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Modes of Heat and Mass Transfer. P M V Subbarao Associate Professor Mechanical Engineering Department IIT Delhi. Just as Intelligent as an Human Being. Modes of Heat & Mass Transfer. Conduction or Diffusion Convection Radiation. Conduction Heat Transfer.
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Modes of Heat and Mass Transfer P M V Subbarao Associate Professor Mechanical Engineering Department IIT Delhi Just as Intelligent as an Human Being .. ..
Modes of Heat & Mass Transfer • Conduction or Diffusion • Convection • Radiation
Conduction Heat Transfer • Conduction is at transfer through solids or stationery fluids. • When you touch a hot object, the heat you feel is transferred through your skin by conduction. • Two mechanisms explain how heat is transferred by conduction: lattice vibration and particle collision. • Conduction through solids occurs by a combination of the two mechanisms; heat is conducted through stationery fluids primarily by molecular collisions.
The Chip Carrier • The chip is housed in a chip carrier or substrate made of ceramic, plastic, or glass in order to protect its delicate circuitry from detrimental effects of the environment. • The basic part: • The chip • Bond • Lead frame • Bond Wires • Lid • Pins • The design of the chip carrier is the first level of thermal control of electronic devices.
Heat Generation & Conduction through A Basic Chip • The circuitry of an electronic component through which electrons flow and thus heat is generated is referred as the junction • Junctions are the sites of heat generation and thus the hottest spots in a component. • Modern chips can with stand a Maximum Junctions temperature of 150 oC. • Lower Junction temperatures are desirable for extended and lower Maintenance costs. • The basic thermal issue is : Heat generation in 1 micron chip made of silicon wafer.
Fourier’s law of heat conduction A Constitutive Relation • The rate of heat transfer through the wall increases when: • The temperatures difference between the left and right surfaces increase, • The wall surface area increases, • The wall thickness reduces, • The wall is changed from brick to aluminum. • If we measure temperatures of the wall from left to right and plot the temperature variation with the wall thickness, we get: This is called as Fourier Law of Conduction
Statement of Fourier’s Law The (mod of) heat flux, q’’, (the flow of heat per unit area and per unit time), at a point in a medium is directly proportional to the temperature gradient at the point. Temperature gradient across the slab of thickness Dx: T The heat flux across the slab x
Local Heat flux in a slab: Global heat transfer rate:
Mathematical Description • Temperature is a scalar quantity. • Heat flux is defined with direction and Magnitude : A Vector. • Mathematically it is possible to have: Using the principles of vector calculus:
Further Physical Description • Will k be same in all directions? • Why k cannot be different each direction? • Why k cannot be a vector? Will mathematics approve this ? What is the most general acceptable behavior of k, approved by both physics and mathematics?
Most General form of Fourier Law of Conduction We are at cross roads !!!!!
Physical-mathematical Feasible Model • Taking both physics and mathematics into consideration, the most feasible model for Fourier’s Law of conduction is: Thermal conductivity of a general material is a tensor.
Fire Resistant Wood • Among the assessment properties of wood composite of structural members in building construction, fire performance is important and getting more attention nowadays. • A new composite called molded carbon phenolic spheres (CPS), a mixture of sugi wood charcoal powders and phenol formaldehyde resin molded with a hot press is developed by a research group in Japan. • The heat due to a fire accident should be thrown out fast outside the building.