Infrared emissions from turbofans with high aspect ratio nozzles

A mixed flow turbofan is analyzed from the point of view of i.r. emission characteristics. A simple absorption coefficient model of the core flow gas displays the potential effectiveness of high aspect ratio nozzles as a variable in the design of aircraft engine installations for low i.r. signature. The simplicity of the model described and the limited view perspectives used to assess the signature restrict the usefulness of the results to that of guiding preliminary design. In general, more precise solutions are complex and depend on specification of a relatively large number of independent variables to describe the source—seeker geometry, weather, etc. The level of effort to carry out more accurate analysis may be inconsistent with the preliminary design process where simple criteria such as those described here may be sufficient to select the best of several candidate designs. The influence of cycle parameters is assessed using an /zth power dependence of radiation on temperature and a simple mixing model to estimate the core length. The analysis shows that cycle parameters which improve cycle efficiency and thus fuel consumption also reduce i.r. emissions. Bypass ratio near unity gives low i.r. signature for both optically thin and thick spectral regions.