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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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