Heat transfer (Thermodynamics.)

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Heat Transfer (In the Thermodynamics)

INTRODUCTION

Heat transfer is a subject of widespread interest to the students of engineering curriculum, practicing engineering and technicians engaged in the design, construction, testing and operation of the many diverse forms of heat exchange equipment required in our scientific and industrial technology. While launching ourselves into the frontiers of heat transfer, let us recall the following basic concepts and definitions

In thermodynamics that when two systems are brought into contact through some kind of wall, energy transfers such as heat and work take place between them, Work is a transfer of energy to a particle which is evinced by changes in its position when acted upon by a force. Heat like work, is energy in the process of being transferred.  

The transfer and conversion of energy from one form to another is basic to all heat transfer process, and hence they are governed by the first as well as the second law of thermodynamics. The major difference between thermodynamics and heat transfer is that the former deals with the relation between heat and other forms of energy, whereas the latter is concerned with the analysis of rate of heat transfer. Heat transfer is commonly associated with fluid dynamics by providing supplements the law of thermodynamics by providing additional experimental rules to establish energy transfer rates

MODES OF HEAT TRANSFER

The energy transfer as heat takes place by three distinct modes: conduction, convection and radiation.

Conductions :                                                                                                                                                Thermal conduction is a mechanism of heat propagation from a region of higher temperature to a region of low temperature within a medium (solid, liquid or gaseous) or between two mediums which are in direct physical contact with each other. Example: the heating of the metal rod from one end by heat source. In this case, the heat is transferred from one end to other end by conduction.

Convections :                                                                                                                                                Thermal convection is a process of energy transport affected by the circulation medium (gas, liquid or a powder substance). Convection is possible only in a fluid medium and is directly linked with the transport of medium itself. In natural convection, the circulation of the fluid medium is caused by buoyancy effects, i.e. by the difference in densities of the cold and heated particles. In forced convection, the flow of fluid is caused by a pump, fan or by atmospheric winds. These mechanical devices provides a definite circuit for the circulating currents and that speeds up the heat transfer rate.

Radiation :                                                                                                                                                     Thermal radiation is the transmission of heat in the form of radiant energy or wave motion from one body to another across an intervening space.

UNSTEADY HEAT CONDUCTION 

A solid body is said to be in a steady state if its temperature does not vary with time. If however there is an abrupt change in its surface temperature or environment it takes some time before the body attains an equilibrium temperature or a steady state. During this interim period the temperatures varies with time and the body is said to be in an unsteady or transient state. The steady state is thus the limit of transient temperature distribution for large values of times

The analysis of transient heat transfer is of great interest to engineers because of its widespread occurrence such as boilers tubes, rocket nozzles, electric irons, automobile engines, cooling of IC engines; cooling and freezing of food, heat treatment of metals by quenching, heating and cooling of building etc. for practical purposes is necessary to know the time taken to attain a certain temperature when the environment suddenly changes.