HEAT TRANSFER FROM FINNED SURFACES - Unipamplona€¦ · s by attaching to the surface extended...
Transcript of HEAT TRANSFER FROM FINNED SURFACES - Unipamplona€¦ · s by attaching to the surface extended...
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HEAT TRANSFER FROM
FINNED SURFACES
Mehmet Kanoglu
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fundamentals of Thermal-Fluid Sciences, 3rd EditionYunus A. Cengel, Robert H. Turner, John M. Cimbala
McGraw-Hill, 2008
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HEAT TRANSFER FROM FINNED SURFACES
When Ts and T are fixed, two ways to increase the rate of heat transfer are
• To increase the convection heat transfer coefficient h. This may require the
installation of a pump or fan, or replacing the existing one with a larger one, but
this approach may or may not be practical. Besides, it may not be adequate.
• To increase the surface area As by attaching to the surface extended surfaces
called fins made of highly conductive materials such as aluminum.
Newton’s law of cooling: The rate of heat transfer from
a surface to the surrounding medium
The thin plate fins of a car radiator greatly
increase the rate of heat transfer to the air.
Some innovative fin designs.
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FIGURE 3–34
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FIGURE 3–34
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Performance Characteristics
In this section we provide the performance
characteristics:
Temperature distribution,
Rate of heat transfer,
Fin efficiency
For convecting, radiating, and convecting-radiating fins.
Configurations considered include longitudinal fins,
radial fins, and spines.
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Fin Equation
Differential equation
Temperature excess
if
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The general solution of the differential equation
Boundary conditions at the fin base and the fin tip.
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Constant base temperature and convecting
tip
maxfin,
finfin
finfin
fin
conditionsboundary
.
.
max,
.
.
sinh)/(cosh
cosh)/(sinh
sinh)/(cosh
)(sinh)/()(cosh
/)(
Q
Q
hAQ
mLmkhmL
mLmkhmLhPkAQ
mLmkhmL
xLmmkhxLm
dxdkLh
b
bc
b
Lx
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Constant base temperature and insulated tip
mL
mL
Q
Q
hAQmLhPkAQ
mL
xLm
dxd
bbc
b
Lx
tanh
tanh
cosh
)(cosh
/
.
.
max,
..
maxfin,
finfin
finfinfin
0 conditionsboundary
Fins are not likely to be so long that their temperature
approaches the surrounding temperature at the tip. A more
realistic assumption is for heat transfer from the fin tip to be
negligible since the surface area of the fin tip is usually a
negligible fraction of the total fin area.
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Convection (or Combined
Convection and Radiation)
from Fin Tip
A practical way of accounting for the
heat loss from the fin tip is to replace
the fin length L in the relation for the
insulated tip case by a corrected
length defined as
Corrected fin length Lc is defined such
that heat transfer from a fin of length Lc
with insulated tip is equal to heat transfer
from the actual fin of length L with
convection at the fin tip.
t the thickness of the rectangular fins
D the diameter of the cylindrical fins
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Constant base and tip temperatures
maxfin,
finfin
finfin
fin
L(L) conditionsboundary
.
.
max,
.
.
sinh
)/(cosh
sinh
)(sinhsinh)/(
Q
Q
hAQ
mL
mLhPkAQ
mL
xLmmx
b
btbc
bt
b
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Infinitely high fin with constant base
temperature
cfin
b
bc
mx
b
hA
kP
hAQ
hPkAQ
e
finfin
fin
0(L) (L :conditionsboundary
max,
.
.
)
Revisar
eficiencia!
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Conduction and convection in a straight fin of uniform
cross-sectional area. (a) Rectangular fin. (b) Pin fin. (c)
Four common tip boundary conditions. (d) Temperature
distribution for the infinite fin (x →∞)
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Temperature distribution, and loss
heat of uniform sectional fins
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Fin Efficiency
Fins enhance heat
transfer from
a surface by
enhancing surface
area.
Ideal and actual
temperature
distribution
along a fin.
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Zero thermal resistance or infinite
thermal conductivity (Tfin = Tb)
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• Fins with triangular and parabolic profiles contain less material
and are more efficient than the ones with rectangular profiles.
• The fin efficiency decreases with increasing fin length. Why?
• How to choose fin length? Increasing the length of the fin
beyond a certain value cannot be justified unless the added
benefits outweigh the added cost.
• Fin lengths that cause the fin efficiency to drop below 60 percent
usually cannot be justified economically.
• The efficiency of most fins used in practice is above 90 percent.
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Fin
Effectiveness
The
effectiveness
of a fin
• The thermal conductivity k of the fin
should be as high as possible. Use
aluminum, copper, iron.
• The ratio of the perimeter to the cross-
sectional area of the fin p/Ac should be
as high as possible. Use slender pin fins.
• Low convection heat transfer coefficient
h. Place fins on gas (air) side.
• The use of fins are recommended when
f ≥ 2. (Incropera)
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Efficiency of straight fins of rectangular, triangular, and parabolic profiles.
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Efficiency of annular fins of constant thickness t.
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Various surface areas associated
with a rectangular surface with
three fins.
Overall effectiveness for a finned surface
The overall fin effectiveness depends
on the fin density (number of fins per
unit length) as well as the
effectiveness of the individual fins.
The overall effectiveness is a better
measure of the performance of a
finned surface than the effectiveness
of the individual fins.
The total rate of heat transfer from a
finned surface
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Work Class:
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Proper Length of a Fin
Because of the gradual temperature drop along the fin, the region near the fin
tip makes little or no contribution to heat transfer.
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Proper Length of a Fin
mL = 5 an infinitely long fin mL = 1 offer a good compromise between heat
transfer performance and the fin size.
0,100
0,197
0,462
0,762
0,905
0,9640,987 0,995 0,999 1,000
0,0
0,2
0,4
0,6
0,8
1,0
1,2
0 0,5 1 1,5 2 2,5 3 3,5 4 4,5 5
Tan
h(m
L)
mL
Longitud ideal de una aleta
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• Heat sinks: Specially
designed finned surfaces
which are commonly used in
the cooling of electronic
equipment, and involve one-
of-a-kind complex
geometries.
• The heat transfer
performance of heat sinks is
usually expressed in terms of
their thermal resistances R.
• A small value of thermal
resistance indicates a small
temperature drop across the
heat sink, and thus a high fin
efficiency.
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Fin DesignThe measures ηf and εf probably attract the interest of
designers not because their absolute values guide the
designs, but because they are useful in characterizing fins
with more complex shapes. In such cases the solutions are
often so complex that ηf and εf plots serve as labor saving
graphical solutions.
The design of a fin thus becomes an open-ended matter of
optimizing, subject to many factors. Some of the factors
that have to be considered include:
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Fin Design
The weight of material added by the fin. This might be a
cost factor or it might be an important consideration in this
own right.
The possible dependence of h on (T – T), flow velocity
past the fin, or other influences
The influence of the fin (or fins) on the heat transfer
coefficient, h, as the fluid moves around it (or them)
The geometric configuration of the channel that the fin
lies in
The cost and complexity of manufacturing fins
The pressure drop introduced by the fins
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Exercise, Cengel
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Example, Cengel
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Class work:The extent to which the tip condition affects the thermal
performance of a fin depends on the fin geometry and thermal
conductivity, as well as the convection coefficient. Consider an
alloyed aluminun (k=180 W/ m.K) rectangular fin whose base
temperature is Tb= 100 °C. The fin is exposed to a fluid of
temperature T∞= 25°C, and the uniform convection coefficient of h=
100 W/m2.K, may be assumed for the fin surface (tip condition).
* For a fin of length L= 10 mm, w= 5 mm, thickness t = 1 mm,
determine the efficiency and effectiveness.
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HEAT TRANSFER IN COMMON CONFIGURATIONSSo far, we have considered heat transfer in simple geometries such as large
plane walls, long cylinders, and spheres.
This is because heat transfer in such geometries can be approximated as one-
dimensional.
But many problems encountered in practice are two- or three-dimensional and
involve rather complicated geometries for which no simple solutions are
available.
An important class of heat transfer problems for which simple solutions are
obtained encompasses those involving two surfaces maintained at constant
temperatures T1 and T2.
The steady rate of heat transfer between these two surfaces is expressed as
S: conduction shape factor
k: the thermal conductivity of the medium between the surfaces
The conduction shape factor depends on the geometry of the system
only. Conduction shape factors are applicable only when heat transfer
between the two surfaces is by conduction.
Relationship between the conduction
shape factor and the thermal resistance
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Example, Cengel
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Example, Cengel
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Numerical Methods in Steady
Heat conduction
Mehmet Kanoglu
Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display.
Fundamentals of Thermal-Fluid Sciences, 3rd EditionYunus A. Cengel, Robert H. Turner, John M. Cimbala
McGraw-Hill, 2008
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Numerical Methods
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Limitations Better modeling
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Flexibility Complications
Computers and numerical methods are
ideally suited for such calculations, and a
wide range of related problems can be
solved by minor modifications in the code or
input variables. Today it is almost
unthinkable to perform any significant
optimization studies in engineering without
the power and flexibility of computers and
numerical methods
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Human Nature
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Numerical Simulation
scale of simulation:
100
10-3
10-6
10-9
10-12 10-9 10-6 10-3 100
Quantum systems
Atomic scale:Molecular dynamics
bott
om
-up
Macroscopic scales: Finite elementsFV, FD.
top
-dow
n
Mesoscale, microscopic and macroscopic scales
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Numerical Simulation
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Numerical Simulation
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Numerical Simulation
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Numerical Simulation
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Numerical Simulation
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Finite difference formulation of differential
equations
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Finite difference formulation of differential
equations
The wall is subdivided into M sections
of equal thickness x = L/M in the x-
direction, separated by planes passing
through M+1 points 0, 1, 2, ..., m-1, m,
m+1, ..., M called nodes or nodal points.
The x-coordinate of any point m is
simply xm = mx, and the temperature at
the point is simply T(xm) = Tm.
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Finite difference formulation of differential
equations
The first derivate of temperature dT/dx at the midpoints m-½ and m+½
of the sections surrounding the node m can be expressed as
Noting that the second derivate is simply the derivate of the first
derivate, the second derivate of temperature at node m can be
expressed as
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Finite difference formulation of differential
equations
The governing equation for steady one-
dimensional heat transfer in a plane wall
with heat generation and constant
thermal conductivity, can be expressed in
the finite difference form as
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Finite difference formulation of differential
equations The finite difference formulation for
steady two-dimensional heat
conduction in a region plane wall with
heat generation and constant thermal
conductivity, can be expressed in
rectangular coordinates as
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One-dimensional steady heat conduction
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One-dimensional steady heat conduction
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Boundary conditions
Boundary conditions most commonly
encountered in practice are the
specified temperature, specified heat
flux, convection, and radiation
boundary conditions
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Boundary conditions
1. Specified Heat Flux Boundary
Condition
Special case: Insulated Boundary
2. Convection Boundary Condition
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Boundary conditions
3. Radiation Boundary Condition
4. Combined Convection and Radiation
Boundary Condition
5. Combined Convection, Radiation and
Heat Flux Boundary Condition
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Boundary conditions
6. Interface Boundary Condition
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Boundary conditions
Treating Insulated Boundary Nodes
as Interior Nodes: The Mirror Image
Concept
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EXAMPLE
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EXAMPLE
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QUIZ
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Two-Dimensional Steady Heat Conduction
A logical numbering scheme for two-
dimensional problems is the double
subscript notation (m,n) where m = 0,
1, 2, ..., M is the node count in the x-
direction and n = 0, 1, 2, ..., N is the
node count in the y-direction. The
coordinates of the node (m,n) are
simply x = mx and y = ny, and the
temperature at the node (m,n) is
denoted by Tm,n.
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Two-Dimensional Steady Heat Conduction
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Boundary nodes
For the heat transfer under steady
conditions, the basic equation to
keep in mind when writing na energy
balance on a volume element is
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Example:
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73
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74
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Symmetry sections
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Ejercicio
81