A Proposed Methodology for Optimized Diffuser Design

For emissary design the following characteristics are decisive for maximum efficiency: diffuser length, number of risers, number of ports per riser, diffuser, riser and ports diameters and port height above the bottom.

These characteristics are the only ones that the designer can control, since environmental variables such as: current velocity, discharge, environmental density and bathymetry cannot be changed, as well as effluent flow rate and effluent concentration, which are adopted as premise.

However, these definitions are usually obtained through a heuristic methodology based on specialist program such as CORMIX or VISJET, and in the designer`s experience. The procedure basically consists of testing several configurations until finding the one with the highest dilution, the lowest head loss, the most economical, or the best possible combination. At the end, one is not sure if the configuration chosen is in fact the optimal one.

In this paper, we propose a methodology based on empirical relations to determine the optimum configuration. The methodology consists in verifying the dimensions of some characteristics and the limits of some parameters, in a defined sequence. Throughout the procedure, configurations are discarded and the number of possible combinations are reduced significantly. The remainder are analyzed considering cost criteria.

In this methodology, the characteristics to be analyzed are: diffuser length, number of risers, diffuser and ports diameters and ports height above the bottom. The parameters and check sequence are: (1) ratio between the effluent flow and the length of the diffuser; (2) ratio between the spacing of the risers (or ports) and the distance below the surface; (3) ratio between the sum of the areas of the ports and the area of the diffuser; (4) Froude number of the jet through the port; (5) application of a criterion to determine if the behavior of the plume is single or line; (6) calculation of the initial dilution, using the specific equation to the plume behavior; (7) Calculation of the extent of sedimentation inside the diffuser tube; (8) Calculation of head loss; and (9) cost analysis.

A case study will be presented to demonstrate the validity of this methodology.