The Effect of Gas Bubbles on the Flow Structure and Turbulence in a Downward Two-Phase Flow in a Vertical Pipe
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The Effect of Gas Bubbles on the Flow Structure and Turbulence in a Downward Two-Phase Flow in a Vertical Pipe I. A. Evdokimenko1, P. D. Lobanov1* , M. A. Pakhomov1** , V. I. Terekhov1*** , and P. K. Das2 1
Kutateladze Institute of Thermophysics, Siberian Branch, Russian Academy of Sciences, pr. Akad. Lavrent’eva 1, Novosibirsk, 630090 Russia 2 Department of Mechanical Engineering, Indian Institute of Technology Kharagpur, Kharagpur, India Received January 30, 2020; in final form, June 1, 2020; accepted July 21, 2020
Abstract—This paper presents the experimental and numerical results of modeling the local structure of gas-liquid downflow in a vertical pipe. The measurements were carried out using the PIV/PLIF system “Polis-PIV,” developed at the Institute of Thermophysics SB RAS. The numerical model is based on the use of a two-fluid Eulerian approach. The effect of changes in the diameter of air bubbles on the local flow structure and wall friction in a two-phase bubbly flow is studied. DOI: 10.1134/S1810232820030066
1. INTRODUCTION Turbulent bubbly downflows in pipes or channels often occur in the nuclear and power engineering, chemical, food industries, and other applications. Data about the local structure of turbulent bubbly flow are important for designing of modern power plant equipment, which explains the great interest in experimental [1–9] and numerical [10–13] studies of such flows over the past few decades. These flows are usually turbulent with strong interfacial interaction between the carrier fluid and gas bubbles. It was previously reported that a downward bubbly flow has a number of unique features as compared with an upward flow. Profiles of void fraction in a downflow are characterized by the appearance of a near-wall region free of bubbles, and the concentration of gas bubbles in the turbulent flow core is almost constant [4, 6–8]. The absence of bubbles near the wall is explained by the action of transverse forces, such as lift (Saffman), turbophoresis, and wall forces [7]. The fluid velocity in a downflow can have a local maximum at some distance from the wall [4–7]. The intensity of longitudinal fluctuations of the carrier fluid velocity near the pipe wall is less than the respective value for a single-phase flow, and it is higher in the turbulent flow core [6, 7]. The suppression of fluctuation intensity near the wall is explained by the absence of bubbles. The fluid turbulence in the axial part of the pipe is generated by vortex formation, when the fluid flow flows around gas bubbles [7]. Addition of small bubbles to the carrier fluid flow leads to decrease in the intensity of the turbulent pulsations. Small bubbles come closer to the wall surface than large bubbles do. [9]. The void fraction and pressure drop in upward and downward flows were studied in [1]. The void fraction in a downflow is higher than that in an upflow with identical gas volumetric flow rate ratios at the inlet. The void fraction, hydraulic resistance, heat transfer, and local structure of bubbly downflows were studied experimentally in the regimes of co
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