Optimization of the AdBlue e vaporation module for Scania V8 engines

Sammanfattning: The aftertreatment techniques introduced to follow the emission legislations require a constant improvement process to comply with the gradually more stringent demands. SCR is the system used nowadays to deal with NOx emissions in most heavy-duty vehicles. An aqueous-urea solution, AdBlue, is sprayed into the evaporation unit, where urea should decompose to ammonia, the reducing agent. This is a critical step because the NH3 amount available heavily affects the final nitrous oxides reduction to nitrogen. Moreover the urea decomposition’s sides reactions are likely to occur, forming deposits that increase the pressure drop and in a certain time period could even foul the system. The evaporation module used in the silencer for Scania trucks equipped with V8 engines consists of a pipe in pipe configuration made in stainless steel 1.4509, where the exhaust gases flow heating up the inner pipe finned on its outer surface. The AdBlue is sprayed on the inner pipe’s inner surface, creating a wall film and cooling down the tube. The production of the evaporation pipe however involves a costly manufacturing process, being made of 144 flanges laser welded on a 0.355 m length, for a total of more than 52 m of welding. The goal of this thesis is to analyse the heat transfer from the exhaust gases to the pipe and how to improve it, in order to achieve a lower temperature drop on the pipe due to the AdBlue dosing, reducing at the same time the risk of building up deposits. The application of different materials for the evaporation unit is also considered. Furthermore many manufacturing processes are evaluated as a cost-effective alternative to the current one. Although the operating points have a wide range of variation, the analysis is focused on the worst conditions for urea evaporation which are low mass flow and low flow temperature. Stainless steel is the best trade-off between cost, thermal conductivity and corrosion resistance but the much higher conductivity of copper alloys would justify the investigation of a copper evaporation pipe coated with stainless steel. Different designs of the heat flanges are assessed, first with correlations and FEA and then through a CFD analysis, where 62 different solutions are compared. The fins height results to be the most influencing parameter, requiring an increment from 7.5 mm to 11 mm to improve the heat transfer performances of the evaporation unit. The gap between each fin is also important, leading to a flanges quantity reduction suggestion. With the current fin design and half of the number of flanges, 11 mm high, the performances would improve by almost 40% (at 800 kg/h and 300℃). Furthermore both the Abstract pipe thickness and thermal conductivity are affecting the temperature drop, with different weight depending on the design and the operating point. It is however always advantageous to use a thicker wall and a material with a higher thermal conductivity. Lastly the tests performed on the specifically developed test rig show a good accordance with the simulations in comparing different materials but are not suitable to compare finned designs.