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Alekseev M.V., Vozhakov I.S., Lezhnin S.I. Non-stationary process characteristics of the gas outflow into a liquid. Multiphase Systems. 14 (2019) 2. 82–88 (in Russian).
2019. Vol. 14. Issue 2, Pp. 82–88
URL: http://mfs.uimech.org/mfs2019.2.012,en
DOI: 10.21662/mfs2019.2.012
Non-stationary process characteristics of the gas outflow into a liquid
Alekseev M.V.∗, Vozhakov I.S.∗,∗∗, Lezhnin S.I.∗,∗∗
∗Institute of Thermophysics SB RAS, Novosibirsk, Russia
∗∗Novosibirsk State University, Novosibirsk, Russia

Abstract

A numerical simulation of the process of the outflow of gas under pressure into a closed container partially filled with liquid was carried out. For comparative theoretical analysis, an asymptotic model was used with assumptions about the adiabaticity of the gas outflow process and the ideality of the liquid during the oscillatory one-dimensional motion of the liquid column. In this case, the motion of the liquid column and the evolution of pressure in the gas are determined by the equation of dynamics and the balance of enthalpy. Numerical simulation was performed in the OpenFOAM package using the fluid volume method (VOF method) and the standard k-e turbulence model. The evolution of the fields of volumetric gas content, velocity, and pressure during the flow of gas from the high-pressure chamber into a closed channel filled with liquid in the presence of a ”gas blanket“ at the upper end of the channel is obtained. It was shown that the dynamics of pulsations in the gas cavity that occurs when the gas flows into the closed region substantially depends on the physical properties of the liquid in the volume, especially the density. Numerical modeling showed that the injection of gas into water occurs in the form of a jet outflow of gas, and for the outflow into liquid lead, a gas slug is formed at the bottom of the channel. Satisfactory agreement was obtained between the numerical calculation and the calculation according to the asymptotic model for pressure pulsations in a gas projectile in liquid lead. For water, the results of calculations using the asymptotic model give a significant difference from the results of numerical calculations. In all cases, the velocity of the medium obtained by numerical simulation and when using the asymptotic model differ by an order of magnitude or more.

Keywords

gas outflow,
high pressure chamber,
OpenFOAM,
gas injection into water,
gas injection into liquid lead

Article outline

Problem: Analytical and numerical study of the process of outflow of gas under pressure into a closed container with liquid. Obtaining characteristic process parameters and comparing the calculation results by the asymptotic model and the results of numerical simulation.

Methods: For the theoretical analysis, an asymptotic model was used, in which assumptions were made about the adiabatic process of gas outflow and fluid ideality during oscillatory motion of the liquid column, and the movement of the liquid column and the evolution of pressure in the gas are determined by the dynamics equation and the enthalpy balance. As a numerical method for solving a system of model differential equations in partial derivatives, a finite-difference scheme was chosen, based on the use of the Euler method in the OpenFOAM package. The resolution of the interphase surface was carried out by the volume of fluid method (VOF method).

In a study was determined:

  1. The dynamics of pulsations in the gas cavity arising during the flow of gas into the closed region substantially depends on the physical properties of the liquid in the volume. In the case of water, the growth of a slug occurs vertically displacing the liquid and compressing the gas volume in the upper region. At a time of 5 ms, the compression of the gas volume ceases and then the expansion phase follows. The movement of the liquid forms a compression of the gas shell in its central part in the radial direction (8 ms). In the case of lead, the growth of a slug occurs both in the radial and vertical directions. At the same time, the shape of the gas shell takes on a “bell-shaped” appearance. Liquid displacement by compression of the upper gas volume ends in 12 ms. Then there is a return motion of the liquid and compression of the slug. A cumulative jet is formed at the upper boundary, which, when moving downward, breaks down from the oncoming gas stream inside the slug, and the interphase boundary of the slug becomes unstable, leading to the separation of small bubbles from the slug and the formation of jets and drops inside the slug.
  2. The difference in the numerical calculation of pressure from the asymptotic model during the injection of gas into water is characterized by uneven pulsations. This non-uniformity is because, when injected into water, a gas stream is formed that passes through the liquid column. In this case, an unsteady gas-dynamic structure of pressure surges is formed inside the gas shell. When the gas flows into the water “at the first pulsation”, the pressure near the nozzle is always lower than the critical pressure, which characterizes the possible blocking of the flow. In the numerical calculation of gas injection into the lead, the jet gas flow is not observed. Calculation of the pressure at the first pulsation of the slug is in good agreement with the pressure obtained from the asymptotic model.

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