HEAT TRANSFER CHAPTER 9 Free Convection 们au Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 1 HEAT TRANSFER CHAPTER 9 Free Convection
Natural Convection Where we’ ve been ·。 Up to now, have considered basic concepts of natural convection, the governing equations and laminar free convection on vertical surface Where were going Consider empirical correlations for natural convection Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 2 Natural Convection Where we’ve been …… • Up to now, have considered basic concepts of natural convection, the governing equations and laminar free convection on vertical surface. Where we’re going: • Consider empirical correlations for natural convection
Empirical Correlations Typical correlations for heat transfer coefficient developed from experimental data are expressed as hl CRa L k L RO G 8B(T,-T)L3 AL=ori 1/4 For T n=1/3 For L Rayleigh number Vertical plate For an plate at constant Ts Coordinales or recommended curve 1 ⊥.9 2.63 3.89 6.03 0.5 Logo NuL, 1.5 33.1 108 14 501.0 11 1.0 .2 Log1o rar Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 3 Empirical Correlations • Typical correlations for heat transfer coefficient developed from experimental data are expressed as: Vertical Plate For an plate at constant Ts ( ) 3 Pr g T T L Ra Gr s L L − = = Log10RaL Log10NuL n L CRaL k hL Nu = = Rayleigh number = = 1/ 3 1/ 4 n n For T For L
Empirical Correlations(Contd Vertical Plate(Contd Alternative applicable to entire Rayleigh number range(for constant Ts) 0.387Ra16 E 0.825+ 9/68/27 9.26 +(0.492/Pr) Vertical cylinders Use same correlations for vertical flat plate f D≈35 ≥ 1/4 L Inclined plate 7 See figure 9.7 d Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 4 Empirical Correlations (Cont’d) Vertical Plate (Cont’d) • Alternative applicable to entire Rayleigh number range (for constant Ts) Vertical Cylinders • Use same correlations for vertical flat plate if: Inclined Plate 2 8/ 27 9/16 1/ 6 1 (0.492/ Pr) 0.387 0.825 + = + L L Ra Nu 1/ 4 ~ 35 GrL L D Eq 9.26 See Figure 9.7
Empirical Correlations(Contd Horizontal plate Cold plate Plate, T, (S< To late, Is Plate, T Hot plate (S>T Plate, T Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 5 Empirical Correlations (Cont’d) Horizontal Plate Cold Plate (Ts < T) Hot Plate (Ts > T)