Particle-Laden Pipe Flows at High Volume Fractions Show Transition Without Puffs

Using ultrasound imaging velocimetry, we are able to present unique insight in transitional particle-laden flows. Together with a Moody diagram of time-averaged properties, we demonstrate that the laminar-turbulent transition behavior at high volume fractions is distinct from the single-phase case and cases with low volume fractions. For low volume fractions, a sharp transition is found with the presence of turbulent puffs, similar to the single-phase case. Seemingly, particles in this regime trigger subcritical transition. For high volume fractions a smooth transition is discovered without turbulent puffs in the transition regime. For this regime, particles cause a supercritical transition.

Figure 1

The friction factor as function of Reynolds number (panel a, selected cases) and as function of Reynolds number and volume fraction (panel b, where each marker represents a measurement).

Figure 2

Radial ($v$) velocity data as function of time for five different Reynolds numbers for $\varphi =1\%$. The intermittency $\gamma$ represents the fraction of puffs and is obtained from the pressure drop signal. The velocity data are normalized using the centerline velocity. A bar of length $30D$ (based on the averaged centerline velocity for each Re) is shown in the top right corner for each panel. Only the top half of the pipe is shown; the radial positions (r) are normalized with the pipe radius R.

Figure 3

Radial ($v$) velocity data as function of time for six different Reynolds numbers for $\varphi =14\%$.