heat transfer 1 dai hoc bach khoa ha noi.pdf

Heat transfer

1. HEAT TRANSFER
DR. NGUYEN NGOC HOANG
Dept. of Food & Biological Process & Equipment
Hanoi University of Science and Technology
[email protected]
mobile: [phone]

H E A T T R A N S F E R

2. Course Objectives
HEAT TRANSFER PRINCIPLES 1
2
BASIC HEAT EXCHANGER TYPES 3
HEAT EXCHANGER DESIGN METHOD 4
SPECIFIC BASIC HEAT TRANSFER CASES

3. Course Objectives
REFRIGERATION PRINCIPLES 5
6
TWO STAGE CYCLE 7
SINGLE STAGE CYCLE

4. b
Prescribed Textbook
Frank Kreith, Raj M. Manglik, Mark S. Bohn. Principles of Heat
Transfer. 7th ed. Singapore. Cengate Learning Inc., 2011
T. W. Fraser Russell, Anne S. Robinson, Norman J. Wagner, Mass
and heat transfer. New York, Cambrigde University Press, 2008
Reference Literature
[1] William S. Janna. Engineering Heat Transfer. 2nd ed. Boca Raton.
CRC, 2000
[2] John H. Lienhard. A Heat Transfer Textbook. 3rd ed.
Massachusetts. Plogiston, 2008
[3]. Purdue University Thermophysical Properties Research Center.
Thermophysical Properties of Matter. New York, NY: IFI/Plenum, 1970-
1979
[4]. Reid, R. C. The Properties of Gases and Liquids. 4th ed. New York,
NY: McGraw-Hill, 1987

5. LECTURE PRESENTATION

6. HEAT TRANSFER
WHAT
Text
HEAT
TRANSFER
WHEN
HOW
WHAT

7. WHAT IS HEAT TRANSFER?

Heat Transfer is the transmission of energy
in form of heat from one body or region to
another

8. HEAT TRANSFER
WHEN
HOW
WHAT
WHAT
Text
HEAT
TRANSFER
WHEN
HOW

9. WHEN DOES HEAT TRANSFER?
Second law of thermodynamics
Heat can flow from a higher temperature region to
a lower temperature region, but not the other way
around



difference of temperature
is driving force of the transmission
The transmission of heat is from the
higher temperature region to
the lower one

10. Q&A
touch to an ice piece
feel cold
Does the heat
transfer from the
ice piece to the
hand???
Definitely wrong
conclusion

11. HEAT TRANSFER
WHEN
HOW
WHAT
WHAT
Text
HEAT
TRANSFER
WHEN
HOW

12. Diagram
1
CONDUCTION
2
CONVECTION
3
RADIATION
HEAT TRANSFER

13. HOW DO HEAT TRANSFER?

There are three fundamental methods of heat
transfer

14. HEAT MEASUREMENT
Concept
Heat transfer
Dimension
Heat quantity
Heat transfer rate
Unit
J, Btu
J/s, W, Btu/h
Heat flux
Temperature
J/m
2
, Btu/h.ft
2

o
C,
o
K,
o
F
Dimensions are our
basic concepts of
measurements
such as …..

Units are the means of
expressing dimensions
numerically, for
instance...

15. HEAT MEASUREMENT
The heat quantity is proportional to mass of material, the
temperature difference and the specific heat capacity

Q=mCT (J)

Where:
The specific heat capacity of a material is the quantity of
heat needed to raise the temperature of a unit mass
through a unit degree (J/Kg
o
K).

16. HEAT MEASUREMENT
Heat flow (or heat transfer rate) is heat quantity
transmitting per unit time

??????=
??????
??????
(W)

Heat flux is heat quantity transmitting through an unit
heat transfer area per unit time.

??????
"
=
??????
??????.??????
(W/m
2
)

17. DEMENSIONS & UNITS

18. Example
Problem: A 500-g copper coffee cup is filled with 200-g of coffee.
How much heat was required to heat cup and coffee from 20 to 96
0
C?
Where the specific heat of copper C
m = 390 J/Kg C
o
and that of coffee
water C
w = 4186 J/Kg C
o

Solution:
Total heat is that required to raise temperature of mug and coffee-water
().

Q
T = m
cc
c t + m
wc
w t
Coffee-water: (0.20 kg)(4186 J/kgC
0
)(76 C
0
)
Copper cup: (0.50 kg)(390 J/kgC
0
)(76 C
0
)
Q
T = 63,600 J + 14,800 J

19. Problem: A cylindrical resistor on a circuit board dissipates 0.6 W of power.
The diameter and length of the resistor are 0.4 cm and 1.5 cm respectively.
The amount of heat dissipated in 24 h and the heat flux are to be determined.
Assumes that heat is transferred uniformly from all surfaces

Solution:
The amount of heat this resistor dissipates during a 24-hour period is


The surface of the resistor (heat transfer area)



The heat flux on the surface of the resistor:

CLASS PROBLEMS kJ 51.84= Wh14.4 h) W)(246.0(tqQ 2
22
cm 136.2885.1251.0cm) cm)(1.5 4.0(
4
cm) 4.0(
2
4
2  



DL
D
A
s 2
W/cm0.2809
2
"
cm 136.2
W60.0
sA
q
q

20. HEAT MEASUREMENT
Change of phase
The latent heat of fusion (h
f) of a substance is the heat per
unit mass required to change the substance from the solid to the liquid
phase of its melting temperature.
h
f =Q/m

The latent heat of vaporization (h
v) of a substance is the heat
per unit mass required to change the substance from a liquid to a vapor
at its boiling temperature
h
v =Q/m

21. LATENT HEAT EXCHANGE
Problem: What is the heat in Joules required to convert
25 grams of 0 °C ice into 0 °C water ?
heat of fusion of water = 334 J/g

Solution:
Heat required to convert 0 °C ice to 0 °C water
Use the formula q = m·h
f
q = (25 g)x(334 J/g)

22. Class problem
Problem: What is the heat in Joules required to convert 25 grams of
-10 °C ice into 150 °C steam?
heat of fusion of water = 334 J/g; heat of vaporization of water = 2257 J/g
specific heat of ice = 2.09 J/g·°C; specific heat of water = 4.18 J/g·°C
specific heat of steam = 2.09 J/g·°C
Solution:
The total energy required is the sum of the energy to heat the -10 °C ice to
0 °C ice, melting the 0 °C ice into 0 °C water, heating the water to 100 °C,
converting 100 °C water to 100 °C steam and heating the steam to 150 °C.
Step 1: Heat required to raise the temperature of ice from -10 °C to 0 °C.
Use the formula q = mcΔT
q = (25 g)x(2.09 J/g·°C)[(0 °C - -10 °C)]
Step 2: Heat required to convert 0 °C ice to 0 °C water
Use the formula q = m·h
f
q = (25 g)x(334 J/g)
Step 3: Heat required to raise the temperature of 0 °C water to 100 °C water
q = (25 g)x(4.18 J/g·°C)[(100 °C - 0 °C)]

23. LATENT HEAT EXCHANGE
Problem: What is the heat in Joules required to convert 25 grams of
-10 °C ice into 150 °C steam?
heat of fusion of water = 334 J/g; heat of vaporization of water = 2257 J/g
specific heat of ice = 2.09 J/g·°C; specific heat of water = 4.18 J/g·°C
specific heat of steam = 2.09 J/g·°C
Solution:
Step 4: Heat required to convert 100 °C water to 100 °C steam
q = m·ΔH
v
q = (25 g)x(2257 J/g)
Step 5: Heat required to convert 100 °C steam to 150 °C steam
q = mcΔT
q = (25 g)x(2.09 J/g·°C)[(150 °C - 100 °C)]
Step 6: Find total heat energy
Heat
Total = Heat
Step 1 + Heat
Step 2 + Heat
Step 3 + Heat
Step 4 + Heat
Step 5
Heat
Total = 522.5 J + 8350 J + 10450 J + 56425 J + 2612.5 J
Heat
Total = 78360 J

24. CONDUCTION
WHAT IS HEAT CONDUCTION?

Heat is transferred by conduction within a body
or substance by direct molecular communication

25. WHAT IS CONDUCTION MECHANISM?

In solids, heat conduction is due to two effects:
- The lattice vibrational waves induced by the
vibrational motions of the molecules positioned at
relatively fixed positions in a periodic manner called a
lattice
- The energy transported via the free flow of electrons
in the solid

26. CONDUCTION
The lattice vibrational waves?

- Every atom is physically bonded to its neighbours in
some way.
- If heat energy is supplied to one part of a solid, the
atoms vibrate faster.
- As they vibrate more, the bonds
Between atoms are shaken more.
- This passes vibrations on to the
next atom, and so on

27.  In metals, some of the electrons (often one per atom)
are not stuck to individual atoms but flow freely among
the atoms.
Now if one end of a bar is hot, and the other is cold, the
electrons on the hot end have a little more thermal
energy- random jiggling- than the ones on the cold end.
So as the electrons wander around, they carry energy
from the hot end to the cold end, which is another way of
saying they conduct heat.
The energy transported via the free electrons

28. CONDUCTION
Fourier's Law of Heat Conduction

- Joseph Fourier developed the mathematical theory of
heat conduction in the nineteenth century

29. CONDUCTION
Fourier's Law of Heat Conduction
The rate of heat flow, dQ/dt, through a homogeneous
solid is directly proportional to the area, A, of the section
at right angles to the direction of heat flow, and to the
temperature difference along the path of heat flow,
dT/dx, and to the thermal conductivity

30. CONDUCTION
Thermal conductivity
is a material property that indicates the amount
of heat that will flow per unit time across an unit
area when the temperature gradient is unity
k (W/m)

31. CONDUCTION
Thermal conductivity

32. CONDUCTION
Thermal conductivity & temperature
- The thermal conductivity of pure
metals decrease when temperature
increase
- For non metals solids the
conductivity increases with
increase in temperature
- The thermal conductivities of
most liquids decrease with
increasing temperature, with water
being a notable exception
- The thermal conductivity of a gas
increases with increasing
temperature

33. CONDUCTION
Thermal conductivity & temperature
. The relatively high thermal
conductivities of pure metals are
primarily due to the electronic
component.
- As the temperature increases, the
molecular vibrations increase (in
turn decreasing the mean free path of
molecules). So, they obstruct the flow
of free electrons, thus reducing the
conductivity.
- In case of non metals, there are no
free electrons. So, only the molecular
vibrations are responsible for
conduction of heat

34. Process Heat Transfer Principles and Applications - Robert W. Serth - Elsevier Science &
Technology Books

The thermal conductivity of pure metals decrease when
temperature increase over 200
o
K

35. Example
Problem: The roof of an electrically heated home is 6 m long, 8 m
wide, and 0.25 m thick, and is made of a flat layer of concrete whose
thermal conductivity is k 0.8 W/m · °C. The temperatures of the
inner and the outer surfaces of the roof one night are measured to
be 15°C and 4°C, respectively, for a period of 10 hours. Determine
(a) the rate of heat loss through the roof that night and (b) the cost of
that heat loss to the home owner if the cost of electricity is
$0.08/kWh.

36. Example
Solution:
Assumptions : Steady operating conditions exist during the entire
night since the surface temperatures of the roof remain constant at the
specified values.
The area of the roof is A = 6 m x 8 m = 48 m2,
The heat transfer through the roof is



The amount of heat lost through the roof during a 10-hour period


The cost of the heat loss

37. Class problem
Problem: A refrigerator consumes 600 W of power when operating, and its motor
remains on for 5 min and then off for 15 min periodically. Refrigerator has height of 1.8
m, width of 0.8 m and length of 1.2 m. the thickness of the refrigerator walls is 0.03m.
What is the average thermal conductivity of the refrigerator walls?
Assumptions 1 Quasi-steady operating conditions exist. 2 The inner and outer surface
temperatures of the refrigerator remain constant. The coefficient of performance (COP)
of the refrigerator is 2.5. The temperature difference between two side of the
refrigerator’s walls is constant and equal to 30
o
C.
Solution:
The total surface area of the refrigerator where heat transfer takes place is

Since the refrigerator has a COP of 2.5, the rate of heat removal from the refrigerated
space, which is equal to the rate of heat gain in steady operation, is

But the refrigerator operates a quarter of the time (5 min on, 15 min off). Therefore, the
average rate of heat gain is

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