Air-water two-phase flow in circular pipes has been studied by many investigators. Because of the important effect of flow patterns on the flow and heat transfer, investigations on flow patterns are still very active. On the other hand, investigations of flow patterns in noncircular pipes are still very rare. Triangular tubes have found a great deal of applications in many fields of industry, especially in heat transfer equipment, such as compact heat exchanges, solar heat collectors, miniature heat pipes, nuclear reactor cores and electronic cooling systems. While a large number of studies have been made of turbulent single-phase flow in triangular geometries, very little has been done with two-phase flow. The present investigation was to study the effects of triangular pipe geometry and orientation on flow patterns, slug frequency, and liquid film dryout in gas-liquid flows.

Experimental investigations on the flow patterns of air-water two-phase flow in horizontal circular and triangular pipes were carried out. The circular test section consists of a 2-inch (50.8 mm) ID, 28 ft (8.53 m) long clear PVC pipe followed by an equilateral triangular pipe 20 ft (6.1 m) in length having a side length of 2.7-inch (68.6 mm), corresponding to hydraulic diameter of 1.56-inch (39.6 mm). In addition, the effect of corners on the liquid distribution was investigated using two different orientations of triangular channel (apex upward and downward) with comparison to round tubes. Video cameras were positioned 60 and 120 diameters from the inlet of each test section to investigate the flow development. Superficial liquid and gas velocities that ranged from 0.3 to 6.6 ft/s and from 3.1 to 65.6 ft/s, respectively. Different understandings of two-phase flow regimes affected by the shapes of the cross-sections of the channels were obtained by flow visualization. The following flow patterns at different air and water superficial velocities were recognized for both pipe geometries: slug flow, pseudo-slug flow, stratified-wavy flow, and annular flow. Visual observations revealed that the slug frequency in the triangular channel is lower than the frequency obtained in the circular pipe.

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