Particle-laden exchange flows in inclined pipes*

The buoyancy-driven lock-exchange flow of a suspension mixture and a pure fluid in inclined pipes is studied experimentally. We Investigate the interpenetration of the heavy particle-laden fluid and the light pure fluid in Boussinesq limit while considering the effects of the initial volume fraction of particles φ₀ and inclination angle of pipe β on the flow. Three distinct regimes are identified: I) Sedimentary: at near-horizontal inclinations and for φ₀ close to dilute and packed limits, the flow comes to a halt as particles settle out of the mixture. II) Mixing: away from the horizontal angle and for the intermediate volume fraction of particles, flow advances steadily and particles stay mixed within the suspension. III) Transitionary: a novel non-linear and unsteady behavior occurs as flow transitions from sedimentary to mixing regime. Through a scaling analysis, It is revealed that enhanced convection of heavy and light mixtures in inclined pipes (Boycott effect) facilitates the streamwise advancement rate of the particle-laden front V_f of order cosβ². Furthermore, the impact of particle size and fluid's viscosity on the classified flow regimes is studied in detail.