Research Projects

Gas turbines are used in the aero-transport and power generation sectors. International regulatory bodies enforce legislative standards for gas turbine emissions. To reduce NOx emissions, a combustion staging with the rich-quench-lean (RQL) concept has been implemented commercially. New soot emission standards have been introduced recently. Thus, it is worthwhile to investigate soot formation in RQL-type combustors. We have commissioned an optically accessible RQL rig at IISc which is operated with ethylene gaseous fuel. The rig has been characterized in detail through velocity field, air/fuel mixing field, reaction zone imaging, and soot concentration measurements. We aim to advance this work through spray flame investigations by applying simultaneous quantitative laser diagnostics. The present rig will be modified to enable spray flame operation by pre-heating primary air and by optimizing the fuel-injector design. The proposed work will examine both the soot and nitric oxide (NO) formations in the RQL combustor operating with liquid fuels. The aim is to understand soot/NO formation processes under RQL conditions with liquid fuels. At present, there are only a few in-situ soot and NO studies in spray flames. A detailed understanding of the soot/NO formation mechanisms in RQL combustors is lacking. This gap will be filled by the proposed work. We will also examine the heat release rate (HRR) by using the product of OHxCH2O concentrations as a marker for HRR to examine flame structure. The secondary aim is to understand the influence of swirler geometry and operating conditions on soot and NO emissions. To ensure better atomization, a high-shear swirler will be used, which incorporates two concentric swirlers with a designed air split between inner and outer swirlers. Traditionally, emissions have been measured using ex-situ probe-based gas analyzers which only provide net exhaust emissions. To gain local insights into soot and NO formation/consumption, laser-based simultaneous diagnostics will be used. To know the soot/NO formation sites relative to the flame front, the OH radical distribution will be imaged simultaneously. Thus, there are three simultaneous measurements, namely OH/NO, OH/Soot, and OH/CH2O for HRR imaging. The main techniques are laser-induced incandescence (LII, for soot) and laser-induced fluorescence (LIF, for NO, OH, and CH2O). The major challenge lies in the quantification of LIF and LII signals. The expertise gained by our lab will be utilized to correct for soot-maturity led LII biases. LIF signal will be corrected for local temperature and quenching species through spectroscopic simulations based on previously gained competence. The outcome of the project will be a detailed database of soot/NO emissions and flame structure trends with a variation of fuel type, swirler design, and flow conditions. The following three fuels will be examined: n-heptane, jet fuel 4-component surrogate, and Jet-A commercial fuel. A second-generation multi-university-research-initiative (MURI-2) surrogate will be used which is a blend of: n-dodecane, iso-octane, 1,3,5-trimethylbenzene, and n-propylbenzene. Three swirlers with varying air-split ratios (between inner/outer swirlers) will be examined. Three quench-to-primary airflow splits will be tested. After preliminary measurements, a select test cases will be examined in detail through simultaneous quantitative laser diagnostics. All the data and understanding will be synthesized to propose a comprehensive mechanism for soot/NO formation in this complex RQL spray flame. The research outcome will be used to establish a specialized RQL network of international colleagues to facilitate knowledge exchange between experimental and simulation communities. The fundamental insights from this work will eventually guide combustor designers in developing low-emission technologies. Overall, both fundamental and technological advances are anticipated through the proposed research.