8. Heat and Mass Transfer free notes for Diploma / BTech.

1. What is Heat and Mass Transfer?

The energy in transit is call heat. Heat is a form of energy. The molecules of a substance are in parallel motion. The mean kinetic energy per molecule of the substance is proportional to its absolute temperature.

Heat and Mass Transfer is a combined area of study in Transport Phenomena that deals with the movement of energy (heat) and matter (species) due to driving forces such as temperature and concentration differences. It is a core subject in Mechanical Engineering and is widely used in thermal and chemical process design.

2. What is Heat Transfer ?

Heat transfer defined as ” The transmission of energy from one region to another as a result of temperature gradient”. In heat transfer the driving potential is temperature difference whereas in mass transfer the driving potential is concentration difference. In mass transfer we concentrate upon mass motion which result in changes in composition, and are caused by the variations in concentration of the various constituent species.

3. Modes of Heat Transfer

3.1. Conduction

Conduction is the transfer of heat within a solid or between bodies in direct contact, without any bulk motion of the material.

• Energy transfer occurs due to molecular vibration and electron movement.
• Common in solids, especially metals.

Governing Law:

Fourier’s Law$$q = -kA \frac{dT}{dx}$$

• $q$ = heat transfer rate
• $k$ = thermal conductivity
• $A$ = area
• $dT/dx$= temperature gradient

Examples:

• Heat flow through a metal rod
• Heating of a spoon placed in hot tea

3.2. Convection

Definition:
Convection is heat transfer between a surface and a moving fluid due to combined effects of fluid motion and conduction at the surface.

Types:

• Natural (Free) Convection: Fluid motion due to buoyancy forces (density differences)
• Forced Convection: Fluid motion due to external devices like fans or pumps

Governing Relation:

$$q = hA(T_s – T_\infty)$$

Where:

• $h$ = heat transfer coefficient
• $T_s$​ = surface temperature
• $T_\infty$​ = fluid temperature

Examples:

• Cooling of a hot cup of coffee in air
• Water cooling in a radiator
• Air conditioning systems

3.3. Radiation

Definition:
Radiation is heat transfer in the form of electromagnetic waves without requiring any medium.

Mechanism:

• Energy emitted due to temperature of a body
• Can travel through vacuum

Governing Law:

Stefan–Boltzmann Law$$q = \sigma \epsilon A T^4$$

• $\sigma$ = Stefan–Boltzmann constant
• $\epsilon$ = emissivity
• $T$ = absolute temperature

Examples:

• Heat from the Sun reaching Earth
• Heat from a fire felt at a distance
• Thermal imaging systems

Uses of Heat Transfer:

• To estimate the rate the flow of energy as heat through the boundary of a system under study.
• To determine the temperature field under steady and transient condition.

Applications of heat transfer:

• Design of thermal and nuclear power plants.
• Refrigeration and air-conditioning.
• Internal combustion engines.
• Heat treatment of metals, Design of furnaces.

You can further study for Heat Transfer

4. What is Mass Transfer?

Mass transfer is the movement of mass (atoms, molecules, or particles) from one location to another due to concentration differences.

• Occurs in gases, liquids, and solids
• Driven by concentration gradient

5. Modes of Mass Transfer

4.1. Diffusion

Diffusion is a spontaneous process driven by a concentration gradient, continuing until uniform concentration is achieved throughout the system. Movement of particles from higher concentration to lower concentration.

Example: When perfume is sprayed in a room, its molecules gradually spread everywhere without any external force.

• Governed by Fick’s Law of Diffusion

Formula:$$J = -D \frac{dC}{dx}$$

• J: Diffusion flux
• $D$: Diffusion coefficient
• $dC/dx$: Concentration gradient

Negative sign indicates flow from high to low concentration.

Factors Affecting Diffusion

• Concentration gradient
• Temperature
• Pressure (especially in gases)
• Nature of medium (gas > liquid > solid)

2. Convection (Mass Transfer)

Convection mass transfer is the process in which mass is transported between a surface and a moving fluid (or within a moving fluid) due to combined effects of fluid motion and concentration gradient. Simply Mass transfer due to bulk fluid motion.

• Often occurs simultaneously with heat transfer
• Example: Evaporation of water into air

6. Similarity Between Heat and Mass Transfer

Heat Transfer	Mass Transfer
Driven by temperature difference	Driven by concentration difference
Fourier’s Law	Fick’s Law
Thermal conductivity (k)	Diffusion coefficient (D)
Heat flux	Mass flux

Both follow similar mathematical principles and are often studied together.

7. Difference between Heat and Mass transfer

Aspect	Heat Transfer	Mass Transfer
What is transferred	Energy (heat)	Physical substance (mass)
Driving force	Temperature gradient	Concentration gradient
Governing law	Fourier’s Law	Fick’s Law
Units	Watts (W)	kg/s or mol/s
Medium required	May or may not require medium (radiation doesn’t)	Requires material medium
Modes	Conduction, Convection, Radiation	Diffusion, Convection

8. Applications of heat and mass transfer

• Heat exchangers in industries
• Air conditioning and refrigeration
• Drying and evaporation processes
• Chemical reactors
• Environmental engineering (pollution control)

9. Key Terms related to heat and mass transfer

• Temperature Gradient → Change in temperature with distance
• Concentration Gradient → Change in concentration with distance
• Thermal Conductivity (k) → Ability to conduct heat
• Diffusivity (D) → Ability to transfer mass

Continue Your Reading With Next Chapter Below

Chapter 1 : Conduction

Chapter 2 : Convection

Chapter 3 : Boiling and Condensation

Chapter 4 : Heat Exchangers

Chapter 5 : Radiation

Chapter 6 : Mass Transfer

Chapter wise Question & Answer