Comparison of Calculation Methods for Head Losses in Multiport Diffuser Outfall Design

Multiport diffusers are efficient components of outfall infrastructure for river or submarine effluent disposal. They are utilized to avoid pollutant accumulation and provide rapid dispersion. A multiport diffuser design program has been developed to improve the current level of design technology for such installations.

Good multiport diffuser outfall design must address the hydraulics outside and inside the diffuser. Several methodologies for the analysis of the diffuser internal hydraulics have been adopted by various authors. These include a 1-D pipe flow port-to-port analysis (Fischer et al., 1979, Wood et al., 1993). Fischer et al. (1979) define a bulk loss coefficient C_d to estimate the loss from simple riser geometries for sharp-edged and bell-mouthed ports. These discharge coefficients are empirical and do not consider diameter ratios and flow separation effects in detail.

Other methodologies include a fictitious porous conduit and a 2 or 3-D field Eulerian grid for every point of the diffuser (Shannon, 2002, Mort, 1989). These two have the advantage that unsteady, stratified flow (i.e. saltwater intrusion) calculations are easier to implement than the port-to-port analysis. However, they do not specify appropriate local loss formulations for common or complex diffuser and port geometries.

The present approach involves calculating the numerous local losses due to expansions, contractions, bends, friction, etc., as well as additional flow forcing due to density differences. This approach accounts for more complex geometries. The CorHyd model presented here computes the flow distribution along the diffuser and the related pressure losses in the pipe system. It considers different pipe materials and geometric configurations and releases restrictions of previous diffuser programs by considering flexible geometry specifications, high risers, and variable area orifices, all with the automatic definition of loss coefficients. CorHyd employs this approach using a steady state pipe flow analysis. It employs the continuity equation at each flow division and the work-energy equation along pipe segments, along with detailed loss calculations. The diffuser pressure upstream and downstream of each port is calculated.

Additional features regarding blocked ports, sensitivity analysis and performance evaluation for varying parameters ensure proper diffuser design and reduced costs for installation, operation, and maintenance. Coupling with the regulatory mixing zone model, CORMIX, allows for outfall design, optimization, and computation of the subsequent mixing zone characteristics in the receiving waters.