Tutorial 7: Transient Flow Analysis using Feed Forward Controller Model

Purpose/Objective:

This exercise will show the user how to create a Feed Forward Controller that will adjust a valve opening position based on the valve upstream pressure. The user will learn how to:

Create a Feed Forward Controller
Setup a transient run
Check the model
Run the model
Post-process the model
Elements used: Conventional Orifice, Standard Tube, Ball Valve, FFWD Controller, Generic Fixed Volume Accumulator.

Step 1: Create plan

The goal of this exercise is to determine time it takes a volume to pressurize up to 400 psi from a small leak and then how long it takes to depressurize the volume after a valve is opened by a feed forward controller.
1. Volume initial pressure = 14.7 psi
2. Fluid = air; Use compressible gas elements

Figure 1.01: Model Plan

Step 2: Building Model

Create Chambers (2 Boundary Plenum and 2 Internal Momentum) and the Accumulator
Figure 1.02: Chamber and Accumulator
Create Elements: 1 Conventional Orifice, 2 Standard Tubes, and 1 Ball Valve
Figure 1.03: Elements
Create Feed Forward Controller
Figure 1.04: Feed Forward Controller
Assign Chamber Pressure and Temperature
1. Upstream Boundary Plenum: P= 400 psi, T= 80 F
2. Downstream Boundary Plenum: P= 14.7 psi, T= 80 F
3. 2 Internal Momentum: Ptotal = 14.7 psi and Pstatic = 14.7 psi, T=80F.
  
  These are the initial conditions for the transient.
Assign Accumulator Properties: Total Volume = 4 ft³
- Chamber Initialization for the accumulator: Plenum, Pstatic = 14.7 psi, T = 80F
Figure 1.05: Accumulator Properties
Assign Element properties
1. Orifice: Area = 0.05 in²
2. Upstream Tube: Dia = 1 in., Length = 1 ft, Heat Transfer = Adiabatic
3. Downstream Tube: Dia = 1 in, Length = 100 ft, Heat Transfer = Adiabatic
4. Ball Valve: Pipe Dia = 1 in, Valve Position = 0% open (controller will adjust this)
Assign Feed Forward Controller Properties
1. Controller will use a python script.
  # Valve position will be set to 100% open if the upstream pressure> 400 psi.
  # After the valve position is 100% open it will stay 100% open until the run ends.
  if VALVEPOS_CUR == 0.0:
  if PS > 400:
  VALVEPOS_NEW=100
  else:
  VALVEPOS_NEW=0
  else:
  VALVEPOS_NEW=100
Copy and Paste or retype the text. Lines with # are comments. Python uses indentation to identify control logic (if and else statements).
2-gauge variables: Upstream pressure and the ball valve position
- Ball valve position will be used as a gauge and manipulated variable.
1 manipulated variable: the ball valve position
Figure 1.06: FFWD Controller properties
Analysis Setup:
1. Analysis Type: Transient Flow
2. Initial Condition Option: User-defined Pressure and Temperature Data
  - Solver will use P and T that have been assigned to the 2 internal momentum chambers and the accumulator for the time = 0 sec. solution.
3. Transient Run Control Parameters (Figure 1.07)
  1. Run Start Time: 0 sec
  2. Run Stop Time: 100 sec
  3. Solution Time Step Option: Automatic Time Stepping
    - Initial, Minimum and Maximum Time Step: 0.01, 0.1, 0.5 sec
  4. Output Time Step Option: Constant Output Intervals
    - Output Time Step: 0.5 sec
Figure 1.07: Transient Run Control Parameters

Step 3: Check Model and Run

Select checkmark icon from the top toolbar to check the model for warnings/errors.
- No errors or warnings should appear.
Select run icon from toolbar. Run Flow Simulator.

Figure 1.08: Run Panel

Step 4: Post-process

The first results file (*.res) of the transient should automatically be loaded into GUI. If not, it can be selected via File → Load Result File
By default, both chamber and elemental results are displayed in the graphical workspace.
Figure 1.09: Results at 0.5 seconds
Use the Results Table for post processing results Versus Time.
1. Pick the Accumulator Static Pressure
  Figure 1.10: Accumulator Static Pressure versus Time
2. Pick the Orifice element upstream of the accumulator and the tube downstream and plot Total Flow
  Figure 1.11: