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The automotive industry has been restricted by several regulations to reduce nitrogen oxide emissions from diesel engines. To adhere to the emission regulation, advanced engine technology aims to control oxygen access, compression, and ignition engine, while achieving both reduced fuel consumption and lowering unburnt HC, CO2, and CO levels. However, it is difficult to meet the NOx limits of the European standard (EURO VI) and American standard (US TIER 2 BIN 5). Selective catalytic reduction (SCR) with ammonia (NH3) has been successfully employed to remove NOx from diesel engines. However, uniform ammonia distribution is usually difficult to achieve. Therefore, a study on ammonia uniformity to predict NOx reduction efficiency in SCR systems is needed. The engine utilized in the experiment was turned on for a few minutes to obtain the desired temperature and airflow, and the simulation was performed using a commercial code of STAR-CCM+. This study was performed at engine operating conditions of 1500 rpm, 2000 rpm, and 3000 rpm; simulation was performed with a fixed pressure and velocity, as in the experiment, thus creating a similar turbulent swirl flow in the SCR system. Numerical results were validated using experimental results of ammonia concentration distribution. [ABSTRACT FROM AUTHOR]

Urea aqueous solution is injected into the hot exhaust flow which acts as the source of ammonia in SCR systems to reduce the NOx emissions of diesel vehicles. However, urea decomposition thus ammonia generation is not always perfect, resulting in solid urea deposit formation accumulated in exhaust pipe wall, on the catalyst surface and mixing element. Decomposition of urea and mitigation of solid deposit formation is a great challenge to implement automotive SCR systems. This study presents a combined experimental and numerical investigation of UWS spray, droplet breakup, urea decomposition and deposit formation of SCR systems. The investigation firstly focused on fundamental spray characteristics of UWS and its effects on urea decomposition. The experiment was conducted with a commercial pressure driven SCR injector into an optically accessible test chamber and z-type shadowgraph imaging was implemented to capture the spray images. Relevant dimensions of spray images were estimated by digital image processing. The injection quantity of the experimental injector was measured to get the injection rate. Temperature distribution in the exhaust pipe and ammonia mass fraction was measured for different exhaust gas temperatures to estimate the urea decomposition. Finally, FTIR was used to analyze the chemical composition of solid deposit formed due to incomplete urea decomposition. The numerical assessment was carried out by using STAR CCM+ CFD code. The Eulerian-Lagrangian approach was implemented for the modeling of multiphase flow and urea water spray droplets. The investigation reveals that higher injection pressure increases the risk of wall impingement and wall wetting that leads to deposit formation. Though injection pressure is dominant on droplet size, exhaust gas temperature has also significant effects on droplet size and evaporation. Spray injection causes local cooling that effects on urea decomposition and relatively higher exhaust gas temperature is required for the complete decomposition of urea. Major chemical composition identified in the deposit sample are unreacted urea, biuret, and cyanuric acid. [ABSTRACT FROM AUTHOR]

Selective catalytic reduction is the most promising technique for NOx reduction from diesel engine exhaust emissions. The greatest challenge to implementing the system is solid deposit formation. Spray wall impingement of UWS was investigated in a constant volume chamber by numerical simulation using STAR CCM+ CFD code. The realizable k-ε two-layer approach together with standard wall functions and all y+ treatment was implemented for the modeling of turbulence flow. The two-phase flow was modeled by using Eulerian- Lagrangian approach. The heated wall temperature was maintained from 338 K to 573 K during the UWS spray wall impingement. The development of the spray after impinging on the heated wall was visualized and measured. UWS droplet size distribution, wall film formation and droplet evaporation rates were estimated, which are vital parameters for the system performance but not well researched. User-defined functions (UDF) were developed to input the Leidenfrost temperature limit in the calculation and to estimate the desired parameters. The investigation reveals that wall temperature is the most important parameter that significantly affects spray development after impingement, droplet size distribution, wall film formation and droplet evaporation. Increasing the wall temperature leads to longer spray front projection length, smaller droplet size, faster droplet evaporation and smaller film thickness, which are preconditions for urea crystallization reduction. The numerical model and parameters were validated by comparing with experimental data. [ABSTRACT FROM AUTHOR]