Dune flow

Following slug flow, the particles, instead of forming stationary dunes, gradually build up over the cross-section until they eventually cause a blockage. This type of flow is less common than dune flow. 

One major difference between pneumatic transport and hydraulic transport is that the gas-solid interaction for pneumatic transport is generally much smaller than the particle-particle and particle-wall interaction. There are two primary modes of pneumatic transport dense phase and dilute phase. In the former, the transport occurs below the saltation velocity (which is roughly equivalent to the minimum deposit velocity) in plug flow, dune flow, or sliding bed flow. Dilute phase transport occurs above the saltation velocity in suspended flow. The saltation velocity is not the same as the entrainment or pickup velocity, however, which is approximately 50% greater than the saltation velocity. The pressure gradient-velocity relationship is similar to the one for hydraulic transport, as shown in... 

Most existing models are suited only to pure dilute-phase or dense-phase (high m ) applications, whereas the possible modes of flow over long distances occur between these two extremes (e.g., dune-flow, sliding beds, irregular slugging, etc.—usually at moderate m ). 

Discontinuous dense phase flow can be divided into three fairly distinct flow patterns discrete plug flow in which discrete plugs of solids occupy the full pipe cross-section dune flow in which a layer of solids settled at the bottom of... 

Saltating flow is encountered at gas velocities just below the saltation velocity. Particles are conveyed in suspension above a layer of settled solids. Particles may be deposited and re-entrained from this layer. As the gas velocity is decreased the thickness of the layer of settled solids increases and eventually we have dune flow. 

Immature dune flow Degenerate slug flow... 

In experiments to measure the properties (length, spacing and velocity) of dunes undergoing non-suspension flow [23], several sensors have been utilised, namely pressure transducers, vibration detectors and capacitance sensors. The first two sensors were off-the-shelf and the final one was developed for the purpose. In all cases, cross correlation was used between transducer pairs to measure velocity. The conclusion of the study was that the pressure transducers provided important data on the fluctuations in gas pressure as the dunes flowed and the vibration sensors were able to detect the waves passing - albeit with a considerable spatial filtering effect. The capacitance system was able to resolve the structure... 

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