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. Natural Convection in Enclosed Spaces. Natural Convection in Enclosed Spaces. Boiling.
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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.
A: Free convection; B: Nucleate boiling; C: Transition Boiling;
D: Film boiling
Type Excess Temperature (deg. C)
Free Convection: < 5
Nucleate Boiling: 5 – 25
Transition Boiling: 25 – 120
Film Boiling: > 120
The excess temperature is the surface temperature above the boiling point of the liquid.
Bubbles 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.
Film begins to form at surface. Surface changes from nucleate to film boiling.
Surface completely covered with film blanket.
Film boiling not desirable! Better to have nucleate!
Film 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.
Droplet 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.
Homogeneous condensation or fog formation resulting from increased pressure due to expansion.
Direct contact condensation.
For tubes, the liquid thickens as the condensate flows down the tube.
For banks of tubes, the liquid may flow as a film to the lower tube, or it may drip to the lower tubes.