Forced-convective steady-state heat transfers from shrouded vertical fin arrays,aligned parallel to an undisturbed air-stream

An experimental investigation of the steady-state rates of heat loss from an array of vertical rectangular fins of 3 mm thickness and 250 mm length, when in the presence of an almost adiabatic horizontal shroud, situated adjacent to and above the horizontal fin tips, is reported. With the fin's horizontal base at a uniform temperature of 40 ± 0·1° C above that of the ambient environment, the optimal fin separation—corresponding to the maximum rate of heat loss—is deduced. As the ratio of the shroud height above the vertical fins to the fin height decreases from unity to zero, this optimal value decreases by approximately 17%.The frictional characteristics of the air flow through the fin array have also been studied in the Reynolds number range of 4·0 × 10⁴ to 2·0 × 10⁵. Large streamwise pressure drops and high heat-transfer rates resulted when the fins were closely spaced and no clearance gap was present above the vertical fins. In reasing the shroud clearance resulted in smaller overall pressure drops and decreasing heat-transfer rates from the heat exchanger.     

Heat exchanger design: Optimal thickness (under natural convective conditions) of vertical rectangular fins protruding upwards from a horizontal rectangular base

Steady-state rates of heat transfer from an array of vertical, rectangular polished duralumin fins under natural convective conditions have been measured. The horizontal base, which was manufactured of the same material, was kept at the uniform temperature of either 20·0 (±0·2)°C or 40·0 (±0·2)°C above the local air temperature of 20 (±0·2)°C.

The optimal thickness of the fins in this array, corresponding to a maximum rate of heat dissipation, was deduced. For a base of width 190 mm and length 500 mm, the optimal thickness for fins of 60 mm protrusion rose from 2·0 mm to 4·5 mm when the fin separation was increased from 20 mm to 60 mm. This optimal fin thickness was almost invariant with respect to the change of the considered base temperature.     

Heat transfer performances of vertical rectangular fins protruding from rectangular bases: Effect of fin length

The effects of increasing the fin length from 250 to 375 mm on (i) the steady-state rate of heat loss and (ii) the optimal uniform fin separation of vertical rectangular fins protruding from a horizontal or a vertical rectangular base, have been investigated experimentally. A constant base temperature, 40 (±0·3)°C above that of the ambient environment, was used.     

Heat exchanger design: Optimal uniform separation between rectangular fins protruding from a vertical rectangular base

Steady-state rates of heat loss, from an array of 3 mm thick, 250 mm long, horizontal rectangular duralumin fins extending 60 mm perpendicularly out of a 250 mm × 190 mm vertical rectangular duralumin base, have been measured. With the base, at a uniform temperature of between 40°C and 80°C, in a 20°C ambient environment, two separations of the parallel fins, corresponding to maxima in the rate of heat loss versus fin separation curves ensued, respectively at 12 ± 1 mm and 38 ± 1 mm. The use of the latter maxima (i.e. the optimal separation) leads to the higher rate of heat loss. The heat transfer performances of vertical and horizontal rectangular fin arrays on a vertical rectangular base are compared: using the same geometrical configuration and identical base temperatures in both cases, the vertical fin orientation has the more rapid, steady-state heat loss.     

The influence of internal fins on mixed convection inside horizontal tubes

Results of a numerical analysis of fully developed, laminar, mixed convection in horizontal tubes with two vertically oriented fins are presented. Fins are found to suppress the free convective currents and thus the enhancements in Nusselt number and friction factor decrease as the fin height increases. For all values of Grashof number and relative fin height considered, the percentage rise in Nusselt no. due to free convection exceeded the percentage increase in friction factor.     

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).     

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.     

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.     

Enhancing heat transfer rates from closed-sided,open-topped heat exchangers,each having vertical rectangular fins extending upwards from a horizontal base

The almost two-dimensional steady-state rates of heat loss from arrays of uniformly-spaced vertical rectangular fins, extending upwards—in otherwise stagnant air—from horizontal heated bases, have been measured. (The vertical air gaps between the fins were closed at their sides, by insulated vertical end-barriers.) The effects of various combinations of height, thickness and spacing of the fins, for different base temperatures (in the range 40 to 100°C), have been studied.For the configuration considered, in a normal ambient environment (～ 20°C), there is an optimal fin spacing (? 16 mm) corresponding to the greatest steady-state rate of free convective/conductive heat loss through the air from the finned system, and this is almost independent of the temperature of the heat exchanger base (in the range 40–100°C). At this optimal spacing for base temperatures not greater than 50°C, the convective/conductive heat transfer rate from the array increases with the fin height up to about 60 mm, so that it would be uneconomic to employ taller fins if convection/conduction is dominant compared with radiation.If the radiation contribution is also considered, then the optimal spacing corresponding to the maximum total steady-state rate of heat loss through the air is somewhat less than the optimal spacing for which, under the same temperature conditions, the maximum steady-state rate of convective/conductive heat leak occurs. The greater the emissivity of the heat exchanger surfaces, the narrower the optimal uniform gaps between the fins.A two-dimensional finite-difference computer program has been composed to predict the temperature distribution throughout the heat exchanger for a stipulated ambient environmental temperature and experimentally-determined distribution of the heat transfer coefficient over the surfaces of the exchanger. This enables, for instance, any hot spots to be located prior to a proposed design being built.     

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.     

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.     

Maximising the performances of flat-plate heat exchangers experiencing free or forced convection

Steady-state free and forced convective cooling of vertical, rectangular, 3 mm thick, 250 mm long fins, uniformly separated and protruding vertically upwards from a 250 mm × 190 mm isothermal horizontal base was investigated.For each combination of specified fin protrusion, horizontal forced air flow-rate in the direction along the fins and fin base temperature, the optimal fin spacing—corresponding to the maximum rate of heat loss—has been deduced. As the fin protrusion of the heat exchanger increases, this optimal value rises significantly in forced convection conditions, but declines slightly in the presence of free convection alone.The temperature distributions over the fins' surfaces were also studied, when the fin base was maintained at constant temperatures of either 40°C, 60°C or 80°C above that of the ambient environment. Large temperature depressions occurred near the leading edges and tips of the fins in forced convection, whereas much nearer isothermal temperature distributions were present under free convection conditions.     

NATURAL CONVECTION AND ENTROPY GENERATION FROM A CYLINDER WITH HIGH CONDUCTIVITY FINS

The problem of laminar natural convection from a horizontal cylinder with multiple equally spaced high conductivity fins on its outer surface was investigated numerically. The effect of several combinations of number of fins and fin height on the average effective Nusselt number was studied over a wide range of Rayleigh numbers. The results showed that there was an optimal combination of number of fins and fin height for maximum heat transfer from the cylinder for a given value of Rayleigh number. A high number of short fins slightly decreased the heat transfer from the cylinder. The calculated velocity and temperature profiles also were used to study the total entropy generation. The total entropy production was dominated by entropy generation due to thermal effects. The exception was at Ra _D = 10³ and a large cylinder diameter where entropy generation was dominated by entropy generation due to viscous effects. This information can be used to access the changes in the thermodynamic efficiency due to the addition of fins to enhance the natural convection heat transfer from a horizontal cylinder.     

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.     

Mixed Convection from a Cylinder with Highly Conductive Fins in Cross Flow

The problem of laminar mixed convection from an isothermal cylinder with highly conductive fins in cross flow was solved numerically. The average Nusselt number was calculated at different combinations of number of fins (0–17), fin height (0–2), Reynolds number (10–200), and buoyancy parameter (0–5). The fins were most effective at low Reynolds numbers and low buoyancy. The addition of short fins at high values of the Reynolds number and buoyancy parameter resulted in a slight reduction in the Nusselt number. There was an optimal number of fins beyond which additional fins did not increase or even reduce the average Nusselt number. This number was Reynolds number- and buoyancy parameter-dependent. When using a small number of long fins at low buoyancy, an even number of fins was better than an odd number of fins.     

Cooling of Vertical Shrouded-Fin Arrays of Rectangular Profile by Natural Convection: An Experimental Study

Abstract

Experiments have been performed to determine the natural-convection heat transfer characteristics of vertically oriented shrouded heat sinks (finned surfaces) of rectangular profile under uniform heat flux condition applied to the base. The size and configuration of the heat sink, the power dissipated, and the clearance gap between the shroud and the fin tips were varied during the experiments. The heat transfer medium was air. The temperatures were maintained below 150° during the experiments, which is the maximum allowable operation temperature for most silicon-based electronic components. It was found that shrouding, in general, significantly enhances heat transfer from the heat sinks. For a fixed heat flux and heat sink configuration, the maximum temperature on the heat sink dropped as the clearance was increased, attained a minimum, and then started to rise again. The effect of the shroud on 'the maximum temperature and the average Nusselt number is illustrated     

Optimal location of a single horizontal pipeline in a rectangular, horizontal air-filled enclosure to achieve maximum thermal insulation

An experimental investigation into the effect of the displacement ratio of a heated horizontal pipeline within an atmospheric pressure air-filled, relatively cold, horizontal rectangular cavity was made. The optimal position of the pipeline to achieve maximum energy thrift occurred at a displacement ratio approximately equal to 0·7, i.e. in the upper region of the cavity. This result is of significance with respect to the reduction of distribution heat losses from underground district-heating pipelines.     

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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.     

Numerical investigation of a PCM-based heat sink with internal fins

The present study explores numerically the process of melting of a phase-change material (PCM) in a heat storage unit with internal fins open to air at its top. Heat is transferred to the unit through its horizontal base, to which vertical fins made of aluminum are attached. The phase-change material is stored between the fins. Its properties used in the simulations, including the melting temperature of 23-25 °C, latent and sensible specific heat, thermal conductivity and density in solid and liquid states, are based on a commercially available paraffin wax.A detailed parametric investigation is performed for melting in a relatively small system, 5-10 mm high, where the fin thickness varies from 0.15 mm to 1.2 mm, and the thickness of the PCM layers between the fins varies from 0.5 mm to 4 mm. The ratio of the PCM layer to fin thickness is held constant. The temperature of the base varies from 6 °C to 24 °C above the mean melting temperature of the PCM.Transient three- and two-dimensional simulations are performed using the Fluent 6.0 software, yielding temperature evolution in the fins and the PCM. The computational results show how the transient phase-change process, expressed in terms of the volume melt fraction of the PCM, depends on the thermal and geometrical parameters of the system, which relate to the temperature difference between the base and the mean melting temperature, and to the thickness and height of the fins.In search for generalization, dimensional analysis of the results is performed and presented as the Nusselt numbers and melt fractions vs. the Fourier and Stefan numbers and fin parameters. In some cases, the effect of Rayleigh number is significant and demonstrated.     

Thermal performance analysis and optimum design parameters of heat exchanger having perforated pin fins

This paper reports the heat transfer enhancement and corresponding pressure drop over a flat surface equipped with circular cross section perforated pin fins in a rectangular channel. The channel had a cross section area of 100–250 mm². The experiments covered the following ranges: Reynolds number 13500–42,000, clearance ratio (C/H) 0, 0.33 and 1 and interfin spacing ratio (S_y/D) 1.208, 1.524, 1.944 and 3.417. Correlation equations were developed for the heat transfer, friction factor and enhancement efficiency. The experimental results showed that the use of circular cross section pin fins may lead to heat transfer enhancement. Enhancement efficiencies varied between 1.4 and 2.6 depending on clearance ratio and interfin spacing ratio. Using a Taguchi experimental design method, optimum design parameters and their levels were investigated. Nusselt number and friction factor were considered as performance parameters. An L₉(3³) orthogonal array was selected as an experimental plan. First of all, each goal was optimized separately. Then, all the goals were optimized together, considering the priority of the goals, and the optimum results were found to be Reynolds number of 42,000, fin height of 50 mm and streamwise distance between fins of 51 mm.