INTRODUCTION OF HEAT AND MASS TRANSFER :
The present standard of living is made possible by the energy available in the form of heat from various sources like fuels. The process by which this energy is converted for everyday use is studied under thermodynamics, leaving out the rate at which the energy is transferred. In all applications, the rate at which energy is transferred as heat, plays an important role. The
design of all equipments involving heat transfer require the estimate of the rate of heat transfer. There is no need to list the various equipments where heat transfer rate influences their operation.
The driving potential or the force which causes the transfer of energy as heat is the difference in temperature between systems. Other such transport processes are the transfer of momentum, mass and electrical energy. In addition to the temperature difference, physical parameters like geometry, material properties like conductivity, flow parameters like flow
velocity also influence the rate of heat transfer.
The aim of this text is to introduce the various rate equations and methods of determination of the rate of heat transfer across system boundaries under different situations.
The study of heat transfer is directed to (i) the estimation of rate of flow of energy as heat through the boundary of a system both under steady and transient conditions, and (ii) the determination of temperature field under steady and transient conditions, which also will provide the information about the gradient and time rate of change of temperature at various
locations and time. i.e. T (x, y, z, τ) and dT/dx, dT/dy, dT/dz, dT/dτ etc. These two are interrelated, one being dependent on the other. However explicit solutions may be generally required for one or the other.
The basic laws governing heat transfer and their application are as below:
First law of thermodynamics : postulating the energy conservation principle: This law provides the relation between the heat flow, energy stored and energy generated in a given system. The relationship for a closed system is: The net heat flow across the system bondary + heat generated inside the system = change in the internal energy, of the system. This will also apply for an open system with slight modifications.
The second law of thermodynamics : establishing the direction of energy transport as heat. The law postulates that the flow of energy as heat through a system boundary will always be in the direction of lower temperature or along the negative temperature gradient.
Newtons laws of motion : used in the determination of fluid flow parameters.
Law of conservation of mass : used in the determination of flow parameters.
The rate equations : as applicable to the particular mode of heat transfer.
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