- The document investigates how a vertical-downward elbow affects two-phase flow distribution compared to a vertical-upward elbow through CFD simulation and experimental data. - For two-phase flow through a vertical-downward elbow, the void fraction has a single peak along the inner wall that dissipates downstream, with little influence from secondary flow patterns. - This behavior is dramatically different than for a vertical-upward elbow and shows elbow-specific flow models are needed to capture these differences critical to reactor safety analysis.

1. The Advanced Multi-phase Flow Laboratory, Department of Mechanical and Nuclear Engineering
Vertical-Downward Elbow Restriction on Two-Phase Flow
Sponsored by Toshiba-Westinghouse Fellows Program
OBJECTIVE: To investigate the geometric effects of a vertical-downward elbow on
two-phase flow by comparing single-phase CFD simulation and two-phase flow data.
SIGNIFICANCE
SUMMARY
• Void fraction has a single-peaked distribution which dissipates
after a vertical-downward elbow.
• Void fraction distribution shows weak relationship with
secondary flow downstream of vertical-downward elbow
• Vertical-downward elbow shows dramatically different trends
than vertical-upward; thus elbow-specific models are required
RESULTS
METHODS AND FACILITY
FUTURE WORK
• Obtain additional data at different locations and flow rates for database
• Develop predictive models for two-phase flow restrictions
• Implement new models to reactor thermal-hydraulic system analysis code
• In disaster situations, two-phase flow is induced during a loss of coolant
accident (LOCA) when a leak or pipe break depressurize the primary loop of
pressurized water reactors (PWRs).
• Little is known about two-phase flow restrictions; database creation vital
• Improve reactor thermal-hydraulic codes for enhanced safety analysis by
implementing models created from database of restricted flow
Single-Phase Simulation
● Counter-rotating vortices
produce swirling
● Low velocity water emanates
from inner wall; high velocity
water concentrated near
outer wall.
Two-Phase Experiment
● Single peak of void fraction
along the inner wall.
● Void fraction ‘ridge’ along
pipe wall at 0D
● Partial dissipation of peak
at 3D
● Evidence of secondary
bimodal peaking at 3D
● Negligible bubble
entrainment by swirling
for vertical-downward
elbow.
● Upstream conditions
and pressure
distribution cause
single-peak void fraction
distribution
● Previous research for
vertical-pward elbow
shows bimodal peaking
as bubbles are entrained
in secondary flow. (jf
=3.00 m/s; jg, atm =0.14
m/s)
Comparison
Test Facility
CFD Methods
Experimental Methods
● ANSYS CFX computational fluid
dynamics solver used for analysis
● Entire loop geometry simulated
● RNG k-ε turbulence model
● High quality mesh
● 50.8 mm ID acrylic test
section
● 90° glass elbows
● Development Length:
○ Vertical: ~60D (~3 m)
○ Horizontal: ~180D (~9 m)
Four-Sensor Conductivity Probe
Measurement principle
Kim et al., 2014
120 data points per cross-section
Andrew Hardison, Robert Morris University Philip Graybill, Grove City College
Mesh
cross-section
Probe captures time-averaged
local two-phase flow parameters
(Nb, fb, Vb, α, ai, & Dsm)
Four-Sensor Conductivity
Probe Schematic
● Adiabatic air-water
● Dual injection
● 24 measurement
locations
Measurement PointsVolumetric liquid flux (jf ) : 4.00 m/s
Volumetric gas flux (jg, atm ) : 0.23 m/s
L=3DL=0D
Bulk velocity (Ubulk ) : 4.00 m/s
Void
Fraction L=0D L=3D
Previous
Work
Vertical-UpwardVertical-Downward
L=3D
L=0D
Previous
Work
(Yadav, 2013)