2 Real Aircraft Environmental Hazards Experiment
2.1 Cockpit Electronic Flight Bag Fire Experiment
In the risk experiment of thermal runaway of lithium-ion batteries in the cockpit, a high air replacement rate was set, and an electronic flight bag (EFB) was heated by a heater. The EFB lithium-ion battery with a 7.2Ah storage capacity (SOC of 100%) immediately activated the fire extinguishing system to control the fire when an open flame occurred. The results show that the levels of CO, CO2, and O2 in the cabin vary slightly, but the maximum temperature can reach 600 ℃. The opacity of the smoke is 10% and lasts for 5 minutes. A huge pressure pulse can push open the unlocked cabin door. Even at a high air replacement rate (once per minute), the thermal runaway of a single EFB device’s lithium-ion battery will seriously affect safe flight and driving, posing a potential catastrophic hazard.
2.2 Cabin E-tablet Fire Experiment
Tablets are work and entertainment devices carried by crew members and passengers, with significant potential hazards. In 2013, FAA placed the tablet computer on a kitchen trolley and heated it by a heater in the cabin panel fire experiment. In the initial stage, flames continuously emerge from the gaps, then burn fiercely, with a sudden increase in pressure and rush open the pusher door. Although the ventilation system is in normal use, the cabin is still filled with thick smoke. When the premixed gas is ignited, the released flame and a large amount of smoke are sufficient to render the ventilation system in the cabin ineffective.
In another cabin tablet fire experiment, the tablet was placed in the 727 kitchen storage box. The experimental results showed that a huge pressure shock occurred before the thermal runaway was fully carried out, and the maximum temperature outside the box reached 81 ℃. But if multiple tablets catch fire, it will lead to a higher risk.
2.3 Large scale lithium battery experiment in cargo hold
Continuously conducting and updating large-scale lithium battery fire experiments in real environments from outdoor to Class E and Class C cargo holds, studying the hazards and thermal runaway characteristics of large-scale lithium battery fires. A large-scale lithium battery experiment was conducted in the cargo hold. 5000 18650 lithium-ion batteries and 4800 SF123A lithium-metal batteries were grouped and placed in the middle of the Class C cargo hold of the 737 cargo plane. Ventilation was set, and after a period of time, the water fire extinguishing system was activated to control the fire appropriately. A heat flow meter is arranged in front of and directly above the battery, and a thermocouple (with heights of 15, 92, and 152cm respectively) and gas detector are vertically arranged next to it to record the environmental gas composition and temperature changes inside the cargo and cockpit.
The experiment found that the thermal runaway of lithium-metal batteries ejects metallic lithium and electrolyte, causing severe combustion that can penetrate the iron bottom plate. The temperature at the top of the cabin can reach up to 1000 ℃, which is much more dangerous than lithium-ion batteries. When only a portion of the batteries are involved in the reaction and the fire is properly controlled, the oxygen volume fraction in the cargo hold can reach a minimum of 3%, and the cabin top temperature can reach up to 927 ℃. At the same time, gas is released and seeps into the cockpit, causing an increase in the toxic gas volume fraction and temperature, which affects the normal driving of the pilot. If all batteries participate in the reaction, it will cause catastrophic damage
Research on thermal runaway of lithium batteries under changing environments
3.1 Experimental content
Research on thermal runaway of lithium batteries is mostly based on ground static environment, lacking research on flight dynamic environment. Using a research method combining small-scale observation experiments, large-scale similarity experiments, theoretical analysis, and numerical simulation, this study investigates the factors that affect the occurrence of thermal runaway in lithium batteries under normal aviation transportation conditions. It also investigates the characteristics of thermal runaway propagation, temperature field, and gas release in changing environments, as well as the control of gas concentration, temperature, and explosion suppression of thermal runaway in lithium batteries.
Conduct analysis on the factors influencing the temperature characteristics of lithium battery thermal runaway, lithium battery packaging, and the fire extinguishing ability of onboard fire extinguishing systems. Use simulation software to simulate the dynamic changes in pressure, oxygen environment, and flow field during normal and emergency flights, establish a single lithium battery thermal runaway heat generation and dissipation model, and analyze the temperature field distribution, energy release, combustion and explosion process, as well as the required fire extinguishing ability caused by thermal runaway. Establish a thermal runaway heat propagation model for multiple lithium-ion batteries, analyze the direction and thermal resistance of heat propagation between batteries
(2) Small scale experiments.
Using the already built small-scale low-pressure compartment, by controlling the pressure and temperature inside the compartment, a small amount of lithium batteries are placed in a certain frequency oscillator to simulate the changing pressure, temperature, and vibration conditions of normal aviation transportation. The factors and key conditions that affect the thermal runaway of lithium batteries, as well as the temperature field and gas release characteristics of lithium batteries, are studied.
(3) Large scale validation experiment.
Using experimental equipment, simulate the pressure changes of civil aircraft during the lifting and lowering process, reproduce the low-pressure, low oxygen, and dynamic pressure environment in the cargo hold, and measure parameters such as smoke composition, density, temperature, thermal radiation flux, and explosion height during the thermal runaway process of lithium batteries.
With the increase of SOC, the maximum temperature, maximum heat release rate, total mass loss, and total combustible gas release during the thermal runaway process of lithium batteries gradually increase; When SOC is around 50%, it is most likely to cause thermal runaway propagation between batteries, which poses the greatest risk; When the SOC is lower than 30%, the possibility of thermal runaway propagation will stop. When transporting lithium-ion batteries, controlling the battery level to be lower than 30% will reduce the severity of fire incidents; The danger and difficulty of extinguishing lithium-ion battery fires are much higher than general cargo fires.
Whether lithium-ion battery fires occur in the cargo, cabin, or cockpit, they will cause huge or even catastrophic accidents. Therefore, aircraft fire extinguishing systems and ventilation systems should pay attention to lithium-ion battery fire prevention and control. The research on the fire characteristics of lithium batteries and the risk experiments of air transportation are mostly based on static ground environmental conditions, and there is a lack of relevant experimental research on simulating flight conditions. The normal flight environment of an aircraft is different from the ground stationary conditions, and changes in pressure, temperature, and vibration frequency under normal air transportation conditions all have a significant impact on lithium battery fires. Relevant experimental research needs to be verified and further supplemented.