Boiling crisis dynamics: low gravity experiments and modeling

Transcription

1 Boiling crisis dynamics: low gravity experiments and modeling V. Nikolayev (1), Y. Garrabos (2), C. Lecoutre (2), D. Hitz (3), D. Chatain (3), R. Guillaument (2), V. Janeček (4,5), D. Beysens (3,4) (1) Dept. of Condensed Matter Physics, CEA-Saclay, France (2) Inst. of Condensed Matter Chemistry of Bordeaux, France (3) Dept. of Low Temperatures, CEA-Grenoble, France (4) Lab. of physics and mechanics of heterogeneous media, ESPCI, Paris, France (5) Present address: ArcelorMittal Global R&D, Maizières-lès-Metz, France Presented by Vadim Nikolayev 1

2 Importance of the boiling crisis Evaporation, boiling very efficient heat transfer transition from nucleate to film boiling Film boiling causes very inefficient cooling e.g. of injectors of cryogenic propellants Phenomenon needs to be mastered inside the propellant tanks Nucleate boiling: (separate vapor bubbles) Film boiling: (quasi-continuous vapor film covers heater) Presented by Vadim Nikolayev 2

3 Triggering mechanism of the boiling crisis: Vapor bubble spreading over the heater liquid saturated vapor (bubble) θ app θ app Apparent contact angle increases with substrate heating Spreading causes increase of the bubble adhesion to the solid increase of bubble residence time coalescence of bubbles vapor film creation Presented by Vadim Nikolayev 3

4 Why apparent contact angle may vary Macroscopic scale ~ mm Microscopic scale (nm - µm) < 1 µm θ app vapor bubble liquid saturated vapor liquid θ micro heater T sat + T heater q = k T / hx ( ) L L Two effects may cause the interface curvature at evaporation: vapor recoil force viscous pressure drop due to microscale liquid flow toward the contact line Presented by Vadim Nikolayev 4

5 Apparent angle increase at evaporation θ app calculated for 10 Mpa and 1 bar Apparent contact angle compared with experimental data of Raj et al. Int. J. Heat Mass Transfer, 2012) (Janeček, Nikolayev, Phys. Rev. E, 2013 ) Presented by Vadim Nikolayev 5

6 Vapor bubble growth simulation with gravity Two vapor bubble growth regimes are identified: bubble departure regime : at low heat flux, the bubble volume increases with no or small apparent angle increase until bubble departure from the heater bubble spreading regime : at high heat flux, the apparent angle grows, dry area under the bubble grows and the bubble departure is retarded due to its stronger adhesion to the heater; lateral coalescence with neighboring bubbles becomes a likely event heater; heater temperature rises boiling crisis bubble departure regime bubble spreading regime color: temperature Presented by Vadim Nikolayev 6

7 Weghtlessness: CHF=0 No bubble departure from the heater Bubble spreading always occurs provided the experiment time is long enough Experiments are needed to prove this theory Presented by Vadim Nikolayev 7

8 Experiment near gas-liquid critical point. What for? Slow down the boiling: D T 0 near critical point (critical slowing down) to observe the boiling crisis mechanism Achieve the boiling crisis with small heat flux: q CHF p c System pressure p But: surface tension 0, bubbles are inexistent in Earth gravity reduced gravity is necessary Presented by Vadim Nikolayev 8

9 ISS experiments: DECLIC Dispositif pour l Etude de la Croissance et des LIquides Critiques Facility for Studies of Crystal Growth and Critical Fluids CNES + NASA, International Space Station (ISS) Experiment Locker Insert (exchangeable) Electronic Locker Flight model of DECLIC DECLIC integrated into Express Rack. One of the inserts (ALI, Alice-like insert) is designed for the critical boiling studies Presented by Vadim Nikolayev 9

10 Dryout in the interferometry cell of ALI/DECLIC The apparent contact angle growth with heating is clearly visible. It causes a complete heater dryout. As soon as the heating stops, the bubbles return to its initial shape. Bubble growth particularity near T c : the bubble volume remains constant, while the vapor mass grows Presented by Vadim Nikolayev 10

11 Direct observation SF 6 cell Transparent film heater and connection plots 10.6 mm LIGHT IN vapor liquid LIGHT OUT CCD cameras Presented by Vadim Nikolayev 11

12 Dry spot growth and coalescence regime Triple contact line receding in direct observation cell of DECLIC/ALI : effect of apparent contact angle growth One can see also the dry spot growth and coalescence until the moment where the heating is stopped. The time in hexadecimal format is shown at the upper left corner (in the internal to DECLIC units 1/23s). Pulse 300 µw at Tc-600µK ends at 2bb2040 Presented by Vadim Nikolayev 12

13 Magnetic gravity compensation experiments Magnetic force acting on a diamagnetic substance: F 2 magn ~ ( ρl ρv ) grad( B ) The field can be configured to compensate for a single substance; we use H 2 at 33K. The exact compensation is achieved at one point chosen to be the heater center. In the rest of the cell there are residual effective gravity (magnetic) forces. In the present experiment the gravity is compensated within 10-2 g in the volume of 25 cm 3. The effective gravity is centripetal (axially symmetric), tends to create a liquid column in the middle of the cell. Presented by Vadim Nikolayev 13

14 Experimental H 2 cell for 2014 LNCMI experiment vapor liquid lateral portholes Cu cell structure insulating ring ITO film lower porthole 45 mirror Presented by Vadim Nikolayev 14

15 Similarity of drying dynamics in H 2 and SF 6 SF mk to critical point (DECLIC ) H 2 10 mk to critical point Bubble spreading over the heater is visible. Dynamics of coalescence to be studied Presented by Vadim Nikolayev 15

16 Recent experiments (Oct 14) at LNCMI Side view Bottom view H 2 at 80mK to T c and 44mW heating Presented by Vadim Nikolayev 16

17 Conclusions Experiments near the critical point present a powerful tool and reveal new facts about boiling at high heat flux otherwise difficult to observe. Our observations are incompatible with hypotheses of macroscopic origin of the boiling crisis (in particular with the Zuber and Katto theories). They show that the triggering phenomenon of the boiling crisis is the dry spot spreading. Two regimes of dry spot growth are identified: separate circular dry spot growth observed far from the boiling crisis) and chain dry spot coalescence regime The objective of the future studies is to find the crossover between these regimes. Due to their versatility, the magnetic gravity compensation experiments are important to bridge the gap between DECLIC and conventional boiling experiments. Presented by Vadim Nikolayev 17

18 Acknowledgements CNES/DECLIC team LNCMI support team CNES fundamental microgravity grant EuroMagnet grant for usage of high magnetic fields Presented by Vadim Nikolayev 18