Study on high-pressure water mist fire extinguishing for urban underground substations: full-scale experiments and mechanism analysis

Underground substations housing oil-immersed transformers present critical fire risks in urban environments, where confined-space pool fires can escalate into cascading structural failures. This study investigates the effectiveness and mechanisms of high-pressure water mist fire suppression in such environments. Based on the structural and fire characteristics of actual substations, a full-scale experimental facility incorporating a 110 kV transformer was developed to assess the influence of the water mist flow rate (112 and 224L/min) and obstructions on extinguishing performance. The experimental results demonstrate that the extinguishing process in confined spaces can be categorized into three distinct stages: flame intensification, flame suppression, and flame extinction. The interplay between the water mist flow rate and obstructions significantly affects the underlying extinguishment mechanisms. Analysis of temperature and gas concentration data revealed that at a lower water mist flow rate (112 L/min), the suppression stage persisted longer, with extinguishment primarily driven by the suffocation effect. However, the presence of an obstruction above the pool fire hindered effective extinguishment. Conversely, at a higher flow rate (224 L/min), minimal changes in oxygen concentration were observed, indicating that fire suppression was predominantly attributed to flame cooling and disruption of the oil boiling layer. The surface temperature of the obstruction remained below 100 °C, highlighting the effective thermal cooling effect by the water mist to the wall surfaces. These findings offer valuable insights into the design and optimization of high-pressure water mist fire suppression systems for underground substations, contributing to enhanced safety measures in confined thermal systems.