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New Initiatives at Fluent Inc.^ Condensation of WaterVapor from Humid Air Brian Bell, Fluent Inc

Overview • Condensation of water vapor from humid air (or

other mixture of water vapor and non-condensablegas) plays an important role in many applications ß ß Air conditioning equipmentHeat exchangers in nuclear power plants

• Transport of non-condensable gases, which can^ ß^ ß^ Latent heat recovery from wet flue gasFogging on windows
• be calculated with CFD, plays an important role inthese applicationsNon-condensable species transport can be usedto predict condensation rates under some

conditions

Film CondensationModel

• Liquid-vapor interface isimpermeable to air, socondensation mass fluxcan be written as
• Condensation represented bysource term in boundary cells ß Mass fraction set to saturation valueat wall when temperature < saturation ß ß temperatureSource term is function of wall speciesmass fraction and gradientImplemented with User-Defined Functions

ωH 2 O ωairair^ T m& ′h′ 2 o =?v = (? (^) h2o^1 − 1 ) ?D∂?∂nh^2 o^ Condensate FilmWall

m& ′h′^ ′ 2 o=m&h′^ ′ 2 oAV cellface^ Wall:^ Boundary Cellwhen Tω^ =^ ωsatw(T,P)^ < Tsat

Typical mass fractionand temperatureprofiles for condensationin the presenceof a noncondensable Tw Ti T"

Condensation ModelAssumptions

• • The vapor phase is a binary mixture of air and water vaporDiffusion in the vapor controls the condensation rate ß This allows the assumption that Tliquid film in the condensation process is neglectedi" Tw, therefore the role of the
• No droplet formation in the vapor phase^ ß^ Valid under some conditions, but not all. Validity of assumptionimproves with decreasing condensation rates or increasing airmass fraction
• •• Filmwise condensationVelocity of liquid-vapor interface equal to zeroLaminar flow of vapor and condensate film

Condensation ModelResults

Air Mass Fraction Contours (ωh2o, " = 0.15660) Sensible and Latent Heat TransferComponents (ωh2o, " = 0.15660) Condensation Mass Flux (u" = 1.0 m/s) Condensation Mass Flux (u" = 0.1 m/s)

Position (m) Condensation Mass Flux (kg/m²·s) Position (m) Condensation Mass Flux (kg/m²·s) Position (m)

Heat Flux (W/m²)^ Latent Heat TransferSensible Heat Transfer ω^ ω^ ω^ ωh2o, "h2o, "h2o, "h2o, " = 0.15660= 0.33267= 0.78643= 0.47967 ω^ ω^ ω^ ωh2o, "h2o, "h2o, "h2o, " = 0.15660= 0.33267= 0.78643= 0.

Comparison withAnalytical Solution

Analytical Solution from Sparrow et al., 1967, “Forced Convection Condensation in thePresence of Noncondensables and Interfacial Resistance”, Int. J. Heat Mass Transfer , Vol. 10, pp. 1829-

Condensation Mass Flux (kg/m²·s) Condensation Mass Flux (kg/m²·s)

Condensation Mass Flux (kg/m²·s) Condensation Mass Flux (kg/m²·s)

Position (m) Position (m) Position (m)

Position (m)

ωh2o, " = 0.15660 ωh2o, " = 0. ωh2o, " = 0.47967 ωh2o, " = 0.