Combustion instability poses a negative impact on the performance and structural durability of both land-based and aircraft gas turbine engines, and early detection of combustion instabilities is of paramount importance not only for performance monitoring and fault diagnosis, but also for initiating efficient decision and control of such engines. Combustion instability is, in general, characterized by self-sustained growth of large-amplitude pressure tones that are caused by a positive feedback arising from complex coupling of localized hydrodynamic perturbations, heat energy release, and acoustics of the combustor. This paper proposes a fast dynamic data-driven method for detecting early onsets of thermo-acoustic instabilities, where the underlying algorithms are built upon the concepts of symbolic time series analysis (STSA) via generalization of D-Markov machine construction. The proposed method captures the spatiotemporal co-dependence among time series from heterogeneous sensors (e.g. pressure and chemiluminescence) to generate an information-theoretic precursor, which is uniformly applicable across multiple operating regimes of the combustion process. The proposed method is experimentally validated on the time-series data, generated from a laboratory-scale swirl-stabilized combustor, while inducing thermo-acoustic instabilities for various protocols (e.g. increasing Reynolds number (Re) at a constant fuel flow rate and reducing equivalence ratio at a constant air flow rate) at varying air-fuel premixing levels. The underlying algorithms are developed based on D-Markov entropy rates, and the resulting instability precursor measure is rigorously compared with the state-of-the-art techniques in terms of its performance of instability prediction, computational complexity, and robustness to sensor noise.
Significant efforts have been and are being spent on developing intensified tubular reactors for continuous manufacturing of fine and specialty chemicals. In this work, we have proposed a new design of passive mixer-cum-reactor for process intensification and development of continuous processes. The mixer/reactor consists of threaded inserts with cone-shaped ends, placed concentrically in the tube such that fluid flows through the annular region between the inserts and the tube. The proposed design is easy to fabricate, maintain, and overcomes the limitations of scale up/scale down compared to most of the commercial passive mixers. The split and recombine of flow around inserts, the swirling effect generated by threads, change in the swirl direction due to change in the direction of screw threads, and pinching effect/expansion at the cone-cone shaped ends realize desired enhancements in mixing and heat transfer. A detailed computational study has been carried out on the mixer-cum-reactor to characterize flow, mixing and heat transfer at different operating conditions using a verified and validated CFD model. Various designs and configurations of threaded inserts were considered: 5-channel, 7-channel and 9-channel, smooth surface (no threading) and smooth surface-extended rear end inserts. The flow, mixing and heat transfer were characterized over the Reynolds number range of 100 to 1600. Structure of the generated swirling flow, effect of pinching/expansion, direction reversal of flow, tracer fraction, temperature and path lines were investigated systematically to gain new insights. Threaded inserts could achieve excellent mixing (>99 % of mixing intensity) and heat transfer (7 times smooth inserts and 20 times without inserts). The presented results will provide a sound basis for selecting appropriate threaded inserts for intensifying mixing and heat transfer in tubular reactors. The work also provides a useful starting point for further work on multiphase flows in a tubular reactor with threaded inserts.
A laboratory-scale swirl-stabilized combustor is experimentally characterized for various configurations involving variable air flow rates and different fuel injection locations. Unsteady pressure and heat release rate measurements were obtained simultaneously in order to determine the stability map of the combustor for the experimented configurations. It is observed that a sharp rise in pressure amplitude coincides with a break in the dominant spectral content variation with the inlet Reynolds number. The time series data were analyzed by using the tools of symbolic dynamic filtering and the divergences among the outputs of each sub-class of observations were obtained as anomaly measures. In the proposed method, symbol strings are generated by partitioning the (finite-length) time series to construct a special class of probabilistic finite state automata (PFSA) that have a deterministic algebraic structure. The anomaly measures are defined based on the probabilistic state vectors distribution across each sub class. The method which is based on representing a given time series data as a set of PFSA is observed to be capable of predicting an impending combustion instability as well as to distinguish between the symbol-state distribution among various instability conditions. The measure also successfully captures changes in the thermoacoustic regime as a function of the fuel injection location.
Flame dynamics and combustion instability is a complex problem involving different non-linearities. Combustion instability has several detrimental effects on flight-propulsion dynamics and structural integrity of gas turbines and any such spaces where combustion takes places internally, primarily in internal combustion engines. The description of coherent features of fluid flow in such cases is essential to our understanding of the flame dynamics and propagation processes. A method that is able to extract dynamic information from flow fields that are generated by a direct numerical simulation or visualized in a physical experiment (like in the case discussed in this paper) is Dynamic Mode Decomposition. This paper presents such a feature extraction and stability analysis of hi-speed combustion flames using Dynamic Mode Decomposition and it’s sparsity promoting variant. Extensive experimental data was collected in a swirl-stabilized dump combustor at various operating conditions (e.g. premixing level and flow velocity) for analysing the flame stability conditions.
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