force convection experiment
DESCRIPTION
Fluid dynamic lab experiment entitle Force ConvectionTRANSCRIPT
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.