OBJECTIVE
Demonstrate the use of a fin (extended surface) to improve the heat transfer in forced convection.
INTRODUCTION
Convection Convection is the movement of molecules within fluids. fluids. It cannot cannot take place in solids, since either bulk current flows or significant diffusion can take place in solids. Convection is one of the major modes of heat transfer and mass transfer. orced orced convec convectio tion n is a mechan mechanism ism,, or t!pe t!pe of heat heat transp transport ort in which which fluid fluid motion motion is generated b! an external source, like a pump, fan, and suction device. It should be considered as one of the main methods of useful heat transfer as significant amounts of heat energ! can be transported ver! efficientl! and this mechanism mechanis m is found ver! commonl! in ever!da! life, includ including ing central central heatin heating, g, air condit condition ioning ing,, steam steam turbin turbines es and in man! man! other other machin machines. es. orced convection is often encountered b! engineers designing or anal!"ing heat exchangers, pipe flow, f low, and flow over a plate at a different temperature te mperature than the stream. #owever, in an! forced convection situation, some amount of natural convection is alwa!s present whenever there are g$forces present. %hen the natural convection is not negligible, such flows are t!picall! referred to as mixed convection. &he removal of excessive heat from s!stem components is essential to avoid damaging effects of burning or overheating. &herefore, the enhancement of heat transfer is an important subj subject ect of therm thermal al engi enginee neeri ring ng.. 'xten 'xtende ded d surfa surfaces ces (fins (fins)) are fre freue uentl ntl! ! use use in heat heat exchanging devices for the purpose of improve the heat transfer between a primar! surface and the surrounding fluid. arious arious t!pes of heat exchanger fins ranging from relativel! simple shapes, such as rectangular, suare, c!lindrical, annular, tapered or pin fins, to a combination of different geometries, have been used. &he stud! of improving heat transfer performance is referred to as heat transfer augmentation, enhancement or intensification. &he heat transfer augmentation is ver! important subject in industrial heat exchangers and other other thermal thermal applica applicatio tion. n. 'xtend 'xtended ed surface surfaces, s, which which are popula popularl! rl! known known as fins, fins, are extensivel! used in air$cooled automobile engines and in air$cooled aircraft engines. ins are also used for the cooling of computer processors, and other electronic devices. In various applications heat from the fins is dissipated b! natural as well as forced convection and radiation. ins are used as arra!s in all the applications.
THEORY
#eat transfer from an object can be improve b! increasing the surface area in contact with the air b! adding fins or pins normal to the surface. &his can be seen in *ewton+s aw of Cooling that states that the rate of heat loss of a bod! is proportional to the difference in temperatures between the bod! and its surroundings, which defines the convection heat transfer rate-
&he constant of proportionalit! h is termed the convection heat$transfer coefficient. &he heat transfer coefficient h is a function of the fluid flow, so, it is influenced b! the surface geometr!, the fluid motion in the boundar! la!er and the fluid properties as well. &he effect of the surfaces can be demonstrated b! comparing finned and unfinned surfaces with a flat plate under the same conditions of power and flow. heated surface dissipates heat to the surrounding fluid primaril! through a process called convection. #eat is also dissipated b! conduction and radiation, however these effects are not considered in this experiment. ir in contact with the hot surface is heated b! the surface and rises due to reduction in densit!. &he heated air is replaced b! cooler air, which is in turn heated b! the surface, and rises. &his process is called free convection. Convection heat transfer from an object can be improved b! increasing the surface area in contact with the air. In practical it ma! be difficult to increase the si"e of the bod! to suit. In these circumstances the surface area in contact with the air ma! be increased b! adding fins or pins normal to the surface. &hese features are called extended surfaces. t!pical example is the use of fins on the c!linder and head on an air$cooled petrol engine. &he effect of extended surfaces can be demonstrated b! comparing finned and pinned surfaces with a flat under the same conditions of power input and airflow.
APPARATUS
&he finned surface consists of / fins that are each 0.1m high and 0.023m wide. &he pinned surface consists of 14 pins that each have a diameter of 0.015m and are 0.023mlong.
Finned Surface
Pinned Surface
Convection Tet !ac"ine
Bae %eo&etr'
Sto#$atc"
Finned Surface
Pinned Surface
%ide 6 0.11 m
%ide 6 0.023 m
Diameter 6 0.015m
ength 6 0.1 m
ength 6 0.1 m
ength 6 0.023 m
PROCEDURE
1. 7lace the heat exchanger into the test duct an record the ambient temperature (T∞) 8. 7lace the finned and set the heater power control to () * . llow the temperature to rise to +, -C then adjust the heater power control to .,*/ 5. 9tart stopwatch wait ) &inute and recode the heated plate surface temperature (Ts). :. ;epeat step 0.1 then set the fan speed control to give reading of 2/, &3 one the thermal anemometer. llow ) &inute for the reading stabili"e and then again record surface temperature(Ts) <. &hen adjust the fan speed to ./, &3 recording the Ts for each speed.
RESU4T AND DATA ANA4YSIS
ir elocit!
#eater &emperature
&s > &?
#
(&s)(=C) 2/.< <3.5 :3.4
(=C) :<.< 5:.5 8:.4
(%@(m.=C)) 18.40 12.3< 85.:0
#eater &emperature
&s > &?
#
(&s)(=C) 22.1 <4.2 <8.8
(=C) :8.1 55.2 83.8
(%@(m.=C)) 2.<3 3.8: /.38
0 1 8 Pinned P5ate
Finned P5ate
ir elocit! 0 1 8
Sae ca5cu5ation6
Aase rea 6 x w 6 0.1 x 0.11 6 0.011 mB Finned #5ate
t air velocit! 6 0 &s > &?
6 22.1 > 8: 6 :8.1
Finned #5ate area 6 / x w Aase rea
6 / x 0.1 x 0.023 0.011
6 0.0488 mB
#eat transfer # 6 @s (&s > &?) # 6
[email protected] (:8.1) H 7 8/)+ *30&/-C1
Pinned #5ate
t air velocit! 6 0 &s > &?
6 2/.< $ 8:
6 :<.< =C Pinned #5ate area 6 14 x ED@8 Aase rea
6 14 x E x 0.015 x 0.023@8 0.011 6 0.05:2 mB #eat transfer # 6 @s (&s > &?) # 6
[email protected]:2 (58.3) H 7 2(/8.*30&/-C1
DISCUSSION
0 AMIRUL ASHRAF BIN HASSAN, 2013767009 1
Aased on the force convection experiment that we have been conducted, the heat transfer for a fixed surface temperature, are affected b! the heat transfer coefficient and t!pe of surface area. Aoth experiments are using the same value of air velocities which are 0, 1 and 8 m@s. Fn the first experiment that we did, we started with 0 m@s and b! using the finned platesG the heat transfer coefficient obtained from the calculation are 2.<3 %@ (m.c). s for 1 m@s the value that we obtained are 3.8: %@ (m.c), and lastl! for the 8 m@s, the heat transfer coefficients are /.38 %@ (m.c). *ext, the finned plates were removed and replace b! pinned plates. &he heat transfer coefficients that we managed to get for 0, 1 and 8 m@s are 18.40, 12.3<, and 85.:0 %@ (m.c). rom the result above, we plotted the graphs of air velocit! versus surface temperature (&s > &?). Aoth plates show same trend which are, as we increase the value of air velocities, the temperature difference value will decrease. &hrough m! observation,finned plate disseminated heat b! convection faster compared to the pinned surface plate.&hese happens due to the difference of the surface area that exposed to the air flow.&he larger surface area that exposed to the air flow,the greater heat convection occur on a heated bod!. &he more heat can be disseminate when surface area increased, thus prove the *ewton+s aw of Cooling where the heat transfer is proportional with the area of the surface. or this experiment, we have detected the few error that can affect the data that we got. &he error that we detect was the pinned and finned plate didn+t cooled b! the air velocit! but ma! cooled b! the air condition in the lab. It is because lab temperature in the lab is below the room tmeperature that is less than 82=. &he other error is, the value of the data is not so constant and because of that it can affect the result of the experiment.
CONC4USION 0 AMIRUL ASHRAF BIN HASSAN, 2013767009 1
Aased on the experiment,we can conclude that the finned plate can provide more surface area that exposed to the air flow which can improve the heat convection.&he fan in this experiment can be considered as Hforced convection element+ as it provides different velocit! of the air flow which gives different rate of heat transfer.astl!,we can obviousl! see the difference between the natural heat convection and forced convection as we can anal!se based on the data obtained. s a conclusion,we able to achieve our objective thus these experiment is successfull.
REFERENCES
•
http-@@en.wikipedia.org@wiki@orcedconvection
•
Jumar, ., K 9ah, L. (800:). &hermal 'ngineering. lpha 9cience
•
http-@@www.efunda.com@formulae@heattransfer@convectionforced@overviewforced.c fm
•
&.D. 'astop, . McConke!, pplied &hermod!namics or 'ngineering &echnologists
|
•
Nunus .Cengel, #eat and Mass &ransfer, &hird 'ditions (9I Onits) Mc Lraw #ill.
