Author(s): Sebnem Elci; Oguz Hazar; Nisa Bahadiroglu; Derya Karakaya

Linked Author(s): Sebnem Elçi

Keywords: Two-phase flow; Stratified reservoirs; Aeration; ANSYS FLUENT model

Abstract: Water managers worldwide are facing the serious problem of dense blooms of cyanobacteria in reservoirs. Although some of the control measures (including mitigation) have a direct impact on cyanobacterial blooms by biomass removal, flushing, or mixing; the number of proven technologies is limited. This study aims at designing an autonomous system for artificial destratification to control cyanobacteria growth in the reservoirs. Five steps are followed for this purpose. First, a system is setup that mimics a thermally stratified reservoir experiencing hypoxia. For the generation of controlled stratified layers, a cooling system composed of a compressor, condenser, filter, capillary tube, copper pipes as an evaporator, a thermostat, and a monitor is integrated to the bottom section of the tank. The copper pipe which is approximately 20 meters is placed at a spiral shape at the bottom of the water tank enabling the passage of the cooling gas during operation which is controlled via a thermostat attached to the system. The heating of the top layers is maintained by strip heaters placed on top of the tank to heat the top layer as desired. Next, mixing efficiency of air diffusers is investigated via experiments and numerical modeling. Custom-made air diffusers are used in all experiments. A set of replaceable plates are designed and a total of 20 plates are custom made those having one hole, two holes, five holes and twelve holes, with diameters of d=1,2,3,4,5 mm. The air to the diffusers is provided by air pumps at flow rates of Q=100l/h, 200l/h and 400l/h. Effects of bubble size, bubble slip velocity, and other parameters on destratification efficiency are studied. Nondimensional numbers involving bubble diameter, bubble diffusing area, air rate, and stratification rates are utilized to quantify destratification efficiency for the best design of aeration systems. Then, an Arduino-controlled autonomous monitoring system for water quality parameters - temperature and dissolved oxygen is introduced to the tank. This system is used to begin the aeration process when predefined conditions for water quality are satisfied. Following the experiments, the destratification hydrodynamics in the tank is modeled using a 3D numerical model - ANSYS FLUENT program utilizing Reynolds-averaged Navier-Stokes motion for incompressible and time-varying fluid motion. Calibration and validation of the numerical model with observations enabled the testing of the various configurations of the aeration system. Finally, an optimum artificial mixing system for thermally stratified reservoirs is proposed based on experimental and numerical results.

DOI: https://doi.org/10.3850/IAHR-39WC252171192022867

Year: 2022