Modeling and Experimental Studies on the Absorption of Entrained Gas and the Influence on Fluid Compressibility

Abstract

The bulk modulus of hydraulic fluid is dependent on the quantity of entrained gas in the fluid. In this paper, an effective fluid bulk modulus model that captures dynamic gas absorption during pressure transients is derived from the overall mass transfer theory. Optical measurement of a microgas bubble volume is used to determine the interfacial mass transport. Compared to traditional models, the proposed model is able to capture the 10% gap in the pressure profile between the first and second cycles, when simulating multiple compression cycles of an oil sample with 0.65% entrained gas by volume at 8 MPa.

Novel Methods for Modeling and Online Measurement of Effective Bulk Modulus of Flowing Oil

“…The change of bubble size in the oil can be obtained by combining Eqs. (12), (14) and (15). However, it is a challenge to determine d by the traditional method since Eq.…”

“…Although the hydraulic oil has different grades, its viscosity values are very close. Therefore, the bubble microscope photo published by literature [14] represents the general law that reflects the distribution of bubble size, and can be used to estimate the distribution style and parameters of bubble size in the hydraulic oil.…”

“…The Literature [14] pointed out that on the premise that the bubble radius is less than 130um, it only takes about 0.1s to compress the bubble to temperature equilibrium. According to the log-normal distribution of bubble size, the proportion of bubbles with the radius greater than 130um is less than 1%.…”

“…During the operation, free air can be mixed into the hydraulic fluid in multiple ways, including air drawn into oil through the leakage of the pipeline or the accumulators, oil exposed in air, and even the dissolved air separated from the oil under low-pressure condition. All of those aforementioned factors produce bubbles [14], [15]. The low-density and the large molecular gap of bubbles bring in large volume change under the varying pressure, and affects the compressibility of the liquid.…”

“…Zhou et al [22] determined the time of inter-phase mass transfer by the Rayleigh-Plesset equation. Tian and Van de Ven [14] simplified the physical meaning of the mass transfer coefficient, and pointed out the dissolving rate of air is affected by the size of bubble.…”