Presentation on theme: "Natural Convection in Enclosed Spaces"— Presentation transcript:
1 Natural Convection in Enclosed Spaces Two vertical plates separated by a distance. Each plate at a different temperature.Ends are insulated.Convective heat transfer occurs in the fluid within the space.
5 Boiling Type Excess Temperature (deg. C) Free Convection: < 5 Nucleate Boiling: – 25Transition Boiling: – 120Film Boiling: > 120The excess temperature is the surface temperature above the boiling point of the liquid.
6 Nucleate BoilingBubbles form at surface and separate, mixing the fluid. As temperature increases, the bubbles join to form bubble columns or jets. These eventually merge to form slugs. This type of boiling is the most desirable for a chemical process due to the high heat rates.
7 Transition BoilingFilm begins to form at surface. Surface changes from nucleate to film boiling.
8 Film BoilingSurface completely covered with film blanket.
11 Film Boiling Correlations Film boiling not desirable! Better to have nucleate!
12 CondensationFilm Condensation: Liquid film covers surface and flows continuously from the surface. Characteristic of clean, uncontaminated surfaces.The presence of the liquid film reduces the heat transfer.
13 CondensationDroplet Condensation: drops form in cracks, pits and cavities on the surface and may grow and coalesce thru condensation. More than 90% of the surface is covered by drops, ranging from a few micrometers in diameter to agglomerations visable to the naked eye. The droplets flow from the surface due to gravity.Heat transfer for droplet condensation is higher than film. To be conservative, most design calculations assume film.