UC Irvine

Liquid fuel injection in combustion systems involves multi-phase flows generated by fuel sprays. Studying the fluid mechanical phenomena integrated in these flows that influence the fuel’s combustion, such as liquid jet break up into droplets, can benefit from experimental approaches for imaging them. Such information can help with design choices to control stability and increase the efficiency of combustion systems. However, the high optical scattering from the cloud of small droplets obscures traditional imaging methods. In addition, conventional imaging does not provide depth information that is often critical in spray understanding. Digital holography (DH) is a well-established and powerful technique for high-resolution 3D imaging. Monochromatic laser beams and digital cameras are used in this technique to record digital holograms (diffracted object wave interfered with the non-diffracted reference wave) of the 3D objects. The object field is then numerically reconstructed using the digital hologram recorded on a CCD detector but it is not clear how the light scattered from small particles, as in the spray situation, will complicate the hologram analysis. Consequently, the current work examines the use of

ultra-short laser pulses for digital holography in multi-phase flows of combustion interest. A state-of-the-art Ti-Sapphire amplified laser system is used, with pulses as short as 100 fs and up to 1mj per pulse energy. Using this system creates the opportunity for coherence filtering and time-gating filtering, which can improve signal-to-noise ratio in highly scattering media like dense sprays. In this work, scattering media with optical conditions similar to the condition in combusting dense sprays is replicated with hydrosols. USAF resolution chart is used as test target to demonstrate quantitatively the imaging resolution and contrast of the system. The atomizing spray diagnostics literature study shows that optical Kerr effect (OKE) time-gating is the only promising technique that has been demonstrated to image the liquid structure inside the transient diesel sprays. Experimental results presented in this dissertation show high potential of ultra-short pulse DH for microscopic imaging though highly scattering media with competitive resolution and quality compared to OKE time-gating, and holography provides the additional feature of resolving the internal structure of dense sprays in 3D. The technique can potentially image an intact liquid core of diesel sprays, veiled behind the optically deep cloud of droplets, which has never been done before.