Leaking flammable liquids that come in contact with hot surfaces aboard aircraft can create flammability hazards that can pose risks to the aircraft and personnel on-board. To properly assess these risks, research on the operating parameters that influence the hot surface ignition (HSI) process is required. HSI events occur when a flammable liquid impinges on a hot surface, evaporates/boils, and mixes with the surrounding air. If this mixture is between the upper flammable limit (UFL) and lower flammable limit (LFL) of the fuel and the temperature of the mixture is above the auto-ignition temperature (AIT), this mixture can ignite and, if sustained, cause damage to the aircraft and its occupants, as shown in Figure 1.
The probability of ignition by a hot surface is dependent on several factors that include but are not limited to:
- Test fluid temperature
- Test fluid volume injected onto the hot surface
- Injection method (e. contacts the hot surface through a stream, spray, or discrete droplets)
- Air current over the hot surface
- Temperature of the surrounding air
- Equivalence ratio of the air/fuel mixture due to ambient air pressure and high altitude conditions
- Vapor pressure and density of the flammable liquid
The evaluation of the influence of each parameter on the HSI process requires that ability to control and monitor each condition. However, stray air currents from environmental influences and temperature gradients on hot surfaces used for testing can produce errors that result in large deviations of the influencing factors to HSI events, as shown in Figure 2.
As shown in Figure 2, there is a significant deviation in the experimentally determined minimum hot surface ignition temperature (MHSIT) (i.e. the minimum temperature at which an HSI event will occur) across multiple research groups which is attributed to the lack of reproducibility and control of influencing environmental conditions. Furthermore, the American Petroleum Industry (API) recommends that a hot surface ignition event is not to be considered as a hazard until the hot surface temperature is 182°C above the AIT of the flammable liquid. As shown in Figure 2, HSI events can occur even when the recommendation by API is followed (see MIL-H-83282 in Figure 2).
In order to provide the repeatable test conditions that are required to properly assess HSI risks aboard aircraft, Advanced Cooling Technologies, Inc. (ACT) developed an HSI test apparatus, shown in Figure 3, under an Air Force funded Phase I Small Business Innovative Research (SBIR) program. This test apparatus was developed with a chamber that isolates the hot surface from the environment. A vapor chamber designed to operate in a temperature range from 500-700°C provides the isothermal hot surface. The isothermality of the vapor chamber compared to an equivalent metal block and the temperature response of both surfaces to a steady stream of injected fluid on the surface is presented in Figure 4 and Figure 5, respectively. The vapor chamber was designed to be removable from the test apparatus such that vapor chambers with various geometries can be integrated into the chamber to evaluate the dependence of hot surface geometry on the HSI process. Additionally, the test apparatus was developed with a fluid injection system that can deliver flammable liquids to the hot surface in either a stream, spray, or droplet.
Figure 5. Videos of a 0.5 mL injection of water onto the surface of a (top) metal block and (bottom) vapor chamber exhibiting isothermal performance of the vapor chamber throughout the duration of the fluid injection
This test apparatus was outfitted with a photodiode and an exposed thermocouple to provide response to the ignition process and the time to ignition (i.e. the time required for ignition to occur after delivery of the flammable liquid) of various types of fuel at surface temperatures from 500-700°C were evaluated in static air conditions. Single components fuels (n-decane, n-heptane, and isopropyl alcohol (IPA), were evaluated in addition to multicomponent JP-8 fuel. The probability of ignition given a length of exposure of the flammable liquid to the hot surface is presented in Figure 6.
As shown in Figure 6, the probability of ignition of n-decane, n-heptane, and JP-8 are very similar given the amount of exposure to a given surface temperature and is related to the similar physical properties of the fuel, such as normal boiling point, AIT, and vapor density (Figure 7). However, IPA exhibited markedly different HSI behavior showing almost zero probability of ignition at surface temperatures less than 600°C. This is attributed to the higher evaporation rates of the fuel from the hot surface thereby producing a lower probability of achieving an ignitable mixture.
The repeatability of this test apparatus was evaluated by developing a standardization curve for the time to ignition of n-decane at surface temperatures varying from 500-700°C and re-evaluating the time to ignition of n-decane before and after JP-8 trials (standardization checks) to ensure the HSI test apparatus provided repeatable results.
As shown in Figure 8, the time to ignition evaluated in ACT’s HSI test apparatus provided repeatable results across multiple trials at the same surface temperature. Thus, indicating the test apparatus provided a repeatable environment for evaluating the influencing parameters of HSI events.
In current research efforts, ACT has expanded the capabilities of the HSI test apparatus by incorporating a wind tunnel style test chamber that enables various air flow conditions to be evaluated over hot surfaces with varying geometries (i.e. inclined, curved, etc.); see Figure 9 below.
 Colwell, Jeff D., and Ali Reza. “Hot Surface Ignition of Automotive and Aviation Fuels.” Fire Technology. 2005.
 American Petroleum Institute, “Ignition Risk of Hydrocarbon Liquids and Vapors by Hot Surfaces in the Open Air.” API Recommended Practice 2216. Third Edition. Washington, D.C. December 2003.