MODULE-1 According to thermodynamic concept ,the energy that is transferred as a result of a temperature difference is called HEAT THREE DIFFERENT TYPES OF HEAT TRANSFER: 1.CONDUCTION 2.CONVECTION 3.RADIATION
Itmportances of heat transfer: 1.electrical engineerg:cooling system for motors,generators and transformor 2.chemical engineering:evaporation,condensation,heating and cooling of fluid 3. civil engineering: constructions dams,structures,building design. 4.mechanical engineering:heat transfer in internal combustion engine,steam generations,refrigeration and heating and ventillation.
Conduction:it is the process by which heat flows from a region of higher temperature to a region of lower temperature within the substance. Conduction take place by two mechanism:a)by lattice vibration:molecules at higher temperature imparts energy to adjacent molecules at lower temperature. b)by free electrons: transfer of heat by free electrons, the free electrons concentration in non-metals is very low.Hence conduction is found in solids
Convection:it is possible only in the presence offluids( liquid and gases).As fluid passes over hotobjects,they pick up heat energy and carry it to colder area. Two types of convection: 1.free convection 2.forced convection
RADIATION:RADIATION HEAT TRANSFER BETWEEN TWO SUBSTANCES TAKEPLACE EVEN WITHOUT ANY MEDIUMTHROUGH ELECTROMAGNETIC WAVES
Heat flux: it is the amount of heat flow per unit area through a body=Q/A W/m2 the materials having higher thermal conductivity are conductors, while material having lower thermal conductivity are called as insulators
Radiation is the only way that heat canmove through a vaccum.Radiant heatcan be felt by holding a hand near anyhot object,higher the temperature ofthe object, more energy beingreleased. The denser the solid, thefaster heat will be transferred from thehottest regions to coldest.
Convection: Is the way heat flowsthrough fluids, whether they are liquids or gases.
THE LAWS OF HEAT TRANSFER. 1.Fourier’s law of heat conduction.The rate of heat flow is proportional tothe product of the area of flow A, andthe temperature gradient (-dt/dx),the constant of proportionality being the thermal conductivity k, which is a property of material. Unit is j/s or W Qx=-kA(dt/dx)
2.NEWTONS LAW OF HEAT CONVECTION Q=hA(t1-t2) heat transfer is occuring from asurface area A at temperature t1, to a fluid at a lower temperature t2. h is the convection co-efficient in w/m2K
STEFAN-BOLTZMANN LAW OF RADIATION: Q=EσAT4 T-absolute temperature, A-surface area, E-emissivity σ-stefan boltzmann constant,5.67*10-8w/m2k4It states that the radiant energy emitted by a blackbody is proportional to the surface area and fourth power of its absolute temperature.
Thermal conductivity:thermal conductivity, k is the property of a materials ability to conduct heat PURE METALS HAVE HIGHER VALUES OF THERMAL CONDUCTIVITY WHILE GASES AND VAPORS HAVE LOWEST.
K for a pure metals decreases with temperature. K=ko(1+bф+cф2) where ф=T-Tref.K for a non homogeneous materials increases both with increasingtemperature and increasing density.
Thermal conductivity: it is defined asthe ability of a substance to conduct heat. It is expressed in w/m-k or w/moC.Heat transfer co-efficient(h): ability of the fluid carry away heat from thesurfaces which in turn depends upon velocities and other thermal properties.unit w/m2 k or w/m2oC