This paper reports the results of a joint experimental and numerical study of the ow characteristics and flame stabilization of a hydrogen rich jet injected normal to a turbulent, vitiated cross ow of lean methane combustion products. Simultaneous high-speed stereoscopic PIV and OH PLIF measurements were obtained and analyzed alongside three-dimensional direct numerical simulations of inert and reacting JICF with detailed H2/CO chemistry. Both the experiment and the simulation reveal that, contrary to most previous studies of reacting JICF stabilized in low-to-moderate temperature air cross ow, the present conditions lead to an autoigniting, burner-attached flame that initiates uniformly around the burner edge. Significant asymmetry is observed, however, between the reaction zones located on the windward and leeward sides of the jet, due to the substantially different scalar dissipation rates. The windward reaction zone is much thinner in the near field, while also exhibiting significantly higher local and global heat release than the much broader reaction zone found on the leeward side of the jet. The unsteady dynamics of the windward shear layer, which largely control the important jet/cross flow mixing processes in that region, are explored in order to elucidate the important flow stability implications arising in the reacting JICF. Vorticity spectra extracted from the windward shear layer reveal that the reacting jet is globally unstable and features two high frequency peaks, including a fundamental mode whose Strouhal number of ~0.7 agrees well with previous non-reacting JICF stability studies. The paper concludes with an analysis of the ignition, ame stabilization, and global structure of the burner-attached flame. Chemical explosive mode analysis (CEMA) shows that the entire windward shear layer, and a large region on the leeward side of the jet, are highly explosive prior to ignition and are dominated by non-premixed flame structures after ignition. The predominantly mixing limited nature of the flow after ignition is confirmed by computing the Takeno flame index, which shows that ~70% of the heat release occurs in non-premixed regions.
The ever increasing amount of data generated by scientific simulations coupled with system I/O constraints are fueling a need for in-situ analysis techniques. Of particular interest are approaches that produce reduced data representations while maintaining the ability to redefine, extract, and study features in a post-process to obtain scientific insights. This paper presents two variants of in-situ feature extraction techniques using segmented merge trees, which encode a wide range of threshold based features. The first approach is a fast, low communication cost technique that generates an exact solution but has limited scalability. The second is a scalable, local approximation that nevertheless is guaranteed to correctly extract all features up to a predefined size. We demonstrate both variants using some of the largest combustion simulations available on leadership class supercomputers. Our approach allows state-of-the-art, feature-based analysis to be performed in-situ at significantly higher frequency than currently possible and with negligible impact on the overall simulation runtime.
52nd AIAA Aerospace Sciences Meeting - AIAA Science and Technology Forum and Exposition, SciTech 2014
Lietz, C.; Hassanaly, M.; Raman, V.; Kolla, Hemanth K.; Chen, J.; Gruber, A.
In the design of high-hydrogen content gas turbines for power generation, ashback of the turbulent ame by propagation through the low velocity boundary layers in the premix- ing region is an operationally dangerous event. Predictive models that could capture the onset of ashback would be indispensable in gas turbine design. For this purpose, modeling of the ashback process using the large eddy simulation (LES) approach is considered here. In particular, the goal is to understand the modeling requirements for predicting ashback in confined goemetries. The ow configuration considered is a turbulent channel ow, for which high-fidelity direct numerical simulation (DNS) data already exists. A suite of LES calculations with different model formulations and filterwidths is considered. It is shown that LES predicts certain statistical properties of the ame front reasonably well, but fails to capture the propagation velocity accurately. It is found that the ashback process is invariant to changes in the initial conditions and additional near-wall grid refinement but the LES filterwidth as well as the subfilter models are found to be important even when the turbulence is almost fully resolved. From the computations, it is shown that for an LES model to predict ashback, suffcient resolution of the near-wall region, proper represen- tation of the centerline acceleration caused by ame blockage, and appropriate modeling of the propagation of a wrinkled ame front near the center of the channel are considered the critical requirements.