Heat Transfer: Convection

Concept

Convection is the mechanism of heat transfer occurring between a solid surface and an adjacent moving fluid (liquid or gas). It inherently involves two phenomena: the conductive heat transfer through the thin stationary fluid boundary layer near the surface, and the macroscopic transport of heat by fluid motion. Convection is classified as forced convection (fluid motion is driven by external means like a pump or fan) or natural/free convection (fluid motion is caused by buoyancy forces due to temperature-induced density differences).

Formula & Method

The rate of convective heat transfer is governed by Newton's Law of Cooling: $q = h A (T_s - T_\infty)$ Where:

• $q$ = Heat transfer rate
• $h$ = Convective heat transfer coefficient
• $A$ = Surface area available for heat transfer
• $T_s$ = Surface temperature
• $T_\infty$ = Fluid bulk temperature

The parameter $h$ is not a property of the fluid itself, but rather an experimentally determined parameter that depends on fluid properties (density, viscosity, thermal conductivity, specific heat), surface geometry, and the nature of fluid motion (laminar or turbulent flow).

Engineering Application

In thermal design, convection is often the limiting resistance in heat exchangers. Engineers use dimensionless numbers like the Nusselt ($Nu$), Reynolds ($Re$), and Prandtl ($Pr$) numbers to correlate empirical data and estimate $h$ for different geometries. Enhancing convection involves increasing fluid velocity (increasing $h$) or increasing the surface area ($A$) using fins or extended surfaces, which is standard practice in radiators and electronic heat sinks.