Search Results

Three time integration schemes and four finite volume interpolation schemes for the convection term in momentum equation were tested under turbulent planar gas jet and Sandia non-reacting vaporizing Spray-H cases. The three time integration schemes are the first-order Euler implicit scheme, the second-order backward scheme, and the second-order Crank-Nicolson scheme. The four spatial interpolation schemes are cubic central, linear central, upwind, and vanLeer schemes. Velocity magnitude contour, centerline and radial mean velocity and Reynolds stress profiles for the planar turbulent gas jet case, and fuel vapor contour and liquid and vapor penetrations for the Diesel spray case predicted by the different numerical schemes were compared. The sensitivity of the numerical schemes to mesh resolution was also investigated. The non-viscosity based dynamic structure subgrid model was used. The numerical tool used in this study was OpenFOAM.

The paper reports the application of a look-up table approach within a LES combustion modelling framework for the prediction of knock limit in a highly downsized turbocharged DISI engine. During experimental investigations at the engine test bed, high cycle-to-cycle variability was detected even for relatively stable peak power / full load operations of the engine, where knock onset severely limited the overall engine performance. In order to overcome the excessive computational cost of a direct chemical solution within a LES framework, the use of look-up tables for auto-ignition modelling perfectly fits with the strict mesh requirements of a LES simulation, with an acceptable approximation of the actual chemical kinetics. The model here presented is a totally stand-alone tool for autoignition analysis integrated with look-up table reading from detailed chemical kinetic schemes for gasoline.

Fundamental simulations using DNS type procedures were used to investigate the ignition, combustion characteristics and the lift-off trends of a spatially evolving turbulent liquid fuel jet. In particular, the spatially evolving n-Heptane spray injected in a two-dimensional rectangular domain with an engine like environment was investigated. The computational results were compared to the experimental observations from an optical engine as reported in the literature. It was found that an initial fuel rich combustion downstream of the spray tip is followed by diffusion combustion. Investigations were also made to understand the effects of injection velocity, ambient temperature and the droplet radius on the lift-off length. For each of these parameters three different values in a given range were chosen. For both injection velocity and droplet radius, an increase resulted in a near linear increase in the lift-off length.

Fundamental simulations in a quiescent cell under adiabatic conditions were made to understand the effect of temperature, equivalence ratio and the components of the recirculated exhaust gas, viz., CO2 and H2O, on the combustion of n-Heptane. Simulations were made in single phase in which evaporated n-Heptane was uniformly distributed in the domain. Computations were made for two different temperatures and four different EGR levels. CO2 or H2O or N2was used as EGR. It was found that the initiation of the main combustion process was primarily determined by two competing factors, i.e., the amount of initial OH concentration in the domain and the specific heat of the mixture. Further, initial OH concentration can be controlled by the manipulating the ambient temperature in the domain, and the specific heat capacity of the mixture via the mixture composition. In addition to these, the pre combustion and the subsequent post combustion can also be controlled via the equivalence ratio.

Diesel particulate samples were collected from a light duty engine operated at a single speed-load point with a range of biodiesel and conventional fuel blends. The oxidation reactivity of the samples was characterized in a laboratory reactor, and BET surface area measurements were made at several points during oxidation of the fixed carbon component of both types of particulate. The fixed carbon component of biodiesel particulate has a significantly higher surface area for the initial stages of oxidation, but the surface areas for the two particulates become similar as fixed carbon oxidation proceeds beyond 40%. When fixed carbon oxidation rates are normalized to total surface area, it is possible to describe the oxidation rates of the fixed carbon portion of both types of particulates with a single set of Arrhenius parameters. The measured surface area evolution during particle oxidation was found to be inconsistent with shrinking sphere oxidation.

A swept-volume method of calculating the volume swept by the flame during each time step is developed and used to improve the calculation of fuel reaction rates. The improved reaction rates have been applied to the ignition model and coupled with the level set G-equation combustion model. In the ignition model, a single initial kernel is formed after which the kernel is convected by the gas flow and its growth rate is determined by the flame speed and thermal expansion due to the energy transfer from the electrical circuit. The predicted ignition kernel size was compared with the available experimental data and good agreements were achieved. Once the ignition kernel reaches a size when the fully turbulent flame is developed, the G-equation model is switched on to track the mean turbulent flame front propagation.

Modeling ignition kernel development in spark ignition engines is crucial to capturing the sources of cyclic variability, both with RANS and LES simulations. Appropriate kernel modeling must ensure that energy transfer from the electrodes to the gas phase has the correct timing, rate and locations, until the flame surface is large enough to be represented on the mesh by the G-Equation level-set method. However, in most kernel models, geometric details driving kernel growth are missing: either because it is described as Lagrangian particles, or because its development is simplified, i.e., down to multiple spherical flames. This paper covers the geometric aspects of kernel development, which makes up the core of a Triangulated Lagrangian Ignition Kernel model. One (or multiple, if it restrikes) spark channel is initialized as a one-dimensional Lagrangian particle thread.

Knock in a Rotax-914 engine was modeled and investigated using an improved version of the KIVA-3V code with a G-equation combustion model, together with a reduced chemical kinetics model. The ERC-PRF mechanism with 47 species and 132 reactions [1] was adopted to model the end gas auto-ignition in front of the flame front. The model was validated by a Caterpillar SI engine and a Rotax-914 engine in different operating conditions. The simulation results agree well with available experimental results. A new engineering quantified knock criterion based on chemical mechanism was then proposed. Hydroperoxyl radical (HO2) shows obvious accumulation before auto-ignition and a sudden decrease after auto-ignition. These properties are considered to be a good capability for HO2 to investigate engine knock problems.

In most large-eddy simulation (LES) applications to two-phase engine flows, the liquid-air interactions need to be accounted for as source terms in the respective governing equations. Accurate calculation of these source terms requires the relative velocity “seen” by liquid droplets as they move across the flow, which generally needs to be estimated using a turbulent dispersion model. Turbulent dispersion modeling in LES is very scarce in the literature. In most studies on engine spray flows, sub-grid scale (SGS) models for the turbulent dispersion still follow the same stochastic approach originally proposed for Reynolds-averaged Navier-Stokes (RANS). In this study, an SGS dispersion model is formulated in which the instantaneous gas velocity is decomposed into a deterministic part and a stochastic part. The deterministic part is reconstructed using the approximate deconvolution method (ADM), in which the large-scale flow can be readily calculated.