Table of Content
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Summary/Abstract 2
Introduction and Objective 2
Theory 3
Description of Equipment Apparatus 4
Procedure 4
Data, Observations and Results 6
Analysis and Discussions 8
Discussions 13
Conclusions 15
References 15
Abstract
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An experiment was conducted to perform and demonstrate free and force convection heat transfer using different type extended surface plate. This following experiment outlines the proper procedure for determining these temperature distribution along an extended surface and consequently helps the student to demonstrate them graphically by doing the analysis based on the all the data and readings obtained.Heat transfer by convection between a surface and the surrounding fluid can be increased, by attaching thin strips of metal fins to the surface. When heat transfer takes place by convection from both interior and exterior surfaces of a tube or a plate, generally fins are used on the surfaces where the heat transfer coefficients are low.
Heat transfer by simultaneous conduction and convection, whether free or forced, forms the basis of most industrial heat exchangers and related equipment. The measurement and prediction of heat transfer coefficients for such circumstances is achieved in the free and forced convection heat transfer apparatus by studying the temperature profiles and heat flux in an air duct with associated flat and extended transfer surfaces.
Introduction and Objectives
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In this particular laboratory experiment, students are required to demonstrate the use of extended surfaces to improve heat transfer from a surface and to demonstrate convection heat transfer by using different type of extended surface. Besides that, the temperature distribution along an extended surface is also to be determined at the end of the experiment.In addition to that, this experiment is also useful in such a way that it helps to provide some exposure to the students so that they are able to interpret the obtained test data and at the same time are able to apply the theory they have learned in class.
Theory
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A heated surface dissipates heat to the surrounding fluid primarily through a process called convection. Heat is also dissipated by conduction and radiation, however these effects are not considered in this experiment. Air in contact with the hot surface is heated by the surface and rises due to reduction in density. The heated air is replaced by cooler air, which is in turn heated by the surface, and rises. This process is called free convection.In free convection small movements of air generated by this heat limit the heat transfer rate from the surface. Therefore more heat is transfer if the velocity is increase over the heated surface. This process of assisting the movement of air over the heated surface is called forced convection. A heated surface experiencing forced convection will have a lower surface temperature than that of the same surface in free convection, for the same power input.
Convection heat transfer from an object can be improved by increasing the surface area in contact with the air. In practical it may be difficult to increase the size of the body to suit. In these circumstances the surface area in contact with the air may be increased by adding fins or pins normal to the surface. These features are called extended surfaces. A typical example is the use of fins on the cylinder and head on an air-cooled petrol engine. The effect of extended surfaces can be demonstrated by comparing finned and pinned surfaces with a flat under the same conditions of power input and airflow.
Description of Experimental Apparatus
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Figure 1: Sketch diagram of Convention Heat Transfer Rig
Procedure
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Experiment 1
1. The fan assembly is removed from the top of the duct 2. The finned heat exchanger is placed into the test duct.
3. The power cable and temperature sensor cable is connected to the heat exchanger.
5. To accelerate the heating time, the heater power control is set to 70 watts (90 watts for pinned heat exchanger). Once the plate temperature reaches about 550C (700C for pinned heat exchanger), the heater power is set to 50 watts. 6. After a sufficient time is allowed for the heated plate to achieve steady state
condition, the heated temperature plate (tH) is recorded.
7. The distance of the access holes on the pinned and finned heat exchanger is measured from the black plate.
8. The temperature probe is inserted into the duct through the hole nearest to the heated plate, T1, ensuring that the tip of the probe is in contact with the pin.
9. The temperature of the next two holes on the fins. (T2 and T3) are measured.
Experiment 2
1. The fan assembly is placed on to the top of duct.
2. Insert the temperature probe is inserted into the duct through the hole nearest the heated plate, T1, ensuring that the tip of the probe is in contact with the pin
3. The fan speed control is set to low speed.
4. After the temperature has achieve steady state condition (~3 minutes), the temperature probe reading is noted
5. This procedure is repeated at medium and maximum speed
6. The finned heat exchanger is removed and is then replaced with the pinned heat exchanger.
Data and Observations
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Input Power = 70 WAmbient air temperature (tA) = 28.6 0C
TEMPERATURE LOCATION T (0C)
FINNED PLATE PINNED PLATE
(T- tA) (T-tA)
HEATED PLATE TEMP, tH 50.0 21.4 50.0 21.4
T1 53.7 25.1 50.8 22.2
T2 52.9 24.3 57.9 29.3
T3 52.1 23.5 60.1 31.5
TABLE 2.1: FREE-CONVECTION RESULTS
FAN SPEED FINNED HEAT EXCHANGER PINNED HEAT EXCHANGER T1 T2 T3 T1 T2 T3 0 30.9 38.5 43.2 50.8 57.9 50.8 LOW 32.0 38.4 45.3 57.2 53.3 41.9 MEDIUM 31.6 33.6 38.6 48.1 42.6 35.5 HIGH 31.6 32.6 36.4 43.5 43.6 32.8
TABLE 2.2 EXTENDED SURFACE TEMPERATURE AT DIFFERENCE FAN SPEED FAN SPEED FINNED HEAT EXCHANGER PINNED HEAT EXCHANGER HEATER TEMP, TH (TH-TA) HEATER TEMP, TH (TH-TA) 0 31.8 3.2 50.0 21.4 LOW 34.6 6.0 60.6 32.0 MEDIUM 37.0 8.4 58.2 29.6 HIGH 38.1 9.5 58.3 29.7
FAN SPEED
RATE OF HEAT TRANSFER, q (W) FINNED HEAT EXCHANGER PINNED HEAT EXCHANGER 0 0.896 2.447 LOW 1.680 3.659 MEDIUM 2.352 3.385 HIGH 2.660 3.396
TABLE 2.4 RATE OF HEAT TRANSFER AT DIFFERENCE FAN SPEED
Sample of Calculation
For finned heat exchanger:
For pinned heat exchanger:
Analysis and Results
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All the required graphs are shown on the following pages:Graph 1: Graph of Temperature (T-tA), versus distance for each plate
Graph 2: Graph of Temperature (tH-tA) against fan speed for each of the plates
Graph 3: Graph of extended surface temperature against distance from the black plate for both heat exchangers at various fan speed
Graph 1: Graph of Temperature (T-tA), versus distance for each plate 0 5 10 15 20 25 30 35
heated plate temp. t1 T2 T3
Tem p e ratu re Distance
Graph of Temperature against Distance
Finned plate Pinned plate
Graph 2: Graph of Temperature (tH-tA) against fan speed for each of the plates 0 5 10 15 20 25 30 35
0 LOW MEDIUM HIGH
Tem p e ratu re Fan Speed
Graph of Temperature against Fan Speed
Finned heat exchanger Pinned heat exchanger
Graph 3: Graph of extended surface temperature against distance from the black plate for both heat exchangers at various fan speed 0 10 20 30 40 50 60 70
0 LOW MEDIUM HIGH
Tem p e ratu re Extendard Surface
Graph of Extended Surface against Temperature.
T1 (finned) T2 (finned) T3 (finned) T1 (pinned) T2(pinned) T3(pinned)
Graph 4: Graph of heat transfer rate against fan speed for both exchangers 0 0.5 1 1.5 2 2.5 3 3.5 4 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 R ate o f H e at Tr an sf e r Fan Speed
Graph of Heat Transfer Rate against Fan Speed
FINNED HEAT EXCHANGER PINNED HEAT EXCHANGER
Discussions
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Data and Graph AnalysisFrom Data, Observation and Results section of this report, we had plotted the graph where we can see the relationship between the temperature and distance for each plate. From Graph 1, for a finned plate, initially when we heated the plate the temperature is increase. After a few moment, the temperature drop linearly until T3 distance. On the other hand, for the pinned plate the temperature increases constantly until T3.
As for Graph 2, it can clearly be shown we can see that pinned heat exchanger have high temperature difference compared to finned heat exchanger. This indicates that, pinned plate release less heat compare to finned plate.
Besides that, we can come up with a few relationships by observing the pattern of the curves illustrates by Graph 3. For both pinned and finned heat exchanger, at position T2 and T3, the temperature seems to be decreasing as the speed of the fan increases. In contrary, at position T1, as the speed of the fan went up from 0 to the highest speed, the temperature for finned heat exchanger increases while for pinned heat exchanger, the temperature decreases.
Comments on the correlation between total surface area of the heat exchanger and the temperature achieved and which of the extended surfaces has greater surface area.
For finned plate, it is square in shape. The heat release from the plate is high, and thus the temperature stored is low. On the other hand, for pinned plate, it is cylinder in shape. The heat release from the plate is low compared to finned plate. Thus, the temperature stored in the pinned plate is high.
For a heat exchanger with 100 % efficiency, the whole of the extended surface should be at the same temperature as the backplane, why this is not achievable in the experiment?
According to thermodynamic law of conduction of heat transfer, total amount of energy transferred will not be 100 % efficiency because they will be have some minor loses during the heat conduction. Technically, when the heat is transferred from one medium to another, there will be some friction that will cause some significant heat losses.
Which extended surface have higher heat transfer rate? Why?
Pinned plate have high heat transfer rate. It is because it is cylinder in shape. The cylinder shape basically made the pinned plate released heat in slow rate.
Conclusions
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From this experiment, we can see that the use of extended surfaces enhance heat transfer from a surface. It is found that finned extended surface releases heat faster than pinned extended surface due to higher surface area.In theory, higher flow velocity would encourage convection heat transfer. This theory is clearly illustrated in Graph 4 where rate of heat transfer increases as the fan speed increases.
Generally, the temperature decreases as we measure from position 1 to 3. This temperature distribution pattern is depicted in Graph 3. However, discrepancy may occur due to some errors. For instance, the inconsistency of the fan speed that caused by worn out equipment may greatly affect the entire experiment.
In conclusion, this experiment can be consider as successful since all the objectives have been covered.
References
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Instruction manual from the Heat Transfer & Applied Thermodynamics Lab2012, Convection Heat Transfer,
http://www.engineeringtoolbox.com/convective-heat-transfer-d_430.html
2012, Wikipedia, Convection
http://en.wikipedia.org/wiki/Convection
2012, Wikipedia, Heat Exchanger
2012, The theory behind heat transfer
http://www.distributionchalinox.com/produits/alfa-laval/echangeurs/heat-transfer-brochure.pdf
Incropera, DeWitt, Bergmann, Lavine, Fundamentals of Heat and Mass Transfer, 7th Edition, Wiley Asia Student Edition
Yunus A. Cengel, Michael A. Boles, Thermodynamics An Engineering Approach, 7th Edition, Mc Graw Hill
