Diffuse back-illumination setup for high temporally resolved extinction imaging

Abstract

This work presents the development of an optical setup for quantitative, high-temporal resolution line-of-sight extinction imaging in harsh optical environments. The application specifically targets measurements of automotive fuel sprays at high ambient temperature and pressure conditions where time scales are short and perceived attenuation by refractive index gradients along the optical path (i.e., beam steering) can be significant. The illumination and collection optics are optimized to abate beam steering, and the design criteria are supported by well-established theoretical relationships. The general effects of refractive steering are explained conceptually using simple ray tracing. Three isolated scenarios are analyzed to establish the lighting characteristics required to render the observed radiant flux unaffected by the steering effect. These criteria are used to optimize light throughput in the optical system, enabling minimal exposure times and high-temporal resolution capabilities. The setup uses a customized engineered diffuser to transmit a constant radiance within a limited angular range such that radiant intensity is maximized while fulfilling the lighting criteria for optimal beam-steering suppression. Methods for complete characterization of the optical system are detailed. Measurements of the liquid-vapor boundary and the soot volume fraction in an automotive spray are presented to demonstrate the resulting improved contrast and reduced uncertainty. The current optical setup reduces attenuation caused by refractive index gradients by an order of magnitude compared to previous high-temporal resolution setups.

A Progress Review on Soot Experiments and Modeling in the Engine Combustion Network (ECN)

“…The original diffused illumination source used at SNL [3,11] followed the design of Ghandi and Heim [12], and this configuration was also implemented in the DBIEI measurements presented here from UPV; however, a new optical configuration characterized by a much larger region of spatial uniformity and more diffused rays at the imaging plane has been developed at SNL as described in Westlye et al [13]. The improved optical setup reduces perceived attenuation due to beam steering.…”

Development of limited-view tomography for measurement of Spray G plume direction and liquid volume fraction

“…As explained in the setup and in Ref. (Westlye et al 2017), for the imaging setup, an engineered diffuser is utilized to produce a ray bundle over a certain angle. This angle depends on the overall dimensions of the optical setup as illustrated in Fig.…”

“…This angle depends on the overall dimensions of the optical setup as illustrated in Fig. 9b (Westlye et al 2017), with parameters for the current setup given in the embedded table. The "schlieren-free" image zone S does not extend all of the way to the window edge but depends upon a given DBIEI setup.…”

“…Engineered diffusers supply a homogeneous light field within a certain diffusion angle. The optical fundamentals for the setup are discussed in (Westlye et al 2017). The Engine Combustion Network (ECN) recommends extinction techniques (that also account for beamsteering) for liquid penetration measurements, because it is found to be the more reproducible technique over different laboratories in comparison to Mie-scattering imaging and the experimental results may be more closely compared to CFD simulations .…”

“…This is the key step to transform the limited data to a "full-view"-like dataset. The inputs also enforce Sketch and data to calculate the schlieren-free image zone of a given DBIEI setup according to (Westlye et al 2017) spline interpolation method. The final sinogram is shown in Fig.…”