Low order modeling of thermoacoustic instabilities

Abstract

Pollution and The Greenhouse Effect are a concern in our society. The high levels of CO2, NOx, ozone, and methane gasses are alarming and have pushed gas turbine industries to develop more environmentally friendly combustors. In the past years, Lean Premixed Combustors have been identified as a potential solution. The temperature of the flame determines the release of emissions under certain conditions. However, Lean Premixed Combustors are more susceptible to thermoacoustic instabilities. The prediction of these instabilities requires a study of the acoustic behavior of the flame, for instance, through the analysis and development of Flame Transfer Functions. The approach to analyzing Flame Transfer Functions is to study the effects of changing the length where the screws are located from 45mm to 30mm. With this change, the flame properties such as flame height and bulk velocity are modified to see if the flame is unstable. The Flame Transfer Function for this project thesis was provided by the Combustion Group at NTNU (Norwegian University of Science and Technology). It is called the Distribution Time Lag model, or DTLG model for short. The first step to analyze the DTLG model is to carry out a Stability Analysis that assesses the flame properties to control the amount of H2 entering the combustor. The main objective of the analysis is to see if the DTLG model could stabilize the combustor. Initial results show that it could not stabilize. Therefore, a Geometry Analysis must be implemented. It consists of changing the geometry of the two ducts in the combustor and study if the combustor could stabilize with such change. The results show that the combustor does not stabilize for the chosen geometry. It was not found in this study a geometry that makes the combustor stable for any of the flame heights nor bulk velocities implemented