Numerical Investigation of a Swirl Induced Flameless Combustor for Gas Turbine Applications

Flameless combustion (FC) is characterized by the presence of well distributed reaction zones, absence of temperature peaks and a strong turbulence-chemistry interaction. High efficiency, low NOx, low combustion noise and stability are some advantages of FC. This study is a continuation of previous FC studies carried out at the University of Cincinnati (UC). The distributed vortex burner, designed to operate at typical gas turbine conditions was found to exhibit FC at lean operating conditions. The current study aims to scale down this burner while retaining FC characteristics. Three new designs, C2B, C3B and C9B, were proposed to improve fuel entrainment in the central recirculation zone (CRZ) with non-intrusive fuel injection tubes while retaining flameless characteristics. These designs differ only in the angle and location of fuel injection. For this study, non-reacting and reacting flow simulations were carried out using STAR-CCM+. Turbulence was modeled using the Realizable k-epsilon model while complex chemistry Laminar Flame Concept (LFC) approach was used for the transport of chemical species. San Diego mechanism with 57 species and 268 reactions was used to provide a high-fidelity chemistry model for the combustion of gaseous Propane. The choice of numerical models was validated against experimental data from UC. Non-reacting simulations with air and fuel were performed to determine the differences in fuel flow path between the designs considered. Any difference in the flow field is caused by changes in fuel line configuration, and since the fuel flow rate is about two orders of magnitude lower than the total mass flow rate, all designs exhibit similar flow velocities and flow structures with few changes. The fuel streamlines reveal that C9B exhibits better fuel entrainment into CRZ. For reacting flow, an integral, non-dimensional parameter R independent of phi is defined to quantify the normalized amount of fuel consumed over any volume of interest, in this case-entire combustion chamber ( R-global) and only in the CRZ (R-Recirculation). R-global is similar for all designs implying that R-Recirculation of different designs are directly comparable. For phi= 0.36, C9B exhibits the highest value of R-Recirculation, further reiterating the observation of higher fuel entrainment in the non-reacting flow analysis. Baseline (scaled) and C3B exhibit markers of FC at phi= 0.36 with well distributed reaction zones as indicated by mass fraction distributions of species like CO2, CO and NOx, absence of temperature peaks and simultaneous reduction of CO and NOx. For the same phi, C2B and C9B show higher values of these quantities with the peak values concentrated over a small region. At high phi =0.7, all designs exhibit clear gradients in temperature and species mass fractions distribution along with much higher NOx. At this higher phi, baseline and C3B transition from the distributed combustion regime observed at phi=0.36 to turbulent diffusion flame. In conclusion, C3B is found to retain the FC characteristics of the original design while also providing a non-intrusive way to inject fuel.