Heat transfer

09 April 2025 19:46

Introduction to Heat Transfer

Heat transfer can be defined as a branch of thermal engineering that deals with the production, use, change, and
transportation of thermal energy from one physical system to another. This process is carried out by means of
conduction, convection, and radiation.
In solids, liquids, and gases, heat is transferred in different forms. This process plays a critical role in various applications,
from HVAC systems to electronics cooling, and even in everyday cooking. Understanding this enables improvements in
system performance, the extension of energy applications, and research breakthroughs in an array of different fields. In
this article, the basic definition will be presented together with other forms of heat transfer and the mathematical
principles of these phenomena.

Explanation of Three Basic Modes of Heat Transfer (Conduction, Convection and Radiation)
conduction heat transfer
Conduction is defined as The process of transmission of energy from one particle of the medium to another with the particles being
in direct contact with each other.Conduction can take place in solids, liquids, or gases. In gases and liquids, conduction is due to the
collisions and diffusion of the molecules during their random motion. In solids, it is due to the combination of vibrations of the
molecules in a lattice and the energy transport by free electrons.
Example: A cold canned drink in a warm room, for example, eventually warms up to the room temperature as a result of heat
transfer from the room to the drink through the aluminum can by conduction.
Convection heat transfer
Convection is defined as The movement of fluid molecules from higher temperature regions to lower temperature regions.
Example: heat house with radiator, Gulf stream transports Heat from Caribbean to Europe.
There are two types of convection, and they are:
Natural convection: When convection takes place due to buoyant force as there is a difference in densities caused by the
difference in temperatures it is known as natural convection. Examples of natural convection are oceanic winds.
Forced convection: When external sources such as fans and pumps are used for creating induced convection, it is known as forced
convection. Examples of forced convection are using water heaters or
geysers for instant heating of water and using a fan on a hot summer day.
radiation heat transfer
Radiation is when electromagnetic waves (radiation) carry heat from one
object to another. Example: heat you feel when you are near a fire, Heat
from the sun, Formation of frost (ice) at night T(air) > 0ºC
Radiation heat transfer is measured by a device known as thermocouple.
Fourier’s law of heat conduction
it states that "The rate of heat flow through a simple homogeneous solid is
directly proportional to the area of section at right angle to the direction of
heat flow & to the change of temp with respect to length of path of heat flow."
Where,
Q -Quantity of heat conducted through a Wall in unit time.
A - Area exposed to heat flow i.e. at right angle to direction
of heat flow.
dt- temperature difference between two sides of wall.
dx- Thickness of wall .
dt/dx-Temp gradient

Thermal conductivity (k)

→ Amount of Energy conducted through a body of unit area & unit thickness in unit time when difference in the temperature
between the faces causing heat flow is unit temperature difference.

Thermal resistance (R-value)

Thermal resistance represents the ability of a material layer to resist heat transmission.
Thermal resistance is calculated as:

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Thermal resistance (R-value)
Thermal resistance represents the ability of a material layer to resist heat transmission.
Thermal resistance is calculated as:

Thermal resistance is also called R-value and its unit is (m2K)/W. The greater the thermal resistance, the better the thermal
insulation performance.
HEAT CONDUCTION IN A HOMOGENEOUS SLAB (WALL)
• L = Thickness of Plane wall
• A = C/s Area
• K = Thermal Conductivity
• T₁, T₂ = Temp. maintained at two faces

Suppose it is steady state, one dimension, without heat generation, then the eqn is:

}
Again on integration

Now apply boundary condition in eqn (i) Specified Temp. Boundary Conditions

We get:

Put the value of C₁ & C₂ in eqn. (i)

Since

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thermal resistance of heat conduction

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Stefan-Boltzmann Law
Stefan Boltzmann Law relates the temperature of the blackbody to the amount of power it emits per unit area. The law states that,
“The total energy emitted/radiated per unit surface area of a blackbody across all wavelengths per unit time is directly
proportional to the fourth power of the black body’s thermodynamic temperature. ”

According to this Law:-

σ=Stefan constant

Or for real (non-ideal) bodies:

ce
Explanation of Terms:

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Absorptivity, Reflectivity and Transmissibility
Let Qi → incident radia on energy
Qa → radia on energy absorbed by the body
Qr → radia on energy reflected by the body
Qt → radia on energy transmi ed by the body.
By conservation of energy principle, total sum must be equal to the incident radiation:
Qi = Qa + Qr + Qt
⇒ 1 = Qa/Qi + Qr/Qi + Qt/Qi • α (alpha) = absorptivity
⇒α+β+γ=1 • β (beta) = reflectivity
Absorptivity of body: • γ (gamma) = transmissibility
Absorptivity of a body is the ratio of radiation heat absorbed by the body to the total radiation heat received by the body.

absorptive power

Reflectivity of body:
Reflectivity of a body is the ratio of radiation heat reflected by the body to the total radiation received by the body.

reflective power

Transmissibility of body:
Transmissibility of a body is the ratio of radiation heat transmitted by the body to the total radiation received by the body.

transmissive power

BLACK BODY
• Black Body is a body which absorbs all the thermal radiation incident or falling upon the body over all wavelengths.
• A tiny hole in a furnace wall is treated as a black body.
For black body

Example:
• Sun is also treated as a black body.
• Ice is also treated as black body.

What is a Heat Exchanger?

A heat exchanger is a device that allows the heat to be transferred from one fluid or medium to another. Liquids, gases, or solids of
various temperatures can be used as the medium. The heat transfer process can be among two gases, two liquids, or one gas and
one liquid. The fluids can be separated or in direct contact according to the type of heat exchanger. The heat is transferred
between the two fluids because of their temperature difference without gaining or losing any heat from the surroundings.

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between the two fluids because of their temperature difference without gaining or losing any heat from the surroundings.
Working Principles of Heat Exchanger
Heat exchanger functions by transferring heat from higher to lower temperatures. Heat can thus be transferred from the hot fluid
to the cold fluid if a hot fluid and a cold fluid are separated by a heat-conducting surface.
The operation of a heat exchanger is governed by thermodynamics. Heat can be transferred with the help of conduction,
convection, or radiation. Conduction is the transfer of thermal energy from one material to another through the motion of a fluid
such as heated air or water.
Convection is the transfer of thermal energy from one surface to another through the motion of a fluid such as heated air or water,
and thermal radiation is a heat energy transfer mechanism characterised by the emission of electromagnetic waves from a heated
surface or object.
The laws of thermodynamics are the fundamental concepts that underpin heat exchangers.
1. The Zeroth Law of Thermodynamics states that in thermal equilibrium, thermodynamic systems have the same temperature.
If two systems are in thermal equilibrium with a third system, the two former systems must also be in thermal equilibrium
with one another; hence, all three systems are at the same temperature.
2. The First Law of Thermodynamics states that energy cannot be created or destroyed, but it can be transmitted from one
medium to another, such as heat.
3. The Second Law of Thermodynamics establishes entropy (S) as an additional property of thermodynamic systems, which
describes a closed thermodynamic system’s natural invariable tendency to increase in entropy over time.
classification of heat exchangers based on two aspects: (a) Based on Flow Arrangement
1.Co-current or Parallel Flow Heat Exchanger
A parallel flow heat exchanger is a device where the hot and cold fluids move in the same direction, starting from one end and
exiting at the other end, enabling heat transfer between the two. It is less efficient than counterflow heat exchangers because the
temperature difference between fluids decreases along the flow path, reducing the rate of heat exchange. Understanding the
design and function of parallel flow heat exchangers is crucial for optimizing thermal systems found in industries like power
generation and HVAC.
2. Counter Flow Heat Exchanger
A counter-flow heat exchanger is a device where fluids flow in opposite directions, enhancing heat transfer efficiency. In this
configuration, the hot fluid enters one end and the cold fluid enters the opposite end, ensuring maximum temperature difference
between them, which maximizes heat transfer. This design leads to higher overall effectiveness compared to parallel-flow heat
exchangers.

3. Cross Flow Heat Exchanger

A cross-flow heat exchanger is a type of heat exchanger where fluids flow perpendicularly
to each other, meaning they are not in the same direction. This design is commonly used
in applications where one fluid flows through tubes or channels and another flows across them.
(b) Based on Constructional Features
1. Shell and Tube Heat Exchanger

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