1. Objective
The objective of this experiment is to study the effects of condensing surfaces by observing the modes of condensation and determining the heat flux and surface heat transfer coefficient at constant pressure.
2. Introduction
The previous experiments have dealt with the boiling of 1,1-Dichloro-1-fluoroethane (R141b) CCl2 F-CH3 fluid in great detail and observed how the heat transfer rate and coefficient change throughout each stage of the boiling process. So, that experiments focus on the boiling process of the fluid as well as the heating element. This experiments focus on the condensation processes as well as on the condensing surface. The fluid that will be used in this experiment is water.
There are basically two major modes of condensation. They are film and dropwise condensation.
In this experiment we will look at the condensation of water in great detail and observe how the heat transfer rate and coefficient change throughout each stage of the condensation process.
3. Theory
- Filmwise Condensation
- Dropwise Condensation.
3.1 Filmwise Condensation
As briefly described, during filmwise condensation a layer of condensation covers the cool surface and this offers resistance to the transfer of heat. Depending upon the height of the condensing surface, the rate of condensation and the local vapour velocity, flow in this layer, which is flowing in a generally downward direction, may be laminar, or mildly turbulent with a "rippled surface". When flow is laminar as in the Hilton Film and Dropwise Condensation Unite, the rate of heat transfer can be determined from theoretical considerations provided a number of assumptions are made. These include,
i.
Flow in the condensate film is entirely laminar.
ii.
Heat is transmitted through the layer by conduction only.
iii.
The temperature in the layer falls uniformly from the saturation temperature of the vapour on the outside to the temperature of the cooling surface on inside.
iv.
The condensate flows under the action of gravity only.
Using these assumptions Nusselt derived the following equation for vertical surfaces with film wise condensation.
It will be noted that the properties k, r, and m used are at saturated liquid conditions , although within the layer of condensate , the liquid will be slightly sub-cooled. The derivation of this equation and descriptions of more recent work which have resulted in slightly different expressions are given.
3.2 Dropwise Condensation
The surface heat transfer coefficient achieved during dropwise condensation is usually between 5 and 10 times greater than with filmwise condensation under the same conditions. Dropwise condensation does not provide the same opportunity for theoretical analysis as does filmwise, but a considerable amount of experimental data has been accumulated. This has related to both heat transfer coefficients and to methods of promoting dropwise condensation in practical plants. Unfortunately common drpwise condensation promoters are effective for a relatively short time and at present suitable permanent surface treatments are either too expensive or impractical.
If dropwise condensation can be reliably achieved in practical plants, the heat transfer surface area required in steam to water heat exchangers can be reduced to about 60% of the present size. It therefore seems probable that efforts to achieve this will continue.
4. Apparatus
The apparatus for this experiment is the Film and Dropwise Condensation Unit H910. This unit is a very simple to understand. Study the diagram attached to the equipment to gain an understanding of the operation of the unit.
Data:
Fluid: Distilled Water
Quantity: 500 cm3
Heat transfer to surroundings: approx. 2.5 W/K
Specific heat of water: 4.18 kJ/kgK
Dimensions of heating surface (fin)
Surface Area of heating element: 14.4 x 10-3 m2
Condenser Surface Area: 3.7 x 10-3 m2
Diameter: 12.7 mm
Length: 90 mm
Temperature drop across copper shell of condenser: 2 x 10-6 f (K), where f is in W/m2
Glass Chamber
Nominal Internal Diameter: 76 mm