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The PHOENICS-Direct Simulation Scenario: Slurry

by James Stewart (CHAM Ltd.) and Gianandrea Messa (Politecnico di Milano)
March - April 2013

Click here for general information about Simulation Scenarios.

Contents

  1. Purpose
  2. Menu items
    1. Geometry
    2. Material properties
    3. Boundary conditions
    4. Output Settings
    5. Computational Grid
    6. Other numerical
  3. Some results

    1. Purpose

    The Slurry scenario simulates the steady, two-phase flow of a sediment-laden fluid through a straight pipe of circular cross-section.

    The purpose of the Slurry simulation scenario (SimScene) is to enable non-CFD experts to predict the pipe pressure gradient, the solids volume fraction distribution and the velocity distribution within the pipe for such a flow set-up.

    2. Menu items

    Each SimScene opens with a 'Top Page containing a descriptive document detailing the features of the specific scenario and its interface. Upon launching the Slurry SimScene the 'Top Page' content is seen as shown below:

    Clicking on the 'Inspect or modify input data' tab from the menu-bar of the Slurry SimScene interface accesses the menu structure for the scenario. It is within this section of the interface that the input data for the case can be viewed and adjusted. The 'Inspect or modify input data' tab for the Slurry Simscene can be seen below:

    Within the left-hand panel of this section of the interface, as seen above, there are six 'parameter groups' into which the input parameters have been divided, these are:

    1. Geometry
    2. Material properties
    3. Boundary conditions
    4. Output settings
    5. Computational Grid
    6. Numerical

    Each of the groups listed above contains parameter(s) governing the case.

    2.1 Geometry

    The previous image shows the contents of the 'Geometry' parameter group, as is indicaed by the blue highlighting of the 'Geometry' button on the left. This group contains a single parameter, namely, 'pipe diameter, m'. The Slurry SimScene user can adjust this setting by editting the contents of the 'white box'. When the simulation is activated, the value within the 'white box' is that which will be used.

    The length of the pipe section for the Slurry scenario is set as 100*D, where D is the user-set pipe diameter. This is to ensure that fully-developed flow conditions are successfully formed within the latter section of the pipe.

    2.2 Material Properties

    Clicking on the 'Material properties' button from the left-and panel of the interfaces causes the following to appear:

    As can be seen from the image above the parameters, and their default settings, governing the material properties for the scenario are displayed within the right-hand panel of the interface. The first two parameters relate to the properties of the fluid used for the case. The user is free to adjust the fluid density and kinematic viscosity to represent their fluid of interest by simply editing the contents of the 'fluid density, kg/m^3' and 'fluid kinematic viscosity, m^2/s' boxes respectively.

    The remaining two parameters govern the properties of the sediment with which the fluid is laden to form a slurry. Again, by entering the desired values appropriately, the user can adjust the 'particle density, kg/m^3' and the 'particle diameter, m'.

    2.3 Boundary Conditions

    Clicking on the 'Boundary Conditions' button from the left-hand panel of the interface accesses the following:

    As can be seen from the above image, the 'Boundary Conditions' group contains two parameters, namely: 'slurry superficial velocity, m/s' and 'delivered solids volume fraction'. To adjust these values, the SimScene user need simply to enter the desired value in the white box adjacent to the parameter they wish to change.

    2.4 Output Settings

    Clicking on the 'Output settings' button from the left-hand panel of the interface accesses the parameters governing the output for the scenario as seen within the image below:

    The parameters seen above determine what will be printed to the inforout file at the end of the simulation. The first parameter, 'print input settings?', simply prints the input data for the case when set to true.

    The 'print deposition velocity and suspended flow indicator?', when set to true, 'tells' EARTH to calculate the particle deposition velocity, based on the user-set data, and then compare this value with the user-set slurry superficial velocity. If the inflow velocity exceeds the deposition velocity, the case is deemed to be in the fully-susended flow regime. In such a case, the suspended flow indicator is 1.0, if the case is not in the fully suspended flow regime, the suspended flow indicator takes the value 0.0. Both the deposition velocity and the suspended flow indiator are then printed to the inforout file as DEP_VELOCITY and FULLY_SUSPENDED respectively.

    The final parameter within the 'Output settings' group, 'print pipe pressure gradient?', will print the calculated pressure gradient along the lenght of the pipe to the inforout file at the end of the simulation run.

    2.5 Computational Grid

    Clicking on the 'Computational Grid' button from the left-hand panel of the slurry SimScene interface will cause the following to be displayed:

    This parameter group contains two parameters relating to the computational grid for the case, namely, 'no. of cells in circumferential direction (x-direction)' and 'no. of cells in axial direction (z-direction)'. By adjusting these values, the grid cell distribution throughout the domain will be altered for the subsequent run.

    The grid distribution in the radial direction is not available to be user-set since the numerics of the case are dependent upon a strict criterion bounding the dimensionless wall distance to the first grid point. As a result, the case is set-up to auto-grid in the radial (y) direction to ensure that this criterion is satisied for all choices of pipe diameter.

    2.6 Numerical

    Clicking on the 'Numerical' button from the left-hand panel of the interface causes the following to appear:

    The 'Numerical' parameter group contains some settings for the PHOENICS solver - EARTH. It is from within this parameter group that the SimScene user can set the 'number of iterations (sweeps)' to be performed by EARTH during the calculation. In addition, the user can also set the '% error cut-off criteria (%)' which is the convergence criteria the solver will adopt for the calculation. When the sums of the errors for each solved for variable within the simulation drop below the '% error cut-off criteria (%)' value, the solver will terminate the calculation.

    The final parameter within the 'Numerical' parameter group, 'restart from end of previous run?', allows the user to specify and active restart for the subsequent run. Doing this 'tells' EARTH to load the phi file from the end of the previous run to obtain the initial settings for all variables within the case. The solver will then perform the specified number of iterations on top of those already computed. For example, performing a preliminary run of 600 iterations followed by a restart run of 600 iterations corresponds to 1200 iterations in total.

    3. Some results

    Upon completion of the calculation, the EARTH - Monitor Plot window will close and the 'View File' tab section of the Slurry SimScene interface will become active, by default displaying the inforout file, as seen below:

    As can be seen from the above image, the first seven values, from 'PIPE_DIAMETER' to 'BULK_VELOCITY provide an echo of the input data for the case. The Slurry SimScene calculates the deposition velocity based on the case settings in a hidden-from-user manner. This value is then printed to the inforout file as 'DEP_VELOCITY' as seen above.

    In addition, the SimScene also compares the user-set slurry inflow velocity with the calculated deposition velocity and provides an indication of whether the case is dealing with fully-suspended sediment load or not. This is displayed by printing, into the inforout file, 'FULLY_SUSPENDED' as 1.0 to indicate fully suspended flow conditions or 0.0 otherwise. As an be seen from above, the default settings for the Slurry SimScene gives a fully suspended flow condition.

    The final value printed to the inforout file, 'PRESS_GRADIENT', gives the computed pressure gradient along the length of the pipe.

    Clicking on the 'Display scenario' button from the SimScene interface menu-bar, as shown within the image below, launches the post-processor - PHOTON.

    The Slurry SimScene contains an in-built PHOTON macro file which, when the post-processor is activated, will automatically produce an array of images as well as saving each image to file within the %PHOENICS%\d_sapps\Slurry\working directory.

    Once activated, the first image produced by PHOTON will show the cross-sectional computational grid used for the simulation. The following image displays the cross-sectional grid for the default mesh size:

    Either clicking the left-hand mouse button or pressing any key on the keyboard will move the PHOTON macro on to the next image, namely the contours showing the solids volume fraction at a point 80% of the pipe length from the inlet. For the default settings, this plot appears as follows:

    As can be seen from the above image, the solids volume fraction is greatest at the base of the pipe, indicating some particle deposition as the slurry flows along the pipe.

    Continuing through the post-processing macro as previously mentioned, you will see the fluid axial velocity and the particle axial velocity, as shown by the following two images respectively:

    As can be seen from the images above, the slurry mixture flows more quickly through the centre of the pipe, as is to be expected.

    The final image of the macro sequence displays the flow vectors within the pipe, as seen by the following image:

    The above image clearly shows the recirculation occurring as the slurry passes along the pipe.