convezione

Convezione is the transfer of heat by the movement of a fluid. It occurs when a fluid carries thermal energy from one place to another as it flows, and is driven by temperature-induced density differences or by external forces.

Natural convection arises from buoyancy: warmer, less dense fluid tends to rise, while cooler, denser fluid sinks, creating a circulation that enhances heat transfer without mechanical aid. Forced convection results when an external agent, such as a fan, pump, or moving fluid, drives the flow and increases the rate of heat transfer.

The rate of convective heat transfer depends on fluid properties (density, viscosity, thermal conductivity, specific heat), the temperature difference, and the flow geometry and velocity. It is commonly described by Newton's law of cooling: q = h A ΔT, where q is the heat transfer rate, A the surface area, ΔT the temperature difference, and h the convective heat transfer coefficient. The coefficient h encapsulates complex flow behavior and is usually determined from experiments or numerical correlations.

Convection is characterized by dimensionless numbers that help predict heat transfer behavior. The Reynolds number (Re) compares inertial to viscous forces and relates to flow regime. The Prandtl number (Pr) compares momentum and thermal diffusion. The Grashof number (Gr) and the Rayleigh number (Ra = Gr·Pr) describe natural convection. The Nusselt number (Nu) links h to the fluid’s thermal conductivity, Nu = hL/k, with L a characteristic length and k the conductivity.

Applications include cooling of electronic components, heat exchangers, ventilation and HVAC systems, solar collectors, and various industrial processes. Convection often works alongside conduction and radiation in thermal systems.