OBJECTIVEDemonstrate the use of a fin (extended surface) to improve the heat transfer in forced convection.

INTRODUCTIONConvectionis the movement of molecules withinfluids. It 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 ofheat transferandmass transfer.Forcedconvectionis a mechanism, or type of heat transport in which fluid motion is generated by 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 energy can be transported very efficiently and this mechanism is found very commonly in everyday life, includingcentral heating,air conditioning,steam turbinesand in many other machines. Forced convection is often encountered by engineers designing or analyzingheatexchangers, pipe flow, and flow over a plate at a different temperature than the stream. However, in any forced convection situation, some amount of natural convection is always present whenever there areg-forces present. When the natural convection is not negligible, such flows are typically referred to as mixed convection.The removal of excessive heat from system components is essential to avoid damaging effects of burning or overheating. Therefore, the enhancement of heat transfer is an important subject of thermal engineering. Extended surfaces (fins) are frequently use in heat exchanging devices for the purpose of improve the heat transfer between a primary surface and the surrounding fluid. Various types of heat exchanger fins ranging from relatively simple shapes, such as rectangular, square, cylindrical, annular, tapered or pin fins, to a combination of different geometries, have been used. The study of improving heat transfer performance is referred to as heat transfer augmentation, enhancement or intensification.The heat transfer augmentation is very important subject in industrial heat exchangers and other thermal application. Extended surfaces, which are popularly known as fins, are extensively used in air-cooled automobile engines and in air-cooled aircraft engines. Fins are also used for the cooling of computer processors, and other electronic devices. In various applications heat from the fins is dissipated by natural as well as forced convection and radiation. Fins are used as arrays in all the applications.

THEORYHeat transfer from an object can be improve by increasing the surface area in contact with the air by adding fins or pins normal to the surface. This can be seen in Newtons Law of Cooling that states that the rate of heat loss of a body is proportional to the difference in temperatures between the body and its surroundings, which defines the convection heat transfer rate:

The constant of proportionalityhis termed theconvection heat-transfer coefficient.The heat transfer coefficient h is a function of the fluid flow, so, it is influenced by the surface geometry, the fluid motion in the boundary layer and the fluid properties as well. The effect of the surfaces can be demonstrated by comparing finned and unfinned surfaces with a flat plate under the same conditions of power and flow.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.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.

APPARATUSThe finned surface consists of 9 fins that are each 0.1m high and 0.068m wide. The pinned surface consists of 17 pins that each have a diameter of 0.013m and are 0.068mlong. Finned Surface Pinned Surface

Convection Test Machine Stopwatch

Base GeometryFinned SurfacePinned Surface

Wide = 0.11 mWide = 0.068 mDiameter = 0.013m

Length = 0.1 mLength = 0.1 mLength = 0.068 m

PROCEDURE1. Place the heat exchanger into the test duct an record the ambient temperature (T)2. Place the finned and set the heater power control to 75 W. Allow the temperature to rise to 80 C then adjust the heater power control to 20W.3. Start stopwatch wait 5 minute and recode the heated plate surface temperature (Ts).4. Repeat step (2) then set the fan speed control to give reading of 1.0 m/s one the thermal anemometer. Allow 5 minute for the reading stabilize and then again record surface temperature(Ts)5. Then adjust the fan speed to 2.0 m/s recording the Ts for each speed.

RESULT AND DATA ANALYSISAir VelocityHeater Temperature (Ts)(C)Ts T(C)H(W/(m.C))

069.545.512.70

158.334.316.85

248.724.723.40

Pinned Plate

Finned PlateAir VelocityHeater Temperature (Ts)(C)Ts T(C)H(W/(m.C))

066.142.16.58

157.633.68.24

252.228.29.82

Sample calculation: Base Area = L x w = 0.1 x 0.11 = 0.011 mFinned plateAt air velocity = 0Ts T = 66.1 24= 42.1Finned plate area = 9 x Lw + Base Area = 9 x 0.1 x 0.068 + 0.011 = 0.0722 m

Heat transfer H = Q/As (Ts T)H = 20/0.0722 (42.1)H = 6.58 W/(m.C)

Pinned plateAt air velocity = 0Ts T= 69.5 - 24 = 45.5 CPinned plate area = 17 x DL/2 + Base Area = 17 x x 0.013 x 0.068/2 + 0.011 = 0.0346 mHeat transfer H = Q/As (Ts T)H = 20/0.0346 (32.8)H = 17.62W/(m.C)

DISCUSSION (AMIRUL ASHRAF BIN HASSAN, 2013767009)

Based on the force convection experiment that we have been conducted, the heat transfer for a fixed surface temperature, are affected by the heat transfer coefficient and type of surface area. Both experiments are using the same value of air velocities which are 0, 1 and 2 m/s. On the first experiment that we did, we started with 0 m/s and by using the finned plates; the heat transfer coefficient obtained from the calculation are 6.58 W/ (m.c). As for 1 m/s the value that we obtained are 8.24 W/ (m.c), and lastly for the 2 m/s, the heat transfer coefficients are 9.82 W/ (m.c). Next, the finned plates were removed and replace by pinned plates. The heat transfer coefficients that we managed to get for 0, 1 and 2 m/s are 12.70, 16.85, and 23.40 W/ (m.c).From the result above, we plotted the graphs of air velocity versus surface temperature (Ts T). Both plates show same trend which are, as we increase the value of air velocities, the temperature difference value will decrease. Through my observation,finned plate disseminated heat by convection faster compared to the pinned surface plate.These happens due to the difference of the surface area that exposed to the air flow.The larger surface area that exposed to the air flow,the greater heat convection occur on a heated body. The more heat can be disseminate when surface area increased, thus prove the Newtons Law of Cooling where the heat transfer is proportional with the area of the surface.For this experiment, we have detected the few error that can affect the data that we got. The error that we detect was the pinned and finned plate didnt cooled by the air velocity but may cooled by the air condition in the lab. It is because lab temperature in the lab is below the room tmeperature that is less than 26. The other error is, the value of the data is not so constant and because of that it can affect the result of the experiment.

CONCLUSION (AMIRUL ASHRAF BIN HASSAN, 2013767009)

Based 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.The fan in this experiment can be considered as forced convection element as it provides different velocity of the air flow which gives different rate of heat transfer.Lastly,we can obviously see the difference between the natural heat convection and forced convection as we can analyse based on the data obtained. As a conclusion,we able to achieve our objective thus these experiment is successfull.

REFERENCES

http://en.wikipedia.org/wiki/Forced_convection Kumar, A., & Sah, G. (2004). Thermal Engineering. Alpha Science http://www.efunda.com/formulae/heat_transfer/convection_forced/overview_forced.cfm T.D. Eastop, A. McConkey, Applied Thermodynamics For Engineering Technologists 5th Edition, Pearson Prentice Hall, 1993. Yunus A.Cengel, Heat and Mass Transfer, Third Editions (SI Units) Mc Graw Hill.