Development of a hybrid probe for gas turbine combustor measurements

An innovative approach has been used to provide optical access to a combustor exhaust flow for nonintrusive measurements in the high temperature flow. This hybrid probe system combines a line-of-sight optical measurement system with an intrusive, high temperature - capable probe for emission, absorption, or transmission spectral measurements in the high temperature flow. A single 0.94 inch diameter probe concept was selected to be compatible with existing pitot / emission combustor exit rakes and to avoid the significant design challenge of maintaining optical alignment across long path lengths. The optical components were positioned at the top and bottom of a 0.25 inch flow-through slot and protected by a rearward facing step and a local circumferential air purge flow. The optical components housed inside the probe included two sets of lenses and fiber optic cables. The probe was designed for easy installation and removal of the optical assemblies from each end of the probe. Water cooling passages running longitudinally within 0.050-inch from the surface provided a thermal barrier for the optical components. A hydraulic and thermal analysis of the probe identified the water flow rate required to maintain the probe surface temperature at 1200 °F. The aerodynamic shape of the flow-through slot in the probe was designed so that the gas properties at the optical line of sight were minimally disturbed fi-om fi-ee stream conditions. A numerical solution of the flow field through the slot predicted a maximum density change from the free stream density of less than 1.0 percent and a Mach number change of 0.05. The first application of the hybrid probe was to obtain transmission measurements of a continuum light source in a natural gas / air combustor exhaust at 200 psia and 3000 °F. The source fiber optic cable was severed during installation prior to the test and no optical transmission data were obtained. However, probe durability was satisfactory with no signs of external thermal distress. The use of purged orifices as windows to protect the internal optical components of the probe from damage or contamination was successful. The presence of the hybrid probe was shown to have a negligible effect on flow conditions at the optical path of the transmitted beam - a necessary condition for directly inferring free-stream conditions from hybrid probe measurements. The hybrid probe was also shown to be dimensionally stable since the optical alignment repeated pre- and post- test.