REVIEW OF HEAT TRANSFER
ENHANCEMENT IN DIFFERENT TYPES
OF EXTENDED SURFACES
A.B.GANORKAR1* 1*
Student, IV Semester M.Tech (Heat Power Engineering)
Mechanical Engineering Department, G.H.Raisoni College of Engineering, Nagpur. 440016 (India)
1*
Corresponding author
V.M.KRIPLANI 2
Professor and Head 1* 2
Mechanical Engineering Department, G.H.Raisoni College of Engineering, Nagpur. 440016 (India)
Abstract
Extended surface heat exchangers are simple in construction and extensively used in many of the industries. Continuous Research is going on to improve its effectiveness by increasing fluid turbulence, generating secondary fluid flow patterns, reducing the thermal boundary layer thickness and increasing the heat transfer surface area. The present paper is a review of different types of arrangements of extended surfaces.
Keywords: Extended surfaces, heat exchangers, heat transfer enhancement
1. Introduction
Heat exchangers have several industrial and engineering applications. The design procedure of heat exchangers is quite complicated, as it needs exact analysis of heat transfer rate and pressure drop estimations apart from the issues such as long-term performance and the economic aspect of the equipment. The heat transfer can be increased by the following different Augmentation Techniques .
They are broadly classified into three different categories: (i) Passive Techniques
(ii)Active Techniques (iii)Compound Techniques.
1.1 Passive Techniques
These methods generally use surface or geometrical modifications to the flow channel by incorporating inserts or additional devices. They promote higher heat transfer coefficients by disturbing or altering the existing flow behavior except for extended surfaces. Heat transfer augmentation by these techniques can be achieved by using;
1.1.1 Treated Surfaces
Such surfaces have a fine scale alteration to their finish or coating which may be continuous or discontinuous. They are primarily used for boiling and condensing duties.
1.1.2 Rough surfaces
These are the surface modifications that promote turbulence in the flow field in the wall region, primarily in single phase flows, without increase in heat transfer surface area.
1.1.3 Extended surfaces
1.1.4 Displaced enhancement devices
These are the inserts that are used primarily in confined forced convection, and they improve energy transport indirectly at the heat exchange surface by displacing the fluid from the heated or cooled surface of the duct with bulk fluid from the core flow.
1.1.5 Swirl flow devices
They produce and superimpose swirl flow or secondary recirculation on the axial flow in a channel. These include helical strip or cored screw type tube inserts, twisted tapes. They can be used for single phase and two-phase flows.
1.1.6 Coiled tubes
These lead to relatively more compact heat exchangers. It produces secondary flows and vortices which promote higher heat transfer coefficients in single phase flows as well as in most regions of boiling.
1.1.7 Surface tension devices
These consist of wicking or grooved surfaces, which direct and improve the flow of liquid to boiling surfaces and from condensing surfaces.
1.1.8 Additives for liquids
These include the addition of solid particles, soluble trace additives and gas bubbles in single phase flows and trace additives which usually depress the surface tension of the liquid for boiling systems.
1.1.9 Additives for gases
These include liquid droplets or solid particles, which are introduced in single-phase gas flows either as dilute phase (gas-solid suspensions) or as dense phase (fluidized beds).
1.2 Active Techniques.
In these cases, external power is used to facilitate the desired flow modification and the concomitant improvement in the rate of heat transfer. Augmentation of heat transfer by this method can be achieved by
1.2.1 Mechanical Aids
Such instruments stir the fluid by mechanical means or by rotating the surface. These include rotating tube heat exchangers and scrapped surface heat and mass exchangers.
1.2.2 Surface vibration
They have been applied in single phase flows to obtain higher heat transfer coefficients.
1.2.2 Fluid vibration
These are primarily used in single phase flows and are considered to be perhaps the most practical type of vibration enhancement technique.
1.2.3 Electrostatic fields
1.2.4 Injection
Such a technique is used in single phase flow and pertains to the method of injecting the same or a different fluid into the main bulk fluid either through a porous heat transfer interface or upstream of the heat transfer section.
1.2.5 Suction
It involves either vapor removal through a porous heated surface in nucleate or film boiling, or fluid withdrawal through a porous heated surface in single-phase flow.
1.2.6 Jet impingement
It involves the direction of heating or cooling fluid perpendicularly or obliquely to the heat transfer surface.
1.3 Compound Techniques
When any two or more of these techniques are employed simultaneously to obtain enhancement in heat transfer that is greater than that produced by either of them when used individually, is termed as compound enhancement. This technique involves complex design and hence has limited applications.
2. EXTENDED SURFACE GEOMETRIES
A large number of extended surface geometries have been proposed for use in compact heat exchangers, and more are still being developed. A high-performance surface will enhance the heat transfer that takes place within the heat exchanger, without incurring penalties on friction and pressure drop that are severe enough to negate the benefits of heat transfer augmentation. In this section, the following types of plate-fin geometries are examined: plain fins, wavy and corrugated channels, offset-strip fins, louvered fins, and vortex generators
2.1 Plain Fins
Plain fin surfaces are characterized by long uninterrupted flow passages with performance comparable to that obtained inside long circular tubes wavy and Corrugated Channel. The plain fins that are most commonly used have flow channels with either a rectangular or triangular cross-section (Fig-1). The enhancement in heat transfer achieved with plain fins is due mainly to increased area density, rather than any increase in the heat transfer coefficient. Plain fins require a smaller flow frontal area than interrupted surfaces (i.e. offset strip fins and louvered fins) for given values of heat duty, pressure drop, and flow rate, but the flow length with plain fins will be greater, resulting in a higher overall heat exchanger volume
Fig.1. Rectangular Plain Fin
M.J. Sable1, S.J. Jagtap , P.S. Patil , P.R. Baviskar & S.B. Barve [1] have investigated for natural convection adjacent to a vertical heated plate with a multiple v- type partition plates (fins) in ambient air surrounding. As compared to conventional vertical fins, this v-type partition plate’s works not only as extended surface but also as flow turbulator. In order to enhance the heat transfer, V-shaped partition plates (fins) with edges faced upstream were attached to the two identical vertical plates. They observed that among the three different fin array configurations on vertical heated plate, V-type fin array design performs better than rectangular vertical fin array and V-fin array with bottom spacing design. The performance was observed to improve further, with increase in the height of the V-plates (fin height).
floor-mounted heater. Two kinds of perforations were investigated (hole and slot).Equations were solved with software FLUENT 6.3 ®. Ranges of current work are as follow: Reynolds number: 6000 to 40000, open-area perforation ratio 0.05 to 0.15, perforation’s inclination angle: 0< Ѳ <45. He found that, by applying perforation, in all cases there was significant enhancement in heat-transfer. Also by increasing perforation angle to 45, more fresh air reaches the base plate and augments the local heat transfer more considerably. The more inclination angles the more heat transfer enhancement. Increasing open-area ratio more area is exposed to fresh air and causes better heat transfer but perforation’s diameter is another parameter and perforations with same open-area ratio with different hole diameter behaves differently. This is due to capability of spreading out the flow. If the distance of perforations is close enough in respect to distance of the ribs, the jets might collide in the downstream and the result would be more local turbulence intensity and higher heat transfer coefficient. Perforation cause less resistance of blocks against the stream and the result is reduction of friction factor in comparison with solid cases. Inclination angle of perforation did not exhibit an appreciable effect on pressure drop. Increasing perforation diameter improves both heat transfer and pressure drop over the channel. Friction factor generally decreases with increasing Reynolds number for both solid and perforated ribs. Slot and hole perforations generally behave the same. However, for same open-area ratio for current study geometry, more overall thermal performance was achieved for hole perforation case. Thus, slot perforations are considerable for spot cooling purposes.
Ahmad Khoshnevis, Faramarz Talati, Maziyar Jalaal, Esmaeil Esmaeilzadeh [3] has investigates numerically the effects of attached perforated ribs on heat-transfer enhancement of a rectangular channel (Fig-2). Different Open-area ratio of perforation (Number and diameter of holes), Perforation inclination angle, Number of ribs for Reynolds range of 6000 up to 40000 were examined. Equations were solved by FLUENT® (ver.6.3), using standard k-ω turbulence model. They concluded the following points
Increasing perforation’s open-area ratio augments heat transfer and decreases pressure-drop.
Increasing inclination angle enhances thermal performance while insignificant in pressure-drop.
Increasing perforation diameter improves both heat transfer and pressure drop over the channel.
Heat-transfer enhancement generally decreases with increasing Reynolds number.
By decreasing the distance between the perforated ribs (increasing number of the ribs), more enhancement achieved both heat-transfer and pressure drop.
Friction factor generally decreases with increasing Reynolds number for both solid and perforated ribs.
Fig.2. Study model
Abdullah H. AlEssa and Mohammed Q. Al-Odat [4] has numerically investigated natural convection heat transfer enhancement from a horizontal rectangular fin embedded with equilateral triangular perforations. The heat dissipation rate from the perforated fin is compared to that of the equivalent solid one. The effect of geometrical dimensions of the perforated fin and thermal properties of the fin was studied in detail. They concluded the following points.
For certain values of triangular dimensions, the perforated fin can result in heat transfer enhancement. The magnitude of enhancement is proportional to the fin thickness and its thermal conductivity.
The extent of heat dissipation rate enhancement for perforated fins is a complicated function of the fin dimensions, the perforation geometry, and the fin thermophysical properties.
The gain in heat dissipation rate for the perforated fin is a strong relation of both, the perforation dimension, and lateral spacing. This relation attains a maximum value at given perforation diameter and lateral spacing, which is called the optimum perforation dimension and optimum spacing, respectively.
The perforation of fins enhances heat dissipation rates and at the same time decreases the expenditure of the fin material.
of heat transfer coefficients due to perforations. They concluded that, for certain values of rectangular perforation dimension, the perforated fin enhances heat transfer. The magnitude of enhancement is proportional to the fin thickness and its thermal conductivity. Also, the extent of heat dissipation rate enhancement for perforated fins is a function of the fin dimensions, the perforation geometry and the fin thermophysical properties. Also, the gain in heat dissipation rate for the perforated fin is a strong relation of both, the perforation dimension and the lateral spacing.
M.R. Shaeri, M. Yaghoubi, K. Jafarpur [6] has done three-dimensional numerical computation is made for turbulent fluid flow and convective heat transfer around an array of rectangular solid and new design of perforated fins with different no. and two various sizes of perforations. Experiments were conducted for the range of Reynolds no. from 2000 to 5000 based on fin thickness and pr= 0.71 and following main conclusions are drawn.
For fins with perforations the region of recirculation over the faces of perforated fins at a fixed altitude of fin is different than solid fin but this region over the top surface of fins is nearly the same for all types of fins studied.
With increase of perforations flow becomes complicated, average friction coefficient decreases and solid fin has the highest value of cf.
For fins with perforation, drag force reduces. Also drag ratio becomes smaller by increasing Reynolds no.
Average Nu. No. decreases by increasing no. of perforations. Solid fin has the largest average Nusselt no. for each Reynolds no. for practical application following co-relation are proposed.
For large size windows:
Fins with same porosity but larger window sizes have higher Nusselt no. than fin with smaller window.
By increasing no. of perforations, temperature difference between the fin base and fin tip becomes larger.
By making window perforations, and especially with increasing no. of perforations, lighter fins that are more economical, will be achieved. The main advantage of these new kinds of perforated fins is their considerable lower weight.
For perforated fin, of type 2-7 with increase of Re. no. the percentage of heat transfer enhancement with respect to solid fin becomes smaller but opposite trend is observed.
Dharma Rao, S.V. Naidu, B. Govinda Rao and K.V. Sharma [7], The problem of laminar natural convection heat transfer from a fin array containing a vertical base and horizontal fins is theoretically formulated by treating the adjacent internal fins as two fin enclosures. The governing equations of mass, momentum and energy balance for the fluid in the two fin enclosure together with the heat conduction equations in the fins are numerically solved using ADI method. The heat transferred to the ambient fluid from the two end fins is also computed separately. Heat transferred by radiation is considered in the analysis. The numerical results are compared with the experimental data available in literature. The effects of system parameters such as base temperature, fin height, fin spacing on heat transfer rate from the fin array are studied. They concluded the following points
with decreasing of fin spacing heat transfer rates are increasing sharply.
convective heat transfer rates from a fin array are increasing with increasing of base temperature of the fin array at all fin spacings.
convective heat transfer rates from a fin array are increasing with increasing of fin length at all fin spacings.
It is observed that average Nusselt numbers are increasing with increasing of Rayleigh number and emissivities for all fin lengths.
average Nusselt number for short fins more than long fins.
David J. Kukulka, Kevin G. Fuller [8] have investigated experimentally heat transfer surface like solid plain, solid textured, solid with openings (louvered), and textured with openings (Fig-3). He used different types of material like copper, stainless steel (304), aluminum, copper alloys, nickel alloys.
Type-1
38 % Increase in Heat Transfer with
Type-2
40 % Increase in Heat Transfer with Excellent Flow Distribution
Type-3
18 % Increase in Heat Transfer with Directional Flow Distribution
Type-4
25 % Increase in Heat Transfer with Average Flow Distribution
Type-5
30 % Increase in Heat Transfer with Excellent Flow Distribution
Fig.3. Types of textures
Mohammad Mashud, Md. Ilias Inam, Zinat Rahman Arani and Afsanul Tanveer [9] research, a solid cylindrical fin and two other cylindrical fins with circular grooves and threads on their outside surface are investigated experimentally. The heat input to the fin is varied such that the base temperature is maintained constant under steady state. Based on a study of effect of pressure reduction, using available resources, the chamber is designed for a vacuum of 680 mm Hg. The experimental result shows that for cylindrical fin with circular grooves (depth 3.5mm) heat loss is a maximum. The grooved cylindrical fin loses approximately1.23 times greater heat per unit area, compared to the threaded cylindrical fin, and 2.17 times greater heat per unit area, respectively compared to the solid pin fin at a pressure lower than atmospheric pressure. As pressure decreases heat loss reduces and contribution of radiation heat transfer on total heat loss increases.
modeling program developed based on empirical expressions. In addition to the thermal measurements, the effect of air flow bypass characteristics in open duct con-figuration was investigated. As expected, the straight fin experienced the lowest amount of flow bypass over the heat sink. For this particular application, where the heat source is localized at the center of the heat sink base plate, the overall thermal resistance of the straight fin was lower than the other two designs mainly due to the combined effect of enhanced lateral conduction along the fins and the lower flow bypass characteristics.
2.2 Wavy and Corrugated Channels
Wavy and corrugated channels both enhance heat transfer by promoting mixing due to complex recirculatory flows and boundary layer separation. However, less friction is expected in wavy channels because the sharp corners of the corrugated channel are not present.
Fig.4a. Corrugated plate with tile angles of 200
Fig.4b. Corrugated plate with tile angles of 400
Fig.4c. Corrugated plate with tile angles of 600
Paisarn Naphon [11] have investigated the heat transfer characteristics and pressure drop in the channel with V corrugated upper and lower plates under constant heat flux. He has carried out the analysis on channel with two opposite corrugated plates on which all configuration peaks lie in a staggered arrangement. Corrugated plates with three different corrugated tile angles of 200 (Fig-4a), 400 (Fig-4b) and 600 (Fig-4c) for Reynolds number and heat flux in the ranges of 2000–9000 and 0.5–1.2 kW/m2, respectively. He found that The corrugated surface has a significant effect on the enhancement of heat transfer and pressure drop. The heat transfer rate is higher as the air mass flow rate increases. For a given air Reynolds number and heat flux, the average plate temperatures at higher wavy angle are lower than those from lower wavy angle. However, the increase of heat transfer rate is less than that of the air mass flow rate. Therefore, the outlet air temperature tends to decrease as the air mass flow rate increases. The pressure drop continues to increase with Reynolds number. the pressure drops obtained from the channel with higher wavy angle are significantly higher than those with lower angles. The measured pressure drops obtained from the channel with corrugated surfaces are 1.96 times higher than those from the plane surfaces.
2.3 Offset Strip Fins
Fig.5. Offset Strip Fins
J. M. Corberan, E. Cuadros, K. Gonzalez [12] have done experimental study about the single phase pressure drop along a typical channel of a plate and fins heat exchanger has been performed for three different fin pads: plain fin, and two offset strip fin pads with different serration lengths.
2.4 Louvered Fins
Louvered fins (Fig-6), typically found in many compact heat exchanger designs, increase the average heat transfer by interrupting the boundary layer formation and by providing more surface area. Louvered fin surfaces are commonly used in automobile radiators. The louvered fm geometry consists of an interrupted surface similar to that of the offset-strip fin. However, the slit strips of louvered fins are not completely offset. Instead, the slit fin is rotated between 200 and 600 relative to the direction of the airflow. Most radiators use a louver strip width of 1.0 to 1.25 mm. For equal strip width, the louvered fin geometry provides enhancement comparable to that of offset strip fins. Moreover, louvered fins are less expensive than offset strip fins for large-quantity production, because of their ease of manufacture using high-speed mass production technology
Fig.6. Louvered Fins
A. C. Lyman, R. A. Stephan, and K. A. Thole, L. W. Zhang and S. B.[13] Memory have conducted an experiment in a number of large-scale louver models with varied fin pitch and louver angle over a range of Reynolds numbers. They have given a method for evaluating the spatially-resolved louver heat transfer coefficients using various reference temperatures, such as the bulk flow temperature and adiabatic wall temperature, to define the convective heat transfer coefficients. They found that, there were large variations in the performance of the various louver models at the lower Reynolds numbers relative to the higher Reynolds numbers.
Re=230 case did not. Time-resolved velocity measurements were also made in the wake region of a fully developed louver for a range of Reynolds numbers. For 1000 < Re < 1900, there was an identifiable peak frequency for the velocity fluctuations giving a constant Strouhal number of St=0.17. The velocity measurements indicated that the boundary layer on the ownstream side of the louver was slightly thicker than on the upstream side of the louver.
4.5 Vortex Generators
Vortex generators (Fig-7) do not significantly change the effective heat transfer surface area of the plate, but they increase the heat transfer coefficient by creating longitudinally spiraling vortices which promote mixing between the wall and core regions of the flow Vortex generators are a relatively new type of enhancement device, and an optimum geometry has not yet been arrived at. There are any number of possibilities for different vortex generator surfaces, since one can vary the size, angle of attack, aspect ratio, and/or arrangement of the vortex generators.
Fig.7. Vortex Generators
Teerapat Chompookham, Chinaruk Thianpong, Sutapat Kwankaomeng, Pongjet Promvonge[15] have
done Experimental investigations to study the effect of combined wedge ribs and winglet type vortex generators (WVGs) on heat transfer and friction loss behaviors for turbulent airflow through a constant heat flux channel. To create a reverse flow in the channel, two types of wedge (right-triangle) ribs were introduced. Wedge ribs were pointing downstream and pointing upstream. The arrangements of both rib types placed inside the opposite channel walls are in-line and staggered arrays. To generate longitudinal vortex flows through the tested section, two pairs of the WVGs with the attack angle of 60° were mounted on the test channel entrance. The test channel had an aspect ratio, AR=10 and height, H=30 mm with a rib height, e/H=0.2 and rib pitch, P/H=1.33. The flow rate in terms of Reynolds numbers is based on the inlet hydraulic diameter of the channel ranging from 5000 to 22,000. The presence of the combined ribs and the WVGs shown the significant increase in heat transfer rate and friction loss over the smooth channel. The Nusselt number and friction factor values obtained from combined the ribs and the WVGs were found to be much higher than those from the ribs/WVGs alone. In conjunction with the WVGs, the in-line wedge pointing downstream provides the highest
Increase in both the heat transfer rate and the friction factor while the staggered wedge pointing upstream yields the best thermal performance.
References
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Technology, 28-30 April 2009
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[7] Dharma Rao, S.V. Naidu, B. Govinda Rao and K.V. Sharma “Combined Convection and Radiation Heat Transfer from a Fin Array with a Vertical Base and Horizontal Fins”, Proceedings of World Congress on Computer science 2007 WCECS 2007, october 24-26, 2007, San Francisco, USA
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