8/11/2019 Lecture 14 - Convective Heat Transfer -1

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Lecture 14 : Heat Transfer via ConvectionContents:1. Heat conduction and convection in composite walls, cylinders, spheres (in series)2. Over-all heat transfer coefficient. !imensionless num"ers in Heat transfer #. $orced Convection %nside &ipes'. $orced Convection Outside &ipes. atural Convection

*. Heat + chan ers--------------------------------------------------------------------------------------------------------------------------------------------------

Learning objectives:1. !ifferentiate "etween conduction and convection2. %dentify if convection is via forced or natural. + press heat transfer e uation for convection in terms of individual film coefficients or over-all heat transfer

coefficients#. &lot temperatures of fluids flowin in a dou"le-pipe heat e chan er as a function of len th'. /olve pro"lems in applications involvin heat transfer via convection

a. !ou"le-pipe heat e chan er

Heat transfer by Convection- Heat transfer involvin fluids- Heat is transferred due to mi in process "etween cold and hot portions of a fluid

2 types:- Forced Convection 0 mi in due to mechanical means such as pumps fans blo!ers compressors agitators

etc .- "atural convection 0 mi in due to density difference via "ouyancy arisin from temperature gradient

#ssumptions:1. /teady-state heat transfer, rate of heat % rate of heat O 3 4567s, 67s, 83 7hr etc.92. o eneration, no accumulation

. Heat transfer flow from#. $or sphere and cylinder, heat transfer direction is radial.'. 12, 5 2 are different conductin materials. Convection in fluid i and fluid $ , conduction in materials 5 12 and 5 2

*. Constant area across the sla" with the fluid

%lab:

;t steady-state T 2 > T 3

12 23

T1

T2

T3

x1

x2

Ti

To

fuidfuidq% qO 3

8/11/2019 Lecture 14 - Convective Heat Transfer -1

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T h Aq =

. where =h heat transfer coefficient similar to mass transfer coefficient,

s K m J

..2

/ame as in conduction