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Boiling Condensation and Two Phase Flow
Lecture 3
Prof. Sarit Kumar Das
Department of Mechanical Engineering
Indian Institute of Technology Madras
BOILING
Process of changing the liquid to vapor at a constant temperature called as ‘saturation temperature’ or ‘boiling
point’
Many ways in which boiling can be classified –Many ways in which boiling can be classified –
• Mechanism of phase change• Geometry of fluid container• External/Internal Boiling
Before discussing these classification let us look at ‘evaporation’also known as ‘silent boiling’
EVAPORATION
• Nature’s mechanism to turn liquid into vapor(Evaporation from lakes, rivers, oceans)• Does not require liquid to reach Tsat
(occurs at ambient conditions – makes the air humid)
Applications:• Cooling Towers• Comfort Air Conditioners• Humidification/Dehumidification Desalination Systems
Vaporization takes place once the Tamb is higher than Tsat
corresponding to the partial pressure of water vapor in air
Evaporation in practical applications –• Heat flux is such that wall is just above Tsat at the given
pressure • Wall temperature is not sufficient for vapor bubbles to be
formed
To ~ TsatVapor
Boundary layer where
Pool heated from bottom
Liquid y
Twall
Tliquid
Temperature profile
1 mm
Boundary layer where Twall drops to Tliquid is typically 1mm in thickness
CLASSIFICATION OF BOILING
Most popular way of classification – based on geometry
Major modes –• Pool Boiling• Flow Boiling
Classification based on mechanism of boiling
Major modes –• Nucleate Boiling• Convective Boiling• Film Boiling
Nucleate Boiling: • Vapor bubbles are formed on a surface in contact with the liquid• Can happen both with stagnant liquid or with liquid flowing on surface
Convective Boiling:• Liquid forms a thin film on the wall and heat is conducted through this
thin film• Finally evaporates the liquid at the vapor interface• Finally evaporates the liquid at the vapor interface
Film Boiling:• Due to high flux a vapor film blankets the heater surface• Results in very poor heat transfer• Raises the temperature of the heating surface; causes melting resulting
in ‘burn out’
Mechanisms are not always unique; several mechanisms can co-exist
So the geometry based classification is more common
Pool Boiling: • When liquid is stagnant and a heater is immersed into the pool• However liquid circulation due to bubbles & natural convection its not
really stagnant
Electrodes
Geometry Based Classification
Pt wire
Heater
Heating surface
VARIOUS CONFIGURATIONS FOR STUDYING POOL BOILING
Some studies consider pool boiling as ‘external boiling’ and flow boiling as ‘internal boiling’
Analogy with external and internal convection of single phase
Vapor
Water Surface
100o
100.4o
TEMPERATURE PROFILE IN NUCLEATE BOILING
Water
Temperature
y
110o
FLOW BOILING
• In industrial equipment, boiling takes place under conditions of forced flow
• Vaporization can be complete or partial depending on application• Entire process undergoes change in two aspects
� Vapor/Liquid content� Pattern in which vapor/liquid co-exist
Regimes of Flow boiling
• Forced convection• Subcooled boiling• Saturated Boiling• Convective Boiling Dryout• Spray Cooling
Physically various fluid structures exist –
• Bubbly flow• Slug flow• Churn flow• Annular flow• Mist flow Transitions from
-subcooled to nucleate-nucleate to convective-nucleate to convective
BUBBLY FLOW BUBBLY/SLUG FLOW
SLUG FLOW SLUG/SEMI-ANNULAR FLOWSLUG FLOW SLUG/SEMI-ANNULAR FLOW
SEMI ANNULAR FLOW WAVY ANNULAR FLOW
ANNULAR FLOW
• Subcooled liquid enters tube ; h depends on mass flux• As nucleate boiling starts all curves merge• Nucleate boiling has higher rate of heat transfer –
independent of mass flux
• Nucleate boiling depends on heat flux; q1, q2…• As the constant heat flux line reaches region of convective
boiling, they follow constant mass flux lines
Transition from nucleate to convective boiling
Nucleate boiling is heatflux dependent
Convective boiling is mass flux dependent
In both cases heat transfer coefficient expressed as-
c is dependent on fluid propertiesx* is the qualitym is the mass flow rate
h=cqnmpf(x*)
m is the mass flow rate
For nucleate boiling0.1 < p < 0.3 n = 0.75 (approx.)
For convective boiling0.6 < p < 0.8n=0 (approx.)
End of Lecture 3End of Lecture 3