Example:Heat Loss from Smokestack Given:Hot gas enters a exhaust stack shown at 150°C. Find: Rate of heat loss from stack △I of the exhaust gas T=40°C T=0°C V=5m/s 20m Assume: Ignore radiation heat loss 2m ·Steady state conditions Tin=150°C Solution Method? V=1m/s #11 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 (Cont'd) Compute Reynolds T=40°C From table, T.=0°C二 V=5 m/s 20m =1.364×10-5 m21s VD 5×2 Rep= 2m+ y1.364×10-5 =7.35×105 Tin=150°C V=1m/s 。 Values of C and m From Table:C=0.076 and m-0.7 Air properties (from Tables) Pr of freestream=0.714 ；Pr3=0.705 1/4 Nup hD=CRep Pr Pr K Pr. W h m2K #12 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