Spatially and Temporally Resolved Diagnostics of Dense Sprays Using Gated, Femtosecond, Digital Holography

This is a proposal to develop a unique, gated, picosecond, digital holography system for characterizing dense particle fields in high pressure combustion environments; a critical requirement clearly defined in the NASA solicitation. Most imaging methods fail to provide this capability because noise from multiple scattering buries the signal needed to acquire a useful image. Solutions to this problem are expensive, difficult to implement, and not ideal candidates for field experiments. The proposed innovation combines digital holography and picosecond, optical gating to limit the amount of optical noise sufficiently to enable high resolution, 3D imaging, effectively generalizing existing pseudo-ballistic imaging systems that have been used for imaging through dense particle fields. Storing the complete wavefront in a hologram enables use of a wide range of optical diagnostics methods including image processing and interferometry to improve image and information quality. The result is a new sensor concept that will be extremely useful in the experimental study of dense sprays and other particles fields, providing a detailed, instantaneous look at the structure and position of all of the particles as well as density field information in a large three dimensional sample volume. Moreover, the system can record dynamic information at high frequency.

The tool developed in this research can provide data in combustion research that is not currently attainable with other methods and therefore can have an impact on future aircraft engine combustor designs. The system is applicable in studies requiring fuel air ratios, particle size and number density and refractive index gradients in the gases surrounding the particles.

The resulting tool can enhance combustion research in commercial engine development leading to lower emission, control instabilities and may have commercial applications in other gas-turbine based industries (such as power generation and industrial burners). The modeling and results can be and will be employed in current and future hydrocarbon rocket engine designs (improving combustion efficiency, ignition, stability, etc.).

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