Fluidic low-frequency oscillator with vortex spin-up time delay

•New principle of no-moving-part fluidic oscillator.•Monostable turn-down fluidic amplifier.•Relaxation switching by controlled by vortex spin-up.•Frequency exponentially dependent on flow rate.

The paper describes a fluidic oscillator developed for supplying microbubbles of CO2 to algae in photobioreactors. Having no moving parts and powered solely by the flow of the supplied gas the oscillator can be made robust and reliable. The advantage it brings is the flow can undergo a direction reversal during each cycle. Freshly made gas microbubbles together with some surrounding liquid thus can return into micropores of the aerator – and subsequently be accelerated away from the aerator vicinity. This prevents microbubbles merging just after their formation, a phenomenon that so far has been the main obstacle in generation of very small bubbles. The oscillation mechanism is based on a switched inlet vortex amplifier combined with a relaxation process involving the spin-up time of the vortex flow. Experiments relate to design development, the back-and-forth outflow, frequency, and overall performance.