For this case, R&R Consult assisted Aalborg Engineering with a plant for Siemens in Malta. We simulated the flow behaviour of hot exhaust gas from a heat recovery steam generator (HRSG) as it was ejected from a by-pass exhaust stack. The CFD simulation investigated the flow and temperature field of ejected exhaust gas — specifically, whether it could compromise the nearby main exhaust duct during worst-case wind conditions. This could pose a problem for the electronic equipment installed at the main exhaust duct. The main objective was to evaluate the heating of the main exhaust duct in case strong winds carried the by-pass exhaust gas towards it. During the simulation, we recreated the behaviour of hot exhaust gas ejected at high altitude and the influence of buoyancy forces, temperature- dependent density and external wind conditions.
The setup was evaluated at various wind speeds, HRSG operating loads, and exhaust gas temperatures. The results mapped the height at which by- pass exhaust gas might come into contact with the main exhaust duct, and determined peak temperatures on the outer surface of the main exhaust duct. With this information, we quantified the potential risk of damaging equipment, given a worst-case scenario combination of operating- and ambient conditions.