The world's first test facility that allows the visualization of cavitation on a rotating inducer in both cryogen and water was used for comparing the cavitation features in liquid nitrogen at 77.9 K and water at 292.5 and 333.5 K. The test inducer was a triple-threaded helical one with a diameter of 65.3 mm and a rotational speed range of 3500-6000 rpm. The backflow vortex cavitation on the rotating inducer was quantitatively measured in the considered fluids. From the results, it was inferred that the backflow vortex cavitation orbital rate and its orbital diameter around the inducer axis depend on the head coefficient but are almost independent of the cavitation number and kind of fluid. Furthermore, the diameter of each backflow vortex cavitation column depends on the head coefficient, cavitation number, and kind of fluid. At the same head coefficient and cavitation number, the diameter in liquid nitrogen is 2.4 times smaller than that in water because of the thermodynamic effects. Moreover, the appearance of the cavitation is "foggy" in nitrogen and "foamy" in water. This difference can be explained by the maximum stable size of cavitation bubbles derived from the critical Weber number theory. Based on this theory, each cavitation bubble at the tip vortex regions in nitrogen is four times smaller than that in water.

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NOx conversion performance of a urea-selective catalytic reduction (SCR) system comprising V2O5/TiO2 catalyst under steady state operating conditions of an 8-litre, common-rail turbo direct injection (TDI) diesel engine was investigated. It was shown that the urea-SCR system achieves 70–90 per cent NOx conversion under medium and high load conditions at 1440 r/min and that NOx conversion is low under low load conditions because of the low catalyst temperatures and the NO/NO2 ratio being higher than unity. It was also shown that NOx conversion exceeds 90 per cent when the catalyst temperature is higher than 530 K. To investigate the details of the chemistry and thermofluid dynamics within the urea-SCR system, a computational fluid dynamics (CFD) code that incorporates detailed surface chemistry was developed based on the modified subroutines of CHEMKIN-II. The spatial variations of chemical species including NO and NH3 in a thin catalyst channel was calculated using the model. The calculated result of NO conversion showed relatively good agreement with experimental results.

Backflow vortices occasionally occur in the annular mixing zone between the main and axially reverse whirling flows from the impeller tip clearance on an axial pump or compressor. A number (N) of tornado-like backflow vortices rotate around themselves and revolve around the casing axis with a diameter (d) and a revolving angular velocity (ω). To investigate the factors determining N and the movement of the backflow vortices, theoretical analyses are performed. Each backflow vortex is generated in the mixing zone; the core region of each backflow vortex is considered to be a forced vortex, while the outer region is considered to be a free vortex. The ratio (f) of the forced vortex to the distance between the backflow-vortex center and the casing is defined. Each backflow vortex has a circulation and induces movements of all the other backflow vortices depending on the distance between the vortices. The casing restricts the movements of all the backflow vortices, and imaginary image vortices are considered on the other side of the casing. Consequently, for d, ω, N, and f, any parameter can be determined if the other three parameters are specified. As an application of the present theory to an inducer representing an axial pump or compressor, the number (Ncav) of “backflow-vortex cavitations,” which occur around the backflow-vortex center, is predicted. Cavitation is visible; therefore, Ncav is quantitatively measurable. In the parameter ranges studied for the tested inducer, the predicted value of N accurately agrees with the experimentally measured value of Ncav.

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