[quarterlbr]

Hello all-

I have a steady-state (multiphase) model of a pipe carrying superheated steam into which I am injecting water via the particle injection feature in CFX, with particle evaporation. Without secondary breakup enabled, I can get convergence with reasonable results with 200 particles or so. I'm using IAPWS steam as the continuous fluid, with the CFX H2Og/l combo as my evaporating fluid. My mesh quality I think is ok:
+--------------------------------------------------------------------+
| Domain Name | Orthog. Angle | Exp. Factor | Aspect Ratio |
+----------------------+---------------+--------------+--------------+
| | Minimum [deg] | Maximum | Maximum |
+----------------------+---------------+--------------+--------------+
| Default Domain | 56.0 OK | 7 ok | 5 OK |
+----------------------+---------------+--------------+--------------+
| | %! %ok %OK | %! %ok %OK | %! %ok %OK |
+----------------------+---------------+--------------+--------------+
| Default Domain | 0 0 100 | 0 <1 100 | 0 0 100 |
+----------------------+---------------+--------------+--------------+

With secondary breakup enabled (I've tried a few different models), however, I cannot seem to achieve a similar convergence/stability. I tried remeshing, changing timesteps, and weighting. I use the input from the solved non-secondary model as initial conditions. I have changed the number of particles down to 5. Maybe I just haven't hit the right combination. I've read here elsewhere that I shouldn't have to enable relaxation on any of the equations, so I've tried to avoid that (I do use them for the particle equations). The static temperature and temperature variables during iterations appear to rise well outside of their expected range (shouldn't be above 1350R at the most, but are hitting 3000R+).

I am stumped. I've explored this site (fantastic site, btw), as well as the CFX manuals and internet. I think maybe I just haven't happened upon someone with a similar issue. I've just started a run with the model that successfully solved without secondary breakup using a timestep of 1.0e-6 (this is the smallest I've gone yet) hoping this will help (to see the effect), at the risk of growing old.

At this point, I'm wondering if I have switched an option on here to cause issues and don't realize it. Just baffled that turning on secondary breakup has such an effect.

Any help is appreciated. Beginning of out file posted below.

Thanks in advance,
Charlie

-------------

FLOW: Flow Analysis 1
   SOLUTION UNITS:
     Angle Units = [rad]
     Length Units = [ft]
     Mass Units = [lb]
     Solid Angle Units = [sr]
     Temperature Units = [R]
     Time Units = [s]
   END
   ANALYSIS TYPE:
     Option = Steady State
     EXTERNAL SOLVER COUPLING:
       Option = None
     END
   END
   DOMAIN: Default Domain
     Coord Frame = Coord 0
     Domain Type = Fluid
     Location = B204, B225, B250, B256
     BOUNDARY: Default Fluid Fluid Interface Side 1
       Boundary Type = INTERFACE
       Location = F212.204,F222.225,F249.250
       BOUNDARY CONDITIONS:
         COMPONENT: H2O
           Option = Conservative Interface Flux
         END
         HEAT TRANSFER:
           Option = Conservative Interface Flux
         END
         MASS AND MOMENTUM:
           Option = Conservative Interface Flux
         END
         TURBULENCE:
           Option = Conservative Interface Flux
         END
       END
     END
     BOUNDARY: Default Fluid Fluid Interface Side 2
       Boundary Type = INTERFACE
       Location = F224.225,F252.256,F255.256
       BOUNDARY CONDITIONS:
         COMPONENT: H2O
           Option = Conservative Interface Flux
         END
         HEAT TRANSFER:
           Option = Conservative Interface Flux
         END
         MASS AND MOMENTUM:
           Option = Conservative Interface Flux
         END
         TURBULENCE:
           Option = Conservative Interface Flux
         END
       END
     END
     BOUNDARY: Outlet
       Boundary Type = OUTLET
       Location = OutletPlane
       BOUNDARY CONDITIONS:
         FLOW REGIME:
           Option = Subsonic
         END
         MASS AND MOMENTUM:
           Option = Static Pressure
           Relative Pressure = 112.5 [psi]
         END
       END
     END
     BOUNDARY: Steam Inlet
       Boundary Type = INLET
       Location = InletPlane
       BOUNDARY CONDITIONS:
         COMPONENT: H2O
           Mass Fraction = 0.0
           Option = Mass Fraction
         END
         FLOW DIRECTION:
           Option = Normal to Boundary Condition
         END
         FLOW REGIME:
           Option = Subsonic
         END
         HEAT TRANSFER:
           Option = Static Temperature
           Static Temperature = 704 [F]
         END
         MASS AND MOMENTUM:
           Mass Flow Rate = 55.6 [lb s^-1]
           Mass Flow Rate Area = As Specified
           Option = Mass Flow Rate
         END
         TURBULENCE:
           Option = Medium Intensity and Eddy Viscosity Ratio
         END
       END
       FLUID: Spray Water
         BOUNDARY CONDITIONS:
         END
       END
     END
     BOUNDARY: Walls
       Boundary Type = WALL
       Location = AllWalls
       BOUNDARY CONDITIONS:
         HEAT TRANSFER:
           Option = Adiabatic
         END
         MASS AND MOMENTUM:
           Option = No Slip Wall
         END
         WALL ROUGHNESS:
           Option = Smooth Wall
         END
       END
       FLUID: Spray Water
         BOUNDARY CONDITIONS:
           PARTICLE WALL INTERACTION:
             Option = Equation Dependent
           END
           VELOCITY:
             Option = Restitution Coefficient
             Parallel Coefficient of Restitution = 1.0
             Perpendicular Coefficient of Restitution = 1.0
           END
         END
       END
     END
     DOMAIN MODELS:
       BUOYANCY MODEL:
         Buoyancy Reference Density = 0.1642 [lb ft^-3]
         Gravity X Component = 0 [m s^-2]
         Gravity Y Component = -9.81 [m s^-2]
         Gravity Z Component = 0 [m s^-2]
         Option = Buoyant
         BUOYANCY REFERENCE LOCATION:
           Option = Automatic
         END
       END
       DOMAIN MOTION:
         Option = Stationary
       END
       MESH DEFORMATION:
         Option = None
       END
       REFERENCE PRESSURE:
         Reference Pressure = 0 [psi]
       END
     END
     FLUID DEFINITION: RH Stm
       Material = Steam5 and H2Og VMix
       Option = Material Library
       MORPHOLOGY:
         Option = Continuous Fluid
       END
     END
     FLUID DEFINITION: Spray Water
       Material = H2Ol
       Option = Material Library
       MORPHOLOGY:
         Option = Dispersed Particle Transport Fluid
         PARTICLE DIAMETER CHANGE:
           Option = Mass Equivalent
         END
         PARTICLE DIAMETER DISTRIBUTION:
           Option = Rosin Rammler
           Rosin Rammler Power = 3.76
           Rosin Rammler Size = 566 [micron]
         END
       END
     END
     FLUID MODELS:
       COMBUSTION MODEL:
         Option = None
       END
       FLUID: RH Stm
         COMPONENT: H2O
           Option = Transport Equation
         END
         COMPONENT: Steam5
           Option = Constraint
         END
         FLUID BUOYANCY MODEL:
           Option = Non Buoyant
         END
         HEAT TRANSFER MODEL:
           Include Viscous Dissipation Term = Off
           Option = Thermal Energy
         END
         WALL CONDENSATION MODEL:
           Option = None
         END
       END
       FLUID: Spray Water
         EROSION MODEL:
           Option = None
         END
         FLUID BUOYANCY MODEL:
           Option = Density Difference
         END
         HEAT TRANSFER MODEL:
           Option = Particle Temperature
         END
         PARTICLE ROUGH WALL MODEL:
           Option = None
         END
       END
       HEAT TRANSFER MODEL:
         Option = Fluid Dependent
       END
       THERMAL RADIATION MODEL:
         Option = None
       END
       TURBULENCE MODEL:
         Option = k epsilon
         BUOYANCY TURBULENCE:
           Option = None
         END
       END
       TURBULENT WALL FUNCTIONS:
         Option = Scalable
       END
     END
     FLUID PAIR: RH Stm | Spray Water
       Particle Coupling = Fully Coupled
       Surface Tension Coefficient = 0.04855 [N m^-1]
       COMPONENT PAIR: H2O | H2Ol
         Option = Liquid Evaporation Model
         LATENT HEAT:
           Option = From Material Properties
         END
       END
       INTERPHASE HEAT TRANSFER:
         Option = Ranz Marshall
       END
       MOMENTUM TRANSFER:
         DRAG FORCE:
           Option = Schiller Naumann
         END
         PRESSURE GRADIENT FORCE:
           Option = None
         END
         TURBULENT DISPERSION FORCE:
           Option = Particle Dispersion
         END
         VIRTUAL MASS FORCE:
           Option = None
         END
       END
       PARTICLE BREAKUP:
         Critical Weber Number for Bag Breakup = 6.0
         Option = Reitz and Diwakar
         Time Factor for Bag Breakup = 3.1415927
         Time Factor for Stripping = 20
         Weber Number Factor for Stripping = 0.5
       END
     END
     INITIALISATION:
       Option = Automatic
       INITIAL CONDITIONS:
         Velocity Type = Cartesian
         CARTESIAN VELOCITY COMPONENTS:
           Option = Automatic
         END
         COMPONENT: H2O
           Option = Automatic
         END
         STATIC PRESSURE:
           Option = Automatic with Value
           Relative Pressure = 114 [psi]
         END
         TEMPERATURE:
           Option = Automatic with Value
           Temperature = 700 [F]
         END
         TURBULENCE INITIAL CONDITIONS:
           Option = Medium Intensity and Eddy Viscosity Ratio
         END
       END
     END
     PARTICLE INJECTION REGION: Particle Injection Region 1
       Coord Frame = Coord 0
       FLUID: Spray Water
         INJECTION CONDITIONS:
           INJECTION METHOD:
             Option = Cone
             CONE DEFINITION:
               Injection Centre = 9.23 [m], 0 [m], -1.05 [m]
               Option = Hollow Cone
               Radius of Injection Plane = 0.5 [in]
               INJECTION DIRECTION:
                 Injection Direction X Component = 0
                 Injection Direction Y Component = 0
                 Injection Direction Z Component = 1
                 Option = Cartesian Components
               END
             END
             INJECTION VELOCITY:
               Cone Angle = 30 [deg]
               Injection Velocity Magnitude = 71.58 [ft s^-1]
               Option = Velocity Magnitude
               DISPERSION ANGLE:
                 Dispersion Angle = 5 [deg]
                 Option = Dispersion Angle
               END
             END
             NUMBER OF POSITIONS:
               Number = 200
               Option = Direct Specification
             END
           END
           PARTICLE DIAMETER DISTRIBUTION:
             Option = Rosin Rammler
             Rosin Rammler Power = 3.759
             Rosin Rammler Size = 580.96 [micron]
           END
           PARTICLE MASS FLOW RATE:
             Mass Flow Rate = 8.333 [lb s^-1]
           END
           TEMPERATURE:
             Option = Value
             Temperature = 304 [F]
           END
         END
       END
     END
   END
   DOMAIN INTERFACE: Default Fluid Fluid Interface
     Boundary List1 = Default Fluid Fluid Interface Side 1
     Boundary List2 = Default Fluid Fluid Interface Side 2
     Interface Type = Fluid Fluid
     INTERFACE MODELS:
       Option = General Connection
       FRAME CHANGE:
         Option = None
       END
       MASS AND MOMENTUM:
         Option = Conservative Interface Flux
         MOMENTUM INTERFACE MODEL:
           Option = None
         END
       END
       PITCH CHANGE:
         Option = None
       END
     END
     MESH CONNECTION:
       Option = Automatic
     END
   END
   OUTPUT CONTROL:
     MONITOR OBJECTS:
       MONITOR BALANCES:
         Option = Full
       END
       MONITOR FORCES:
         Option = Full
       END
       MONITOR PARTICLES:
         Option = Full
       END
       MONITOR POINT: Outlet Temp
         Coord Frame = Coord 0
         Expression Value = areaAve(Temperature)@Outlet
         Option = Expression
       END
       MONITOR POINT: Particle Ave Temp
         Cartesian Coordinates = 0.0[m],0.0[m],0.0[m]
         Coord Frame = Coord 0
         Option = Cartesian Coordinates
         Output Variables List = Spray Water.Averaged Temperature
         MONITOR LOCATION CONTROL:
           Interpolation Type = Nearest Vertex
         END
         POSITION UPDATE FREQUENCY:
           Option = Initial Mesh Only
         END
       END
       MONITOR POINT: Particle RMS Temp
         Cartesian Coordinates = 0.0[m],0.0[m],0.0[m]
         Coord Frame = Coord 0
         Option = Cartesian Coordinates
         Output Variables List = Spray Water.RMS Temperature
         MONITOR LOCATION CONTROL:
           Interpolation Type = Nearest Vertex
         END
         POSITION UPDATE FREQUENCY:
           Option = Initial Mesh Only
         END
       END
       MONITOR RESIDUALS:
         Option = Full
       END
       MONITOR TOTALS:
         Option = Full
       END
     END
     RESULTS:
       File Compression Level = Default
       Option = Standard
       Output Equation Residuals = All
     END
   END
   SOLVER CONTROL:
     Turbulence Numerics = High Resolution
     ADVECTION SCHEME:
       Option = High Resolution
     END
     CONVERGENCE CONTROL:
       Maximum Number of Iterations = 2500
       Minimum Number of Iterations = 20
       Physical Timescale = 1e-006 [s]
       Timescale Control = Physical Timescale
     END
     CONVERGENCE CRITERIA:
       Conservation Target = 0.0001
       Residual Target = 0.00001
       Residual Type = RMS
     END
     DYNAMIC MODEL CONTROL:
       Global Dynamic Model Control = On
     END
     PARTICLE CONTROL:
       PARTICLE INTEGRATION:
         First Iteration for Particle Calculation = 40
         Iteration Frequency = 40
         Option = Forward Euler
       END
       PARTICLE TERMINATION CONTROL:
         Maximum Number of Integration Steps = 20000
         Maximum Tracking Distance = 70 [foot]
       END
       PARTICLE UNDER RELAXATION FACTORS:
         Energy Under Relaxation Factor = 0.5
         Mass Under Relaxation Factor = 0.5
         Velocity Under Relaxation Factor = 0.5
       END
     END
   END
 END
 COMMAND FILE:
   Version = 18.1
   Results Version = 18.1
 END

[ghorrocks]

I would recommend:
* Having a close look at the results with particle breakup enabled. Write a backup file before it crashes (if it diverges). The results will not be converged but might give a clue as to why particle breakup is causing problems. Check that they are behaving as you would expect.
* Pay close attention to the region where new particles are being generated, I suspect the spurious temperatures are occurring in this region.
* To check whether there is anything to be gained by improving mesh quality, I would make a simplified geoemetry which can be meshed with 1:1 hexas or close to it. Run that and see if the particle breakup works or not. I know of some models where an aspect ratio of 1.2 is too much, and for these models the CFX built in mesh quality checks are useless. Your model already has IAPWS (which is well known to be numerically unstable), and adding particle breakup might push it over the edge.

[quarterlbr]

Thank you for the advice, Mr. Horrocks. Based on your input, I actually tried to gain a numerical advantage by swapping to the H2O/v/l material set, replacing my IAPWS material (my test run w/o breakup shows this would work for me). So, I hope this gives me more chance for success.

I interrupted a run and took a look at the paused solution with the particle breakup enabled. The issue appears to occur at the outlet of my pipe elbow where it connects to a short straight piece (the geometry is a straight pipe with particle injection, a downstream 90 degree elbow, and a short straight pipe from the elbow outlet). The particle/droplet temperature, according to the particle track, drops below the water temperature coming into the pipe (which I think impossible). This is also the area of the lowest temperature in the entire domain. I didn't see anything suspicious at the injection location except the particle temperature is too low. Image attached of the elbow trace.

I have not set up a simple geometry with the 1:1 hexas yet, but will work on something today if I can't resolve the issues. Based on the interrupted solution observations and suggestions you made, I did go ahead and refine the meshing in the elbow to test it and am trying a smaller physical timestep for the steady-state simulation as well. The residuals have dropped a bit and the monitor points (outlet temps) don't oscillate as much. I am also wondering if the concentration of coincident droplets in a particular volume/element in the elbow would overwhelm the solution, and would I benefit from a coarser mesh at the elbow.

I included some images of residuals and outlet temp monitor that may provide more insight and clarity to my problem that I don't necessarily recognize: first signature is no breakup solution; second is breakup enabled (left it running overnight); and the last part is the refined elbow mesh with smaller physical timestep.

I did notice that when plotting particle temperatures, some strange values appear and maybe I don't understand the plotted variables. For example, on a particle track, if I plot Particle.Temperature, the values are near 70 degrees lower than what should be the lowest temperature in the solution; if I plot Temperature along the particle track, the results seem reasonable. Could this also be an issue for me, or is Particle.Temperature simply a variable used by the solver and has no physical meaning? Or maybe I do understand, and this is actually the root of my problem?

Thanks,
Charlie

[ghorrocks]

You better check the documentation on this, but I suspect Particle.Temperature is the particle temperature and Temperature is the temperature of the continuous phase at that same point. That may explain the difference you are seeing.

I do not have an answer for your main question on why the temperature is not realistic. You are going to have to do some trial and error to work that one out.

[quarterlbr]

I think you are correct. I have been working with a simpler model via your suggestion, and have discovered further questions about the particle fluid temperature effects and the sensitivity to pressure and inlet mass fraction of the fluid. This is a different question than I had before, so I will post in a newer thread concentrating on the particle temperatures themselves.

Thanks for your help.

Charlie