FREE AND
FORCED
CONVECTI0N
TEAM
MECHANICS
Members: Mohammed
Nadeem, Asilika Seru,
Ruben Molu, Priyansha
Sharma, Teeta Titti
Supervisor(s): Deepak
Prasad
FREE AND FORCED CONVECTION
AIM/GOALS/PURPOSE
•
To understand the characteristics of heat
transfer between free convection and forced
convection
•
Determine how the surface area and factors that
hinders the heat transfer in convection.
TABLE OF CONTENT
Introduction
Literature Review

The heat transfer mechanisms involved

Factors that influence the heat transfer

Equations related to the project

What are the expected results you might obtain?
Equations related to the project
Methodology
Conclusion
Reference
FREE AND FORCED CONVECTION
INTRODUCTION
• The study of heat transport processes that are influenced by the
flow of fluids is known as CONVECTIVE HEAT TRANSFER or
simply CONVECTION.
• Convectional heat transfer is the transfer of heat from one
place to another by the movement of fluids (liquids or gases).
This can occur through natural convection, where the
movement of fluid is caused by temperature differences within
the fluid, or through forced convection, where the fluid is
forced to move by an external source such as a fan or pump.
5
FREE AND FORCED CONVECTION
TYPES OF CONVECTION
• Natural (Free) Convection
• Forced Convection
6
FREE AND FORCED CONVECTION
LITERATURE REVIEW
 The heat transfer mechanisms involved
 Factors that influence the heat transfer
 Equations related to the project
 What are the expected results you might obtain?
7
FREE AND FORCED CONVECTION
FREE CONVECTION
• It is a mechanism, or kind of mass and heat transfer,
where fluid motion is produced solely by density
differences within the fluid that occur as a result of
temperature gradients, not by any external source.
(like a pump, fan, suction device, etc.).
8
FREE CONVECTION
• It uses the natural buoyancy of the fluid to drive the heat transfer process.
• Free convection originates when a body force (gravity) acts on a fluid containing
density gradients that are caused by temperature difference.
Free and Forced Convection
9
FREE AND FORCED CONVECTION
FREE CONVECTION
 HEAT TRANSFER MECHANISMS INVOLVED
• To increase the heat transfer surface area and thereby decrease the resistance of the boundary layer, plate-fin
heat sinks are frequently used in natural convection.
• Fluid motion due to buoyancy force
• Figure:
10
FREE AND FORCED CONVECTION
EXAMPLES
Natural convection is used
to cool a boiled egg in a
cooler environment.
Natural convection to
warm a cold drink in a
warmer environment
11
EXAMPLES
• Electronic Cooling Package (Fins)
Presentation title
12
EXAMPLES
• Cooling coil at the back of the refrigerator
FREE AND FORCED CONVECTION
13
EXAMPLES
• Electric / Water / Steam Radiators
Presentation title
14
FREE AND FORCED CONVECTION
FACTORS THAT INFLUENCE THE HEAT
TRANSFER
•
Fin Geometry
Determining width and length must be considered in the selection of the number of fins.
More fins result in
 more surface area and boundary layer resistance is reduced
 Decrease fin spacing, therefore, decrease the heat transfer
•
Fluid Properties
Every time the flow rate increases, the heat transfer also increases.
•
The difference in temperature between the surface and the surrounding fluid
As a result, the greater the temperature difference between an object and its surroundings
(air, for example), the faster heat energy is transferred.
15
FREE AND FORCED CONVECTION
EQUATION
• 𝑸 = 𝒉𝑨 𝑻𝑾 − 𝑻∞
•
Q = heat transfer rate [w or J/s]
•
h = convective heat transfer coefficient [w/m. K]
•
A = surface or exposed area
•
𝑇𝑊 = wall of temperature [℃ ]
•
𝑇∞ = freestream fluid temperature [℃]
The convective heat transfer coefficient ‘h’ is a measure of how easily heat can be
transferred between the fluid and the object, and depends on properties of the
fluid such as its density, viscosity and thermal conductivity as well as the velocity
of the fluid and the geometry of the project.
16
DIMENSIONLESS PARAMETERS
Grashof Number
ratio of the buoyancy force to the vicious force.
Where;
g - gravity.
β - same as expansion coefficient.
Ts - temperature of the surface of the plate.
L- length of the plate.
V - kinematic viscosity.
Presentation title
17
DIMENSIONLESS PARAMETERS
Reynolds Number
Reynolds number can be defined as the ratio of inertial forces to viscous forces.
Where:
ρ – Density
v – Velocity.
d – Diameter
μ –Viscosity
The Reynolds number is used to determine whether a fluid is in laminar or turbulent
flow [1].
Presentation title
18
DIMENSIONLESS PARAMETER
Prandtl Number
It is defined as the ratio of momentum diffusivity (kinematic viscosity) to thermal
diffusivity.
Where;
V – Momentum diffusivity.
α – thermal diffusivity.
Presentation title
19
DIMENSIONLESS PARAMETER
Raleigh Number
It is a measure of the instability of a layer of fluid due to differences of temperature and
density at the top and bottom.
Where;
g- gravity
β- thermal expansion coefficient
ΔT – Temperature difference
L – length
V - kinematic viscosity coefficient
α – thermal diffusivity
Presentation title
20
Expected Results
The fins made of aluminum are expected to lose
heat at a very slow rate compared to a forced
convection system.
21
FREE AND FORCED CONVECTION
FORCED CONVECTION
• In a forced convection mechanism, fluid motion is
produced by an outside source (such as a pump, fan,
suction device, etc.).
22
FREE AND FORCED CONVECTION
FORCED CONVECTION
HEAT TRANSFER MECHANISMS INVOLVED
• Heat Transfer in Forced Convection is enhanced by Fluid Motion (the higher the velocity the higher
the heat transfer).
Figure
• The convective heat transfer coefficient “h” is strongly influenced by fluid properties, solid surface
roughness, and fluid flow (Laminar & Turbulent)
23
FREE AND FORCED CONVECTION
FACTORS THAT INFLUENCE THE
HEAT TRANSFER
• LAMINAR FLOW
• Is the motion of a fluid in which each particle follows the same path as the particles
before it.
• Its viscosity is high
• Its velocity is low
• Eg. Honey
• Figure:
24
FREE AND FORCED CONVECTION
FACTORS THAT INFLUENCE THE
HEAT TRANSFER
• TURBULENT FLOW
• Type of fluid (gas or liquid) flow in which the fluid experiences
irregular fluctuations or mixing.
• Its viscosity is low
• The speed of the fluid at a point continuously changes in both
magnitude and direction
Figure:
25
FREE AND FORCED CONVECTION
SURFACE ROUGHNESS
• Surface roughness can have a significant impact on fluid dynamics & heat
transfer in convective flows by causing perturbations in the velocity profile,
which affect surface drag, turbulent mixing & heat transfer.
•
Figure:
16
FREE AND FORCED CONVECTION
EXAMPLES
27
METHODOLOGY
1. The heat exchanger will be placed into the test duct.
2. Heat the electrical coil and leave it to cool at room temperature without any external sources
with free convection whereas forced convection heats the electrical coil heater and cools it using
a centrifugal fan with a throttle plate.
3. Measure the flow rate using an air velocity sensor by placing both at the ends of the velocity
sensor.
4. The anemometer will be placed on top of the air duct to measure air velocity from the outlet.
5. The centrifugal fan with the throttle plate blows air into the duct from the bottom then it
absorbs air from its surrounding and blows it into the channel above the inlet side and is
controlled by the throttle plate.
6. The heat transfer service unit is turned on and the voltage is increased, which increases the
current through the heater, therefore, results in heat loss.
FREE AND FORCED CONVECTION
28
CONCLUSION
 The aim of the Project is achieved whereby factors affecting the heat has been
identified and analysed.
 Two commonly used heat transfer mechanisms are free and natural convection.
 The movement of heat from a high temperature location to a lower temperature
area is a natural phenomenon that integrates the laws of thermodynamics.
 Both cases the actual heat transfer depends on the surface ability to accept heat.
 Temperature differential between two states is necessary for heat transfer to take
place. After thermal equilibrium is reached, this will stop.
 The larger the surface area of the material, the more work is required from the
surface particles to transfer heat.
 However, it was found that the heat transfer effect also depends on its volume.
Therefore, the surface/volume ratio must be taken into account.
FREE AND FORCED CONVECTION
29
THANK YOU
FREE AND FORCE COVECTION
REFERENCE