Scaling of Gas Diffusion Into Limited Partial Cavities

Abstract

Bubbles populations in the wake of a partial cavity resulting from gas diffusion were measured to determine the noncondensable gas flux into the cavity. The diffusion rate is related to the dissolved gas content, the local cavity pressure, and the flow within and around the cavity. The measurements are used to revisit various scaling relationships for the gas diffusion, and it is found that traditional scaling that assumes the presence of a gas pocket overpredicts the gas diffusion. A new scaling based on diffusion into the low void fraction bubbly mixture within the partial cavity is proposed, and it is shown to adequately scale the observed production of gas bubbles for dissolved air saturation from 30% to 70% at 1 atm, limited cavities on the order of 0.3–3 cm in length at a freestream speed of 8 m/s (σ = 2.3–3.3 and Reynolds number based on the cavity length of order 105).

The dynamics of partial cavity formation, shedding and the influence of dissolved and injected non-condensable gas

“…The dissolved oxygen content was varied using a closed loop deaeration system, and the freestream value was measured using an Orion Start A113 dissolved oxygen meter to a precision of ±2%. The dissolved oxygen values during the experiment varied from 30% to 75% of saturation at STP, and here the dissolved oxygen is assumed to be a suitable proxy for the total dissolved gas content, as discussed by Lee et al (2016).…”

“…Dissolved gas within the water channel can influence the freestream nuclei population as discussed in Ceccio & Brennen (1991). In addition, dissolved gas within the bulk flow gas can be exchanged with a partial cavity, as discussed recently by Lee et al (2016). Typically, the liquid near the cavity may be supersaturated with dissolved gas, and mass transfer will take place into the cavity at the gas-liquid cavity interface (Parkin & Ravindra, 1991) or at the interfaces of the bubbly mixture that forms the cavity (Yu & Ceccio, 1997;Lee et al, 2016).…”

Void fraction measurements in partial cavitation regimes by X-ray computed tomography

“…We should not ignore the fact that some part of these void fractions may be attributed to non-condensable gas, instead of just water vapor. The diffusion rate into the cavity is related to the dissolved gas content, the local cavity pressure, and the flow within and around the cavity ( Lee et al, 2016 ). However, to minimize this effect, the measurements were performed at relatively low dissolved gas content (described in Section 2.2 ).…”

Experimental Methods for the Study of Hydrodynamic Cavitation

“…In order to relate the pressure change to the gas concentration, the chemical composition of the dissolved gases must be known, and some systems combine the probe with a separate instrument to sample and characterize the composition of the gas released into the headspace. For air dissolved in water, it is often assumed that the ratio of the dissolved gases track the ratio of nitrogen and oxygen in air at standard conditions (Yu and Ceccio 1997;Lee et al 2016). In fact, in many test facilities, only the dissolved oxygen is measured, and it is assumed that the level of oxygen saturation parallels the overall dissolved air concentration.…”