Dynamic Response Characteristics of Ceiling Temperature to Jet Fan Ventilation Volume in Highway Tunnel Fires

Ventilation control in highway tunnels is a crucial measure for suppressing the spread of high-temperature smoke during fires and ensuring tunnel operational safety. Aiming at the problem that existing control systems rely on the assumption of uniform cross-section wind speed and are thus difficult to directly detect and control, this paper proposes to take ventilation volume as the control input and ceiling temperature as the feedback to achieve real-time and effective control of smoke backflow. By constructing a dynamic response system of ceiling temperature under the action of ventilation, and based on the assumption of a First Order Plus Dead Time (FOPDT) system model, the system response is carried out by exciting the system with step ventilation volume, and the response characteristics of ceiling temperature under different fire scenarios and ventilation volumes are investigated. According to the regulatory effect of ventilation volume on ceiling temperature, a dynamic temperature response classification method is proposed, and the ventilation volume ranges corresponding to ceiling temperature responses under different fire heat release rates are derived. The results show that when the step ventilation volume is close to the critical value (Classification II), the length of smoke backflow is effectively controlled, and the response of ceiling temperature to ventilation volume conforms to the characteristics of the FOPDT system, with the coefficient of determination R² reaching up to 0.96. The gain coefficient |K| is positively correlated with ventilation volume, while the lag time τ is negatively correlated with ventilation volume.