Thermoacoustic analysis of the dynamic pressure inside a model combustor during limit cycle oscillations

In this work comprehensive experimental and numerical studies incorporating the most relevant physical mechanisms causing limit cycle pressure and combustion rate oscillations (LCO) in a laboratory scale combustor will be discussed. The strong interaction between the aerodynamics-combustion-acoustic oscillations (ACA), and under specific conditions the aerodynamics-combustion-structural vibrations (ACS), is studied by a careful selection of experiments and numerical simulations performed using commercially available computational models. It is shown predominantly that the convective time scales due to the aerodynamics at the flame stabilizer and the time period related to acoustic propagation have to be of the same order in magnitude to be able to drive the system into LCO. The measurements indicated that the frequency spectrum of the oscillations of the LCO has a distinct peak close to the natural mode of the combustor along with higher order “harmonics” due to non-linear effects. Some non-harmonic higher order peaks are observed that are associated with the structural (liner) natural frequencies of vibration. A numerical simulation has been performed using the commercial code (ANSYS V13.0) that includes the effects of fluid-structure interaction by means of pressure load transfer on to the structure and vice-versa. The fluid domain is modeled using CFX and the structural domain is represented by ANSYS. The information is exchanged between the two domains dynamically at every time step computed. In order to reduce the computational effort and quickly gain insight into the problem only a 2 mm slice of the whole geometry has been considered making it essentially a 2D analysis. The good agreement between the model and measured instability frequencies shows a very promising approach in predicting the limit cycle oscillations in this kind of configurations

Sensitivity of the Numerical Prediction of Turbulent Combustion Dynamics in the LIMOUSINE Combustor

The objective of this study is to investigate the sensitivity and accuracy of the reaction flow-field prediction for the LIMOUSINE combustor with regard to choices in computational mesh and turbulent combustion model. The LIMOUSINE combustor is a partially premixed, bluff body-stabilized natural gas combustor designed to operate at 40–80 kW and atmospheric pressure and used to study combustion instabilities. The transient simulation of a turbulent combusting flow with the purpose to study thermoacoustic instabilities is a very time-consuming process. For that reason, the meshing approach leading to accurate numerical prediction, known sensitivity, and minimized amount of mesh elements is important. Since the numerical dissipation (and dispersion) is highly dependent on, and affected by, the geometrical mesh quality, it is of high importance to control the mesh distribution and element size across the computational domain. Typically, the structural mesh topology allows using much fewer grid elements compared to the unstructured grid; however, an unstructured mesh is favorable for flows in complex geometries. To explore computational stability and accuracy, the numerical dissipation of the cold flow with mixing of fuel and air is studied first in the absence of the combustion process. Thereafter, the studies are extended to combustible flows using standard available ansys-cfx combustion models. To validate the predicted variable fields of the combustor's transient reactive flows, the numerical results for dynamic pressure and temperature variations, resolved under structured and unstructured mesh conditions, are compared with experimental data. The obtained results show minor dependence on the used mesh in the velocity and pressure profiles of the investigated grids under nonreacting conditions. More significant differences are observed in the mixing behavior of air and fuel flows. Here, the numerical dissipation of the (unstructured) tetrahedral mesh topology is higher than in the case of the (structured) hexahedral mesh. For that reason, the combusting flow, resolved with the use of the hexahedral mesh, presents better agreement with experimental data and demands less computational effort. Finally, in the paper, the performance of the combustion model for reacting flow is presented and the main issues of the applied combustion modeling are reviewe

Seguimiento de las guías españolas para el manejo del asma por el médico de atención primaria: un estudio observacional ambispectivo

Objetivo Evaluar el grado de seguimiento de las recomendaciones de las versiones de la Guía española para el manejo del asma (GEMA 2009 y 2015) y su repercusión en el control de la enfermedad. Material y métodos Estudio observacional y ambispectivo realizado entre septiembre del 2015 y abril del 2016, en el que participaron 314 médicos de atención primaria y 2.864 pacientes. Resultados Utilizando datos retrospectivos, 81 de los 314 médicos (25, 8% [IC del 95%, 21, 3 a 30, 9]) comunicaron seguir las recomendaciones de la GEMA 2009. Al inicio del estudio, 88 de los 314 médicos (28, 0% [IC del 95%, 23, 4 a 33, 2]) seguían las recomendaciones de la GEMA 2015. El tener un asma mal controlada (OR 0, 19, IC del 95%, 0, 13 a 0, 28) y presentar un asma persistente grave al inicio del estudio (OR 0, 20, IC del 95%, 0, 12 a 0, 34) se asociaron negativamente con tener un asma bien controlada al final del seguimiento. Por el contrario, el seguimiento de las recomendaciones de la GEMA 2015 se asoció de manera positiva con una mayor posibilidad de que el paciente tuviera un asma bien controlada al final del periodo de seguimiento (OR 1, 70, IC del 95%, 1, 40 a 2, 06). Conclusiones El escaso seguimiento de las guías clínicas para el manejo del asma constituye un problema común entre los médicos de atención primaria. Un seguimiento de estas guías se asocia con un control mejor del asma. Existe la necesidad de actuaciones que puedan mejorar el seguimiento por parte de los médicos de atención primaria de las guías para el manejo del asma. Objective: To assess the degree of compliance with the recommendations of the 2009 and 2015 versions of the Spanish guidelines for managing asthma (Guía Española para el Manejo del Asma [GEMA]) and the effect of this compliance on controlling the disease. Material and methods: We conducted an observational ambispective study between September 2015 and April 2016 in which 314 primary care physicians and 2864 patients participated. Results: Using retrospective data, we found that 81 of the 314 physicians (25.8%; 95% CI 21.3–30.9) stated that they complied with the GEMA2009 recommendations. At the start of the study, 88 of the 314 physicians (28.0%; 95% CI 23.4–33.2) complied with the GEMA2015 recommendations. Poorly controlled asthma (OR, 0.19; 95% CI 0.13–0.28) and persistent severe asthma at the start of the study (OR, 0.20; 95% CI 0.12–0.34) were negatively associated with having well-controlled asthma by the end of the follow-up. In contrast, compliance with the GEMA2015 recommendations was positively associated with a greater likelihood that the patient would have well-controlled asthma by the end of the follow-up (OR, 1.70; 95% CI 1.40–2.06). Conclusions: Low compliance with the clinical guidelines for managing asthma is a common problem among primary care physicians. Compliance with these guidelines is associated with better asthma control. Actions need to be taken to improve primary care physician compliance with the asthma management guidelines

Nonlinear behavior of the thermoacoustic instabilities in the limousine combustor

The topic of this dissertation are the large amplitude pressure perturbations which are sometimes observed in gas turbine burners and boilers. The pressure oscillations are the result of unstable feedback between the combustion process and pressure waves. The amplitude of these oscillations reach such large amplitudes that the oscillation is limited by non-linear phenomena. This phenomenon is known as Limit Cycle Oscillation (LCO) of pressure and has a negative impact on the technical lifetime of the gas turbine engine. This research is part of the LIMOUSINE project, funded by the European Commission under the Marie Curie FP7 Initial Training Network, grant agreement number 214905. The flame dynamics have been studied empirically with laboratory scale, atmospheric combustion setups specifically designed for this project. These combustion chambers have rectangular cross-section and they use a wedge-shaped "bluff body" to stabilize the flame. The burner uses methane as fuel and the thermal power ranges between 30 and 70 kW. The burner can sustain different combustion regimes, depending on the fuel and air flows. The combustion process may be stable without any remarkable dynamics or may show pressure Limit Cycle Oscillations. The frequency of the LCO is related to the acoustic resonance frequency of the combustion chamber, as can be seen from the sensitivity of the Helmholtz and Strouhal dimensionless numbers. One of the features of the limit cycle is the non-linearity of the signal. The pressure spectrum shows regularly spaced secondary peaks which do not match any of the acoustic modes. The origin of these dynamics lies on the temporal waveform. The characteristics of the limit cycle were investigated with non-linear methods, such as the dimension of the attractor of the measured time series for the pressure and the heat release rate in the phase space. In general, stable combustion have chaotic motion while LCO cases present an associated dimension of 3. Nevertheless, the motion of the structural elements is deterministic for both combustion regimes. Last but not least, the interaction between wall vibration and combustion induced oscillations was examined. The coupling of the two physical systems is visible to some extent