Numerical Investigation of Air Bubbles Evolution and Coalescence from Submerged Orifices Based on OpenFOAM Pan Feng, He Ying, Li-zhong Mu 2018-7-6 Dalian University of Technology, China
Outline Background Objective and content Numerical model Results and discussion Summary 2
Background of Nucleate Boiling Higher heat flux can be transferred by lower temperature gradients in boiling system. a potential cooling solution for high-performance micro-processors Hydrodynamic factors and thermal factors interplay in boiling system. Fluid side Bubble growth Bubble coalescence Bubble departure Bubble rising Hydrodynamic factors 0-0.5s, T=7.4K (Wang, Bachelor thesis, 2018) (Dalian University of Technology) Solid side Liquid layer evaporation Nucleation site activation Heater conduction Thermal factors Conjugate interactions in boiling system Conjugate problems in boiling system 3
Bubble coalescence in boiling system near the boiling surface Low heat-flux Only occurs in High heat-flux (Hutter,2009) (Cheng,2018) Horizontal coalescence Declining coalescence Vertical coalescence Three basic coalescence type during boiling experiments (Zhang Lei,2002 ; Zhang Lu, 2007; Cheng, 2018) A non-dimensional factor: S/D S: the distance between two active sites D: single bubble departure diameter Pool boiling artificial surface; (Zhang, 2002) natural surface polished by sandpaper; (Cheng, 2018) The horizontal and declining coalescence only occur in the range when S/D<1.5 4
Air-water isothermal system Shoji(2001) and Zhang(2002) investigated the bubble behaviors from a single orifice submerged in water with different air flow rates, to investigate the interactions between the successive bubbles. Tange (2004) analyzed the bubbling features from twin submerged orifices. Comparing with the bubble behavior in boiling system Experimental setups of twin orifices (Tange, 2004) Isothermal system Boiling system In analogy with Orifices Active sites Chamber pressure Air flow rate Chamber Volume Heater superheat Vapor generation rate Thermal capacity of heater surface 5
Objective: To investigate the bubble behaviours from submerged orifice in isothermal system, and will help better understanding the boiling mechanisms from the perspective of hydrodynamic interaction. Content: In current work, a series of numerical experiments based on OpenFOAM have been carried out. The bubble dynamics from single submerged orifice and the interaction between successive bubbles. The horizontal coalescence processes of bubbles from twin orifices are simulated. 6
Numerical Model Governing equations u 0 (Continuity equation) t u uu p g fσ (Momentum equation) t u 0 ( Liquid volume function equation) t OpenFOAM version :V3.0.x Solver: interfoam Model: VOF track the two-phase interface Closure relations Two-phase mixture density: 1 1 g 1 l Continuum Surface tension: Curvature in the two-phase interface: f The solver flow chart 7
Conditions for the Simulations Computational domain Width 30mm air space Initial water level total pressure Atmosphere 10mm Boundary conditions Walls and bottom : no-slip Atmosphere: total pressure Air inlet : fixed air flow rate 100 cc/min~2000cc/min Height 60mm water Physical parameters Air density, ρ g 1.1691 kg/m 3 y z Walls no-slip x D o =2mm air inlet fixed flow rate Bottom no-slip Water density, ρ l 998.2 kg/m 3 Air kinematic viscosity, ν g 1.48e 10-5 m 2 /s Water kinematic viscosity, ν l 1.05 10-6 m 2 /s Surface tension, σ 0.073 N/m 8
Examination of Grid Independency Examination of Grid Independency Grid Mesh Departure diameter D b Bubble periods τ d 60 120 0.5mm 3.5mm 205ms 120 240 0.25mm 4.3mm 220ms 150 300 0.2mm 4.6mm 225ms 180 360 0.167mm 4.6mm 225ms 240 480 0.125mm 4.6mm 225ms the accuracy the computational time Mesh 0.2mm,the bubble departure with the same diameter 4.6 mm at the same time, 225ms. 9
Bubbles Evolution from Single Orifice 10 Zhang s Experiment,2001 1 100cc/min No interaction between bubbles 1 1 2 3 3 3 1 1 2 Comparison of Bubble behaviors in different air flow 1 2 2 800cc/min Bubble collision Bubble2 is accelerated by the wake flow of bubble1 and collides with bubble1. 1500cc/min Coalescence in vertical Bubble2 elongates quickly in vertical and is pulled up and absorbed into the upper bubble rapidly within a very short time interval of 20ms. 100cc/min 900cc/min 1500cc/min
Bubble Evolution from Single Orifice : existent in the case; : non-existent in the case. Air flow rate (cc/min) Without interaction Bubble collision Vertical coalescence 100 200 500 800 1000 1500 2000 Evolution of bubble behaviors with different air flow rate q 200cc/min without interactions between bubbles. 500cc/min q 1000cc/min bubble collision will occur between two or several successive bubbles. q 1500cc/min vertical coalescence will bring about near the orifice. This phenomenon is similar with the boiling experiment results that, the vertical coalescence will only occur at high heat flux region. (Hutter,2009;Cheng,2018) 11
Departure diameter The effect of the air flow rate on bubble departure diameter As the air flow rate increases from 100cc/min to 2000cc/min, the bubble departure diameter climbs from 5.4mm to 14.0mm. The present prediction obtains a good agreement with Zhang s experiment (2001). 12
Twin Bubbles Coalescence From Double Orifices D b =5.4mm S=9mm (a): S=9mm, D b =5.4mm, S/D=1.67 Time sequence of simulated result q=100cc/min,d b =5.4mm S: the distance between orifices D: the single bubble departure diameter when S/D=1.67, bubbles grow and rise as isolated bubbles without coalescence which are similar to the bubbles in two single orifices. 13
Twin Bubbles Coalescence From Double Orifices S=8mm (b): S=8mm, D b =5.4mm, S/D=1.48 Coalescence in horizontal occurs near the surface As coalescence occurs, two bubbles merged into one bigger bubble and departures form the orifices as a whole. After the coalescence bubble departures at 635ms, two new bubbles grow from the orifices as shown at 720ms. 14
Twin Bubbles Coalescence From Double Orifices S=5mm (c): S=5mm, D b =5.4mm, S/D=0.93 Coalescence occurs in horizontal at 215ms Only one new bubble forms from two orifices. The two inlets cooperate and offer air for the single bubble. 15
Summary A numerical simulation has been presented with VOF model based on OpenFOAM to reveal the evolution mechanisms of bubble behaviors from single or twin orifices. Results show that: Vertical coalescence and collision between successive bubbles only occurs in cases with larger air flow rate, which should been owing to the strong wake flow from the previous bubble. Horizontal coalescence will not occur when S/D > 1.5, supporting the results of boiling experiments recently. Although the current research is based on isothermal system without phase change, the results may be helpful to understand the mechanism of the coalescence during boiling process. 16
Thank you! Acknowledgements: This work is financially supported by National Key Technology R&D Program (NO.2013GB11005B).
Next Work
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