Charles A. Ward Thermodynamics and Kinetics Laboratory, University of Toronto
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Transcript of Charles A. Ward Thermodynamics and Kinetics Laboratory, University of Toronto
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Charles A. Ward
Thermodynamics and Kinetics Laboratory,
University of Toronto
Fluid Behavior In Absence
Of Gravity: Confined Fluids and Phase Change
Second g-jitter Meeting
Victoria, British Columbia
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Configuration of a Confined Fluid at gConfiguration of a Confined Fluid at g0
Liquid
g
Prediction from thermodynamics
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Apparatus Used on the Space Shuttle
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Position of the Apparatus and Observations on the Space Shuttle
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Thermodynamic predictions
Measure the contact angle at the upper and lower interface...
Average SAMS reading
Average OARE reading
Average values from a confined fluid
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ge>0
Pl >Pu
nSV)l >nSV)u
γSV)l <γSV)u
θl >θu
Summary of the Proposed Mechanism
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Examine the Effect of Adsorption on the Contact Angle of the Water-Glass System
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New Theory
Gibbs adsorption equation, Young Eq.
Statistical mechanics
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Comparison of Isotherms with Measurements
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Mechanism by Which Large Contact Angles on the Space Shuttle are Produced
5°C Space shuttle
observations compared to those in a ground-based laboratory.
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Way it looks and the Way It Should Look!
€
nSV = f (T ,PV )⇒ θ = g(T ,PV )
€
PV − PL = γ LV (1R1
+1
R2)
€
μL = μ V = μ SV = μ SL
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Experimental Apparatus Used to Study Liquid-Vapour Phase Change Processes
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1. Measure in one horizontal direction.
A. No evaporation when pressure was 820 Pa.
B. Pressure in the vapor 775Pa,
j = 0.407±0.006 g/m2s
2. Without opening the system, rotate the 3- dimensional positioner 90° and measure in the second horizontal direction.
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Near the Interface During Steady State Water Evaporation
PIV =593±34Pa
TIL =−0.4±0.05°C
TIV =2.6±0.05°C
j =1.017g/sm2
PIL =617.3Pa
Psat(TIL )=593Pa
Psat(TIV )=766.6Pa
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Temperature During Steady State Evaporation of Water
1. Uniform temperature layer in the liquid near the interface.
2. Thermal conduction below the uniform temperature layer.
3. How does the energy cross the uniform temperature layer?
°
PIV =181.0±0.5Pa
TIL =−16.20±0.02°C
TIV =−10.45±0.01°C
j =1.520±0.003g/ sm2
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Does Marangoni Convection Alone Explain the Uniform Temperature Layer?
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Interfacial Properties During Steady State Evaporation
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Assumed Velocity Profile Near the Interface
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€
σ (R0 ,θ ) = η (1r
∂vr
∂θ+
∂vr
∂r−
vθ
r) r=R0
€
∇γLV • iθ =1R0
(dγ LV
dTIL
)(dTI
L
dθ)€
∇γLV • iθ = σ (R0 ,θ )
€
vθ (R0 ,θ ) = −1η
(dγ LV
dTIL
)(dTI
L
dθ) ln(1−
2δu
R0
)
Determine Tangential Speed from Measured Temperature Profile
Equate tangential surface tension gradient with viscous shear stress
Surface Tension is only a function of temperature
Viscous Shear Stress
Expression for the fluid speed:
€
v(2δu ,θ ) = 0
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Tangential Speed Determined from Thickness of the Uniform-Temperature Layer and Measured Interfacial Temperature Gradient
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Image of Interface and Probe During Steady State Evaporation
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Results Suggest Marangoni Flow is Unstable
€
j = 0.407g
m2sVapor-phase pressure: 776.1 Pa
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Effect of Marangoni Convection on Evaporation
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Comparison of Speed Determined by Two methods
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Probe Position as a Function of Time
When Evaporation is Occurring at Different
(Steady) Rates
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Power Spectra of Probe Oscillations
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If there is no Marangoni
convection, energy conservation is not satisfied!
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Conclusions
1. A fluid confined in a cylindrical container and exposed to the acceleration field of the Shuttle adopts the two-interface configuration, but not the configuration it would be expected to adopt if the system were in equilibrium and the acceleration were ~10-6g0. The configuration adopted corresponds to the configuration expected under equilibrium conditions if the acceleration were greater than 10-4g0.
2. During water evaporation, thermocapillary (or Marangoni) convection exists at the interface. Even in a ground-based laboratory the flow parallel to the interface is oscillatory. At higher evaporation rates, the thermocapillary convection can become turbulent.