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Title: Selective catalytic reduction : an optical investigation of sprays, mixing and deposit formation mechanisms in urea dosing systems
Author: Lockyer, Thomas
ISNI:       0000 0004 7971 0696
Awarding Body: Loughborough University
Current Institution: Loughborough University
Date of Award: 2016
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Selective catalytic reduction has become the mainstream exhaust after-treatment approach for removing the harmful nitrogen oxides emitted by heavy-duty diesel engines. The realisation of a highly-efficient selective catalytic reduction system has the opportunity to be a key enabling technology for a future generation of low carbon dioxide producing engines; freeing an engine to combust fuel in a much more efficient manner while still ensuring that mandatory legislated emissions compliance is maintained. Successful implementation of a high-efficiency system for such an engine is reliant on the ability to significantly increase the amount of ammonia which can be delivered to the catalyst. As the accepted precursor to ammonia, this necessitates more urea to be dosed into the exhaust flow and fully decompose into ammonia whilst simultaneously avoiding the formation of problematic deposits within the exhaust hardware. This thesis details an experimental study into the various mechanisms which occurred when a urea spray was injected into a hot exhaust flow. This work was conducted using a state-of-the-art after-treatment system; taken from a heavy-duty diesel engine and installed into a hot flow test bench. The particular system utilised a mixing device in the exhaust hardware to accelerate the stages of urea decomposition. Modifications to the hardware allowed optical access facilitating the use of selected optical techniques to examine a plethora of interactions and mechanisms which proceeded between the spray, the flow and the mixer at a range of representative exhaust flow conditions. Results obtained using phase Doppler interferometry demonstrated that at low dosing rates, using the hot surfaces of a two-stage mixer for droplet impaction enabled 70 % of the spray mass to decompose readily. Particle imaging velocimetry was used to identify a dominant flow structure downstream of the mixer characterised by a pair of counterrotating vortices, generated by turning vanes at the back of the mixer. This strong mixing flow helped improve the distribution of the remaining droplets and facilitated full decomposition of the spray mass within a downstream distance of 200 mm. However, at higher dosing rates excessive surface cooling, brought about by increased spray impingement, resulted in the emergence of liquid films on the surfaces of the mixer. High-speed shadowgraphic imaging captured the stripping of large droplets from the liquid films measuring up to 5 mm in diameter. Over sustained durations of steady-state dosing, time-lapse imaging demonstrated that these large stripped droplets accumulate inside the exhaust to form significant deposits, even at exhaust gas temperatures of 450 °C. This sequence of mechanisms, which culminated in the propagation of widespread deposits, was attributed to localised surface cooling of the mixer, largely due to the three-jet spray structure produced by the original injector. By changing to a pressure-swirl injector the spray better utilised the surfaces of the mixer available for impaction, resulting in a decrease in the amount of deposits formed at higher dosing rates. Unfortunately, the same sequence of film formation and droplet stripping processes which resulted in deposit growth eventually occurred as the dosing rate was further increased. These findings illustrated the need to promote increased heat transfer rates between the mixer and the impacting spray. This was realised by substituting the original mixer with a very high-surface area, novel mixing device in the form of a cordierite ceramic substrate. When coupled with the correct spray type, the high-surface area mixer supressed the development of liquid films on the surfaces at these high dosing rates, thereby preventing the procession of the previously seen series of deposits formation processes. However, the extruded profile of the substrate mixer also eliminated any desirable downstream flow mixing and allowed a proportion of the spray droplets to be entrained in the flow, forming small deposits on surfaces much further downstream. By reinstalling the original twostage mixer immediately downstream of the substrate, the remaining droplets were successfully removed from the flow. Ultimately, this new hardware arrangement facilitated up to a five-fold increase in the deposit-free dosing rates relative to the original state-of-the-art system at a number of exhaust conditions, and thus would be suitable for a highly-efficient selective catalytic reduction system.
Supervisor: Not available Sponsor: EPSRC ; Energy Technologies Institute
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Mechanical Engineering not elsewhere classified ; Urea ; SCR ; Selective catalytic reduction ; NOx ; Diesel emissions ; Deposits ; DEF ; AdBlue