Analysis of the influence of different real flow effects on computational fluid dynamics boundary conditions based on the method of characteristics

Nowadays, turbocharged internal combustion engines (ICEs) are very common in automotive powerplants, monopolizing the Diesel sector and having a steadily increasing percentage in the gasoline one. In this frame, the interest in modeling the behavior of the turbomachinery components involved, with the ultimate goal of characterizing the performance of the turbocharged ICE, seems clear. A turbomachine can be simulated using 3D-CFD software, but its computational cost does not allow to reproduce the whole turbocharger test rig. Moreover, the existence of long ducts requires a considerable computational time until the pressure reflections at the boundaries dissipate in order to reach a periodic solution. The use of non-reflecting boundary conditions reduces the needed length of ducts without introducing spurious wave reflections. An anechoic boundary condition (BC) based on the Method of Characteristics has been previously developed, considering the case of an inviscid and adiabatic 1D flow of a perfect gas. However, real flows do not behave in such ideal manner. In this paper, the extension of the scope of the previous BC is sought. In this way, a methodology to evaluate the performance of the anechoic BC under these real flow situations is shown. The consideration of ideal gas instead of perfect gas, the flow viscosity and the non-homentropic flow makes it necessary to modify the Method of Characteristics, since the Riemann Invariants are not constant any more. In this frame they are referred to as Riemann Variables. An additional issue that has been considered is the effect of swirl flow, as the one in the turbine outlet, on the anechoic BC. Some improvements to be implemented in the BC are proposed in order to have a better performance in these real flow situations.Galindo, J.; Tiseira Izaguirre, AO.; Fajardo, P.; Navarro García, R. (2013). Analysis of the influence of different real flow effects on computational fluid dynamics boundary conditions based on the method of characteristics. Mathematical and Computer Modelling. 57(7-8):1957-1964. doi:10.1016/j.mcm.2012.01.016S19571964577-

Exhaust Pressure Waves in Diesel Engines and the Impacts on Aftertreatment Sprays

Aftertreatment devices are commonly used in diesel engine systems for emission control. To ensure the proper function of the aftertreatment devices, it is often necessary to inject additional fuel or a reducing agent into the exhaust flow. The conditions in the exhaust flow are affected by the exhaust pressure wave, caused by the reciprocating movement of the piston and valves. Therefore, this research investigates the factors that affect the exhaust pressure wave and the consequent impacts of this wave on a liquid spray. First, the exhaust pressure wave action was studied experimentally on a single cylinder diesel engine at various engine operating conditions such as different load, EGR, and exhaust backpressure levels. Then, the gas flow velocity in the exhaust system was estimated with the simulation tool AVL BOOST. A stand-alone shock tube was used to create a condition comparable to the exhaust flow. The impacts of the pressure wave and the gas flow on the aftertreatment spray distribution and the droplet breakup were studied via optical methods