1. RELAP5-3D© to Fluent CFD Software Coupling
Richard R. Schultz
Workshop: VHTR R&D Plan
Hosted by: Academic Center of Excellence for Thermal Fluids and Reactor Safety, Oregon State University
September 19-20, 2005

2. Project Summary & Other Considerations…
RELAP5-3D© & Fluent coupling.
Importance
Benefits
How it was done
Status
Ongoing analysis of VHTR
Other considerations

3. Importance of Coupling Systems Analysis Codes to CFD Codes widely recognized …
IAEA recently hosted a meeting to discuss: “…interest in the application of 3-dimensional CFD software as a supplement to or in combination with system codes, which provide the boundary conditions for CFD codes.” Problems under discussion include:  
·         Evaluation of performance of passive safety features;
·         Local phenomena leading to cladding ruptures;
·         Multidimensional TH in various components;
·         Liquid/gas stratification and interface tracking; and
·         Bubble dynamics in suppression pools.”

4. To Address Such Issues…
Fluent Corporation & INEEL embarked on project to couple Fluent CFD and RELAP5-3D©
Fluent CFD was selected as collaborator by INEEL* because:
Fluent CFD demonstrated high quality—see CFD Forum results: C. J. Freitas, “Perspective: Selected Benchmarks from Commercial CFD Codes,” Journal of Fluid Mechanics, 117, 1995, pp. 208 to 218
Extremely cooperative
High quality standard: ISO 9001:2000
Largest user base and largest suite of applications (& probably largest suite of V&V cases).
____________________
* after contacting CFX, Fluent, and STAR-CD

5. Fluent to RELAP5-3D© Coupling: Advantages
Utilize system-wide macro modeling capability of RELAP5-3D
Model some components of the system in detail using the micro-modeling features of FLUENT
Courtesy of Fluent

6. Benefits of Fluent & RELAP5-3D© Coupling
The performance of the system depends on the flow through each component, and vice versa
Boundary condition information is transferred back and forth between the two codes
Both the system and component behaviors are more accurately predicted
By coupling the calculation, the effect of one on the other can be better understood
Courtesy of Center for Multiphase Research, RPI

7. An Executive program controls the coupling of RELAP5-3D© to other codes using PVM
PVMEXEC
PVM
RELAP-3D
FLUENT
CONTAIN
Synchronous or Asynchronous coupling

8. Coupling Protocol has been extensively checked out..
Most extensively by Bettis Atomic Power Laboratory
Performed calculations with up to three codes coupled to RELAP5-3D© simultaneously, including the CFX CFD code.
Grind time defined by CFD code when coupled with RELAP5-3D©

9. Coupling of RELAP5-3D© & Fluent is Semi-Implicit
RELAP5-3D© & Fluent move forward time step-by-time step to perform calculation.
Calculation only moves forward when convergent solution is achieved in both left and right computational domains.
RELAP5-3D reactor physics can be coupled to Fluent (as one domain—with small additional of work). Completion of this task will create only known example of commercial CFD code with reactor physics.

10. Blowup of Fluent model linked to RELAP5-3D© model

11. Validation Underway Using Gas-Cooled Reactors as Basis
Prismatic reactor is focus
Working fluid: helium
Collaborators: Fluent, General Atomics, and INEEL
Fluent model of lower plenum* will couple with RELAP5-3D© model of balance-of-system.
________________
* Lower plenum CFD calculations provided by Fluent Corporation.

12. RELAP5-3D© coupled to enable detailed analysis of lower plenum flow patterns
Upper
Plenum
RELAP5
model
Core
Balance Of
Plant
RELAP5
model
Lower
Plenum
Fluent model

13. Fluid Behavior in Lower Plenum
Need: Preliminary evaluation of turbulent intensity and mixing characteristics likely to be present in lower plenum (LP) at rated condition.
Use existing CFD model of LP with most representative turbulence model and perform preliminary evaluation of fluid behavior as function of location in LP.
Helium gas entering domain around core support posts
Helium gas exiting domain via Hot Duct
Fluid Domain
Core support post

14. Validation of LP Flow Model
Objective: The flow in the lower plenum of the NGNP will involve multiple jets entering from the core into a crossflow moving toward the exit duct and having to negotiate the presence of rows of support pillars. The modeling strategies (turbulence model, grid characteristics, time stepping, etc.) for simulating this complex turbulent flow must be validated.

15. RELAP5-3D© & Fluent Coupling Summary
Fluent selected as collaborator—Fluent CFD may be capable of analyzing large fraction of required problems for gas-cooled and liquid metal advanced systems
RELAP5-3D© & Fluent are coupled.
The two codes will move forward time-step-by-time step after each code has converged.
Coupling software has been extensively checked.
Presently performing calculations to study mixing in prismatic lower plenum.

16. Relationship of RELAP5-3D© to CFD Codes as a Group…
RELAP5-3D© can be coupled with most, if not all, CFD codes.
Bettis Atomic Power Lab has coupled CFX* with RELAP5-3D© using our Executive—consequently expertise to couple these codes for Gen IV use exists.
STAR-CD has offered us access to their source code—however we haven’t had the resources to pursue their offer.
No problems foreseen in coupling with non-commercial CFD codes.
______________________
* A variant of CFX—source code owned and modified by Bettis

17. Potential Non-Commercial CFD Candidates…
LANL’s T-3 CFD codes.
NPhase: Rensselaer Polytechnic Institute