Chapter 9 - Boiling - Chapter Notes, Chemical Engineering, Semester Notes | EduRev

: Chapter 9 - Boiling - Chapter Notes, Chemical Engineering, Semester Notes | EduRev

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BOOKCOMP, Inc. — John Wiley & Sons / Page 635 / 2nd Proofs / Heat TransferHandbook / Bejan
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CHAPTER9
Boiling
JOHN R. THOME
Laboratory of Heat and Mass Transfer
Faculty of Engineering Science
Swiss Federal Institute of Technology Lausanne
Lausanne,Switzerland
9.1 Introduction to boiling heat transfer
9.2 Boiling curve
9.3 Boiling nucleation
9.3.1 Introduction
9.3.2 Nucleation superheat
9.3.3 Size range of active nucleation sites
9.3.4 Nucleation site density
9.4 Bubble dynamics
9.4.1 Bubble growth
9.4.2 Bubble departure
9.4.3 Bubble departure frequency
9.5 Pool boiling heat transfer
9.5.1 Nucleate boiling heat transfer mechanisms
9.5.2 Nucleate pool boiling correlations
Bubble agitation correlation of Rohsenow
Reduced pressure correlation of Mostinski
Physical property type of correlation of Stephan and Abdelsalam
Reduced pressure correlation of Cooper with surface roughness
Fluid-speci?c correlation of Goren?o
9.5.3 Departure from nucleate pool boiling (or critical heat ?ux)
9.5.4 Film boiling and transition boiling
9.6 Introduction to ?ow boiling
9.7 Two-phase ?ow patterns
9.7.1 Flow patterns in vertical and horizontal tubes
9.7.2 Flow pattern maps for vertical ?ows
9.7.3 Flow pattern maps for horizontal ?ows
9.8 Flow boiling in vertical tubes
9.8.1 Chen correlation
9.8.2 Shah correlation
9.8.3 Gungor–Winterton correlation
9.8.4 Steiner–Taborek method
9.9 Flow boiling in horizontal tubes
9.9.1 Horizontal tube correlations based on vertical tube methods
635
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CHAPTER9
Boiling
JOHN R. THOME
Laboratory of Heat and Mass Transfer
Faculty of Engineering Science
Swiss Federal Institute of Technology Lausanne
Lausanne,Switzerland
9.1 Introduction to boiling heat transfer
9.2 Boiling curve
9.3 Boiling nucleation
9.3.1 Introduction
9.3.2 Nucleation superheat
9.3.3 Size range of active nucleation sites
9.3.4 Nucleation site density
9.4 Bubble dynamics
9.4.1 Bubble growth
9.4.2 Bubble departure
9.4.3 Bubble departure frequency
9.5 Pool boiling heat transfer
9.5.1 Nucleate boiling heat transfer mechanisms
9.5.2 Nucleate pool boiling correlations
Bubble agitation correlation of Rohsenow
Reduced pressure correlation of Mostinski
Physical property type of correlation of Stephan and Abdelsalam
Reduced pressure correlation of Cooper with surface roughness
Fluid-speci?c correlation of Goren?o
9.5.3 Departure from nucleate pool boiling (or critical heat ?ux)
9.5.4 Film boiling and transition boiling
9.6 Introduction to ?ow boiling
9.7 Two-phase ?ow patterns
9.7.1 Flow patterns in vertical and horizontal tubes
9.7.2 Flow pattern maps for vertical ?ows
9.7.3 Flow pattern maps for horizontal ?ows
9.8 Flow boiling in vertical tubes
9.8.1 Chen correlation
9.8.2 Shah correlation
9.8.3 Gungor–Winterton correlation
9.8.4 Steiner–Taborek method
9.9 Flow boiling in horizontal tubes
9.9.1 Horizontal tube correlations based on vertical tube methods
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9.9.2 Horizontal ?ow boiling model based on local ?ow regime
9.9.3 Subcooled boiling heat transfer
9.10 Boiling on tube bundles
9.10.1 Heat transfer characteristics
9.10.2 Bundle boiling factor
9.10.3 Bundle design methods
9.11 Post-dryout heat transfer
9.11.1 Introduction
9.11.2 Thermal nonequilibrium
9.11.3 Heat transfer mechanisms
9.11.4 Inverted annular ?ow heat transfer
9.11.5 Mist ?ow heat transfer
9.12 Boiling of mixtures
9.12.1 Vapor–liquid equilibria and properties
9.12.2 Nucleate boiling of mixtures
9.12.3 Flow boiling of mixtures
9.12.4 Evaporation of refrigerant–oil mixtures
9.13 Enhanced boiling
9.13.1 Enhancement of nucleate pool boiling
9.13.2 Enhancement of internal convective boiling
Nomenclature
References
9.1 INTRODUCTIONTO BOILINGHEATTRANSFER
When heat is applied to a surface in contact with a liquid, if the wall temperature
is suf?ciently above the saturation temperature, boiling occurs on the wall. Boiling
may occur under quiescent ?uid conditions, which is referred to as pool boiling,
or under forced-?ow conditions, which is referred to as forced convective boiling.
In this chapter a review of the fundamentals of boiling is presented together with
numerous predictive methods. First the fundamentals of pool boiling are addressed
andthenthoseof?owboiling.Tobetterunderstandthemechanicsof?owboiling,a
section is also presented on two-phase ?ow patterns and ?ow pattern maps. Then the
effects of mixture boiling are described. Finally, the topic of enhanced heat transfer
is introduced.
For more exhaustive treatments of boiling heat transfer, the following books are
recommended for consultation: Tong (1965), Wallis (1969), Hsu and Graham (1976),
Ginoux (1978), van Stralen and Cole (1979), Delhaye et al. (1981), Whalley (1987),
Thome (1990), Carey (1992) and Collier and Thome (1994). In addition, Rohsenow
(1973) provides a detailed historical presentation of boiling research.
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BOOKCOMP, Inc. — John Wiley & Sons / Page 635 / 2nd Proofs / Heat TransferHandbook / Bejan
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CHAPTER9
Boiling
JOHN R. THOME
Laboratory of Heat and Mass Transfer
Faculty of Engineering Science
Swiss Federal Institute of Technology Lausanne
Lausanne,Switzerland
9.1 Introduction to boiling heat transfer
9.2 Boiling curve
9.3 Boiling nucleation
9.3.1 Introduction
9.3.2 Nucleation superheat
9.3.3 Size range of active nucleation sites
9.3.4 Nucleation site density
9.4 Bubble dynamics
9.4.1 Bubble growth
9.4.2 Bubble departure
9.4.3 Bubble departure frequency
9.5 Pool boiling heat transfer
9.5.1 Nucleate boiling heat transfer mechanisms
9.5.2 Nucleate pool boiling correlations
Bubble agitation correlation of Rohsenow
Reduced pressure correlation of Mostinski
Physical property type of correlation of Stephan and Abdelsalam
Reduced pressure correlation of Cooper with surface roughness
Fluid-speci?c correlation of Goren?o
9.5.3 Departure from nucleate pool boiling (or critical heat ?ux)
9.5.4 Film boiling and transition boiling
9.6 Introduction to ?ow boiling
9.7 Two-phase ?ow patterns
9.7.1 Flow patterns in vertical and horizontal tubes
9.7.2 Flow pattern maps for vertical ?ows
9.7.3 Flow pattern maps for horizontal ?ows
9.8 Flow boiling in vertical tubes
9.8.1 Chen correlation
9.8.2 Shah correlation
9.8.3 Gungor–Winterton correlation
9.8.4 Steiner–Taborek method
9.9 Flow boiling in horizontal tubes
9.9.1 Horizontal tube correlations based on vertical tube methods
635
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9.9.2 Horizontal ?ow boiling model based on local ?ow regime
9.9.3 Subcooled boiling heat transfer
9.10 Boiling on tube bundles
9.10.1 Heat transfer characteristics
9.10.2 Bundle boiling factor
9.10.3 Bundle design methods
9.11 Post-dryout heat transfer
9.11.1 Introduction
9.11.2 Thermal nonequilibrium
9.11.3 Heat transfer mechanisms
9.11.4 Inverted annular ?ow heat transfer
9.11.5 Mist ?ow heat transfer
9.12 Boiling of mixtures
9.12.1 Vapor–liquid equilibria and properties
9.12.2 Nucleate boiling of mixtures
9.12.3 Flow boiling of mixtures
9.12.4 Evaporation of refrigerant–oil mixtures
9.13 Enhanced boiling
9.13.1 Enhancement of nucleate pool boiling
9.13.2 Enhancement of internal convective boiling
Nomenclature
References
9.1 INTRODUCTIONTO BOILINGHEATTRANSFER
When heat is applied to a surface in contact with a liquid, if the wall temperature
is suf?ciently above the saturation temperature, boiling occurs on the wall. Boiling
may occur under quiescent ?uid conditions, which is referred to as pool boiling,
or under forced-?ow conditions, which is referred to as forced convective boiling.
In this chapter a review of the fundamentals of boiling is presented together with
numerous predictive methods. First the fundamentals of pool boiling are addressed
andthenthoseof?owboiling.Tobetterunderstandthemechanicsof?owboiling,a
section is also presented on two-phase ?ow patterns and ?ow pattern maps. Then the
effects of mixture boiling are described. Finally, the topic of enhanced heat transfer
is introduced.
For more exhaustive treatments of boiling heat transfer, the following books are
recommended for consultation: Tong (1965), Wallis (1969), Hsu and Graham (1976),
Ginoux (1978), van Stralen and Cole (1979), Delhaye et al. (1981), Whalley (1987),
Thome (1990), Carey (1992) and Collier and Thome (1994). In addition, Rohsenow
(1973) provides a detailed historical presentation of boiling research.
BOOKCOMP, Inc. — John Wiley & Sons / Page 637 / 2nd Proofs / Heat TransferHandbook / Bejan
BOILING CURVE 637
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9.2 BOILING CURVE
When heating a surface in a large pool of liquid, the heat ?ux q is usually plotted
versus the wall superheat ? T
sat
, which is the temperature difference between the
surface and the saturation temperature of the liquid. First constructed by Nukiyama
(1934), theboilingcurve depicted in Fig. 9.1 is also referred to asNukiyama’scurve,
where four distinct heat transfer regimes can be identi?ed:
1. Natural convection. This is characterized by single-phase natural convection
from the hot surface to the saturation liquid without formation of bubbles on
the surface.
2. Nucleate boiling. This is a two-phase natural convection process in which
bubbles nucleate, grow, and depart from the heated surface.
3. Transitionboiling. This is an intermediate regime between the nucleate boiling
and ?lm boiling regimes.
Figure 9.1 Nukiyama’s boiling curve.
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CHAPTER9
Boiling
JOHN R. THOME
Laboratory of Heat and Mass Transfer
Faculty of Engineering Science
Swiss Federal Institute of Technology Lausanne
Lausanne,Switzerland
9.1 Introduction to boiling heat transfer
9.2 Boiling curve
9.3 Boiling nucleation
9.3.1 Introduction
9.3.2 Nucleation superheat
9.3.3 Size range of active nucleation sites
9.3.4 Nucleation site density
9.4 Bubble dynamics
9.4.1 Bubble growth
9.4.2 Bubble departure
9.4.3 Bubble departure frequency
9.5 Pool boiling heat transfer
9.5.1 Nucleate boiling heat transfer mechanisms
9.5.2 Nucleate pool boiling correlations
Bubble agitation correlation of Rohsenow
Reduced pressure correlation of Mostinski
Physical property type of correlation of Stephan and Abdelsalam
Reduced pressure correlation of Cooper with surface roughness
Fluid-speci?c correlation of Goren?o
9.5.3 Departure from nucleate pool boiling (or critical heat ?ux)
9.5.4 Film boiling and transition boiling
9.6 Introduction to ?ow boiling
9.7 Two-phase ?ow patterns
9.7.1 Flow patterns in vertical and horizontal tubes
9.7.2 Flow pattern maps for vertical ?ows
9.7.3 Flow pattern maps for horizontal ?ows
9.8 Flow boiling in vertical tubes
9.8.1 Chen correlation
9.8.2 Shah correlation
9.8.3 Gungor–Winterton correlation
9.8.4 Steiner–Taborek method
9.9 Flow boiling in horizontal tubes
9.9.1 Horizontal tube correlations based on vertical tube methods
635
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9.9.2 Horizontal ?ow boiling model based on local ?ow regime
9.9.3 Subcooled boiling heat transfer
9.10 Boiling on tube bundles
9.10.1 Heat transfer characteristics
9.10.2 Bundle boiling factor
9.10.3 Bundle design methods
9.11 Post-dryout heat transfer
9.11.1 Introduction
9.11.2 Thermal nonequilibrium
9.11.3 Heat transfer mechanisms
9.11.4 Inverted annular ?ow heat transfer
9.11.5 Mist ?ow heat transfer
9.12 Boiling of mixtures
9.12.1 Vapor–liquid equilibria and properties
9.12.2 Nucleate boiling of mixtures
9.12.3 Flow boiling of mixtures
9.12.4 Evaporation of refrigerant–oil mixtures
9.13 Enhanced boiling
9.13.1 Enhancement of nucleate pool boiling
9.13.2 Enhancement of internal convective boiling
Nomenclature
References
9.1 INTRODUCTIONTO BOILINGHEATTRANSFER
When heat is applied to a surface in contact with a liquid, if the wall temperature
is suf?ciently above the saturation temperature, boiling occurs on the wall. Boiling
may occur under quiescent ?uid conditions, which is referred to as pool boiling,
or under forced-?ow conditions, which is referred to as forced convective boiling.
In this chapter a review of the fundamentals of boiling is presented together with
numerous predictive methods. First the fundamentals of pool boiling are addressed
andthenthoseof?owboiling.Tobetterunderstandthemechanicsof?owboiling,a
section is also presented on two-phase ?ow patterns and ?ow pattern maps. Then the
effects of mixture boiling are described. Finally, the topic of enhanced heat transfer
is introduced.
For more exhaustive treatments of boiling heat transfer, the following books are
recommended for consultation: Tong (1965), Wallis (1969), Hsu and Graham (1976),
Ginoux (1978), van Stralen and Cole (1979), Delhaye et al. (1981), Whalley (1987),
Thome (1990), Carey (1992) and Collier and Thome (1994). In addition, Rohsenow
(1973) provides a detailed historical presentation of boiling research.
BOOKCOMP, Inc. — John Wiley & Sons / Page 637 / 2nd Proofs / Heat TransferHandbook / Bejan
BOILING CURVE 637
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9.2 BOILING CURVE
When heating a surface in a large pool of liquid, the heat ?ux q is usually plotted
versus the wall superheat ? T
sat
, which is the temperature difference between the
surface and the saturation temperature of the liquid. First constructed by Nukiyama
(1934), theboilingcurve depicted in Fig. 9.1 is also referred to asNukiyama’scurve,
where four distinct heat transfer regimes can be identi?ed:
1. Natural convection. This is characterized by single-phase natural convection
from the hot surface to the saturation liquid without formation of bubbles on
the surface.
2. Nucleate boiling. This is a two-phase natural convection process in which
bubbles nucleate, grow, and depart from the heated surface.
3. Transitionboiling. This is an intermediate regime between the nucleate boiling
and ?lm boiling regimes.
Figure 9.1 Nukiyama’s boiling curve.
BOOKCOMP, Inc. — John Wiley & Sons / Page 638 / 2nd Proofs / Heat TransferHandbook / Bejan
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4. Film boiling. This mode is characterized by a stable layer of vapor that forms
between the heated surface and the liquid, such that the bubbles form at the free
interface and not at the wall.
Between these four regimes are three transition points. The ?rst is calledincipience
of boiling (IB) or onset of nucleate boiling (ONB), at which the ?rst bubbles appear
on the heated surface. The second is the peak in the curve at the top of the nucleate
boiling portion of the curve, referred to as departure from nucleate boiling (DNB),
the critical heat ?ux (CHF), or peak heat ?ux. The last transition point is located at
thelowerendofthe?lmboilingregime(attheletter E) and is called the minimum
?lmboiling (MFB)point. These are all denoted in Fig. 9.1, while a representation of
these regimes is shown in Fig. 9.2.
In the natural convection part of the curve, the wall temperature rises as the heat
?ux is increased until the ?rst bubbles appear, signaling the incipience of boiling.
These bubbles form (or nucleate) at small cavities in the heated surface, which are
callednucleationsites. The active nucleation sites are located at pits and scratches in
the surface. Increasing the heat ?ux, more and more nucleation sites become activated
until the surface is covered with bubbles that grow and depart in rapid succession.
The heat ?ux increases dramatically for relatively modest increases in? T
sat
(de?ned
as T
w
- T
sat
), noting that the scale is log-log. Increasing the heat ?ux even further,
departing bubbles coalesce into vapor jets, changing the slope of the nucleate boiling
curve. A further increase in q eventually prohibits the liquid from reaching the heated
surface, which is referred to as the DNB or CHF, such that complete blanketing of
the surface by vapor occurs, accompanied by a rapid rise in the surface temperature
to dissipate the applied heat ?ux.
Following DNB, the process follows a path that depends on the manner in which
the heat ?ux is applied to the surface. For heaters that impose a heat ?ux at the surface,
such as electrical-resistance elements or nuclear fuel rods, the process progresses on
a horizontal line of constant heat ?ux so that the wall superheat jumps to point D

,
where ?lm boiling prevails as shown in Fig. 9.1, and whose vapor bubbles grow and
depart from the vapor–liquid interface of the vapor layer, not from the surface. A
ulterior increase in q may bring the surface to the burnout point (letter F), where
the surface temperature reaches the melting point of the heater. Reducing the heat
?ux, the ?lm boiling curve passes below point D

until reaching point E, the MFB
point. Here again, the process path depends on the mode of heating. For an imposed
heat ?ux, the process path jumps horizontally at constant q to the nucleate boiling
curve B

C. Consequently, a hysteresis loop is formed when heating a surface up
past the DNB and then bringing it below MFB when q is the boundary condition
imposed.
When the wall temperature is the externally controlled variable, such as by varying
the saturation temperature of steam condensing inside a tube with boiling on the
outside, the process path moves from the DNB to the MFB point, and vice versa,
following the transition boiling path. In transition boiling, the process vacillates
between nucleate boiling and ?lm boiling, where each mode may coexist next to the
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CHAPTER9
Boiling
JOHN R. THOME
Laboratory of Heat and Mass Transfer
Faculty of Engineering Science
Swiss Federal Institute of Technology Lausanne
Lausanne,Switzerland
9.1 Introduction to boiling heat transfer
9.2 Boiling curve
9.3 Boiling nucleation
9.3.1 Introduction
9.3.2 Nucleation superheat
9.3.3 Size range of active nucleation sites
9.3.4 Nucleation site density
9.4 Bubble dynamics
9.4.1 Bubble growth
9.4.2 Bubble departure
9.4.3 Bubble departure frequency
9.5 Pool boiling heat transfer
9.5.1 Nucleate boiling heat transfer mechanisms
9.5.2 Nucleate pool boiling correlations
Bubble agitation correlation of Rohsenow
Reduced pressure correlation of Mostinski
Physical property type of correlation of Stephan and Abdelsalam
Reduced pressure correlation of Cooper with surface roughness
Fluid-speci?c correlation of Goren?o
9.5.3 Departure from nucleate pool boiling (or critical heat ?ux)
9.5.4 Film boiling and transition boiling
9.6 Introduction to ?ow boiling
9.7 Two-phase ?ow patterns
9.7.1 Flow patterns in vertical and horizontal tubes
9.7.2 Flow pattern maps for vertical ?ows
9.7.3 Flow pattern maps for horizontal ?ows
9.8 Flow boiling in vertical tubes
9.8.1 Chen correlation
9.8.2 Shah correlation
9.8.3 Gungor–Winterton correlation
9.8.4 Steiner–Taborek method
9.9 Flow boiling in horizontal tubes
9.9.1 Horizontal tube correlations based on vertical tube methods
635
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9.9.2 Horizontal ?ow boiling model based on local ?ow regime
9.9.3 Subcooled boiling heat transfer
9.10 Boiling on tube bundles
9.10.1 Heat transfer characteristics
9.10.2 Bundle boiling factor
9.10.3 Bundle design methods
9.11 Post-dryout heat transfer
9.11.1 Introduction
9.11.2 Thermal nonequilibrium
9.11.3 Heat transfer mechanisms
9.11.4 Inverted annular ?ow heat transfer
9.11.5 Mist ?ow heat transfer
9.12 Boiling of mixtures
9.12.1 Vapor–liquid equilibria and properties
9.12.2 Nucleate boiling of mixtures
9.12.3 Flow boiling of mixtures
9.12.4 Evaporation of refrigerant–oil mixtures
9.13 Enhanced boiling
9.13.1 Enhancement of nucleate pool boiling
9.13.2 Enhancement of internal convective boiling
Nomenclature
References
9.1 INTRODUCTIONTO BOILINGHEATTRANSFER
When heat is applied to a surface in contact with a liquid, if the wall temperature
is suf?ciently above the saturation temperature, boiling occurs on the wall. Boiling
may occur under quiescent ?uid conditions, which is referred to as pool boiling,
or under forced-?ow conditions, which is referred to as forced convective boiling.
In this chapter a review of the fundamentals of boiling is presented together with
numerous predictive methods. First the fundamentals of pool boiling are addressed
andthenthoseof?owboiling.Tobetterunderstandthemechanicsof?owboiling,a
section is also presented on two-phase ?ow patterns and ?ow pattern maps. Then the
effects of mixture boiling are described. Finally, the topic of enhanced heat transfer
is introduced.
For more exhaustive treatments of boiling heat transfer, the following books are
recommended for consultation: Tong (1965), Wallis (1969), Hsu and Graham (1976),
Ginoux (1978), van Stralen and Cole (1979), Delhaye et al. (1981), Whalley (1987),
Thome (1990), Carey (1992) and Collier and Thome (1994). In addition, Rohsenow
(1973) provides a detailed historical presentation of boiling research.
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BOILING CURVE 637
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[637], (3)
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9.2 BOILING CURVE
When heating a surface in a large pool of liquid, the heat ?ux q is usually plotted
versus the wall superheat ? T
sat
, which is the temperature difference between the
surface and the saturation temperature of the liquid. First constructed by Nukiyama
(1934), theboilingcurve depicted in Fig. 9.1 is also referred to asNukiyama’scurve,
where four distinct heat transfer regimes can be identi?ed:
1. Natural convection. This is characterized by single-phase natural convection
from the hot surface to the saturation liquid without formation of bubbles on
the surface.
2. Nucleate boiling. This is a two-phase natural convection process in which
bubbles nucleate, grow, and depart from the heated surface.
3. Transitionboiling. This is an intermediate regime between the nucleate boiling
and ?lm boiling regimes.
Figure 9.1 Nukiyama’s boiling curve.
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638 BOILING
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[638], (4)
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4. Film boiling. This mode is characterized by a stable layer of vapor that forms
between the heated surface and the liquid, such that the bubbles form at the free
interface and not at the wall.
Between these four regimes are three transition points. The ?rst is calledincipience
of boiling (IB) or onset of nucleate boiling (ONB), at which the ?rst bubbles appear
on the heated surface. The second is the peak in the curve at the top of the nucleate
boiling portion of the curve, referred to as departure from nucleate boiling (DNB),
the critical heat ?ux (CHF), or peak heat ?ux. The last transition point is located at
thelowerendofthe?lmboilingregime(attheletter E) and is called the minimum
?lmboiling (MFB)point. These are all denoted in Fig. 9.1, while a representation of
these regimes is shown in Fig. 9.2.
In the natural convection part of the curve, the wall temperature rises as the heat
?ux is increased until the ?rst bubbles appear, signaling the incipience of boiling.
These bubbles form (or nucleate) at small cavities in the heated surface, which are
callednucleationsites. The active nucleation sites are located at pits and scratches in
the surface. Increasing the heat ?ux, more and more nucleation sites become activated
until the surface is covered with bubbles that grow and depart in rapid succession.
The heat ?ux increases dramatically for relatively modest increases in? T
sat
(de?ned
as T
w
- T
sat
), noting that the scale is log-log. Increasing the heat ?ux even further,
departing bubbles coalesce into vapor jets, changing the slope of the nucleate boiling
curve. A further increase in q eventually prohibits the liquid from reaching the heated
surface, which is referred to as the DNB or CHF, such that complete blanketing of
the surface by vapor occurs, accompanied by a rapid rise in the surface temperature
to dissipate the applied heat ?ux.
Following DNB, the process follows a path that depends on the manner in which
the heat ?ux is applied to the surface. For heaters that impose a heat ?ux at the surface,
such as electrical-resistance elements or nuclear fuel rods, the process progresses on
a horizontal line of constant heat ?ux so that the wall superheat jumps to point D

,
where ?lm boiling prevails as shown in Fig. 9.1, and whose vapor bubbles grow and
depart from the vapor–liquid interface of the vapor layer, not from the surface. A
ulterior increase in q may bring the surface to the burnout point (letter F), where
the surface temperature reaches the melting point of the heater. Reducing the heat
?ux, the ?lm boiling curve passes below point D

until reaching point E, the MFB
point. Here again, the process path depends on the mode of heating. For an imposed
heat ?ux, the process path jumps horizontally at constant q to the nucleate boiling
curve B

C. Consequently, a hysteresis loop is formed when heating a surface up
past the DNB and then bringing it below MFB when q is the boundary condition
imposed.
When the wall temperature is the externally controlled variable, such as by varying
the saturation temperature of steam condensing inside a tube with boiling on the
outside, the process path moves from the DNB to the MFB point, and vice versa,
following the transition boiling path. In transition boiling, the process vacillates
between nucleate boiling and ?lm boiling, where each mode may coexist next to the
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BOILING CURVE 639
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[639], (5)
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Figure 9.2 Pool boiling regimes.
another on the heated surface or may alternate at the same location on the surface. In
?lm boiling, the wall is blanketed completely by a thin ?lm of vapor, and therefore
heat is conveyed by heat conduction across the vapor ?lm and by radiation from
thewalltotheliquidortothewallsofthevessel.Thevapor?lmisstableinthat
liquid does not normally wet the heater surface and relatively large bubbles are
formed by evaporation at the free vapor–liquid interface, which then depart and rise
up through the liquid pool. In the next sections, important aspects of the boiling curve,
its phenomena, and predictive methods are described.
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