studied. It was shown that the thermal resistance of the whole heat exchanger can be reduced by increasing the fin radius and decreasing the fin thickness, while the fin volume is kept fixed. In the second part of the study, a 2-D numerical simulation was performed in order to validate the results of the analytical study. The effects of two parameters (cooling tube diameter, the fin length) on the hydrogen charging time were highlighted. The increasing in the tube diameter from 2.5 mm to 5 mm results to 25% reduction of the charging time, which is very noticeable. On the other hand, given a reactor radius, increasing the length of fin reduces the overall thermal resistance of the reactor-heat exchanger. The results showed that the decreasing of the thermal resistance of 13% leads to a decreasing in charging time of 42%. Finally, it was found that the results of the numerical simulation agreed qualitatively with those of analytical study. Therefore, the analytical solution presented can be used for a quick assessment of the finned tube heat exchanger design without significant errors.     

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%.     

Experimental study on the heat transfer at the heat exchanger of the thermoacoustic refrigerating system

Oscillatory flow heat transfer at the heat exchanger of the thermoacoustic refrigeration system was studied. The study identified significant factors that influence this heat transfer as well as the construction of the system. The results from the experimental study were correlated in terms of Nusselt number, Prandtl number and Reynolds number to obtain a useful new correlation for the heat transfer at the heat exchangers. Results show that using straight flow heat transfer correlations for analyses and design of this system could result in significant errors. Results also show the relationship between the oscillatory heat transfer coefficient at the heat exchangers, the mean pressure and frequency of oscillation. Higher mean pressures result in greater heat transfer coefficients if the thermoacoustic refrigerating system operates at the corresponding resonant frequency. However, a compromise has to be reached to accommodate construction of the stack.     

Heat and mass transfer in plate-fin sinusoidal passages with vapor-permeable wall materials

Laminar forced flow and heat mass transfer in sinusoidal plate-fin small passages encountered in compact heat mass exchangers are investigated. The duct is similar to a traditional plate-fin heat exchanger, but vapor-permeable materials like polymer membranes, paper, and ceramics can be used as the duct materials so both sensible heat and moisture can be exchanged simultaneously. Heat conduction and mass diffusion in the fins and heat and moisture convection in the fluid are analyzed simultaneously as a conjugate problem. Their fully developed Nusselt and Sherwood numbers under various aspect ratios and fin conductance parameters are calculated. The results found that though fins extend the heat transfer area, they are less effective compared to a traditional compact heat exchanger with metal foils. Most unfortunately, fin efficiencies for moisture transfer are even much smaller than those for heat transfer due to the low fin mass conductance parameters. For such heat mass exchangers, the use of fins can be regarded mostly as supporting materials, rather than as mass intensification techniques.     

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.     

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.     

A Simple Air-Side Data Analysis Method for Partially Wet Flat-Tube Heat Exchangers

An air-side data analysis method is developed for flat-tube heat exchangers under partially wet conditions. In order to simplify the combined sensible and latent heat transfer, it is assumed that condensate drainage paths develop such that, at steady state, water does not spread to noncondensing surfaces, which therefore remain dry. The air dew point is compared to local fin-tip and fin-base temperatures, and a partially wet flat-tube heat exchanger is partitioned into fully wet, partially wet, and dry-fin regions, which are subsequently analyzed as separate heat exchangers. Using an enthalpy-based effectiveness–NTU (number of transfer units) method, average fin efficiency is calculated for each region, and the locations of region boundaries are determined iteratively. The proposed data analysis method is demonstrated with experimental data for a flat-tube louver-fin heat exchanger under various latent loads. The general approach presented can be extended to other heat exchanger geometries.     

Thermoacoustics and related oscillatory heat and fluid flows in micro heat exchangers

Classical linear thermoacoustic theory is applied to compact micro heat exchangers and the validity of such calculus applied to micro scale is discussed. Expressions for the radial profiles and average values of the fluid axial velocity and temperature are demonstrated, formulations for first order friction and heat transfer coefficients of oscillating flows are deduced. It is shown how aerodynamic and thermal performances of a micro heat exchanger in pulsed flow regime can be characterized with three factors: a thermal characteristic time, an aerodynamic matrix of transfer and a thermal efficiency. The model proposed is devoted to design the micro heat exchangers of micro refrigerators dedicated to the cooling of electronic components.     

单向开缝翅片管换热器传热与阻力性能的数值模拟及试验研究

对一种单向开缝翅片管换热器进行了数值模拟及试验研究,分析了不同翅片间距及管径下单向开缝翅片管换热器的传热与阻力性能的变化规律。数值模拟和试验结果的对比表明,采用数值模拟方法研究单向开缝翅片管换热器的传热与阻力性能是可行的。     

Estimation of heat transfer coefficients in oscillating flows: The thermoacoustic case

The classical linear thermoacoustic theory is integrated through a numerical calculus with a simple energy conservation model to allow estimates of the optimal length of thermoacoustic heat exchangers and of the magnitude of the related heat transfer coefficients between gas and solid walls. This information results from the analysis of the temperature and heat flux density distributions inside a thermally isolated thermoacoustic stack. The effects of acoustic amplitude, plate spacing, plate thickness and Reynolds number on the heat transfer characteristics are examined. The results indicate that a net heat exchange between the acoustically oscillating gas and the solid boundary takes place only within a limited distance from the stack edges. This distance is found to be an increasing function of the plate spacing in the range (0 ⩽ y₀/δ_κ ⩽ 2), becoming constant for y₀/δ_κ ⩾ 2. The calculated dimensionless convective heat transfer coefficients, the Nusselt numbers, between gas and solid wall are comparable to those evaluated from classical correlations for steady laminar flow revised under the “Time-Average Steady-Flow Equivalent” (TASFE) and “root-mean-square Reynolds number” (RMSRe) models. Numerical results agree with measurements of the heat transfer coefficient found in literature to within 20%.     

Heat transfer and pressure drop characteristics of peripheral-finned tube heat exchangers

The peripheral-finned tube is a new geometry aimed at avoiding moisture-condensate blockage hindering of the air-side heat transfer, by allowing for robust air flow pathways. It consists of a porous structure formed by periodic, radial-hexagonal fin arrangements of different radial extents mounted with a 30° offset from its neighboring level. Here, the air-side pressure drop and the heat transfer characteristics of five different heat exchanger prototypes with different geometric characteristics, such as the radial fin length, fin distribution, and heat exchanger length, were evaluated experimentally in an open-loop wind-tunnel calorimeter. The results demonstrate the effective performance, i.e., the pressure drop and heat transfer characteristics, of this new heat exchanger. A one-dimensional theoretical model based on the porous media treatment was also developed to predict the thermal-hydraulic behavior of the heat exchanger. The model incorporates the actual fin geometry into the calculation of the air-side porosity. The air-side permeability is calculated according to the Kozeny–Carman model and the particle-diameter based analysis. The model predicts the experimental data within a few percent RMS, depending on the correlations used for the friction coefficient and interstitial Nusselt number.     

Studies on pumping power in terms of pressure drop and heat transfer characteristics of compact plate-fin heat exchangers—A review

Renewable energy sources like solar energy, wind energy, etc. are profusely available without any limitation. Heat exchanger is a device to transfer the energy from one fluid to other fluid for many applications in buildings, industries and automotives. The optimum design of heat exchanger for minimum pumping power (i.e., minimum pressure drop) and efficient heat transfer is a great challenge in terms of energy savings point of view. This review focuses on the research and developments of compact offset and wavy plate-fin heat exchangers. The review is summarized under three major sections. They are offset fin characteristics, wavy fin characteristics and non-uniformity of the inlet fluid flow. The various research aspects relating to internal single phase flow studied in offset and wavy fins by the researchers are compared and summarized. Further, the works done on the non-uniformity of this fluid flow at the inlet of the compact heat exchangers are addressed and the methods available to minimize these effects are compared.     

Experimental study on heat and mass transfer characteristics of louvered fin-tube heat exchangers under wet condition

An experimental study was conducted to investigate the effect of a tube row, a fin pitch and an inlet humidity on air-side heat and mass transfer performance of louvered fin-tube heat exchangers under wet condition. Experimental conditions were varied by three fin pitches, two rows, and two inlet relative humidities. From the experimental results, it was found that the heat transfer performance decreased and the friction increased with the decrease of a fin pitch, for 2 row heat exchanger. The effect of a fin pitch on heat transfer performance was negligible with 3 row heat exchanger. The change in a relative humidity was not affected heat transfer and friction. However, the mass transfer performance was slightly decreased with the increase of a relative humidity and with the decrease of a fin pitch. The mass transfer performance decreased with the decrease of a fin pitch. The mass transfer performance of the louvered fin-and-tube heat exchanger was different according to the number of a tube row.     

A Large Eddy Simulation of Plate-Fin and Tube Heat Exchangers with Small Diameter Tubes

Plate-fin and tube heat exchangers are extensively studied both experimentally and numerically. However, data on the fluid flow and heat transfer in the exchanger passage with small diameter tubes have not been accumulated enough. With a large eddy simulation technique (LES), this study performs a detailed investigation of the fluid flow and heat transfer in a plate-and-tube channel with tubes of diameters as small as 5.2 mm. The conservation equations for mass, heat, and momentum were solved by the proposed LES model. It was found that the LES model is appropriate to predict the fluid flow and heat transfer. Compared to heat exchangers of larges tubes, the heat exchangers exhibit much higher heat transfer coefficients with small tubes. The fin efficiencies are improved with small tubes.     

Thermal design of multi-stream heat exchangers

The thermal design of multi-stream heat exchangers of the plate and fin type is presented. Although originally used in low temperature processes, their application is extrapolated to above temperature processes and it is shown that, conceptually, multi-stream exchangers could replace whole heat recovery networks. The approach is based on the use of temperature vs. enthalpy diagrams or composite curves, which show how a multi-stream exchanger can be subdivided into block sections that correspond to enthalpy intervals and indicate the entry and exit points of the streams. A design methodology for plate and fin exchangers in countercurrent arrangement, characterized by the maximization of allowable pressure as a design objective is extended to the design of multi-fluid exchangers. The methodology uses a thermo-hydraulic model which relates pressure drop, heat transfer coefficient and exchanger volume. The potential applicability of the methodology is demonstrated on a case study.     

开缝布置方式对开缝翅片管换热器 传热与阻力特性的影响

为了获得开缝布置方式对开缝翅片管换热器传热与阻力特性的影响规律,对5种不同翅片管换热器进行了数值模拟研究,并进行了模化试验验证。结果表明：增加开缝会提高翅片管换热器的传热性能,但阻力也随之增加;与开缝位置相比,开缝数量对开缝翅片管换热器传热与阻力特性的影响更大;在Re=4800~7500日时,开缝翅片管换热器综合流动传热性能 随着Re数的增大而增大;在5种翅片中,开缝翅片的综合流动传热性能高于普通平直翅片;数值模拟与试验结果偏差较小,采用数值模拟方法能够比较准确地分析开缝翅片管换热器的传热与阻力特性。     

Experimental Determination of the Heat Transfer Coefficient of a Plate-Fin Heat Exchanger

Heat transfer in compact plate-fin heat exchangers is augmented by the introduction of complex fin patterns in the channels. Kays and London presented a lot of experimental data for several types of fin configurations, and many authors followed their example with other types of fins. For some fin types, the heat transfer correlation for the Nusselt number cannot be found in literature. Most of the data are given for large scale model fins in good controlled laboratory environments—little data is available for real heat exchangers.

A test rig was constructed at Ghent University to verify the performance of several fin types. Measurements were done on a real heat exchanger and not on a large scale model in order to determine the performance under real operational conditions.

The measurement setup consists of a hot water circuit and an air circuit with a fan. In the heat exchanger, 40 thermocouples are introduced on the air side and the wall. This way, the convection coefficient of the fins can be determined for a broad range of Reynolds numbers.

In the paper the measurement set-up is discussed and the measurements are presented. An in depth error analysis is performed on the measurements. This way a heat transfer correlation is provided with a tight error margin for compact plate-fin air coolers.     

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.