Heat and Mass Transfer: Module 1

Heat and Mass Transfer: Module 1

Module 1

BASICS OF HEAT TRANSFER
1.1 Difference between heat and temperature
In heat transfer problems, we often interchangeably use the terms heat and temperature. Actually,there is a distinct difference between 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 the 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 SI system, common units for measuring heat are the Joule and calorie.

1.2 Thermodynamics and heat transfer
in the course of thermodynamics you have studied the interaction of heat and work and the laws of the thermodynamics. first law deals with energy balance and leads to the concept of enthalpy, whereas second law deals with availability balance and determines the direction in which heat energy will flow and leads to the concept of entropy. in fact, it is the second law which says that heat flows from a location of high temperature to a location of low temperature. then, you may be wondering as to what is the need for a separate science of heat transfer. the answer is, thermodynamics deals with equilibrium processes; total heat transfered from one equilibrium state to another state can be easily calculated by the laws of thermodynamics. however, the rate of heat transfer and the temperature variation with time and position cannot be calculated by these laws alone and to do this, we need the laws of heat transfer.

Application of Heat Transfer

Mechanical Engineering: In boilers, heat exchangers, turbine systems,I.C. engine, etc.
Metallurgical Engineering: In furnace, heat treatment of components etc.
Electrical engineering: Cooling system for electrical motors, generators
Chemical Engineering: In process equipment used in refineries, chemical Plant etc.
Nuclear Engineering: In removal of heat generated by nuclear fission using liquid metal coolants, design of nuclear fuel rods against possible burnout etc.
Aerospace Engineering and Space Technology: in the design of aircraft system and components, rockets, missiles etc.
Cryogenic Engineering: in the production, storage, transportation and utilisation of cryogenic liquids(at very low temperature ranging from 100k to 4 k or even lower) for various industrial, research and defence applications.
Civil Engineering: in the design of suspersion bridges, railway tracks, air conditioning and insulation of buildings etc.

MODES OF HEAT TRANSFER
Conduction
-needs matter
-molecular phenomenon (diffusion process)
-without bulk motion of matter
Convection
-heat carried away by bulk motion of fluid
-needs fluid matter
Radiation
-does not needs matter
-transmission of energy by electromagnetic waves




CONDUCTION
Conduction is a microscopic phenomenon. Here, more energetic particle of a substance transfer their energy to their less energetic neighbors. Conduction can occurs in a solid, liquid or gas. In a solid, transfer of energy occurs by lattice vibration and or free electrons. In a liquid or gas, the transfer of energy occurs by collision and diffusion of molecules. It should be noted that in solid energy transfer occurs only in conduction whereas in liquid and gases other modes of energy transfer are also possible

Governing rate equation for conduction is given by Joseph Fourier as Fourier's law. It states that the rate of heat flow by conduction in a given direction is proportional to the area normal to the direction of heat flow and to the gradient temperature in that direction

Differential Form
q = kA dT/dx, W
k = Thermal Conductivity, W/mK
A = Cross sectional
Area, m 2
T = Temperature, K or o C
x = Heat flow path, m
q Difference Form
q = k A (T 1 - T2 ) / (x 1 - x2 )