3 Thermal radiation When two bodies at different temperature are separated by perfect vacuum, heat transfer between them by conduction or convection is not possible, but it takes place(occurs by thermal radiation The radiation energy emitted by a body because of its temperature. The radiation energy is transmitted in the space in the form of electromagnetic waves or in the form of discrete photons Maxwell' s theory and planck's hypothesis
3 Thermal radiation When two bodies at different temperature are separated by perfect vacuum, heat transfer between them by conduction or convection is not possible, but it takes place(occurs) by thermal radiation. The radiation energy emitted by a body because of its temperature. The radiation energy is transmitted in the space in the form of electromagnetic waves or in the form of discrete photons. Maxwell’s theory and Planck’s hypothesis
3 Thermal radiation a Surface process thermal radiation is absorbed or emitted by surface of body for solids or fluids a Bulk process -thermal radiation is absorbed or emitted by the bulk of gases Stefan-Boltzman law-radiation energy emitted by a body is proportional to the fourth power of absolute temperature for black body, as emitted A aNet exchanged energy of radiation for black body q=Ao(TH aNet exchanged energy of radiation for practical body 9=a+-)
Surface process—thermal radiation is absorbed or emitted by surface of body for solids or fluids; Bulk process—thermal radiation is absorbed or emitted by the bulk of gases. Stefan-Boltzman law—radiation energy emitted by a body is proportional to the fourth power of absolute temperature for black body,as 3 Thermal radiation 4 qemitted = AσT ( )42 4 q=Aσ T1 −T ( )42 4 q=Aσεn T1 −T Net exchanged energy of radiation for black body Net exchanged energy of radiation for practical body
1-2 Heat transfer process and overall coefficient of heat transfer The heat flow transfers from a hot fluid to a cool fluid through a solid wall 热流体 a for example: a plane wall exposed at a hot fluid a on one side and a cooler fluid b on the other side,.图2通过平的传热过程 In steady state the heat transfer is expressed by
1-2 Heat transfer process and overall coefficient of heat transfer The heat flow transfers from a hot fluid to a cool fluid through a solid wall For example: a plane wall exposed at a hot fluid A on one side and a cooler fluid B on the other side. In steady state the heat transfer is expressed by 热流体 冷流体 tf1 q tw2 tw1 α2 α1 tf2 图8-2通过平壁的传热过程
1-2 Heat transfer process and overal coefficient of heat transfer 6×、1=U(7 fI ∫2 h A Where U called overall heat transfer coefficient, and A0 Ah A k A,h, Lead into a concept of thermal resistance f2 1/(A0) R+r+ r R
Where U called overall heat transfer coefficient,and 1-2 Heat transfer process and overall coefficient of heat transfer ( ) 1 1 1 2 1 1 2 2 1 2 f f w f f UA T T h A A h A T T q = − + + − = κ δ 1 1 2 2 1 1 1 AU A h Aw A h = + + κ δ R T T R R R T T AU T T q f f w f f f f 1 2 1 2 1 2 1 2 1 /( ) − = + + − = − = Lead into a concept of thermal resistance
Chapter 2 Steady state heat conduction 2-1 Basic law of heat conduction 2-2 Differential equation of heat conduction 2-3 One-Dimensional steady-state conduction 2-4 Conduction through the fin Contain parts of 1-1 and 2-1v 11
Chapter 2 Steady state heat conduction 2-1 Basic law of heat conduction 2-2 Differential equation of heat conduction 2-3 One-Dimensional steady-state conduction 2-4 Conduction through the fin Contain parts of 1-1 and 2-1~11