This document provides an overview of topics related to heat and mass transfer, including: - Fins and their applications (Unit I) - Convection boundary layer concepts including velocity and thermal boundary layers (Unit II) - Heat exchangers including concentric tube, cross flow, and shell and tube designs (Unit III) - Boiling and condensation processes including boiling curves and regimes (Unit IV) - Mass transfer concepts and analogies to heat transfer including diffusion, convection, and concentration boundary layers (Unit V) It defines key terms and concepts for each topic and provides illustrations of processes like boundary layer development, boiling curves, and mass transfer mechanisms like diffusion and convection.

1. UNIT I
FINS
(EXTENDED SURFACES)

2. Fin Applications
2

3. Fins Applications
3

4. Fins in Motor Body
4

5. Fin configurations
5
(a) straight fin of uniform cross-section on plane wall
(b) straight fin of uniform cross-section on circular tube
(c) annular fin
d) straight pin fin

6. UNIT III
HEAT EXCHANGERS

7. Concentric Tube Parallel &
Counter
7

8. Concentric Tube : parallel, Counter
8

9. Cross Flow : Mixed & Unmixed
9

10. Cross Flow : Mixed & Unmixed
10

11. Shell & Tube HX.
11

12. UNIT II
CONVECTION
BOUNDARY LAYER
CONCEPT
Heat and Mass Transfer: Fundamentals & Applications
Fourth Edition
Yunus A. Cengel, Afshin J. Ghajar
McGraw-Hill, 2011

13. 1313
VELOCITY BOUNDARY LAYER
Velocity boundary layer: The region of the flow above
the plate bounded by δδ in which the effects of the viscous
shearing forces caused by fluid viscosity are felt.
The boundary layer thickness, δδ, is typically defined as the
distance y from the surface at which u = 0.99V.
The hypothetical line of u = 0.99V divides the flow over a
plate into two regions:
Boundary layer region: The viscous effects and the
velocity changes are significant.
Irrotational flow region: The frictional effects are
negligible and the velocity remains essentially constant.

14. 1414
THERMAL BOUNDARY LAYER
Thermal boundary layer on a flat plate (the
fluid is hotter than the plate surface).
A thermal boundary layer develops when a fluid at a specified temperature
flows over a surface that is at a different temperature.
Thermal boundary layer: The flow region over the surface in which the
temperature variation in the direction normal to the surface is significant.
The thickness of the thermal boundary layer δt at any location along the
surface is defined as the distance from the surface at which the temperature
difference T − Ts equals 0.99(T∞− Ts).
The thickness of the thermal
boundary layer increases in the
flow direction, since the effects
of heat transfer are felt at
greater distances from the
surface further down stream.
The shape of the temperature
profile in the thermal boundary
layer dictates the convection
heat transfer between a solid
surface and the fluid flowing
over it.

15. 1515
Development of the velocity profile in a circular pipe
• A flow field is best characterized by its velocity distribution.
• A flow is said to be one-, two-, or three-dimensional if the flow velocity
varies in one, two, or three dimensions, respectively.
• However, the variation of velocity in certain directions can be small
relative to the variation in other directions and can be ignored.
The development of the velocity profile in a circular pipe. V = V(r, z)
and thus the flow is two-dimensional in the entrance region, and
becomes one-dimensional downstream when the velocity profile fully
develops and remains unchanged in the flow direction, V = V(r).

16. UNIT III
BOILING AND
CONDENSATION

17. 17
BOILING HEAT TRANSFER
• Evaporation occurs at the liquid–vapor interface
when the vapor pressure is less than the saturation
pressure of the liquid at a given temperature.
• Boiling occurs at the solid–liquid interface when a
liquid is brought into contact with a surface
maintained at a temperature sufficiently above the
saturation temperature of the liquid.

18. 18
Classification of boiling
• Boiling is called pool boiling in
the absence of bulk fluid flow.
• Any motion of the fluid is due to
natural convection currents and
the motion of the bubbles under
the influence of buoyancy.
• Boiling is called flow boiling in
the presence of bulk fluid flow.
• In flow boiling, the fluid is forced to
move in a heated pipe or over a
surface by external means such
as a pump.
excess temperature
Boiling heat flux from a solid surface to the fluid

19. 19
Subcooled Boiling
• When the
temperature of the
main body of the
liquid is below the
saturation
temperature.
Saturated Boiling
• When the
temperature of the
liquid is equal to
the saturation
temperature.

20. 20
POOL BOILING
Boiling takes different forms,
depending on the ∆Texcess = Ts − Tsat
In pool boiling, the fluid is not forced to flow
by a mover such as a pump.
Any motion of the fluid is due to natural
convection currents and the motion of the
bubbles under the influence of buoyancy.
Boiling Regimes and
the Boiling Curve

21. 21

22. 22
Natural Convection Boiling
(to Point A on the Boiling Curve)
• Bubbles do not form on the heating surface until the liquid is heated
a few degrees above the saturation temperature (about 2 to 6°C for
water)
• The liquid is slightly superheated in this case (metastable state).
• The fluid motion in this mode of boiling is governed by natural
convection currents.
• Heat transfer from the
heating surface to the fluid
is by natural convection.
• For the conditions of Fig.
10–6, natural convection
boiling ends at an excess
temperature of about 5°C.

23. 23
• The bubbles form at an
increasing rate at an increasing
number of nucleation sites as we
move along the boiling curve
toward point C.
Nucleate Boiling (between
Points A and C)
• Region A–B ─ isolated
bubbles.
• Region B–C ─
numerous continuous
columns of vapor in the
liquid.
Point A is referred to as
the onset of nucleate
boiling (ONB).

24. 24
• In region A–B the stirring and agitation caused by the entrainment of the
liquid to the heater surface is primarily responsible for the increased heat
transfer coefficient.
• In region A–B the large heat fluxes obtainable in this region are caused by
the combined effect of liquid entrainment and evaporation.
• For the entire nucleate boiling range, the heat transfer coefficient ranges
from about 2000 to 30,000 W/m2
·K.
• After point B the heat
flux increases at a
lower rate with
increasing ∆Texcess, and
reaches a maximum at
point C.
• The heat flux at this
point is called the
critical (or maximum)
heat flux, and is of
prime engineering
importance.

25. 25
Transition Boiling
(between Points C and D)
• When ∆Texcess is increased past point C,
the heat flux decreases.
• This is because a large fraction of the
heater surface is covered by a vapor
film, which acts as an insulation.
• In the transition boiling
regime, both nucleate and
film boiling partially occur.
• Operation in the transition
boiling regime, which is
also called the unstable
film boiling regime, is
avoided in practice.
• For water, transition boiling
occurs over the excess
temperature range from
about 30°C to about
120°C.

26. 26
Film Boiling (beyond Point D
• Beyond point D the
heater surface is
completely covered by a
continuous stable vapor
film.
• Point D, where the heat
flux reaches a minimum
is called the Leidenfrost
point.
• The presence of a vapor
film between the heater
surface and the liquid is
responsible for the low
heat transfer rates in the
film boiling region.
• The heat transfer rate
increases with increasing
excess temperature due
to radiation to the liquid.

27. 27
Burnout Phenomenon
• A typical boiling process
does not follow the boiling
curve beyond point C.
• When the power applied to
the heated surface exceeded
the value at point C even
slightly, the surface
temperature increased
suddenly to point E.
• When the power is reduced
gradually starting from point
E the cooling curve follows
Fig. 10–8 with a sudden drop
in excess temperature when
point D is reached.

28. 28
Any attempt to increase the heat
flux beyond qmax will cause the
operation point on the boiling
curve to jump suddenly from
point C to point E.
However, surface temperature
that corresponds to point E is
beyond the melting point of most
heater materials, and burnout
occurs.
Therefore, point C on the boiling
curve is also called the burnout
point, and the heat flux at this
point the burnout heat flux.
Most boiling heat transfer
equipment in practice operate
slightly below qmax to avoid any
disastrous burnout.

29. 29
Heat Transfer Correlations in Pool Boiling
• Boiling regimes differ considerably in their character.
• Different heat transfer relations need to be used for different boiling regimes.
• In the natural convection boiling regime heat transfer rates can be accurately
determined using natural convection relations.
• No general theoretical relations for heat
transfer in the nucleate boiling regime is
available.
• Experimental based correlations are
used.
• The rate of heat transfer strongly
depends on the nature of nucleation
and the type and the condition of the
heated surface.
Nucleate Boiling

30. 30
Film condensation
• The condensate wets the surface and
forms a liquid film.
• The surface is blanketed by a liquid
film which serves as a resistance to
heat transfer.
Dropwise condensation
• The condensed vapor forms droplets
on the surface.
• The droplets slide down when they
reach a certain size.
• No liquid film to resist heat transfer.
• As a result, heat transfer rates that
are more than 10 times larger than
with film condensation can be
achieved.
Condensation occurs when the temperature of a vapor is reduced below
its saturation temperature.
CONDENSATION HEAT TRANSFER

31. 31
• Liquid film starts forming at the top
of the plate and flows downward
under the influence of gravity.
• δ increases in the flow direction x
• Heat in the amount hfg is released
during condensation and is
transferred through the film to the
plate surface.
• Ts must be below the saturation
temperature for condensation.
• The temperature of the condensate
is Tsat at the interface and decreases
gradually to Ts at the wall.
FILM CONDENSATION

32. 32
DROPWISE
CONDENSATIONDropwise condensation, characterized by
countless droplets of varying diameters on the
condensing surface instead of a continuous
liquid film and extremely large heat transfer
coefficients can be achieved with this
mechanism.
The small droplets that form at the nucleation
sites on the surface grow as a result of
continued condensation, coalesce into large
droplets, and slide down when they reach a
certain size, clearing the surface and exposing it
to vapor. There is no liquid film in this case to
resist heat transfer.
As a result, with dropwise condensation, heat
transfer coefficients can be achieved that are
more than 10 times larger than those associated
with film condensation.
The challenge in dropwise condensation is not to
achieve it, but rather, to sustain it for prolonged
periods of time.
Dropwise condensation of
steam on copper surfaces:

33. UNIT V
MASS TRANSFER

34. 34
INTRODUCTION
Whenever there is an imbalance of a commodity in a medium, nature tends to
redistribute it until a “balance” or “equality” is established. This tendency is often referred
to as the driving force, which is the mechanism behind many naturally occurring
transport phenomena.
The commodity simply creeps away during
redistribution, and thus the flow is a diffusion
process. The rate of flow of the commodity is
proportional to the concentration gradient
dC/dx, which is the change in the concentration
C per unit length in the flow direction x, and the
area A normal to flow direction.
kdiff is the diffusion coefficient of the medium, which is a measure of how fast a
commodity diffuses in the medium, and the negative sign is to make the flow in the
positive direction a positive quantity (note that dC/dx is a negative quantity since
concentration decreases in the flow direction).

35. 35
The diffusion coefficients and thus diffusion rates of gases
depend strongly on temperature.
The diffusion rates are higher at higher temperatures.
The larger the molecular spacing, the higher the diffusion rate.
Diffusion rate: gases > liquids > solids

36. 36
ANALOGY (SIMILARITY) BETWEEN HEAT
AND MASS TRANSFER
We can develop an understanding of mass transfer in a short time with
little effort by simply drawing parallels between heat and mass transfer.
Temperature
The driving force for mass transfer is
the concentration difference.
Both heat and mass are transferred
from the more concentrated regions
to the less concentrated ones.
If there is no difference between the
concentrations of a species at
different parts of a medium, there will
be no mass transfer.

37. 37
Conduction
Mass is transferred by conduction (called diffusion) and convection only.
Rate of mass
diffusion
Fick’s law
of diffusion
DAB is the diffusion coefficient (or mass
diffusivity) of the species in the mixture
CA is the concentration of the species in the
mixture

38. 38
Heat Generation
Heat generation refers to the conversion of some form of energy such
as electrical, chemical, or nuclear energy into sensible thermal energy
in the medium.
Some mass transfer problems involve chemical reactions that occur
within the medium and result in the generation of a species throughout.
Therefore, species generation is a volumetric phenomenon, and the
rate of generation may vary from point to point in the medium.
Such reactions that occur within the medium are called homogeneous
reactions and are analogous to internal heat generation.
In contrast, some chemical reactions result in the generation of a
species at the surface as a result of chemical reactions occurring at the
surface due to contact between the medium and the surroundings.
This is a surface phenomenon, and as such it needs to be treated as a
boundary condition.
In mass transfer studies, such reactions are called heterogeneous
reactions and are analogous to specified surface heat flux.

39. 39
Convection
Mass convection (or convective mass transfer) is the mass transfer mechanism
between a surface and a moving fluid that involves both mass diffusion and bulk
fluid motion.
Fluid motion also enhances mass transfer considerably.
Newton’s law
of cooling
Rate of mass
convection
In mass convection, we define a
concentration boundary layer in an
analogous manner to the thermal boundary
layer and define new dimensionless
numbers that are counterparts of the
Nusselt and Prandtl numbers.
hmass the mass transfer coefficient
As the surface area
Cs − C∞ a suitable concentration difference
across the concentration boundary layer.

40. 40
MASS DIFFUSION
Fick’s law of diffusion states that the rate of diffusion of
a chemical species at a location in a gas mixture (or
liquid or solid solution) is proportional to the
concentration gradient of that species at that location.
1 Mass Basis
On a mass basis, concentration is expressed in
terms of density (or mass concentration).
The density of a mixture at a location is equal to the
sum of the densities of its constituents at that location.
The mass fraction of a species ranges between
0 and 1, and the sum of the mass fractions of
the constituents of a mixture be equal to 1.

41. 41
TYPE I STEADY MASS DIFFUSION THROUGH A WALL
Many practical mass transfer problems involve the diffusion of a species through a
plane-parallel medium that does not involve any homogeneous chemical reactions under
one-dimensional steady conditions.
diffusion resistance
of the wall

42. 42
TYPE I : STEADY ONE-DIMENSIONAL MASS
TRANSFER THROUGH NONREACTING
CYLINDRICAL AND SPHERICAL LAYERS
On a molar basis

43. 43
TYPE II : Equimolar Counterdiffusion
equimolar counterdiffusion

44. 44
TYPE III: DIFFUSION OF VAPOR THROUGH A
STATIONARY GAS : STEFAN FLOW
Many engineering applications such as heat pipes, cooling ponds, and the familiar
perspiration involve condensation, evaporation, and transpiration in the presence of a
noncondensable gas, and thus the diffusion of a vapor through a stationary (or stagnant)
gas.
To understand and analyze such processes, consider a liquid layer of species A in a tank
surrounded by a gas of species B, such as a layer of liquid water in a tank open to the
atmospheric air at constant pressure P and temperature T.

45. 45
This relation is known as Stefan’s
law, and the induced convective flow
described that enhances mass
diffusion is called the Stefan flow.

46. 46
CONVECTIVE MASS TRANSFER
Now we consider mass convection (or convective mass transfer), which is the
transfer of mass between a surface and a moving fluid due to both mass diffusion
and bulk fluid motion.
The analogy between heat and mass convection holds for both forced and natural
convection, laminar and turbulent flow, and internal and external flow.
Mass convection is also complicated because of the complications associated with
fluid flow such as the surface geometry, flow regime, flow velocity, and the variation
of the fluid properties and composition.
Therefore, we have to rely on experimental relations to determine mass transfer.
Mass convection is usually analyzed on a mass basis rather than on a molar basis.
Concentration boundary layer: In mass
convection, the region of the fluid in which
concentration gradients exist.

47. 47
In internal flow, we have a concentration
entrance region where the concentration
profile develops, in addition to the
hydrodynamic and thermal entry regions.
The concentration boundary layer
continues to develop in the flow direction
until its thickness reaches the tube center
and the boundary layers merge.
The distance from the tube inlet to the
location where this merging occurs is
called the concentration entry length Lc,
and the region beyond that point is called
the fully developed region.

48. 48
SIMULTANEOUS HEAT
AND MASS TRANSFER
Many mass transfer processes encountered in
practice occur isothermally, and thus they do not
involve any heat transfer.
But some engineering applications involve the
vaporization of a liquid and the diffusion of this
vapor into the surrounding gas.
Such processes require the transfer of the latent
heat of vaporization hfg to the liquid in order to
vaporize it, and thus such problems involve
simultaneous heat and mass transfer.
To generalize, any mass transfer problem
involving phase change (evaporation,
sublimation, condensation, melting, etc.) must
also involve heat transfer, and the solution of
such problems needs to be analyzed by
considering simultaneous heat and mass
transfer.

49. UNIT IV
FUNDAMENTALS OF
THERMAL RADIATION

50. 50
INTRODUCTION Radiation differs from conduction and
convection in that it does not require the
presence of a material medium to take place.
Radiation transfer occurs in solids as well as
liquids and gases.
The hot object in vacuum
chamber will eventually cool
down and reach thermal
equilibrium with its
surroundings by a heat transfer
mechanism: radiation.

51. 51
Accelerated charges or changing electric currents give rise to electric and
magnetic fields. These rapidly moving fields are called electromagnetic waves or
electromagnetic radiation, and they represent the energy emitted by matter as a
result of the changes in the electronic configurations of the atoms or molecules.
Electromagnetic waves transport energy just like other waves and they are
characterized by their frequency ν or wavelength λ. These two properties in a
medium are related by
c = c0 /n
c, the speed of propagation of a wave in that medium
c0 = 2.9979×108
m/s, the speed of light in a vacuum
n, the index of refraction of that medium
n =1 for air and most gases, n = 1.5 for glass, and n = 1.33 for water
It has proven useful to view electromagnetic radiation as the propagation
of a collection of discrete packets of energy called photons or quanta.
In this view, each photon of frequency n is considered to have an energy of
The energy of a photon is inversely
proportional to its wavelength.

52. 52
THERMAL RADIATION
The
electromagnetic
wave spectrum.
The type of electromagnetic radiation that is pertinent
to heat transfer is the thermal radiation emitted as a
result of energy transitions of molecules, atoms, and
electrons of a substance.
Temperature is a measure of the strength of these
activities at the microscopic level, and the rate of
thermal radiation emission increases with increasing
temperature.
Thermal radiation is continuously emitted by all matter
whose temperature is above absolute zero.
Everything
around us
constantly
emits thermal
radiation.

53. 53
In heat transfer studies, we are interested in
the energy emitted by bodies because of their
temperature only. Therefore, we limit our
consideration to thermal radiation.
The electrons, atoms, and molecules of
all solids, liquids, and gases above
absolute zero temperature are constantly
in motion, and thus radiation is
constantly emitted, as well as being
absorbed or transmitted throughout the
entire volume of matter.
That is, radiation is a volumetric
phenomenon.

54. 54
BLACKBODY RADIATION
• Different bodies may emit different amounts of radiation per unit surface area.
• A blackbody emits the maximum amount of radiation by a surface at a given
temperature.
• It is an idealized body to serve as a standard against which the radiative
properties of real surfaces may be compared.
• A blackbody is a perfect emitter and absorber of radiation.
• A blackbody absorbs all incident radiation, regardless of wavelength and
direction.
Stefan–Boltzmann constant
Blackbody emissive power
The radiation energy
emitted by a blackbody:

55. 55
Spectral blackbody emissive Power:
The amount of radiation energy emitted
by a blackbody at a thermodynamic
temperature T per unit time, per unit
surface area, and per unit wavelength
about the wavelength λ.
Boltzmann’s constant
Planck’s
law

56. 56
The wavelength at which the
peak occurs for a specified
temperature is given by
Wien’s displacement law:

57. 57
Observations from the figure
• The emitted radiation is a continuous function of wavelength.
At any specified temperature, it increases with wavelength,
reaches a peak, and then decreases with increasing
wavelength.
• At any wavelength, the amount of emitted radiation increases
with increasing temperature.
• As temperature increases, the curves shift to the left to the
shorter wavelength region. Consequently, a larger fraction of
the radiation is emitted at shorter wavelengths at higher
temperatures.
• The radiation emitted by the sun, which is considered to be a
blackbody at 5780 K (or roughly at 5800 K), reaches its peak
in the visible region of the spectrum. Therefore, the sun is in
tune with our eyes.
• On the other hand, surfaces at T < 800 K emit almost entirely
in the infrared region and thus are not visible to the eye
unless they reflect light coming from other sources.

58. Intensity of Emitted Radiation
58

59. 59

60. Incident
Radiation
60

61. Radiosity
61

62. 62
RADIATIVE PROPERTIES
Most materials encountered in practice, such as metals, wood, and bricks, are opaque
to thermal radiation, and radiation is considered to be a surface phenomenon for such
materials.
Radiation through semitransparent materials such as glass and water cannot be
considered to be a surface phenomenon since the entire volume of the material
interacts with radiation.
A blackbody can serve as a convenient reference in describing the emission and
absorption characteristics of real surfaces.
Emissivity
• Emissivity: The ratio of the radiation emitted by the surface at a given temperature
to the radiation emitted by a blackbody at the same temperature. 0 ≤ ε ≤ 1.
• Emissivity is a measure of how closely a surface approximates a blackbody (ε = 1).
• The emissivity of a real surface varies with the temperature of the surface as well as
the wavelength and the direction of the emitted radiation.
• The emissivity of a surface at a specified wavelength is called spectral emissivity
ελ. The emissivity in a specified direction is called directional emissivity εθ where θ
is the angle between the direction of radiation and the normal of the surface.

63. 63

64. 64
Absorptivity,
Reflectivity, and
Transmissivity
Irradiation, G:
Radiation flux
incident on a
surface.
for opaque surfaces

65. 65
Kirchhoff’s Law
The total hemispherical emissivity of
a surface at temperature T is equal
to its total hemispherical absorptivity
for radiation coming from a
blackbody at the same temperature.
Kirchhoff’s law
spectral form of
Kirchhoff’s law
The emissivity of a surface at a specified wavelength, direction, and
temperature is always equal to its absorptivity at the same wavelength,
direction, and temperature.

66. 66
The Greenhouse Effect
Glass has a transparent window in the wavelength range 0.3 µm < λ < 3 µm in which
over 90% of solar radiation is emitted. The entire radiation emitted by surfaces at room
temperature falls in the infrared region (λ > 3 µm).
Glass allows the solar radiation to enter but does not allow the infrared radiation from the
interior surfaces to escape. This causes a rise in the interior temperature as a result of
the energy buildup in the car.
This heating effect, which is due to the nongray characteristic of glass (or clear plastics),
is known as the greenhouse effect.

67. ATMOSPHERIC AND SOLAR RADIATION
Atmospheric radiation: The radiation energy emitted or
reflected by the constituents of the atmosphere.
The energy of the sun is due to the continuous
fusion reaction during which two hydrogen
atoms fuse to form one atom of helium.
Therefore, the sun is essentially a nuclear
reactor, with temperatures as high as
40,000,000 K in its core region.
The temperature drops to about 5800 K in the
outer region of the sun, called the convective
zone, as a result of the dissipation of this
energy by radiation.
Total solar irradiance Gs:
The solar energy reaching
the earth’s atmosphere is
called the
Solar constant: The total solar irradiance. It
represents the rate at which solar energy is
incident on a surface normal to the sun’s rays
at the outer edge of the atmosphere when the
earth is at its mean distance from the sun 67

68. The value of the total solar irradiance can be used
to estimate the effective surface temperature of
the sun from the requirement that
The sun can be treated
as a blackbody at a
temperature of 5780 K.
68

69. 69