Heat Transfer

Heat transfer is a scientific discipline that studies the movement of thermal energy between materials due to temperature differences. The types of heat transfer are:

1. Heat transfer by conduction
2. Heat transfer by convection
3. Heat transfer by radiation

^{Heat Transfer Example (Generated by Artificial Intelligence)}

Heat transfer by conduction is the transfer of energy through molecular interactions within a material due to a temperature difference; this transfer occurs via molecular collisions and the motion of free electrons. The process takes place without any macroscopic movement of the material, relying solely on internal interactions. Conduction is quantitatively described by Fourier’s law:

$q=-k\ast A\ast dT/dx$

• q is the heat flux
• A is the surface area
• k is the thermal conductivity
• dT/dx is the temperature gradient

Heat transfer by convection is the transfer of heat between a surface and a moving fluid in contact with it; this transfer occurs through both fluid motion (bulk transport) and molecular interactions (conduction). In convection, heat is first conducted from the surface to the fluid, then carried away by the fluid’s motion. This process can be either natural (driven by density differences) or forced (driven by external agents such as pumps or fans). Convection is generally divided into two subcategories:

• Forced convection: Fluid motion is induced by an external source such as a fan or pump.
• Natural convection: Fluid motion arises entirely from buoyancy forces caused by density differences due to temperature variations.

The amount of heat transfer is expressed by Newton’s law of cooling:

$q=hA(T_s-T_{\infty })$

• h is the convective heat transfer coefficient
• A is the surface area
• T_s is the surface temperature
• T_∞ is the ambient temperature of the fluid

Radiation is the only mechanism of heat transfer that transmits energy in the form of electromagnetic waves; it can occur even in a vacuum. The amount of radiation emitted depends on the temperature and surface properties of the object. The ideal surface that emits the maximum possible radiation is called a blackbody.

Heat transfer by radiation is proportional to the fourth power of the absolute temperature of the object. All objects emit radiation; however, the rate of emission is directly related to the object’s emissivity. The ideal emitter is defined as a blackbody, for which maximum radiation occurs. There are two key equations:

• Blackbody radiation (Stefan–Boltzmann law): This formula is used to calculate the energy emitted by a body via radiation into its surroundings.
• Radiation heat transfer between two surfaces

$qkaracisim=\epsilon \ast \sigma \ast A\ast T^4$

• q: Heat emitted by radiation (W)
• ε: Emissivity of the body, 0≤ε≤1
• σ: Stefan–Boltzmann constant, 5.67×10^(−8) W/m²K⁴
• A: Surface area emitting radiation (m²)
• T: Absolute temperature (K)

 $qikiy\ddot{u}zeyarasında=\sigma \ast A\ast F_1{\to }_2\ast (T_1{}^4-T_2^4)$

• F_1→2: View factor from surface 1 to surface 2
• T₁, T₂: Absolute temperatures of the respective surfaces (K)