Image optimization for chemical species tomography with an irregular and sparse beam array

Abstract

High-speed tomographic imaging of hostile engineering processes using absorptionbased measurements presents a number of difficulties. In some cases, these challenges include severe limitations on the number of available measurement paths through the subject, and the process of designing the geometrical arrangement of those paths for best imaging performance. This paper considers the case of a chemical species tomography system based on near-IR spectroscopic absorption measurements, intended for application to one cylinder of a multicylinder production engine. Some of the results, however, are applicable also to other hardfield tomographic modalities in applications where similar constraints may be encountered. A hitherto unreported design criterion is presented for optimal beam geometry for imaging performance, resulting in an irregular array with only 27 measurement paths through the subject for the engine application. Image reconstruction for this severely limited geometry is considered at length, using both simulated and experimental phantom data. Novel methods are presented for the practical generation of gaseous phantoms for calibration and testing of the system. The propane absorption coefficient at 1700nm is measured. Quantitative imaging of propane plumes in air is demonstrated, showing good localisation of circular plumes with diameter as small as 1/5 of the subject diameter and excellent imaging of multiple plumes.

Tomographic imaging of the liquid and vapour fuel distributions in a single-cylinder direct-injection gasoline engine

“…The above treatment of the first term in exponential form is a departure from the previous practice in [5,21,23], and is found to be helpful below. Both terms are explicitly stated in terms of their deviation from their values at θ ref .…”

“…This absorption feature of long-chain hydrocarbons is very broad (~15 nm half-width at half-maximum) and unstructured, as discussed in [21], when compared with the very narrow spectral width of the diode laser sources used, and when compared with the sub-nm width of absorption features of small molecules that are typically measured in tunable diode laser absorption spectroscopy (TDLAS) systems [22]. The spectral absorption properties of air/HC fuel mixtures are relatively easy to calibrate in terms of temperature and pressure, as reported in [21,23]. For a stoichiometric mixture of air and iso-octane, at about 10 bar pressure in a typical gasoline engine cylinder, a beam of light at 1700 nm passing diametrically through the cylinder will undergo around 5% absorption by HC molecules.…”

“…More limiting is that, by using individual fibres and collimators, optical mounting requirements around the engine cylinder dictate that only a few tens of distinct PCI measurements can be obtained, which is challenging in terms of image reconstruction. Extensive development of image reconstruction methods and robust laboratory tests of IMAGER [23,24] demonstrated its fundamental imaging capability, with spatial resolution about 4% of the subject cross-sectional area, which is clearly much poorer than PLIF.…”

“…9(b). In principle, the CST technique yields quantitative images of fuel distribution, as discussed in section 2 and demonstrated in [5,23]. However, the absence of calibration data, i.e.…”

Robust, spatially scanning, open-path TDLAS hygrometer using retro-reflective foils for fast tomographic 2-D water vapor concentration field measurements

“…These setups allow for very fast measurements, since all channels can be recorded simultaneously and the field measurement rate equals the rate at which the concentrations along each projection are measured. However, such highly parallel, bi-static setups either require large amounts of light-source-detectorpairs and the appertaining data acquisition channels (Wright et al, 2005;Terzija et al, 2008;Terzija and McCann, 2011;Ma et al, 2013) or are somewhat limited regarding spatial resolution (Ma et al, 2009;Deguchi, 2012). Additionally, a disadvantage of bi-static setups in field applications is the cumbersome alignment of many detectors.…”