Hot Surface Ignition Temperatures of Hydrocarbon Fuels

摘要

Hot surface ignition temperatures are measured for n-heptane, turbine engine based fuels namely Jet-A, JP-8, JP-5 and piston-engine based fuel 100 low lead aviation gasoline (Avgas) using a newly developed apparatus. Hot surface ignition is defined as the process of a flammable liquid coming in contact with a hot surface and evaporating, mixing and reacting with the surrounding oxidizer with self-supporting an exothermic heat release process (combustion). If all the conditions are adequate, the fuel completely turns into the combustion products following the ignition process. The ignition and combustion processes depend on vaporization rate, heat of vaporization, mixing rate with the oxidizer, a critical balance between the energy release during the initiation reactions and the rate of heat transfer to the surrounding mixture and the boundaries. Under most practical circumstances, the ignition process is probabilistic because of small variations in key parameters. One of the most important parameters determining die probability of successful ignition is the temperature of the hot surface. A test apparatus is established to reproducibly observe the hot surface ignition process by dispensing ~25 µL drops of fuel on an electrically heated surface with a temperatu re uniformity within ±5°C. Ignition and flame propagation events are recorded using regular and high speed visible cameras. The ignition event is transient and initiates at randomly distributed locations on the hot surface. Initial tests confirmed that the experimental variations in the drop size, drop velocity, plume characteristics, surface properties including temperature changes, and the nonlinear dependence of temperature of the initial reaction rate lead to the probabilistic nature of the ignition event Ignition probability is defined as a ratio of the number of ignitions and the number of dispensed drops, 20 drops are dispensed at each defined hot surface temperature for each fuel. The number of ignitions is counted by visual observation of a luminous flame. Curve fits are plotted in which the probabilities of ignition are graphed as a function of the hot surface temperature using Logistic functions whose two parameters are evaluated using the lowest maximum and the highest minimum temperatures for ignition.