Due to strict emission legislation, the trend in the development of aero-engine gas turbine combustion is heading towards lean burning approaches. Lean combustion reduces the combustion temperatures and therefore also the nitrogen oxides emissions. Unfortunately, lean combustion suffers from instabilities and the operation close to the point of lean blowout increases the risk of imminent blowoff. Active stability control is therefore inevitable. The objective of this work is to evaluate the signal obtained from an ion current measurement technique to enable combustion control for aircraft propulsion applications in the near future.
In the past ion current measurements have been used in several studies as flame turbulence analyzer and to detect the reaction rate. However, investigations in lean burning and swirl stabilized airblast injection combustors for future propulsion concepts are rare. The signal obtained from an ion current detector inside a combustor depends strongly on the measurement position. In this experimental investigation field measurements at atmospheric conditions of the ion concentration in a tubular combustor with a sampling rate of 8 kHz are compared with 4 kHz time resolved temperature and OH* chemiluminescence measurements in order to determine the position of the reaction inside the combustor. Variations were performed of the air to fuel ratio (AFR), the air preheating temperature and the pressure drop across the injection system to clarify the interpretation of the ion current signal. The results indicate a strong dependence of the ion current signal on the AFR and that the technique has distinct advantages compared to OH* chemiluminescence measurements: The measurement equipment is comparable non-expensive and the results reveal that the reaction rate is measured directly and are not interpreted from a 3D image. A transition in flame shape from a compact to a tornado flame can be clearly identified with the applied probe. Furthermore, regions with high temperature fluctuations do not necessarily reveal the reaction zone in a recirculating flow field.
The Potential of Using the Ion-Current Signal for Optimizing Engine Stability - Comparisons of Lean and EGR (Stoichiometric) Operation
