• Heat convection is a mode of heat transfer by the mass motion of a fluid such as air.
• This movement causes the fluid to circulate, allowing heat to be transferred from one place to another.
• Convection can also occur in solids, but it is most commonly associated with fluids.
• Heat convection occurs to the surface of an object where the surrounding fluid of the object is heated and moved energy away from the source of heat.

Thermal resistance is a heat property and a measurement of a temperature difference by which an object or material resists a heat flow.

In conduction thermal resistance is given as

Q = Δx/kA

Where

Δx = distance between two-point

A = Cross-section area

K = Thermal Conductivity

⇒ A greater thickness means less heat flow and so does a lower conductivity. Together these parameters form the thermal resistance of the construction. The thermal resistance is proportional to the thickness of a layer of the construction and inversely proportional to its conductivity. Thus reduction in thickness and an increase in the area results in a reduction of thermal resistance.

⇒ Convective stirring develops when heating up fluid from below. The heating decreases the fluid’s density near the bottom and produces fluid being less dense than the surrounding fluid. Stirring of fluid and cleaning the heating surface increases the h value (convective heat transfer), and thus reduces thermal resistance.

⇒ In radiation, heat flow increases with an increase in temperature and reduces with the reduction in emissivity. Thus thermal resistance reduces.

δ = the thickness of the hydrodynamic boundary layer; the region of flow where the velocity is less than 99% of the far-field velocity.

δ_T = the thickness of the thermal boundary layer; the region of flow where the local temperature nearly reaches the value (99%) of the bulk flow temperature

\(\begin{array}{*{20}{l}} \begin{array}{l} \dfrac{\delta }{{{\delta _T}}} = {\left( {{\rm{Pr}}} \right)^{\dfrac{1}{3}}}\\ \: \end{array}\\ {\dfrac{{{\delta _T}}}{\delta } = {{\left( {{\rm{Pr}}} \right)}^{ - \dfrac{1}{3}}} \Rightarrow n = - \dfrac{1}{3}} \end{array}\)

The ratio of the thickness of the thermal boundary layer to the thickness of the hydrodynamic boundary layer is equal to -1/3.

Heat flows from one body to other when they have different temperatures.

The rate at which temperature changes is proportional to the rate at which heat is transferred. The temperature of a sample changes more rapidly if heat is transferred at a high rate and less rapidly if heat is transferred at a low rate.

• In the regenerator-type heat exchanger, heat transfer takes place by the flow of hot and cold fluids alternately over a surface.
• Regenerators are compact heat exchangers in which heat is alternately stored and removed using a heat storage matrix.
• During the hot period, heat is transferred from the hot fluid and is stored in the packing of the regenerator.
• In the subsequent cold period, this heat is regenerated and is transferred to the cold fluid passing through the exchanger.

Nusselt number is the dimensionless heat transfer coefficient and appears when you are dealing with convection. It, therefore, provides a measure of the convection heat transfer at the surface.

The Nusselt number is the ratio of convective to conductive heat transfer across a boundary.

The Nusselt number is the ratio of convective to conductive heat transfer across a boundary. The convection and conduction heat flows are parallel to each other and to the surface normal of the boundary surface and are all perpendicular to the mean fluid flow in the simple case.

In free convection heat transfer, the Nusselt number is the function of the Grashoff number, and the Prandtl number is the Nusselt number.

For free convection

Nu = C(Gr Pr)^m

The ratio of the emissive power and absorptive power of all bodies is the same and is equal to the emissive power of a perfectly black body. This statement is known as Kirchhoff's law.

Kirchhoff's law of thermal radiation states that the emissive power to the coefficient of absorption is constant for all the substances at a given temperature. We can say that at a given temperature, the emissivity of a substance is equal to the coefficient of absorption.

This leads to the observation that if an object absorbs 100 percent of the radiation incident upon it, it must reradiate 100 percent.

Kirchhoff’s law of thermal radiation states that the emissivity of a body is equal to its absorptivity when the body remains in thermal equilibrium with its surrounding.

Note:-  A blackbody allows all incident radiation to pass into it (no reflected energy) and internally absorbs all the incident radiation (no energy transmitted through the body).

Joule sec is the unit of Planck's constant.

Planck's constant, symbolized h, relates the energy in one quantum (photon) of electromagnetic radiation to the frequency of that radiation.

In the International System of Units (SI), the constant is equal to approximately 6.626176 x 10^-34 joule-seconds.

The process of heat transfer from one particle of the fluid to another by the actual movement of the fluid particles caused by some mechanical means is known as Forced convection.

Forced convection is a special type of heat transfer in which fluids are forced to move, in order to increase the heat transfer. This forcing can be done with a ceiling fan, a pump, a suction device, or other.

Forced convection is used in heating and air conditioning systems, electronics cooling, and in numerous other technologies. Forced convection is used in designing heat exchangers, in which one fluid stream is used to heat or cool another fluid stream.