MULTI GAS SIMULATION (Advanced)
[Ventsim Visual™ Advanced]
Gas Simulation is a subset of contaminant simulation with some specialised properties to allow for multiple gases and air density estimation based on gas composition. This can have application for natural ventilation changes for simulation.
Gas Simulation is based on the same linear spread algorithm as other contaminant simulations, however it allows for the simultaneous release and distribution of up to 15 different gas types, and automatically volumetrically balances other gases to ensure a 100% total concentration mixture is maintained. This function may be useful when tracking multiple contaminants of different levels from different sources, or when the density effects of different gas types need to be modelled in a simulation.
Gases can be placed in models using an injection method, an inline gas concentration method, or by using a gas emission rate. The first two methods use the same type of simulation, however the construction of the airways in Ventsim differs. The last method ‘emits; gas along predefined airway routes.
Inline Concentration Method
Specifically sets the concentration of gases in an airway and distributes the gas concentration downstream. All air entering the set airway is reset to the specified gas concentration. This method would normally be used when a known or measured concentration of gas is present in an airway.
Injection Method
This method allows the user to ‘inject’ a gas into an airway and observe the changes downstream. It does not reset the concentration of gas passing the injection point, and will therefore allow accumulation of gas concentration though the model.
This method involves constructing a separate dummy ‘airway’ to inject the gas into the model. The injection dummy airway should connect to the ‘surface’ to allow the injection to draw a quantity of gas separate from the model simulation. The injected gas should then be set to the actual concentration (eg 90% methane) and a fixed flow amount set to specify the rate of injection into the model. If very small fixed flow values need to be specified, the unit accuracy for airflow may need to be increased in the Conversion Settings.
The model can then be simulated to show the results downstream. To show the smaller concentrations of gas, the colour legend display may need to be manually adjusted to show the full range of colours over a much smaller range.
Linear Emission Method
This method allows the user to specify a rate that gas may be emitting from an airway surface into the mine atmosphere (for example coal seam gas). The emission is entered in litres per second per metre (l/sec/m) along the airway, and gradually replaces the atmosphere in the airway over time. Note that the method used is ‘replacement’ which is not strictly accurate as in reality the gas adds to the atmospheric volume causing a change in ventilation flows. In most cases however, the ventilation flow change is minute and can be ignored. If the emission rate is significant, and is likely to independently cause a change in airflows and perhaps push gas into other airways, then the more cumbersome ‘injection’ method should be considered instead.
Placing Gas Sources
The EDIT form has a facility to fix the concentration of gas compositions in any airway (injected or in-line) in the ventilation model. All gas concentrations should be entered as a volume based concentration value (not mass based) and then ‘FIXED’ to instruct the simulation to use the specified amount.
If gas mixtures are to be commonly used throughout a model, it is recommended to establish PRESET gas mixtures, which can be defined in the PRESET table and will automatically apply the gases to the airway when set.
Gases which are not fixed will be automatically adjusted to ensure the total volume concentration of all combined gases remain at 100%. The adjustment is done proportionally so gases which are not fixed (for example Nitrogen at 79%) will be adjusted proportionally more than lower concentration gases (for example Oxygen as 21%)
Simulating Results
Pressing the simulation directly from the Edit form or the Contamination Gas Simulation sub-button on the toolbar will run a Steady State simulation. Alternatively, gas simulation can be performed dynamically, using the Dynamic Simulation > Gas button.
To display the results, the text or colour display may need to be changed to select the type of gas to be reviewed. The colour legend values may also need to be manually changed to show the full range of colours for the desired concentrations.
To clear the results of a gas simulation and reset the model back to a standard atmospheric gas composition, use the Clear Contaminants options.
Density Based Simulation of Gas
Ventsim Visual™ Advanced has the ability to simulate the effect of different air densities on airway resistance and natural ventilation pressures.
This ability is extended to include the effect of gas composition on total air density and natural ventilation pressures. To use this option, the following options need to be enabled in the Settings > Simulation > Gas
- Compressible Airflows (available from Settings > Simulation > Airflow Menu)
- Natural Ventilation Pressure (available from Settings > Simulation > Airflow Menu)
- Gas Density for Simulation (available from the Settings > Simulation > GAS Menu)
Once enabled, provided Natural Ventilation Pressure (NVP) option is chosen, any gas distribution simulated in the model will result in changes to airflow simulation based on the effect of the gas density on air buoyancy, and the change in effective airway resistance.
For example, if 5% methane is simulated through an area of a mine with a Gas Simulation, then providing the Gas Density Simulation option is turned on, subsequent airflow simulation will predict the steady state buoyancy effect of methane through that region of the mine.
Another option may be to simulate gas drainage through pipe models, although this process has yet to be validated in Ventsim Visual
Warning : After completing simulation work using Gas Density simulation options, remember to restore the settings and turn off the option, otherwise future simulation will be permanently affected by gas through a model, until the gas it cleared out and removed with the clear contaminant option.