Optical Measurements for Ramjet Engine Development

INTRODUCTION Advanced propulsion systems involve highly complex combustion flow processes. Extensive and accurate flowfield measurements can help to guide and support engine development. For example, in the hypersonic dual-combustor ramjet, the subsonic combustion chamber operating at fuel-rich conditions generates a complex multiphase combustion flow comprising liquid fuel droplets, soot particles, and reacting gases. The complexity of this flow is intensified by its subsequent downstream interaction with air at supersonic speeds. In such reacting flows, conventional mechanical probes can disturb the local flow environment as well as be subjected to impact and thermal damage. Optical instruments that have been developed and used in laboratory experiments of less complex combustion flows avoid these difficulties. However, their application in the harsh flow environment of ramjet engine combustion is more difficult and needs to be approached cautiously. In contrast to mechanical probes, optical instruments generally are nonintrusive; produce minimum flow disturbance; have a faster response, higher data rate capability, and better spatial resolution; and, in some cases, can measure more than one flow parameter per instrument. Flow modeling requires accurate flowfield measurements of stream velocity, temperature and pressure distributions, and, when chemical reactions are present, the chemical species concentration distributions. For a reacting flow containing a considerable amount of particulates, the characteristics and distributions of these particles need to be measured. The particulate flow is important because it can strongly affect both the performance of optical diagnostic devices and the combustion efficiency. In relatively clean combustion flows, such as most laboratory flames, the particulates are generally of submicrometer size. In the fuel-rich ramjet flows, however, soot particles and unburned fuel droplets can extend the particle size range from sub micrometer size to as large as 50 micrometers. In principle, all of the above flow parameters may be determined by optical instruments. APL's Aero-