No abstract

Coherent structures and mixing in the flow field of a jet in crossflow have been studied using computational (Large Eddy Simulation) and experimental (Particle Image Velocimetry and Laser-Induced Fluorescence) techniques. The mean scalar fields and turbulence statistics as determined by both are compared for circular, elliptic, and square nozzles. For the latter configurations, effects of orientation are considered. The computations reveal that the distribution of a passive scalar in a cross-sectional plane can be singleor double-peaked, depending on the nozzle shape and orientation. A proper orthogonal decomposition of the transverse velocity indicates that coherent structures may be responsible for this phenomenon. Nozzles which have a single-peaked distribution have stronger modes in transverse direction. The global mixing performance is superior for these nozzle types. This is the case for the blunt square nozzle and for the elliptic nozzle with high aspect ratio. It is further demonstrated that the flow field contains large regions in which a passive scalar is transported up the mean gradient (counter-gradient transport) which implies failure of the gradient diffusion hypothesis.

Concerning the efforts in reducing the impact of fossil fuel combustion on climate change for power production utilizing gas turbine engines Flue Gas Recirculation (FGR) in combination with post combustion carbon capture and storage (CCS) is one promising approach. In this technique part of the flue gas is recirculated and introduced back into the compressor inlet reducing the flue gas flow (to the CCS) and increasing CO2 concentrations. Therefore FGR has a direct impact on the efficiency and size of the CO2 capture plant, with significant impact on the total cost. However, operating a GT under depleted O2 and increased CO2 conditions extends the range of normal combustor experience into a new regime. High pressure combustion tests were performed on a full scale single burner reheat combustor high-pressure test rig. The impact of FGR on NOx and CO emissions is analyzed and discussed in this paper. While NOx emissions are reduced by FGR, CO emissions increase due to decreasing O2 content although the SEV reheat combustor could be operated without problem over a wide range of operating conditions and FGR. A mechanism uncommon for GTs is identified whereby CO emissions increase at very high FGR ratios as stoichiometric conditions are approached. The feasibility to operate Alstom’s reheat engine (GT24/GT26) under FGR conditions up to high FGR ratios is demonstrated. FGR can be seen as continuation of the sequential combustion system which already uses a combustor operating in vitiated air conditions. Particularly promising is the increased flexibility of the sequential combustion system allowing to address the limiting factors for FGR operation (stability and CO emissions) through separated combustion chambers.

No abstract

Large eddy simulations (LES) are performed for single and multiphase jets in crossflow (JICF). The multiphase JICF are compared to the single-phase case for the same momentum and mass flow ratios but with droplets of different sizes. Multiphase JICF have stronger counterrotating vortex pairs (CVPs) than a corresponding single-phase JICF. Moreover, their trajectories are higher and their induced wakes weaker. The smaller the Stokes number of the droplets, the more the solution approaches the solution for single-phase flow. The computed results show the formation of a CVP and horseshoe vortices, which are convected downstream. LES also reveals the intermittent formation of upright wake vortices from the horseshoe vortices on the ground toward the CVP. The dispersion of polydisperse spray droplets is computed using the stochastic parcel method. Atomization and droplet breakup are modeled by a combination of the breakup model by Reitz and the Taylor analogy breakup model (see Caraeni, D., Bergström, C., and Fuchs, L., 2000, Flow, Turbul. Combust., 65(2), pp. 223–244). Evaporation and droplet collision are also modeled. The flow solver uses two-way coupling. Averages of the velocity and gaseous fuel mass fraction are computed. The single-phase JICF is validated against experimental data obtained by PIV. Additionally, the PDFs and frequency spectra are presented.