Transfer learning (TL) is a machine learning (ML) tool where the knowledge, acquired from a source domain, is 'transferred' to perform a task in a target domain that has (to some extent) a similar setting. The underlying concept does not require the ML method to analyse a new problem from the beginning, and thereby both the learning time and the amount of required target-domain data are reduced for training. An example is the occurrence of thermoacoustic instability (TAI) in combustors, which may cause pressure oscillations, possibly leading to flame extinction as well as undesirable vibrations in the mechanical structures. In this situation, it is difficult to collect useful data from industrial combustion systems, due to the transient nature of TAI phenomena. A feasible solution is the usage of prototypes or emulators, like a Rijke tube, to produce largely similar phenomena. This paper proposes symbolic time series analysis (STSA)-based transfer learning, where the key idea is to develop a capability of discrimination between stable and unstable operations of a combustor, based on the time series of pressure oscillations from a data source that contains sufficient information, even if it is not the target regime, and then transfer the learnt models to the target regime. The proposed STSA-based pattern classifier is trained on a previously validated numerical model of a Rijke-tube apparatus. The knowledge of this trained classifier is 'transferred' to classify similar operational regimes in: (i) an experimental Rijke-tube apparatus and (ii) an experimental combustion system apparatus. Results of the proposed transfer learning have been validated by comparison with those of two shallow neural networks (NN)-based TL and another NN having an additional long-short-term-memory (LSTM) layer, which serve as benchmarks, in terms of classification accuracy and computational complexity.

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