Impact characteristics of liquid nitrogen droplets...แถ / แถ โข แถ ~๐ด ร ๐ โ๐...
Transcript of Impact characteristics of liquid nitrogen droplets...แถ / แถ โข แถ ~๐ด ร ๐ โ๐...
Impact characteristics of liquid nitrogen droplets
Michiel AJ van Limbeek,
Thomas Nes, Marcel ter Brake and Srinivas Vanapalli
Spray cooling
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Hot plate
Drop
แถ๐ ?
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Heat transfer
Phase change
Impact dynamics
Impact Target
โข Roughness
โข Material
โข Geometry
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Frustrated Total Internal Reflection (FTIR)
Camera
Drop
Prism
Laser
ฮธ
Shirota, M., van Limbeek, M. A. J., Lohse, D., & Sun, C. (2017). Measuring thin films using quantitative frustrated total internal reflection (FTIR). The European Physical Journal E, 40(5), 54.
๐ ๐ค๐๐ก
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Impact dynamics: short timescales
Ethanol drop impacting at U=2m/s on a cold sapphire surface
Shirota, Minori, et al. "Dynamic Leidenfrost effect: relevant time and length scales." Physical review letters 116.6 (2016): 064501. 5
Impact dynamics: heatingLiquid nitrogen on heated sapphire (smooth surface)
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Two types of measurements
โข Single drop โข Drop stream
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Experimental Set-up
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Results
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U=1.3 m/s T=82 K
Slowed down 500x
ResultsIncreasing plate temperature
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U=1.3 m/s T=92 K
Slowed down 500x
ResultsIncreasing plate temperature
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U=1.3 m/s T=102 K
Slowed down 500x
Results
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Ethanol vs LN2
๐๐ก๐
๐๐ฟ
ฮ๐ก๐ โก๐๐ฟ โ ๐๐ก๐๐๐๐๐๐ก
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Rescaling
Exp Data from multiple referenced papers14
Impact timescale
๐ท = ๐ก๐โขHow long does the drop take heat?
โข Impact timescale ๐ = ๐ท/๐
โข Contact time (non LF) drop sticks until evaporated
โข Capillary timescale (LF)
๐Hot plate
Drop
๐Hot plate
Drop
๐๐๐๐ =๐๐ท3
8๐พ
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Dominant heat transfer mechanism
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แถ๐๐๐๐๐/ แถ๐๐๐ฃ๐๐
โข แถ๐๐๐๐๐~๐ด๐ค๐๐ก ๐๐๐๐๐๐๐โ๐๐
๐ผ๐๐ก
โข แถ๐๐๐ฃ๐๐~๐ฟ๐๐ แถ๐๐๐ โ ๐ฟ๐๐0.2 (๐๐๐๐๐ โ ๐๐ ) [1]
โขแถ๐๐๐๐๐แถ๐๐๐ฃ๐๐
~๐ด๐ค๐๐ก
๐ฟ๐๐
๐๐
0.2 ๐ผ๐๐ก
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[1] S. Herbert, S. Fischer, T. Gambaryan-Roisman, and P. Stephan, Local heat transfer and phase change phenomena during single drop impingement on a hot surface, IJHMT, vol. 61, 2013
Area vs contact line
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Dominant heat transfer mechanism
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Role of impact target: thermal propertiesIs the target isothermal during the impact?
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Thermal timescale
โข 1D Heat equation ๐๐ก๐ = ๐ผ๐๐ฅ
2๐
โข Heat flux across the vapour layer๐๐ ๐๐ฅ๐ = โ ๐ โ ๐๐ ๐๐ก
๐ฅ๐ ๐ฅ, ๐ก
๐ ๐ฅ, 0 = ๐0๐๐ , ๐๐ , ๐ถ๐,๐
๐๐ ๐๐ก
ฮ = expt
๐๐กโerfc เตt ๐๐กโ
where ฮ =๐๐ฅ=0โ๐๐ ๐๐ก
๐0โ๐๐ ๐๐กand ๐๐กโ = ๐๐ ๐๐ ๐ถ๐,๐ โ
โ2
0 ๐
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Thermal properties of impactor
โข Thermal timescaleฮค๐๐๐ถ๐ โ2~0.1 s โช ๐ท/๐
โข Isothermal behaviour
MAJ van Limbeek et al. Vapour cooling of poorly conducting hot substrates increases the dynamic Leidenfrost temperatureInt J Heat&Mass Transfer 97 (2016): 101-109
300K
Sapphire100K
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Test setup
J.W. Ekins Experimental Techniques for Low-TemperatureMeasurements Oxford University Press 2006
D
U
TemperatureSensor
30x
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Droplet stream
D
U
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Droplet stream
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Results
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U=1.6 m/s
Slowdown 5x
Results
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Varying the velocity Zoom
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Thermal effect of plate changes ๐๐ฟ
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Solid properties
Property Sapphire 80K Sapphire 300K Glass 300K unitsDensity ๐๐ 4000 4000 2520 kg/m3
Specific heat ๐ถ๐,๐ 100 776 816 kJ/kg K
Thermal conductivity ๐๐ 1000 32 1 W/K m
Thermal diffusivity ๐ผ๐ = เต๐๐
๐๐ ๐ถ๐,๐ 1โ10-3 1โ10-5 4โ10-7 m2/s
Thermal timescale ๐๐กโ 100 15 3 ms
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Delayed touchdown on glass
T=292oC U=3.8m/s31
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Glass Phase diagram
van Limbeek, Shirota, Sleutel, Prosperetti, Sun and Lohse, IJHMT 2016
Leidenfrost boiling
Transition boiling
Contact boiling
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Surface cooling
๐๐กโ = ๐๐ ๐๐ ๐ถ๐,๐ โโ2
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Competition of two timescales
โข Impact timescale ๐๐๐๐ = โ1 ms
represents the contact or residence time of the drop near the surface
โข Cooling effects become relevant when ๐๐กโ โ ๐๐๐๐
which is the case for a glass surface:0.3 ms โ1 ms
Drop diameterImpact velocity
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Micro droplet
T=265oC T=305oC
U=10 m/s, slowed 20k
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Conclusion/Overview
โข We studied the impact of liquid nitrogen drops and obtained the phase diagram for the dynamic Leidenfrost effect
โข The high thermal conductivity of the sapphire impact target enables us to measure the temperature of the plate during spray cooling and relate it to the wetting behavior using FTIR imaging.
โข A strong correlation between the cooling of the sapphire and the wetting behavior was observed.
โข Conductive cooling is stronger than evaporation
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