(A) direct numerical simulation study of flow and heat transfer in particle-laden turbulent convection|RISS 상세보기

다국어 초록 (Multilingual Abstract)

We observed convection phenomena in the channel using Direct Numerical Simulation (DNS). There are two channel domains, vertical and horizontal. In the horizontal channel, interaction with fluid, including particles, was investigated. First, the flow according to the temperature gradient was observed in a vertical channel emphasizing the influence of gravity. Considering the downward or upward flow due to buoyancy, the aspect ratio of the vertical channel was 6 : 1 : 3 for the vertical axis, width, and horizontal axis, respectively, and a vertically long domain was used, and the Prandtl number (Pr) is set to 7, assuming that the internal fluid was water at room temperature decided. In the case of water, the change in physical properties depending on temperature is quite large, so the temperature gradient provided for convection can affect the Prandtl number. To exclude this, the maximum Rayleigh number (Ra) and channel width were considered. In the case of water, Pr > 1, the fluid boundary layer occurs higher than the thermal boundary layer, so it was difficult to predict what kind of fluid phenomenon would occur. To implement and analyze this, the flow was implemented by increasing Ra from still flow level flow to soft turbulence. At low Ra, laminar and quasi-periodic flows were observed, and at high Ra, chaotic components began to appear in the flow field and the symmetry of the flow collapsed. This phenomenon was quantified using various statistical methods, and the mechanism was inferred by examining the stress distribution. The second study is the observation of Rayleigh-Benard convection involving a large number of particles. The effect of temperature was emphasized to understand fluid movement and heat transfer characteristics in a flow field containing a large number of particles. We implemented convection in a channel with a relatively large aspect ratio(= 6) and set the internal fluid to Pr =0.7 and Ra=1000000 to generate weak turbulence. The particles were assumed to be droplets, and nine cases were created by varying the size and mass fraction of the particles. It has been reported that a cell-shaped flow structure formed by upward and downward flow due to buoyancy appears when a fluid with a high Prandtl number is in a laminar state. However, this study observed that particles settling in a weak turbulent flow field modify the flow structure into a cell shape only through mechanical interaction with the fluid. Feedback between particles and fluid was implemented using 2-way coupling, and point-particle methodology was applied to particle tracking considering the non-dimensionalized size of the particles. The particles created a polygonal cell-shaped flow structure by suppressing the horizontal component of the fluid and reducing the large-scale flow structure. Investigated results were ecplained using figures and tables.

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국문 초록 (Abstract)

본 학위논문은 수직채널대류와 입자가 가득한 수평채널대류를 조사한 결과를 보고한다. 유동은 직접수치모사(Direct Numerical Simulation)를 이용해서 구현하였으며, 입자는 라그란지안 입자 추적 기법을 적용해서 구현됐다.
먼저 수직채널대류를 조사하기 위해, 서로 다른 온도로 유지되는 수직채널에 상온의 물이 가득 찬 상태를 수치적으로 구현했다. 벽사이의 온도 차이로 대변되는 레일리 수를 낮은 값(Ra=50000)부터 높은 값(Ra=1400000)까지 변화시키면서 층류부터 난류까지 관찰했다. 유동은 층류 영역에서 다른 연구자들이 이전에 보고한 공기 유동과 동일한 특성을 보여줬다. 레일리 수를 점점 증가시켜서 유동이 난류까지 발달하게 되면 흐름의 대칭성이 무너지게 된다. 여기서 흐름의 대칭성이란, 뜨거운 벽과 차가운 벽에서 온도에 의한 부력으로 유동이 각각 상승과 하강을 하는데, 이때 상승하는 유량과 하강하는 유량이 같은 값으로 대칭을 이루는 것을 말한다. 다시 돌아와서, 무너진 대칭흐름은 비교적 낮은 레일리 수 영역(C1, C2)에서 간헐적으로 회복하거나 반대 방향 흐름으로 역전된다. 레일리 수를 더 높이게 되면 무너진 유동의 대칭성은 더 이상 회복되거나 반대방향 흐름으로 역전되지 않는다. 본 연구는 레일리 수에 따른 유동 특성의 변화를 보여주고 레일리 수와 넛셀 수의 근사식을 도출해서 특이점을 제시했다. 또한 유동장 내 응력분포를 통해서 대칭 무너짐 현상을 설명했다.
다음으로 입자가 가득한 수평채널을 연구하기 위해, 레일리-버나드 대류로 알려진 대류 모델을 공기로 구현하고 액적을 상정한 무거운 입자를 무작위 위치에 분배시켰다. 입자는 유동장에서 종단속도로 침강하며, 유체와 입자의 직접적인 연관성에 주목하기 위해 바닥에 가라앉은 입자는 제거했다. 입자와 유체의 인과관계를 조사하기 위해 입자의 크기와 질량분율을 다르게 해서 9개의 케이스를 계산했다. 하강하는 입자는 수평방향 유동성분을 억제시키고 수직방향 유동성분의 거대구조를 약화시키면서 버나드 셀로 알려진 세포형태의 플럼구조를 만든다. 버나드 셀은 입자가 작을수록 커지고 입자의 질량분율이 클수록 개수가 많아지는 현상을 관찰했다. 에너지 예산 분배를 통해 입자와 유체의 에너지 피드백을 제시했으며 입자의 영향을 시간에 따라 관찰하면서 정성적, 정량적으로 버나드 셀의 형성 과정을 설명했다.

번역하기

본 학위논문은 수직채널대류와 입자가 가득한 수평채널대류를 조사한 결과를 보고한다. 유동은 직접수치모사(Direct Numerical Simulation)를 이용해서 구현하였으며, 입자는 라그란지안 입자 추적...

목차 (Table of Contents)

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x
ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
Chapter 1. Numerical Methodology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Convection in Vertical Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

List of Figures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . iii
List of Tables . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . x
ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . xi
Chapter 1. Numerical Methodology. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1 Convection in Vertical Channel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.1 Governing equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.1.2 Numerical method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2 Particle-laden Rayleigh B´enard convection . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.1 Governing equations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.2.2 Particle tracking method. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Chapter 2. Heat transfer in infinity parallel walls . . . . . . . . . . . . . . . . . . . . . . . . 12
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
2.2 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2.1 Flow regime classification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
2.2.2 Steady or quasiperiodic flow regimes . . . . . . . . . . . . . . . . . . . . . . . 20
2.2.3 Chaotic flow regimes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.3 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28
Chapter 3. Particle-laden flow in Rayleigh B´ enard convection . . . . . . . . . . . . 42
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
3.2 Investigated Parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
3.3 Results and Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
3.3.1 Time-averaged flow structures in particle-laden flows . . . . . . . . 48
3.3.2 Reduction of turbulence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59
3.3.3 Heat flux . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.3.4 Flow changing mechanism at transitional period . . . . . . . . . . . . . 66
3.3.5 Energy budget analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
3.4 Summary. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Chapter 4. Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
4.1 Heat transfer in infinity parallel walls. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
4.2 Particle-laden flow in Rayleigh B´ enard Convection . . . . . . . . . . . . . . . . . . 101
Bibliography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Abstract in Korean . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

참고문헌 (Reference)

1. On predicting particle-laden turbulent flows, ELGHOBASHI, S., 52 (4), 309–329, , 1994

2. Turbulent free convection in a vertical slot, ELDER, J. W., 23 (01), 99, , 1965

3. Heat transport in bubbling turbulent convection, LAKKARAJU, R., STEVENS, R. J. A. M., ORESTA, P., VERZICCO, R., LOHSE, D. & PROSPERETTI, A, 110 (23), 9237–9242, , 2013

4. Lagrangian statistics in turbulent channel flow, LEE, C., CHOI, J.-I., YEO, K., 16 (3), 779–793, , 2004

5. On the spatial structure of cellular convection, KIRDIASHKIN, A., BERDNIKOV, V., 15, 561–565, , 1979

6. Rayleigh number scaling in numerical convection, KERR, R. M., 310, 139–179, , 1996

7. Scaling in thermal convection: A unifying theory, GROSSMANN, S., LOHSE, D., 407, 27–56, , 2000

8. Breakdown of wind in turbulent thermal convection, DU PUITS, R., THESS, A., RESAGK, C., 75 (1), 1–4, , 2007

9. Large-scale mean patterns in turbulent convection, EMRAN, M. S., SCHUMACHER, J., J. Fluid Mech. 776, 96–108, , 2015

10. Multicellular natural convection in a vertical slot, KORPELA, SEPPO A, LEE, YEE, 126, 91–121, , 1983