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In the marine environment, the discharge of wash water from Exhaust Gas Cleaning systems is regulated so that the lower pH levels—caused by sulphur dioxide concentrations—are controlled. Specifically, the Marine Environment Protection Committee regulation MEPC.259(68) states that the discharged wash water plume should be equal to or above a pH of 6.5 at a distance of 4m from the discharge point. CFD modeling is a great tool to assess the discharge conditions so that the regulatory conditions are met and effects on marine life are minimized.

A recent research project required such CFD simulations, where the detailed hull profile in the vicinity of the discharge was utilized to create the wall boundary condition and then several hundred cubic meters of surrounding sea water were modeled to contain the discharge plume. Typically, the piping upstream of the discharge is also modeled as it can affect the discharge plume shape. Also, diffuser inserts are commonly used in the mouth of the discharge pipe. Using CFD simulation, it was determined that a simple cross made up of intersecting angle profile pieces mounted flush with the hull, was the most effective jet diffusion device. The bend of the angle profile faces upstream, into the discharge pipe, so that the discharging wash water splits around the vee-like cross section and forms a recirculation region immediately downstream of the insert. This causes a rapid break-up of the discharge plume so that it doesn’t penetrate very far from the hull, while also incurring a reasonably small pressure drop.

Using CFD to model the discharge, the detailed concentration profiles outside of the hull were determined to monitor pH levels, and the size of the diffuser inserts was selected to minimize pressure drop, while still meeting the regulatory conditions.