How is the heat transfer? Mechanism of Convection Applications . Mean fluid Velocity and Boundary and their effect on the rate of heat transfer. Fundamental equation of heat transfer o!arithmic"mean o!arithmic"mean temperature difference. Heat transfer Coefficients. Coefficients. Heat flu# and $usselt correlation %imulation pro!ram for Heat &#chan!er
How is the heat transfer? • Heat can transfer 'etween the surface of a solid conductor and the surroundin! medium medium whenever temperature !radient e#ists. Conduction Convection $atural convection Forced Convection
Natural and forced Convection $atural
convection occurs whenever heat flows 'etween a solid and fluid( or 'etween fluid layers.
As
a result of heat e#chan!e
Chan!e in density of effective fluid layers ta)en place( which causes upward flow of heated fluid. *f this motion is associated with heat transfer mechanism only( then it is called $atural Convection
Forced Convection
*f this motion is associated 'y mechanical means such as pumps( !ravity or fans( the movement of the fluid is enforced. And in this case( we then spea) of Forced convection.
Heat &#chan!ers
• A device whose primary purpose is the transfer of ener!y 'etween two fluids is named a Heat &#chan!er.
Applications of Heat &#chan!ers Heat Exchangers prevent car engine overheating and increase efficiency Heat exchangers are used in Industry for heat transfer
Heat exchangers are used in AC and furnaces
• +he closed"type e#chan!er is the most popular one. • ,ne e#ample of this type is the -ou'le pipe e#chan!er.
• *n this type( the hot and cold fluid streams do not come into direct contact with each other. +hey are separated 'y a tu'e wall or flat plate.
rinciple of Heat &#chan!er •
First aw of +hermodynamic/ “Energy is conserved.”
dE dt
0
0 0 0 0 0 .hin − ∑ m .hout + q + w s + e generated = ∑ m out in
h .C ph .∆T h Qh = A.m c .C pc .∆T c Qc = A.m
•Control Volume
∑ m .h0 = −∑ m .h0 in
out
CO!" Q h
HO
Cross Section Area
hermal #oundary !ayer
THERMAL
$egion III% Solid . Cold !i&uid Convection
BOUNDARY LAYER
Energy moves from hot fluid to a surface by convection, through the wall by conduction, and then by convection from the surface to the cold fluid.
dq x Th
(E)O(*S !A) O+ CCO!I(,
= h .( T − T ).dA c
ow
Ti,wall To,wall
c $egion I % Hot !i&uid' Solid Convection
Q hot
Q cold
(E)O(*S !A) O+ CCO!I(,
dq x
= hh .( T h − T iw ).dA
$egion II % Conduction Across Copper )all +O-$IE$*S !A)
dq x
= − k .
dT dr
c
• elocity distribution and boundary layer 1hen fluid flow throu!h a circular tu'e of uniform cross" suction and fully developed( +he velocity distri'ution depend on the type of the flow. *n laminar flow the volumetric flowrate is a function of the radius. r = D 4 2
V =
∫ u2π rdr r =3
V = volumetric flowrate u = average mean velocity
*n tur'ulent flow( there is no such distri'ution.
• +he molecule of the flowin! fluid which ad5acent to the surface have 6ero velocity 'ecause of mass"attractive forces. ,ther fluid particles in the vicinity of this layer( when attemptin! to slid over it( are slow down 'y viscous forces. Boundary layer
r
• Accordin!ly the temperature !radient is lar!er at the wall and throu!h the viscous su'"layer( and small in the tur'ulent core.
= hA∆ T q x = hA(T w − T ) q x
Tube wall heating
1arm fluid
Metal wall
δ
h
+wh cold fluid
cooling +c
+wc
k q x = A(T w − T )
δ
• +he reason for this is 78 Heat must transfer throu!h the 'oundary layer 'y conduction. 28 Most of the fluid have a low thermal conductivity 9)8 :8 1hile in the tur'ulent core there are a rapid movin! eddies( which they are equali6in! the temperature.
