17-09-2016, 12:38 PM
CRITICAL HEAT FLUX IN SUBCOOLED FLOW
BOILING – AN ASSESSMENT OF CURRENT
UNDERSTANDING AND FUTURE
DIRECTIONS FOR RESEARCH
[attachment=71200]
Abstract. Critical Heat Flux, or CHF, is an important condition that
defines the upper limit of safe operation of heat transfer equipment
employing boiling heat transfer in heat flux controlled systems. Although
significant research has been conducted in this field, a clear understanding
of the basic mechanisms leading to the CHF condition is still lacking. The
present article covers the subcooled flow boiling CHF and reviews the
parametric trends and photographic studies reported by earlier
investigators. An in-depth review of the existing models is presented in
light of these studies, and further research needs are identified.
INTRODUCTION
Dissipation of large heat fluxes at relatively small temperature differences is possible in
systems utilizing boiling phenomenon as long as the heated wall remains wetted with the
liquid. With the wetted wall condition at the heated surface, heat is transferred by a
combination of two mechanisms: (i) bubbles are formed at the active nucleation cavities
on the heated surface, and heat is transferred by the nucleate boiling mechanism, and (ii)
heat is transferred from the wall to the liquid film by convection and goes into the bulk
liquid or causes evaporation at the liquid-vapor interface. The large amount of energy
associated with the latent heat transfer (compared to the sensible energy change in the
liquid corresponding to the available temperature potential in the system) in the case of
nucleate boiling, or the efficient heat transfer due to liquid convection at the wall, both
lead to very high heat transfer coefficients in flow boiling systems. Removal or depletion
of liquid from the heated wall therefore leads to a sudden degradation in the heat transfer
rate.
The way in which the heated surface arrives at the liquid starved condition in a flow
boiling system determines whether it is termed as Critical Heat Flux or Dryout condition. The evolution of the terminology itself is quite interesting. From a
mechanistic viewpoint, the following definitions seem to be appropriate and are
therefore recommended:
Critical Heat Flux condition represents the upper limit of heat flux (in heat flux
controlled systems) followed by a drastic rise in wall temperature, or considerable
degradation in heat flux with an increase in wall temperature (in temperature controlled
systems) in the nucleate boiling heat transfer. A vapor blanket covers the heated surface
separating the surface from the liquid.
Dryout condition represents the termination of continuous liquid contact with the wall. It
follows the gradual depletion of liquid due to evaporation and entrainment of the liquid
film. The vapor, from the continuous vapor phase in the bulk flow, covers the heated
surface, and the discrete liquid droplets flowing in the vapor core may make occasional
contact with the heated surface.
1.1 Historical Perspective of Critical Heat Flux
As early as 1888, Lang (1888) recognized through his experiments with high pressure
water that as the wall temperature increased beyond a certain point, it resulted in a
reduction in the heat flux. However, it was Nukiyama (1934) who realized that the
“maximum heat transmission rate” might occur at relatively modest temperature
differences. An excellent summary of the historical developments in this area was
presented by Drew and Mueller (1937).
Another aspect of historical significance is the evolution of the term Critical Heat
Flux. Early investigators used various terminology to describe this condition, e.g.,
maximum or peak heat flux, maximum boiling rate (Drew and Mueller, 1937), and
burnout heat flux. Nukiyama (1934) described it as the critical point on the boiling
curve. The earliest usage of the term Critical Heat Flux is seen by Zuber (1959). Further
investigation is needed to determine if publications in other languages (by investigators
such as Kutateladze and Fritz) used this terminology. Well into 1980s, there was no
consensus on the use of a single term. However, in the mid-1980s, the term critical heat
flux, and its acronym, CHF, became widely accepted.
1.2 Application Areas for CHF Studies
After establishing the terminology, let us examine the relevance and application of CHF
and Dryout in the fields of current interest. The historical applications such as quenching
are still valid. The major impetus for the CHF studies in the recent past was the nuclear
reactor core cooling. The catastrophic nature of the disaster associated with the CHF in a
nuclear reactor, leading to core meltdown, put a high premium on the CHF studies. The
urgency of the problem led to exhaustive experimentation in geometries similar to the
reactor core. The safe operating limits were established through compilation of data from
various experiments – developing the lookup tables. In his exhaustive literature survey
report, Boyd (1983a, b) points out the severe inadequacies in the theoretical modeling of
the CHF phenomena leading to empiricism.
CRITICAL HEAT FLUX IN SUBCOOLED FLOW BOILING 209
Another major impetus for research in CHF was provided by the refrigeration and
power industry in determining the Dryout point in a refrigeration evaporator and the safe
operating limit in a boiler. The concerns in these cases were largely regarding safety and
economic optimization of the systems.
The focus for CHF and Dryout studies as we enter the new millennium has
somewhat shifted. The issues related to the nuclear industries are still valid. However,
the emphasis has now moved toward gaining the basic understanding of the mechanisms
leading to the CHF condition. Developments in new augmentation techniques have
opened a whole new area where extensive CHF data for specific systems are not
available. For example, in spite of its superior performance in the flow boiling
application, the microfin tubes have not been tested for their upper limits in CHF. Many
new compact heat exchanger geometries are now being employed in flow boiling
applications, but their dryout characteristics have not been established. These and many
more challenges have emerged with the advancement in the heat transfer enhancement
technologies and their microscale applications. A major evolving area is the CHF in
narrow channels employed in fast response evaporators for fuel cell applications. In the
present paper, the current status of our understanding of the CHF in subcooled flow
boiling is reviewed, and recommendations for future work in this area are presented.
2. OVERVIEW OF PREVIOUS REVIEW ARTICLES
Subcooled flow boiling has received considerable attention due to its potential for
sustaining high heat fluxes in nuclear fusion applications. One of the most
comprehensive reviews on this subject was presented by Boyd (1983a and b) in a twopart
survey article addressing the fundamental issues, modeling, and correlations for
CHF in subcooled flow boiling. Boyd (1983a) presents a list of various fusion machines
and the heat flux levels sustained in them at steady state levels. He also provides an
exhaustive table with the details of the experimental studies available in literature. In the
second part, Boyd (1983b) presents a comprehensive table listing available correlations
for predicting CHF. It is clear that the available exhaustive experiments and correlations
were aimed at obtaining the CHF limits under specific operating conditions. A
comprehensive model was not yet developed; the parametric trends were however
identified from the data.
As mentioned in the Introduction section, the vast data available on CHF have been
compiled as look-up tables by various investigators. A relatively recent paper by
Groeneveld et al. (1996) presents the summary of the latest 1995 look-up table
developed jointly by AECL Research (Canada) and IPEE (Obninsk, Russia). It is based
on an extensive database of CHF values in tubes with vertical upflow of steam-water
mixtures. The table is designed to provide CHF values for 8 mm diameter tubes at
discrete values of pressure, mass flux, and dryout qualities covering the ranges 0.1 to 20
MPa, 0 to 8 Mg/m2
s, and –0.5 to 1 respectively. Linear interpolation is provided for
intermediate values, with an empirical correction factor for diameters different from 8
mm. The look-up table provides a tool capable of predicting data with an rms error of
7.82 percent for the 22,946 data points.
BOILING – AN ASSESSMENT OF CURRENT
UNDERSTANDING AND FUTURE
DIRECTIONS FOR RESEARCH
[attachment=71200]
Abstract. Critical Heat Flux, or CHF, is an important condition that
defines the upper limit of safe operation of heat transfer equipment
employing boiling heat transfer in heat flux controlled systems. Although
significant research has been conducted in this field, a clear understanding
of the basic mechanisms leading to the CHF condition is still lacking. The
present article covers the subcooled flow boiling CHF and reviews the
parametric trends and photographic studies reported by earlier
investigators. An in-depth review of the existing models is presented in
light of these studies, and further research needs are identified.
INTRODUCTION
Dissipation of large heat fluxes at relatively small temperature differences is possible in
systems utilizing boiling phenomenon as long as the heated wall remains wetted with the
liquid. With the wetted wall condition at the heated surface, heat is transferred by a
combination of two mechanisms: (i) bubbles are formed at the active nucleation cavities
on the heated surface, and heat is transferred by the nucleate boiling mechanism, and (ii)
heat is transferred from the wall to the liquid film by convection and goes into the bulk
liquid or causes evaporation at the liquid-vapor interface. The large amount of energy
associated with the latent heat transfer (compared to the sensible energy change in the
liquid corresponding to the available temperature potential in the system) in the case of
nucleate boiling, or the efficient heat transfer due to liquid convection at the wall, both
lead to very high heat transfer coefficients in flow boiling systems. Removal or depletion
of liquid from the heated wall therefore leads to a sudden degradation in the heat transfer
rate.
The way in which the heated surface arrives at the liquid starved condition in a flow
boiling system determines whether it is termed as Critical Heat Flux or Dryout condition. The evolution of the terminology itself is quite interesting. From a
mechanistic viewpoint, the following definitions seem to be appropriate and are
therefore recommended:
Critical Heat Flux condition represents the upper limit of heat flux (in heat flux
controlled systems) followed by a drastic rise in wall temperature, or considerable
degradation in heat flux with an increase in wall temperature (in temperature controlled
systems) in the nucleate boiling heat transfer. A vapor blanket covers the heated surface
separating the surface from the liquid.
Dryout condition represents the termination of continuous liquid contact with the wall. It
follows the gradual depletion of liquid due to evaporation and entrainment of the liquid
film. The vapor, from the continuous vapor phase in the bulk flow, covers the heated
surface, and the discrete liquid droplets flowing in the vapor core may make occasional
contact with the heated surface.
1.1 Historical Perspective of Critical Heat Flux
As early as 1888, Lang (1888) recognized through his experiments with high pressure
water that as the wall temperature increased beyond a certain point, it resulted in a
reduction in the heat flux. However, it was Nukiyama (1934) who realized that the
“maximum heat transmission rate” might occur at relatively modest temperature
differences. An excellent summary of the historical developments in this area was
presented by Drew and Mueller (1937).
Another aspect of historical significance is the evolution of the term Critical Heat
Flux. Early investigators used various terminology to describe this condition, e.g.,
maximum or peak heat flux, maximum boiling rate (Drew and Mueller, 1937), and
burnout heat flux. Nukiyama (1934) described it as the critical point on the boiling
curve. The earliest usage of the term Critical Heat Flux is seen by Zuber (1959). Further
investigation is needed to determine if publications in other languages (by investigators
such as Kutateladze and Fritz) used this terminology. Well into 1980s, there was no
consensus on the use of a single term. However, in the mid-1980s, the term critical heat
flux, and its acronym, CHF, became widely accepted.
1.2 Application Areas for CHF Studies
After establishing the terminology, let us examine the relevance and application of CHF
and Dryout in the fields of current interest. The historical applications such as quenching
are still valid. The major impetus for the CHF studies in the recent past was the nuclear
reactor core cooling. The catastrophic nature of the disaster associated with the CHF in a
nuclear reactor, leading to core meltdown, put a high premium on the CHF studies. The
urgency of the problem led to exhaustive experimentation in geometries similar to the
reactor core. The safe operating limits were established through compilation of data from
various experiments – developing the lookup tables. In his exhaustive literature survey
report, Boyd (1983a, b) points out the severe inadequacies in the theoretical modeling of
the CHF phenomena leading to empiricism.
CRITICAL HEAT FLUX IN SUBCOOLED FLOW BOILING 209
Another major impetus for research in CHF was provided by the refrigeration and
power industry in determining the Dryout point in a refrigeration evaporator and the safe
operating limit in a boiler. The concerns in these cases were largely regarding safety and
economic optimization of the systems.
The focus for CHF and Dryout studies as we enter the new millennium has
somewhat shifted. The issues related to the nuclear industries are still valid. However,
the emphasis has now moved toward gaining the basic understanding of the mechanisms
leading to the CHF condition. Developments in new augmentation techniques have
opened a whole new area where extensive CHF data for specific systems are not
available. For example, in spite of its superior performance in the flow boiling
application, the microfin tubes have not been tested for their upper limits in CHF. Many
new compact heat exchanger geometries are now being employed in flow boiling
applications, but their dryout characteristics have not been established. These and many
more challenges have emerged with the advancement in the heat transfer enhancement
technologies and their microscale applications. A major evolving area is the CHF in
narrow channels employed in fast response evaporators for fuel cell applications. In the
present paper, the current status of our understanding of the CHF in subcooled flow
boiling is reviewed, and recommendations for future work in this area are presented.
2. OVERVIEW OF PREVIOUS REVIEW ARTICLES
Subcooled flow boiling has received considerable attention due to its potential for
sustaining high heat fluxes in nuclear fusion applications. One of the most
comprehensive reviews on this subject was presented by Boyd (1983a and b) in a twopart
survey article addressing the fundamental issues, modeling, and correlations for
CHF in subcooled flow boiling. Boyd (1983a) presents a list of various fusion machines
and the heat flux levels sustained in them at steady state levels. He also provides an
exhaustive table with the details of the experimental studies available in literature. In the
second part, Boyd (1983b) presents a comprehensive table listing available correlations
for predicting CHF. It is clear that the available exhaustive experiments and correlations
were aimed at obtaining the CHF limits under specific operating conditions. A
comprehensive model was not yet developed; the parametric trends were however
identified from the data.
As mentioned in the Introduction section, the vast data available on CHF have been
compiled as look-up tables by various investigators. A relatively recent paper by
Groeneveld et al. (1996) presents the summary of the latest 1995 look-up table
developed jointly by AECL Research (Canada) and IPEE (Obninsk, Russia). It is based
on an extensive database of CHF values in tubes with vertical upflow of steam-water
mixtures. The table is designed to provide CHF values for 8 mm diameter tubes at
discrete values of pressure, mass flux, and dryout qualities covering the ranges 0.1 to 20
MPa, 0 to 8 Mg/m2
s, and –0.5 to 1 respectively. Linear interpolation is provided for
intermediate values, with an empirical correction factor for diameters different from 8
mm. The look-up table provides a tool capable of predicting data with an rms error of
7.82 percent for the 22,946 data points.