Stephen C. Bates

The design and testing of a gas cooled probe protector shell is described. Standard optical probes are limited to relatively low temperature and mild environmental conditions. By enclosing such probes in concentric air- cooled stainless steel tubes with a brazed sapphire window at the end, their operating limits can be greatly extended. Gas cooling eliminates the facility requirements and leak hazards of water-cooled probes. Low gas flow rates have been demonstrated to maintain a probe at less than 100 degree(s)C in a 600 degree(s)C quiescent environment.

An experimental demonstration of fiber optic imaging inside a furnace at 1000°C is described, A low-resolution sapphire fiber optic video system was designed, fabricated, and tested for basic performance in a small tube furnace. The imaging fiber bundle was assembled using 100 high-quality sapphire fibers aligned and bonded at each end. Experiments to achieve a high-temperature cladding are described. Reference imaging experiments at room temperature were performed with the sapphire fiber bundle and a commercial glass fiber bundle of comparable sue. Imaging experiments at 1000°C are described and discussed.

Direct flame imaging through a transparent cylinder liner has bee´n used to study the final phase of lean combustion in a spark-ignition four-stroke optical engine. Two image-intensified video cameras record instantaneous lean flame radiation at two preset times to examine the structure and development of the flame as it approaches the wall to complete the flame travel. There is one peripheral spark plug on the side of the cylinder opposite the cameras, so that combustion completion images capture only one flame thickness and provide a unique map of flame intensity. These flames have highly structured intensity patterns on all of the scales associated with in-cylinder turbulence. Although the large-scale pattern of burning results from the flame shape early in the cycle, intensity variations of the flame images indicate that the burning rate varies significantly over the flame area on both large and small scales, indicating that the flame front is not a uniformly propagating sheet.

Sets of sequential-cycle instantaneous flame images are used to study cycle-by-cycle variation of lean combustion in a spark-ignition four-stroke optical engine. Stereo gated image-intensified NTSC video cameras record flame radiation to show three-dimensional structure, while flame development is measured in each cycle by super-imposing early and later flame images. A variety of physical causes for cyclic variation are identified from the images. Correlations of flame geometry with mass burned fraction show that a larger initial flame kernel size results in a faster early burn in the cycle, and that flames that are flatter on a large scale cause lower peak burn rates than round flames. The early flame kernel is shown to vary greatly in size, shape, and location. This kernel has a major effect on combustion by setting the basic flame shape in mid-cycle. Large scale unstable flows appear to cause major cyclic variation in flame shape and combustion, an effect seen at 500 rpm, but which has disappeared at 1000 rpm.