HeatTransfer-I-Section-1

Convecuon heat transfer is classied according to

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Unformatted text preview: Boltzmann Eqn. q' '1 = σT14 , q' '2 = σT24 q' ' = h (Ts − T∞ ) q' '12 = σ(T14 − T24 ) € € € € 10 11 ConducUon Heat Transfer •  Fourier’s Law q' ' = − k € $T − T ' dT ≈ − k& 2 1 ) %L( dx #T − T & #T − T & q = −kA% 2 1 ( = kA% 1 2 ( $L' $€ ' L "W % 2' $ # m& [W ] •  k is the thermal conducUvity and € depends on the type of material € separaUng the two surfaces: –  Metals ~ 10 – 400 W/mK –  Non- Metals ~ 0.1 – 500 W/mK –  Liquids ~ 0.1 – 10 W/mK –  Gases ~ 0.01 – 0.1 W/mK ConducUon Heat Transfer 12 13 ConvecUon Heat Transfer •  Newton’s Law of Cooling q' ' = h (Ts − T∞ ) q = hA(Ts − T∞ ) "W % $ m2' # & [W ] € •  h is the convecUon heat transfer coefficient and € € depends on many things: € –  Process –  Fluid ProperUes –  Geometry –  LocaUon ConvecUon Heat Transfer •  Convec6on Heat Transfer is controlled by a thin hydrodynamic fluid layer at the heat transfer surface. •  A thermal boundary layer is also present and can be smaller, larger or equal in thickness to the hydrodynamic boundary layer. •  ConvecUon Heat Transfer coefficients are someUmes called “film coefficients” as a result. •  ConvecUon Heat Transfer is classified according to: –  Single Phase versus Two Phase (boiling/condensaUon) –  External Flow versus Internal Flow –  Forced Flow (pressure driven flow) versus Natural Flow (density driven flow) 14 •  •  •  •  15 RadiaUon Heat Transfer 4 Stefan- Boltzmann EquaUon q' 'rad = σ(Ts4 − Tsur ) "W m % $ ' # & σ = 5.67 × 10 − 8 [ W m 2 K ] is the Stefan- Boltzmann constant / 4 4 More generally, we write: q' 'rad = σε (Ts4 − Tsur ) "W m % $ ' # & 2 € € 4 qrad = σεA(Ts4 − Tsur ) 2 [W ] ε is the surface emissivity (a property). We will examine € € this later in more detail. When ε = 1 we have a “black body” or perfect emiSer...
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