Experimental evaluation on PID-based adaptive longitudinal ventilation control of tunnel fire smoke

In actual fire scenarios, the heat release rate (HRR) is unknown, and excessive longitudinal ventilation velocity can compromise the safety of evacuations downstream from fire source. Therefore, PID-based adaptive longitudinal ventilation is introduced to maintain optimal backlayering lengths upstream while ensuring favorable thermal stratification downstream. A series of experiments under various HRR and longitudinal ventilation were conducted to derive a semi-empirical formula for predicting critical velocity. Subsequently, experiments with PID-based ventilation were conducted to determine a set of control parameter combinations for practical reference. The performance of PID-based adaptive ventilation in controlling tunnel fire smoke at different HRRs was investigated. Compared to critical ventilation, PID-based ventilation significantly enhances smoke thermal stratification downstream. The parameter accessing the stratification stability, $\Delta T_{cf}/\Delta T_{avg}$, can be increased by up to 53.2%. As the HRR increases, the delay and stabilization time of the PID-based ventilation system decrease, while the overshoot increases. Nonetheless, the upstream steady-state temperature, controlled by PID-based ventilation, remains impervious to HRR fluctuations. $\Delta T_{cf}/\Delta T_{avg}$ are all greater than the threshold value of 1.7, which means excellent smoke thermal stratification. PID-based ventilation effectively controls fire smoke in stable, step-change and t-squared changing HRRs, thereby optimizing the management of ventilation system and enhancing the rescue efficiency.