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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
vapour starts to turn into liquid, releasing its latent energy. This process is known as
Condensation.

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.



Homogeneous condensation. Vapour condenses out as droplets suspended in a gas phase to form a fog.



Direct contact condensation. Vapour condenses when brought to contact with a cool liquid.

Which mode of condensation is most frequently encountered in engineering applications?


surface condensation

Describe the two different types of surface condensation and explain which one is
assumed in thermal analysis.

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



Dropwise Condensation: The surface is covered with drops of different sizes, which grow, coalesce and due to the action of gravity, flow from the surface. It develops over surfaces coated with substances that inhibit wetting.



Because it is difficult to maintain dropwise condensation and because film condensation heat flux coefficients are lower, design calculations are usually carried our for film condensation.

What general principle needs to be followed in the design of condensers, in order to
increase the condensation effectiveness?

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.

List the dimensionless groups relevant in condensation thermal analysis.


*

For laminar film condensation over a vertical plate: Produce the momentum equation that describes the flow within the liquid film


*

For laminar film condensation over a vertical plate: State what assumptions are normally made and show the resulting simplified
momentum equation.


*

For laminar film condensation over a vertical plate:What are the simplest boundary conditions that can be applied to the momentum
equation?


*

For laminar film condensation over a vertical plate:What further physical principles are used and what assumption can be made about
the temperature variation, in order to produce analytical expressions for the Nusselt number, the flow rate and the film thickness?


*

For laminar film condensation over a vertical plate: In the equations described in 7d) above, at what temperature are liquid properties
evaluated?



*

For laminar film condensation over a vertical plate: In the equations described in 7d) above, what is the definition of the augmented
latent heat of vapourisation h'fg and why is it used?


*

To what other surfaces can the equations derived for vertical flat plates can be extended
to and what are the relevant limitations?


a) Condensation over vertical cylinders, provided, R>>delta



b) Condensation over inclined plates, for small angles of inclination, by replacing g by
g.cos(theta), where theta is the angle between the plate and the vertical direction


Which dinensionless parameter is used to determine when a liquid film on a vertical
surface becomes turbulent?


The dimensionless parameter used to determine when a liquid film on a vertical surface becomes turbulent is a Reynolds number, defined as:



where is the mass flow rate per unit width through a cross-section of the condensate film.

For condensation over a vertical column of horizontal tubes, the average Nusselt number
is lower than for condensation over a single horizontal tube. Explain why.


The reduction in the average Nu for condensation over a vertical column of horizontal
tubes, is due to the increase in the average film thickness for each successive tube, which in turn increases the thermal barrier between the tube surface and the vapour.

Why for condensation over the upper side of horizontal surfaces, Nusselt numbers are
lower than for condensation over vertical surfaces?


because for a horizontal surface which faces
upwards, it is more difficult for the condensate to flow away from the surface and hence the condensate film is thicker than for a vertical surface, thus increasing the surface thermal resistance.

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.

What are the main characteristics of internal forced convective condensation in straight
pipes?

In internal forced convective condensation in straight pipes, the progress of the liquid is
assisted by the vapour that flows through the core of the pipe. The liquid film therefore
becomes thinner. Consequently the Nusselt number increases.

What are the dimensionless parameters which appear in empirical correlations for the
Nusselt number in internal forced convective condensation in straight pipes?


The dimensionless parameters which appear in empirical correlations for the Nusselt
number in internal forced convective condensation in straight pipes are:

What is significant about internal forced convective condensation in straight pipes when
the vapour Reynolds number,
,becomes greater than 3.5x10^4?


Then the pipe orientation and flow direction no longer matter.

The condensation correlations provided are based on data collected when the vapour is
pure, meaning that gas side consists of only the substance that eventually condenses into
liquid. What happens on occasions, like water vapour in the atmosphere, where the vapour is in a mixture also containing one or more non-condensable gases?


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

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.


What are the advantages of a heat pipe?


Heat Pipes function almost isothermally and can thus be used:



To achieve high heat transfer rates between a source and a sink.



To level out temperatures in a system

What are the properties desirable in a heat pipe working fluid?


The properties desirable in a heat pipe working fluid are:



High latent heat.
High surface tension.
Low liquid viscosity.
High thermal conductivity


What is the dimensionless number used to quantify the potential of a fluid as a heat pipe
working fluid?

Merit number: