« Back
  • Using CFX in situations where air transport is an issue

    Modelling the transport of air in pipes using CFX
  • Modelling the transport of air in pipes using CFX
  • Modelling the transport of air in pipes using CFX
  • Modelling the transport of air in pipes using CFX
  • Modelling the transport of air in pipes using CFX

Modelling the transport of air in pipes using CFX

The computational fluid dynamics (CFD) software package ANSYS CFX is widely used at Wallingford for 3D numerical simulations. We used CFX to carry out a study into the simulation of air pocket transport in pipes.

This could provide a first step towards using CFX for a range of situations where air transport is an issue.

Air transport could be an issue in:

In cooling water systems, for example, there might be an accumulation of air in the pipework following a pump shut-down or air could also be entrained in flows plunging from seal weirs and entering into pipework. Sewers and water pipelines might show reduced flow capacity and air can get entrained in drop shafts or be released at outfalls with negative environmental consequences.

Existing data

We already held an extensive experimental database from a previous research study (Escarameia et al, 2005, Escarameia, 2006). We were able to draw on this to assess whether CFX could reproduce the hovering velocity (the flow velocity for which an air pocket just remains stationary/quasi stationary) and the critical velocity (the minimum flow velocity required to evacuate an air pocket from a pipe) associated with particular pipe slopes and sizes of air pocket.

Programme of work

Our work programme was originally set up to include:

However, we encountered difficulties with the use of CFX, for example in the setting of modelling parameters to ensure conservation of air mass. Work was therefore re-scoped to involve a thorough investigation and selection of the basic modelling parameters.

There are difficulties in modelling air pockets, in that a very fine mesh is required at the air/water/wall interface. It is also hard to ensure conservation of mass. Velocity increases tend to shear air pockets rather than flushing them downstream intact - a feature that was observed in laboratory experiments.

Research revealed that, even with the most refined approach (over 3 million elements) the hovering velocity was achieved to within 15% of the experimental findings but the critical velocity was fundamentally not achieved. Solver settings regarding timestep and convergence led to long and impractical solver times for consultancy projects. These were in the region of 5 days for each second of simulation.

Conclusions

Valid lessons were learned about the selection of modelling parameters in this complex area and these may be useful for future CFD investigations. However, the limitations described led us to recommend that modelling the movement of air pockets in pipelines should not currently (2008) be offered using ANSYS CFX on consultancy projects at HR Wallingford. The design formula in Escarameia et al (2005) remains a suitable tool for determining flushing velocities. We briefly assessed other CFD software programmes with open source, but these did not appear to have the required functionality either. Further software programs, which are not full 3D CFD codes but which may offer practicable computational times, should be considered.

Authors

Amanda Chapman; Manuela Escarameia

Keywords

CFD modelling; Air transport in pipes; Multi-phase flows

Completed

2008

References

Escarameia M, Dabrowski C, Gahan C and Lauchlan C (2005). Experimental and numerical studies on movement of air in water pipelines. HR Wallingford Report SR 661, April 2005.

Escarameia M (2006). Investigating hydraulic removal of air from water pipelines, Proc. Institution of Civil Engineers, Water Management 160 Issue WMI, pp25-34

Chapman A and Escarameia M. (2008) Modelling the transport of air in pipes using CFX; an exploratory study. IT567 HR Wallingford, December 2008.

Read more information at

"There are difficulties in modelling air pockets, in that a very fine mesh is required at the air/water/wall interface."

Contact

Project information

Related projects