Experimental investigation of effect of the efficiency block ramp on increasing the dissolved oxygen in semi-uniform regime

The dissolved oxygen concentration in water is one of the most important parameters of water quality and plays an important role in improving the environmental conditions of rivers as well as the processes of removing iron and manganese from drinking water in water treatment plant. One of method to increase level of the dissolved oxygen in water is installation of structures that by increasing turbulence of water lead to entrainment of small bubbles of air into water body. Block ramps are one of the gravity-falling structures that in addition to the optimal performance in the stability of riverbeds, due to the structure of over passing flow and the possibility of applying various roughness’s in size and arrangement, can also act as an aerator structure.

Experimental Setup and procedure

In this study, the effect of different arrangements of intermediate-scale roughness on increasing the performance of dissolved oxygen in water in the downstream of the block ramp with slopes of 1:3, 1:5 and 1:7 was investigated experimentally.Experimental measurements were conducted in the hydraulics modeling laboratory of the University of Guilan, Iran, in a flume with a rectangular cross-section with, 0.88 m width, 1 m depth and 8.6 m length. A centrifugal pump controlled with an adjustable frequency AC motor supplied a range of discharges in semi-uniform regimes. It should be noted that the flow rate was adjusted using an ultrasonic flow meter with an accuracy of ±0.1 l/s.The DO measurements were conducted with two calibrated portable HQ30D Model DO meters 0.7 m upstream and 2.5 m downstream from the toe of the weirs where air bubbles completely disappeared from view. . During this process, the DO meters continuously recorded the DO measurements.To investigate the performance the block ramp with different slope and arrangements of roughness particles with intermediate scale (2.5<yc/ks<6.5), flow discharges (3.7<H/yc<6.9), and free and submerged hydraulic jump, 248 tests were carried out.

Experimental observations and analysis of the results showed that by creating intermediate-scale roughness on block ramps, the flow overpasses in different directions on the rough elements at lower approaching flow depth. Therefore, diffusion of flow along the structure intensifies and performance of increase of dissolved oxygen increases. Experimental observation showed that for smooth block ramp and high level of overpassing flow over roughed surface of block ramp, flow regime is skimming and there is no aeration occurred along structures. Comparison of results showed that for upstream dissolved oxygen of 2 to 5 mg/l at both free and submerged hydraulic jump, performance of the blokramps to increase of Ef increase by increase of flow discharges and corresponding H/yc. However, more increase of flow discharge caused to slightly decrease of Ef.Comparison of results indicated that at Cu = 2mg/l and submerged hydraulic jump, by increasing the slope of smooth block ramps from 1:3 to 1:5 and 1:7, Ef changed from 0.99 to 1.1 and 1.36, respectively. Furthermore, for free hydraulic jump, reduction of slope of smooth blockramp from 1:3 to 1:5 and 1:7, Ef changed from 1.35 to 1.53 and 1.44, respectively.By installing intermediate-scale roughness with various arrangement, on average by increasing the slope of block ramps from 1:3 to 1:5 and 1:7, Ef changed from 1.1 to 1.2 and 1.47, respectively. Furthermore, for free hydraulic jump, reduction of slope of smooth block ramp from 1:3 to 1:5 and 1:7, Ef changed from 1.43 to 1.34 and 1.61, respectively.Comparison of results showed that by installing intermediate-scale roughness with various arrangement Ef increases 7 percent on average for submerged hydraulic jump.

Comparison of performance of tested structures in all upstream dissolved oxygen concentrations shows that for block ramp with a slope of 1:3, installation of intermediate-scale roughness with different arrangements, performance of dissolved oxygen concentration (Ef) compared to no roughness conditions in ramp hydraulic jump conditions, it has improved by 8%, however, by changing the hydraulic jump to free, the performance of these structures decreased by 3%. Comparison of the results shows that by changing the slope of the structure to 1:5, create intermediate-scale roughness with different arrangements in ramp hydraulic jump conditions a 6% increase in performance of dissolved oxygen concentration was observed compared to no roughness conditions, however, by changing the hydraulic jump to free, the performance of these structures decreased by 14%. By changing the slope of the block ramp to 1:7 and create intermediate-scale roughness with different arrangements like other slopes studied, the performance of dissolved oxygen concentration for ramp and free hydraulic jump conditions, improved by 7 and 3%, respectively, compared to no roughness conditions.