Heat Transfer Mechanisms
Heat transfer occurs through three mechanisms: conduction (direct contact), convection (fluid motion), and radiation (electromagnetic waves). In most real situations, all three mechanisms operate simultaneously, but one usually dominates depending on the geometry, materials, and temperature ranges involved.
Conduction transfers heat through solid materials or stationary fluids via molecular collisions. Convection involves bulk fluid motion carrying thermal energy. Radiation transfers heat through electromagnetic waves and requires no medium, which is how the Sun heats the Earth across the vacuum of space.
Heat Transfer Equations
Thermal Conductivity Values
| Material | k (W/(m·K)) |
|---|---|
| Copper | 401 |
| Aluminum | 237 |
| Steel | 50 |
| Glass | 1.0 |
| Wood | 0.12 |
| Air | 0.026 |
Frequently Asked Questions
Which heat transfer mode is most efficient?
Conduction through metals is fastest for short distances due to high thermal conductivity. Convection is most effective for transferring heat over larger distances in fluids. Radiation dominates at high temperatures and across vacuums. For building insulation, minimizing all three modes is the goal.
What is the h coefficient in convection?
The convective heat transfer coefficient h (W/(m²·K)) depends on fluid properties, flow velocity, and geometry. Natural convection in air gives h = 5-25 W/(m²·K), forced air convection 25-250, water natural convection 100-900, and boiling water up to 100,000 W/(m²·K).