【4/24 09:50~10:50 Keynote Speech】Multi-pronged Framework to Identify and Understand Combustion Dynamics under Wide Range of Operating Conditions
2021 Taiwan Combustion and Energy Conference,Huwei Township, Yunlin County, Taiwan, 24 April. 2021
Multi-pronged Framework to Identify and Understand Combustion Dynamics under Wide Range of Operating Conditions
Satyanarayanan R. Chakravarthy*1, Vikram Ramanan1
1Department of Aerospace Engineering and National Centre for Combustion Research and Development, IIT-Madras, Chennai, India
ABSTRACT: Combustion dynamics results from coupling between flow, flame and potential unsteady sources, and is typically excited at the duct natural frequencies. In the present work, we demarcate our findings related to combustion dynamics based on a) Application of reduced order models to determine the flow-flame coupling across multiple geometries; b) Identification of key control parameters in delaying/exciting combustion dynamics; c) Modelling and discovery of mechanisms employing high fidelity computations. The lower order models involve application of linear and non-linear mapping methods to estimate the dominant spatio-temporal hydrodynamic phenomena and associated growth/ decay rates. The linear techniques, namely, Proper Orthogonal Decomposition and Dynamic Mode Decomposition display variable growth/decay rates that are associated with multiple self-excited hydrodynamic and acoustic instabilities. These interactions result in intermittency and mixed mode oscillations. Non-linear techniques involve stacked convolutional auto-encoders, which in a semi-supervised manner, learn to differentiate stable and unstable combustion and hence, identify even short-scale precursors to combustion dynamics. Key control parameters identified in the present work include the effect of turbulence intensity in delaying combustion instability, and addition of hydrogen in exciting higher harmonics. Analysis of the former points to the requirement of a flow parameter (recirculation zone length) in addition to flame parameter to identify the non-dimensional group elements. The excitation of higher acoustic modes is related to the presence of two heat release rate zones with a stagger. The findings are crucial in the light of development of future engines based on hydrogen combustion. The computational results derived from multi-scale asymptomatic expansion and LES simulations highlight the role of coherent structures promoted by non-linear acoustic streaming. In the laminar framework, synchronization of the unsteady pressure to the duct acoustic mode is achieved by the locking-on of the acoustic mode to the hydrodynamics. We extend the study to liquid fuels by measurement and characterization of self-excited flow-instabilities influencing atomization under realistic engine conditions. The role of processing vortex core in droplet impingement and the effect of wall-film thickness on the secondary atomization is emphasized. These unsteady features underscore the role of such processes in controlling atomization and subsequently, combustion performance and dynamics. Extension of the study to forced acoustics, both longitudinal and transverse, reveals the effect of frequency on primary atomization.
Keywords: Combustion dynamics, reduced order models, high fidelity computation, spray dynamics.
Corresponding Author: Satyanarayanan R. Chakravarthy(Email: src@ae.iitm.ac.in)
Department of Aerospace Engineering & National Centre for Combustion Research and Development, IIT-M