Boiling and Condensation

BOILING AND CONDENSATION

BOILING
• Phase change phenomenon where in transformation of liquid to vapour occurs at the saturation temperature of the fluid. It occurs at a solid/liquid interface due to convection heat transfer from the solid and usually occurs at surface temperatures higher than the saturation temperature of the fluid Agitation of fluid by vapor bubbles provides large convection coefficients Modified Newton’s law of cooling qs''  h Ts  Tsat   Te

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BOILING – CLASSIFICATON
– Pool Boiling - the liquid is quiescent and its motion near the surface is due to free convection and mixing induced by bubble growth and detachment – Flow Boiling - fluid motion is induced by external means as well as by free convection and bubble induced mixing – Subcooled Boiling – fluid at a mean temperature below the saturation temperature – Saturation Boiling - fluid at a mean temperature equal to the saturation temperature

BOILING CURVE – POOL BOILING
• • • • Nukiyama’s experiment on Pool Boiling Experiments at atmospheric conditions Using nichrome wire (melting temperature ~ 1500 k) Heat Flux controlled

BOILING CURVE – POOL BOILING

REGIMES OF POOL BOILING

• Free Convection Boiling   Te  5 C   Little vapor formation  Liquid motion is principally due to single phase natural convection  Onset of Nucleate Boiling – ONB (ΔTe~ 5oC)

• Nucleate Boiling  5   Te  30 C 

 Isolated Vapor Bubbles  5   Te  10 C  – Liquid motion is strongly influenced by nucleation of bubbles at the surface – Heat transfer coefficient increases rapidly with increase in heat flux – Heat transfer is principally due to contact of liquid with the surface (single-phase convection) and not to vaporization  Jets and Columns 10   Te  30 C  – Increasing number of nucleation sites causes bubble interactions and coalescence into jets and slugs – Liquid/surface contact is impaired and h starts to decrease

  • Critical Heat Flux - CHF, qmax   Te  30 C 

 Maximum attainable heat flux in nucleate boiling and is of the order of 1 MW/m2 at atmospheric pressure  An increase in heat flux beyond CHF causes the surface to be blanketed by vapor, and the surface temperature can spontaneously achieve a value that potentially exceeds its melting point  If the surface survives the temperature shock, conditions are characterized by film boiling

• Film Boiling
 Heat transfer is by conduction and radiation across the vapor blanket

• Transition Boiling (only for Temperature-Controlled Heating)
 Surface conditions oscillate between nucleate and film boiling, but portion of surface experiencing film boiling increases with excess temperature  Te .  Also termed unstable or partial film boiling.

Nucleate Boiling

Film boiling

REGIMES OF FLOW BOILIN

CONDENSATION
• Phase change phenomenon where in transformation of vapour to liquid occurs at the saturation temperature of the fluid. Heat transfer to a surface occurs by condensation when the surface temperature is less than the saturation temperature of an adjoining vapor. Classification
– Film condensation – Drop wise condensation

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FILM CONDENSATION
• Entire surface is covered by the condensate, which flows continuously from the surface and provides a resistance to heat transfer between the vapor and the surface Characteristic of clean, uncontaminated surfaces

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FILM CONDENSATION

DROP WISE CONDENSATION
• Surface is covered by drops ranging from a few micrometers to agglomerations visible to the naked eye Thermal resistance is greatly reduced due to absence of a continuous film Surface coatings may be applied to inhibit wetting and thus formation of film

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