1. Heat Transfer Carlos Silva December 9 th 2009

2. Energy transference Energy can be transferred between a system and its surroundings Work Heat Mass flow

3. HEAT TRANSFER

4. What is it? Transition of thermal energy from a hotter body to a cooler body occurs in such a way that the body and the surroundings reach thermal equilibrium always occurs from a higher-temperature object to a cooler- temperature one (second law of thermodynamics) heat transfer between them can never be stopped; it can only be slowed Heat transfer is energy in transit due to a temperature difference

5. How does it happen? Energy can be transferred as heat in 3 modes: Conduction transfer by direct contact of particles of matter Convection transfer between a solid surface and the nearby fluid in motion natural forced Radiation transfer of heat through empty space

6. CONDUCTION

7. Definition Tranfer of heat within a substance, molecule by molecule solid or fluid (greater in solids, specially metals) molecules gain energy and begin to vibrate faster temperature increases and they begin to bump into the molecules next to them heat is being transferred from the warm end to the cold end

8. Fourrier’s law (law of heat conduction) time rate of heat transfer through a material is proportional to the negative gradient in the temperature and to the area at right angles, to that gradient, through which the heat is flowing. Heat flow (W/m 2 )

9. Conductivity k (W/mK) characteristic of the substance

10. Thermal resistance or resistivity R-value (m 2 K/W) reciprocal of conductivity measure of insulation

11. CONVECTION

12. Definition movement of molecules within fluids Occur between a surface and a moving or stationary fluid when they are at different temperatures Two mechanisms Free or natural When the flow is induced by buoyancy forces Forced when the flow is caused by external forces

13. Newton’s law of cooling the rate of change of the temperature of an object is proportional to the difference between its initial temperature and the ambient temperature Convection heat transfer coefficient h (W/m 2 K) Depends on conditions in the boundary layer, which is influenced by surface geometry, the nature of fluid motion,

14. Natural convection mass movement of a fluid in the vertical direction warm air is less dense than cold air, making cold air heavier than warm air layer of molecules touching the surface are heated by conduction

15. Forced convection transfer of heat in the horizontal direction (advection) larger transfer than in natural convection caused by a fan or a pump

16. RADIATION

17. Definition transfered through wave energy energy is released when these waves are absorbed by an object All surfaces of finite temperature emit energy in the form of electromagnetic waves

18. Radiation Emitted radiation can be: Absorbed Increasing the internal energy of the gas molecules. Reflected Radiation is sent backward. The Albedo represents the reflectivity of an object and describes the percentage of light that is went back. Scattered deflected in all directions Transmitted radiation passes through the gas unchanged.

19. Stefan-Boltzman law The temperature of an object is related to the emitted radiation Emissivity (W/m 2 ) σ = 5.67x10-8W/m2K4 0<ε<1 (radiative property) Blackbody ideal radiator ( ε=1 ) Radiation exchange

20. Earth Albedo Sun shines on Earth 341 W/m 2 of surface area The Albedo has been declining earth reflects less energy

21. Earth budget

22. EXAMPLES

23. Boiling water

24. Heat transfer in houses Infrared image

25. Conduction in a house

26. Convection in a house

27. Radiation in a house