Development of spectral microscopy imaging technique for time-resolved imaging of primary breakup in high-pressure fuel sprays
The primary breakup of high-pressure sprays has been a topic of study for several decades, and while several theoretical models have been proposed to describe this process, the exact mechanisms remain unknown. To verify proposed theories, high-resolution visualization of breakup phenomena is necessary. However, in practical fuel sprays, liquid surface instability and breakup features in the near-nozzle area are expected to be less than 5 μm in size and moving at speeds comparable to the orifice exit velocity (~600 m/s in a diesel fuel spray). Though modern high-speed cameras can operate at extremely high frame rates, providing adequate temporal resolution of this problem, the available image sensor area is dramatically reduced at these acquisition speeds, leading to poor image resolution and an inability to resolve objects in the required micron scale. By contrast, modern still-frame cameras have very high image sensor resolutions, enabling good resolution of micron-scale objects when coupled with a suitable lens, but temporal resolution of the atomization process is lost. To date, it has been impossible to realize the simultaneous spatial and temporal resolution required to capture time-resolved images of primary breakup in practical fuel sprays. In this work, we introduce the development of a new high-resolution imaging technique, termed ‘spectral microscopy,’ which can simultaneously resolve the temporal and spatial scales of primary breakup in high-pressure sprays. The new imaging technique employs high-power pulsed color LEDs and an affordable consumer-grade DSLR color camera coupled to a long-range working-distance microscopic lens. Red, green, and blue LEDs are selected to overlap with the detection bandwidths of the Bayer filter overlaying the camera image sensor and are pulsed with a 20-ns duration at different times to capture a temporal sequence of three images within a single frame. Post-processing enables separation of distinct temporally-separated images from each color channel. In this paper, we introduce and characterize the optical performance of the spectral microscopy imaging system and demonstrate preliminary efforts to image the primary breakup of high-pressure sprays.