Kirchoff's law Monochromatic emissivit monochromatic absorptivity However, emissivity and absorptivity for a given material vary with wavelength E.g. white paint a =0.4, GIr=0. 8 At a given frequency, Kirchoff's law says that the emissivity and absorbtivity will be the same White paint on spacecraft will have a low equilibrium temperature because it absorbs little energy in the frequencies of the solar spectra, but may have a high emissivity at the ir frequencies associated with the bodies temperature Called a selective surface
11 Kirchoff’s Law • Monochromatic emissivity = monochromatic absorptivity • However, emissivity and absorptivity for a given material vary with wavelength • E.g., white paint αs=0.4, εIR=0.8 At a given frequency, Kirchoff’s law says that the emissivity and absorbtivity will be the same. White paint on spacecraft will have a low equilibrium temperature because it absorbs little energy in the frequencies of the solar spectra, but may have a high emissivity at the IR frequencies associated with the bodies temperature. Called a selective surface
View factor F,a is the view factor from surface 1 with area A, to surface 2 with area A2 A,F=AF The view factor is a function of the size, geometry, relative position, and orientation of two surfaces in all directions and emmissivity and absorptivity are not strong functions of all Assume a diffuse gray surface condition, I.e., that a particular surface emits ec wavelength
12 View Factor • F1-2 is the view factor from surface 1 with area A1 to surface 2 with area A2 • A1 F1-2 = A2F2-1 A2 A1 The view factor is a function of the size, geometry, relative position, and orientation of two surfaces. Assume a diffuse gray surface condition, I.e., that a particular surface emits equally in all directions and emmissivity and absorptivity are not strong functions of wavelength
Radiation equation Into deep space q=oEAT4 · Between two surfaces q=oe1E2A1F12(T1-T24) If one surface is earth assume a=l and T=250to260K Technically, q= a(t-T)but t deep space is 4K, <<T4 We can use these equations to calculate the heat transfer between two surfaces, once we calculate the view factor
13 Radiation Equation • Into deep space q=σεAT4 • Between two surfaces q=σε1ε2 A1 F1-2 (T1 4- T2 4) • If one surface is Earth, assume ε=1 and T=250 to 260 K Technically, q= A(T4- T4) but T deep space is 4K, <<T4 We can use these equations to calculate the heat transfer between two surfaces, once we calculate the view factor
Thermal analysis Conservation of energy lingo +qdissipatedsoeint And incident absorbed reflected +transmitted for transparent materials with no internal dissipation Thermal balance energy absorbed energy dissipated -energy emitted=0 in steady state Dissipated energy is primarily referring to the heat generated by electrical For transparent materials with no internal dissipation, energy is either absorbed reflected or transmitted
14 Thermal Analysis • Conservation of energy • And qincident=qabsorbed + qreflected +qtransmitted for transparent materials with no internal dissipation t E qin qout qdissipated ∂ ∂ − + = int Thermal balance energy absorbed + energy dissipated – energy emitted = 0 in steady state. Dissipated energy is primarily referring to the heat generated by electrical equipment For transparent materials with no internal dissipation, energy is either absorbed, reflected or transmitted