BOILING HEAT TRANSFER

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BOILING HEAT TRANSFER

• P M V Subbarao
• Associate Professor
• Mechanical Engineering Department
• IIT Delhi

A Means to induct Bountifulness to a Fluid. A
Basic means of Power Generation A science which
made Einstein Very Happy!!!
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James Watt A Statue !!!

• Mrs. Campbell, Watt's cousin and constant
  companion, recounts, in her memoranda, written in
  1798
• Sitting one evening with his aunt, Mrs. Muirhead,
  at the teatable, she said
• "James Watt, I never saw such an idle boy take a
  book or employ yourself usefully
• for the last hour you have not spoken one word,
• but taken off the lid of that kettle and put it
  on again,
• holding now a cup and now a silver spoon over the
  steam, watching how it rises from the spout,
• and catching and connecting the drops of hot
  water it falls into.
• Are you not ashamed of spending your time in this
  way? "

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Natural Convection in A Pool of Saturated Liquid
Tsat
Onset of Convection
Tsurface
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Further Behavior of Saturated Liquid
Increasing DT
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High Overshoots !!!
A
B
A Onset of Natural convection
B Onset of Nucleate Boiling
Heat Flux
Overshoot
Wall Superheat (DTTs Tsat)
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Boiling

• In a steam power plant convective heat transfer
  is used to remove heat from a heat transfer
  surface.
• The  liquid  used  for  cooling  is  usually  in
   a  compressed  state,  (that   is,  a  subcooled
   fluid)  at pressures higher than the normal
  saturation pressure for  the given temperature.
• Under certain conditions some type of boiling can
  take place.
• It is  an important  process  in nuclear  field
   when  discussing convection heat transfer.
• More  than  one  type  of  boiling  can  take
   place  within  a  
• nuclear facility.

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Nuclear Power Plant
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Steam Boiler
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Classification of Boiling

• Microscopic classification or Boiling Science
  basis
• Nucleated Boiling
• Bulk Boiling
• Film Boiling
• Macroscopic Classification or Boiling Technology
  basis
• Flow Boiling
• Pool Boiling

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Nucleate Boiling

• The most common type of local boiling encountered
  in nuclear facilities is nucleate boiling.
• In nucleate boiling, steam bubbles form at the
  heat transfer surface and then break away and are
  carried into the main stream of the fluid.  
• Such movement enhances heat transfer because the
  heat generated at the surface is carried directly
  into the fluid stream.   
• In the main fluid stream, the bubbles collapse
  because the bulk temperature of the fluid is not
  as high as the heat transfer surface  temperature
   where  the  bubbles  were  created.   
• This  heat  transfer  process  is  sometimes
  desirable  because  the  energy  created  at  the
   heat  transfer  surface  is  quickly  and
   efficiently "carried" away.

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Bulk Boiling

• As  system  temperature  increases  or  system
   pressure drops,  the  bulk  fluid  can  reach
   saturation conditions.  
• At this point, the bubbles entering the coolant
  channel will not collapse.  
• The bubbles will tend to join together and form
  bigger steam bubbles.  
• This phenomenon is referred to as bulk boiling.
• Bulk  boiling  can  provide  adequate  heat
   transfer  provided  that  the  steam  bubbles
   are carried  away  from  the  heat  transfer
   surface  and  the  
• surface  is  continually  wetted  with
   liquid water.   
• When this cannot occur film boiling results.

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Film Boiling

• As the temperature continues to increase, more
  bubbles are formed  than  can  be  efficiently
   carried  away.   
• The  bubbles  grow  and  group  together,
   covering small  areas  of  the  heat  transfer
   surface  with  a  film  of  steam.    
• This  is  known  as  partial  film boiling.    

• Steam  has  a  lower  convective  heat  transfer
   coefficient  than  water.  
• The  steam patches on the heat transfer surface
  act to insulate the surface.
• As  the  area  of  the  heat  transfer  surface
   covered  with  steam  increases,  the
   temperature  of  thesurface  increases
   dramatically,  while  the  heat  flux  from  
  the  surface  decreases.   
• This  unstable situation continues until the
  affected surface is covered by a stable blanket
  of steam.   
• The condition after the stable steam blanket has
  formed is referred to as film boiling.

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Boiling Curve
W/m2.s
0C
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• Four regions are represented in Figure.   
• The first and second regions show that as heat
  flux increases, the temperature difference
  (surface to fluid) does not change very much.
• Better heat transfer occurs during nucleate
  boiling than during natural convection.  
• As the heat flux increases, the  bubbles  become
   numerous   enough  that  partial  film  boiling
   (part  of  the  surface  beingblanketed  with
   bubbles)  occurs.    
• This  region  is  characterized  by  an  increase
   in  temperature difference and a decrease in
  heat flux.   
• The increase in temperature difference thus
  causes total film boiling, in which steam
  completely blankets the heat transfer surface. 

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Departure from Nucleate Boiling and Critical
Heat Flux

• In practice, if the heat flux is increased, the
  transition from nucleate boiling to film boiling
  occurs suddenly, and the temperature difference
  increases rapidly, as shown by the dashed line in
  the figure.    
• The  point  of  transition  from  nucleate
   boiling  to  film  
• boiling  is  called  the  point  of
  departure from nucleate boiling, commonly written
  as DNB.  
• The heat flux associated with DNB is  commonly
   called  the  critical  heat  flux  (CHF).    
• In  many  applications,  CHF  is  an  important
  parameter.  

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• For example, in a reactor, if the critical heat
  flux is exceeded and DNB occurs at any location
• in  the  core,  the  temperature  difference
   required  to  transfer  the  heat  being
   produced  from  the surface  of  the  fuel  rod
   to  the  reactor  coolant  increases  greatly.
     
• If,  as  could  be  the  case,  the temperature
  increase causes the fuel rod to exceed its design
  limits, a failure will occur.
• The amount of heat transfer by convection can
  only be determined after the local heat transfer
  coefficient  is  determined.   
• Such  determination  must  be  based  on
   available  experimental  data.
• After experimental data has been correlated by
  dimensional analysis, it is a general practice to
  write   an   equation   for   the   curve   that
    has   been   drawn   through   the   data   and
    to   compare experimental results with those
  obtained by analytical means.  

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Flow Boiling

• Flow boiling occurs when all the phases are in
  bulk flow together in a channel e.g., vapor and
  liquid flow in a pipe.
• The multiphase flow may be classified as
  adiabatic or diabatic, i.e., without or with heat
  addition at the channel wall.
• An example of adiabatic flow would be oil/gas
  flow in a pipeline, or air/water flow.
• In these cases the flow patterns would change as
  the inlet mass flow rates of the gas or liquid
  are altered or as the velocity and void
  distributions develop along the channel.

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Adiabatic Flow Through A Pipe
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Diabatic Flow Through A Pipe
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Diabatic Boiling Along A Furnace Wall Tube
Furnace Exit
Hot Exhaust gases
Heat Radiation Convection
Flame
Burner
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Natural Circulation Steam Generator
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Natural Circulation Nuclear Reactor
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Forced Circulation Steam Generator
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Forced Circulation Nuclear Reactor
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Once Through Steam Generator
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Super Critical Nuclear Reactor
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Slip in Multi-Phase flow

• For example in a riser tube of a steam generator
  the vapor rises faster than the liquid due to
  buoyancy effects.
• One may term this velocity inequality as "slip"
  between the vapor and the liquid.
• The ratio of these velocities is called the "slip
  ratio".
• A better description of the phenomena is to
  consider it as a relative velocity difference
  between the phases, Vg - Vl .
• Flow boiling heat transfer can occur under two
  different boundary conditions, either a specified
  wall heat flux or a specified wall temperature.
• The former case is an idealized example of a
  boiler tube in a fossil fuel boiler and the
  latter case is an idealized example of a riser
  tube in a nuclear steam generator.

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Multi Phase Heat Transfer

• One of the many applications of multiphase
  heat-transfer is to be able to predict the
  temperature of the wall of a boiling surface for
  a given heat flux or the variation of wall heat
  flux for a known wall temperature distribution.
• In this section we focus on the methodology to
  estimate the wall temperature or the wall heat
  flux depending on the appropriate boundary
  condition.
• We focus on describing the regions of heat
  transfer, locating the onset of nucleate boiling
  and finally estimating the wall condition.

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Single-Phase Liquid Heat Transfer

• Figure shows an idealized form of the flow
  patterns and the variation of the surface and
  liquid temperatures in the regions designated by
  A, B, and C for the case of a uniform wall heat
  flux.
• Under steady state one-dimensional conditions the
  tube surface temperature in region A (convective
  heat transfer to single-phase liquid), is given
  by

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• and where
• q is the heat flux,
• P is the heated perimeter,
• G is the mass velocity,
• A is the flow area and is the liquid specific
  heat.
• Also DTfw is the temperature difference between
  the wall surface and the mean bulk liquid
  temperature at a given length z from the tube
  inlet,
• h is the heat transfer coefficient to
  single-phase liquid under forced convection.

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• The liquid in the channel may be in laminar or
  turbulent flow, in either case the laws governing
  the heat transfer are well established for
  example, heat transfer in turbulent flow in a
  circular tube can be estimated by the well-known
  Dittus-Boelter equation.

• This relation is valid for heating in fully
  developed vertical
• upflow in z/D gt 50 and Re gt 10,000.
• For the case of a given constant wall
  temperature, the temperature difference will
• decrease, as well as the heat flux.
• From an energy balance this is represented by a
  logarithmic decrease in the
• temperature difference.

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The Onset of Nucleate Boiling

• If the wall temperature rises sufficiently above
  the local saturation temperature pre-existing
  vapor in wall sites can nucleate and grow.
• This temperature, TONB, marks the onset of
  nucleate boiling for this flow boiling situation.
  
• From the standpoint of an energy balance this
  occurs at a particular axial location along the
  tube length, ZONB.
• Once again for a uniform flux condition,

We can arrange this energy balance to emphasize
the necessary superheat above saturation for the
onset of nucleate boiling
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Now that we have a relation between DTONB and
ZONB we must provide a stability model for the
onset of nucleate boiling. one can formulate a
model based on the metastable condition of the
vapor nuclei ready to grow into the world. There
are a number of correlation models for this
stability line of DTONB. Using this approach,
Bergles and Rohsenow (1964) obtained an equation
for the wall superheat required for the onset of
subcooled boiling.
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Their equation is valid for water only, given by
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Subcooled Boiling

• The onset of nucleate boiling indicates the
  location where the vapor can first exist in a
  stable state on the heater surface without
  condensing or vapor collapse.
• As more energy is input into the liquid (i.e.,
  downstream axially) these vapor bubbles can grow
  and eventually detach from the heater surface and
  enter the liquid.
• Onset of nucleate boiling occurs at an axial
  location before the bulk liquid is saturated.
• Likewise the point where the vapor bubbles could
  detach from the heater surface would also occur
  at an axial location before the bulk liquid is
  saturated.
• Now this axial length over which boiling occurs
  when the bulk liquid is subcooled is called the
  "subcooled boiling" length.
• This region may be large or small in actual size
  depending on the fluid properties, mass flow
  rate, pressures and heat flux.
• It is a region of inherent nonequilibrium where
  the flowing mass quality and vapor void fraction
  are non-zero and positive even though the
  thermodynamic equilibrium quality and volume
  fraction would be zero since the bulk
  temperature is below saturation.

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The first objective is to determine the amount of
superheat necessary to allow vapor bubble
departure and then the axial location where this
would occur. A force balance to estimate the
degree of superheat necessary for bubble
departure.
this conceptual model the bubble radius rB, is
assumed to be proportional to the distance to the
tip of the vapor bubble,YB , away from the heated
wall. One can then calculate this distance
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The superheat temperature, is then found by
using the universal temperature profile relation.
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Now using the local energy balance one can relate
the local bulk temperature, TfB, to the superheat
temperature difference,
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Saturated Boiling and the Two-PhaseForced
Convection Region

• Once the bulk of the fluid has heated up to its
  saturation temperature, the boiling regime enters
  saturated nucleate boiling and eventually
  two-phase forced convection.
• We once again want to be able to find the wall
  condition for this situation e.g., wall
  temperature for a given heat flux.
• One should note that the heat transfer
  coefficient is so large that the temperature
  difference between the wall and the bulk fluid is
  small allowing for large errors in the
  prediction, without serious consequences.
• The saturated nucleate boiling and two-phase
  forced convection regions may be associated with
  an annular flow pattern.
• Heat is transferred by conduction or convection
  through the liquid film and vapor and is
  generated continuously at the liquid film/vapor
  core interface as well as possibly at the heat
  surface.
• Extremely high heat transfer coefficients are
  possible in this region values can be so high as
  to make accurate assessment difficult.
• Typical figures for water of up to 200 W/m2 K
  have been reported.

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Following the suggestion of Martinelli, many
workers have correlated their experimental
results for heat transfer rates in the two-phase
forced convection region in the form
The convection heat transfer coefficient is
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POOL BOILING

• Pool boiling is the process in which the heating
  surface is submerged in a large body of stagnant
  liquid.
• The relative motion of the vapor produced and the
  surrounding liquid near the heating surface is
  due
• primarily to the buoyancy effect of the vapor.
  Nevertheless, the body of the liquid as a whole
  is essentially at rest.
• Though the study on the boiling process can be
  traced back to as early as the eighteen century,
  the extensive study on the effect of the very
  large difference in the temperature of the
  heating surface and the liquid, DT, was first
  done by Nukiyama (1934).

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Onset of Nucleate Boiling

• Vapor may form from a liquid
• (a) at a vapor-liquid interface away from
  surfaces,
• (b) in the bulk of the liquid due to density
  fluctuations, or
• (c) at a solid surface with pre-existing vapor or
  gas pockets.
• In each situation one can observe the departure
  from a stable or a metastable state of
  equilibrium.
• The first physical situation can occur at a
  planar interface when the liquid temperature is
  fractionally increased above the saturation
  temperature of the vapor at the vapor pressure in
  the gas or vapor region.
• Thus, the liquid "evaporates" into the vapor
  because its temperature is maintained at a
  temperature minimally higher than its vapor
  "saturation" temperature at the vapor system
  pressure.
• Evaporation is the term commonly used to describe
  such a situation which can also be described on a
  microscopic level as the imbalance between
  molecular fluxes at these two distinctly
  different temperatures.

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• To find the particular heat flux and superheat
  pair natural convection mode of heat transfer
  that would exist prior to boiling is considered.

• For water at atmospheric pressure this model
  predicts an "onset of nucleate boiling" for a
  superheat less than 10C, with a cavity size of
  about 50 microns.
• In practice the superheat may be as high as 100C
  for very smooth, clean metallic surfaces.

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Pool Boiling Critical Heat Flux

• Critical heat flux (CHF) in pool boiling is an
  interesting phenomenon.
• If one controls the input heat flux, there comes
  a point where as the heat flux is increased
  further the heater surface temperature undergoes
  a drastic increase.
• This increase originally was not well understood.
• Kutateladze (1951) offered the analogy that this
  large abrupt temperature increase was caused by a
  change in the surface geometry of the two phases.
  
• In fact, Kutateladze first empirically correlated
  this phenomenon as analogous to a gas blowing up
  through a heated porous plate cooled by water
  above it.
• At a certain gas volumetric flow rate (or
  superficial velocity, ) the liquid ceases to
  contact the heated surface and the gas forms a
  continuous barrier.

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• where the constant, Co, is found to be in the
  range of 0.12 to 0.18.

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Film Boiling and the Minimum FilmBoiling Point

• Once the critical heat flux is exceeded the
  heater surface is blanketed by a continuous vapor
  film i.e., film boiling.
• Under this condition one must find the heat
  transfer resistance of this vapor film as well as
  consider the additional effect of radiation heat
  transfer at very high heater surface temperatures
  through this vapor film (gt 10000 C).
• Bromley (1950) used the approach first developed
  by Nusselt for film condensation to predict the
  film boiling heat transfer coefficient for a
  horizontal tube

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