NURADIBAH BINTI MOHD AMER

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Presentation transcript:

NURADIBAH BINTI MOHD AMER RADIATION PART 1 NURADIBAH BINTI MOHD AMER

INTRODUCTION AND BASIC EQUATION FOR RADIATION (1) NATURE OF RADIANT HEAT TRANSFER In radiant heat transfer, the medium through which the heat is transferred usually is not heated. Radiation heat transfer is the transfer of heat by electromagnetic radiation. In an elementary sense the mechanism of radiant heat transfer is composed of three distinct steps or phases: The thermal energy of a hot source, such as wall of a furnace at T1, is converted into energy in the form of electromagnetic-radiation waves. These waves travel through the intervening space in straight line and strike a cold object at T2 such as furnace tube containing water to be heated. The electromagnetic waves that strike the body are absorbed by the body and converted back to the thermal energy or heat.

(2)ABSORPTIVITY AND BLACK BODIES -When thermal radiation falls upon a body, part is absorbed by the body in the form of heat, part is reflected back into space, and part may actually transmitted through body. -For most cases, bodies are opaque to transmission so this will be neglected. For opaque bodies  = absorptivity or fraction absorbed Ρ = reflectivity or fraction reflected

A BLACK BODY is defined as one that absorbs all radiant energy and reflects none. ρ = 0 and  = 1.0 for BLACK BODY Such a black body also emits radiation depending on its temperature and does not reflect any. The ratio of the emissive power of a surface to that of a black body is call emissivity and is 1.0 for black body. Kirchhoff’s Law states that at the same temperature T1, 1 and ξ1 of a given surface are the same or 1 = ξ1

(3) RADIATION FROM A BODY AND EMISSIVITY The basic equation for heat transfer by radiation from a perfect black body with an emissivity =1.0 is q = heat flow (W) A= surface area of body ( 𝑚 2 ) σ = 5.676 X 10 −8 𝑊 𝑚 2 𝐾 4 T = temperature in K For a body that is not a black body and has emissivity ξ < 1, the emissive power is reduced by ξ or Substances that have emissivities of less than 1 are called gray bodies. All real materials have an emissivity ξ < 1

RADIATION TO A SMALL OBJECT FROM SURROUNDINGS

RADIATION TO A SMALL OBJECT FROM SURROUNDINGS Example 1 A small oxidized horizontal metal tube with an OD of 0.0254 m (1 in), 0.61 m (2ft) long, and with a surface temperature at 588 K (600 °F) is in a very large furnace enclosure with fire-brick walls and the surrounding air at 1088 K (1500 °F). The emissivity of the metal tube is 0.6 at 1088 K and 0.46 at 588 K. Calculate the heat transfer to the tube by radiation using SI and English units.

COMBINED CONVECTION PLUS RADIATION HEAT TRANSFER

A convenient chart giving values of ℎ 𝑟 in English unit with ξ =1 A convenient chart giving values of ℎ 𝑟 in English unit with ξ =1.0 is given in figure below.

COMBINED CONVECTION PLUS RADIATION FROM A TUBE Example 2 Recalculate Example 1 for combined radiation plus natural convection to the horizontal 0.0254 m tube.