The flow patterns and pressure gradient of immiscible liquids are still subject of immense research interest. This is partly because fluids with different properties exhibit different flow behaviors in different pipe[U+05F3]s configurations under different operating conditions. Recently, Yusuf et al. (2012) investigated experimentally the flow patterns and pressure gradient of horizontal oil-water flow in 25.4. mm acrylic pipe. This paper describes similar works in 19. mm ID pipe to examine how significant is the effect of a small decrease in pipe diameter on flow patterns and pressure gradient. The results reveal a remarkable influence of pipe diameter on flow patterns and pressure gradient. The region of dual continuous and dispersed oil in water flows are enlarged as the pipe diameter increases from 19 to 25.4. mm while the extent of stratified, bubble and annular flow regions are found to decrease as the pipe diameter increases. The pressure gradient values obtained in the 19. mm pipe are greater than those measured in the 25.4. mm pipe at similar superficial oil and water velocities. The differences in pressure gradient results become bigger with higher oil and water velocities. The experimental pressure gradient results were compared with the two-fluid, homogenous and drift-flux models. The drift-flux model showed a good prediction to the experimental results while the two-fluid and the homogenous models were found to highly overpredict the experimental results especially for the smaller pipe diameter.
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