Modes of Heat Transfer
As we all know Heat Transfer is nothing but the transfer of energy (i.e. Heat) from the body which is at higher temperature to the body which is at lower temperature, this all happens since there is differences of temperature between both bodies.
Aricle written By : Rayyan Shaikh
In thermodynamics we have studied about the process of conversion of low grade energy (heat) into high grade energy (work). The basic difference between thermodynamic and heat transfer is that, In thermodynamics we study about magnitude or amount of the heat transfer While in Heat Transfer we study about the rate through which the transfer of heat takes place Means how much time it takes to flow the heat from hotter body to colder one.
Heat transfer mechanisms. Heat flow from hotter body to colder One is actually by means three modes.
- Conduction: This mode of energy transfer occurs due to temperature difference within a body or between bodies in thermal contact without the involvement of mass flow and mixing. Or, Conduction is the mode of heat transfer in which transfer of heat takes without the actual movement of the molecules.
Conduction takes places in Solid, Liquid & Gases all three phases But Mostly conduction takes place in solid phase because of two most important reasons :
- Molecular vibration / Lattice Vibration (30%)
- Free electron transfer(70%).
We all know that atoms/molecules always vibrate on their mean position due to its molecular thermal energy so generally 30% of conduction take place due to inter molecular vibration and remaining 70 % is because of free electron transfer(Metals). On a macroscopic scale it seems there is no conduction in liquid and gases but on a microscopic scale molecules of liquid and gases are vibrating over a short distance due to somewhat of its thermal energy as a result conduction comes into picture.
Conduction in liquid: conduction in liquid takes place because of process called diffusion that means heat will diffuse from one layer of fluid to another layer fluids.
Conduction in gases: conduction in a gas take place because of molecular collision or molecular momentum transfer. when high temperature, high velocity molecule collides with low temperature, low velocity molecule.
Conduction is governed by Fourier’s law.
Fourier’s law of heat conduction which states that the heat flow by conduction in any direction is proportional to the temperature gradient and area perpendicular to the flow direction and is in the direction of the negative gradient. The proportionality constant obtained in the relation is known as thermal conductivity, k, of the material. The mathematical formulation is given in equation
Q = – kA dT/dx
Q= heat flow in watt (Joule /sec) W.
dT/ dX=temperature gradient in X direction in 0C.
k=thermal conductivity of material in (Unit = Wm/K )
A= perpendicular area of heat in (m2
What is a thermal conductivity ?
Thermal conductivity is nothing but ability of material that allows the heat energy to pass through it Or, Ability of a material to conduct Heat. Thermal conductivity denoted by K and unit is W m/k. It is a constant of proportionality in Fourier’s law of heat conduction and play very important role in heat transfer.
2. Convection: It is the mode of heat transfer in which transfer of heat takes place with the actual movement of the molecules. On a macroscopic scale there is the movement of molecules takes place.
It is an another mode of heat transfer, which takes place between solid surfaces and surrounding. Here the energy which is getting transfer is combined by molecular diffusion and bulk mass flow.
The heat flow is independent of the properties of material and depends only on the fluid properties, flow properties & geometry (i.e. the shape and nature of the surface will influence the flow and hence the heat transfer significantly effected by the geometry of surface).
If flow is caused by buoyant forces due to temperature difference than it is known as natural convection And, if flow is produced by some external means like pump and fan than it is known as force convention.
In most applications heat is transferred from one fluid to another fluid which is separated by a solid surface also In that situation heat is transferred from the hot fluid to the solid surface (By Convection), then in between solid surface (By Conduction) and Again from the surface to the cold fluid by Convection only.
Figure 2 : Heat Flow From Hot Plate to Cold Fluid Bulk
Here in Figure 2, the heat is getting transferred from hot plate (Temp of Plate > Temp Of Fluid) to cold fluid layer(almost nearby of the plate) by conduction and then one layer of fluid to another layer of fluid or between the fluid bulk in y direction by convection.
The heat transfer rate equation is given by Newton, who clubbed all the parameters into a single equation called convective heat transfer coefficient (h) as given in equation :
Q = hA (T1 – T2)
Where, Q = rate of heat transfer h= convective heat transfer coefficient A = surface area. T1,T2 is temperature of surface and fluid respectively.
3. Radiation: It is the mode of heat transfer in which the transfer of heat takes place in the form of electro-magnetic waves. Radiation heat transfer does not require any material medium for its propagation, it may also occurs through vacuum. Also, Till these electro-magnetic particles impinges to another surfaces it doesn’t not releases its heat content (That is why a significant amount of heat we obtain after being sun so far from Earth).
Every body except 0oK emits thermal energy in the form of electromagnetic waves traveling with Speed of light called radiation. Such radiation incident on surfaces is absorbed and thus radiation heat transfer takes place between surfaces at different temperatures.
No material medium is required for radiative transfer but the surfaces should be in visual contact for direct radiation transfer.
Radiation heat transfer rate is given by Stefan-Boltzmann law which states that heat radiated out is proportional to the fourth power of absolute temperature of surface and heat transfer rate between surfaces is given in equation as following.
Q = F σ A (T14 – T24)
Q = heat transfer rate by radiation
where, F— a factor depending on geometry and surface properties,
σ— Stefan Boltzmann constant (5.67 × 10-8W/m2K4 (SI units))
A— surface area in m2 ,
T1 and T2 are surface temperatures.
Here, In Figure 3 heat transfer from coil to surface of the vessel is by means of radiation. Within the Surface of vessel by conduction and from container to fluid by means of natural convection because flow of fluid is due to buoyant forces.
Image source: Machine Design