# Heat Transfer – Introduction and Formulas

### What is heat transfer?

Exchange of thermal energy between the physical systems is known as heat transfer. However, the rate of heat transfer is based upon 2 aspects which are properties of intervening medium & the system’s temperatures as heat is transferred through these systems. While there are 3 fundamental modes of this heat transfer which are – (1) conduction (2) convection & (3) radiation.

### Details of heat transfer along with the laws of thermodynamics

In other words, this heat transfer is nothing but the flow of energy which is in the form of heat. Besides this, it is a process with the help of which the internal energy of a system happens to change. This is the sole reason for this heat transfer to be used in applications like that of the 1st law of thermodynamics. As far as the 2nd law of thermodynamics is concerned then the direction of heat transfer is stated to be from the region which is of high temperature to that of another region which is stated to be of lower temperature. And this is governed by the 2nd law of thermodynamics.

As it has been written quite clearly earlier, this heat transfer tends to change the internal energy. This happens to hold ground even when energy moves from one system to the other system. Therefore, one can say that this heat transfer can occur in a direction which tends to increase entropy of collection of the systems.

### When does heat transfer cease?

However, there are times when this heat transfer ceases & it so happens when the thermal equilibrium has reached. At this stage, the bodies involved along with the surroundings happen to reach the same temperature. In addition to this, the thermal expansion or the thermal contraction for that matter is nothing but it is (the) tendency of matter to be changed in volume & this is done in order to respond to the change that takes place in the temperature.

### Notes on heat transfer

In physics, heat is generally defined to be the transfer of thermal energy across some well – defined boundary, this is a boundary which is around a thermodynamic system. Additionally, this heat transfer is the process function & not functions of the state.

### Formula for thermal hydraulics –

In the above equation,  is the heat flux which is (W / m2) while  is the density (kg / m3) & is said to be the heat capacity at the constant pressure (J / kg. K). turns out to be the change in the temperature (K) & finally  is the velocity (m / s).

Rayleigh number when determining relative strength of convection & conduction –

In the above equation,  = acceleration due to gravity,  = density,  = density difference between lower & upper ends, = dynamic viscosity,  = thermal diffusivity,  = volume thermal expansivity,  = temperature,  = characteristic length and  = kinematic viscosity

Stefan – Boltzmann equation to rate transfer of radiant energy –

Equation for radiative transfer between the 2 objects –

In the above equation,  = heat flux,  = emissivity,  = Stefan – Boltzmann constant,  = absolute temperature.