module-_06_-_Convection.ppt - MECH 451 Heat Transfer Convection Ref Çengel Yunus A Heat Transfer A Practical Approach 2nd Edition John Wiley Sons

module-_06_-_Convection.ppt - MECH 451 Heat Transfer...

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MECH 451 - Heat Transfer Convection Ref: Çengel, Yunus A., Heat Transfer, A Practical Approach, 2nd Edition, John Wiley & Sons, Inc. (2003).
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Lesson Outline The basics of convection heat transfer analysis will be covered Nusselt Number Prandtl Number Reynold’s Number Blasius Solution
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Convection Convection is transfer of energy between a solid surface and a moving fluid adjacent to it (liquid or gas) In the absence of motion, the heat transfer will be by conduction. As fluid motion increases, the heat transfer rate increases. Therefore, convection will always transfer heat more efficiently than conduction
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Convection Fluid motion can be either forced or natural Forced is due to an external power source Natural results from density differences created by temperature differences
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Convection – Newton’s Law of Cooling Convection can be very difficult to analyze because of fluid motion Most analyses will use a Newton’s Law of Cooling with the convection heat transfer coefficient (h – W/m 2 K) temp bulk fluid T temp surface T area surface A coeff HXFR convection h T T hA Q s s s s conv
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Convection – Newton’s Law of Cooling The convection heat transfer coefficient is NOT a material property. It is experimentally determined and is dependent upon geometry, the fluid properties and flow conditions temp bulk fluid T temp surface T area surface A coeff HXFR convection h T T hA Q s s s s conv
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Convection Big Picture Convection does not get any more complex than Newton’s Law of Cooling The difficulty lies in how to determine the convection heat transfer coefficient temp bulk fluid T temp surface T area surface A coeff HXFR convection h T T hA Q s s s s conv
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Determination of convection heat transfer coefficient Convection is heat transfer by molecular interaction caused by bulk fluid motion As a result, the study of convection is very dependent upon the fluid properties and analysis of fluid boundary layer conditions
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