Underground » Ventilation, Gas Drainage and Monitoring
Effective goaf management requires a thorough understanding of goaf gas behaviour under varying conditions such as mine ventilation systems, goaf drainage, inertisation, barometric pressure change and goaf consolidation. However, there is a lack of a practical tool that mine ventilation engineers can use to model and assess goaf gas flow behaviour at a site level.
While computational fluid dynamics (CFD) has been extensively used by researchers to model goaf gas behaviour, it is impractical to use at mines due to its complexity and time-consuming nature. Ventsim, which uses a one-dimensional network solver, is currently one of the industry standard mine ventilation simulation software. The potential of using ventilation software such as Ventsim to model goaf gas flow at a site level has gained interest.
This project, conducted in two stages, aimed to test the concept of using the Ventsim methodology for modelling goaf gas flow behaviours. To conduct this test, various new methods and Ventsim tools were developed within the project, through a collaborative effort between CSIRO and Howden Ventsim along with valuable industry inputs.
The work and key findings are summarised within the report in the following areas:
- Goaf structure, flow types and simulation techniques
- Development of an equivalent goaf resistance model
- Preliminary development and testing of goaf modelling using Ventsim
- Development of a functional goaf resistance model
- Development of an add-on goaf simulation module (alpha version) in Ventsim
- Assumptions and Parameters of Goaf-airway interfaces
- Evaluating Goafsim against field scenarios
- Preliminary study on incorporating diffusion in Goafsim
An approach was developed using Ventsim to model the 3D porous flow in the longwall goaf. The approach includes the following key components:
- A method to build 2D and 3D goaf grid with recognition of the caving parameters
- Mathematical translation between permeability in porous flow and resistance in pipe flow
- A functional goaf resistance model enabling adjustment of goaf permeability distribution
- A method for injecting gas into the goaf grid in Ventsim, considering gas emission profile
- Selection of approximate resistances for airway-goaf interfaces.
A test module called Goafsim was developed in this project using a trial version of Ventsim.
Simulations of various field scenarios demonstrated that Goafsim can predict general trends of goaf gas distribution patterns. However, the results do not adequately represent CFD modelling results, nor do they qualitatively agree with the goaf gas distribution measured by a Tube Bundle system. In other words, the results may not be quantifiably reliable. There are inherent limitations of Goafsim, including:
- The simplification of using 1D pipe flow methods for simulating 3D porous flow in the goaf
- Limited resolution due to constrained number of airways in Ventsim
- The neglect of diffusion in solving species transport.
At this stage, Goafsim may be used as an educational or training visualisation tool for understanding goaf and general patterns of goaf gas flows. It may be used by experienced ventilation engineers to gain rough ideas on likely goaf gas responses to changes and/or to visualise new concepts on goaf management. However, the results should not be relied upon for guiding any goaf management operations.
Recommendations
The project has primarily focused on the theoretical development of methods and tools for using Ventsim to model goaf gas flows. It will be critical to let end users, particularly ventilation engineers, to carefully review the development of this method as detailed in this report. Their feedback and comments will play a crucial role in assessing the feasibility and potential of Goafsim, as well as determining areas for future development.
Since the physics of goaf gas flow has been simplified to enable the use of 1D pipe flow approach to simulate 3D porous flow, it is also important to have academics review the underlying science behind this modelling approach. This can be achieved through publishing the scientific aspects of this development in relevant journals.
Final disclaimers
This project aimed to explore the concept of using Ventsim to model goaf gas behaviours. The development within the project represents an initial output and does not ensure the functionality in modelling goaf gas flow and distributions. The methods, tools and simulation results described in this report should be interpreted carefully and not used for operational use or seeking any guidance on goaf gas management.