Thermal Conduction, Convection, Radiation -Heat Transfer

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Thermal Conduction, Convection, Radiation -Heat Transfer

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Thermal conduction, convection & radiation are the modes of heat transfer and it is a nice topic that will be discussed here. Here, we will learn all the details about heat, heat transfer, and modes of heat transfer.

Heat is an energy that can be transferred from one body to another body and it is driven by the temperature. Heat always transfers from the body which maintained at high temperature to the body which is kept at the lower temperature.

Now, how the transfer happens and what are the modes of heat transfer, we will learn in this session.

Thermal Conduction, Convection, Radiation

Before discussing thermal conduction, convection, radiation heat transfer, we need to learn the basics of heat transfer.

What is heat transfer?

Heat transfer means the transfer of energy from two different bodies kept at different temperatures.

  • Heat is always transferred from the high-temperature regions to low temperatures.
  • The same temperature objects cannot transfer heat.
  • Temperature difference increases the heat transfer rate.
  • The heat transfer rate is different for different materials.
  • Thin material transfers heat fast.
  • Thick material transfer heat slows.
  • A large surface area transfers more heat.
  • Very good conductors can transfer a large quantity of heat

Heat Transfer – Example & Explanation 

Example of Heat Transfer

Do you like tea or coffee in the morning? I like to have a cup of coffee. When coffee is made, it is at a very high temperature. So, we cannot immediately take it as it is very hot.

What do we do to make the coffee? We simply wait a little time at the normal room temperature or keep the coffee in front of a fan.  Gradually, coffee becomes a little less temperature and we can take.

Explanation of Heat Transfer

When coffee is made, it consists of high heat content and temperature high, say, 70 deg. C. So, we cannot take it. The normal temperature of the atmosphere is about 30 deg. C (for example).

When we keep it sometimes in the room, heat is released into the atmosphere. Coffee released the temperature to the atmosphere, and it comes at the same to the atmosphere, like 30 deg. C. This release of heat means a transfer happens between coffee and the atmosphere.

This transfer is called heat transfer.

Units of Heat Transfer

Are discussing the heat transfer, hence, the unit of heat transfer should be known,

  • SI system: Joule (J)
  • G.S system: cal.
  • Heat Transfer Rate: watt or kW

Relationship, 1 Calorie = 4.186 J, 1000 watt = 1 kW

Types of Heat Transfer (Thermal Conduction, Convection & radiation)

We have learned the basic of heat transfer, that means heat can transfer from one object to another object or one object to surrounding to any objects.

There are three modes of heat transfer,

  • Thermal Conduction
  • Convection
  • Radiation

Thermal Conduction Heat Transfer

Conduction Meaning

When two solids of different temperatures come into contact, heat transfer happens from high-temperature solid to low-temperature solid.

  • Conduction happens when objects are in direct contact.
  • Normally conduction happens in solids.
  • It is called thermal conduction also.
  • Solid means molecules are strongly bonded by intermolecular forces. Hence, no transfer of molecules happens during heat transfer.
  • Heat transfer happens by the vibration or the agitation of the molecules.
  • Since thermal conduction is directly depending on the contact surface, hence, if the contact surface is more, heat transfer will also be more.

Thermal Conduction Definition

Conduction is defined as the transfer of heat by the vibration or the agitation of molecules instead of motion.

It depends on a few factors,

  • The type of material used, for example, copper transfer heat first than iron.
  • The thickness of the material, if the thickness is more, the transfer will be slow and thinness is less, transfer will be first.
  • If the temperature difference between the two materials is more, heat transfer will be more and if the difference is less, the transfer will be less.

Mathematical Derivation

Heat transfer occurs due to direct contact of matter. As per “Fourier’s Law“, we know that,

  • Q/t proportional to A
  • Q/t proportional to T2-T1
  • Q/t proportional to 1/d

Rate of heat transfer pro to A (T2-T1)/d

Q/t = kA (T2=T1) / d

q  = U A (T2-T1)               (1)

Where,

  • q = heat transfer rate (W, J/s, Btu/hr)
  • k = Thermal Conductivity of material (W/m K or W/m oC, Btu/(hr oF ft2/ft))
  • d = material thickness (m, ft)
  • A = heat transfer area (m2, ft2)

U = k / d

=  Coefficient of Heat Transfer (W/(m2K), Btu/(ft2 h oF)

dT = T2 – T1

= temperature gradient – difference – over the material (oC, oF)

Factors affecting the rate of conduction

Based on the equation as well as example, the following factors are affecting the rate of conduction,

  • Thickness
  • Area
  • Temperature difference
  • Thermal conductivity

Thickness
If you take a metal plate and heat it in an oven, what will happen? The metal plate will be heated.

  • Now, if the plate thickness is more, it takes more time to be heated.
  • If the plate thickness is less, it takes very little time to be heated.
  • If the thickness is more, the temperature difference between molecules will be more, and resistance to heat transfer will be more.
  • Hence, heat transfer will be less.
  • So, if the thickness is more heat transfer will be less and if the thickness is less, heat transfer will be more.
  • Factors affecting the rate of heat conduction through a material

Conduction examples

  • Refrigerant pipe thickness 0.6mm has more heat transfer rate than 0.8mm thickness.

Area
If the area increases, contact surfaces will increase.

  • Contact surface increase means; heat transfer will increase.

Conduction examples

  • A metal plate 10 sq.m has more heat transfer than the plate area 5 sq.m.

Temperature difference
If the temperature difference is more, heat flow will be more.

  • If the temperature difference is less, heat transfer will be less.
  • If the temperature difference is zero, there will not be any heat transfer. To understand this concept, the zeroth law of thermodynamics is the best.

Conduction examples

  • Let us consider two plates at different temperatures. Case-1, 1st plate is 25 deg. C and 2nd plate 10 deg. C, hence, the difference is 25-10 = 15 Deg. C.
  • Case-2, 1st plate is 35 deg. C and 2nd plate 10 deg. C, hence, the difference is 35-10 = 25 Deg. C.
  • In this above example, heat transfer will be more in case-2 (temperature difference =25 Deg. C), as there is more heat transfer.

Thermal conductivity

Thermal conductivity is one of the main properties associated with heat transfer. It is property, by which we can understand the heat transfer rate. If the value of thermal conductivity is more, heat transfer will be more.

Various metals have different thermal conductivity.

MaterialThermal Conductivity (W/m-K)
Copper397.5
Stainless Steel13.4
Water0.55
Wood0.15 to 0.2
Rubber0.14

Convective Heat Transfer

Convection heat transfer happens in the liquids or gases if there is any temperature difference. In this method, heat transfer happens partly by molecule movements or mass transfer.

It may be between solid and moving fluids also.

Convection Meaning

Convection is defined as the method of heat transfer by the movement of molecules or transfer of the mass in liquids or gasses

Example

from a region of higher temperature to a region of lower temperature. Convection heat transfer occurs partly due to the actual movement of molecules or due to the mass transfer.

  • Boiling water in a container is an example of convection. Water at the bottom of the container becomes heated first and become light and goes to the upside. Cold water which is heavy than the hot water comes down and continues the cycles. Here, water molecules are moving from one place to another and gradually water becomes hot.

Factors affecting the rate of convection

The convection rate depends on the following factors,

  • Type of fluids
  • Type of flow
  • Velocity of fluids
  • Viscosity of fluids
  • Surface roughness
  • Heating medium
  • Heat flux

Mathematical Expression

Let us see the expression for convection heat transfer. We will see here, heat transfer between a solid and fluid in motion.

The rate of convection is calculated as,

Q = hc ∙ A ∙ (Ts – Tf)

Where,

  • Q – heat transferred
  • Q – Q/t – heat transferred per unit time
  • Hc –   coefficient of convective heat transfer
  • A – area of heat transfer
  • Ts – surface temperature
  • Tf – fluid temperature

This law is known as Newton’s Law of Cooling.

Type of Convective heat transfer

  • Forced Convection
  • Natural Convection or Free convection

Forced Convection

  • It happens when convection occurs by an external force.
  • Normally pumps, fans, blowers, etc. are used as an external force.
  • It helps to transfer more heat due to fluid velocity.
  • This is also known as Assisted Convection

Natural or Free Convection

  • Natural convection means there will not be any external forces.
  • When the temperature difference is in the fluid, its density changes.
  • Due to the change in density, there will be a change between the high-temperature zone to the low-temperature zone.
  • Buoyancy forces are created.
  • This buoyancy force creates natural convection.
  • This is known as free or natural convection.
  • This is Newton’s Law of Cooling

Radiation Heat Transfer

Radiation heat transfer happens when heat transfer happens between two bodies, which are not in contact.

Definition

Radiation is defined as the method of heat transfer by which heat is transferred from a body to another body, which doesn’t have any contact.

  • In this case, no movement of the molecules is involved.
  • It doesn’t depend on the medium; it can work even in the vacuum also.
  • Charged protons and electrons got the movement, which creates the electromagnetic waves.
  • This wave is generated from the emitter.

Example

The light, as well as the heat, comes from the sun to the earth, however, there is no medium or only air. This is an example of radiation.

In the micro oven, radiation is the main process to make the food hot.

Mathematical Derivation

Thermal radiation is expressed by Stefan-Boltzmann law.  It is as follows,

q = e ∙ σ ∙ A· (Tr – Tc)4

Where,

  • q  – thermal radiation heat transfer rate or heat transfer per unit time (W)
  • A is the area of radiation or area of the emitting body (m2)
  • Tr is the radiator temperature
  • Tc is the surrounding temperature
  • e is emissivity
  • σ= 5.6703 10-8 (W/m2K4) – TheStefan-Boltzmann Constant

We have learned the basics of conduction heat transfer, convection heat transfers, and radiation heat transfer,

Difference Between Thermal Conduction Convection and Radiation

ConductionConvectionRadiation
Heat transfer happens between objects when they are in contactHeat transfer happens with the fluid itself.Heat transfer happens by electromagnetic waves
It is a very slow processFasterVery fast
Transfer due to temperature difference.Transfer happens due to density difference or buoyancyIt is by waves.
A hot solid transfers heat by conductionBoiling milk in a container transfer heat by convectionSun provides thermal radiation to the earth.

Conclusion

We have learned the basics of modes of heat transfer, its basic, understanding, simple mathematical derivations.

Further Study

Check out few interesting articles,

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