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Direct numerical simulations of turbulent core-annular flows with water-lubricated high viscosity oil in vertical and horizontal pipes : 수직과 수평 원형관 내 물 윤활작용을 가지는 고점성 기름으로 이루어진 난류 중심-환형 유동의 직접수치모사

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Authors

김기영

Advisor
최해천
Major
공과대학 기계항공공학부(멀티스케일 기계설계전공)
Issue Date
2018-08
Publisher
서울대학교 대학원
Description
학위논문 (박사)-- 서울대학교 대학원 : 공과대학 기계항공공학부(멀티스케일 기계설계전공), 2018. 8. 최해천.
Abstract
The water-lubricated transport has received attention as the drag reduction technology to deliver high viscosity oil. Previous stability analyses have shown that the flow arrangement is stable when the low viscous water is located near the wall encapsulating the high viscosity oil in the core. However, irregular wavy shapes of the phase interface appear due to the large viscosity difference and turbulence in the annulus. Therefore, in the present study, the characteristics of a turbulent core-annular flow with water-lubricated high viscosity oil in vertical and horizontal pipes are investigated using direct numerical simulation, in conjunction with a level-set method to track the phase interface between oil and water. The pressure drop as well as the instantaneous shape of the phase interface of the present study agree well with those from the experiment in the literature at the same flow condition.

For a vertical pipe, five different oil volume fractions are examined for a fixed mean wall friction (Re_τ = 720, where u_τ is the friction velocity, R is the pipe radius and ν_w is the kinematic viscosity of water). The total volume flow rate of a core-annular flow is similar to that of a turbulent single-phase pipe flow of water, indicating that water lubrication is an effective tool to transport high viscosity oil in a pipe. The high viscosity oil flow in the core region is almost a plug flow due to its high viscosity, and the water flow in the annular region is turbulent except for the case of large oil volume fraction (e.g., 0.91 in the present study). With decreasing oil volume fraction, the mean velocity profile in the annulus becomes more like that of turbulent pipe flow, but the streamwise evolution of vortical structures is obstructed by the phase interface wave. In a reference frame moving with the core velocity, water is observed to be trapped inside the wave valley in the annulus, and only a small amount of water runs through the wave crest. The phase interface of the core-annular flow consists of different streamwise and azimuthal wavenumber components for different oil holdups. The azimuthal wavenumber spectra of the phase interface amplitude have the largest power at the smallest wavenumber whose corresponding wavelength is the pipe circumference, while the streamwise wavenumber having the largest power decreases with decreasing oil volume fraction. The overall convection velocity of the phase interface is slightly lower than the core velocity. Finally, we suggest a predictive oil holdup model by defining the displacement thickness in the annulus and considering the boundary layer characteristics of water flow. This model predicts the variation of the oil holdup with the superficial velocity ratio very well.

For a horizontal pipe, six different superficial velocity ratio's (j_w/j_o = 0.057 ~ 0.41) are examined by changing the water superficial velocity (j_w=q_w/πR^2) for the fixed oil superficial velocity (j_o=q_o/πR^2), where q_w and q_o are volume flow rates of water and oil, respectively. The pressure drop as well as the shape of the phase interface agree well with those from the experiment in the literature at the same flow rates of oil and water. The core flow is almost a plug flow and rises due to the buoyancy, and thus the gap between the phase interface and wall is narrow and wide near the upper and lower surfaces of the pipe, respectively. By defining the clearance Reynolds number (Re_c) based on the core velocity and the local gap size, the annular flow is characterized into three different regimes: laminar Couette flow driven by the core for Re_c <= 600, transition flow for 600 < Re_c < 2000, and turbulent flow for Re_c >= 2000. The transition from laminar to turbulent flows is observed with the azimuthal direction because the local gap size varies, which is shown with fluctuations of the local wall shear stress. For laminar and turbulent flow regions, the local wall shear stress at an azimuthal location is proportional to Re_c^(-1) and Re_c^(-1/4), respectively. The local minimum of the local wall shear stress is in the transitional region, and the minimum pressure drop occurs at j_w/j_o = 0.11, where most of Re_c with an azimuthal direction are located in the transitional regime. The dynamics of the phase interface are examined by calculating the pressure and viscous shear contributions on instantaneous stress and mean lift and drag coefficients on the core. For the lift coefficient, the pressure force almost balances the buoyancy, and the contribution from the viscous shear stress is very small. For the drag coefficient (normalized with the wall friction), the contribution of the viscous shear is large for low j_w/j_o but decreases with increasing j_w/j_o, while that of the pressure is not much changed. With increasing j_w/j_o, the drag force of the core becomes less than the wall friction, where the wall friction of the core-annular flow is comparable to that of the water only flow and much smaller than that of the oil only flow.
Language
English
URI
https://hdl.handle.net/10371/143077
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