Jeffrey Bernard Fedison

Dual-SiC Photodiode Devices for Simultaneous Two-Band Detection

SiC is a highly useful material for optical sensing devices in the UV due to its large bandgap. It also exhibits a robust physical nature, making it suitable for harsh environment applications such as flame sensing. Here we report the first demonstration of SiC photodiode devices with multiple, independent detection components for sensing more than one radiation band simultaneously. The fabrication process and layout of the devices is described along with optical and electrical measurements of their performance. Individual sensing components exhibited peak responsivities greater than 60 mA/W at wavelengths of 290 and 320 nm. Optically tailored SiC devices, such as the ones described here, may further be applied to other applications such as missile plume detection, engine control, and communications in the solar-blind window. The application of particular interest for these devices was for flame temperature sensing. Testing the dual-SiC photodiode devices in front of a combustion flame showed a significant and linear dependence of the photodiodes output ratio with temperature, demonstrating its feasibility. Successful determination of flame temperature would be a major improvement in the control and optimization of combustion processes.

Silicon carbide photodiode sensor for combustion control

A dual silicon carbide photodiode chip was developed to determine the temperature of a natural gas combustion flame. The concept uses the change in shape of the 260-350 nm OH band with temperature. One half of the chip was covered with a long-pass multiple layer dielectric filter with a short wavelength cutoff at about 315 nm. After amplification, the two signals produced by the filtered and unfiltered halves of the chip are divided to produce a ratio, which is very sensitive to changes in flame temperature. Sensitivity is about 0.35% per 20/spl deg/F change in flame temperature for temperatures between 2700 and 3000/spl deg/F. The temperature measured is the specific average temperature encompassed by the field of view of the sensor assembly.

Determination of Lean Burn Combustion Temperature Using Ultraviolet Emission

Measurements of the ultraviolet emission spectrum emitted from a lean burn premixed natural gas flame were taken over a range of flame temperatures using a fiber-optic/CCD spectrometer. Combustion temperatures were determined by two methods: by measuring the unburned oxygen in the exhaust and by calculating the temperature using the fuel and airflows. These temperatures were correlated to ratios composed of the integrated intensity of the long wavelength region of the OH band between 310 to 340 nm (ratios numerator) and that between 305 and 310 nm (ratios denominator). Average local combustor flame temperatures at the end of the combustion zone may then be determined by tracking these ratios during combustor operation. The sensitivity of these ratios yields a 0.8% change in the ratios every 20 degF with a precision of plusmn30 degF or plusmn1% at 3000 degF with 95 % confidence bounds demonstrating the feasibility of this technique for use as a potential control parameter for gas turbine combustors burning natural gas and air mixtures. This method is well suited for the low equivalence ratios (< 1) required to reduce NOx and CO emissions. Other methods using peak ratios of different emission bands exhibit nonlinearity, lower sensitivity and greater uncertainty.