What is Heat Transfer ? Thermodynamics? ,Heat and Temperature ? conduction ,convection ,radiation -MECHANICAL

Heat transfer / Heat v/s Temperature/Modes of Heat transfer .....

BASICS OF HEAT TRANSFER

While teaching heat transfer ,one of the first questions students commonly ask is the difference between heat and temperature .Another common questions concerns the difference between the subjects of heat transfer and thermodynamics . Let me begin this unit by trying to address these two questions.also three modes of heat transfer ,conduction convection and radiation.

What is Heat?

Heat is a form of energy. It derives its origins at the molecular scale. Molecules of a substance vibrate at their positions either fixed or not when energy is supplied to them. As they vibrate they transfer their energy to the surrounding molecules causing them to vibrate as well.

 

What is Temperature?

The temperature of a body is the measure of the amount of heat content possessed by it. It is measured in degree Celcius (°C) or Kelvin(°K). The temperature of a substance is a physical quantity that measures the degree of hotness or coldness of a body.

Difference between Heat andTemperature

In heat transfer problems , we often interchangeably use the terms heat and temperature . actually there is a distant difference the two. Temperature is a measure of the amount of energy possessed by the molecules of a substance. It manifests itself as a degree of hotness and can be used to predict the direction of heat transfer. The usual symbol for temperature is T .the scales for measuring temperature in SI units are Celsius and Kelvin temperature scales .heat on the other hand is energy in transit .Spontaneously heat flows from a hotter body to a colder one. The usual symbol for heat is Q. In the system common units for measuring heat are the joule and calorie.

Difference between Thermodynamics and Heat transfer

Thermodynamics tells us:	Heat transfer tells us:
Ø  how much heat is transferred (dQ)	Ø  how (with what modes) dQ is transferred
Ø  how much work is done (dW)	Ø  at what rate dQ is transferred
Ø  final state of the system	Ø  temperature distribution inside the body

Heat Transfer

Heat energy can be transferred from one body to the other or from one location in a body to the other. Study of the techniques and methods adopted to transfer heat energy is known as ‘Heat Transfer’. To facilitate heat transfer between two bodies there needs to be a temperature difference between them. This means that these bodies must be a two different temperatures one higher than the other to allow heat to flow from one body to the other.

This means that no heat transfer occurs between two bodies which are at the same temperature. At the same time, it is very important to note that heat only flows from a body at higher temperature to a body at a lower temperature. Although this may look obvious, this law is very important from the point of view of thermodynamics.

Example : Take a Cup of Tea 

Let us say that you have prepared a cup of tea for yourself. The tea is very hot say at 80 °C and so you leave it in a room with a temperature of 25 ° C for some time to cool down. This is the first law of heat transfer. Heat transfer will only take place between 2 bodies when they have a substantial temperature difference.

Now, after some time you come back to find that the tea in the cup has cooled down to say 50°C and you have a sip of the same. This is the second law of heat transfer. Heat will only flow from a body at higher temperature to a body at a lower temperature. It is not possible to have a scenario where the heat flows from the room at 25° C to the cup of tea at 80° C and heat it even further.

These techniques and methods discussed below are observed in nature and thus have been generalized for all things while under consideration for the purpose of the study. However, no observation against these has been ever recorded or observed thus establishing their credibility as truthful and applicable at all times.

Heat transfer takes place in ONE of the three ways namely: Conduction, Convection and Radiation We will discuss each of these methods in detail.

Heat transfer Methods finds a variety of applications in the chemical process industries.

Heating and Cooling of Batch Tanks This application will allow the user to calculate the time it takes to heat up and then cool a batch vessel or tank.

The heating methods supported are:

1. Tempered Water Jacket or Coil
2. Direct Steam Injection
3. Electrical Heating Jacket
4. Steam Jacket or Coil

The cooling methods supported are:

1. water jacket or coil
2. External cooler (heat exchanger)
3. Direct water mixing

Ø Points to remember : Heat Transfer take place  as Per Second Law of Thermodynamics

As per the second law of thermodynamics the transfer of heat takes place from the body of high temperature to the body of low temperature. There won’t be spontaneous transfer of heat from the body at low temperature to the body at high temperature. For heat transfer from low temperature body to high temperature body, external work has to be done.

Modes of heat transfer

Heat transfer is the physical act of thermal energy being exchanged between two systems by dissipating heat. Temperature and the flow of heat are the basic principles of heat transfer. The amount of thermal energy available is determined by the temperature, and the heat flow represents movement of thermal energy.

On a microscopic scale, the kinetic energy of molecules is the direct relation to thermal energy. As temperature rises, the molecules increase in thermal agitation manifested in linear motion and vibration. Regions that contain higher kinetic energy transfer the energy to regions with lower kinetic energy. Simply put, heat transfer can be grouped into  three modes of heat transfer between the two bodies:  conduction, convection and radiation.

                             

Conduction 

Conduction is the transfer of heat from one part of a substance to another part of the same substance, or from one substance to another in physical contact with it, without appreciable displacement of molecules forming the substance, In solids, the heat is conducted by the following two mechanisms

(i) By lattice vibration (the faster moving molecules or atoms in the hottest part of a body transfer heat by impacts some of their energy to adjacent molecules).

(ii) By transport of free electrons (Free electrons provide an energy flux in the direction of decreasing temperature For metals, especially good electrical conductors, the electronic mechanism is responsible for the major portion of the heat flux except at low temperature).

In case of gases, the mechanism of heat conduction is simple. The kinetic energy of a molecule is a function of temperature. These molecules are in a continuous random motion exchanging energy and momentum. When a molecule from the high temperature region collides with a molecule from the low temperature region, it loses energy by collisions.

In liquids, the mechanism of heat is nearer to that of gases. However, the molecules are more closely spaced and inter molecular forces come into play.

Convection

"Convection" is the transfer of heat within a fluid by mixing of one portion of the fluid with another .

Convection is possible only in a fluid medium and is directly linked with the transport of medium itself. the fluid

Convection constitutes the macro form of the heat transfer since macroscopic particles of a fluid moving in space cause the heat exchange.

The effectiveness of heat transfer by convection depends largely upon the mixing motion of the fluid.

This mode of heat transfer is met with in situations where energy is transferred as heat to flowing fluid at any surface over which flow occurs. This mode is basically conduction in a very thin fluid layer at the surface and then mixing caused by the flow. The heat flow depends on the properties of fluid and is independent of the properties of the material of the surface. However, the shape of the surface will influence the flow and hence the heat transfer.

Free or natural convection. Free or natural convection occurs when the fluid circulates by virtue of the natural differences in densities of hot and cold fluids; the denser portions of the fluid move downward because of the greater force of gravity, as compared with the force on the less dense.

Forced convection. When the work is done to blow or pump the fluid, it is said to be forced convection.

Radiation

"Radiation" is the transfer of heat through space or matter by means other than conduction or convection.

Radiation heat is thought of as electromagnetic wives or quanta (as convenient) an emanation of the same nature as light and radio waves. All bodies radiate hear, so a transfer of heat by radiation occurs because hot body emits more heat than it receives and a cold body receives more heat than it emits. Radiant energy (being electromagnetic radiation) requires no medium for propagation and will pass through vaccum.

Heat Transfer uses following laws,

• Fourier's Law (Heat conduction)
• Newtons Law of Cooling (Heat convection)
• Planck's Law (Thermal Radiation)
• Stephan Boltzman's Law (Thermal Radiation)
• Wein's Law (Thermal Radiation)
• Kirchoff's Law (Thermal Radiation)
• Lambart's Law (Thermal Radiation)