Heat Transfer 1 | Basics

Heat Transfer 1 | Basics In This Blog we will learn about Basics of heat transfer.

Content of this blog
  • Definitions
  • Basics of Heat Transfer
  • Modes of Heat Transfer
  • Practical example

Interview Questions from this Heat transfer basics

  1. What is definition of Heat transfer?
  2. What is definition of Temperature?
  3. What is Definition of Heat?
  4. Which data predicted from Heat transfer and Thermodynamics?
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1. Definitions 

Following definitions must known before study of heat transfer

A. Heat Transfer
  • Heat Transfer is energy in transit due to temperature difference.
B. Temperature
  • Measure of the amount of energy possessed by the molecules of a substance
  • Measure of how hot or cold a substance is
  • Predict the direction of heat transfer
  • Symbol is T and its unit is Celsius and Kelvin 
c. Heat
  • Energy in transit
  • The transfer of energy as heat occurs at the molecular level as a result of a temperature difference
  • The usual symbol for heat is Q.
  • Common unit of heat is Joule and calorie in the SI system.
  • Thermal energy is related to the temperature of matter. 
 For a given material and mass:
  •  Higher the temperature
  • Greater its thermal energy. 
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2. Basics of Heat Transfer

Heat transfer is a study of:

  • Exchange of thermal energy through a body or between bodies. This occurs there is a temperature difference.
  • When two bodies are at different temperatures. Thermal energy transfers from the one with higher temperature to the one with lower temperature
  • Heat always transfers from hot to cold. Heat always moves from a warmer place to a cooler place.
  • Hot objects in a cooler room will cool to room temperature.   Cold objects in a warmer room will heat up to room temperature.
  •   If a cup of coffee and a ice cube were left on the table in this room what would happen to them? Why?
The cup of coffee will cool until it reaches room temperature.
The ice cube will melt and then the liquid will warm to room temperature.

 Thermodynamics tells us:

            • How much heat is transferred (δQ)
            • How much work is done (δW)
            • Final state of the system

Heat transfer tells us:

            • How (with what modes) δQ is transferred
            • At what rate δQ is transferred
            • Temperature distribution inside the body

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3. Modes of Heat transfer

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There are three modes of heat transfer:

         Any energy exchange between bodies occurs through one of these modes or a combination of them.
         An energy transfer across a system boundary due to a temperature difference

  • Transfer of heat through solids or stationery fluids by the mechanism of inter-molecular interactions. It needs a matter It does not require any bulk motion of matter.
  • When you touch a hot object, the heat you feel is transferred through your skin by conduction.
  • When you heat a metal strip at one end, the heat travels to the other end.As you heat the metal, the particles vibrate, these vibrations make the adjacent  particles vibrate, and so on and so on, the vibrations are passed along the metal and so is the heat. 
  • The outer electron of metal atoms drifts, and is free to move.When the metal is heated, this ‘sea of electrons’ gain kinetic energy and transfer it throughout the metal.
  •  Insulators, such as wood and plastic do not have this ‘sea of electrons’ which is why they do not conduct heat as well as metals.
  • Metal is a conductor, wood is an insulator. Metal conducts the heat away from your hands. Wood does not conduct the heat away from your hands as well as the metal, so the wood feels warmer than the metal.

Heat Transfer 1 | Basics
It is a thermodynamic property of a material.b Unit is  W/m K 

Thermal Conductivity of Selected Materials at Room Temperature.
         Copper            401     
         Silver               429     
         Gold                 317     
         Aluminum       237     
         Steel                60.5    
         Limestone       2.15    
         Bakelite           1.4      
         Water              0.613  
         Air                   0.0263

An energy transfer across a system boundary due to a temperature difference


Convection uses the motion of fluids to transfer heat  

  • By the combined mechanisms of Intermolecular interactions Bulk transport. 
  • Convection needs fluid matter. 
  • In a typical convective heat transfer
  • Hot surface heats the surrounding fluid
  • This is then carried away by fluid movement such as wind. 
  • The warm fluid is replaced by cooler fluid
  • Which can draw more heat away from the surface?
  • Since the heated fluid is constantly replaced by cooler fluid
  • Rate of heat transfer is enhanced.
Free or natural convection  induced by buoyancy forces 

Natural convection

The fluid movement is created by the warm fluid itself. 
The density of fluid decrease as it is heated
Hot fluids are lighter than cool fluids
Warm fluid surrounding a hot object rises
It is replaced by cooler fluid. 
The result is a circulation of air above the warm surface

Forced convection 
Induced by external means
May occur with phase change  boiling condensation 

What happens to the particles in a liquid or a gas when you heat them?
The particles spread out and become less dense.
This effects fluid movement

Modes of Heat Transfer - Conduction convection Radiation

Cooler, dense fluids sink through warmer less dense fluids. 
In effect, warmer liquids and gases rise up. 

  • Heat transfer involves the transfer of heat by electromagnetic radiation 
  • That arises due to the temperature of the body. 
  • Radiation does not need matter. 
  • There are no particles between the Sun and the Earth so it CANNOT travel by conduction or by convection. 

Practical example

Four containers were placed equidistant from a heater. Which container would have the 
Warmest water after ten minutes?

Practical example of heat transfer

 Black container would be the warmest after ten minutes because its surface absorbs heat radiation the best. The shiny metal container would be the coolest because it is the poorest at absorbing heat radiation.

         Conduction rate equation is described by the Fourier Law:

qconduction = - k A dT/dx
                 q = heat flow:
                  k = thermal conductivity,  (W/m K)
                  A = Cross-sectional area through which the heat is conducting
                  dT/dx = dT is the change in temperature
  • By adding more insulation to a wall always decreases heat transfer.
  • The thicker the insulation, the lower the heat transfer rate.
  •  This is expected, since the heat transfer area A is constant
  • Adding insulation always increases the thermal resistance of the wall without affecting the convection resistance.
  •  Adding insulation to a cylindrical piece or a spherical shell, however, is a different matter
  • The additional insulation increases the conduction resistance of the insulation layer
  •  But decreases the convection resistance of the surface because of the increase in the outer surface area for convection.
  •   The heat transfer from the pipe may increase or decrease, depending on which effect dominates.
  •   The value of r2 at which heat transfer rate reaches maximum is determined from the requirement that   (zero slope).
  •  Performing the differentiation and solving for r2 yields the critical radius of insulation for a cylindrical body to be  rc = k/h
  • Note that the critical radius of insulation depends on  The thermal conductivity of the insulation k
  • The external convection heat transfer coefficient h . 

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Thanks for reading - Heat Transfer 1 | Basics
Naitik Patel
Industrial Guide

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