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25 Cards in this Set
- Front
- Back
What is condensation. |
When the temperature of a vapour is reduced below its saturation temperature, then the |
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List and describe the three different modes of condensation. |
Surface condensation. The vapour comes to contact with a cool surface and as it condenses the surface absorbs the latent heat released.
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Which mode of condensation is most frequently encountered in engineering applications?
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surface condensation |
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Describe the two different types of surface condensation and explain which one is |
Film Condensation: A liquid film covers the entire condensing surface and under the action of gravity and sometimes also of forced convection, it continuously flows away from the surface. It develops over clean, uncontaminated surfaces
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What general principle needs to be followed in the design of condensers, in order to |
The condensate forms a thermal barrier between the cool surface and the warm vapour. Since in Film Condensation, as can be seen in the diagram of page 1, the film thickness increases in the vertical direction, it is desirable to use short vertical surfaces, or horizontal cylinders. Most condensers therefore consist of horizontal tube bundles, with liquid coolant flowing through them and vapour condensing over their external surfaces. |
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List the dimensionless groups relevant in condensation thermal analysis.
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For laminar film condensation over a vertical plate: Produce the momentum equation that describes the flow within the liquid film
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For laminar film condensation over a vertical plate: State what assumptions are normally made and show the resulting simplified
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For laminar film condensation over a vertical plate:What are the simplest boundary conditions that can be applied to the momentum
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For laminar film condensation over a vertical plate:What further physical principles are used and what assumption can be made about
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For laminar film condensation over a vertical plate: In the equations described in 7d) above, at what temperature are liquid properties
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For laminar film condensation over a vertical plate: In the equations described in 7d) above, what is the definition of the augmented
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To what other surfaces can the equations derived for vertical flat plates can be extended
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a) Condensation over vertical cylinders, provided, R>>delta
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Which dinensionless parameter is used to determine when a liquid film on a vertical
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The dimensionless parameter used to determine when a liquid film on a vertical surface becomes turbulent is a Reynolds number, defined as:
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For condensation over a vertical column of horizontal tubes, the average Nusselt number
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The reduction in the average Nu for condensation over a vertical column of horizontal |
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Why for condensation over the upper side of horizontal surfaces, Nusselt numbers are
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because for a horizontal surface which faces |
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What is forced convective condensation heat transfer and what are its main characteristics? |
The vapour is forced to flow over the cooled surface. The moving vapour then drags liquid in the direction of motion. The overall condensation heat transfer process is one of natural convection mixed with forced convection. |
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What are the main characteristics of internal forced convective condensation in straight |
In internal forced convective condensation in straight pipes, the progress of the liquid is |
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What are the dimensionless parameters which appear in empirical correlations for the
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The dimensionless parameters which appear in empirical correlations for the Nusselt |
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What is significant about internal forced convective condensation in straight pipes when
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Then the pipe orientation and flow direction no longer matter. |
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The condensation correlations provided are based on data collected when the vapour is
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The heat transfer coefficients are significantly lower than for the cases that involve only pure vapour. This is because the condensing species must first diffuse through the concentration boundary layer that covers the gas side of the interface. The condensing species must therefore first overcome the mass transfer resistance posed by the concentration boundary layer |
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What are the main features of a heat pipe? |
The heat pipe is a tube the inside of which is lined with a layer of wicking material. The wick is wetted with an appropriate liquid. One end of the tube is exposed to a heat source, while the other is in contact with a cold reservoir. The liquid evaporates at the hot end, and the resulting vapour flows to the cold end of the tube, where it condenses. Capillary action then moves the condensed liquid axially along the wick back to the evaporator where it is again vapourised.
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What are the advantages of a heat pipe?
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Heat Pipes function almost isothermally and can thus be used:
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What are the properties desirable in a heat pipe working fluid?
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The properties desirable in a heat pipe working fluid are:
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What is the dimensionless number used to quantify the potential of a fluid as a heat pipe |
Merit number: |