Difference B/W Evaporation & Boiling

Evaporation-occurs at liquid-vapor interface

when vapor pressure is less than saturation pressure of the liquid at a given temperature e.g. evaporation from water at 20 C.

- No bubble formation or bubble movement involved

Cont…..

Boiling - occurs at solid-liquid interface

when liquid is brought into contact with a surface at temp Ts, above saturation temp Tsat of the liquid

- It involves rapid bubble formation at solid-surface that detach from surface and rise to top surface

Boiling occurs when a liquid is brought into contact with a surface at a temperature above the saturation temp of the liquid

Laws Applicable to Boiling

Newton's law of cooling

q boiling=h(Ts-Tsat)=h ∆Texcess

∆Texcess =Ts-Tsat =Excess temperatureTs=temperature of heating surfaceTsat= saturation temp of liquid

Classification of Boiling

1) Based on Bulk Fluid Motiona. Pool Boiling b. Flow Boiling

2) Based on Bulk liquid temperature

a. Sub-cooled Boiling b. Saturated Boiling

Pool Boiling Boiling in absence of bulk fluid

flowFluid body is stationeryAny possible fluid motion will be

due to natural convection currents

E.g boiling of water in a pan on stove

Flow Boiling/Forced convection boiling Boiling in presence of bulk fluid

flowFluid is forced to flow in a heated

pipe or over a surface by pump etc

Convection effects will be present

Sub-cooled/Local Boiling Boiling is sub-cooled if temperature of

main body of fluid is below the saturation temp Tsat (i.e. bulk of liquid is sub-cooled)

It occurs at early stages of boilingBubbles formation and disappearance

near hot surface Bubbles disappear as they transfer

heat to surrounding sub-cooled liquid Boiling is confined to locality of hot

surface so also called local boiling

Cont……

Bubbles serve as energy movers and transfer heat to fluid by condensing

Saturated/Bulk Boiling Boiling is saturated if

temperature of main body of fluid is equal to the saturation temp Tsat (i.e. bulk of liquid is saturated)

It occurs when entire liquid body reaches saturation temperature

Bubbles rise to the top

Boiling Curve for Pool Boiling

Four regimes/phases for pool boiling with change in excess temperature are

1. Natural Convection Boiling 2. Nucleate Boiling 3. Transition Boiling 4. Film Boiling

Natural Convection Boiling

Fluid motion in this regime is by natural convection currents

Heat transfer from heating surface to fluid is by natural convection

Liquid is slightly superheated

Nucleate Boiling Bubbles form at nucleation sites

(rough surface)Bubbles form, travel & collapse in

liquidVacated space near heated

surface is filled by liquid Increased stirring & agitationIncreased h and more heat flux Further rise in temp cause the

formation of more bubbles and these move to free surface and break up and release vapor

Cont….

At large values of ∆Texcess a larger fraction of heater surface is covered with bubbles and make difficult for liquid to reach heating surface so max heat flux is reached called as Critical/Maximum heat flux point c.

Most desirable in industry because with small

∆Texcess, high heat transfer rates occur

Transition Boiling/Unstable regime

At this phase larger fraction of heating surface is covered with bubbles, so heat flux decrease

vapor film reduces heat transfer b/c its k value is less than liquid

Partial nucleate and film boiling occur Usually this regime is avoided in

industry

Film Boiling

Heating surface is completely covered with stable film, heat flux is minimum

By increasing temp further, heat transfer occur through vapor film by radiation which is significant at high temperature

Boiling Regimes during mecthanol boiling on steam heated copper tube

a) Nucleate Boiling b) Transition Boilingc) Film Boiling

Boiling Curve

Enhancement of heat transfer in pool boiling In nucleate boiling rate of heat transfer

depends on active nucleation sitesIncreasing nucleation sites will

increase heat transfer Surface roughness and dirt increase

heat transfer Heat flux in nucleate boiling can be

increased by a factor of 10 Coat surface with thin

layer/Thermoexcell-E

Cont…..

Mechanical agitation and surface vibration also increase heat transfer

Flow Boiling

In pool boiling vapor bubbles rise due to buoyancy forces, but in flow boiling, fluid is forced to move by external source such as pump as it undergoes a phase change process

Combined effects of pool boiling and convection

Flow Boiling Types

A) External Flow Boiling If fluid is forced to move over a

heated surface (on external side)

B) Internal Flow Boiling If fluid is forced to move inside

a heated surface/tube (on internal side)

External Flow Boiling It is similar with pool boiling, but

added fluid motion increases the nucleate boiling heat flux and critical boiling heat flux

Velosity ∞ h ∞ q max

Internal Flow Boiling

It is complicated as there is no space for vapor for escape, so vapor and liquid flow together, hence two-phase flow occurs.

Different floe regimes are present depending on relative amounts of vapor and liquid present

Flow regimes in Internal Flow Boiling

FC= Total vapors

MF=liquid drops suspended in vapor

TF= complete dry spot form on tubes inner side

AF=core of flow consist of vapors and liquid flow in annular space b/w vapor and tube

SF=Bubbles grow and coalesce into slugs of vapor

BF= when bubbles appear in liquid

FC=Total liquid

Flow regimes in Internal Flow Boiling

Applications of Boiling

-House hold refrigerator(refrigrant boiling)

-steam power plants boilers-chemical industries boilers -cooling of nuclear reactor by

coolant boiling- industrial kettles - electronic component cooling by

boiling of liquid in which these are placed

- Regenerative cooling of rocket motors

Condensation Heat Transfer Condensation occurs when

temperature of a vapor is reduced below its saturation temperature.

Vapor is contacted by a solid surface at a temp well below vapor saturation temp

Condensation Types A) Film Condensation B) Drop-wise Condensation

Film Condensation

Condensate wet the surface and form a liquid film on the surface which fall due to gravity

Thickness of film increases as it moves down due to more condensation

Film Condensation

Drop-wise Condensation

Condensed vapors form droplets on surface instead of film, surface is covered by drops

Droplets slide down the surface, when they reach a certain size

Drop-wise Condensation

Heat Exchanger

LMTD Definition Log mean temp difference is

defined as that temperature difference which, if constant, would give the same rate of heat transfer as actually occurs under variable conditions of temperature difference

Industrial Temp Control of Heat ExchangersE-2163 control