Heat management on rectangular metal hydride tanks for green building applications

A numerical study fully validated with solid experimental results is presented and analysed, regarding the hydrogenation process of rectangular metal hydride tanks for green building applications. Based on a previous study conducted by the authors, where the effective heat management of rectangular tanks by using plain embedded cooling tubes was analysed, in the current work the importance of using extended surfaces to enhance the thermal properties and the hydrogenation kinetics is analysed. The studied extended surfaces (fins) were of rectangular shape; and several combinations regarding the number of fins and the fin thickness were examined and analysed. The values for fin thickness were 2-3-5 and 8 mm and the number of fins studied were 10-14-18 and 20. To evaluate the effect of the heat management process, a modified version of a variable named as Non-Dimensional Conductance (NDC) is introduced and studied. A novel AB₂-Laves phase intermetallic was considered as the metal hydride for the study. The results of the hydrogenation behaviour for the introduced parameters (fin number and thickness) showed that the rectangular tank equipped with the cooling tubes in combination with 14 fins of 5 mm fin thickness has the capability of storing hydrogen over 90% of its theoretical capacity in less than 30 min.     

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 .     

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.     

Heat exchanger design: Thermal performances of rectangular fins protruding from vertical or horizontal rectangular bases

An experimental investigation of the steady-state rates of heat transfer from an array of vertical rectangular fins of 3 mm thickness and 250 mm length, protruding 60 mm perpendicularly upwards from a 250 mm × 190 mm horizontal rectangular base, is reported. For constant (to ±0·1°C) base temperatures between 40°C and 80°C, in an ambient environment of 20±0·2°C, the optimal separation of the parallel fins, corresponding to the maximum rate of heat loss, is 10·5±1·0 mm.The effects of the extent of the fin protrusions on the thermal performances of such vertical fins, on the same base, which was arranged to be either vertical or horizontal, have been studied. The experiments were performed with three different fin protrusions, namely 32 mm, 60 mm and 90 mm, for a base temperature of 40°C above that of the ambient environment. The steady-state rate of heat dissipation from the fin array increased slightly less than linearly with the fin protrusion for both orientations, but the relationship became closer to linear as the fin spacing was increased.A comparison of the abilities to dissipate heat to the room air from the same geometrical configuration having a rectangular fin array but positioned with vertical fins on a vertical base, vertical fins protruding upwards from a horizontal base, or horizontal fins on a vertical base, has been made. The orientation with vertical fins protruding upwards from the horizontal base, is the preferred option because of the relatively high rates of heat transfer that can then be achieved.     

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.     

Some New Solutions for Extended Surface Heat Transfer Using Symbolic Algebra

The paper reports some new solutions for heat transfer through extended surfaces or fins using the symbolic algebra package Maple 8, which is widely available. The four specific problems chosen for the present study are: (a) a rectangular convection fin with the heat transfer coefficient varying either linearly or exponentially with the distance from the base, (b) a truncated conical spine with convection at both ends, (c) a heat-generating annular fin with a constant base heat flux and an adiabatic tip, and (d) a convection fin array made of a rectangular fin and two triangular fins. Each problem is formulated in a manner that makes its solution novel and distinct from what is available in the literature. Solutions are provided in symbolic forms. Using the numerical and graphical capabilities of Maple, the results are presented in the form of numerical data as well as graphical displays. The paper demonstrates that Maple provides an effective and convenient tool for the analysis of extended surface heat transfer problems that otherwise demand tedious algebraic manipulations.     

Numerical prediction of flow and heat transfer in a channel in the presence of a built-in circular tube with and without an integral wake splitter

A numerical investigation was carried out to study the heat transfer behavior of a circular tube in cross-flow configuration with a longitudinal fin attached at the rear of the tube. The investigated configuration is intended to model either an element of a cross-flow heat exchanger or an element of the array of pin fins. The longitudinal finning of a circular tube is assumed to be in a configuration where the fin is attached at the back of the circular tube. The longitudinal fins, built-in with the tubes, are called integral splitter plates. The splitter plate creates a streamlined extension of the circular tube. It brings about enhancement of heat transfer from the tube surface. A reduction in the size of the wake zone in comparison with the wake of a circular tube is observed. Narrowing of the wake zone reduced convective heat transfer from the tube surface but the splitter plate itself generated an extra fin area for conduction. Overall, there is an improvement in heat transfer past the circular tube with an integral splitter plate compared with the case of flow past a circular tube without a splitter plate. Flow and heat transfer results are presented for three different chord lengths of the splitter plate and three different values of the Reynolds numbers (500, 1000 and 1500). The heat transfer enhancement obtained by finning was compared with that obtained by increasing the diameter of the unfinned tubes.     

Exact,analytical heat transfer from longitudinal radiating fins of rectangular profile in a tube/fin ensemble

In this technical note, the problem concerning the quantification of heat transfer rates from an array of longitudinal radiating fins of rectangular profile in a tube/fin ensemble to a nonzero temperature sink is investigated. Radiating fins constitute essential elements in the thermal control of spacecrafts and satellites. We consider quasi one-dimensional heat conduction in the longitudinal radiating fins and neglect radiative exchange between the fins and the tubes carrying a hot fluid. It is demonstrated that the governing nonlinear differential equation of second order with constant coefficients and nonhomogeneous can be solved in exact, analytical implicit form. The pertinent temperature distributions eventually provide the magnitudes of heat transfer rates and fin efficiencies influenced by the radiation–conduction parameter and the sink temperature.     

Effect of lateral fin profiles on turbulent flow and heat transfer performance of internally finned tubes

Heat transfer performance of internally finned tubes with blocked core-tube was numerically investigated by the realizable k–ε turbulence model with wall function method using a commercial software FLUENT. Three kinds of lateral fin profiles, that is, S-shape, Z-shape and V-shape, were studied and compared. The corresponding correlations of Nusselt number and friction factor for different-shape internally finned tubes were obtained. The comprehensive performances of the studied tubes were compared under identical mass flow rate, identical pumping power and identical pressure drop conditions. It was found that tubes with S-shape fins and Z-shape fins were superior to that with V-shape fins, and moreover, tube with Z-shape fins had the best performance. The fin outer curvature radius R near the inner surface of out-tube for the S-shape finned tube had appreciable effect on heat transfer, whereas the fin inner curvature radius r near the outer surface of blocked core-tube had little impact on heat transfer. Hence, when manufacturing the internally finned tube with S-shape fins, it is better to select the outer curvature radius R as smaller as possible.     

Fin performance ratios greater than unity: Not just a theoretical aspiration

Previous work using the fin performance ratio has shown that it may not exceed unity. This study proves that this is not the case under certain conditions relating to the heat transfer from the tip of the fin. Equations for longitudinal rectangular fins have been used to demonstrate how this can be achieved and a performance ratio chart is provided showing performance ratios exceeding unity when the ratio of the heat transfer coefficient at the tip to that along the length of the fin exceeds the maximum effectiveness (a parameter of the problem). Under these conditions the maximum performance ratio is given by the ratio of the heat transfer coefficients (α_e/α) divided by the maximum effectiveness. This has relevance for fins used in boiling and condensing systems where different heat transfer coefficients may exist on different parts of the fins due to the existence of different phases.     

CFD analysis on heat transfer through different extended surfaces

The present study includes computational fluid dynamics analysis and comparison of heat enhancement through different extended surfaces, especially in rectangular and square conductive and nonconductive fins. Computational and numerical analysis of heat transfer from a rectangular extended surface and a pin-finned plate studied to calculate the average Nusselt number in parallel, vertical direction placed along the sidewall. The total rise of the mean Nusselt number is noticed around 36% in pin-finned plate with respect to a plain plate. This is examined with optimal fin spacing of S_v with L ratio equals to 0.2 and S_h with W ratio equals to 0.25, height of extended surfaces 24 mm with 45° angle of inclination. The mean Nusselt number reduces with a rise in the angle of inclination and also increases with a rise in aspect ratio. The present study reveals that inline and staggered arrangements do not yield appreciably different results. The maximum average Nusselt number difference between conductive and nonconductive fins is around 5% for S_h per W ratio 0.33 and S_v per L ratio 0.2 at an angle of inclination 45°, fin height of 6 mm (height to thickness ratio 2).     

CFD Analysis of Steady Laminar Natural Convection Heat Transfer from a Pin Finned Isothermal Vertical Plate

Heat transfer behavior with both the conductive and nonconductive fins have been analyzed by examining variations of the local and average Nusselt numbers in two‐dimensional flow. The main objective of this study is to quantify and compare the natural convection heat transfer enhancement of fin array with different fin aspect ratio and at different angles of inclination. It is found that significant heat transfer augmentation is obtained for both conductive and nonconductive fins. For conductive fins 20% higher augmentation factor is obtained when the fin aspect ratio is 6, angle of inclination is 60° and the pitch‐to‐length ratio is 0.2. For nonconductive fins, 10% higher augmentation factor is obtained when fin aspect ratio is 8, angle of inclination is 45° and pitch‐to‐length ratio at 0.5. A general correlation has been developed to predict the average Nusselt number and heat transfer augmentation factor for conductive and nonconductive fin arrays as a function of different fin configurations.     

Filmwise condensation of a pure fluid and a binary mixture in a bundle of enhanced surface tubes

An experimental study has been carried out to investigate the characteristics of condensation in a bundle of horizontal tubes. These tubes have different types of external surfaces: smooth surface (1D), low trapezoidal fins with several fin pitches (2D) and specific fins (3D, C+ tube). The used fluids are either pure refrigerant (HFC 134a) or binary mixture of refrigerants (HFC 134a/HFC23).For the pure fluid and a single tube, the influence of fin spacing has been studied (11, 19, 26, 32 and 40 fins/inch) and a comparison has been made with the Gewa C+ tube. The results were analysed with the Beatty and Katz theory and compared to a specific model, taking into account both gravity and surface tension effects, developed for the Gewa C+ tube.For the bundle and for a pure fluid, the inundation of the lowest tubes has a strong effect on the Gewa C+ tubes performances contrary to the finned tubes. For the mixture the heat transfer coefficient decreases dramatically especially for Gewa C+ tube.     

Performance of annular fins with different profiles subject to variable heat transfer coefficient

Performance of annular fins of different profiles subject to locally variable heat transfer coefficient is investigated in this paper. The performance of the fin expressed in terms of fin efficiency as a function of the ambient and fin geometry parameters has been presented in the literature in the form of curves known as the fin-efficiency curves for different types of fins. These curves, that are essential in any heat transfer textbook, have been obtained based on constant convection heat transfer coefficient. However, for cases in which the heat transfer from the fin is dominated by natural convection, the analysis of fin performance based on locally variable heat transfer coefficient would be of primer importance. The local heat transfer coefficient as a function of the local temperature has been obtained using the available correlations of natural convection for plates. Results have been obtained and presented in a series of fin-efficiency curves for annular fins of rectangular, constant heat flow area, triangular, concave parabolic and convex parabolic profiles for a wide range of radius ratios and the dimensionless parameter m based on the locally variable heat transfer coefficient. The deviation between the fin efficiency calculated based on constant heat transfer coefficient, reported in the literature, and that presently calculated based on variable heat transfer coefficient, has been estimated and presented for all fin profiles with different radius ratios.     

Natural convection heat transfer in horizontal and vertical closed narrow enclosures with heated rectangular finned base plate

Natural convection heat transfer and fluid flow characteristics in horizontal and vertical narrow enclosures with heated rectangular finned base plate have been experimentally investigated at a wide range of Rayleigh number (Ra) for different fin spacings and fin lengths. Quantitative comparisons of finned surface effectiveness (ε) and heat transfer rate between horizontal and vertical enclosures have been reported. In comparison with enclosure of a bare base plate, insertion of heat conducting fins always enhances heat transfer rate. Optimization of fin-array geometry has been addressed. The results gave an optimum fin spacing at which Nusselt number (Nu_H) and finned surface effectiveness (ε) are maximum. It has been found that: (1) increasing fin length increases Nu_H and ε; (2) increasing Ra increasesNu_H for any fin-array geometries and (3) for any fin-array geometry and at Ra > 10,000, increasing Ra decreases ε while for fin-array geometries of large fin spacing and at Ra < 10,000, increasing Ra increases ε. Useful design guidelines have been suggested. Correlations of Nu_H have been developed for horizontal and vertical enclosures. Correlations predictions have been compared with previous data and good agreement was found.     

Numerical and experimental study of a solar equipped with offset rectangular plate fin absorber plate

A mathematical model that allows the determination of the thermal performances of the single-pass solar air collector with offset rectangular plate fin absorber plate is developed. The model can predict the temperature profile of all the components of the collector and of the air stream in the channel duct. Into the latter are introduced the offset rectangular plate fins, which increase the thermal heat transfer between the absorber plate and the fluid. The offset rectangular plate fins, mounted in a staggered pattern, are oriented parallel to the fluid flow and are soldered to the underside of the absorber plate. They are characterized by high heat transfer area per unit volume and generate the low pressure losses. The experimental results of the air stream temperature will be compared with the results obtained by the theoretical model suggested.     

Experimental study of thermal performance of offset rectangular plate fin absorber-plates

A new technique was developed to enhance the heat transfer more than with fully developed turbulent flow. Commonly the offset rectangular plate fins are used in heat exchangers; they alternate with the periodically interruptions. The fluid homogenization depends on the length of interruption, and new velocity and thermal boundary layers were developed at each encountered offset fin located downstream of interruption. This technique characterizes a higher heat transfer than in fully developed turbulent flow. In the present work this technique of periodically interrupted offset fins is adapted to solar collector. An experimental investigation carried out showed the generated enhancement of thermal performance. The offset rectangular plate fins, mounted in staggered pattern, are oriented parallel to the fluid flow and are soldered to the underside of absorber plate. High thermal performances are obtained with low flow friction and in consequence a low electrical power consumption by the fan in comparison to the flat plate collector.     

Maximization of heat transfer density from radially finned tubes in cross-flow using the constructal design method

The maximization of volumetric heat transfer density from radially finned tubes in cross-flow is investigated in this study based on the constructal design method. A row of radially finned tubes is placed in cross-air flow. The tubes and the radial fins are heated at uniform temperatures and cooled by the air cross-flow. The cross-air flow is generated by a finite pressure difference. Two dimensionless pressure differences (Bejan number) are considered (Be = 10³ and Be = 10⁵). The objective function, the degrees of freedom, and the constraints in the constructal design method should be identified. The objective function is the maximization of the heat transfer density from the finned tubes. The degrees of freedom are; the fin tip-to-fin tip spacing, the number of fins, the tube diameter, the fin thickness, and the angle between the fins. The constraints are the length and height of the space occupied by the finned tubes. The pressure-driven flow and energy equations (steady, two-dimensional, and incompressible) are solved by means of the finite volume method. The ranges of the dimensionless fin tip-to-fin tip spacing are (0.2 ≤ S ≤ 1 for Be = 10³ and 0.05≤ S ≤ 0.3 for Be = 10⁵). The number of fins is changed as (N = 2, 4, 6, 8, 10, and 12). The dimensionless tube diameter is changed as (D = 0.25, 0.5, and 0.75). The dimensionless fin thickness is changed as (T = 0.001, 0.01, and 0.05). The results showed that for both (Be = 10³) and (Be = 10⁵), the highest value of the maximum volumetric heat transfer density is for (N = 2) and decreases as the number of fins increases. In addition, the minimum values of the maximum volumetric heat transfer density occur when the vertical fins exist at (N = 4, 8, and 12).     

A model on the basis of analytics for computing maximum heat transfer in porous fins

This study presents an analytical work on the performance and optimum design analysis of porous fin of various profiles operating in convection environment. Straight fins of four different profiles, namely, rectangular, convex parabolic and two exponential types are considered for the present investigation. An analytical technique based on the Adomian decomposition method is proposed for the solution methodology as the governing energy equations of porous fins for all the profiles are non-linear. A comparative study has been carried out among the results obtained from the porous and solid fins, and an appreciable difference has been noticed for a range of design conditions. Finally, the result shows that the heat transfer rate in an exponential profile with negative power factor is much higher than the rectangular profile but slightly higher than the convex profile. On the other hand, the fin performance is observed to be better for exponential profiles with positive power factor than other three profiles. A significant increase in heat transfer through porous fins occurs for any geometric fin compared to that of solid fins for a low porosity and high flow parameter.     

Numerical validation of natural convection heat transfer with horizontal rectangular fin array using straight knurling patterns on fins—correlation for Nusselt number

Natural convection heat transfer from horizontal rectangular fin array with various knurling patterns has been studied experimentally by the present authors to investigate the effect of knurl‐produced surface roughness of fin on the heat transfer rate. The parameters like knurling height from base, knurling depth, fin spacing, and supply wattage were considered for parametric study. In the present paper, numerical method (CFD) is used to simulate natural convection phenomena with knurled fins and results are validated with the experimental data available from literature. The numerical results show similar trends compared with experimental data and one can use this method to study various fin configurations for knurling patterns. The flow patterns from experiments and numerical method are compared for different supply wattages and fin spacing to back up the conclusion. It is also observed that the variation in nondimensional roughness depth and spacing (D _kn/H and S/H) have more effect on Nusselt number than roughness height parameter (H _kn/H). Further, the method is extended to study numerically large number of fin configurations with knurling patterns to gather sufficient data for Nusselt number with respect to fin geometric parameters as above and establish correlation for heat transfer coefficient for such type of fins.