An Experimental Study of Roman Dropshaft Operation : Hydraulics, Two-Phase Flow, Acoustics

A Dropshaft is an energy dissipator connecting two channels with different invert elevations. In the present study, the hydraulics of vertical, rectangular dropshafts was systematically investigated in two prototypes and five smaller models. Experimental observations showed three basic flow regimes for dropshaft configurations with 180 degree outflow direction. For dropshaft configurations with 90 degree outflow direction, only two flow regimes were observed. At low flow rates, the rate of energy dissipation in the dropshafts was nearly 95%. In the models, the pool depth had little effects on the hydraulic properties of the dropshaft, but larger rate of energy dissipation was consistently observed with 90 degree outflow direction. Neutrally-buoyant particles were used to record particle residence times in the shafts. For low flow rates and deep shaft pools, the probability distribution functions of residence times exhibited a bi-modal distribution in both model and prototype. Detailed air-water flow and acoustic measurements were conducted in one prototype dropshaft under controlled flow conditions. Void fraction measurements demonstrated a strong aeration of the shaft pool for all flow regimes, but regime R2. The air-water content distributions were basically two-dimensional . At low flow rates, the data were successively compared with an analytical solution of the air bubble advective diffusion equation. Pseudo-bubble chord size measurements showed a broad range of entrained bubble sizes, from less than 0.5 mm to more than 25 mm, with mean pseudo-bubble chords between 10 and 20 mm. The distributions of chord sizes were skewed with a preponderance of small bubbles compared to the mean. Analysis of the streamwise distributions of bubbles indicated a fair proportion of bubbles associated with a cluster structure and the large majority of clusters consisted of 2 bubbles only. Acoustic signatures of the bubbly flow characterised accurately the changes in flow regimes. However the transformation from acoustic frequencies to bubble radii underestimated the entrained bubble sizes and it did not predict the shape of bubble size probability distribution functions measured with intrusive conductivity probes. Although basic hydraulic characteristics were similar between model and prototype based upon a Froude similitude, observations of dimensionless bubble penetration depths and neutrally-buoyant particle residence times showed some discrepancies between model and prototype results. It is believed that these differences highlight the limitations of a Froude similitude for studies of air entrainment, residence times and mass transfer in dropshaft even with a geometric scaling ratio LR = 3.1 as in the present work