<p>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 (V_A) 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 (T_F, T_A) is important.</p> <p>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.</p> <p> </p> <p>MHSIT is measured including the effects of V_A, T_F, T_Aand 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.</p><p>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.</p><p></p>

Grassland fuels quickly respond to moisture changes in the environment, and successfully ignite more readily compared with other wildland fuel types. In recent years in New Zealand grasslands, wildfire ignitions have increased due to recreational activities on public conservation land. Ignition sources have included off-road vehicles, sparks from machinery, and campfires, cooking stoves, etc. This research investigated ignition thresholds for fully cured tussock (Festuca novae-zelandiae) and exotic (Agrostis capillaris) grasses, with the aim of providing a scientific basis for wildfire prevention through decision-support tools for activity controls. Five ignition sources of concern to the Department of Conservation were tested in the laboratory, and results were validated against field experiments. Experiments were innovative, and were designed to simulate ignitions from: hot exhaust systems on off-road vehicles (hot metal); sparks from vehicle exhausts (carbon emissions); grinding operations (metal sparks); smouldering debris dropped onto grass fuels from hot vehicle parts (organic embers); and ordinary cigarette lighters (open flame). Fuel moisture content (MC), and wind speed were varied, but ambient temperature and relative humidity were kept relatively constant in the laboratory. Logistic regression was used to analyse data for each ignition source, except organic embers because no ignitions occurred. Ignition thresholds were determined for a probability of ignition success of 50%, and all models were statistically significant. The thresholds are listed in terms of model accuracy for each experiment: open flame was 28% MC without wind, and 55% MC with light wind (1 m/s); metal sparks was 37% MC; hot metal, with a wind speed of 2 m/s and MC of 1%, was 398ºC hot metal temperature; and carbon emissions was 65% MC. The results represent a significant contribution to knowledge of the ignition behaviour of grassland fuels. Further research is required to verify and extend the results; but, initial findings provide a scientific basis for management, investigations of wildfire causes, and decisions around controls on recreational activities to protect highly sensitive ecosystems and natural areas from damaging wildfires.

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La certification des moteurs aéronautiques impose des fortes réductions des émissions polluantes. Une des solutions adoptées par les constructeurs est d'introduire la combustion pauvre dans les nouvelles chambres. Cette configuration pose des problèmes de stabilité de flamme et de ré-allumage en altitude. Le ré-allumage des moteurs doit être garanti et il y a une nécessité réelle de mieux comprendre les interactions complexes et multi-physiques guidant ce processus. Cette étude expérimentale vise les différentes phases de l'allumage aéronautique dans deux chambres swirlées, confinées diphasiques. Un foyer mono-injecteur permet d'analyser le développement du noyau de flamme dans ses premiers instants et de décrire comment les interactions avec l'écoulement local peuvent conduire à une réussite d'allumage ou à une ratée, via différents mécanismes. Une chambre multi-injecteur est dédiée à analyser la propagation de la flamme entre injecteurs pour différents espacements et carburants. / Jet engine certification undergoes more and more stringent controls that impose a strong reduction of pollutant emissions. As a response, designs move towards lean combustion, which raises difficulties relative to combustion stability and re-ignition capabilities in high altitude. The use of liquid fuels in real chambers introduces new variables into the ignition process, which involves complex simultaneous multi-physical interactions. The present experimental investigation addresses different phases of aeronautical ignition in two different confined, swirled, spray jet chambers. A single-injector facility is used to study the initial flame kernel development and interaction with the flow leading to successful ignition or misfire, following different mechanisms. A multi-injector facility enables the investigation of flame propagation between injectors, which is also governed by the local flow. Here, inter-injector distances are varied and fuels of different volatilities are tested.