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An experimental investigation was conducted to determine the air-forced flame response of a five-nozzle, 250 kW, lean premixed gas turbine can combustor. Operating conditions were varied over a range of inlet temperatures, inlet velocities, and equivalence ratios, while the forcing frequency was varied from 100 to 450 Hz with constant normalized velocity fluctuations of approximately 5%. The response of the flame’s rate of heat release to inlet velocity fluctuations is expressed in terms of the phase and gain of a flame transfer function. In addition, chemiluminescence imaging is used to characterize the time-averaged and phase-averaged spatial distribution of the flame’s heat release. The resulting flame transfer functions and chemiluminescence flame images are compared to each other to determine the effects of varying the operating conditions. In addition, they are compared to data obtained from a single-nozzle combustor with the same injector. The forced response of the multi-nozzle flame demonstrates a similar pattern to those obtained in a single-nozzle combustor with the same injector. An exception occurs at high frequency where the multi-nozzle flame responds to a greater degree than the single-nozzle flame. At low frequency the multi-nozzle flame dampens the perturbations while the single-nozzle flame amplifies them. A number of minima and maxima occur at certain frequencies which correspond to the interference of two mechanisms. The frequency of these minima is nearly the same for the single- and multi-nozzle cases. When plotted with respect to Strouhal number instead of frequency there is a degree of collapse that occurs around the first observed minima.

The objective of this study was to develop knock criteria for aviation diesel engines that have experienced a number of malfunctions during flight and ground operation. Aviation diesel engines have been vulnerable to knock because they use cylinder wall coating on the aluminum engine block, instead of using steel liners. This has been a trade-off between reliability and lightweighting. An in-line four-cylinder four-stroke direct-injection high-speed turbocharged aviation diesel engine was tested to characterize its combustion at various ground and flight conditions for several specially formulated Jet A fuels. The main fuel property chosen for this study was cetane number, as it significantly impacts the combustion of the aviation diesel engines. The other fuel properties were maintained within the MIL-DTL-83133 specification. The results showed that lower cetane number fuels showed more knock tendency than higher cetane number fuels for the tested aviation diesel engine. In this study, maximum pressure rise rate, or Rmax, was used as a parameter to define knock criteria for aviation diesel engines. Rmax values larger than 1500 kPa/cad require correction to avoid potential mechanical and thermal stresses on the cylinder wall coating. The finite element analysis model using the experimental data showed similarly high mechanical and thermal stresses on the cylinder wall coating. The developed diesel knock criteria are recommended as one of the ways to prevent hard knock for engine developers to consider when they design or calibrate aviation diesel engines.

A tomographic image reconstruction technique has been developed to measure the 3D distribution of CH* chemiluminescence of unforced and forced turbulent premixed flames. Measurements are obtained in a lean premixed, swirl-stabilized multi-nozzle can combustor. Line-of-sight images are acquired at equally spaced angle increments using a single intensified charge-coupled device camera. 3D images of the flames are reconstructed by applying a filtered back projection algorithm to the acquired line-of-sight images. Methods of viewing 3D images to characterize the structure, dynamics, interaction and spatial differences of multi-nozzle flames are presented. Accuracy of the reconstruction technique is demonstrated by comparing reconstructed line-of-sight images to measured line-of-sight downstream-view images of unforced flames. The effect of the number of acquired projection images on the quality of the reconstruction is assessed.The reconstructed 3D images of the unforced multi-nozzle flames show the structure of individual flames as well as the interaction regions between flames. Forced flame images are obtained by phase-synchronizing the camera to the forcing cycle. The resulting 3D reconstructions of forced flames reveal the spatial and temporal response of the multinozzle flame structure to imposed velocity fluctuations, information which is essential to identifying the underlying mechanisms responsible for this behavior.

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