Heat and Mass Transfer Assignment Help

Conduction, Convection and Thermal Radiation

Conduction- Heat conduction in solids is by photons and free electrons and some transitional movements of atoms, in liquids is by phonons and free electrons and some transitional movements of atoms, while in gases is by random collision.

Convection- It possesses much larger heat transfer rate than heat conduction. There are two types of convection. Free convection and forced convection.Free convection is normally due to the temperature- density relationship. Forced convection is accelerated by external forces, such as using fan to blow the air in oven.

Thermal Radiation- thermal radiation is by electromagnetic propagation. There is no needed for thermal radiation, although the electromagnetic waves can be transferred through gases.

Examples on Heat and Mass Transfer

In 1798, Count Rumford observed that the amount of heat is proportional to the amount of mechanical work done. If W is the amount of work done and Q is the quantity of heat produced, then

W~ Q

W= JQ, where J is the constant of proportionality known as Joule’s mechanical equivalent of heat or Joule’s constant

If Q=1, then J=W,

So, Joule’s mechanical equivalent of heat may be defined as amount of work required to be done to produce a unit quantity of heat.

If Q is measured in calorie and W in erg, then J= 4.186 * 10^ 7 erg/cal

If Q is measured in calorie and W in joule, then J= 4.186 joule /cal

In SI, both work and heat are two equivalent forms of energy. However the following distinction between work and heat must be clearly noted.

Work is the transfer of mechanical energy irrespective of temperature difference whereas heat is the transfer of thermal energy because of temperature difference only.

Units of Specific Heat

In CGS system the unit of heat is calorie and is defined as the quantity of heat required to raise the temperature of one gram of water through 1˚c.

1 kilo calorie=1000calories.

1 kilo calorie=4200 joules (J)

Or 1 calorie=4.2 joules (J)

Heat capacity

The heat required to raise the temperature through 1˚c depends upon the mass and also on the material of the substance. For example, if equal masses of water and copper are heated through 1˚c, then a large amount of heat has to be supplied to water.

Heat Capacity

Different quantities of heat are required to change the temperature of same mass of different materials through the same range of temperatures. This shows that the different materials must have a property by virtue of which the same mass when heated through the same range of temperatures would absorb different quantities of heat.

This thermal property of the substance is called the specific heat capacity of the substance. It is also called the specific heat of the substance. It is defined as the quantity of heat energy required to raise the temperature of a unit mass of a substance through 1˚c.

Heat capacity=mass x specific heat capacity.

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