Forced Convection Heat Transfer

Formula

Description

Newton's law of cooling describes convective heat transfer from a surface to a moving fluid (air). The convection coefficient h depends on the fluid velocity, fluid properties, and surface geometry. Forced convection (using fans) dramatically increases h compared to natural convection, enabling much higher power dissipation from the same heatsink. This formula is fundamental to heatsink design, enclosure thermal analysis, and cooling system sizing for electronics.

Variables

• Q — Heat transfer rate (W)
• h — Convection heat transfer coefficient (W/(m²·°C))
• A — Surface area exposed to airflow (m²)
• ΔT — Temperature difference between surface and ambient air (°C)

Practical Notes

Typical h values: natural convection in air 5-25 W/(m²·°C), forced convection in air 25-250 W/(m²·°C), liquid cooling 100-20000 W/(m²·°C). Doubling airflow velocity increases h by roughly 40-50%. Fin spacing must account for boundary layer thickness to avoid diminished returns from closely spaced fins.