Radiation Heat Transfer

1. Radiation Heat Transfer P M V Subbarao Associate Professor Mechanical Engineering Department IIT Delhi Medium or Mediator is not a Must .. .. If Potential is High Enough….. Mediator is a Nuisance ……

2. Thermal Radiation • Radiation does not require a medium to pass through; • It is the only form of thermal energy travel present in vacuum. • It uses electromagnetic (photons) means of transportation, which travels at the speed of light. • Thermal radiation is emitted by any matter with temperature above 0 degree Kelvin (-273 °C). • The electromagnetic spectrum classifies radiation according to wavelengths of the radiation. • Main types of radiation are (from short to long wavelengths): gamma rays, x-rays, ultraviolet (UV), visible light, infrared (IR), microwaves, and radio waves. • Radiation with shorter wavelengths are more energetic and contains more thermal energy. • Radiative heat transfer occurs when the emitted radiation strikes another body and is absorbed. • We all experience radiative heat transfer everyday; solar radiation, absorbed by our skin, is why we feel warmer in the sun than in the shade.

3. Means for Radiation Heat Transfer • Depending on the degree of their permeability for heat rays, substances are said to be diathermal or athermal – • Diathermal if they let radiation pass more or less unhindered. • Athermal, if they partly absorb it and heat up themselves. • For example, air and rock salt are diathermal; • Glass for long waves (>4mm), metals and lampblack are athermal. • We sense the action of the Sun's rays as heat, because our skin absorbs the radiation, while the air remains cold, because is lets the rays pass. • The same explanation applies to the difference between the temperatures of thermometers and air.

4. Thermal Energy Balance for A Satellite

5. Recent hazard is that microwave radiation emitted by cell-phones has the potential to heat up human tissue in the area of our head where the phone is pressed against. This thermal (heating) effect can cause headaches, fatigue, tissue swelling, tingling and other severe potential long-term damages. Younger people are more vulnerable to EMF waves due to skull-thickness, which is maturing and growing. Cell Phones: Invisible Hazards in the Wireless Age The Era of Microwave Radiation

6. The World’s Energy Future Belongs in Orbit Space Studies Institute

7. P M V Subbarao Mechanical Engineering, IIT Delhi 1 GW Solar Space Power Station

8. Microwave Oven

9. Radiation Theory • No single theory - both wave theory and quantum mechanics is required to explain all radiative phenomena. • For our purposes we use the wave phenomenon. • Highest temperature in engineering application is about 6000 K. • Emissive power (E): The total amount of energy flux emitted by a surface at a given temperature is the emissive power. • It depends on the temperature of the surface, and the surface characteristics. • At a defined temperature there is a maximum limit to the emissive power of a surface.

10. Black Body Radiation • The maximum emissive power at a given temperature is the black body emissive power (Eb). • The black body emissive power is given by the Stefan-Boltzman Law: • Black body: Theoretical concept, but useful in practice • Gives estimate of maximum absorption and emission for surface • Blackbody emissive power (W/m2) depends on temperature (T) of surface

11. From Where Does Eb=T4 Come? • Identify the monochromatic emissive power, Eb spectral blackbody emissive power  power at each wavelength against wavelength

12. General Body • The amount of radiation emitted by a general object is given by: • ε is a material property called emissivity. • The emissivity has a value between zero and 1, and is a measure of how efficiently a surface emits radiation. • It is the ratio of the radiation emitted by a surface to the radiation emitted by a perfect emitter at the same temperature.

13. where I is the incident radiation. Interaction between a surface and incident radiation I • The emitted radiation strikes a second surface, where it is reflected, absorbed, or transmitted. • The portion that contributes to the heating of the surface is the absorbed radiation. • The percentage of the incident radiation that is absorbed is called the absorptivity, α. • The amount of heat absorbed by the surface is given by:

16. Real Life Heat Transfer Problems : Conjugate Heat Transfer P M V Subbarao Associate Professor Mechanical Engineering Department IIT Delhi Togetherness is More Powerful …….

17. Flexible Baked Filament-based polymer Air Craft Window Tamb = -400C Tamb = 22 0C

18. Surface Temperature Distributions

19. qradiation qconvection qconduction qconduction qconvecrion qradiation qconvection Conjugate Heat Transfer Model

20. Steam pipe Insulation Optimal Insulation for Steam Piping Tsteam Tpipe,in Steam Ambient Air Tpipe,out Design Confusion: Tinsulation,in Tinsulation,out Tair

21. Moist Heat Cooking • Moist heat methods use water in various states to heat food. • The most common state is boiling, where water is heated by conduction through a pot on stovetop, and the heat is transferred to the food through convection. • Various states of water are used for cooking, and there are special names for some of them. • Poaching uses warm water just before boiling. • Simmering refers to cooking in water when it has just begun to boil. Boiling uses vigorously boiling water. • Steaming uses the water in vapor phase to heat the food.

22. HEAT TRANSFE IN TURNING • In recent years there has been a growing interest in modelling of metal cutting process. • It has been accompanied also by the interest in modelling of thermal behaviour of the workpiece. • Thermal expansion affects the machining accuracy, hence the latter interest can be attributed to the demand for higher accuracy and to the effort to improve the ability of its prediction.

23. Heat Transfer in Turning

24. Location of Thermocouples

25. Temperature Distribution Without cooling Jet With Cooling Jet