Post on 06-Oct-2020
Impact characteristics of liquid nitrogen droplets
Michiel AJ van Limbeek,
Thomas Nes, Marcel ter Brake and Srinivas Vanapalli
Spray cooling
π
Hot plate
Drop
αΆπ ?
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Heat transfer
Phase change
Impact dynamics
Impact Target
β’ Roughness
β’ Material
β’ Geometry
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Frustrated Total Internal Reflection (FTIR)
Camera
Drop
Prism
Laser
ΞΈ
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.
π π€ππ‘
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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
β’ Single drop β’ 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
ππ‘π
ππΏ
Ξπ‘π β‘ππΏ β ππ‘ππππππ‘
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Rescaling
Exp Data from multiple referenced papers14
Impact timescale
π· = π‘πβ’How long does the drop take heat?
β’ Impact timescale π = π·/π
β’ Contact time (non LF) drop sticks until evaporated
β’ Capillary timescale (LF)
πHot plate
Drop
πHot plate
Drop
ππππ =ππ·3
8πΎ
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Dominant heat transfer mechanism
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αΆπππππ/ αΆπππ£ππ
β’ αΆπππππ~π΄π€ππ‘ πππππππβππ
πΌππ‘
β’ αΆπππ£ππ~πΏππ αΆπππ β πΏππ0.2 (πππππ β ππ ) [1]
β’αΆπππππαΆπππ£ππ
~π΄π€ππ‘
πΏππ
ππ
0.2 πΌππ‘
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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
β’ 1D Heat equation ππ‘π = πΌππ₯
2π
β’ Heat flux across the vapour layerππ ππ₯π = β π β ππ ππ‘
π₯π π₯, π‘
π π₯, 0 = π0ππ , ππ , πΆπ,π
ππ ππ‘
Ξ = expt
ππ‘βerfc ΰ΅t ππ‘β
where Ξ =ππ₯=0βππ ππ‘
π0βππ ππ‘and ππ‘β = ππ ππ πΆπ,π β
β2
0 π
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Thermal properties of impactor
β’ Thermal timescaleΞ€πππΆπ β2~0.1 s βͺ π·/π
β’ 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 ππΏ
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Solid properties
Property Sapphire 80K Sapphire 300K Glass 300K unitsDensity ππ 4000 4000 2520 kg/m3
Specific heat πΆπ,π 100 776 816 kJ/kg K
Thermal conductivity ππ 1000 32 1 W/K m
Thermal diffusivity πΌπ = ΰ΅ππ
ππ πΆπ,π 1β10-3 1β10-5 4β10-7 m2/s
Thermal timescale ππ‘β 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
ππ‘β = ππ ππ πΆπ,π ββ2
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Competition of two timescales
β’ Impact timescale ππππ = β1 ms
represents the contact or residence time of the drop near the surface
β’ Cooling effects become relevant when ππ‘β β ππππ
which is the case for a glass surface:0.3 ms β1 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
β’ We studied the impact of liquid nitrogen drops and obtained the phase diagram for the dynamic Leidenfrost effect
β’ 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.
β’ A strong correlation between the cooling of the sapphire and the wetting behavior was observed.
β’ Conductive cooling is stronger than evaporation
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