HEAT TRANSFER INTRODUCTION CLASS NOTES FOR MECHANICAL ENGINEERING
INTRODUCTION CLASS NOTES FOR
Compare heat transfer with the transfer
of a Government employee. Heat transfer
is heat in motion. The driving force for
heat transfer is temperature difference.
Heat flows from a body at higher
temperature to a body at lower
temperature. It is an irreversible process.
Greater is the temperature difference,
greater will be the ir-reversibility.
MODES OF HEAT TRANSFER
Heat transfer takes place in three different ways. These are conduction, convection and radiation.
Conduction takes place in solids only. There is no motion of the solid due to it. Driving force for conduction is temperature difference. It takes place at micro level. Therefore it is not visible. It is due to lattice vibrations and free electrons transfer. Main contribution is from free electron transfer. Free electrons are there only in pure metals. Thus metals are most conductive. It takes place from higher temperature to lower temperature.
Convection takes place in fluids i.e. in liquids, vapors and gases. There is a bulk motion of the liquid, vapor or gas in convection. It is visible due to bulk motion. It is at macro level. Driving force for convection is also the temperature difference. It takes place from higher temperature to lower temperature.
Radiation does not require any medium for it. It is due to electromagnetic waves moving with the velocity of light. It is mostly invisible except visible radiations which are a very small part of total radiations. Driving force for radiation involves fourth power of temperature i.e. [Th4–Tl4 and not merely the temperature difference. Heat radiations consist of mainly infrared radiations with very small contributions from ultraviolet radiations and visible radiations. It takes place from higher temperature to lower temperature. The radiation is significant only at higher temperatures. Mostly conduction and convection co-exist.
EFFECTS OF HEAT TRANSFER
Heat transfer causes two effects.
(a) Sensible (Change of temperature)
(b) Latent (phase change), no change of temperature
APPLICATIONS OF HEAT TRANSFER
Heat transfer is from
(i) Human body
(ii) All vehicles having internal combustion engines
(iii) Heat exchangers
(iv) Cooking food
(vi) Air conditioning
(ix) Cooling of electronic equipment, pumps, compressors & motors
(xi) Heat treatment of metals
(xiv) Space vehicles
(xv) Heating and cooling of fluids in chemical industries
Conduction takes place in a spoon dipped in a hot liquid. The other end becomes hot. Conduction is invisible. Conduction takes place under steady and unsteady state conditions.
Convection is by bulk motion of the fluid. Hot goes up on heating & cold comes down. Convection currents start. Land breeze and sea breeze are other examples of convection. Convection is visible. Convection is free and forced types.
Lot of radiation takes place from the Sun which has a temperature of 5900 K. There is radiation from fire and hot engines. Mostly radiation is invisible. Radiation is through electromagnetic waves. All types of radiations are harmful to life. Different types of radiations have different effect.
UTILITY OF HEAT TRANSFER
Study of heat transfer helps in the proper designing of heat transfer equipment. These include fins, furnaces, condensers, evaporators, radiators & boilers. Heat transfer can be from the system to the surroundings and vice versa. Heat transfer is always from the higher temperature to the lower temperature. It is not essential that temperature rises on heat addition as in boiling. Temperature rises without heat addition in adiabatic compression.
LAWS GOVERNING HEAT TRANSFER
(i) Laws of thermodynamics
(ii) Newton’s laws of motion
(iii) Law of conservation of mass
(iv) Rate equations
DIFFERNCE BETWEEN HEAT TRANSFER AND THERMODYNAMICS
Heat is always in motion or transition. It can also find the temperature distribution. It also tells mode of heat transfer. In heat transfer, there is no change in the form of energy. Input is heat, output is heat. Heat lost by high temperature body is equal to the heat gained by low temperature body. Hence the efficiency is 100 %. Heat remains heat before and after heat transfer.
Thermodynamics deals with the conversion of energy from one form to another form. Heat converts into work. It deals with “how much is the heat” and “how much is the work. Thus it deals with efficiency of the process. It works under equilibrium conditions. It also deals with the changing thermodynamic properties of the working substance.