Estimate Gas Parameters inside a Tank

To estimate the gas parameters inside a tank during evacuation follow the steps bellow.

Procedure

  1. Before running the tank evacuation calculation, create a mass flow rate surface goal at the outlet and the average pressure Pv and bulk average temperature Tv volume goals calculated over the entire tank’s volume.
  2. Run the calculation and stop it when the exhaust jet is formed. At this condition, gas parameters inside the tank will be relatively constant.
  3. Calculate the Discharge coefficient (η).

    For many standard outlet orifices, the discharge coefficient is known. But you may have a complex exhaust system with more than one orifice of arbitrary form. If you do not know the discharge coefficient for your exhaust system, you can obtain the discharge coefficient value from the following calculation:



    		(1)

    where is the calculated mass flow rate at the outlet, is the ideal mass flow rate determined via the gas specific heat ratio γ, gas molecular weight μ in the tank, pressure Pv and temperature Tv in the tank calculated over the tank’s volume at the moment you stopped the calculation, effective throat area S, and the ambient pressure Pa from the following system of equations:



    		(2)


    		(3)


    		(4)


    		(5)

    Here Ru is the universal gas constant, M is the Mach number in the throat with area S. You should bear in mind that in this approximation the discharge coefficient is assumed to be independent of pressure, i.e., constant during the evacuation.

  4. The evacuation process itself can now be estimated with an engineering method that allows the determination of the gas parameter changes within the tank versus time. A numeric method can be applied to the closed system of equations for the pressure inside the tank (see “Technical Background”) so at each time moment the gas parameters inside the tank can be obtained. The calculation is done with the Calculator’s Tank evacuation formula:
    1. Open the Calculator  dialog:

      • On the ribbon, click Flow Simulation > Tools > Calculator.

      • In the menu, select Tools > Flow Simulation > Tools > Calculator.

    2. Right-click a cell in the Calculator sheet and select New Formula.
    3. Expand the Numeric Calculation item and select Tank evacuation.
    4. Click OK.
    5. Specify the parameters for Initial tank pressure (P0) in Pa and Initial tank temperature (T0) in K, Molecular mass of the gas inside the tank (μ) in kg/mol, Tank volume (V) in m3, Ambient pressure (Pa) in Pa, Discharge coefficient (η), effective Throat diameter (d) in m, and Specific heat ratio (γ).

      The effective throat diameter d is calculated through the effective throat area S as follows:



      		(6)
    6. Specify Output time moment frequency denoting the number of output time moments: the greater the frequency the smaller the step between the output moments.
    7. Click Edit > Run Numeric Calculation.
Parent Topic: Tank Evacuation