Mechanical Basics

# Heat Transfer

## What is Heat Transfer?

Heat Transfer is the study of temperature distribution and transfer of heat from one system to another system and the study of properties and amount of transfer and rate of transfer.

It is one of the important subjects in the Mechanical Engineering whereas it purely deals with the heat and materials that transfer the heat. It is used to design the heat exchangers such as boilers, heaters, refrigerators, and radiators.

The performance of aircraft also depends upon this subject, for example: how the engines can be cooled. The design of chemical plants is usually done on the basis of the heat transferring and the analogous mass transfer processes.

• Heat Transfer Syllabus.
• Heat Transfer Basics.
• Heat Transfer Textbooks. (for Reference)

## Heat Transfer Syllabus.

The Syllabus will be as follow:

1. Conduction.
2. Transient Conduction & Fins.
3. Convection.
5. Heat Exchangers.

So, the syllabus I am mentioning is the reference taken from the GATE.

## Heat Transfer Basics

• Modes of Heat Transferring.

1. Conduction.

2. Convection.

• Laws of Cooling and  Formula.

• Numbers.

##### Modes Of Heat Transfers:

There are Three heat transferring methods; we call it as heat transfer modes.

Heat Transferring Modes or Modes of heat transferring are also called the Mechanisms of the heat transfer. They are

• Conduction:

In this type of mode, the energy exchange takes place by the kinetic motion or direct impact of molecules. The energy transfer takes place only in the same type of system. It means heat transferring takes place between solid to solid or liquid to liquid or gaseous to gaseous. (not between two different systems).

Ex: If one end of the is heated, the heat transferring to the other end of the rod.

• Convection:

Heat Transferring by Convection can take place between the fluid to the solids or solid to the fluid wherein between two different systems.

Ex: consider a bowl consists of water is heated, then heat is first observed by the bowl, then transfers to the water and get heated.

Heat Transferring by Convection can take place between the fluid to the solids or solid to the fluid wherein between two different systems.

Ex: Consider a bowl consists of water is heated, then heat is first observed by the bowl, then transfers to the water and get heated.

##### Laws of Cooling and Formula:

• Fourier Law or Law of Conduction:

The Fourier Law states that the rate of heat transfers is directly proportional to the temperature gradient and directly proportional to the area that is perpendicular to the direction that heat transferring takes place.

The mathematical expression of the Fourier Law is given as follow:

A = Area of the body.
K = Thermal Conductivity.

• Newton’s Law of Cooling:

This is applicable for the Convection process and it is also called the Law of Convection. It is stated that heat transferring takes place between a hot or cold body is directly proportional to the area and the temperature difference between them.

The Heat Transferring Equation is given as

h = Heat transfer Coefficient unit.
Tb = Temperature of the Material or Body.
Tw = Temperature of the air.
A = Area of the body.

Note: The difference between the two laws is that Fourier considers the Thermal Conductivity (K) and Newton consider the Heat Transfers Coefficient (h).

• Stefan-Boltzmann Law:

This law can be applied only for the Radiation mode. It states that the Emissive power of a black body per unit area and time is directly proportional to the fourth power of the absolute temperature.

The Heat Transferring Equation is given as:

Eb = Emissive Power.
T = Absolute Temperature

##### 4. Numbers

• Reynolds Number (Re):

It is the ratio of the Inertia Force to Viscous Force to which the following fluid is subjected. It is denoted as Re.

• Nusselt Number (Nu):

It is the ratio of the conduction resistance offered by a fluid where stationery to surface convection resistance. It is denoted as Nu.

L = Characteristic Length.

h = Heat Transfer Coefficient for Convection.

K =Thermal conductivity.

• Prandtl Number (Pr):

It is the property of fluid and it is the ratio of the Kinematic Viscosity to the Thermal Diffusivity. It is denoted as Pr.

• Grashof  Number:

It is the ratio of the Buoyance force to the Viscous force acting free convection. It is given by Gr.

• Rayleigh Number (Ra):

It is the product of the Grashof  Number and the Prandtl Number in the natural or free convection boundary layer. It is denoted as Ra.

## Heat Transfer Textbooks (for Reference):

Student needs to refer the textbooks to grab the knowledge that not even found in the classroom teaching. When the student starts referring to the textbooks then the seed of knowledge will start growing in oneself. So, do not stick to the classroom notes but also try to refer to the textbooks that increase the knowledge.