Heat Simulation: temperatures don't converge

[Customer]

In some airways of my model, I get different airflow and temperature results each time I run a Heat Simulation. Is this correct?

[Martin Griffith]

In a model that has Compressible Flow and Natural Ventilation Pressure switched on, this behaviour can occur (for an example, see the Fire Demo model which is included in Ventsim).

The variation can occur because the Airflow and Heat Simulation results are physically-coupled. Each time you run an Air Simulation, the natural ventilation pressures in each airway are recalculated according to the current heat and air density in the airway. Similarly, the Heat Simulation (which runs an Air Simulation first) works by tracking the movement of heated air through the mine, according to the simulated airflow. So, by repeatedly running Heat Simulations, you are each time getting a new airflow, producing a new heat distribution and natural ventilation pressure, which in turn produces a new airflow, and so on. 

If such a variation exists in your mine (and assuming any other errors removed), then a Steady State solution of Airflow, Heat and Natural Ventilation Pressure does not exist; this means the variation is most likely indicative of a natural variation of heat and airflow in your mine. If you want to analyse this variation further, there is a new feature in Ventsim 5.0, called Heat Sensitivity, which gives a measure of this variability. It is available in the drop down menu under the Heat Simulation button.

This tool will run a succession of Heat Simulations and then collate statistical information on the observed variation of heat and airflow in each airway, as well as the number of times the flow reversed direction. These can all be accessed as Color and Text data options. As well as the raw statistical data on means and standard deviations, you can access, for each airway:

• the deviation of the current simulated Heat and Airflow from the statistical average,
• the confidence of the direction of the Airflow, expressed as percentages,
• the confidence in the Airflow Quantity, expressed as percentages,