Contaminant transport in water distribution systems is a growing concern
due to the potential for accidental or intentional contamination events.
Understanding and predicting solute transport through water distribution
pipe networks are crucial to mitigate potential contamination events
through risk assessments, vulnerability assessments, and contaminant
source detection.
Problem
|
The transport of contaminants through water-distribution
pipe networks depends on mixing at pipe junctions, where different
flow rates and contaminant concentrations can exist.
Many water-distribution
network models such as the U.S. Environmental Protection Agency's
EPANET software, an industry standard for modeling hydraulic
and water-quality behavior, assume contaminants mix instantaneously
and completely in pipe junctions.
|
|
 |
The assumption of complete-mixing in pipe junctions is contrary to recent experimental
and computational studies, which show that solute mixing in pipe junctions is
incomplete. The common assumption of complete-mixing thus leads to potentially
inaccurate transport predictions. |
Top of page
Computational Approach
Computational fluid dynamics models of pipe joints were developed to provide
detailed visualizations of the mixing behavior within junctions, in order to
guide the development of our new mixing model.
In agreement with our experimental studies,
these rigorous computational studies found that fluid streams entering
a cross-junction tended to bifurcate, depending on the relative momentum
of the fluid streams, which resulted in incomplete mixing.
3D simulation reveals incomplete mixing |
|
Equal flows bifurcate in a
cross-junction |
Top of page
Experimental Approach
A series of experiments were performed to characterize the
mixing behavior within individual pipe joints. Different
joint configurations (“cross” and “double-T”)
were evaluated, along with different pipe diameters, flow rates,
and spacing between “T” fittings. The results of
these experiments were used to calibrate computational fluid
dynamics models for simulation of larger multi-joint
networks.
|
Single-Joint Experiment |
Double-T Configuration |
|
Incomplete mixing was observed for "T-spacings" less than 10 diameters,
and the most prominent incomplete mixing behavior occurred in cross-joints.
The focus of the research was thus directed toward studying the incomplete
mixing behavior in cross-joints.
Small-scale
multi-joint networks consisting of arrays of cross-joints were
also developed to validate multi-joint computational studies.
3x3 network of cross-joints
We have developed from our experimental and
computational studies a new mixing model to predict concentrations of an
aqueous solute resulting from two pipe flows intersecting at a cross-junction.
|
|
|
Our
new Bulk Advective Mixing (BAM) model honors the observed incomplete-mixing behavior
by retaining bulk fluid momentum. It also neglects turbulent diffusivities and
instabilities at the impinging interface. Therefore, Bulk Advective Mixing is
a lower bound to the amount of mixing that can occur in a junction, whereas complete-mixing
is an upper bound.
EPANET-BAM
We implemented the Bulk Advective Mixing (BAM) model into EPANET
2.00.10, open-source software distributed by the U.S. Environmental Protection
Agency that models flow and contaminant transport through water distribution
pipe networks.
EPANET-BAM, our new augmented BAM-enabled version of EPANET, uses a mixing parameter
to scale between the predictions of the complete-mixing and BAM model solute
concentration predictions.
More information, downloads and tutorials on EPANET-BAM are available from the
website: http://www.sandia.gov/epanet-bam/
Top of page
Publications
- Austin, R.G., B. van Bloemen Waanders, S.A.
McKenna and C.Y. Choi, 2008, Mixing
at Cross Junctions in Water Distribution
Systems. II: Experimental Study,
Journal of Water Resources Planning
and Management, 134(3), 295-302. (SAND2007-4120J)
- Gomez, P.R., C.K. Ho, and C.Y. Choi, 2008,
Mixing at Cross Junctions in Water
Distribution Systems. I: Numerical Study,
Journal of Water
Resources Planning and Management, 134(3), 285-294.
(SAND2007-0774J)
- Ho, C.K., 2008, Solute Mixing Models
for Water Distribution Pipe Networks (provides
original derivation of the Bulk Advective
Mixing model), J. Hydraulic Engineering,
134(9), 1236-1244. (SAND2008-0166J)
- Ho, C.K., and S.S. Khalsa, 2007, An
New Model for Solute Mixing in Pipe Junctions:
Implementation of the Bulk Mixing Model
in EPANET, presentation
to EPA, October 11, 2007 (SAND2007-6646P).
- Ho, C.K., C.Y. Choi, S.A. McKenna, 2007,
Evaluation of Complete and Incomplete Mixing
Models in Water Distribution Pipe Network
Simulations, in Proceedings of the 2007
World Environmental and Water Resources
Congress, May 15-19, 2007, Tampa, FL. (SAND2007-0492C)
- Ho, C.K., L. Orear, Jr., J.L. Wright,
and S.A. McKenna, 2006, Contaminant Mixing
at Pipe Joints: Comparison Between Laboratory
Flow Experiments and Computational Fluid
Dynamics Models, in Proceedings of the
2006 Water Distribution System Analysis
Symposium, Cincinnati, OH, August 27-30,
2006. (SAND2006-3583C)
- Khalsa, S.S. and C.K. Ho, 2007, Improving
Contaminant Mixing Models for Water Distribution
Pipe Networks, presented at the 2007
Sandia National Laboratories Student
Internship Program Symposium, Albuquerque,
NM, August 7, 2007. (SAND2008-0066P)
- McKenna, S.A., L. Orear, J. Wright,
2007, Experimental Determination of Solute
Mixing in Pipe Joints, in proceedings of:
ASCE World Environmental and Water Resources
Congress, Tampa, FL, May, Tampa, FL, May
15-19th. (SAND2007-0496C)
- Orear, L., G. Hammond, S.A. McKenna,
P. Molina, R. Johnson, T. O'Hern, and B.G.
van Bloemen Waanders, 2005, Physical Modeling
of Scaled Water Distribution System Networks,
Sandia National Laboratories, Albuquerque,
NM. (SAND2005-6776)
- Webb, S.W. and B.G. van Bloemen Waanders,
2006, High Fidelity Computational Fluid
Dynamics for Mixing in Water Distribution
Systems, in Proceedings of the 8th Annual
Water Distribution System Analysis Symposium,
Cincinnati, OH, August 27-30, 2006. (SAND2006-3834C)
Top of page |
