Cooling limits of compressed air foam on suppressing lithium-ion battery thermal runaway

Suppressing battery thermal runaway and preventing its propagation have become critical safety challenges. This work investigates the suppression performance and cooling limits of compressed air foam (CAF) on prismatic NCM lithium-ion batteries. Single-cell tests were conducted to compare free burning, water spray, and CAF suppression, while module-scale tests were performed under CAF flow rates up to 12 L/min. Results show that CAF provides more sustained cooling to control battery fire than water spray, because it forms a stable foam blanket, enabling continuous liquid drainage, improved surface wetting, and prolonged evaporative cooling. In the single-cell comparison, CAF achieves a maximum cooling power of 3.0 kW and a maximum equivalent heat transfer coefficient of 350 W/m 2 ⋅K, doubling that of water spray. The module-scale tests show 3 L/min is insufficient due to boil-off crisis, leading to a prolonged quenching process. Increasing the flow rate to 6 L/min improves liquid replenishment but remains insufficient to fully overcome the vapor barrier. At 12 L/min, the high-momentum foam spray penetrates the vapor layer and promotes transition/nucleate boiling at the cell surface, achieving a maximum cooling rate of 8.3 K/s and a maximum equivalent heat transfer coefficient of 7.0 kW/m 2 ⋅K. These findings demonstrate that effective foam suppression of battery fires requires both sufficient coolant volume and high cooling power. Therefore, optimizing foam delivery intensity is essential for improving battery fire suppression and preventing thermal runaway propagation.