Contributed by:
Conduction heat transfer, Thermal conductivity, Convection, Radiant heat transfer
1.
HW/Tutorial # 1
WRF Chapters 14-15; WWWR Chapters 15-16
ID Chapters 1-2
• Tutorial #1
• WRF#14.12, WWWR
#15.26, WRF#14.1,
WWWR#15.2,
WWWR#15.3,
WRF#15.1, WWWR #
16.2.
• To be discussed on Jan.
19, 2021.
• By either volunteer or
class list.
2.
Fundamentals of Heat Transfer
Conduction, Convection, and Radiation Heat Transfer Mode
Ref. ID Figure 1.1 (p 2)
Heat transfer (or heat) is thermal energy in transit due to a temperature difference
3.
Conduction Heat Transfer
•First mechanism - molecular interaction (e.g. gas)
•Greater motion of molecule at higher energy level
(temperature) imparts energy to adjacent molecules at
lower energy levels
•Second mechanism – by free electrons (e.g. solid)
qx dT q
k ; kT
A dx A
4.
Thermal Conductivity
• Physical origins and rate equation
• (Ref. ID; Figure 1.2) Association of conduction heat transfer with
diffusion energy due to molecular activities.
5.
Thermal Conductivity of Gas
• Estimation of the thermal conductivity of gas
• Ref. WWWR pp202-203 ; WRF pp199-200 (Self Study)
• Derived from the gas kinetic theory:
• (1) Considering the summation of the energy flux associated with the
molecules crossing the control surface;
• (2) The number of molecules involved is related to average random
molecular velocity.
• (3) : Boltzmann constant, d: molecular diameter, m: mass per molecule.
1
k 3T / m [Unit = W/(m-K)]
1.5d 2
6.
Thermal Conductivity of Solid
• Estimation of the thermal conductivity of solid
• Ref. WWWR pp204; WRF pp 200 (Self Study)
• (1) Derived from the Wiedemann, Franz, Lorenz Equation (1872).
• (2) The free electron mechanism of heat conduction is directly analogous
to the mechanism of electric conduction.
• ke : electrical conductivity, T: absolute temperature, L: Lorenz number.
L k constant 2.45 10-8W / K 2 @20o C
keT
9.
dT
q x ' ' k
dx
The proportionality constant k is
a transport property known as
the thermal conductivity
(W/mK) and is a characteristic
of the wall material.
dT T2 T1
dx L
T1 T2 T
q x ' ' k k
L L
13.
Finally for the same amount of heat flow the fluxes based on
The inner and out surface areas differ by approximately 42%.
qr 17860
302.7kW / m2; qr 17860 212.6kW / m2
Ai 0.059 A 0.084
0
16.
Heat transfer due to convection involves the
energy exchange between a surface and an adjacent fluid
Forced Convection: When a fluid is made to flow past a
solid surface by an external agent such as a fan or pump
Free/Natural Convection: Warmer (or cooler) fluid next to the
Solid boundary cause circulation because of the density variation
Resulting from the temperature variation throughout a region of
the fluid.
Newton’s Law of Cooling: q/A = hT
q: rate of convective heat transfer (W); A: area normal to direction of
heat transfer; h: convective heat transfer coefficient, T: temperature
Difference between the surface and the fluid.
17.
Convective Heat Transfer Processes: Ref: ID (Figure 1.5; p7)
(a) Forced Convection, (b) Free/Natural Convection,
(c) Boiling, and (d) Condensation.
18.
Boundary layer development in convection heat transfer
Ref. ID (P. 6; Fig. 1.4)
19.
Radiant Heat Transfer
(1) No medium is required for its propagation.
(2) Energy transfer by radiation is maximum when the two
Surfaces are separated by vacuum.
(3) Radiation heat transfer rate equation is given by the
Stefan-Boltzmann law of thermal radiation:
q T 4
A
q: rate of radiant energy emission (W); A: area of emitting
surface (m2); T: absolute temperature; : Stefan-
Boltzmann Constant = 5.676 x 10-8 W/m2-K4
20.
Radiation Exchange. Ref: ID (Figure 1.6; P. 9)
(a) At surface and (b) between a surface and large surroundings.
22.
The three modes of heat transfer have been considered
In real world, different modes of heat transfer are
Consider the case below for steady state conduction through
a plane wall with its surfaces held at constant temperature
T1 and T2.
T1 T2
x
L
28.
10.27 = 2.67+3.8*2
k value for 85%
Magnesia
WWWR Page 676
With interpolation
31.
The heat diffusion equation
Ref. ID (P. 61; Fig. 2.8)
32.
Ref. ID (P. 62)
q x
q x dx q x dx ( 2.7 a )
x
q y
q y dy q y dy ( 2.7b )
y
q z
q z dz q z dz ( 2.7 c )
z
Thermal energy E g q dxdydz (2.8)
generation
T
Energy E st C p dxdydz ( 2.9)
storage t
Conservation
of energy
Ein E g E out E st (1.11a)
33.
T
q x q y q z q dxdydz q x dx q y dy q z dz ρC p dxdydz (2.10)
t
q x q y q z T
dx dy dz q dxdydz ρC p dxdydz (2.11)
x y z t
T
qx kdydz ( 2.12a )
x
T
qy kdxdz ( 2.12b)
y
T
qz kdxdy ( 2.12c )
z
34.
Heat (Diffusion) Equation: at any point in the medium the rate of
energy transfer by conduction in a unit volume plus the
volumetric rate of thermal energy must equal to the rate of
change of thermal energy stored within the volume.
T T T T
k k k q ρC P (2.13)
x x y y z z t
Net conduction heat flux into the controlled volume
T ''
k dx q x ' ' q x dx (2.14)
x x
If the thermal conductivity is constant.
2 2 2
T T T q 1 T
2
2 2 (2.15)
x y z k α t
35.
Where = k/( Cp) is the thermal diffusivity
Under steady-state condition, there can be no change in
the amount of energy storage.
T T T
k k k q 0 (2.16)
x x y y z z
If the heat transfer is one-dimensional and there is no
energy generation, the above equation reduces to
d dT
(k ) 0 (2.17)
dx dx
Under steady-state, one-dimensional conditions with no
energy generation, the heat flux is a constant in the
direction of transfer.
36.
Cylindrical coordinates (1)
When the del operator is expressed in cylindrical coordinates,
the general form of the heat flux vector , and hence the Fourier’s
Law, is
T 1 T T
q" kT k (i j k )
r r Z
T k T T
q " k ; q " ; q " k
r r r z z
37.
Cylindrical coordinates (2)
38.
Spherical coordinates (1)
When the del operator is expressed in spherical coordinates,
the general form of the heat flux vector , and hence the Fourier’s
Law, is
T 1 T 1 T
q" kT k (i j k )
r r r sin
T k T k T
q " k ; q " ; q "
r r r sin r
39.
Spherical coordinates (2)
40.
General Form of The Differential
Energy Equation
DT
kT q C v
Dt
conduction dissipation substantial
derivative
transient +
convective
44.
Initial Conditions
Values of T and v at the start of time interval of interest.
Boundary Conditions
Values of T and v existing at specific positions of the
boundaries of a system
i.e. for given values of the significant space variables
Isothermal Boundaries
Insulated Boundaries
46.
Ref: ID (P. 91,
Table 2.2)
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