The new Gas Simulation feature in Ventsim™ provides a powerful tool for simulating the spread of different gas mixtures through a mine.
Gas simulation works in a similar way to normal contaminant spread simulation. Sources of gas are established and spread through the mine, mixing at junctions with concentrations and mixtures changes depending on diluting air and other sources of gas.
The contaminant routines in Ventsim™ work on a simplified linear average flow, and ignore complexities such as boundaries drag issues, higher center drive velocity profiles, and potential gas densities layering issues. Such complexities introduce a number of fluid dynamic complexities which are highly dependent on geometry and airway profiles, and are outside the scope of large mine ventilation simulation analysis. In most cases, this approximation will be adequate for most mine simulation needs.
Gas concentrations in an airway are set in the EDIT box. A number of different gases can be simultaneously set, providing concentration does not exceed 100%. The remaining standard atmospheric gas concentrations (for example nitrogen) are adjusted to maintenance a 100% volumetric balance. Multiple sources of gas can be entered into different areas of the mine.
There are two methods of introducing and simulating gas to a mine model in Ventsim™.
Method 1: Set the entire airflow to a mixed concentration. All airflow travelling past this point will be set to the specified concentration. This value could be derived from measured data underground, or through theoretical analysis. All simulated airflows downstream from this point will be derived from this source. Unlike general contaminant simulation, gas passing a set source will not accumulate from upstream or recirculation sources (use Method 2 if this is required)
Method 2: Inject a pure concentration of gas into an airway, and allow it to mix with the airflow in the main airway. This method relies on injecting a small concentration of airflow (gas) from a fixed flow source, and normally should be set to a ‘surface’ connection, so that the gas is injected from an external source. The gas level in this injection airway is increase to the pure gas levels (eg 75% methane, 15% CO2 etc). Downstream from the injection point, the mixture will show the diluted levels of gas. To gain a more reasonable colour scale, the legend scale may have to be renumbered to show only the concentration of interest.
Ventsim™ can optionally use the different densities of gases, and incorporate these into the natural ventilation and pressure loss calculation within the simulation. To utilize this feature, ensure that Natural Ventilation is enabled, and the Gas Density option in the Air Simulation Settings is turned on.
An area of a mine high in methane for example, may exert a significant upwards natural ventilation pressure due to the low density of the gas, and Ventsim™ simulation will reflect this if the option is turned on.
Gas distributions through pipes can be done is a similar way. Pipes in Ventsim™ can be constructed with the Ventsim™ ducting function, and high pressure pumps can be installed using Fixed Pressure options in Ventsim™. A lesson on using this method will be included in a future tutorial.