1.0 TITLE

Forced Convection

2.0 OBJECTIVE

1. To demonstrate the use of fin (extended surface) in order to improve the heat

transfer in forced convection.

2. To perform calculation to find the heat transfer coefficient for each finned and pined

surfaces.

3.0 INTRODUCTION

Convection is heat energy transfers between a solid and a fluid when there is a

temperature difference between the fluid and the solid. Generally, convection heat transfer

cannot be ignored when there is a significant fluid motion around the solid. On the other

hand, convection is said to be the flow of heat through a bulk, macroscopic movement of

matter from a hot region to a cool region, as opposed to the microscopic transfer of heat

between atoms involved with conduction.

In order to make it clearer, consider heating up a local region of air. As this air heats,

the molecules spread out, causing this region to become less dense than the surrounding,

unheated air. Being less dense than the surrounding cooler air, the hot air will subsequently

rise due to buoyant forces. This movement of hot air into a cooler region is then said to

transfer heat by convection.

There are two types of convection. The first one is natural convection. Theoretically,

natural convection happens when the temperature of the solid due to an external field such

as fluid buoyancy can induce a fluid motion. It is a strong function of the temperature

difference between the solid and the fluid. Besides that, when the blowing air over the solid

by using external devices such as fans and pumps can also generate a fluid motion. This is

known as forced convection.

This report is going to discuss about forced convection only. In forced convection,

the fluid has a nonzero streaming motion in the farfield away from the body surface, caused

perhaps by a pump or fan or other driving force independent of the presence of the body.

Two major examples are duct flows and bodies immersed in a uniform stream. Since fluid

velocities are forced and may be large, heat transfer via forced convection will usually be

significantly larger than that in natural convection.

Heating a pot of water on a stove is a good example of the transfer of heat by

convection. When the stove is first turned on heat is transferred first by conduction between

the elements through the bottom of the pot to the water. However, eventually the water

starts bubbling - these bubbles are actually local regions of hot water rising to the surface,

thereby transferring heat from the hot water at the bottom to the cooler water at the top by

convection. At the same time, the cooler, more dense water at the top will sink to the

bottom, where it is subsequently heated.

Figure 1 The movement of heat by gas molecules.

4.0 THEORY

Heat transfer from an object can be

improved by increasing the surface area in

contact with the coolant i.e air by adding

fins or pins normal to the surface. From

the Newton’s Law of Cooling, the

convection heat transfer rate is:

Figure 2 The illustration of experimental

equipment.

Where;

Q̇= Power input

h = convection heat transfer coefficient

As = area of plate

Ts = heater temperature

T∞ = air temperature

For this experiment, we use finned plate and pinned plate to compare the effect of heat

transfer by each plates under the same conditions of power and flow.

To calculate the area for finned plate and pinned plate used, we use the equation shown below:

A finned plate=9×Lw+Abase

Apinned plate=17×πDL2

+Abase

Q̇=hA s(T s−T ∞)

5.0 APPARATUS AND EQUIPMENT

The surfaces are shown in the figure below. The finned surface consists of 9 fins that

are each of 0.1 m high and 0.068 m wide. The pinned surface consists of 17 pins that each

have a diameter of 0.013 m and are 0.068 m long.

Figure 3 Dimensions of finned and pinned surfaces.

Figure 4 Finned surfaces. Figure 5 Pinned surfaces.

Figure 6 Full experimental apparatus.

FATIN FASIHAH HARIS2008400898EMD5M3A

9.0 DISCUSSION

In this experiment, we were asked to determine the use of fin (extended surface) in

order to improve the heat transfer in forced convection. The experiment was conducted and all

data is jotted and tabulated in a form of a graph. By comparing the graph between pinned

surface and finned surface, I found that the pinned surface is having a larger slope compared to

finned surface. This means that finned surface is having smaller convection heat transfer

coefficient, h, rather than pinned surface.

There are few factors that needed to be considered in order to improve the heat

transfer in forced convection for finned surface. First is the temperature difference between two

fluids. Second is the heat transfer coefficient between the fluid and the tube wall. Third is the

surface area which the fluid is exposed to. After performing some calculations, we obtained that

the finned surface has larger surface (cross sectional) area compared to pinned surface. As the

heat transfer coefficient of the fluid inside the tube is several times larger than the fluid outside

the tube, the overall heat transfer rate can be improved by increasing the outside surface of the

tube. Although the temperature difference between the temperature surface and ambient

temperature is small, the result still give a small convection coefficient because the effect of

large area.

For pinned surface, according to its sample calculation, it gave a smaller cross sectional

area compared to finned surface. Although the temperature difference between the ambient

temperature and surface temperature is larger than finned surface, the result of coefficient of

convection still gives a small value. It is because, the major effect of coefficient of convection are

cross section area of pinned surface.

10.0 CONCLUSION

The objective of this experiment is to determine the use of fin (extended surface) in

order to improve the heat transfer in forced convection. In conclusion, the heat transfer in force

convection can be improved by using a finned surface compared to pinned surface. This is due to

the larger cross sectional area that is exposed to air. There are several factors that we need to

take in order to improve the heat transfer in force convection which are first, the temperature

difference between the two fluids. Second, the heat transfer coefficient between each of the

fluids and the tube wall and lastly, the surface area to which each fluid is exposed. Therefore, we

can say that the objectives are achieved and this experiment is conducted successfully.