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Sir - Turbulent discussions

Design and Performance of a Passive Dilution Barrier, the paper by Wilson & Shuttleworth (GE January 2002) is an interesting development of the insitu venting trench gender commonly used to control the migration and dispersion of methane and associated gases from the ground.

To determine a mathematical solution, the authors have based the design of the system on the principle of Darcy's formula and laminar gas flow through the ducts.While this approach provides a mathematical solution, laminar flow is certainly undesirable in practice as recent studies suggest that this can lead to conditions where undiluted methane is transported through the ducts of the system to the external vents as 'stream filaments' or a 'stream tube'resulting in potentially hazardous conditions (Card GB & Langdon NJ 2002, to be published).

This phenomenon was initially demonstrated in the 1880s by Reynolds with his early experiments on the flow of fluids in pipes.To prevent this condition it is necessary to create turbulence, or intermittent turbulence, within the ducts so that the methane entering the system from the wells or nodes is mixed and diluted and discharged safely at the vent.Turbulence can be induced simply by including baffles to provide irregular flow pattern or changes in length and cross-sectional area of the ducts.

Obtaining a rigorous mathematical solution for the design of turbulent gas flow in pipes is nearly impossible and relies upon data from experimental observation.Numerical analysis techniques are available but can be overly complex.It is generally true to say that gas flow in any in-ground venting system will occur at a velocity considerably lower than the Mach number (M) so that changes in gas density, due to pressure changes in the ducts, are negligible and can be ignored.

A simplification is to assume that, since the external pressure difference acting on the inground vent is fixed, Darcy's formula can be applied as the governing external equation.The pressure difference can be amended to take account of turbulence by applying pressure loss factors at discrete points in the venting system where turbulent flow takes place.Turbo-streamliner is a simple spreadsheet-based analysis and design tool that can be used to develop a geometric design of the in-ground vent that will ensure that stable and continuous laminar flow conditions cannot exist within the system.

Dr Geoffrey B Card, chief executive, Card Geotechnics

Steve Wilson replies:

Dr Card's comments are welcome.While it is certainly true that laminar flow may induce stream lines, this occurs at a molecular level and will provide extremely thin layers within the flow.This does not mean that the individual gases will separate out.

However, it is important to note that Darcy theory is only used within the design methodology to estimate gas flows within the ground and within the vertical vent nodes.In these locations, dilution is not considered.Darcy theory (laminar flow) is not used within the zone of dilution, ie the horizontal collection/dilution duct.

The air flow through the dilution duct is estimated using a method widely adopted by designers of passive gas protection systems, based on the guidance provided in CIRIA Report 149 and British Standard BS 5925.This method does not assume laminar flow in recognition that turbulence at changes in direction within the system and the intrinsic turbulence within the specific type of duct cause mixing of the gases within the duct.

Dr Card's comments highlight one of the challenging aspects encountered during the development of the system and design methodology.The internal structure of the duct was a key consideration to optimise on flow and turbulence to produce a uniform dilution across the entire length of the dilution duct between bulkheads.This is achieved using high-strength, rectangular, geocellular ducts that incorporate a series of baffles to promote turbulence and mixing.

Furthermore, the most significant simplification in any design method for passive ventilation is that of steady state flow conditions.These rarely occur because wind (and therefore driving pressure) is variable.This effect in itself will also cause mixing within the duct.

Many of the other factors identified by Dr Card are indeed difficult to address in a simple design method.For this reason a computer aided design programme is being developed, based on numerical and CFD modelling techniques, which will be validated against further comprehensively monitored field trials.

Steve Wilson, technical director, The Environmental Protection Group

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