Thermal runaway characteristics of lithium-ion batteries in a branched tunnel: Impact of longitudinal ventilation and cell state of charge

The thermal runaway of power cells in branched tunnel configurations posing unique safety challenges, primarily due to factors like asymmetric airflow warrants, demanding urgent attention amidst expanding electric vehicles and complex urban tunnel networks. This study experimentally investigates the thermal runaway characteristics of lithium iron phosphate power cells in a scaled branched tunnel, focusing on the effects of cell state of charge and longitudinal ventilation velocity. Results reveal intermittent jet fires with multiple eruptive phases. Longitudinal ventilation deflects the jet fire downstream, with the degree of tilt increasing proportionally with airflow velocity. Higher cell state of charge significantly elevates cell surface temperature and radiation heat flux. While the effect of ventilation velocity on cell surface temperature is non-monotonic, radiation heat flux exhibits positive correlations with ventilation velocity. Notably, maximum ceiling temperatures are sharply reduced by longitudinal ventilation, by up to 26.6 % as airflow velocity increases from 0.2 m/s to 0.8 m/s. A normalized model using the sum of two exponential equations is proposed for correlating longitudinal ceiling temperatures under ventilation. These findings offer crucial insights for developing robust fire safety strategies and risk mitigation measures in complex branched tunnel environments.