heat conduction differential equation of heat...
TRANSCRIPT
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Heat Conduction
Differential Equation of Heat Conduction Temperature is not varied with one direction
Temperature is varying with time
Arbitrarily solid
Need differential equation
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Differential Equation of Heat Conduction
Element inside the solid
1) Thermal conductivity is not directly dependent at any point (Isotropic material).
2) Assume that heat generated = (W/m3)
Fourier’s law of heat conduction - For first one dqx
dz.dy.x
Tkdq x
dz.dydx.x
Tk
xx
Tkdq dxx
q
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Differential Equation of Heat Conduction
- For first one dqy
....dqy
....dq dyy
- For first one dqz
....dqz
....dq dzz
Similarly
Now, what is the net rate at which heat is being conducted into the element?
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Differential Equation of Heat Conduction
1) Net amount of heated conducted into dxdydz per unit time
)dqdqdq()dqdqdq( dzzdyydxxzyx
dz.dy.dxz
Tk
zy
Tk
yx
Tk
x
2) Net amount of heated conducted into dxdydz per unit time
)dz.dy.dx(q
3) Rate of change of energy of the element
t
TC)dz.dy.dx.( p
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Differential Equation of Heat Conduction
1 + 2 = 3
t
TCq
z
Tk
zy
Tk
yx
Tk
xp
Therefore, the required differential equation is
Note: dx.dy.dz is cancelled
Apply the 1st law of Thermodynics of closed system
This is the most general form of differential equation for Cartesian coordinate system
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Differential Equation of Heat Conduction
t
TCq
z
T
y
T
x
Tk p2
2
2
2
2
2
If k is constant
In the absence of heat generation
t
T1
k
qT2
is called the thermal diffusivity (unit: m2/s)
t
T1T2
If there is a steady state
0T2 Depending of the nature of the problem
(Laplace’s equation)
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Differential Equation of Heat Conduction
Cylindrical Spherical
cosrx
sinry
zz
cossinrx
sinsinry
cosrz
Zenith angleAzimuth angle
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Differential Equation of Heat ConductionDerivation of different equation of heat conduction in a 2D polar coordinate system ),r(
r
dr drrdq
d
rdq
dq
ddq
1.d.r.r
Tkdq r
ddr.
r
T.r.k
rr
T.r.kdq drr
1.dr.T
r
kdq
drd.r.T
kr
T
r
kdq d
Cylindrical
A
Adistance
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Differential Equation of Heat Conduction
1) Rate at which heat is conducted into the element
d.dr
Tk
r
1
r
Tr.k
r
2) Heat generated per unit time
1drdrq
3) Rate of change of energy of the element
t
TC)1.dr.d.r.( p
volume
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Differential Equation of Heat Conduction
1 + 2 = 3
t
TCq
Tk
r
1
r
Tkr
rr
1p2
Therefore, the required differential equation is
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Differential Equation of Heat Conduction
t
TCq
T
r
1
r
Tr
rr
1k p2
2
2
If k is constant
In the absence of heat generation
If there is a steady state
t
TC
T
r
1
r
Tr
rr
1k p2
2
2
0T
r
1
r
Tr
rr
1k
2
2
2
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Differential Equation of Heat Conduction)z,,r( Derive the differential equation for the 3-D cylindrical system
For constant k
2
2
z
Tk
Additional term on left-hand side
t
TCq
z
TT
r
1
r
Tr
rr
1k p2
2
2
2
2
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Differential Equation of Heat Conduction
Spherical
t
TCq
T
sinr
1Tsin
sinr
1
r
)rT(
r
1k p2
2
2222
2
Now, what do we know?
Coordinate: Cartesian, cylindrical, spherical coordinatesState: steady state, unsteady state Direction: one, two, three directionsMaterial: isotropic material constant k, not constant k
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Differential Equation of Heat Conduction
Solving a problem needs initial and boundary conditions
1. Initial condition
At t = 0, temperature distribution in body is specified
2. Boundary condition1) Prescribed surface temperature2) Prescribed heat flux incident on the surface3) Prescribed heat transfer coefficient at the surface
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Differential Equation of Heat Conduction
1. Prescribed surface temperatureFor example, let the surface be a plane face (at x = L)
T0At x = L T = T0
2. Prescribed heat flux incident on the surface
0Lx A
q
x
Tk
3. Prescribed heat transfer coefficient at the surface
)TT(hx
Tk fLx
Lx
0A
q
hTf
x
x directionOpposite x direction
x = L
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Differential Equation of Heat Conduction
Heat generation
1. Electrical conductor2. Nuclear fuel element3. Setting of concrete4. Agricultural product
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Differential Equation of Heat Conduction
Heat generation (Infinite slab)
Heat generated at uniform rate
x2b
q
We would like to know the temperature distribution at steady state, or equation describing this temperature distribution.
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Differential Equation of Heat ConductionLet put down differential equation (k is constant)
t
TCq
z
T
y
T
x
Tk p2
2
2
2
2
2
• One direction, infinite slab • Steady state
k
q
dx
Td2
2
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Differential Equation of Heat ConductionBoundary conditions
x2b
q
Tf h
hTf
)TT(hdx
dTk fbx
bx
)TT(hdx
dTk bxf
bx
0dx
dT
0x
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Differential Equation of Heat ConductionSolving
1Cxk
q
dx
dT
Integrating it twice
21
2
CxC2
x
k
qT
k
q
dx
Td2
2
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Differential Equation of Heat ConductionTemperature distribution
h
bq)xb(
k2
qTT 22f
h
1
k2
bbqTT fmax
Maximum Temperature (x = 0)
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Differential Equation of Heat Conduction
Heat generation (Infinite solid cylinder)
t
TCq
Tk
r
1
r
Tkr
rr
1p2
rk
q
dr
dTr
dr
d
Reducing the form to
0dr
dT
0r
fRrRr
TThdr
dTk
Boundary condition
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Differential Equation of Heat Conduction
Integrating differential equation and using boundary conditions, we get
h2
Rq)rR(
k4
qTT 22f
h
1
k2
R
2
RqTT fmax
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Differential Equation of Heat ConductionProblemA nuclear fuel element is in the form of a long solid rod (k = 0.85 W/mK) of diameter 14 mm. It generates heat at the uniform rate of 0.45 x 108 W/m3 because of nuclear fusion. The heat is transferred to pressurized cooling water at 300 ºC and the surface heat transfer coefficient is 4500 W/m2K. Calculate the maximum temperature in fuel rod in the steady state.
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Differential Equation of Heat ConductionProblemHeat generated in a slab of 120 mm thickness with a conductivity of 200 W/mK at a rate of 106 W/m3. Determine the temperature at the mid and quarter planes if the surface of the solid on both sides are exposed to convection at 30ºC with a convective coefficient of 500W/m2K. Also find heat flow rate at these planes and the temperature gradients at these planes
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Differential Equation of Heat Conduction
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Differential Equation of Heat Conduction
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Differential Equation of Heat Conduction
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Differential Equation of Heat Conduction
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Differential Equation of Heat Conduction
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Differential Equation of Heat Conduction