Hydrodynamic effects of use of eductors (Jet-Mixing Eductor) for water inlet on circular tank fish culture

- Vertical Spray and Jet-Mixing Eductor were evaluated in terms of tangential velocity, mixing and solids removal.
- For similar values of impulse force and injection velocity, eductors generate better hydraulic performance.
- Eductors have similar or lower energy demand than Vertical Spray at similar hydraulic performance.
- The formation of uniform patterns velocity, mixing phenomena and self-cleaning depend on the water injection angle.

Trials were conducted in circular tanks used in aquaculture (1.2 m diameter and 0.4 of useful depth) in order to evaluate the effect of different water injection devices on the tangential velocity of the water, its uniformity pattern, the mixing time and the removal of solids through the bottom drain. Two injection devices, without fishes, were evaluated at the tank: a Vertical Spray, considered as standard in aquaculture and an eductor (Jet-Mixing Eductor) which is used in chemical and petrochemical industry to homogenize and keep in movement great volumes of water. The devices were evaluated under the same operating conditions: inlet flow of 4, 6 and 8 l/min and water injection angle of 0° and 45°. In each trial, the water velocity inside the tank was measured, also the mixing time and the time in which the 100% of pellets of fish food were eliminated from the tank through the bottom drain. The results indicate that, for all inlet flows, the eductor operating at 45° presents significantly (p < 0.001) better result in terms of hydraulic variables such as tangential velocity, uniformity, mixing time and solids removal time from the tank (self-cleaning effect). In the case of eductors, although an increase of inlet flow produces improvements of hydraulic variables, a change of water injection angle, from 0° to 45°, produces significantly better results (p < 0.001). Differences of hydraulic performance between the eductor and Vertical Spray are mainly owing to the multiplicative effect of the outlet flow that is generated by the eductor. This means that, for similar values of impulse force and water velocity at the exit of the nozzle injection (V2), the eductors generate significantly higher tangential velocity and uniformity, lower mixing times and secondary flow patterns, which ensure the self-cleaning of solid waste. In terms of power consumption, eductors overcome Vertical Spray in the trial performed. However, when comparing them under equal requirements of hydraulic performances (velocity, mix and/or self-cleaning), eductors present the same or lower energy consumption. Comparatively eductors would generate, under similar operating conditions, clear benefits for produced species in aquaculture, by generating hydraulic conditions that ensure a better quality of water and patterns and uniformity of velocity, which are more suitable for their health and normal growth.