4. High Definition Imagery results
In attempts to understand the effect of the pipe surface topology on flow behaviour, a high speed camera is used to record videos of the flows from each pipe used in the investigation. The detailed examination of these videos show small bubbles appeared in the flow. The source of these bubbles could be the dissolved air in water.
High definition recordings of the flow within each pipe were taken and then analysed, with particular attention paid to the shape and trajectory of bubbles. Figure 11 presents the typical flow trajectory recognised throughout the fully coated and non-coated pipes. The bubble can be seen to follow a trajectory parallel to the pipe axis with negligible deflection from this path. The bubble path is almost a straight line for laminar and turbulent flow.
Figure 12 presents the trajectory of bubble in 2mm half-coated pipe. The bubbles appear to oscillate about the axis of the pipe in y direction. This was recognised in all half-coated pipes, and each bubble followed a similar trajectory in figure 12. This shows that the difference in flow interaction with the two half of the pipe wall are creating a non-equilibrium condition which lead to the oscillation of the bubbles around the axis of the pipe. The unbalanced forces on the bubble may lead to a continuous rotation and generation of vortices in flow in the pipe.
Figure 13 presents the variation of average bubble size with pipe diameter. The size and shape of the bubbles in each flow is differed but appeared to present a trend. The 3 mm diameter pipe is not included in this Figure 13 due to the difficulty of observing any bubbles in the recorded images. The average bubble size of the bubbles appears to decrease as the pipe diameter increases. Elongation of the bubbles within turbulent flows was also recognised. Multiple measurements of bubble dimensions were taken for each bubble and then averaged to give an average bubble size. The decrease in bubble size may be attributed to the increase in turbulence experienced by the flow in the larger diameter pipes and further experimentation is required to confirm that the turbulence is the only reason for this trend.