Example: Heat Loss from Smokestack Given: Hot gas enters a exhaust stack shown at 150°C Find: Rate of heat loss from stack △ T of the exhaust gas 40°C T=0°C S 二一 20m Assume Ignore radiation heat loss 2 m · Steady state conditions Tin=150°C Solution method? V=1ms Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 11 Example: Heat Loss from Smokestack • Given: Hot gas enters a exhaust stack shown at 150º C. • Find: – Rate of heat loss from stack – T of the exhaust gas Ts 40 C = Tin = 150º C V = 1 m/s 2 m 20 m T = 0º C V = 5 m/s Assume: • Ignore radiation heat loss • Steady state conditions Solution Method?
Example: Heat Loss from Smokestack(Contd) Compute Reynolds#井 Ts=40C From table T=0°C V=5 m/s v=1.364×10-5m2/s D 5×2 Re 2 D 364×10 735×10 Tmn=150° V=lm/s Values of c and m From table: C=0.076 and m=0.7 Air properties( from Tables) Pr of freestream =0.714. Pr=0.705 hD Pr C Re D k P W K Heat Transfer Su Yongkang School of Mechanical Engineering
Heat Transfer Su Yongkang School of Mechanical Engineering # 12 Example: Heat Loss from Smokestack (Cont’d) • Compute Reynolds # From table, • Values of C and m From Table: C= 0.076 and m=0.7 • Air properties (from Tables) Pr of freestream = 0.714 ; Prs =0.705 Ts 40 C = Tin = 150º C V = 1 m/s 2 m 20 m T = 0º C V = 5 m/s m K W 2 h = 1.364 10 m /s −5 2 = 5 5 7.35 10 1.364 10 5 2 Re = = = − VD D 1 4 1 3 Pr Pr Re Pr = = s m D C D k hD Nu