International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 4, April 2012)
595
Heat Transfer Analysis through Fin Array by Using Natural Convection
Shivdas S. Kharche1, Hemant S. Farkade2
1M Tech Student, Thermal Engineering, Government College of engineering, Amravati,
2Asst. Professor, Department of Mechanical Engineering, Government College of engineering, Amravati,
1shivdasskharche@rediffmail.com
2farkade.hemant@gcoea.ac.in
Abstract- The main purpose of extended surfaces called fins to increase the heat transfer rate. Fins offer an economical and trouble free solution in many situations demanding natural convection heat transfer. Heat sinks in the form of fin arrays on horizontal and vertical surfaces used in variety of engineering applications, studies of heat transfer and fluid flow associated with such arrays are of considerable engineering significance. The main controlling variable generally available to designer is geometry of fin arrays. Considering the above fact, natural convection heat transfer from vertical rectangular fin arrays with and without notch at the center have been investigated experimentally and theoretically. Moreover notches of different geometrical shapes have also been analyzed for the purpose of comparison and optimization. In a lengthwise short array where the single chimney flow pattern is present, the central portion of fin flat becomes ineffective due to the fact that, already heated air comes in its contact.
Many researchers have been studied the heat transfer rate through without notch and notched fins by using aluminum as a material. Verities of researchers were carried out, this paper focuses on heat transfer rate of copper fin for greater heat transfer rate which is need of increased rate of modernization thus extent of copper is tested.
I. INTRODUCTION
Fins are used to enhance convective heat transfer in a wide range of engineering applications, and offer a practical means for achieving a large total heat transfer surface area without the use of an excessive amount of primary surface area. Fins are commonly applied for heat management in electrical appliances such as computer power supplies or substation transformers. Other applications include Internal Combustion engine cooling, such as fins in a car radiator. It is important to predict the temperature distribution within the fin in order to choose the configuration that offers maximum effectiveness.
Natural convection heat transfer is often increased by provision of rectangular fins on horizontal or vertical surfaces in many electronic applications, motors and transformers. The current trend in the electronic industry is miniaturization, making the overheating problem more acute due to the reduction in surface area available for heat dissipation.
Thus heat transfer from fin arrays has been studied extensively, both analytically and experimentally.
Baskaya et al (2000) [3] carried out parametric study of natural convection heat transfer from the horizontal rectangular fin arrays. They investigated the effects of a wide range of geometrical parameters like fin spacing, fin height, fin length and temperature difference between fin and surroundings; to the heat transfer from horizontal fin arrays. However, no clear conclusions were drawn due to the various parameters involved. Finally they concluded that, it is not possible to obtain optimum performance in terms of overall heat transfer by only concentrating on one or two parameters. The interactions among all the design parameters must be considered. This study has shown that each of the variables produces an effect on the overall heat transfer. As a whole, it can be concluded that the overall heat transfer is enhanced with the increase in the height (H), of the fin and decrease in the length (L) the fin.
Fins are generally used to increase the heat transfer rate from the surface. According to Yunus A. Çengel [1] in analysis of fins we consider steady operation with no heat generation in the fin & assume thermal conductivity of material is constant. The heat transfer coefficient is assumed to be constant over the entire surface of the fin. The value of h is much lower at the base than its tip. Because fluid is surrounded by the solid surface near its base. Hence adding too many fins on a surface decrease the overall heat transfer coefficient when the decrease in h offsets any gain resulting from the increase in the surface area.
According to Incropera F. P., DeWitt D. P [2] the term extended surface is commonly used to depict an important special case involving heat transfer by conduction within a solid heat transfer by convection from the boundaries of a solid. Although there are many different situations that involve such combine conduction-convection effect, the most frequent application is one in which an extended surface is used to increase the heat transfer rate between the solid and adjoining fluid. Such an extended surface is called is called as fin.
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 4, April 2012)
596
From the early research work it was clear that, there is
establishment of single chimney pattern (Figure 1) for
lengthwise short fins. There was sidewise entry of air in
case of natural convection cooling of vertical fin array.
The air coming inwards gets heated as it moves towards
the centre of the fin, as well as it rises due to decrease in
density. So, the central portion of the fin becomes
ineffective because hot air-stream passes over that part
and therefore it does not bring about large heat transfer
through that portion. So, if some of the material from that
central portion was removed, and was added at the place
where greater fresh air comes in the contact of the fin
surface, it would increase overall heat transfer coefficient
„h‟. This has been confirmed experimentally by Sane et
al. Figure 2 shows configuration of such fin arrays.
Figure 1: Single Chimney Flow Pattern
II. EXTENDED SURFACES
Heat transfer through the solid to the surface of the
solid takes place through conduction where as from the
surface to the surroundings takes place by convection.
Further heat transfer may be by natural convection or by
forced convection. The rate of heat transfer from a
surface at a temperature „Ts‟ to the surrounding medium
at „T0‟ is given by the Newton‟s law of cooling as:
Q = h AS (Ts −To)
Where: AS - is heat transfer surface area and
h -is the convection heat transfer coefficient.
When the temperatures TS and T0 are fixed by design
considerations, as is often the case, there are two ways to
increase heat transfer rates:
(1) To increase convection heat transfer coefficient [h] &
(2) To increase the surface area [AS].
The alternative is to increase the surface area by
attaching to the extended surfaces called fins made of
highly conductive materials. The main purpose of
extended surfaces is to increase the heat transfer rate.
III. MATERIALS USED FOR FINS
Generally there are two types of materials used for fins
aluminium and copper. The thermal conductivity of
aluminium is 225 W/mK and that of copper is 385
W/mK. The melting and boiling point of copper are
1084˚ and 2595˚ and that of aluminium are 658˚ and
2057˚.
Pure aluminium has silvery colour and it has greater
resistance to corrosion. It is used in deoxidizing molten
irons and steel. It is used to prepare to prepare the metals
from their oxides by heating a mixture of powdered
aluminium and the oxides of the metal to be reduced. Its
electrical resistivity is 2.669 micro ohms/cm.
Copper is reddish brown in colour. Refining of the
metal is usually considered to begin when the copper is
in the blister stage, the surfaces of the cast material being
irregular and blistered due to the generation of gases
during cooling. This copper is 99% pure and is further
refined in the furnace by oxidation process which
removes sulphur and other impurities. The excess of
oxygen is removed from the metal by operation known as
poling. Its electrical resistivity is 1.682 micro ohms/cm.
IV. FINS SHAPES USED FOR HEAT TRANSFER
Different types of fins ware used to increase the heat
transfer rate. The fin shapes used was rectangular, vshapes,
triangular, trapezoidal and circular. Some
researchers were used without notch fins and some uses
notched fins. They were also used different shapes of
notches such as rectangular, v-shapes, triangular,
trapezoidal and circular. The heat transfer rate through
notched fins was more than without notched fins.
According to Senol Baskaya & Murat Ozek [3] they
did the parametric study of fins. They use material for fin
was aluminum. They studied each of the variables of fin
spacing, height, and length and temperature difference
produces an effect on the overall heat transfer.
Figure 2: Configuration of Fin Arrays.
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 4, April 2012)
597
They investigated the effects of a wide range of geometrical parameters like fin spacing, fin height, fin length and temperature difference between fin and surroundings; to the heat transfer from horizontal fin arrays. However, no clear conclusions were drawn due to the various parameters involved. Finally they concluded that, it is not possible to obtain optimum performance in terms of overall heat transfer by only concentrating on one or two parameters. The interactions among all the design parameters must be considered. This study has shown that each of the variables produces an effect on the overall heat transfer. They concluded that the overall heat transfer is enhanced with increase in the height of fin H, and decrease in length L, hence increase in H/L.
According to S.S.Sane, N.K. Sane, G.V Parishwad [5] they did the study of heat transfers through without notch & notch fins. They use fin material was aluminium. They use the length of fin was 150 mm and height of fin 75 mm both was fixed. They take number of fins from 9 to15 and notched portion from 10% to 40%.
They used three different types of arrays by varying the depth of notch. Their set up consist Aluminium fin flats cut with the help of foot shear from Aluminium sheet of 2 mm thickness. In case of notched array, notch was machined with the help of hacksaw and then filed. The fin flats were tied together using tie rods. Spacing was adjusted using Aluminium spacers. Cartridge type rod heaters of 14 mm diameter were used for heating.
They supplied different inputs to fin surface from 50W to 200W. Find the heat transfer coefficient in each case. They concluded that total heat flux as well as the Heat transfer coefficient increases as the notch depth increases. This can be interpreted as below:-
As area removed from the fin is compensated at the air entry ends of the fin it provides chance to get greater amount of fresh cold air (getting sucked into the array through single chimney pattern) to come in contact with hot fin surface. As the air moves inwards along chimney profile, it gets heated and temperature difference between the fin and entering air decreases. This area of fin (near its lengthwise centre) thus becomes relatively less useful for heat transfer. Now when this area is removed and added at place where it is more useful for heat transfer, the heat transfer increases and so does the convective heat transfer coefficient
According to S.D. Suryavanshi, N.K. Sane [6] they did study on natural convection heat transfer through rectangular inverted notched fin arrays. For experimental setup, they used fin flats and spacers are cut from 3 mm and 1 mm thick rolled aluminum sheet and assembled together to form the required fin array. They uses length of fin is 150 mm, height 75 mm, fin spacings from 4-13 mm, number of fins 7-15, notched portion 10-40 % and heat input supplied from 50-200 W.
They fix the length & height of fin & vary fin spacing, number of fins, notched portion, heat input. They concluded that the value of heat transfer coefficient increases as notch area increases and at fin spacing 6mm.
According S.H.Barhatte, M.R.Chopade, and V.N.Kapatkar [4] they did study on heat transfer rate through different types of notches in the fin. They used different notch such as rectangular, circular, triangular and trapezoidal. They compare without notch and notch fin array by supplying different heat inputs. The dimensions of fin were fixed. They concluded that more heat is transfer through triangular notch fin.
V. OBJECTIVE OF WORK
From the literature survey it is observed that different researcher uses different types of fin shapes, different types of notches in the fin. Analyzed the effect of different parameters like length, height, spacing on heat transfer coefficient. But all of them use the material for fin is aluminum. No one use other material than aluminum. Hence I planned to change the material of fin. For this purpose copper is used as a fin material for the experimental work. The dimensions of the fin for experimental work are length 127 mm, height 38 mm and spacing between fins is 9 mm. The thickness of the plate is 1 mm. the length, height and spacing is fixed. The shape of the notch is rectangular. Compare the effect of heat transfer coefficient for notch and without fins.
Fig 3: Experimental set up
VI. EXPERIMENTAL SETUP AND PROCEDURE
Experimental setup consist of base copper plate of 190 ×110 mm having thickness 1 mm. the dimensions of the fin is length 127 mm, height 38 mm and thickness 1 mm. Number of fins are taken as 7.the fins are joined to base plate by brazing operation. The spacing between the fns is 9 mm. the length, height and spacing of the fin is fixed. 8 thermocouples are used for attached to the fins and plate for temperature measurement. A heater coil is used for heating the plate.
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, Volume 2, Issue 4, April 2012)
598
The plate is fixed in a insulating material to avoid heat losses from either sides of the plate. The whole assembly is placed in a wooden block for natural convection. Different heat inputs are given to the plate and note down the temperature at the fins and plate. The experimental set up shown in fig. 3.
Fig 4: Comparison of heat transfer coefficient for without notch
and notch fins
TABLE-I
HEAT TRANSFER COEFFICIENT FOR WITHOUT NOTCHED AND NOTCHED FINS
Sr no
Heat
Input in watt
h
for without notched fin
( W/m2k )
h
for 20% notched fin
( W/m2k )
1
50
8.0595
9.3397
2
60
8.2307
9.6269
3
70
8.5519
10.0100
4
80
8.7130
10.2790
Avg
8.3887
9.8139
VII. CONCLUSION
From the experimental study it is found that the heat transfer rate in notched fins is more than the unnotched fins. The average heat transfer coefficient for without notched fin is 8.3887 W/m2K and for 20% notched fins it is 9.8139 W/m2K. Also the copper gives more heat transfer rate than aluminum plate. As the notch area of fin increases the heat transfer rate also increases. Copper plate gives better heat transfer rate than aluminum plate.
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