Modes of Heat and Mass Transfer

1. Modes of Heat and Mass Transfer P M V Subbarao Associate Professor Mechanical Engineering Department IIT Delhi Just as Intelligent as an Human Being .. ..

2. Modes of Heat & Mass Transfer • Conduction or Diffusion • Convection • Radiation

3. 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.

4. Conduction by lattice vibration

6. Circumstances Leading to Heat Conduction

7. 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.

8. 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.

9. 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

10. 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

11. Local Heat flux in a slab: Global heat transfer rate:

12. 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:

13. 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?

14. Most General form of Fourier Law of Conduction We are at cross roads !!!!!

15. 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.

16. Surprising Inventions !!!

17. 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.

18. Spread of Fire in A Room

19. Micro-structure of CPS

20. Thermal Performance