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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - ITPF (Interfaces in Turbulent Premixed Flames (ITPF))

Teaser

Summary

â€¢ What is the problem/issue being addressed?
The characterization of the interactions between the flame and turbulence is essential in mixing and combustion. In the last decades, the wrinkling of a flame or of a scalar field due to turbulence has been envisioned as the result of vortices interacting with the flame front. However, this preconceived notion should be substantiated through theoretical studies and numerical simulations to accurately understand the nature of the flame-turbulence interactions that are responsible for flame strain and local quenching events. In a turbulent flow, fluid from the irrotational region becomes turbulent through the propagation of the Turbulent/Non-Turbulent (T/NT) enstrophy interface. In many T/NT interfaces, large scale eddy motion folds the separating surface and ingest or engulf irrotational fluid into the turbulent region; subsequently, this â€˜ingestedâ€™ irrotational fluid is â€˜digestedâ€™ and incorporated to the turbulent region by viscous diffusion of vorticity at small scales. These entrainment mechanisms have been studied for the past decades. However, the description and quantification of the importance of small and large scales contributing to entrainment mechanisms are still unsolved. Thus, this project, â€˜Interfaces in Turbulent Premixed Flamesâ€™ (ITPF â€“ EU Project NÂº 706672), aims at improving the physical understanding of the entrainment of hot reacted products in annular co-flows of turbulent premixed flames into jets of fresh reactants. This is an essential point in combusting turbulent free shear flows, because a better knowledge of the dynamics of T/NT and scalar interfaces would lead to better predictions of flame instabilities and field structures.

â€¢ Why is it important for society?
The findings obtained during the ITPF project have had a significant impact and implications in the combustion community. These results can help us understand how the interaction between the scalar and enstrophy interfaces strongly influences the local scalar geometry. A comprehension of these physical mechanisms should better guide the formulation of sound and accurate mixing and combustion models. This can eventually help generating clean, save and cost-effective combustion systems, for burning fossil and sustainable fuels to provide power, heat and transportation.

â€¢ What are the overall objectives?
This project aims at understanding the physics of entrainment in turbulent premixed flames. This research characterizes the entrainment processes through the study and comparison of the T/NT enstrophy interface and scalar interfaces. This study applies statistical and topological methods to identify the contributions to entrainment and engulfment through analyzing flow and scalar fields in high-resolution simulations of turbulent premixed flames. This project develops and post-processes Direct Numerical Simulations (DNS) of turbulent premixed flames and analyzes their results, in conjunction with Large Eddy Simulations (LES). A smart combination of DNSs and LESs permits to unveil contributions of large and small structures to the entrainment process, to better comprehend physical mechanisms and to formulate sound and accurate mixing and combustion models.

Work performed

The findings obtained during the ITPF project have had a significant impact and implications in the combustion community. The relevant new contributions are: 1) Scrutinizing of small-scale flow topologies and local scalar field geometries in the different regions of the computational domain and across the T/NT enstrophy and scalar interfaces of turbulent premixed flames. 2) Exploring the time evolution of flow structures and flame curvature in various flame configurations. 3) Obtaining new results of the local entrainment velocity across the T/NT enstrophy interface and scalar interface using 3-D LES/DNS. The findings obtained shed some light on the nature of flame stabilization because the analysis i) gives information about the engulfment and digestion by molecular diffusion of hot products and fresh gases near the burner exit, ii) can provide details on how the local equivalence ratio of the fresh mixture affects the flame anchoring, and iii) presents new ideas to describe and quantify transport processes across the inner enstrophy and scalar interfaces that influence the flame stabilization. These results are really new and highlight the importance of the entrainment in mixing and probably in local quenching events for turbulent premixed flames.

The data generated in the project have been archived via the central storage system of the University of Duisburg-Essen (UDE). By storing the data in cross-platform and openly documented data formats, it is ensured that the data can be accessed at the research centers as well as possibly by third parties. Data sets with a reference character were made available via the database maintained by Prof. Kempf\'s group. In accordance with the principles of Horizon 2020, the main findings obtained during the ITPF project have been published in â€˜greenâ€™ open access and through peer-reviewed international scientific journals with high Impact Factor. Dr. Cifuentes and Prof. Kempfâ€™s group will continue with the dissemination of results in the general public through formal academic publications, conferences, symposia in different specialized fields and public lectures. The exploitation of results will continue having scientific impact and overarching implications in the international combustion community. In this context, this action has contributed to improve European excellence due to the high level of training, which puts EU researchers in a more competitive position in comparison to others international efforts in the field, and due to the strengthen of the incipient scientific ties that involved the cooperation with national and international partners.

Final results

The physical understanding and results emerged from the ITPF project has offered a robust conceptual framework and has opened new pathways to analyze turbulent combusting flows. These results can help us understand how the interaction between the scalar and enstrophy interfaces strongly influences the local scalar geometry. The insight gained by this analysis will, in turn, be used to propose more physically sound and accurate turbulent mixing and combustion models that can eventually help generating clean, save and cost-effective combustion systems. In a short term, a new study to formulate new molecular micro-mixing models for MILD (Moderate or Intense Low oxygen Dilution) combustion, which is one track for securing clean combustion, will use the data generated by the ITPF project. Dr. Cifuentes has increased his skills in DNS and LES techniques and is very interested in continuing and building up an expertise on modelling applied to MILD combustion. The ITPF project allowed him to acquire all the complementary knowledge that will boost Dr. Cifuentes career to solve old and new challenges in this MILD combustion topic. A new proposal to investigate this topic will be submitted to DFG (Deutsche Forschungsgemeinschaft) German Research Foundation. In summary, the ITPF project has directly addressed the cross-cutting priority of sustainable development in Information Science and Engineering established by the H2020 Work Programme and has reinforced the already large European competitiveness in turbulent combustion research.