Wilfried Edelbauer

Coupled simulations of nozzle flow, primary fuel jet breakup, and spray formation

Presented are two approaches for coupled simulations of the injector flow with spray formation. In the first approach the two-fluid model is used within the injector for the cavitating flow. A primary breakup model is then applied at the nozzle orifice where it is coupled with the standard discrete droplet model. In the second approach the Eulerian multi-fluid model is applied for both the nozzle and spray regions. The developed primary breakup model, used in both approaches, is based on locally resolved properties of the cavitating nozzle flow across the orifice cross section. The model provides the initial droplet size and velocity distribution for the droplet parcels released from the surface of a coherent liquid core. The major feature of the predictions obtained with the model is a remarkable asymmetry of the spray. This asymmetry is in agreement with the recent observations at Chalmers University where they performed experiments using a transparent model scaled-up injector. The described model has been implemented into AVL FIRE computational fluid dynamics code which was used to obtain all the presented results. Copyright

Large Eddy Simulation of Cavitating Throttle Flow

The Reynolds averaged Navier-Stokes equation model (RANS) is state-of-the-art for numerical simulations of cavitating throttle and injector flows. RANS models are based on time-averaged Navier-Stokes equations, and the computational costs are much lower than those for the more advanced Large Eddy Simulations (LES). The principle of LES is low-pass filtering of the Navier-Stokes equations to eliminate the small scales of the solution. In general LES requires higher numerical resolution in space and time, and higher order discretization schemes. In the recent years, clustered processing units provide increased computational resources, and therefore LES simulations became, also for two-phase flows, more and more interesting. The current paper presents Large Eddy Simulations of the cavitating two-phase flow in a rectangular micro-scale throttle operated with Diesel fuel and compares them with RANS simulations. The LES shows interesting new details which cannot be resolved by RANS simulations in general, such as the transition from laminar to turbulent flow in the channel or the phase change caused by turbulent pressure fluctuations in the shear layer. A cavitation erosion model predicts the zones with highest damage probability. All simulations are performed with the commercial CFD code AVL FIRE®. Time-averaged results of the numerically predicted velocity profiles and the liquid–vapor distributions are compared with already published optical measurements performed with the Laser-Induced Fluorescence (LIF) and the light transmission techniques.

Simulation of the oil droplet laden air flow in the crankcase of an Internal Combustion engine

For improving the efficiency of internal combustion engines, the optimisation of the oil and air flow in the crankcase is of particular interest. Due to the complexity of the crank drive motion CFD was rarely used for this application so far. The present work intends to provide a better understanding of the droplet formation and transport processes in the crankcase. Therefore, model correlations have been implemented into a CFD code capable to deal with moving unstructured computational meshes. The simulation shows the oil disintegration at the crank drive and illustrates the propagation of the oil droplets during one crankshaft revolution.

Modeling of Reactive Spray Processes in DI Diesel Engines

Commonly, the spray process in Direct Injection (DI) diesel engines is modeled with the Euler Lagrangian discrete droplet approach which has limited validity in the dense spray region, close to the injector nozzle hole exit. In the presented research, a new reactive spray modelling method has been developed and used within the 3D RANS CFD framework. The spray process was modelled with the Euler Eulerian multiphase approach, extended to the size-of- classes approach which ensures reliable interphase momentum transfer description. In this approach, both the gas and the discrete phase are considered as continuum, and divided into classes according to the ascending droplet diameter. The combustion process was modelled by taking into account chemical kinetics and by solving general gas phase reaction equations. The newly developed method was incorporated into the commercial computational fluid dynamics code AVL FIRE™, and it was used in combination with the previously validated spray sub-models. The method takes into account the liquid jet disintegration, droplet atomization, droplet collision, droplet evaporation, and subsequent vapor combustion.The computed results are validated against available experimental data, and a good agreement of the mean pressure, temperature and rate of heat release was achieved.

Advances in the numerical investigation of the immersion quenching process

A numerical investigation of the immersion quenching process is presented in this paper. Immersion quenching is recognized as one of the common ways to achieve the desirable microstructure, and to improve the mechanical properties after thermal treatment. Furthermore it is important to prevent distortion and cracking of the cast parts. Accurate prediction of all three boiling regimes and the heat transfer inside the structure during quenching are important to finally evaluate the residual stresses and deformations of thermally treated parts. Numerical details focus on the handling of the enthalpy with variable specific heat capacity in the solid. For two application cases, comparison between measured and simulated temperatures at different monitoring positions shows very good agreement. The study demonstrates the capability of the present model to overcome the numerical challenges occurring during immersion quenching and it is capable of predicting the complex physics with good accuracy.

Numerical simulation of compressible cavitating two-phase flows with a pressure-based solver

This work intends to study the effect of compressibility on throttle flow simulations with a pressure–based solver.The simple micro throttle geometry allows easier access for obtaining experimental data compared to a real injector, but still maintaining the main flow features. For this reasons it represents a meaningful and well reported benchmark for validation of numerical methods developed for cavitating injector flows.An implicit pressure–based compressible solver is used on the filtered Navier–Stokes equations. Thus, no stability limitation is applied on the time step. A common pressure field is computed for all phases, but different velocity fields are solved for each phase, following the multi–fluid approach. The liquid evaporation rate is evaluated with a Rayleigh–Plesset equation based cavitation model and the Coherent Structure Model is adopted as closure for the sub–grid scales in the momentum equation.The aim of this study is to show the capabilities of the pressure–based solver to deal with both vapor and liquid phases considered compressible. A comparison between experimental results and compressible simulations is presented. Time–averaged vapor distribution and velocity profiles are reported and discussed. The distribution of pressure maxima on the surface and the results from a semi–empirical erosion model are in good agreement with the erosion locations observed in the experiments. This test case aims to represent a benchmark for furtherapplication of the methodology to industrial relevant cases.

Numerical Simulation of Compressible Multi-Phase Flow in High Pressure Fuel Pump

Modern injection systems utilize high injection pressures to enhance the break-up of the injected fuel and the mixing of fuel with air. Elevated pressure level targets high performance, high efficiency and low tailpipe emissions. Such conditions lead to high internal loads of fuel injection equipment and aggressive conditions within fuel injectors and pumps. The high pressure pump is the most critical component assuring appropriate elevated pressure level. Under certain conditions cavitation can occur within the system, which will affect the performance of the pump and in long term also its durability. Namely, cavitation repeatedly appearing at the same location can lead to erosion damage, which is clearly not desired. Therefore, numerical analyses by means of Computational Fluid Dynamics (CFD), represent a powerful tool in the early stage of component definition or design of the pump itself. As the pressure appearing in such systems exceeds 300 MPa, the liquid fuel needs to be treated as compressible. Moving parts of the investigated fuel pump are displaced due to pressure forces, which means that pressure variations and pressure waves need to be accurately predicted in order to predict accurate part displacements and correct wetted volume shape. In order to achieve this, the liquid fuel is treated as compressible, otherwise exact inlet- and outlet check-valve displacements are not predictable. In present work the liquid compressible Euler-Eulerian multiphase model of the commercial CFD code AVL FIRE® has been applied. The domain has been geometrically discretized using the preprocessing part of the applied CFD tool, moving parts have been handled by a novel, so-called “mesh deformation by formula” methodology. The advantage of the approach is that it does not require the pre-definition of all moving parts but allows for an arbitrary, user-defined movement of all mesh nodes. The motion of internal floating parts is performed automatically during the calculation according to the local pressure forces. Due to high pressure levels local flow velocities are typically very high causing the fuel to undergo phase change from liquid to vapor called cavitation. To accurately account for the effect of cavitation, the applied CFD code offers advanced cavitation modeling options. The applied capability enables estimation of flow aggressiveness and the probability for the onset of cavitation erosion on the surface of the components with the objective to optimize or entirely eliminate cavitation. In the present study two simulations have been performed; (i) part load and (ii) full load condition.Copyright