Lesson 6.1: Modes of Heat Transfer (Conduction, Convection, Radiation)

Heat Transfer is essential in thermal engineering, power plants, and HVAC systems. GATE and PSU exams often test basic concepts, formulas, and applications.

🔹 1. Introduction

• Definition: Heat transfer is energy transfer due to temperature difference.

• Applications: Boilers, heat exchangers, turbines, refrigerators

• Key Concepts:

  1. Temperature gradient drives heat transfer

  2. Modes: Conduction, Convection, Radiation

🔹 2. Conduction

• Definition: Heat transfer through solid or stationary fluid due to temperature gradient

• Fourier’s Law:

q=−kdTdxq = -k \frac{dT}{dx}q=−kdxdT​

Where: k = thermal conductivity, dT/dx = temperature gradient

• Example: Heat through wall of thickness 0.05 m, k = 50 W/m·K, ΔT = 100 K → q?

q=kΔTL=50∗100/0.05=100kW/m2q = k \frac{\Delta T}{L} = 50*100/0.05 = 100 kW/m^2q=kLΔT​=50∗100/0.05=100kW/m2

🔹 3. Convection

• Definition: Heat transfer by fluid motion

• Newton’s Law of Cooling:

q=hA(Ts−T∞)q = h A (T_s – T_\infty)q=hA(Ts​−T∞​)

Where: h = convective heat transfer coefficient

• Types:

  1. Natural Convection: Fluid motion due to buoyancy

  2. Forced Convection: Fluid motion forced by pump/fan

🔹 4. Radiation

• Definition: Heat transfer by electromagnetic waves without medium

• Stefan–Boltzmann Law:

q=ϵσA(Ts4−T∞4)q = \epsilon \sigma A (T_s^4 – T_\infty^4)q=ϵσA(Ts4​−T∞4​)

Where ε = emissivity, σ = Stefan–Boltzmann constant

• Example: Black body, T_s = 500 K, T∞ = 300 K → q?

🔹 5. Solved Examples (PYQ Style)

1. Heat conduction through composite wall

2. Convection from flat plate in air

3. Radiation heat loss from furnace wall

🔹 6. Practice Exercises

1. Compute heat flux in conduction, convection, and radiation

2. Determine convective coefficient from experimental data

3. Compare conduction, convection, and radiation in practical systems

🔹 7. Summary

• Conduction: Through stationary medium, Fourier’s Law

• Convection: Fluid motion, Newton’s Law of Cooling

• Radiation: Electromagnetic waves, Stefan–Boltzmann Law