Resonant Pulsing Frequency Effect for Much Smaller Bubble Formation with Fluidic Oscillation

Abstract

Microbubbles have several applications in gas-liquid contacting operations. Conventional production of microbubbles is energetically unfavourable since surface energy required to generate the bubbles is inversely proportional to the size of the bubble generated. Fluidic oscillators have demonstrated a size decrease for a system with high throughput and low energetics but the achievable bubble size is limited due to coalescence. The hypothesis of this paper is that this limitation can be overcome by modifying bubble formation dynamics mediated by oscillatory flow. Frequency and amplitude are two easily controlled factors in oscillatory flow. The bubble can be formed at the displacement phase of the frequency cycle if the amplitude is sufficient to detach the bubble. If the frequency is too low, the conventional steady flow detachment mechanism occurs instead; if the frequency is too high, the bubbles coalesce. Our hypothesis proposes the existence of a resonant mode or ‘sweet-spot’ condition, via frequency modulation and increase in amplitude, to reduce coalescence and produce smallest bubble size with no additional energy input. This condition is identified for an exemplar system showing relative size changes, and a bubble size reduction from 650 µm for steady flow, to 120 µm for oscillatory-flow, and 60 µm for resonant condition (volume average) and 250 µm for steady-flow, 15 µm for oscillatory-flow, 7 µm for the resonant condition. A 10-fold reduction in bubble size with minimal increase in associated energetics results in a substantial reduction in energy requirements for all processes involving gas-liquid operations. The reduction in the energetic footprint of this method has widespread ramifications in all gas-liquid contacting operations including but not limited to wastewater aeration, desalination, flotation separation operations, and other operations.

Comparison of Bubble Size Distributions Inferred from Acoustic, Optical Visualisation, and Laser Diffraction

“…A single plane is used, as the image analysis software is not designed for a cloud of bubbles. Desai et al [58] is the only other significant work done on 3D cloud bubble dynamics, but no automated image analysis on a 3D bubble population has been developed yet except attempting to do so [7]. It was assumed that it would be an equiresponsive surface and that a single plane could be representative as a microcosm of the overall cloud dynamical system.…”

“…This is used when the number of bubbles is more important than obtaining the size (for example, applications such as dispersed air flotation and froth flotation). Differences in bubble size based on number mean and volume contribution has been described in the literature [58,59].…”

“…Sintered alumina spargers (2 µm average pore size, 2 off-50 cm 2 area), which produces a cloud of fine bubbles approximately 500 µm in size under steady flow [58] is used as the sparger in this study. This sparger, when coupled with the fluidic oscillator, can generate down to 7 µm in average bubble size (number average) [58].…”

Time-Delay Circuits for Fluidic Oscillators and Pulse Shapers

“…Fluidic oscillators-simple, inexpensive and maintenance-free-pulsate the flow quite efficiently. Due to these advantages they found attractive uses in many engineering processes (e.g., References [1][2][3]). Other present uses of fluidics are in the control of fluid flows past aeroplane wings and similar objects (e.g., turbine blades).…”

Special Issue “Fluid Flow and Heat Transfer”

“…The fourth theme of research that emerged can be referred to as "Flow with micro-and nano-scale features". This includes the following papers: There are two papers dealing with microbubbles present in the flow [14,15], a nanofluids application [16], ferromagnetic fluid behaviour [17] and a particle deposition problem [18].…”