# Forced Convection

In this section the correlations used to determine the heat and mass transfer rates are presented. The convection process may be either free or forced convection. In free convection fluid motion is created by buoyancy forces within the fluid. In most industrial processes, forced convection is necessary in order to achieve the most economic heat exchange. The heat transfer correlations for forced convection in external and internal flows are given in Tables 4.8 and 4.9, respectively, for different conditions and geometries.

The mass transfer correlations are obtained by replacing Nu by Sh and Pr by Sc according to the heat and mass transfer analogy.

 Geometry Conditions Correlation Flat plate Laminar, local, 7 3 av e Pr a 0.6, Re, < 10 Nu* = 0.332 Re‘/2 Pr1’3 Laminar, average, Tav Pr a 0.6 , Re* < 105 N~ux = 0.664 Re’/2 Pr1′ ‘ Turbulent, local, Tav 60 a Pr a 0.6 Rex s 10s Nux = 0.0296 Re"2 Pr’/3 Mixed, average, Tav 60 > Pr > 0.6, Re, < 10S = (0.037 Re*/5 — 871 ) Pr1,1 Fully turbulent, average, Tav 5xl05 < Re* < 108 Nax = 0.037 Re*/,S Pr’/; Cylinder Average, , Pr > 0.7 0.4 < Re,/ < 4 x 105 Nurf = C Re” Pr1/J C and m are given in Table 4,10 Average, T„, 500 > Pr > 0.7 1 < Ked < 106 /Pr 1/4 Nu(; = C ReJ Pr’fp^) C and m are given in Table 4.11 Average, Tav Rerf Pr > 0.2 — 0.62 Re,/2 Pr1/3 Nu, i = 0.3 + X [-(%?)] R. j ^ >"Y’1 L V28 200J j Sphere Average, T„, ^80 > Pr > 0.71 3.5 < Re,, < 7.6 xlO4 1 < — < 3.2 Ps Nu,; = 2 + (0.4 Re/’2 + 0.06 Re/’3) x *“grf, , Nu,/ = 2 + 0.6 Rej/2 Prl/3|Z5(|) °" j’’4 Falling drop Average,
 Note: Tav = 0.5(TM+ Ts), where T„ is the free stream temperature and Ts is the surface temperature.

 0 89 З + 0.387(Gr Pr)1/6 V • O m J 1 ( 1 + V 0.492 Pr V. y 9/16′ 8/27
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