Analysis of Flow Boiling in Vertical Tubes

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Analysis of Flow Boiling in Vertical Tubes

• P M V Subbarao
• Professor
• Mechanical Engineering Department

Selection of Optimal Parameters for Healthy and
Safe Furnace Walls with Frictional Flow..
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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.
• Void fraction and Pressure drop are two important
  parameters in real flow boiling.

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Adiabatic Flow Through A Pipe
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Diabatic Flow Through A Pipe
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Selection of Flow rate in Flow Boiling

• This process may either be forced convection or
  gravity driven.
• At relatively low flow rates at sufficient wall
  superheats, bubble nucleation at the wall occurs
  such that nucleate boiling is present within the
  liquid film.
• At high qualities and mass flow rates, the flow
  regime is normally annular.
• As the flow velocity increases, convection in the
  liquid film is augmented.
• The wall is cooled below the minimum wall
  superheat necessary to sustain nucleation.
• Nucleate boiling may thus be suppressed, in which
  case heat transfer is only by convection through
  the liquid film and evaporation occurs only at
  its interface.

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Pressure drop Religious to Secular Attitude
Dphydro Dpfriction
Dphydro Dpfriction
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Pressure Drop in Tubes

• The pressure drop through a tube comprise several
  components friciton, entrance loss, exit loss,
  fitting loss and hydrostatic.

Exact prediction of wall temperature, it is
important to know the pressure Variation along
the flow
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Circulation Vs Once Through
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Auto Control Mechanism in Natural Circulation
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Selection of Steam Mass Flow rate

• A once-through forced circulation furnace with
  high mass flow
• If any tube receives more heat than the average,
  then it will accept receives less flow.
• This can result in further increasing
  temperatures, potentially leading to failures.
• Thus the mass flow per tube must start very high
  to ensure adequate remaining flow after the heat
  upsets.
• Designs with medium mass flow
• These were attempted in once through forced
  circulation boilers with moderate success.
• These exhibit worse consequences than the high
  mass flow designs.
• When the mass flow is degraded during load
  reduction in a tube receiving more heat than the
  average, the remaining flow will have less margin
  to provide acceptable cooling.
• Medium mass flow designs can experience heat
  upsets and/or flow excursions that result in
  flows in individual tubes that are lower than the
  low mass flow design.

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Destructive Mechanisms in Forced Circulation/Onec
through
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• Both the multi-pass and the spiral designs use
  high fluid mass flows.
• High fluid mass flow rates result in high
  pressure losses as well as a once-through .
• Means that strongly heated tubes have a reduction
  in fluid mass flow and a correspondingly high
  increase in fluid and therefore metal temperature
  which can result in excessive tube-to-tube
  temperature differentials.
• This type of behavior is sometimes referred to as
  a "negative flow characteristic.
• In the Vertical design, the furnace enclosure is
  formed from a single, upflow pass of vertical
  tubes.

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• The tube size and spacing is selected to provide
  a low fluid mass flow rate of approximately 1000
  kg/m2-s or less.
• With low mass flow rates, the frictional pressure
  loss is low compared to the gravitational head,
  and as a result, a tube that is heated strongly,
  i.e., absorbs more heat, draws more flow.
• With an increase in flow to the strongly heated
  tube, the temperature rise at the outlet of the
  tube is reduced which limits the differential
  temperature between adjacent tubes.
• This is known as the "natural circulation or
  "positive flow" characteristic.
• Minimize peak tube metal temperatures.
• To minimize peak tube metal temperatures,
  multiple pass and spiral types designs use high
  fluid mass flow rates to achieve good tube
  cooling.
• This results in the "once-through characteristic
  noted above.