What is the flow field of the fermentation tank at different stirring speeds?
Study on the velocity distribution of the flow field in fermentation tanks under different stirring speeds.
1、 Experimental conditions
Experimental equipment: Bairun bioreactor Speed gradient: 200rpm, 300rpm, 400rpm, 500rpm, 600rpm Fixed parameters: Ventilation rate 0.4m ³/h, bottom blade installation height 90mm.
2、 Flow field characteristics
1. Vortex zone displacement phenomenon in low-speed zone (200-300rpm): The overall upward displacement of the circulating vortex zone near the blade is 200rpm: The upper blade vortex zone reaches the top of the tank at 300rpm: The displacement amplitude decreases but there is still a gas velocity dominant mechanism: at low speeds, gas lift dominates, pushing the vortex zone to migrate with bubbles.
Velocity distribution: Average flow velocity: 0.1-0.3m/s, with the high-speed zone concentrated downstream of the blade
2. High speed zone (≥ 400rpm)
Vortex zone stability: The circulating vortex zone is fixed around the edges of each layer of blades. The dominant mechanism of stirring is that the blade shear force overcomes the bubble buoyancy, forming a stable flow field.
Speed distribution: The average flow velocity increases to 0.3-0.6m/s, and the high-speed zone expands to the entire tank, reducing the area of the low-speed zone by more than 50%.
It can be seen that 300rpm is the turning point from the influence of gas velocity to the dominance of stirring. For every 100rpm increase in speed, the standard deviation of the entire tank speed decreases by 12-15%. When the speed is ≥ 400rpm, the dead zone area below the bottom propeller decreases by 70%.
In some engineering projects, it is recommended to operate at a speed of 400-600rpm (balancing mixing efficiency and energy consumption), and when the speed is less than 300rpm, the ventilation rate should be reduced to below 0.3m ³/h to suppress vortex shift.
