Ignition characteristics of fuel droplets with different diameters on a titanium alloy hot surface

The leakage of liquid fuel onto high-temperature components, such as aircraft engine nozzles, poses an ignition hazard, particularly under sudden conditions arising from fuel system aging, mechanical impact, or structural damage. Given that it is a critical issue for aviation fire safety, this study investigates the ignition characteristics of fuel droplets on high-temperature surfaces. Titanium alloys, widely used in modern aircraft for their high strength-to-weight ratio and thermal stability, were selected as the substrate to systematically examine the effects of droplet diameter, surface temperature, and fuel composition on ignition probability and ignition delay time.

An experimental platform was established using a TC4 titanium alloy heating plate to simulate a high-temperature hot surface. RP-3 aviation kerosene and n-heptane droplets with diameters of 3.62-9.49 mm were generated using a precision pipette and released from a height of 30 mm onto the heated surface. Surface temperatures were controlled between 200 ℃ and 800 ℃ with a PID system, and ignition events were recorded with a high-speed camera. Ignition probability was defined as the ratio of successful ignitions to the total number of trials, while the ignition delay time was defined as the time interval from droplet contact to sustained flame appearance. A physics-based logistic model and an energy conservation-Arrhenius model were developed to predict ignition behavior and incorporate dimensionless parameters, such as the Bond and Weber numbers, to account for droplet impact dynamics.

The minimum ignition temperatures for RP-3 aviation kerosene and n-heptane droplets on titanium alloy surfaces are approximately 590 ℃ and 580 ℃, respectively. Ignition probability increased monotonically with surface temperature but displayed nonmonotonic variation with droplet diameter, peaking at a critical diameter of 7.26 mm. For smaller droplets, ignition probability increases with diameter because of enhanced heat transfer and vapor concentration, whereas larger droplets exhibit reduced ignition probability due to weaker internal thermal gradients and limited oxygen diffusion. Ignition delay time decreases with increasing surface temperature and is the shortest at the critical diameter. Under equivalent conditions, RP-3 droplets show a 5%-15% lower ignition probability and a 20%-30% longer ignition delay time than n-heptane. These changes are attributable to RP-3's complex composition, high boiling point, antioxidant additives, and smoke formation. Titanium alloys exhibit ignition temperatures 50-70 ℃ lower than stainless steel because of their lower thermal conductivity, diffusivity, and catalytic activity. The logistic model accurately predicted ignition probability with < 1% error, while the energy conservation-Arrhenius model predicted ignition delay with < 2 s error.

Droplet diameter and surface temperature are the principal factors controlling thermal surface ignition on titanium alloys. The identified critical diameter serves as a useful parameter for monitoring and mitigating fire risks. The developed models offer reliable predictive capability, supporting fire prevention design in aircraft engines. For risk reduction, monitoring droplets 5-9 mm in diameter is recommended, and surface temperatures should be maintained below 600 ℃. Future research should consider additional factors, such as low-oxygen conditions at high altitudes, droplet impact velocity, vibration frequency, and surface characteristics, to further optimize the model and enhance aviation fire safety.