Study on the Formation Mechanism and Critical Criterion of Transformer Jet Fire under External Heat Source

Jet fire is a kind of instability behavior in the combustion process of a transformer. Once the jet fire occurs, the typical characteristic parameters of the fire change nonlinearly, which brings uncertainty and danger to the emergency response. The current research mostly focuses on the combustion instability mechanism of oil-filled equipment under the action of an external heat source and pays little attention to the mechanism and critical criterion of jet fire formed by transformer leakage combustion instability. The leakage combustion of the transformer leads to the dynamic change of thermodynamic state parameters of the medium, and the mechanism of overpressure induced by the phase transition from metastable to unsteady state is not revealed. It is of great significance to study the formation mechanism and critical criterion of transformer jet fire for explaining the dynamic competition behavior between the internal gas production rate and discharge rate during transformer combustion. In this study, the heat loss experiments of transformer oil under different heating rates were carried out, the boiling range of transformer oil was obtained to be 102-300 °C, and the average activation energy was 65.41 kJ/mol. Experiments on the formation mechanism of transformer jet fire under different fire power were carried out, the phase characteristics of the jet release of transformer oil were elucidated, and it was clarified that overpressure was the driving force for the occurrence of the fire. It was found that the key criteria for the formation of a jet fire are a transformer oil temperature of 300 °C and a pressure rise rate of 0.001 kPa/s. The results of this study will help firefighters and rescuers to formulate a more scientific, rational, and safer fire-fighting strategy.