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