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CFD Analysis of Pool Boiling over Microstructures
Final presentation of simulation results
By: Yashar Seyed Vahedein
December 12, 2013
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Pure-Heat conduction model
1.Transient conduction
model
2.Mesh-size is optimized
Pure-Heat conduction model
𝐂𝐨𝐧𝐬𝐭𝐚𝐧𝐭 𝐡𝐞𝐚𝐭 𝐟𝐥𝐮𝐱=𝟏𝟎𝟎𝟎𝐖 /𝐦𝟐
𝑇 𝑐
Insulated Walls
0.1m
0.1
m
3
Transient convection
𝐂𝐨𝐧𝐬𝐭𝐚𝐧𝐭 𝐡𝐞𝐚𝐭 𝐟𝐥𝐮𝐱=𝟏𝟎𝟎𝟎𝐖 /𝐦𝟐
𝑇 𝑐
Insulated Walls
0.1m0.
1 m
Heat convection due to movement of liquid –(temperature dependent density)
• Calculation of the heat transfer coefficient (h) Vs. Time
• Calculation of Nusselt Number from the correlations and simulation
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Natural Convection Results
Temperature over time on the surface
0 500 1000 1500 2000 2500 3000 3500 4000 4500290
300
310
320
330
340
350
360
370
f(x) = 7.76637775931171 ln(x) + 288.925301114463
Temperature over time On heater surface
Logarithmic (Tempera-ture over time On heater surface)
Time/Δt
Tem
pera
ture
(K)
∆ 𝒕=𝟎 .𝟓𝐬
Nu(from simulation)
27.94206885
Nusselt Number (from correlation)
66.4535
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Two-phase Flow Simulation Technique- Volume of Fluid
Cavity-Finer Mesh – 0.715 mm
Rest of the surface – Taken as Y axis
How volume of fluid explains a cell consisting of two fluids (phases)
In this model , phase 0 is liquid water and phase 1 is vapor water
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Current Problem Definition and Objectives
Cavity size
Vapor Inlet-Type: Mass flow rate
Liquid
Bubble interface – use of VOF in this modelW/out Phase Change
VOF, By ‘Hirt and Nichols 1982’.
1. Validating simulation• Match bubble shape and
diameter with experimental data.
2. Finding influence area caused by bubble departure • Use shear stress over the surface.
3. Finding influence on heat transfer • Use heat transfer coefficient of
the surface
1.43 mm
Axis of symmetry
25mm
50mm
Using Axisymmetric model
𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡𝑞 ′ ′=10000𝑊𝑚2
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Experimental data from the literature
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Time and diameter of the first bubble departure (from simulation)
• mm
(observed for 8 departures)
Necking
Onset of Departure
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Validation of the Numerical Results
Theoretical Bubble Departure Diameter
(1)
(2)
(3)
Using Cole and Rohsenow 1969
ExperimentBaines and Mori
Simulation
1 𝐵𝑎𝑖𝑛𝑠𝑎𝑛𝑑𝑀𝑜𝑟𝑖 ,2000
Max. error = 9%
Comparison of Bubble Shape
Comparison of Bubble Departure Diameter
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Comparing Bubble diameter in t= 40 ms -start of the necking- and t=42.25 ms – near departure
Comparing the bubble shape and diameter with experimental results over
time
t= 40.0 ms
t= 42.2 ms
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Temperature distribution during bubble departure and growth
During the growth of next bubble t=280.4 ms
On the moment of departure t=253 ms
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Effect of Necking on Shear Stress
• Direction of shear is coupled with the interface
• Change in velocity enforce the change in shear stress
Receding interface
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Finding influence area on heated wall using shear stress
t= 40.0 ms
t= 42.5 ms
t= 44.5 ms
Influence region –close to (Rohsenow, Mikic, Griffith 1969)
Increase in shear stress
2𝐷𝑏
1.82𝐷𝑏
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Finding the Local heat transfer coefficient on the surface
t= 40.0 ms
t= 42.5 mst= 44.5 ms
t= 49.5 ms
Increase in heat transfer coefficient near departing bubble (micro convection)
h= 𝑞 ′ ′
𝑇 𝑠𝑢𝑟𝑓𝑎𝑐𝑒−𝑇 𝑠𝑎𝑡
Future work: Introducing embedded boiling codes to the same VOF-
method.
Heat generation will be supplied to cavity
surface
Liquid
Cavity with connected
walls-
Modeling Phase change using
embedded boiling code or FT method
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