10 Nombre de Nusselt moyen Nu Pr 1 3 0037Re x 0 8 850 07 1 3 0037 889710 5 08

10 nombre de nusselt moyen nu pr 1 3 0037re x 0 8 850

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Nombre de Nusselt moyen : Nu = Pr 1 / 3 . ( 0.037Re x 0 .8 850 )=( 0.7 ) 1 / 3 . ( 0.037 ∗( 8.89710 5 ) 0.8 850 )= 1133 Coefficient d’échange moyen : 2 8141.55 0 0272 0 5 61.64 - h Nu.k / L * . / . w.m . C Le flux de chaleur échangé est : q = h . S . ( T paroi T f )= 61.64 ∗( 0.5 1 )∗( 50 30 )= 616.4 w Exemple 2 Une plaque verticale de longueur 5 mètre, maintenue à une température égale à 100°C est exposée à de l'air dont la température est 20°C à la pression atmosphérique. Calculer le coefficient d'échange par convection libre pour une largeur de la plaque égale à 20 cm en considérant l'air comme un gaz parfait. 11
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Les propriétés du fluide sont évaluées à la température : T p + T f 2 = 60 ° C ρ = 1.059 Kg/m 3 , μ = 1.9910 5 kg. m -1 . s -1 , K = 0.0287 W/m . °C , P r = 0.7 , ν = μ ρ = 1.879 10 5 m 2 /s Air considéré comme gaz parfait : β = 1 / T = 1 / 333.23 = 3.10 3 G r = ρ 2 . g . d 3 . β . ΔT μ 2 = g . d 3 . β . ΔT ν 2 = 9.81.5 3 .310 3 . 80 1.87910 5 = 1.566710 10 G r . P r = 1.0910 10 ¯ N u = C . ( Pr . G r ) n Formulaire on tire c = 0.10 et n = 1 / 3 ¯ N u = 0.10 . ( Pr . G r ) 1 / 3 = 222.17 Coefficient d’échange est : h = N u . k/L = 222.17 0.0287 / 5 = 1.275 w . m 2 . ° C Le flux de chaleur échangé est : q = h . S . ( T paroi T f )= 1.275 ∗( 5 0.2 )∗( 80 )= 102 w 12
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  • Fall '16
  • Convection, masse volumique, pression, Conductivité thermique, Viscosité cinématique, Nombre de Prandtl

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