Investigate and resolve thermal stream flow reversal warnings

You will investigate the warnings and troubleshoot your model.

Download and extract the part files.

Open the Simulation file

Open the part file and reset the dialog box settings.

  1. Choose File→Open and open heat_exchanger/heat_ex_sim1.sim.
  2. Choose File→Preferences→User Interface and on the Dialog and Precision page, reset the dialog box memory.
  3. Click OK.
    The geometry represents a section of a heat exchanger. The flow inside the heat exchanger is modeled using duct elements. The duct is made of a 1 mm thick aluminum shell. The water inside the heat exchanger moves at a low flow rate of 1 l/min and the channels are exposed to air.

Modify the solution and solver parameters

The solution is already setup with several boundary conditions. You will modify some solution and solver parameters.

  1. In the Simulation Navigator, right-click the Solution 1 node and choose Edit to observe the solution parameters.
  2. On the Initial Condition page, set the Thermal Initial Temperature to Uniform to set specified uniform temperature value.
  3. Click OK.
  4. In the Simulation Navigator, right-click the Solution 1 node and choose Edit Solver Parameters.
  5. On the General tab, set the Scratch Directory to Simulation-Solution Name to generate a subdirectory in the working directory with the name of the current simulation followed by a hyphen and the name of the active solution.
  6. Click OK.
  7. Expand the Solution 1→Simulation Objects, Constraints, and Loads nodes to explore the existing boundary conditions.

Solve the model

Solve the model and inspect the solution messages.

  1. In the Simulation Navigator, right-click the Solution 1 node and choose Solve.
  2. Click OK.
  3. Wait for the solve to end before proceeding.
  4. In the Review Results dialog box, click Yes.
  5. Review the messages in the Solution Monitor.
    Toward the end, you can see a fatal error.

    The solve ended with an error.

  6. Close the Solution Monitor and Information window.
  7. In the Analysis Job Monitor dialog box, click Cancel.

Modify the thermal stream load

Investigate the cause of the fatal error that the solver reports on the thermal stream load. You can specify a reversal threshold value for the mass flow of a thermal stream with no flow reversal condition specified. When the mass flow becomes negative and its absolute value reaches the threshold, the thermal solver issues a fatal error and stops the ongoing solution. When the absolute value of the negative mass flow is below the threshold, the thermal solver substitutes the negative mass flow value with the absolute value. This lets the solution proceed for a very small mass flow value that is close to zero and accounts for numerical round-up errors. You will specify the threshold value using an advanced parameter.

  1. In the Simulation Navigator, right-click the Thermal Stream(1) node and choose Edit.
  2. In the Stream Conditions group, in the Mass Flow row, click Edit.
    The mass flow is defined by a table field, and at time=100min, the mass flow rate becomes negative.
  3. Click Cancel twice to close both dialog boxes.
  4. Choose Home tab→Loads and Conditions group→Simulation Object Type→Advanced Controls .
  5. In the Parameter Definition group, click Catalog .
  6. In the Catalog dialog box, in the Item list, expand the Thermal → Thermal Solver nodes.
  7. Select THERMAL STREAM MASSFLOW REVERSAL THRESHOLD to specify the mass flow reversal threshold value for thermal streams with no flow reversal.
  8. Click OK.
  9. In the Advanced Controls dialog box, in the Value box, type 0.03.
  10. Click OK.

Solve the model with modified mass flow reversal threshold

Solve the model and inspect the solution massages.

  1. In the Simulation Navigator, right-click the Solution 1 node and choose Solve.
  2. Click OK.
  3. Wait for the solve to end before proceeding.
  4. In the Review Results dialog box, click Yes.
  5. Review the messages in the Solution Monitor.
    Toward the end, you can see a warning message.

  6. Close the Solution Monitor and Information window.
  7. In the Analysis Job Monitor dialog box, click Cancel.

Specify flow reversal condition

Specify flow reversal condition for the thermal stream load. You define inlet conditions for the reversal of the flow, and specify the values for mass flow and inlet temperature for the opposite boundary of the stream.

  1. In the Simulation Navigator, right-click the Thermal Stream(1) node and choose Edit.
  2. In the Inlet Conditions - Flow Reversal group, select the Specify Flow Reversal Conditions check box, and set the following properties:
    • Mass Flow=0.012 kg/s
    • Inlet Temperature=20 °C
  3. Click OK.

Solve the model with specified flow reversal condition

Solve the model and inspect the solution messages.

  1. In the Simulation Navigator, right-click the Solution 1 node and choose Solve.
  2. Click OK.
  3. Wait for the solve to end before proceeding.
  4. In the Review Results dialog box, click No.
  5. Close the Information window.
  6. In the Analysis Job Monitor dialog box, click Cancel.
  7. In the Simulation Navigator, right-click the Solution 1 node and choose Browse.
  8. Open the heat_ex_sim1-Solution_1.log file in a text editor to verify that there are no more warning messages for the thermal stream load.
    The solver writes all the information, which appears in the Solution Monitor, in the log file.
You have completed this lab.