Global linear stability analysis of a slit flame subject to intrinsic thermoacoustic instability

The present study makes use of the adjoint modes of the Linearized Reactive Flow (LRF) equations to investigate the Intrinsic Thermoacoustic (ITA) feedback loop of a laminar premixed slit flame. The analysis shows that the ITA feedback loop is closed by vorticity generated in the boundary layer of the slit by impinging acoustic waves penetrating the slit. In this region, adjoint vorticity shows a high sensitivity of the flow. It is also hypothesised that the ITA eigenmode smoothly transitions to a purely hydrodynamic mode -- vortex shedding -- for a passive flame. The computational domain is chosen sufficiently short so as to isolate the ITA feedback loop from cavity modes. This analysis is made possible by the holistic character of the LRF model, i.e. a direct linearization of the non-linear reactive flow equations, including explicit finite rate chemistry and avoiding idealization of the flow.Comment: 11 pages, 6 figures. Presented at the International Congress on Sound and Vibration, July 2023, Pragu

Non-linear simulations of combustion instabilities with a quasi-1D Navier-Stokes code

As lean premixed combustion systems are more susceptible to combustion instabilities than non-premixed systems, there is an increasing demand for improved numerical design tools that can predict the occurrence of combustion instabilities with high accuracy. The inherent non-linearities in combustion instabilities can be of crucial importance, and we here propose an approach in which the one-dimensional Navier-Stokes and scalar transport equations are solved for geometries of variable cross-section. The focus is on attached flames, and for this purpose a new phenomenological model for the unsteady heat release from a flame front is introduced. In the attached flame method (AFM) the heat release occurs over the full length of the flame. The non-linear code with the use of the AFM approach is validated against results from an experimental study of thermoacoustic instabilities in oxy-fuel flames by Ditaranto and Hals [Combustion and Flame, 146, 493-512 (2006)]. The numerical simulations are in accordance with the experimental measurements and both the frequencies and the amplitudes of the resonant acoustic pressure modes are reproduced with good accuracy.Comment: Submitted for publication in "Journal of Sound and Vibration" (30 pages, 8 figures

Effect of hydrogen addition on the consumption speed of lean premixed laminar methane flames exposed to combined strain and heat loss

This study presents a numerical analysis of the impact of hydrogen addition on the consumption speed of premixed lean methane-air laminar flames exposed to combined strain and heat loss. Equivalence ratios of 0.9, 0.7, and 0.5 with fuel mixture composition ranging from pure methane to pure hydrogen are considered to cover a wide range of conditions in the lean region. The 1-D asymmetric counter-flow premixed laminar flame aCFPF with heat loss on the product side is considered as a flamelet configuration that represents an elementary unit of a turbulent flame and the consumption speed is used to characterize the effect of strain and heat loss. Due to the ambiguity in the definition of the consumption speed of multi-component mixtures, two definitions are compared. The definition of the consumption speed based on the heat release results in lower values of the stretched flame speed and even an opposite response to strain rate for some methane-hydrogen-air mixtures compared to the definition based on the fuel consumption. Strain rate leads to an increase in the flame speed for the lean methane-hydrogen mixtures, reaching a maximum value after which the flame speed decreases with strain rate. Heat loss decreases the stretched flame speed and leads to a sooner extinction of the flamelet due to combined strain and heat loss. Hydrogen addition and equivalence ratio significantly impact the maximum consumption speed and the flame response to combined strain rate and heat loss. The effect of hydrogen on the thermo-diffusive properties of the mixture, characterized by the Zel'dovich number and the effective Lewis number, are also analyzed and related to the effect on the consumption speed. Two definitions of the Lewis number of the multi-component fuel mixture are evaluated against the results from the aCFPF.Comment: Submitted to journal Combustion Theory and Modelling - Manuscript ID TCTM-2022-06-6

Learning Hidden States in a Chaotic System: A Physics-Informed Echo State Network Approach

International audienceWe extend the Physics-Informed Echo State Network (PI-ESN) framework to reconstruct the evolution of an unmeasured state (hidden state) in a chaotic system. The PI-ESN is trained by using (i) data, which contains no information on the unmeasured state, and (ii) the physical equations of a prototypical chaotic dynamical system. Non-noisy and noisy datasets are considered. First, it is shown that the PI-ESN can accurately reconstruct the unmeasured state. Second, the reconstruction is shown to be robust with respect to noisy data, which means that the PI-ESN acts as a denoiser. This paper opens up new possibilities for leveraging the synergy between physical knowledge and machine learning to enhance the reconstruction and prediction of unmeasured states in chaotic dynamical systems

Accounting for convective effects in zero-Mach-number thermoacoustic models

This paper presents a methodology to account for some mean-flow effects on thermo-acoustic instabilities when using the zero-Mach-number assumption. It is shown that when a computational domain is represented under the M=0 assumption, a nonzero-Mach-number element can simply be taken into account by imposing a proper acoustic impedance at the boundaries so as to mimic the mean flow effects in the outer, not computed flow domain. A model that accounts for the coupling between acoustic and entropy waves is presented. It relies on a “delayed entropy coupled boundary condition” (DECBC) for the Helmholtz equation satisfied by the acoustic pressure. The model proves able to capture low-frequency entropic modes even without mean-flow terms in the fluctuating pressure equation

Previsão de tempestades severas e de transbordamentos no rio Quitandinha – Petrópolis/RJ

The identification of the atmospheric environment favorable to the formation and development of intense rainfall has been a theme of great relevance by the scientific community in the last decades. This importance is based on the creation of measures which are able to mitigate the consequences related to the rainfall magnitudes, especially those associated with the occurrence of flood events, landslides and, in the most critical scenarios, natural disasters. A strategy for an operational forecasting and nowcasting of intense rainfall, with emphasis on the events of Quitandinha River flooding, located in Petrópolis city, occurred between the years of 2013 and 2016 is presented. The methodology for implementing the strategy involved three stages of analysis. In the first stage, it was proposed the analysis of the Quitandinha River basin characteristics, as well as the identification of precipitation patterns and predominant meteorological systems. The calculation of dynamic and thermodynamic parameters in the synoptic and local scales is presented at the proposal for the second stage. Finally, in the third stage, short-term estimates using weather radars are explored. The results showed the applicability of the operational strategy for the analyzed region and that estimulates the recommendation for the application of the conceived methodology to other regions of Brazil. It is expected that the developed solutions could contribute to and consolidate the use of new tools for operational monitoring and alert systems.A identificação das condições atmosféricas conducentes à formação e ao desenvolvimento de chuvas intensas tem sido tema de grande relevância pela comunidade científica nas últimas décadas. Tal importância firma-se na criação de medidas que possam mitigar as consequências associadas às magnitudes das chuvas, principalmente relacionadas à ocorrência de eventos de cheias, aos deslizamentos de terra e, nos cenários mais críticos, aos desastres naturais. Uma estratégia para a previsão operacional de chuvas intensas, com ênfase nos eventos de transbordamento do rio Quitandinha, localizado em Petrópolis – Rio de Janeiro, ocorridos entre os anos de 2013 e 2016 é apresentada. A metodologia para a aplicação da estratégia envolveu três estágios de estudo. No primeiro estágio, foi proposta a análise das características da bacia do rio Quitandinha, assim como a identificação de padrões de precipitação e sistemas meteorológicos predominantes. O cálculo de indicadores dinâmicos e termodinâmicos nas escalas sinótica e local é apresentado como estágio intermediário. Por fim, no terceiro estágio, são exploradas estimativas de curto prazo utilizando radares meteorológicos. Os resultados mostraram a aplicabilidade da estratégia operacional para a região analisada, estimulando o incentivo de que se empregue a metodologia delineada em outras regiões do Brasil. Espera-se que as soluções desenvolvidas possam contribuir e permitam integrar o uso de novas ferramentas em sistemas de monitoramento e alerta operacionais