Simulating Fuel Spill Fires Under The Wing Of An Aircraft

As part of a probablistic risk assessment program, estimates of the thermal exposure in aircraft crash fires were needed. Tests were conducted to simulate he1 spill fires that might occur under the wing of a C-141 transport aircraft. Measurements were made of the fire temperatures and the heat fluxes to the underside of the wing and the underwing hselage surfaces. Both temperatures and heat fluxes are needed to adequately specify the fire exposure. Direct comparisons between the average centerline temperatures measured at a given elevation in different sizes of fires showed poor agreement A scaling analysis using the total heat release rate gave promising results. Statistical analyses of the temperature and heat flux data were used to develop temperature-to-heat flux mappings. The heat fluxes from the current test series are significantly lower than prior data from large open pool fires. INTRODUCTION As part of a probablistic risk assessment (PRA) program sponsored by the Defense Nuclear Agency, models were needed to estimate the thermal exposure in aircraft crash fires in a large number of different accident scenarios. Predicting the relationship between fire temperatures and the heat flux incident on an object engulfed in a fire is a complex problem. Advanced fire physics model require state-of-the-art submodels (combustion, convection, multidimensional participating radiation, etc.) which are coupled with the flowfield governing momentum solution. Large computer times are required to run advanced models. Simplified models, such as the Gray Gas Model [I], apply first principles only to the dominating physical phenomena and rely on empirical factors to represent the remaining physics. These models, which are partially empirical, are being developed into predictive tools to reduce computer run times. While these computer models are being developed and validated, an analysis of the existing data has been used to develop simplified temperature-to-heat flux mappings which reflect the general trends of the thermal exposure in open pool fires for vertical plates and horizontal cylinders. Data on fire temperature and the heat flux absorbed by different objects have been obtained in large pool fires at Sandia National Laboratories [2] in an attempt to understand FIRE SAFETY SCIENCE-PROCEEDINGS OF THE FOURTH INTERNATIONAL SYMPOSIUM, pp. 1017-1028 1017 Copyright © International Association for Fire Safety Science the physical behavior of fires and to quantify the thermal response of radioactive material (RAM) shipping containers when subjected to transportation accident fire environments. Analyses of these data have demonstrated the importance of the coupling between the object and the fire on the observed heat flux levels [I]. The coupling depends on both the design of the object and the configuration of the test. Overall, this work has shown that both the temperature and the heat flux are needed to describe the thermal exposure of an object in a fire. To account for differences in configuration between the earlier open pool tests and aircraft fires, a series of tests was conducted to simulate fires that might occur under the wing of a C141 transport aircraft as a result of a he1 spill. The objectives were to: 1) Developfire temperature* versus surface heatflux** data for an underwing area that includes the horizontal wing surface and the vertical fuselage surface. 2) Use these data to developfire temperature versus heatflux (T vs q) mappings. 3) Compare this T vs q mapping to the mapping developed from earlier open pool fire tests. 4) Compare thefire temperature data with temperatures from other experiments.