Horizontal two-phase flow of air-water mixtures in small diameter tubes and compact heat exchangers.

摘要

The two-phase flow of air-water mixtures in horizontal pipes was studied in order to determine whether pipe diameter is a significant variable in two-phase flow. Also, the flow patterns that exist in a Compact Heat Exchanger were determined in order to verify or refute the assumption that the flow pattern that exists in the Compact Heat Exchanger is an intimate mixture of the two phases with both the gas and the liquid travelling at the same velocity. This is called the 'Homogeneous' flow assumption. It was found that pipe diameter plays a role in horizontal two-phase flow of air-water mixtures through small diameter tubes. Further, the flow patterns that exist in the Compact Heat Exchanger were found to be annular, intermittent and bubble flow. Other flow patterns which can exist in pipes were absent in the Compact Heat Exchanger.;The flow regime maps for the 5mm, 4mm, 3mm, 2mm and 1mm I.D. precision bored glass tubing have been determined using visual techniques (high speed still and motion picture photography) and an instantaneous pressure trace method developed. The flow maps are shown in Figures 4.2.1, 4.3.1, 4.4.1, 4.5.1, and 4.6.1 respectively. Superficial liquid velocities were varied from 0.0024 m/s to 5.72 m/s, while the superficial gas velocity ranged from 0.015 m/s to 125.34 m/s. The mechanisms responsible for the flow regime transitions in all the tube sizes were identified so that a correct predictive theory may be developed. These are discussed in Chapter 4.;Pressure drop data were obtained for all the tube sizes and the results are appended in Appendix B. It was found that the Lockhart Martinelli correlations does not adequately account for the effect of pipe diameter on the pressure drop.;Finally, the flow map for a finned plate of the Compact Heat Exchanger was determined and is shown in Figure 4.7.1. Here, the superficial liquid velocities were varied from 0.0084 m/s to 8.62 m/s and the superficial gas velocities ranged from 1.05 m/s to 101.2 m/s. The 'homogeneous' flow assumption was found to be only partly correct, since the bubble, annular and intermittent flow regimes were observed in the Compact Heat Exchanger.