Experimental investigation of the influence of water injection on acoustic properties of the exhaust system of a gas turbine combustion test rig

It is a known phenomenon that single can combustion test rigs and gas turbines have a different stability behavior. Real gas turbines are often more stable than their test rigs. One main difference between test rigs and real engines is the injection of cooling water into the test rigs to reduce the temperature of the exhaust gas and thus to protect the exhaust valve. A literature survey showed that the presence of a two phase flow can drastically reduce the sonic velocity and consequently change the acoustic properties of a system. The aim of this project is to study the influence of water injection on the acoustic properties of a test rig representing the exhaust system of a gas turbine. The experimental results clearly show that the sonic velocity does not change in the present test rig because the droplets are too big to follow the acoustic fluctuations. The critical dimensionless number in this context is the Stokes number, which is mainly determined by the droplet diameter and the acoustic frequency. Furthermore, the experimental results point out that the injected water increases the acoustic damping. It can be concluded from this study that the influence of water injection on the acoustic properties and therefore on the stability behavior is very sensitive to the injection conditions, especially the droplet diameter.