Convection correlations: laminar flow in circular tubes 2. The entry region(contd) for the combined entry length ReD pr/3 0.14 Nun=1.86 LID For values of Rep pr/(L/D)](m/y≥2 T=C 0.48<Pr<16.700 00044<(c/,)<975 Allfluid propertiesevaluated at the mean T +T,)2 m, 7 1,O Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 6 Convection correlations: laminar flow in circular tubes • 2. The entry region(cont’d) for the combined entry length • For values of 1/3 0.14 / Re Pr 1.86 = s D D L D Nu Re Pr/( / ) ( / ) 2 1/3 0.14 D L D s All fluid properties evaluated at the mean T Tm = (Tm,i +Tm,o )/ 2 Ts = C 0.48 Pr 16,700 0.0044 ( / s ) 9.75
Convection correlations: turbulent flow in circular tubes A lot of empirical correlations are available For smooth tubes, the fully developed flow Nun=0.023 Re 4/5D0.4 Heating Cooling: Nup=0.023 Re 4/5D-0.3 For rough tubes, coefficient increases with wall roughness. For fully developed flows Nua-1+127/8)(P2 (f/8)ReD-1000 Consider the entry length Short tubes N uD NuD≈ND,fa or =1+ N D, fd (x/D) For liquid metals, see textbook p461 Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 7 Convection correlations: turbulent flow in circular tubes • A lot of empirical correlations are available. • For smooth tubes, the fully developed flow Heating: Cooling: • For rough tubes, coefficient increases with wall roughness. For fully developed flows • Consider the entry length • For liquid metals, see textbook p461. 4/5 0.4 0.023Re Pr NuD = D 4/5 0.3 0.023Re Pr NuD = D 1 12.7( /8) (Pr 1) ( /8)(Re 1000)Pr 1/ 2 2/3 + − − = f f Nu D d NuD NuD, fd or m D f d D x D C Nu Nu ( / ) 1 , = + Short tubes