Influence of outlet width of flume on overflow capacity of weir-flume combination facility.

[Objective] To provide reference for expanding the applicability of weir-flume combination facility, the influence of outlet width of measuring flume on hydraulic performance and flow measurement of weir-flume combination facility was studied. [Method] Under different flow conditions (5-71 L/s), 35 sets of hydraulic performance tests with three measuring flume outlet widths (6 = 25, 35 and 45 cm) were carried. The influence of outlet width on water surface line of middle vertical plane and Froude number (Fr) along the flume was analyzed through the changes of water depth and vertical average velocity of 30 measuring points in the middle vertical plane. Then, the flow thresholds of in-groove flow and weir flow were determined. According to the correlation between inlet-flow (Q) and water depth of measuring point in the flume, the influence of outlet width on the discharge coefficient was analyzed and the flow formula with different outlet widths was established. [Result] Under three different outlet widths, the water depth of the middle vertical plane and Fr showed similar variations along the flume. The water depth of the middle vertical plane was stable in the upstream section and dropped to the thin water layer gradually after entering the measuring flume. Fr was relatively stable in the upstream section (0.1 -0.2) and increased to the maximum value (1.2-1.5) gradually after entering the flume, and the increasing rate increased along the flume. The water depths in the upstream section and in the measuring flume decreased with the increase of outlet width. The water depths in the outlet section, thin water layer and backwater area were not greatly affected by outlet width. The Fr from the upstream section to the torsional transition inside the measuring flume increased with the increase of outlet width, but that of the trapezoidal narrow section decreased with the increase of outlet width. Under same outlet widths, the flow coefficient of the in-flume flow increased with the increase of flow rate, while the flow coefficient of the weir flow decreased. Under same relative acting heads, the discharge coefficient of the in-flume flow decreased with the increase of outlet width, while that of the weir flow increased. Based on the established flow formulas under outlet widths of three measuring flumes, relative errors between calculated and measured flow values were all within 2%, expanding the application scope of weir-flume combination facility. [Conclusion] The outlet width of measuring flume had important influence on water depth in upstream and measuring flume. The increase of outlet width increased the flow threshold and overflow capacity of weir-flume combination facility and pushed the critical depth section downstream. [ABSTRACT FROM AUTHOR]