Indicator gases under thermal effect in a multiphysics coupling model of longwall goaf during coal self-heating

• A Multiphysics coupled model of solid–gas-thermal fields has been developed. • Investigated the transport of CO and H 2 under Multiphysics field conditions. • The Boussinesq equation is introduced to represent the thermal buoyancy effect. • The impact of coal properties is considered on coal spontaneous combustion. Understanding the self-heating behaviour and gas distribution patterns in longwall goaf is crucial for preventing coal mine spontaneous combustion. Unlike previous studies that primarily focus on temperature variations, this study developed a Multiphysics coupled model integrating solid (coal), gas, and thermal processes to systematically investigate the spatiotemporal evolution of temperature fields and indicator gas distributions, with a key point being the comprehensive analysis of their dynamic relationship throughout the spontaneous combustion process. Additionally, the influence of coal properties on self-heating behaviour was analysed. Monitoring points were strategically placed to systematically examine changes in gas concentration centres. The results indicate that temperature gradients drive gas migration and accumulation, with indicator gases moving upward under the effect of thermal buoyancy. As reactions intensify, the gas concentration centre gradually aligns with the temperature field, uncovering the thermal-gas coupled migration mechanism in the spontaneous combustion process. Moreover, different coal ranks exhibit notable differences: lower-rank coals with high reaction heat characteristics are more prone to forming high-temperature regions and gas concentration centres near the working face. This research establishes a theoretical foundation for optimising indicator gas monitoring layouts, such as Tube Bundle System placement, to strengthen fire prevention measures, improve mine safety, and minimise economic losses.