The spatiotemporal coherence as an indicator of the stability in swirling flows

Combustion has played a key role in the development of human society; it has driven the evolution in the manufacturing processes, transportation, and it is used to produce the vast majority of the global energy consumed. The emission of pollutants from the combustion of fossil fuels in power plants lead to the development of advanced clean energy technologies, such as carbon capture and storage. Oxyfuel combustion is part of the carbon capture and storage techniques, and consists in the replacement of the air as oxidiser in the reaction with a mixture of oxygen and recycled flue gas, thus allowing a rich CO2 out-flow stream that can subsequently be compressed, transported and safely stored. The number of phenomena in combustion that are inherently dynamic impede the convention of a unique conception of flame stability. However, the quantification of the flow repeatability can produce insights on the efficiency of the process. This thesis presents the assessment of the stability in swirling flows through the calculation of their spatiotemporal coherence. The experimental data obtained from a 250 kWth combustor allows the assessment of the flame by means of spectral and oscillation severity analyses. A similar methodology is developed to analyse the data from large eddy simulations. The spectral analysis, the proper orthogonal decomposition and the dynamic mode decomposition have been employed to account for the temporal, spatial and spatiotemporal coherence of the flow, respectively. The spatiotemporal coherence is employed as a comprehensive term for the characterisation of the dynamic behaviour in the swirling flows and as a measurable indicator of the stability. This concept can be incorporated into the design of novel combustion technologies that will lead into a sustained reduction in pollutants and to the mitigation of the noxious effects associated to them

Prediction of the radiative heat transfer in small and large scale oxy-coal furnaces

Predicting thermal radiation for oxy-coal combustion highlights the importance of the radiation models for the spectral properties of gases and particles. This study numerically investigates radiation behaviours in small and large scale furnaces through refined radiative property models, using the full-spectrum correlated k (FSCK) model and Mie theory based data, compared with the conventional use of the weighted sum of grey gases (WSGG) model and the constant values of the particle radiation properties. Both oxy-coal combustion and air-fired combustion have been investigated numerically and compared with combustion plant experimental data. Reasonable agreements are obtained between the predicted results and the measured data. Employing the refined radiative property models achieves closer predicted heat transfer properties to the measured data from both furnaces. The gas-phase component of the radiation energy source term obtained from the FSCK property model is higher within the flame region than the values obtained by using the conventional methods. The impact of using non-grey radiation behaviour of gases through the FSCK is enhanced in the large scale furnace as the predicted gas radiation source term is approximately 2-3 times that obtained when using the WSGG, while the same term is in much closer agreement between the FSCK and the WSGG for the pilot-scale furnace. The predicted total radiation source term (from both gases and particles) is lower in the flame region after using the refined models, which results in a hotter flame (approximately 50-150 K higher in this study) compared with results obtained from conventional methods. In addition, the predicted surface incident radiation reduces by using the refined radiative property models for both furnaces, in which the difference is relevant with the difference in the predicted radiation properties between the two modelling techniques. Numerical uncertainties resulting from the influences of combustion model, turbulent particle dispersion and turbulence modelling on the radiation behaviours are discussed