MICROBUBBLES. mechanism of their generation - and application in producing biofuels. SUMMER SCHOOL Prague Václav TESAŘ

Transcription

1 SUMMER SCHOOL Prague 2014 MICROBUBBLES mechanism of their generation - and application in producing biofuels 1 Václav TESAŘ Institute of Thermomechanics Academy of Sciences of the Czech Republic

2 SUMMER SCHOOL Prague Microbubbles OVERVIEW Part I A) Basics of gas bubbles B) Generation of small bubbles 9:00 10:10 90 slides C) Fluidic no-moving moving-part oscillators D) Generating microbubbles by fluidic oscillation 2

3 SUMMER SCHOOL Prague Microbubbles OVERVIEW E) Geopolitics of cheap crude oil Part II 10:15 10:40 30 slides F) Biofuels from algae -- microbubbles supply CO 2 necessary for photosynthesis 3

4 A) Basics of gas bubbles 1. Bubbles for engineering processes 2. Interface between the phases, surface tension 3. Problem of obtaining small bubbles 4. A note on nanobubbles 5. Coalescence of microbubbles 4

5 Bubbles are key factor in : Waste water treatment Bioreactors for growing unicellular organisms Separation by froth flotation Oxidative leaching of plutonium Extracting crude oil from aged oil wells Extracting bituminous tar sands 5

6 Supplying CO 2 Air CLASSICAL APPROACH Problem: generated bubbles are too big, typical diameter ~ 8 mm. Relatively small surface => diffusion transfer into the water is not efficient 6

7 Current effort to make aeration bubbles small: Air percolated through sub- millimetre passages e.g. in sintered aerator bodies Small size: operational problems, easily blocked and clogged 7 Causes problems (clogging) and does not help and does not help Estimates (Hãnel( nel,, 1998): typically less than 40 % of the orifices actually do operate.

8 PRESENT-DAY AERATORS A typical example: Porous top surface : sintered from sub-millimetre HDPE spheres (High-density polyethylene) 8 Pores of 0.12 mm equivalent diameter

9 PRESENT-DAY AERATORS Average bubble size: mm 5.7 mm 9

10 10 MECHANICS OF BUBBLE FORMATION missing!

11 Surface tension σ force per unit length Young-Laplace surface tension law. Pressure difference across the air/water surface is inversely proportional to the curvature radius r. Gravity effect : difference between sessile and pendant shape The same equation for bubbles and drops 11

12 12

13 BUBBLES and DROPS: initial stage in formation Gravity effect : difference between sessile and pendant shape 13

14 BUBBLES: Nonlinear differential equation same for sessile drop or pendant bubble 14

15 BUBBLES: pendant bubble. BUBBLES: Equation solved numerically Syringe 15

16 Same equation: pendant drop or or sessile bubble Important result: bubble spherical for r<1 mm Syringe 16

17 Problem of SMALL BUBBLES Why they are needed in technological processes: Rapid diffusive transport gas-to to-liquid 17 For the same volume: If bubbles are small: large total surface for diffusion transport

18 Also: BUBBLE RISING VELOCITY SLOPE : a 3 times smaller bubble remains 10 times longer in the water 18 Small bubbles: More time for diffusion transport

19 MICROBUBBLES Ideal solution: MICROBUBBLES Definition: less than 1 mm in diameter - but more than 1 μm in diameter Potential advantages were known for decades, but existing microbubble generation methods (e.g., ultrasound,...) were energetically ineffective 19

20 NANOBUBBLES 20 Nanobubbles are extremely stable due to the absorption of ions on their surface. The gas inside the nanobubble does not come in contact with the bulk liquid, allowing the nanobubble to last for a much longer time.

21 Bubble coalescence: Processes too small and too fast, invisible to an eye High-speed camera: frames / sec 21

22 Large bubbles move away fast as soon as they separate from the aerator 22

23 But Small microbubbles are slow - and fail to move away. This causes them to coalesce with the follower bubbles 23

24 Even if generated, microbubbles do not last: CONJUNCTIONS A B A B 24

25 Typical doubling coalescence: 25

26 Actual conjunction is extremely fast : 26

27 Typical chasing type coalescence The larger bubble (below) is faster - it chases the small and slow predecessor and coalesces with it. 27

28 Also fast repeated coalescences: Larger bubble should move faster! 28 Lower speed of oscillating bubbles (note the small slope): they are larger and yet slower

29 Tripling coalescence: 29

30 Multiple small coalescences: 30

31 B) Problems of microbubble generation 1. Initial idea: fragmentation by a fluidic oscillator 2. Instability of parallel bubble formation 3. Bubble self-excited excited oscillation: volume, shape 4. Resonant frequency of shape oscillation 5. Growth by conjunction 31

32 SOLUTION TO THE PROBLEM OF SMALL BUBBLES: It worked but I did not know why OSCILLATOR in the air supply inlet 32

33 IMPORTANT: FLUIDIC (no-moving moving-part) OSCILLATOR PATENT 33

34 Instability of parallel bubble formation Pressure inside inversely proportional to radius 34 Stability lost on reaching the hemispherical shape

35 Instability of parallel bubble formation 35 Stability requires positive slope

36 FRAGMENTATION: Bubbles as well as drops disintegrate due to growing amplitudes of surface waves 36

37 Bubble oscillation following the coalescence: Natural frequency of oscillation: Knowledge was needed for the intended fragmentation by a fluidic oscillator 37 High-speed camera: frames / sec

38 OSCILLATORY MOTIONS after the conjunction No significant volume changes 38

39 OSCILLATORY MOTIONS after the conjunction 39

40 OSCILLATORY MOTIONS after the conjunction Quite different from cavitation studies (Rayleigh 1917): Here is bubble volume constant! 40

41 OSCILLATORY MOTIONS after the conjunction 41

42 LINEAR OSCILLATOR (damping neglected) Known Known 42 Evaluated J

43 SURFACE ENERGY It is possible to evaluate the capacitance C 43

44 44 CAPACITANCE

45 INERTANCE J Bubble inertia is negligible: Mass of oscillating water 45

46 46 EVALUATED INERTANCE:

47 47 Weber number

48 Weber number Evaluated from experimental data 48

49 49 FREQUENCY OF UNDAMPED OSCILLATION

50 C) Fluidic oscillators for microbubble generation 1. Fluidics and microfluidics 2. Micromanufacturing 3. Fluidic amplifiers 3. Fluidic oscillators; feedback loops 4. Master and Slave configuration 50

51 WHY A FLUIDIC OSCILLATOR? NO MOVING PARTS Advantages of moving part absence: - Reliability (no membrane or spring breakages) - No maintenance - Low cost (almost no assembly, single component) - No electrical connections 51

52 Usual layout Matching problem: Large oscillator flow rate (Re) & Small aerator flow Solved by arranging a relief by-pass 52

53 53 Bistable fluidic valve

54 Laser-cut cuttingting 54

55 PRINCIPLE 0F FLUIDIC AMPLIFICATON Flowfield with weak spot - a sensitive location where small input action X produced a large output effect in Y Output is proportional to input 55

56 Self-excited excited oscillation: due to hydrodynamic instability DESTABILISATION BY NEGATIVE FEEDBACK : decreased input decreased output increased input increased output... Continues periodically 56 Negative feedback: it DECREASES the input

57 FLUIDIC OSCILLATOR: Amplifier & Feedback loop(s) Typical size: b = mm main nozzle width Typical frequency: f = Hz 57

58 Principle of an amplifier with feedback loops 58

59 59 Popular recent subject of investigations

60 STOKES NUMBER SIMILARITY in periodic flow processes For steady flows 60

61 General law of nature: HIGH FREQUENCY requires SMALL SIZE Applies also to fluidics 61

62 HIGH FREQUENCY / SMALL SIZE Seach for new oscillator principles 62

63 Seach for new oscillator principles Short feedback loops Not relying on Coanda attachment Easy feedback flow Jet deflected by pressure difference 63 Pathlines coloured by pressure Tortuous path feedback flow

64 HIGH FREQUENCY requires SMALL SIZE Short feedback loops Not relying on Coanda attachment Seach for new oscillator principles 64 Computed pathlines coloured by velocity magnitude

65 CONFLICTING REQUIREMENTS: Very small size needed for high frequency.. But it means too small flow rates => and insufficient blowing power 65

66 Schematic representation MASTER & SLAVE SOLUTION Small high- frequency oscillator Large, high throughput power stage 66

67 MASTER & SLAVE IDEA Small high- frequency oscillator MASTER SLAVE Large, high power amplifier 67

68 68 New principle: ACOUSTIC WAVE FEEDBACK

69 New oscillator principle: ACOUSTIC-WAVE FEEDBACK 69

70 Microbubble generator layout: Currently: OSCILLATOR + AERATOR separate devices Typical illustration from the Patent NEW INTEGRAL SOLUTION 70

71 Integral design: oscillator & aerator in a single submersible body Oscillator 71

72 INTEGRATED: OSCILLATOR + AERATOR in a single body 72

73 73 REMOVAL OF UNSUPPORTED ISLANDS

74 Feed-back action transported by vortices Vortex rotating in resultant cavities carries the feedback signal 74

75 75 Frequency increase by reducing the vortex chamber size

76 76 Frequency dependence on flow rate

77 D) Generation of microbubbles with the fluidic oscillators 1. Real role of fluidics: Preventing the conjunctions 2. Measurement of diffusion transport efficiency 77

78 MICROBUBBLES grow by MULTIPLE CONJUNCTIONS Because of their slow motion, they come into contact and merge O.5 mm 78 Oscillation can prevent the contact Recorded by veryv ery high speed camera

79 Model aerator for discovering the mechanism that prevents microbubble contact and conjunctions Model aerator 79

80 LABORATORY MODEL 80 Tesař V., Hung C.-H., Zimmerman W.: No-Moving-Part Hybrid-Synthetic Jet Actuator, Sensors and Actuators A, Volume 125, Issue 2, p , 10 January 2006

81 Oscillator can prevent conjunction - by temporary flow direction reversal in each cycle Recorded by veryv ery high speed camera 81

82 Hybrid-synthetic jet of water Short-range range suction Long-range outflow 82

83 Experimental results: Steady blowing With fluidic oscillator 83

84 EXPERIMENTS Bubble size measured optically MALVERN Spraytec Laser light diffraction 84 Average bubble diameter

85 85 Typical experimental results

86 Comparison (the same aerator, same air pressure): With fluidic oscillator Steady blowing flow 86

87 EXPERIMENTS: Measured: Concentration of dissolved oxygen: demonstrated 5-times 5 increase in oxidation rate O 2 chosen instead of CO 2.. Because of easier concentration measurement (dissolution only - while CO 2 enters also a chemical reaction) 87

88 EXPERIMENTS: T 88

89 EXPERIMENTS: Measured: concentration of dissolved oxygen: 89

90 EXPERIMENT: No oscillation T 90

91 EXPERIMENT: With fluidic oscillator T 91