Burner flame stabilisation of alternative fuels using nozzle constrictions

Swirl flame stabilisation of lean premixed fuels has been studied and applied to gas turbines for a number of years, giving considerable benefits in terms of reduced pollutant emission, especially of NOx. However, there are still problems that can occur during the combustion process including those related to flashback (especially with hydrogen enriched fuels) and combustion induced instabilities. Swirl of the primary system is optimised to minimize pressure drop, flame contact with the injectors and swirl system, whilst avoiding flashback. Pure hydrogen or hydrogen enriched fuels give rise to especial problems owing to the high flame speed of hydrogen, potential for flashback in conventional or simply modified combustors and often requirement for multi fuel operation. Solutions adopted commercially are normally compromised, leaving considerable room for improvement. This paper describes a combined practical and modelling approach to study and reduce the effect of flashback in practical swirl burners using both a flexible experimental combustor, coupled with extensive Computational Fluid Dynamics modelling to guide experimental progress. The results proved that by adding CO2 the flashback limits can be improved, whilst H2 enriched flames are more difficult to control. Several varied CRZs developed in the field as a consequence of the nozzle configuration, showing high dependence not on the Swirl number, but the type of flow expansion. It was confirmed that the flame can be manipulated to avoid physical contact with the burner solid surfaces, the Swirl number being a particularly important parameter. The size and shape of the CRZ can be readily manipulated to satisfy particular requirements.