lecture 11 - forced convection
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Heat & Mass Transfer
Convection
Heat & Mass TransferTheory & Application
Dr. S. Kamran Afaq
Professor
HITEC University
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Heat & Mass Transfer
Convection
Boiling and Condensation
Introduction
Application : Heat Exchangers
Heat Conduction
Fundamental of Convection
Mass Diffusion
Course Outline
Heat Transfer from Extended Surfaces (FINS)
Convection and Radiation
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Heat & Mass Transfer
Convection
Modes of Heat Transfer
Convection :
In this mode heat is transfer due to the movementof the fluids
FreeConvection
ForcedConvection
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Heat & Mass Transfer
Convection
Heat Convection FreeConvection
ForcedConvection
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Heat & Mass Transfer
Convection
Hot Iron Block
(T1
= 400 C)
Cool Air
(T = 15 C) By Speed
Type of fluid(Water)
, , C, v ..
Roughness, Geometry ofthe object
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Heat & Mass Transfer
Convection
Heat ConvectionMathematical Modeling :
The rate of heat convection over a medium depends on the:
Nature of the flow (Re.Laminar or Turbulent)
Nature of the fluid (Viscosity, k, density, C, etc) Surface Area of the medium
Temperature Difference
Unlike conduction, convection is not concernedwith medium properties
s
AQ
TQ
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Heat & Mass Transfer
Convection
Heat ConvectionMathematical Modeling :
,
,
AQ
TQ
As ThQ
As TQ
Newtons law of
cooling
Convective heattransfer Coefficient(Nature of the fluid)
where;
T = (Ts -T)
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Heat & Mass Transfer
Convection
Heat ConvectionConvective heat transfer Coefficient (h)
Rate ofHeat transferb/w solid surface and a fluid perunit surface areaper
unit temperature difference
Units :
As T
Qh
h : (W/m2/ C)
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Heat & Mass Transfer
Convection
Heat ConvectionNusselt Number
Consider a fluid layer of thickness L, and temperature difference is
T= T2 T1.
Heat transfer through the fluid will be by
convection if the fluid is in motion or by
conduction if it is in rest.
T
LT
hAQ
kAQ
conv
cond
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Heat & Mass Transfer
Convection
Heat Convection
L
T
T
kA
hA
Q
Q
cond
conv
k
hL
Q
Q
cond
conv
k
hLNu
Nusselt Number
Nu represent the enhancement of Heat transfer by convection through
the fluid as compare to conduction.
High value means more effective the convection; Nu=1, means pure
conduction
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Heat & Mass Transfer
Convection
Heat ConvectionPrandtl Number
Velocity Profile
Temperature Profile
The region above the plate where
viscous effect are dominant; known as
velocity boundary layer; and boundary
layer thickness ( ) is defined as y
distance where u=0.99u
Similarly the region in temperature
profile know as the thermal boundary
layer; and boundary layer thickness
( t) is defined as y distance where T -
Ts = 0.99(T-Ts)
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Heat & Mass Transfer
Convection
Heat ConvectionPrandtl Number
In flow over a heated surface, both thermal and velocity boundary
layers develop simultaneously;
Velocity profile have a strong effect on temperature profile; and finallyon thermal boundary layer.
Convective heat transfer coefficient is directly dependent on this
temperature gradient;
The relative thickness of these thermal and velocity boundary layer isdescribe byPrandtl number;
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Heat & Mass Transfer
Convection
Heat Convection
yDiffusivitThermal
viscositykinamaticPr
Prandtl Number
pCkPr
k
CpPr
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Heat & Mass Transfer
Convection
Heat ConvectionPrandtl Number
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Heat & Mass Transfer
Convection
Heat ConvectionReynolds Number
The transition from laminar to turbulent flow depends upon;
Surface Geometry
Surface Roughness
Surface Temperature
Free Stream velocity
Type of fluid
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Heat & Mass Transfer
Convection
Heat ConvectionReynolds Number
Reynolds describe that the flow regime completely depends on the ratio of
the inertiaforces to viscousforces in the fluid;
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Heat & Mass Transfer
Convection
Heat Convection
Re2
2
LV
LV
Reynolds Number
LVRe
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Heat & Mass Transfer
Convection
Heat Convection
Flow over a isothermal flat plate
Depends on
geometry
(m , n = 0 to 1)
k
hLNuk
CpPrLVRe
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Heat & Mass Transfer
Convection
Heat ConvectionAverage Nu for external forced
Convection
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Heat & Mass Transfer
Convection
Heat Convection
k
hLNu
Internal forced Convection
k
hL
Q
Q
cond
conv
k
hDNu h
HydraulicDiameter (Dh)
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Heat & Mass Transfer
Convection
Heat ConvectionInternal forced Convection
Laminar and Turbulent in tubes
LVRe
hDVRe
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Heat & Mass Transfer
Convection
Heat ConvectionInternal forced Convection
hDVRe
Ac = Cross-Sectional Areap = Perimeter
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Heat & Mass Transfer
Convection
Heat ConvectionInternal forced Convection
Laminar Flow
k
hDNu h
C i
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Heat & Mass Transfer
Convection
Heat Convection
Turbulent Flow
Internal forced Convection
C ti
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Heat & Mass Transfer
Convection
Heat Convection
Example
C ti
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Heat & Mass Transfer
Convection
Heat Convection
Tube Side
Con ection
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Heat & Mass Transfer
Convection
Heat Convection
Convection
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Heat & Mass Transfer
Convection
Heat ConvectionAnnulus Side
Convection
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Heat & Mass Transfer
Convection
Heat Convection
Convection
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Heat & Mass Transfer
Convection
Heat Convection
98.2Prs/ft1011.5
FBtu/h.ft.378.0
26
k
k 00150
017 10
0 72
3
.
. /
Pr .
Btu / h. ft. F
ft s2
Properties water at 140 F
Properties of air at 80 F
Example
Convection
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Heat & Mass Transfer
Convection
EXAMPLE 7
5A long 10-cm-diameter steam pipe whose external surface
temperature is 110 Cpasses through some open area that is
not protected against the winds. Determine the rate of heat
loss from the pipe per unit of its length when the air is at 1
atm pressure and 10 C and the wind is blowing across thepipe at a velocity of8 m/s.
Convection
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Heat & Mass Transfer
Convection
EXAMPLE 7
5A long 10-cm-diameter steam pipe whose external surface
temperature is 110 Cpasses through some open area that is
not protected against the winds. Determine the rate of heat
loss from the pipe per unit of its length when the air is at 1
atm pressure and 10 C and the wind is blowing across thepipe at a velocity of8 m/s.
Convection
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Heat & Mass Transfer
Convection
Average Nu for external forced
Convection
Convection
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Heat & Mass Transfer
Convection
31805.04 )7202.0()10219.4(027.0Nu
127Nu
Then the rate of heat transfer from the pipe per unit of its length;
CW/m6.351.0
12702808.0 2
hD
Nukh
W1120)( TThAQ ss
Convection
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Heat & Mass Transfer
Convection
Thank You
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