Heat Transfer
What is Heat Transfer?
Heat transfer through direct molecular collision without bulk movement of the material.
Key formula / rule: Fourier's Law of Conduction
Key points
- Identify and differentiate between conduction, convection, and radiation.
- Apply Fourier's Law, Newton's Law of Cooling, and Stefan-Boltzmann Law.
- Analyze steady-state and transient heat transfer problems.
- Understand the concept of thermal resistance.
Common exam trap
Confusing conduction and convection.
Definitions
- Term
Conduction
- Meaning
Heat transfer through direct molecular collision without bulk movement of the material.
- Term
Convection
- Meaning
Heat transfer through the movement of fluids (liquids or gases).
- Term
Radiation
- Meaning
Heat transfer through electromagnetic waves, independent of a medium.
- Term
Thermal Conductivity (k)
- Meaning
A material property indicating its ability to conduct heat.
- Term
Heat Transfer Coefficient (h)
- Meaning
A measure of the rate of heat transfer between a surface and a fluid.
- Term
Emissivity (ε)
- Meaning
A surface property representing its efficiency in emitting thermal radiation compared to a blackbody.
- Term
Stefan-Boltzmann Constant (σ)
- Meaning
A physical constant relating the total energy radiated per unit surface area of a black body to the fourth power of its absolute temperature.
Learning objectives
Identify and differentiate between conduction, convection, and radiation.
Apply Fourier's Law, Newton's Law of Cooling, and Stefan-Boltzmann Law.
Analyze steady-state and transient heat transfer problems.
Understand the concept of thermal resistance.
Calculate heat transfer rates in simple geometries.
Formulae
- Name
Fourier's Law of Conduction
- Note
Rate of heat conduction is proportional to the area and temperature gradient.
- Expression
$Q = -kA \frac{dT}{dx}$
- Name
Newton's Law of Cooling
- Note
Rate of heat convection is proportional to the surface area and temperature difference.
- Expression
$Q = hA(Ts - Tf)$
- Name
Stefan-Boltzmann Law
- Note
Total energy radiated per unit surface area is proportional to the fourth power of the absolute temperature.
- Expression
$Q = \ε \σ A (Ts^4 - Tsurr^4)$
- Name
Thermal Resistance (Conduction)
- Note
Resistance to heat flow through a solid material.
- Expression
$Rcond = \frac{L}{kA}$
- Name
Thermal Resistance (Convection)
- Note
Resistance to heat flow from a surface to a fluid.
- Expression
$Rconv = \frac{1}{hA}$
- Name
Overall Heat Transfer Coefficient
- Note
Used for composite walls or heat exchangers.
- Expression
$\frac{1}{U} = \sum Rtotal$
Prerequisites
Thermodynamics (temperature, heat, energy, laws of thermodynamics)
Basic Fluid Mechanics (fluid properties, flow regimes)
Calculus (differentiation, integration)
Common mistakes
Confusing conduction and convection.
Incorrectly applying boundary conditions.
Ignoring radiation at lower temperatures.
Unit inconsistencies in calculations.
Assuming constant thermal properties when they vary significantly.
Keywords
Heat Transfer
Conduction
Convection
Radiation
Fourier's Law
Newton's Law of Cooling
Stefan-Boltzmann Law
Thermal Resistance
Thermal Conductivity
Heat Transfer Coefficient
Emissivity
Steady-state
Transient
Practice preview
What is the primary mode of heat transfer in opaque solids where energy is transferred by the vibration and movement of atoms and free electrons?…
easy
A plane wall is composed of two layers of different materials in series. Layer 1 has thickness L1 and thermal conductivity k1, and Layer 2 has thickness L2 and thermal conductivity k2. What is the total thermal resistanc…
medium
Fourier's Law of Heat Conduction states that the rate of heat transfer (Q) through a material is proportional to the negative temperature gradient (-dT/dx) and what other material property?…
easy