Adiabatic vertical downward air–water flow pattern map: Influence of inlet device, flow development length and hysteresis effects

Flow patterns for adiabatic vertical downward air–water flow in an 8.8 mm diameter channel have been studied. In particular, the influence of the inlet device, the flow history and the development length on the flow pattern have been investigated and assessed. Test results with two inlet devices (highly mixed and unmixed) indicate that the inlet device has a major influence on the flow pattern transitions. Furthermore, the transition boundaries between different flow regimes have been found to depend strongly on the flow’s development length, and display hysteresis effects. Not surprisingly, no existing flow pattern map worked well versus the current observations, nor did they agree with one another, thus pointing the need to better characterize the test section inlet geometry, the influence of the test length and the test procedure to obtain genuine flow pattern observations. Presently, it can be concluded that in each flow pattern region there are important differences in the actual flow structures. A future mechanistic flow pattern map should then not only contain flow pattern transition lines but also physical characteristics which describe the flow regime and inlet device effects. This was qualitatively done in this study by superimposing on the flow pattern maps frames which illustrate the characteristics of the flow for two different inlet devices.