007 external forced convection thai
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External forced convection - Thai
007 external forced convection thai
- 1. Ch 7 External Forced Convection 1 King Mongkut¡¯s University of Technology North Bangkok Faculty of Engineering Department of Mechanical and Aerospace Engineering
- 2. Objectives 2 ? ???????????????????????????????????????????? Internal flow ??? External flow ??? ? ????????????????????????????????????????????? (Friction drag) ?????????? ???????????????? (Pressure drag) ??? ?????????????????????????????????? ?????????????????????????????????????????????????? External flow ??? ? ???????????????????????????????????????????????????????????????? ?????????????? Flat plate ???????????????????? Laminar ??? Turbulent ??? ? ??????????????????????????????????????????????????????????????????? ??????????????????????????????????? ???????????????????? Laminar ??? Turbulent ???
- 3. Contents 3 ? Drag and heat transfer in external flow ? Parallel flow over flat plates ? Flow across cylinders and spheres ? Flow across tube banks
- 4. Introduction 4 ? ??????????? ????????????????????????????????? ??????????????????????? ????????????????????????? ??? ?????? Nu, Re ??? Pr ?????????????????????????? ???????????????????????????????????????????????????????????????????????? ????????? ???????????? Flat plate ?????????????????????? ???? ??? ?????????????????? Pr),(ReLL fNu ?Pr),Re*,( LL xfNu ?
- 5. Drag and Heat Transfer in Ext. Flow 5 ? ????????????????????????? ??????????????????? ???????? ???? ? ???????????????????? (Drag force) ???? ?????????????????????????????s ? ???????????????? (Lift force) ???? ????????????????????????????? ? ???????????????????????????????? ? ???????: Free-stream velocity ??? ??????????????????????????? ?????? ???????? ? ??????????????????????????????????? ?????????????????? 0 (??????????) ??? Free- stream value ???????????????????????????????? ? Free-stream velocity ???????????????????????????????
- 6. Drag and Heat Transfer in Ext. Flow 6 ? ???????????????????? (Drag force): ???????????????????????????????? Wall shear forces ????????????????????????????????????? ? ???????????????? (Lift force): ???????????????????????????????? Wall shear forces ??????????????????????????????????????????????? ? Drag Force (FD) ??????????? density, upstream velocity, geometry ? ??????????????????????? (Drag Coefficient, CD) AV F C D D 2 2 1 ? ?
- 7. Drag and Heat Transfer in Ext. Flow 7 ? ??????????????? ???????????????????????????????????? ???????????????????????? ?????????? ?????????????????????????????? ? ??? Nu ??????? x ??? (Nux) ?????? Nu ?????? ????????????????????????? ??? ? ????????????????????????????????????????? ?????????????????????? Film temperature (Tf) ?????? Pr),Re,( * xx xfNu 1? Pr),(ReLfNu 2? )PrRe nm LCNu ? 2 ?? ? TT T s f
- 8. Drag and Heat Transfer in Ext. Flow 8 ? ?????????????????????????? (CDx) ???????????????????????????????? (hx) ?? ????????????????????????????????????? ????????????????????? Velocity boundary layer ????????????????????????? ? ???????????????????????????????? (CD) ???????????????????????????????? ?????? (h) ??????????????????????????????????? ? ????????????????????????????????????????????????????????? Newton¡¯s law of cooling , 0 1 L D D xC C dx L ? ? 0 1 L xh h dx L ? ? )( ??? TThAQ ss ?
- 9. Parallel Flow Over Flat Plates 9 ? ????????????????????????? Flat plate ???????????? L ? ??? Re ??????? x ???????????????????? ? Critical Reynolds number ??? ? ???????????? ??? Critical Re ???????? ??????????? 105 to 3X106 Rex Vx Vx? ? ?? ? 5 Re 5 10cr cr Vx? ?? ? ?
- 10. Friction Coefficient 10 ? ???????????? ???????????????????????? Boundary layer ?????? ?????????????????????????????? x ??? ?? Flat plate ?????? ? Laminar Flow ? Turbulent Flow x x x Re .914 ?? x xfC Re . , 6640 ? 51 380 x x x Re . ?? 51 0590 x xfC Re . , ? 5 105??xRe 75 10105 ??? xRe
- 11. Friction Coefficient 11 ? ???????????? ?????????????????????????????????????????????????????? ??? Flat plate ?????? ? Laminar Flow ? Turbulent Flow , 0 1 L D D xC C dx L ? ? 5 105??xRe x fC Re .331 ? 51 0740 x fC Re . ? 75 10105 ??? xRe
- 12. Heat Transfer Coefficient 12 ? ???????? 6 ???????? Nu ??????? x ??? ?????? ? Laminar Flow ? Turbulent Flow 3150 3320 PrRe. . x x x k xh Nu ?? 60.Pr ? 3180 02960 PrRe. . x x x k xh Nu ?? 6060 ?? Pr. 75 10105 ??? xRe
- 13. Heat Transfer Coefficient 13 ? ????????????? Nux ????????????????????????????????????????????????? ??????? x ??? (hx) ??? ???? hx ????????????????? ???? x-0.5 ???????????? ??? Laminar ? ??????? ????????? Flat plate (x=0), hx ??????????? ¡Þ ???????????? x ??????? ??? hx ?????? ?????????????????????? ? ??? hx ????????????????? Transition ? ?????????????????????? Turbulent ??? hx ??????????????? x ????????? ???? factor x-0.2 50. Rex
- 14. Heat Transfer Coefficient 14 ? ???????? Nu ??????????????????????????????????? ?????? Condition ??? ?????????????? 2 ??? ??? ? ??????????????????????????????? Ts = constant ? ???????????? Uniform heat flux
- 15. Heat Transfer Coefficient (Ts = const.) 15 ? ??? Nu ???????????????????? Flat plate ?????????????? ? Laminar Flow ? Turbulent Flow ? ????????? Flat plate ???????????????????? ?????????????????????????? Laminar ??? Turbulent ?????????????? Nu ?????? 3150 6640 PrRe. . x k hx Nu ?? 3180 0370 PrRe. . x k hx Nu ?? 6060 ?? Pr. 75 10105 ??? xRe 5 105??xRe ? ?0.8 1 3 0.037Re 871 PrLNu ? ? 6060 ?? Pr. 75 10105 ??? xRe
- 16. Uniform Heat Flux 16 ? ?????? Flat plate ????? Uniform heat flux ?????? ??????????? Nux ?????? ? Laminar Flow ? Turbulent Flow 3150 4530 PrRe. . xxNu ? 3180 03080 PrRe. . xxNu ? 6060 ?? Pr. 75 10105 ??? xRe 5 105??xRe
- 17. Flow Across Cylinder and Spheres 17 ? ????????????????????????????????????????????????????????????? ???? ?????? ???? ? shell-and-tube heat exchanger, ? Pin fin heat sinks for electronic cooling. ? ?????????? Re ?????????????????????????????????? ????? ????? ??????????????????????????? (D) ??????? Characteristic length ? ??? Critical Reynolds ?????????????????????????????????? ???????????? Recr=2x105
- 18. Heat Transfer Coefficient 18 ? ??? Nu ?????????????????????????????? (Churchill and Bernstein), ????? Re Pr > 0.2, ??????????????????????????? Tflim ? ??? Nu ??????????????????????????? (Whitaker), ????? 3.5 ¡Ü Re ¡Ü 80,000 ??? 0.7 ¡Ü Pr ¡Ü 380, ???????????????????????? T¡Þ ?????? viscosity ??? Ts ? ? 4 55 81 2 1/3 1 42/3 0.62Re Pr Re 0.3 1 282,0001 0.4 Pr cyl hD Nu k ? ?? ? ? ? ? ?? ?? ? ? ?? ? ? ?? ? ? ? ? 1 4 1 2 2 3 0.4 2 0.4Re 0.06Re Prsph s hD Nu k ? ? ? ? ? ? ?? ? ? ? ? ?? ? ? ?
- 19. Heat Transfer Coefficient 19 ? ????????????????????????????????????????????????? ???????????????????? ????? Nu ?????? ??? ? ?????? n = 1/3 ? Table 7-1 ??????????????????????????????? ???????????????????????????? ???????????? ? ????????????????????????????????????? Tflim nm xC k hx Nu PrRe??
- 22. Flow Across Tube Banks 22 ? ?????????????????????????????????????????????????? ???? Condenser, evaporator, H.E. ?????????? 2 ????????????????? ???????????????????? ??????????? ???????????????????????????????????????????????????????? ???????????? ? ??????????????????????????????????????????????????????????????????? ?????????????????????????????? ???????????????????????????????? ? ??????????????????????????????? ?????????????????????????? ??????????????????????????????????? ????????????????????????????? ??? ?????????????????????? ??????? ????????????????????????????????????? ??????? ?????????????????????????????
- 23. Flow Across Tube Banks 23 ? ??????????????????????????????? ? ??? In-line ?????? Staggered ? Characteristic length: ??????????? ? Three characteristics ? Transverse pitch ST ? Longitudinal pitch SL ? Diagonal pitch SD between tube center ?????? ? ?????????????????????????????? A1 = STL ?????? AT = (ST-D)L ? ??????????????????????????????? In-line arrangement Staggered arrangement ? ?22 2TLD SSS ??
- 24. Flow Across Tube Banks 24 ? ????????? Vmax ?????????????????????????????????????? ????????????????? Re ????? Vmax, ????? ? Vmax ????????????????????? ?????????????? Conservation of mass ? ?????????????????????????? In-line ? ?????????????????????????? Staggered in case of SD < (ST+D)/2 in case of SD < (ST+D)/2 ?? ? DVDV D maxmaxRe ?? V DS S V T T ? ?max V DS S V T T ? ?max V DS S V D T )(2 max ? ?
- 25. Flow Across Tube Banks 25 ? ?????????????????????????????????????????????????????????????????????? ?????????????????????????? ?????????????? ? ?????? 0.7 < Pr < 500 and 0 < ReD < 2x106 ? ?????????????????? ??????????????????? ??? Tm = (Ti + Te)/2 ? ? 0.25 Re Pr Pr Prm n D D s hD Nu C k ? ?
- 26. Flow Across Tube Banks 26 ? Table 7-2 ???????????? Nu ??????????????????????????????????????? ??????? 16 ???
- 27. Flow Across Tube Banks 27 ? ??????????????????????????? 16 ??? ??????????? Nu ?????? ? ?????? ??? F ???? Correction factor ???????? Table 7-3 , LD N DNu F Nu? ?
- 28. Flow Across Tube Banks 28 ? ??????????????????????????????????????????????????? ?????????????? ??? Newton¡¯s Law ???????? conservation of energy ? ?Tln ??? logarithmic mean temperature difference )(ln ieps TTcmThAQ ???? ?? ? ?? ? ? ?ie ie ises ises TT TT TTTT TTTT T ?? ??? ? ?? ??? ?? lnln )()( ln ? ? ? ? ? ? ? ? ? ? ???? p s isse cm hA TTTT ? exp
- 29. Procedures for Calculation 29 ? ????????????????????????????????????? ???? Flat plate, Sphere ???? Cylinder ? ?????????????????????????????? ????????????????????????????? ???????? ????????? Film temperature ? ????? Re ???????????????????????????? Laminar ???? Turbulent ? ?????????????????????????????????????????????????????????????????????? ??????? ? ?????????????????????????????????????????????? ?????????????????????? ??????????? ????? ???? ??????????????????????????? ???????
- 30. Example 1 30
- 31. Example 1 31
- 32. Example 2 32
- 33. Example 2 33
- 34. Example 2 34
- 35. Example 2 35
- 36. Example 3 36
- 37. Example 3 37