Heat Transfer – Method of Heat Transfer

Heat conduction is one of the three basic modes of thermal energy transport. Heat flows from higher temperature to lower temperature. Though it looks simple, heat transfer is quite complex phenomenon. There are three basic modes of heat transfer. This article provides an introduction to modes of heat conduction. Let’s take closer look at heat transfer and three main methods of deployment.

What is Heat?

All matter is made up of molecules and atoms. These atoms are always in different types of motion. This motion of atoms and molecules creates heat or thermal energy. Heat can be defined as process of transfer of thermal energy from or to an object. And as it is a process, objects or bodies do not contain heat but thermal energy.

What is Heat Transfer?

Heat can travel from one place to another in three ways. Heat transfer is process of an item or substance coming into contact with heat source and becoming hot. In more scientific terms, heat transfer is an exchange of thermal energy between two objects. This thermal energy is related to temperature. For given material and mass, temperature is high and thermal energy is greater.

Heat transfer is study of thermal energy exchange through body or between bodies which occurs when there is temperature difference. Therefore, when two bodies are at different temperatures then thermal energy transfers from one with higher temperature to one with lower temperature. Heat always transfers from hot to cold.

Types of Heat Transfer

There are three methods for heat transfer like conduction, convection and radiation. Any energy exchange between bodies occurs through one of these modes or combination of them. Conduction is transfer of heat through solids or stationery fluids. Convection uses movement of fluids to transfer heat. Read also Engineering Materials –Comparison between Crystal Slip and Twinning 

Radiation does not need medium for transferring heat.  So this mode uses electromagnetic radiation by an object for exchanging heat. Knowing what each type is and how it works will give you better understanding of how insulation and weather tripping systems protect your conditioned space. Following are three methods of heat transfer

Conduction

Conduction is most common form of heat transfer and occurs via physical contact. Heat conduction is transfer of heat from warmer to colder object by direct contact. In conduction, heat transfer takes place due to difference temperature in body or between bodies in thermal contact without mixing of mass. Conduction transfers heat via direct molecular collision.

An area of greater kinetic energy will transfer thermal energy to an area with lower kinetic energy. So higher speed particles will collide with slower speed particles. As a result, slower speed particles will increase in kinetic energy.

What is heat conduction process?

Process of heat conduction depends on following factors like temperature, cross-section of material, length of travel path and physical material properties. Heat transfer rate through conduction is governed by Fourier’s law of heat conduction. Check Engineering Materials – Deformation of Solid Materials

This temperature gradient is physical quantity that tells about direction and rate of heat travel. Once there is thermal equilibrium between two temperature differences then thermal transfer stops. And cross-section and path of travel both play an important part in conduction.

So size and length of an object is greater than more energy is needed to heat it and greater surface area that is exposed, the more heat is lost. Physical properties determine which materials transfer heat better than others. On other words, thermal conductivity coefficient tells that metal material will conduct heat better than cloth when it comes to conduction. Following equation calculates rate of conduction.

Q = -kA \(\frac{dT}{dx}\)

  • Where Symbol Q is heat rate flow by conduction
  • K is thermal conductivity of body material.
  • A is cross-sectional area normal to heat flow direction.
  • dT/dx is temperature gradient in direction of the flow.

Minus sign in Fourier’s equation show that flow of heat is from hotter to colder. Heat flow is in direction of negative gradient temperature and that serves to make heat flow positive.

Constant ‘k’ is Thermal conductivity of transport properties. It is used to show that not all materials heat up or retain heat equally well. Other is viscosity associated with transport of momentum, diffusion coefficient associated with transport of mass.

Thermal conductivity provides an indication of rate at which heat energy is transferred through medium by conduction process.

Convection

Heat energy transferred between surface and moving fluid at different temperatures is known as convection. In reality, this is combination of diffusion and bulk motion of molecules. Near the surface fluid velocity is low and diffusion dominates. Away from surface, bulk motion increase influence and dominates.

Convection is means of heat transfer between fluids or by fluid flow. In convection, heat transfer between surface and liquid or gas. As fluid or gas travels faster, as convective heat transfer increases. Convective heat transfer may take form of either forced convection or natural convection. Example of this method is heat transfer by steam to some one hand when place in steam path.

Natural convection

Natural convection refers where fluid movement is created by warm fluid itself. The density of fluid decrease as it is heated thus, hot fluids are lighter than cool fluids. This warm fluid around hot object rises and is replaced by cooler fluid. This continues occurrence is called free or natural convection.

Forced convection

Forced convection occurs when fluid flow is force to travel in surface, such as pump, fan or mixer. Boiling or condensing processes are also referred as convective heat transfer processes. The heat transfer per unit surface through convection is first described by Newton and relation is known as Newton’s law of Cooling.

Newton’s law of Cooling

Newton’s law of Cooling is used for both natural convection and forced convection. Following is normal form of Newton’s law of Cooling as it is used for convection heat transfer is given by-

Q = hA (Ts-T\infty)

  • Ts is surface temperature
  • Where A is surface area of object
  • h is measure of how electively fluid transfers heat by convection.
  • T∞ is outside temperature
  • H is coefficient of convection

Radiation

Radiation is mode of heat transfer which takes place through vacuum and hence, does not need physical medium. Transfer of heat through empty space is known as radiation. In radiative mode, heat transfer takes place in the form of electromagnetic waves. This radiation lies in infrared region of electromagnetic spectrum.

These waves carry energy away from release object. Example of this is head provided to earth by sun. In radiation, heat is transferred in form of wave motion from one body to another body. No medium for radiation to occur. On other word, radiative heat transfer is governed by Stephen-Boltzman law.  Amount of radiation emitted by an object is given by:-

Stephen-Boltzman law

According to Stefan’s law, total radiant heat energy that is emitted by surface is proportional to is absolute temperature. This law is applicable only to black bodies and theoretical surfaces that absorb heat radiation. So this law is described in following manner.

Q = \sigma.T4

  • Where T is temperature of body
  • σ is constant called Stefan-Boltzmann constant.

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