ABSTRACT:The curing reaction of an epoxy resin [diglycidyl ether of bisphenol A (DGEBA)] combined with a methyl-hexahydrophtalic anhydride (MHHPA) hardener and a benzyldimethylamine (BDMA) accelerator was studied over a temperature range of 60 -140°C to build its isothermal time-temperature-transformation (TTT)-cure diagram. This includes gelation and vitrification measurements using rheological measurement techniques, monitoring of the glass transition temperature and the reaction kinetics by differential scanning calorimetry, and determination of the following critical glass transition temperatures: T g0 , Gel T g , and T gϱ . A new curing kinetics model, based on the Sesták-Berggren model, was developed including both chemicaland diffusion-controlled stages of reaction. Then, the TTTcure diagram was built by numerical integration of the kinetics model, and good agreement was obtained by comparison with experimental data. Additionally, for the first time, this model takes into account the incomplete cure, which occurs when the thermosetting system is cured below its ultimate glass transition temperature, leading to a more realistic description of the cure evolution after vitrification.

Natural ester fluids may be used in new transformers as well as to retrofill existing units in order to improve their performance and reliability. Designing or retrofilling power transformers with natural ester fluids requires, however, to account for a number of significant differences in properties, characteristics, and material parameters between natural ester fluids and mineral oil in order to obtain the desired performance (thermally and dielectrically). This paper gives a comprehensive review of natural-ester-fluid main properties and associated values when it comes to environmental impact, fire resistance, and overall performance of transformers filled with such fluids. The key fluid characteristics impacting the design of power transformers are also highlighted.

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This paper introduces a novel approach to monitor pressure dynamics in turbomachinery. This innovation is motivated by the need expressed by machine OEMs and end-users to detect and avoid combustion instabilities, as well as lean-blowout (LBO), in low emission combustion systems. Such situations are often characterised by a marked increase of pressure signals in low frequency range. The piezoelectric technology, conventionally used for pressure measurements, presents sensitivity and stability issues at high temperatures and low frequencies. Here a new paradigm for pressure sensing, based on optical interferometry, is characterised and validated. The interferometric sensing system is designed to provide a larger range of measurement frequencies with better performance, in the low frequency range (< 50Hz), while exposed to high temperatures. This unique feature allows the real-time observation of events, such as the specific behaviour of a low frequency flame dynamic, which is characteristic of an imminent LBO. This improved monitoring system will support an optimisation of the machine performance, leading to a safer, cleaner, more flexible and more cost-efficient operation for the end-user. The novel measurement system has been characterised under non-reactive and reactive conditions within the frame of a joint study between Meggitt SA, Combustion Bay One e.U. and FH Joanneum GmbH. The technology is first described, including the relevant hardware and software components of the measurement chain. The different experimental set-ups and conditions are also illustrated. The results of the test campaign and their subsequent analysis are then presented, supporting the expected advantages over piezoelectric technology. In conclusion, a possible strategy for the detection of LBO precursors based on low frequency data is proposed.

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