RMFM 2016 Presentation_v03_FINAL

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Experimental Study on Foam Generation in Tunneling Industry

Scott Jarriel, Dhrupad Parikh, Yuanli Wu, Dr. Mike Mooney, Dr. Nils Tilton

Foam in Tunneling2

Why condition soil?

Soil conditioning refers to the mixing of soil and conditioning agents, like foam, at the cutter head of the TBM to alter the soil properties.

Control face pressure

Reduce torque

Reduce wear of metallic parts

Improve workability

Reduce clogging

Image courtesy of BASF

Foam Generation

Image courtesy of BASFImage courtesy of BASF

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Foam Generation: The Foam Gun

Air

Water + Surfactant

Inlet

Foam

Outlet

Air and Liquid Mixing

Beads or other filling

Foam Gun

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Desired foam properties

“If the foam sticks to the palm of your hand while it is facing the ground that is great foam.”

• Operator’s Idea of Good Foam

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• Qualitative Analysis of Good Foam Thewes et al. (2012) has defined certain requirements of foams

• High stability• Homogenous bubble size

However, little literature exists on the mechanical system that

generates the foam.

Motivation and Objectives

• The mechanics of foam generation are not fully understood.• Most studies assume foam is generated by turbulent

mixing.• Role of porous material not understood.

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Motivations

Objectives• Understand the mechanisms of foam generation.

• Quantify the influence of system parameters on foam quality.

• Investigate competing role of turbulence and porous material on the creation of foam.

Image courtesy of BASF

Experimental Study on Foam Generation

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Foam generation system:

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Foam gun:

Qair

Q(water + surfactant)

Foam

Porous Material: Bead Diameter Pore Size Permeability

Geometry of the Foam Gun

• Length = 11 cm• Diameter = 1.5 cm

Outlet Inlet

Experimental Study on Foam Generation

Experimental Study on Foam Generation

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Porous media used for experiments Beads (1 mm, 2 mm)

2 mm1 mm

Experimental Study on Foam Generation

Pressurized Air In

Air Mass Flow Controller

Liquid Flow Controller

Tank Containing Water and Surfactant

Foam

G

un

Air to Pressurize Tank

Foam

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Measurement of Foam Properties

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Foam Bubble Size and Distribution:

Measure the foam bubble size and distribution under pressure

Foam Expansion Ratio (FER)

•We use an FER of 15, as an industry.

𝐹𝐸𝑅0=𝑉 𝑓𝑜𝑎𝑚

𝑉 𝑙𝑖𝑞𝑢𝑖𝑑¿𝑄𝑎+𝑄𝑙

𝑄𝑙

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Hypothesis• Many parameters affect foam generation, • Here, we focus on the influence of bead size and

flow rate on the bubble size distribution and foam stability.

• Initial Hypotheses:▫ Increased flow rates should decrease bubble size due to

turbulent mixing.▫ Increased bead size should increase bubble size.▫ Foam stability increases with decreasing bubble size.

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Bubble Size Analysis

• Distributions are “noisy.”

• 1mm beads produce a Gaussian (normal) distribution of bubbles.

• 2mm beads produce a Weibull distribution.

• Weibull distribution possibly due to foam gun edge effects.

0 0.1 0.2 0.3 0.4 0.5 0.60.0%

5.0%

10.0%

15.0%

20.0%

25.0%

30.0%

35.0%

40.0%Frequency distribution for 2mm beads

Qa2.6Qa3.8Qa5.8Qa7.4Qa8.3Qa9.7Qa10.7Qa12.5Qa13.8Qa15.5

Avg. Bubble diameter (mm)

Freq

uenc

y (%

)

-0.4 0.1 0.60.0%

5.0%

10.0%

15.0%

20.0%

25.0%

30.0%

35.0%

40.0%Frequency distribution with 1mm beads

Qa2Qa4Qa6Qa6.6Qa7.5Qa7.8Qa8.5Qa9.7Qa10.9Qa12.5Qa13.5Qa15.5

Bubble Diameter (mm)

Freq

uenc

y (%

)

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Bubble Size Analysis1 mm Bead Size Results

2 4 6 8 10 12 14 16 18-5.55111512312578E-17

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Avg. Bubble diameter VS Air flow rate for 1mm beads

1mm beads

Linear (1mm beads)

Air flow rate (slpm)

Avg.

Bub

ble

diam

eter

(mm

)

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Bubble Size Analysis1 mm Results vs 2 mm Results

0 2 4 6 8 10 12 14 16 18-5.55111512312578E-17

0.05

0.1

0.15

0.2

0.25

0.3

0.35

Avg. Bubble diameter VS Air flow rate for 1mm & 2mm beads

1mm beads

Linear (1mm beads)

2mm beads

Linear (2mm beads)

Air flow rate (slpm)

Avg.

Bub

ble

diam

eter

(mm

)

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Stability of Foam

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Foam Half-Life [ t50 ]

Depends on: Surfactant Concentration Foam Expansion Ratio Surface Tension, Viscosity & Film Thickness

Time required for 50 % liquid to drain from the foam

Measured using a funnel.

Experimental Results: Foam Stability

0 2 4 6 8 10 12 14 16 184

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6

7

8

9

10Half of foam for 1mm and 2mm beads

Half life 2mm beadsHalf life 1mm beads

Air flow rate (SLPM)

Hal

f lif

e (m

ins)

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Conclusions•Bead size significantly affects the bubble

size distributions.

•Flow rates have little to no effect.

•Both bead size and flow rates have an effect on foam stability.

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Looking Forward• Future tests will explore:▫Foam gun geometry (diameter and length)▫Foam gun filling▫Effect of higher pressures encountered in

underground conditions▫FER▫Surfactant concentrations

• Long term, we hope to eventually tailor foam properties to varying soil conditions and allow for more efficient and effective underground tunneling.

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