It is now largely accepted that, subject to careful, well-planned use, CFD can act as a valuable tool in the fields of process design and optimization. The use of CFD, whilst traditionally strong and widespread in the chemical, mechanical and manufacturing industries, has only recently begun to be exploited within the water industry. However, a review of the literature demonstrates that in recent years CFD has been successfully used in water and sewage treatment applications, both in the UK and abroad.

In this work, the simulation of two laboratory and two full scale flocculation unit processes commonly found at water treatment works using CFD is reported. The work provides details of model strategy development covering a broad range of modelling techniques including, inter alia, two-equation and Reynolds stress turbulence modelling, sliding mesh (SM) and multiple reference frames (MRF) approaches to mixing simulation, and mesh density optimisation. This work demonstrates clearly the benefits to be gained from the use of CFD.

Notwithstanding these benefits, the resource implications of complex CFD analyses are considered. The project's early work (MRF, two-equation modelling of one and two litre mechanically mixed vessels) was undertaken using a 1.4GHz, 250MB RAM laptop computer. The most straightforward of these runs took at least 12 hours of CPU time to obtain a converged solution and it became apparent that the modelling of anything more complex would require significantly improved computing facilities. As a result, the high performance computing (HPC) facilities at two UK universities were used for the more complex jar tests scenarios (sliding mesh and Reynolds stress modelling) and all full scale mixing simulations. Even using the HPC facilities, SM simulations of full scale mixers still required up to five days' continuous CPU usage.

Although CFD offers undoubted benefits for those seeking to understand better the flow in and around complex geometries, it is concluded that when using modest computing hardware (e.g. desktop personal computers), CFD modelling is restricted to the analysis of relatively simple flows. More complex scenarios, such as rotating meshes, require greater computing power which is not generally found outside academic environments or highly specialised consultancies. CFD modelling also requires trained staff to generate robust models and solutions. The combination of these two factors is likely to preclude the widespread use of CFD in most water companies and mainstream consultancies. Nevertheless, this work has demonstrated clearly the benefits to be gained from this powerful tool.

purchase the full-text of this paper (price £20)

go to the previous paper
go to the next paper
return to the table of contents
return to the book description
purchase this book (price £120 +P&P)