EXPERIMENTAL INVESTIGATION AND MODELING OF MINIMUM HOT SURFACE IGNITION TEMPERATURE FOR AVIATION FLUIDS
A hot surface is one of the ignition sources which may lead to fires in the presence of aviation fluid leakage. Bleeding ducts and exhaust pipes that are at elevated temperatures are potential sources of ignition. A database of Minimum Hot Surface Ignition Temperatures (MHSIT) resulting from experiments conducted three decades ago at the Air Force Research Laboratory (AFRL), Dayton, OH has served as a valuable source of estimating safe operating temperatures. However, MHSIT for some of the aviation fluids such as Jet-A and MIL-PRF-23699 (lubrication oil) are not readily available. Further, the ranges of the hot surface and flammable liquids’ temperatures and the range of the air stream velocities need to be extended for use in higher pressure ratio and higher performance aircraft engines developed since the generation and interpretation of the original data. The air velocities (VA) in the modern engines have increased by a factor of two and documenting their effects on the MHSIT for a range of test fluid temperatures and air temperatures (TF, TA) is important.
The objectives of this study are to develop a generic test apparatus to study MHSIT and to model an air-fuel mixture space to find the range of temperatures and velocities that lead to ignition. Among various leakage scenarios, the test apparatus simulates spray (atomized particles injected through a nozzle) and stream (dripping from a 3 mm tube) injection. A semiempirical ignition model was developed using an ignition temperature and delay time expression based on an energy balance between the heat lost to the cross-stream flow, the heat added from the hot surface and the heat released by the nascent chemical reactions to estimate the MHSIT.
MHSIT is measured including the effects of VA, TF, TA and the effects of obstacles. Ignition probability is evaluated as a function of the hot surface temperature. The probabilistic nature of the hot surface ignition process was established. New flammable fluids (Jet-A & MIL-PRF-23699) have been tested and MHSIT database was expanded. A large number of ignition experiments were completed to evaluate ignition probability at various flow conditions of aviation fluids: (1) Jet-A, (2) Hydraulic oil (MIL-PRF-5606) and (3) Lubrication oil (MIL-PRF-23699). Uncertainty of the experimental measurements for these tests have been documented. Air velocities were extended up to 7 m/s. Effects of flammable liquid and air temperature on MHSIT were studied. The empirical constants for the semi-empirical model were determined using these experimental data.
The ignition probability is strongly correlated with hot surface temperature and progressively weakly correlated with air velocity, fluid parcel size, air temperature, and test fluid temperature. Parameters investigated in this study are useful design choices considering MHSIT for a given flow condition.