OBJECTIVES
To To demonstrate the relationship between power input and surface
temperature in forced convection. To To demonstrate the use of extended surface to improve heat transfer from
the surface. To To determine determine the temperature temperature distribution distribution along an extended extended surface. surface.
SUMMARY The main purpose of this experiment experiment is to demonstrate the relationship between the power input and the surface temperature in forced convection and also to demonstrate the use of an extended surface to improve heat transfer from the surface. Furthermore, another main purpose of this experiment is to determine the temperature temperature distribution along an extended surface. The variables involved in this experiment are air velocity (m/s), ambient air temperature, t ( !), plate temperature, t " ( !), power (#atts), and surface temperature, t " $ t °
°
( !). %ased on the results ac&uired, the air velocity increases when the surface °
temperature decreases for all three types of plates which are the 'nned, pinned, and at plates. This shows that the air velocity is inversely proportional to the surface temperature. t was proven through experimentation that an ob*ect with a wider area has more surface particles wor+ing to transfer heat. s such, the rate of heat transfer is directly proportional to the surface area through which the heat is being transferred. n experiment 1, at air velocities of .- m/s, 1. m/s, and 1.- m/s, the surface temperatures for 'nned plate are .0 !, .1 !, and 12.1 ! respectively. n experiment , at air velocities of . m/s, 1. m/s, and 1.- m/s, the surface temperatures for at plate are 03.4 !, 04.4 !, and 0-.0 ! respectively. s for the pinned plate, at air velocities of . m/s, 1. m/s, and and 1.1.- m/s, m/s, the the surf surfac ace e temp temper erat atur ures es are are -. -. !, !, 5.5 5.5 !, !, and and 13.0 13.0 ! respectively. Trend graphs were created to demonstrate each of the operation6s relation relationship ship between between the air velocity velocity and the surface surface temperatu temperature re.. From rom that, base based d on the the resul esults ts achi achiev eved ed and and the the theo theory ry stat stated ed,, the the ob*e ob*ect ctiv ives es wer were achieved achieved which were were to demonstra demonstrate te the correlation correlation between between the power input and the surface temperature in forced convection, to demonstrate the use of an extended surface to improve heat transfer from the surface and to determine the temperature temperature distribution along an extended surface.
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INTRODUCTION !onvection, also +nown as !onvective "eat Transfer, is a circulatory motion that occurs in a uid (li&uid/gas) in which, depending on density, the warmer parts move up and the colder parts move down. n this experiment, a speci'c type of convection is investigated on which is forced convection. 7nli+e natural convection (has a slower heat transfer because of limited air velocity), forced convection uses an external source, a fan for example, to aid in the movement of air over a heated surface for a more e8cient heat transfer. This experiment is divided into two parts9 :xperiment 1 and :xperiment . The set;up is as shown in Figure 1 (
°
$ t ( !). °
For the 'rst experiment, a demonstration on the relationship between power input and surface temperature in forced convection is conducted. The experiment begins by 'rstly ensuring that the power supply for the system is switched o= and the wires involved are plugged in. Then, the 'nned heat exchanger is placed into the test duct and the system is switched on. %efore proceeding, record the ambient air temperature, t . The heater power connected to the test duct will then be set to - #atts and is allowed to stabili>e for 1 minutes. fter that, the fan speed is set to .- m/s and is allowed to stabili>e for 5 minutes. #hen the readings are steady, the temperature of the heat exchanger, t" is recorded. ?imilar steps will be conducted for fan speed of 1. m/s and 1.- m/s. For the second experiment, the use of extended surface to improve heat transfer from the surface is demonstrated. The experiment begins by 'rstly ensuring that the power supply for the system is switched o= and the wires involved are plugged in. Then, the at plate heat exchanger is placed into the test duct and the system is switched on. %efore resuming, record the ambient air temperature, t. The heater power connected to the test duct will then be set to 4- #atts and then wait patiently until the temperature rises to 2 !. @ater, tone °
down the heater power to #atts and set the air velocity to m/s and wait for - minutes. fter the readings have stabili>ed, record the temperature of the heat exchanger, t". ?imilar steps are ta+en but with air velocities of 1. m/s and 1.m/s.
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THEORY
NEWTON’S LAW OF COOLING The Aewton6s @aw of !ooling states that the rate of heat loss of a body is proportional to the di=erence in temperatures between the body and its surroundings Bnonymous , 15C. The heat loss due to the forced convection is given by the following e&uation D
) , T ; T ( h E E I
#here D I E dJ/dt is rate at which heat is transferred h
E !onvection heat;transfer coe8cient
E :xposed surface area
T
E Temperature of the immersed ob*ect
The convection heat;transfer coe8cient (h), can only described by e&uations based on empirical analysis. For example, the h of air is approximately e&uals to D h E 1.0- $ v 1Gv #here v is the relative speed of the ob*ect through the air or the speed of the air around the ob*ect. This e&uation is valid for speed from to m/s BHlenn :lert, 1332;15C.
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HOW SURFACE AREA AFFECTS THE RATE OF HEAT TRANSFER nother variable that a=ects the rate of convective heat transfer is the area through which heat is being transferred. n ob*ect with a wider area has more surface particles wor+ing to transfer heat. s such, the rate of heat transfer is directly proportional to the surface area through which the heat is being transferred Bnonymous %, 133;15C. Extended surfaces are extensions of an obect to increase t!e rate of !eat transfer to or fro" t!e en#iron"ent b$ increasin% con#ection& T!e a"ount of conduction' con#ection' or radiation of an obect deter"ines t!e a"ount of !eat it transfers& ($ addin% an extended surface on an obect' increases t!e surface area !ence increasin% t!e a"ount of con#ection )Anon$"ous C' *+,-.&
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DISCUSSION :xperiments to demonstrate the relationship between power input and surface temperature, usage of extended surface to improve heat transfer from the surface, as well as to determine the temperature distribution along an extended surface were conducted using the set;up shown in Figure 1 (?hown in ppendices). The parameters involved in this experiment are air velocity (m/s), ambient air temperature, t ( !), plate temperature, t " ( !), power (#atts), and °
°
the surface temperature, t " $ t ( !). °
Two experiments were conducted during this session9 :xperiment 1 and :xperiment . n :xperiment 1, a 'nned heat exchanger was used to demonstrate the relationship between power input and surface temperature using air velocities of .- m/s, 1. m/s, and 1.- m/s. fter setting up the needed velocity, the plate temperature, t " was recorded. Kn the other hand for :xperiment , at and pinned plate heat exchangers were used to demonstrate the use of extended surface to improve heat transfer with air velocities of m/s, 1. m/s, and 1.- m/s. fter setting up the wanted speed, the plate temperature, t" was recorded. The temperatures between the two types of plates were then compared to see which improves heat transfer better. For both experiments, an ambient air temperature, t of 5. ! was obtained. °
For :xperiment 1, the values for velocity;surface temperature for the 'nned heat exchanger are .- m/s;.0
!, 1. m/s;.1
°
°
!, and 1.- m/s;12.1
!. For
°
experiment , the values for velocity;surface temperature for the at plate are m/s;03.4 !, 1. m/s;04.4 !, 1.- m/s;0-.0 !. #hereas for the pinned plate, °
°
°
values of m/s;- !, 1. m/s;5.5 !, and 1.- m/s;13.0 ! were obtained. °
°
°
Through results, it can be concluded that the plate that has the highest rate of heat transfer is pinned plate, followed by 'nned plate, and lastly at plate.
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Theoretically, Aewton6s @aw of !ooling states that the rate of heat loss (heat transfer) of a body is proportional to the di=erence in temperatures (t " ; t) between the body and its surroundings. nother variable that a=ects the rate of heat transfer is the area through which heat is being transferred. n ob*ect with a wider area has more surface particles wor+ing to transfer heat. s such, the rate of heat transfer is directly proportional to the surface area through which the heat is being transferred. %y adding an extended surface on an ob*ect, will increase the surface area hence increasing the amount of convection. "ence, the 'nned plate has relatively high heat transfer, followed by the pinned plate, and lastly the at plate. The statements were endorsed through the plotting of !orrelation charts/Temperature pro'les. %ased on the results, it can be observed that the results have much complied with the theory stated. #ith that, the ob*ectives of this experiment were achieved. There are several possibilities that might have contributed to the errors that occurred during the experiment. mong those errors is physical errors (caused by experimenters). The experimenters might not have waited for the readings to stabili>e 'rst and have recorded down the wrong readings, which could lead to an abnormal trend of results. Aot *ust that, the experimenter may not have focused well during the experiment and may have recorded down the readings of the parameter in the 'eld of another parameter. %y doing so, it will disrupt the results, and the trend graphs will not result as expected. Kther than that, the experimenter may be careless and accidentally set a higher/lower power supply than it was supposed to thus, resulting in di=erent temperature values. %esides that, the wires or other e&uipment involved may be faulty or not plugged in. #hen this happens, the panel will not display the correct value.
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CORRELATION CHARTS/TE01ERATURE 1ROFILES
A graph between Air Veloit! "#$%& again%t S'r(ae Te#perat're ")C& 1./ 1.0 1. 1.,
Air Veloit! "#$%&
,.2
Finned Ilate
,./ ,.0 ,. ,., 141213,150-/4
S'r(ae Te#perat're ")C&
A graph between Air Veloit! "#$%& again%t S'r(ae Te#perat're ")C& 1./ 1.0 1. 1.,
Air Veloit! "#$%&
Flat Ilate
,.2
Iinned Ilate
,./ ,.0 ,. ,., 1- , - 5, 5- 0, 0- -, --
S'r(ae Te#perat're ")C&
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CONC*USION n conclusion, the correlation chart plotted showed the trend of the surface temperature. Through that, we can determine the plate which has the highest rate of heat transfer to the plate with the lowest rate of heat transfer. Through this experiment, we have learned that surface area and the speed of the air around the body plays a huge role in convection. %y adding an extended surface on an ob*ect will in return increases the surface area hence increasing the amount of convection. This experiment revolves mostly around the Aewton6s @aw of !ooling which states that the rate of heat loss of a body is proportional to the di=erence in temperatures between the body and its surroundings. %ased on results, in descending order of the rate of heat transfer of the plates9 pinned plate has the highest rate of heat transfer, followed by 'nned plate, and lastly at plate. This is because x plate has an extended surface, together with the widest surface area causing it to transfer/release even more heat at an instance. The results mentioned have very much supported by the theory stated, thus it can be deduced that the ob*ectives of this experiment were achieved.
RECOMMENDATIONS There are steps that can be ta+en to prevent these types of errors from occurring. To prevent physical errors (caused by experiments) from occurring, experimenters have to focus and be patient for the readings to stabili>e before recording any data. lso, wor+ together to record data, and not *ust be dependent on *ust a team member. Aext, in order to prevent recording the wrong data, team members should recon'rm with each other on the results to ac&uire the readings which best 't. %esides that, to prevent conducting a slow process, those who conduct the experiment should read the lab manual prior to conducting
the
experiment.
Furthermore,
even
before
conducting
the
experiment, each team should re&uest assistance from available technicians to chec+ whether the experiment is faulty or not, to avoid unwanted results.
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TUTORIA*S
E21ERI0ENT ,
1. Ilot a graph of air velocity against surface temperature (t " ; t) for the 'nned plate.
A graph between Air Veloit! "#$%& again%t S'r(ae Te#perat're ")C& 1./ 1.0 1. 1.,
Air Veloit! "#$%&
,.2
Finned Ilate
,./ ,.0 ,. ,., 141213,150-/4
S'r(ae Te#perat're ")C&
. !omment on the correlation between the velocity of the air and the surface temperature.
t is observed from the results obtained from the actual experiment that as the ir Lelocity (m/s) increases, the ?urface Temperature, t " ; t (!) decreases. #ith that being said, this shows an inversely proportional relationship between ir Lelocity (m/s) against ?urface Temperature.
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E21ERI0ENT *
1. Ilot graphs of air velocity against surface temperature for each at plate and pinned plate.
A graph between Air Veloit! "#$%& again%t S'r(ae Te#perat're ")C& 1./ 1.0 1. 1.,
Air Veloit! "#$%&
Flat Ilate
,.2
Iinned Ilate
,./ ,.0 ,. ,., 1- , - 5, 5- 0, 0- -, --
S'r(ae Te#perat're ")C&
. !omment on the correlation between total surface of the plate and the surface temperature for both at and pinned plate.
The total surface area of the plate a=ects greatly on the surface temperature on both Flat and Iinned Ilates. s observed on the correlation graph generated based on the results obtained from the experiment conducted, the ?urface Temperature, t " ; t (!) for Iinned Ilate has a more drastic increment than the Flat Ilate. This shows that the total surface area of the plate of the Iinned Ilate and the Flat Ilate are both a=ected by the ?urface Temperature. ?ince the Iinned Ilate has an extended surface, it aids tremendously in heat transfer/loss as it has a larger surface area in which increases its rate of convection. s for the Flat Ilate, it has a at surface area in which has a lower rate of heat transfer/loss compared to the plates that have extended surfaces which leads to a lover convection rate.
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5. f the same experiment is run by using the 'nned plate, which one from the three plates will have more e8cient heat transferM Niscuss.
The Finned Ilate would have more e8cient heat transfer compared to the other plates (Iinned and Flat). t is because the Finned Ilate has the widest extended surface compared to the other plates (Iinned and Flat). #ith a wider extended surface, more heat transfer/loss will be released and a faster convection rate is developed.
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RE+ERENCES
B1C Bnonymous , 15C, !onvective "eat Transfer, httpD//en.wi+ipedia.org/wi+i/!onvectiveOheatOtransfer, B14th Kctober 15C BC BHlenn :lert, 1332;15C, The Ihysics "ypertextboo+, httpD//physics.info/convection/, B14th Kctober 15C B5C Bnonymous %, 133;15C, httpD//www.physicsclassroom.com/class/thermalI/u12l1f.cfm , B14th Kctober 15C B0C Bnonymous !, 15C, :xtended ?urface, httpD//en.wi+ipedia.org/wi+i/FinO(extendedOsurface), B14th Kctober 15C B-C Transport Irocess @aboratory Panual, Forced !onvection "eat Transfer (:xperiment ), B14th Kctober 15C
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A,,ENDICES
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