boiling heat transfer
TRANSCRIPT
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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
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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
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Boiling occurs when a liquid is brought into contact with a surface at a temperature above the saturation temp of the liquid
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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
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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
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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
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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
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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
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Cont……
Bubbles serve as energy movers and transfer heat to fluid by condensing
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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
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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
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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
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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
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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
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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
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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
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Boiling Regimes during mecthanol boiling on steam heated copper tube
a) Nucleate Boiling b) Transition Boilingc) Film Boiling
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Boiling Curve
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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
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Cont…..
Mechanical agitation and surface vibration also increase heat transfer
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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
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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)
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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
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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
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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
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Flow regimes in Internal Flow Boiling
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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
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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
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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
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Film Condensation
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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
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Drop-wise Condensation
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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