Tank Components

Tank Component General Description and Quick Guide for Creation in the GUI

There are three tank subtypes available in Flow Simulator under “Incompressible Liquid Elements” section. Each is solved in a similar manner with the only major difference being how the pressure in the tank is determined. It is assumed that each tank is partially filled with a liquid and a gas fills the remainder of the tank.

The tank was developed for transient analysis, but it can be used in a steady state analysis. For a steady state analysis to converge, the model must be setup so the time derivatives in the following equations are 0 (dx/dt=0). For instance, all the liquid exiting the tank must also enter the tank.

Tank Component Inputs

Table of the inputs for the Tank Component.

Tank Component Theory Manual

This section contains a preliminary derivation of how mass flow rate varies with time for simplified tank models.

Mass conservation:

For the open tank and the tank with a bladder the only and are the liquid elements entering and leaving the tank. For the tank with a vent the and are for liquid and gas elements so the mass conservation equation is satisfied for both. This derivative is solved using backward differencing method.

The mass flows and are determined from the elements attached to the tank.

Momentum Conservation:

No momentum is passed from the flows entering the tank to the flows exiting the tank. The tank behaves like a plenum in terms of velocity.

Energy Conservation:

Note that the heat from thermal network or user input, can also be inferred from the user input for polytropic index, n. If n=1, the Q=W and the temperature is only a function of the heat capacity of the fluid in the tank and the energy of the fluid entering the tank. Other properties of the tank are solved as follows

1. Tank Pressure: The tank pressure is the pressure of the gas in the tank, which is the same as the pressure at the top of the liquid surface.
   1. Open tank: The tank pressure is set to a constant value, usually atmospheric pressure.
   2. Tank with a bladder: The expansion or contraction of the trapped gas in the enclosed tank determines the pressure. The polytropic process equation is used: P*V^n=constant. The user can input the polytropic index, n.
   3. Tank with a vent: The gas mass flow through the vent determines the pressure. If the gas mass flow through the tank goes to 0 then the polytropic process equation will be used.
2. Port Pressure: The pressure at each port submerged in the liquid includes the gravitational effect on pressure.

   

   Where;

   ρ=liquid density

   g=gravity acceleration

   h=height from port to top of liquid

3. Fluid Volume:

   

   The is determined from the mass conservation equation.

4. Fluid species: The fluid species mass fraction in the tank are determined by the species in the tank and the species entering the tank.

   

   If an element entering a tank contains both a liquid and a gas the tank can separate the 2 species using the gas separation factor. The factor can be between 0 (no separation) and 1 (100% separation). This factor is used to modify the summation of term in the equation above.

Tank Component Outputs

The following listing provides details about Tank Component output variables.