Heat Transfer Basics

Heat transfer is the movement of energy from a hot place to a cooler place due to a temperature difference. Heat can be transferred in 3 modes: conduction, convection and radiation.

Heat conduction is the transfer of energy within a homogeneous substance, such as a solid, a liquid or a gas, due to temperature gradient within the medium. The basic law governing heat conduction is Fourier’s Law. In a one-dimensional form, the Fourier’s law can be written as: q=-k ΔT/L, where ΔT is the temperature difference, k is the thermal conductivity and L is the thickness of the material. Material with higher thermal conductivity will transfer heat faster. For example, copper is a better conductor than aluminum, and pure aluminum is a better conductor than extruded aluminum (A6063).

Heat convection is the transfer of energy between a solid surface and a moving fluid, such as air and water. It is the combined effect of heat conduction within the fluid and the heat transfer due to the fluid motion. Heat convection can be described by the Newton’s law of cooling: q=hA(Ts-Ta), where Ts is the temperature of the solid surface and Ta is the temperature of fluid away from the surface, h is the heat transfer coefficient, which is not a property of the fluid, but a parameter that depends on the surface geometry, the nature of the fluid flow, the fluid velocity, as well as the fluid properties.

Convection is called forced convection if the fluid flow is driven by external means such as a fan, blower, or the wind. It is called natural convection (or free convection) if the fluid flow is driven by buoyancy forces due to the variation of temperature in the fluid.

Radiation is the exchange of energy through electromagnetic waves between two or more bodies due to their temperature differences. When a hot body is radiating energy to its cooler surroundings, the net radiation heat loss rate can be expressed as: q = εσA (Th4-Tc4), where Th is the absolute temperature of the hot body, Tc is the absolute temperature of the surrounding, A is the area of the object, σ is the Stefan–Boltzmann constant (5.67×10-8 W m-2 K-4), and ε the emissivity of the surface.

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