Forced convection heat transfer and pressure drop for a horizontal cylinder with vertically attached imperforate and perforated circular fins

In this study, the effect of holes placed on perforated finned heat exchangers on convective heat transfer experimentally investigated. Six millimeter-diameter holes were opened on each circular fin on a heating tube in order to increase convective heat transfer. These holes were placed on the circular fins in such a way as to follow each other at the same chosen angle. The holes created turbulence in a region near the heating tube surface on the bottom of the fin. Some experiments were then performed to analyze the effect of this turbulence on heat transfer and pressure drop. These experiments were carried out at six different angular locations in order to determine the best angular location. In addition, a perforated finned heater was compared with an imperforate finned heater to observe the differences. In the cases of the Re above the critical value, Nusselt numbers for the perforated finned positions are 12% higher than the Nusselt numbers for the imperforate state. Moreover, a correlation has been obtained between the Re and Nu in the Re number above the critical value and the Re below the critical value. Meanwhile, correlations regarding pressure drops in the flow areas have been obtained.     

Effect of Flow Direction for the Heat Transfer Performance of Refrigerant R-410A Evaporation in a Plate Heat Exchanger

This article provides an experimental investigation of the effect of flow direction for refrigerant R-410A evaporated in a plate heat exchanger. Parallel-flow and counterflow arrangements with 2°C and 5°C exit superheat conditions were tested. The refrigerant entered the test section at a vapor quality of 0.24 and evaporated at a saturation temperature of 1.1°C. The experimental results were analyzed by the evaporation heat transfer coefficient and overall average heat transfer coefficient separately. The evaporation heat transfer coefficient in parallel-flow arrangement is higher than that in the case of counterflow arrangement. However, the average heat transfer coefficients are affected not only by the flow direction, but also by the exit superheat condition. The interaction of these two effects causes there to be almost no difference of the average heat transfer performance between these two flow arrangements for low exit superheat condition. While the refrigerant exit superheat is high, the overall heat transfer performance of the parallel-flow case is lower than that of the counterflow case.     

3D numerical and experimental analysis for thermal–hydraulic characteristics of air flow inside a circular tube with different tube inserts

Numerical and experimental analyses were carried out to study thermal–hydraulic characteristics of air flow inside a circular tube with different tube inserts. Three kinds of tube inserts, including longitudinal strip inserts (both with and without holes) and twisted-tape inserts with three different twisted angles (α = 15.3°, 24.4° and 34.3°) have been investigated for different inlet frontal velocity ranging from 3 to 18 m/s. Numerical simulation was performed by a 3D turbulence analysis of the heat transfer and fluid flow. Conjugate convective heat transfer in the flow field and heat conduction in the tube inserts are considered also. The experiments were conducted in a shell and tube exchanger with overall counterflow arrangement. The working fluid in the tube side was cold air, while the hot Dowtherm fluid was on the shell side. To obtain the heat transfer characteristics of the test section from the experimental data, the ε-NTU (effectiveness-number of transfer unit) method is applied to determine the overall conductance (UA product) in the analysis.It was found that the heat transfer coefficient and the pressure drop in the tubes with the longitudinal strip inserts (without hole) were 7–16% and 100–170% greater than those of plain tubes without inserts. When the longitudinal strip inserts with holes were used, the heat transfer coefficient and the pressure drop were 13–28% and 140–220%, respectively, higher than those of plain tubes. The heat transfer coefficient and the pressure drop of the tubes with twisted-tape inserts were 13–61% and 150–370%, respectively, higher than those of plain tubes. Furthermore, it was found that the reduction ratio in the heat transfer area of the tube of approximately 18–28% may be obtained if the twisted-tape tube inserts are used.     

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.     

Three-dimensional numerical study of wavy fin-and-tube heat exchangers and field synergy principle analysis

In this paper, 3-D numerical simulations were performed for laminar heat transfer and fluid flow characteristics of wavy fin-and-tube heat exchanger by body-fitted coordinates system. The effect of four factors were examined: Reynolds number, fin pitch, wavy angle and tube row number. The Reynolds number based on the tube diameter varied from 500 to 5000, the fin pitch from 0.4 to 5.2 mm, the wavy angle from 0° to 50°, and the tube row range from 1 to 4. The numerical results were compared with experiments and good agreement was obtained. The numerical results show that with the increasing of wavy angles, decreasing of the fin pitch and tube row number, the heat transfer of the finned tube bank are enhanced with some penalty in pressure drop. The effects of the four factors were also analyzed from the view point of field synergy principle which says that the reduction of the intersection angle between velocity and fluid temperature gradient is the basic mechanism for enhance convective heat transfer. It is found that the effects of the four factors on the heat transfer performance of the wavy fin-and-tube exchangers can be well described by the field synergy principle.     

Experimental study of natural frost formation over a horizontal tube with annular compact fins under natural convection

This paper presents experimental measurements of natural convection heat transfer and frost deposition over a horizontal fin‐tube. Measurements are made for a fin‐tube of diameter 25.4 mm, fin thickness 0.4 mm, fin center diameter 56 mm, and fin spacing 2 mm. For measurements the ambient air temperature and relative humidity are changed from 18 to 25°C and from 35% to 55%, respectively. The tube surface temperature is changed from –5 to –9 °C, and super cooling degrees of 7.5 to 24.5 °C. Results include a visualization of frost deposition growth, frost accumulation rate, and heat transfer rate with respect to time for each experiment. The results show that cold air starts from the upper point and moves downward and frost deposition starts on the fin tips, and grows with time both radially and angularly. Frost growth thickness changes significantly from top to bottom, where the boundary layers of both thermal and concentration increase at the bottom of the fin‐tube section without considerable separation. Frost growth only takes place on the fin's tip and it blocks the heat and mass transfer from the fin surfaces and the tube base which reduces convection and frost growth considerably. © 2011 Wiley Periodicals, Inc. Heat Trans Asian Res; Published online in Wiley Online Library ( wileyonlinelibrary.com/journal/htj ). DOI 10.1002/htj.20397     

Free Convective Heat Transfer in Tilted Longitudinal Open Cavity

Heat transfer experiments were performed to investigate the effects of inclination and channel height-to-gap ratio on free convection in a simulated fin-passage with a strategic aim of devising a criterion for selecting the optimal fin length that could provide the maximum free convective capability. The ranges of parameters investigated include the Grashof number, up to 500,000; channel height-to-gap ratios of 1, 2, and 3; and tilt angles of 0°, 30°, 60°, 90°, 120°, 150°, and 180°. Selections of local and spatially averaged Nusselt number results demonstrate the manner by which the Grashof number, tilt angle, and channel height-to-gap ratio interactively affect the heat transfer. In conformity with the experimentally revealed heat transfer physics, the correlation of a spatially-averaged Nusselt number over two parallel walls and the bottom surface of an open-ended channel is derived that permits the individual and interactive effects of the Grashof number, tilt angle, and channel height-to-gap ratio on heat transfers to be evaluated. A criterion for selecting the optimal height-to-gap ratio of the fin channel is subsequently formulated as a design tool for maximizing the convective capability of a free convective fin assembly.     

Conjugated Heat Transfer Simulation of a Fin-and-Tube Heat Exchanger

Heat transfer and pressure drop of a fin-and-tube heat exchanger are studied by taking into account the conjugated heat transfer between the flow and the fin. The temperature distribution of the fin is calculated in respect to the convective heat transfer of the air flowing through the tube bank channel. Contemporary enhancement methods emphasize the importance of local turbulence augmentation which effects the convective heat transfer. In this paper, the importance of conjugated heat transfer, where the temperature of the flow and fin are coupled together is emphasized and compared with a constant surface temperature boundary condition simulation and experiment, which are found in the literature.     

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.     

General correlation of a natural convective heat sink with plate-fins for high concentrating photovoltaic module cooling

Thermal resistance correlation as a design tool of a natural convective heat sink with plate-fins for concentrating photovoltaic (CPV) module cooling is suggested. For this purpose, extensive experimental investigations are performed for various heat sink geometries, input power, as well as inclination angle. From the comparison between the proposed general correlation and the previous studies, it is shown that the proposed correlation predicts well the effects of inclination angle and fin spacing on the thermal performance of the natural convective heat sink with plate-fins. Also, optimal fin spacing strongly depends on the temperature difference as well as the inclination angle. The optimal fin spacing is significantly changed when the inclination angle is larger than 70°.     

Effect of fins on forced convection heat transfer around a tube

Effect of fins on heat transfer around a tube was investigated experimentally. A test tube of 30 mm diameter was installed in a test section of an open‐type wind tunnel as a single tube, or as a center tube in a single tube row and in a tube bundle of staggered layout. Fins made of paper were put on the test tube having certain fin spacing. It was clarified from the experiment that the local heat transfer coefficient around the tube degrades with decreasing fin spacing, especially on the downstream side of the tube, and the minimum fin spacing where the effect of the fin begins to appear is the largest for the single tube and the smallest for the tube bundle. © 2003 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(5): 445–454, 2003; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.10098     

Effect of angle of attack on the heat transfer coefficient for an annular fin

Wind-tunnel experiments were performed to investigate the sensitivity of the heat transfer coefficient for an annular fin to small angles of attack. Quasi-local measurements were made at three zones on the fin surface: (I) forward of the tube; (II) at the side of the tube; and (III) behind the tube, with separate measurements being carried out on the two faces of the fin. The angle of attack ranged from −2° to 2°. The heat transfer coefficient in zone (I) was highly sensitive to the angle of attack, increasing by about 50% on the face of the fin where leading-edge separation was activated as the arigle was varied from − 2° to 2°. The corresponding increases in zones (II) and (III) were about 10%. On the other hand, for any angle of attack in the investigated range, the two-face average for each zone was not very different from the heat transfer coefficient for zero angle of attack. Therefore, the overall heat transfer performance of an annular fin is not significantly affected by small departures of the angle of attack from zero. However, since the differences between the coefficients for the two faces were found to be significant, measurement of the coefficient at only one face may yield erroneous results for the overall performance.     

Analysis on Heat Transfer Characteristics of Constant‐Temperature Heat Pipe in Waste Heat Utilization for Semi‐Coke

In the coal chemical industry, an internal heating retort furnace is applied to the processing of low‐temperature coal pyrolysis so as to produce semi‐coke. Because the cooling water is used to reduce the temperature of semi‐coke from 500 °C to 60 °C, the waste heat carried by the semi‐coke is released. Meanwhile, the waste water of higher temperature involved with the hazardous substances is discharged into rivers or lakes, causing serious environmental pollution. In the present work, a constant temperature heat pipe is used to recover the waste heat. An iterative method is adopted to numerically solve the thermal resistances and the overall heat transfer coefficients. Results show that the conductivity thermal resistance decreases as the tube diameter increases. In the heating section, the main factors affecting the heat transfer are the thermal resistances of both the radiation heat transfer and the convective heat transfer. As the pressure climbs, the thermal resistance of radiation heat transfer increases, while the thermal resistance of convective heat transfer decreases. In addition, the overall heat transfer coefficients increase with the pressure. The heat transfer efficiency of the heat pipe is about 30%, and a higher economic benefit can be obtained.     

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%.     

A study on heat transfer enhancement using straight and twisted internal fin inserts

The present study investigated the effect of internal aluminum fins with a star-shape cross-section on the heat transfer enhancement and pressure drop in a counterflow heat exchanger. A concentric-tube heat exchanger was used with water as the working fluid. The heat transfer rate increased by 12–51% over a plain tube value, depending on internal fin configurations used. However, the pressure drop also increased substantially by 286–399%. The results showed that a straight-fin configuration is the best to produce a heat transfer increase in a counterflow heat exchanger. Twisted fin configurations did not further increase the heat transfer rate.     

Experimental and modeling analysis of thermal characteristics in carbon fiber wires

To understand the thermal characteristics of carbon fiber heating wires, we tested the heating rules of carbon fiber heating wires with inner core tows of 12k and 24k and different lengths. We found that the surface temperature of the carbon fiber heating wires rose and fell quickly when the power supply was turned on and off by 82.9 and 18.3°C/min, respectively. After the quick rise period, the surface temperature tended to be steady with ±2°C of fluctuation; the steady temperature decreased as the length increased. With an increase in the length from 7 to 12 m, the steady temperature of the 12k and 24k wires reduced from 122°C and 166°C to 53°C and 71°C, respectively. Using the COMSOL Multiphysics software to simulate the carbon fiber heating wires, the simulated surface temperatures were in good agreement with the experimental results, with errors less than 7%. The convective and radiative heat transfer of the simulated carbon fiber electric heating wire decreased with an increase in length and the number of tows, and the convective heat transfer was much higher than the radiative heat transfer to indicate that the convective heat transfer is the main heat transfer mode in the heat dissipation of the carbon fiber electric heating wire. Results indicate that there is a rapid increase in temperature when power is on, and thereafter temperature remains constant. These are the important characteristics of carbon fiber heating wires.     

Effect of Twin Delta-Winged Twisted-Tape on Thermal Performance of Heat Exchanger Tube

This work aims at studying the effect of twin delta-winged twisted-tape insertion on heat transfer, pressure drop, and thermal performance characteristics of a heat exchanger tube. All twisted tapes used in this work were made of aluminum sheets twisted at a single twist ratio of 3.0. The twin delta wings were formed by extrusion of the tape at the center area at every twist length interval. For comparison, three different arrangements of the twin delta wings were: (1) the wing tips pointing upstream of the flow (TTW-up, twin delta-winged twisted tape in counterflow arrangement), (2) the wing tips pointing downstream of the flow (TTW-down, twin delta-winged twisted tape in co-flow arrangement), and (3) the wing tips pointing opposite direction (TTW-o, opposite winged twisted tape). The wing declination was arranged at an angle of 15° with respect to the tape surface. Effects of three different wing-tip angles of 20°, 40°, and 60° for a constant wing base were examined. The experiments were conducted using water as the test fluid in a uniform-heat-flux tube for Reynolds number between 5000 and 15,000. The results demonstrate that the TTW-up consistently provides greater heat transfer rate, friction factor, and thermal performance factor than the TTW-down and the TTW-o, at a similar condition. In addition, the heat transfer rate increases as the wing-tip angle decreases. Over the range investigated, the TTW-up with wing-tip angle of 20° gives the highest thermal performance factor of 1.26 along with a Nusselt number and friction factor of 2.57 and 8.55 times those of the plain tube.     

Numerical Analysis of Jet Impingement Heat Transfer at High Jet Reynolds Number and Large Temperature Difference

Jet impingement heat transfer from a round gas jet to a flat wall was investigated numerically for a ratio of 2 between the jet inlet to wall distance and the jet inlet diameter. The influence of turbulence intensity at the jet inlet and choice of turbulence model on the wall heat transfer was investigated at a jet Reynolds number of 1.66 × 10⁵ and a temperature difference between jet inlet and wall of 1600 K. The focus was on the convective heat transfer contribution as thermal radiation was not included in the investigation. A considerable influence of the turbulence intensity at the jet inlet was observed in the stagnation region, where the wall heat flux increased by a factor of almost 3 when increasing the turbulence intensity from 1.5% to 10%. The choice of turbulence model also influenced the heat transfer predictions significantly, especially in the stagnation region, where differences of up to about 100% were observed. Furthermore, the variation in stagnation point heat transfer was examined for jet Reynolds numbers in the range from 1.10 × 10⁵ to 6.64 × 10⁵. Based on the investigations, a correlation is suggested between the stagnation point Nusselt number, the jet Reynolds number, and the turbulence intensity at the jet inlet for impinging jet flows at high jet Reynolds numbers.     

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.     

Enhancement of natural convection heat transfer from a vertical heated plate using inclined fins

An enhancement technique is developed for natural convection heat transfer from a vertical heated plate with inclined fins, attached on the vertical heated plate to isolate a hot air flow from a cold air flow. Experiments are performed in air for inclination angles of the inclined fins in the range of 30° to 90° as measured from a horizontal plane, with a height of 25 to 50 mm, and a fin pitch of 20 to 60 mm. The convective heat transfer rate for the vertical heated plate with inclined fins at an inclination angle of 60° is found to be 19% higher than that for a vertical heated plate with vertical fins. A dimensionless equation on the natural convection heat transfer of a vertical heated plate with inclined fins is presented. © 2007 Wiley Periodicals, Inc. Heat Trans Asian Res, 36(6): 334–344, 2007; Published online in Wiley InterScience ( www.interscience.wiley.com ). DOI 10.1002/htj.20168