Focused Practice

I. Answer the following questions:

1. What are fundamental characteristics of a fluid?

2. Which state do three phases coexist at?

3. What is the normal boiling point?

4. When are the vapour pressures of nearly all fluids approximately parallel?

5. What is the critical point?

6. What does vapour compression refrigeration equipment do?

II. Analyse the grammar structures underlined in the above text.

III. Speak on: The normal boiling point.

Unit 8 Grammar: The Gerund Word List:

1. axial flow	осевое течение, поток
2. enhancement ratio	степень интенсификации (теплообмена)
3. film condensation	плёночная конденсация
4. fine wire	тонкая проволока
5. fins	пластины радиатора
6. pressure gradient	градиент давления, перепад давления
7. surface tension	поверхностное натяжение
8. thermal conductivity	теплопроводность
9. wire wrap	монтаж проводов накруткой
10. curvature	кривизна
11. heat transfer	теплоотдача
12. refrigerant	охладитель
13. momentum	количество движения

Enhancing Film Condensation Heat Transfer

Several workers have investigated the simple and cheap method of enhancing film condensation heat transfer by winding fine wire on the surface of a condenser tube. The wire wrap does not act in the same way as fins and the wire does not need to have high thermal conductivity. Enhancement is due to thinning of the film between the adjacent turns of the wire caused by the surface tension induced pressure gradient in the condensate. The pressure gradient results from the fact that the interface curvature is higher nearer the wire and gives rise to axial flow of condensate towards the wire.

The presence of the curvature term in the momentum balance for the condensate film leads to significant complication in the theory and no complete solution of the problem has been published to date. An approximate approach involved some empiricism backed by experiments for R11 and ethanol, and naturally the final result was in broad agreement with the data for these fluids. Later measurements for steam do not agree with the approximate theory. The approximate theory has recently been amended to include condensate retention. The theory then involves no empiricism and is in general agreement with the tem data. However, while giving results of the correct order of magnitude, the modified theory predicts a dependence on wire diameter that is opposite to that reported by the approximate approach. There is a report on the first stage of a research programme aimed at resolving these discrepancies. Further theoretical investigations and experiments using a refrigerant are in progress.

The report is given on new measurements for condensation of steam on a horizontal, water-cooled, wire-wrapped tube. The wire diameter and pitch of winding were systematically varied and heat-transfer measurements made for a range of coolant flow rates for each wire diameter and pitch. Data, in the form of heat flux and vapour-to-surface temperature difference were used to determine enhancement ratios (ratio of heat flux for wire-wrapped tube to that for a plain tube at the same vapour-to-surface temperature difference).

The problem of condensation on wire-wrapped tubes is not yet fully understood. Further light should be shed on the problem by the new data using a refrigerant as condensing fluid.