Your Selections

University of Wisconsin
Show Only

Collections

File Formats

Content Types

Dates

Sectors

Topics

Authors

Publishers

Affiliations

Events

 

A Visual Investigation of CFD-Predicted In-Cylinder Mechanisms That Control First- and Second-Stage Ignition in Diesel Jets

University of Wisconsin-Rolf Reitz
University of Wisconsin - Madison-Randy Hessel
Published 2019-04-02 by SAE International in United States
The long-term goal of this work is to develop a conceptual model for multiple injections of diesel jets. The current work contributes to that effort by performing a detailed modeling investigation into mechanisms that are predicted to control 1st and 2nd stage ignition in single-pulse diesel (n-dodecane) jets under different conditions. One condition produces a jet with negative ignition dwell that is dominated by mixing-controlled heat release, and the other, a jet with positive ignition dwell and dominated by premixed heat release.During 1st stage ignition, fuel is predicted to burn similarly under both conditions; far upstream, gases at the radial-edge of the jet, where gas temperatures are hotter, partially react and reactions continue as gases flow downstream. Once beyond the point of complete fuel evaporation, near-axis gases are no longer cooled by the evaporation process and 1st stage ignition transitions to 2nd stage ignition. At this point, for the positive ignition dwell case, all of the fuel has already been injected and the 2nd stage ignition zone is surrounded by a relatively large mass of…
Datasets icon
Annotation icon
 

Evaluating Surface Film Models for Multi-Dimensional Modeling of Spray-Wall Interaction

University of Wisconsin-Rohit Mishra, Christopher Rutland
Published 2019-04-02 by SAE International in United States
Surface film formation is an important phenomenon during spray impingement in a combustion chamber. The film that forms on the chamber walls and piston bowl produces soot post-combustion. While some droplets stick to the wall surface, others splash and interact with the gas present inside the combustion chamber. Accurate prediction of both the film thickness and splashed mass is crucial for surface film model development since it leads to a precise estimation of the amount of soot and other exhaust gases formed. This information could guide future studies aimed at a comprehensive understanding of the combustion process and might enable development of engines with reduced emissions. Dynamic structure Large Eddy Simulation (LES) turbulence model implemented for in-cylinder sprays [1] has shown to predict the flow structure of a spray more accurately than the Reynolds-averaged Navier-Stokes turbulence model. Therefore, using dynamic structure LES turbulence model to analyze the interaction of turbulent gas phase region with the liquid deposited as film and with the splashed droplets is a promising area to study turbulent multiphase flows interacting with…
Datasets icon
Annotation icon
 

Limitations of Sector Mesh Geometry and Initial Conditions to Model Flow and Mixture Formation in Direct-Injection Diesel Engines

Richard C. Peterson
University of Wisconsin-Federico Perini, Rolf Reitz
Published 2019-04-02 by SAE International in United States
Sector mesh modeling is the dominant computational approach for combustion system design optimization. The aim of this work is to quantify the errors descending from the sector mesh approach through three geometric modeling approaches to an optical diesel engine. A full engine geometry mesh is created, including valves and intake and exhaust ports and runners, and a full-cycle flow simulation is performed until fired TDC. Next, an axisymmetric sector cylinder mesh is initialized with homogeneous bulk in-cylinder initial conditions initialized from the full-cycle simulation. Finally, a 360-degree azimuthal mesh of the cylinder is initialized with flow and thermodynamics fields at IVC mapped from the full engine geometry using a conservative interpolation approach. A study of the in-cylinder flow features until TDC showed that the geometric features on the cylinder head (valve tilt and protrusion into the combustion chamber, valve recesses) have a large impact on flow complexity. As a result, errors in near-TDC swirl ratio, vortex structure and turbulence availability were seen when employing sector meshing, even if a 360-degree sector, with direct IVC flow…
Datasets icon
Annotation icon
 

Investigation of Fuel Condensation Processes under Non-reacting Conditions in an Optically-Accessible Engine

University of Wisconsin-Lu Qiu, Rolf Reitz
Sandia National Laboratories-Mark Musculus
Published 2019-04-02 by SAE International in United States
Engine experiments have revealed the importance of fuel condensation on the emission characteristics of low temperature combustion. However, direct in-cylinder experimental evidence has not been reported in the literature. In this paper, the in-cylinder condensation processes observed in optically accessible engine experiments are first illustrated. The observed condensation processes are then simulated using state-of-the-art multidimensional engine CFD simulations with a phase transition model that incorporates a well-validated phase equilibrium numerical solver, in which a thermodynamically consistent phase equilibrium analysis is applied to determine when mixtures become unstable and a new phase is formed. The model utilizes fundamental thermodynamics principles to judge the occurrence of phase separation or combination by minimizing the system Gibbs free energy. It is shown that thermodynamically unstable mixtures are formed during the late expansion stroke for the conditions of the experiments. Close agreement on the beginning of condensation is also observed between the simulations and available experiments.
Datasets icon
Annotation icon
 

Bowl Geometry Effects on Turbulent Flow Structure in a Direct Injection Diesel Engine

University of Wisconsin-Rolf Reitz
University of Wisconsin-Madison-Federico Perini
Published 2018-09-10 by SAE International in United States
Diesel piston bowl geometry can affect turbulent mixing and therefore it impacts heat-release rates, thermal efficiency, and soot emissions. The focus of this work is on the effects of bowl geometry and injection timing on turbulent flow structure. This computational study compares engine behavior with two pistons representing competing approaches to combustion chamber design: a conventional, re-entrant piston bowl and a stepped-lip piston bowl. Three-dimensional computational fluid dynamics (CFD) simulations are performed for a part-load, conventional diesel combustion operating point with a pilot-main injection strategy under non-combusting conditions. Two injection timings are simulated based on experimental findings: an injection timing for which the stepped-lip piston enables significant efficiency and emissions benefits, and an injection timing with diminished benefits compared to the conventional, re-entrant piston.While the flow structure in the conventional, re-entrant combustion chamber is dominated by a single toroidal vortex, the turbulent flow evolution in the stepped-lip combustion chamber depends more strongly on main injection timing. For the injection timing at which faster mixing controlled heat release and reduced soot emissions have been observed experimentally,…
Datasets icon
Annotation icon
 

Automatic Hex-Dominant Mesh Generation for Complex Flow Configurations

SAE International Journal of Engines

University of Wisconsin-Nihar Sawant
Carnegie Mellon University-Soji Yamakawa, Satbir Singh, Kenji Shimada
  • Journal Article
  • 2018-01-0477
Published 2018-04-03 by SAE International in United States
A method for automatically generating hex-dominant meshes for Computational Fluid Dynamics (CFD) applications is presented in this article. Two important regions of the mesh for any CFD simulation are the interior mesh and the boundary layer mesh. The interior mesh needs to be fine in the critical flow regions to ensure accurate solutions. The proposed method uses Bubble Mesh algorithm which packs bubbles inside the geometry to generate the mesh nodes. Algorithm was tested for sample flow problems and improvements were made to interior and boundary layer mesh generation methods. The interior mesh is generated using directionality and sizing control functions specified on the points of a 3D grid generated over the entire geometry. This offers a flexible control over mesh sizing and local mesh refinement. Boundary layer mesh is important to accurately model the physics of boundary layer near the geometry walls. In the proposed method, boundary elements in the mesh are split into multiple divisions with the first division having the smallest thickness to ensure it lies inside the physical boundary layer. The…
Datasets icon
Annotation icon
 

A Triangulated Lagrangian Ignition Kernel Model with Detailed Kinetics for Modeling Spark Ignition with the G-Equation-Part I: Geometric Aspects

University of Wisconsin-Rolf Reitz
University of Wisconsin Madison-Federico Perini, Christopher Rutland
Published 2018-04-03 by SAE International in United States
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. Each channel particle is advected as a Lagrangian tracker plus a turbulent dispersion term, with least-squares field reconstruction to compensate for the lesser mesh resolution. The 1D thread discretization is dynamically updated to stick to the user’s resolution request; plus, particles falling into the…
Datasets icon
Annotation icon
 

Evaluation and Validation of Large-Eddy-Simulation (LES) for Gas Jet and Sprays

University of Wisconsin-Chi-Wei Tsang, Christopher Rutland
ANSYS Inc.-Yue Wang, Cheng Wang, Anthony Shelburn, Long Liang, Karthik Puduppakkam, Abhijit Modak, Chitralkumar Naik, Ellen Meeks
Published 2017-03-28 by SAE International in United States
Large-eddy simulation (LES) is a useful approach for the simulation of turbulent flow and combustion processes in internal combustion engines. This study employs the ANSYS Forte CFD package and explores several key and fundamental components of LES, namely, the subgrid-scale (SGS) turbulence models, the numerical schemes used to discretize the transport equations, and the computational mesh. The SGS turbulence models considered include the classic Smagorinsky model and a dynamic structure model. Two numerical schemes for momentum convection, quasi-second-order upwind (QSOU) and central difference (CD), were evaluated. The effects of different computational mesh sizes controlled by both fixed mesh refinement and a solution-adaptive mesh-refinement approach were studied and compared.The LES models are evaluated and validated against several flow configurations that are critical to engine flows, in particular, to fuel injection processes. These configurations include a turbulent planar gas jet, an evaporating and non-reacting spray, and a reacting spray. In the gas-jet case, predicted time-averaged and root-mean-squared (RMS) flow speed and Reynolds stress are validated against experimental and Direct Numerical Simulation (DNS) data. Qualitative results, including the…
Datasets icon
Annotation icon
 

Pressure-Based Knock Measurement Issues

University of Wisconsin-Arsham J. Shahlari, Jaal Ghandhi
Published 2017-03-28 by SAE International in United States
Highly time resolved measurements of cylinder pressure acquired simultaneously from three transducers were used to investigate the nature of knocking combustion and to identify biases that the pressure measurements induce. It was shown by investigating the magnitude squared coherence (MSC) between the transducer signals that frequency content above approximately 40 kHz does not originate from a common source, i.e., it originates from noise sources. The major source of noise at higher frequency is the natural frequency of the transducer that is excited by the impulsive knock event; even if the natural frequency is above the sampling frequency it can affect the measurements by aliasing. The MSC analysis suggests that 40 kHz is the appropriate cutoff frequency for low-pass filtering the pressure signal. Knowing this, one can isolate the knock event from noise more accurately. Four time windows are identified for a knock event: (1) pressure rise due to flame propagation; (2) a rapid but resolved pressure rise that is ~50 µs in duration; (3) a transducer shock period that is highly contaminated by noise that…
Datasets icon
Annotation icon
 

The Effects of Charge Preparation, Fuel Stratification, and Premixed Fuel Chemistry on Reactivity Controlled Compression Ignition (RCCI) Combustion

SAE International Journal of Engines

University of Wisconsin-Sage Kokjohn, Rolf Reitz
Oakland University-Dan DelVescovo
  • Journal Article
  • 2017-01-0773
Published 2017-03-28 by SAE International in United States
Engine experiments were conducted on a heavy-duty single-cylinder engine to explore the effects of charge preparation, fuel stratification, and premixed fuel chemistry on the performance and emissions of Reactivity Controlled Compression Ignition (RCCI) combustion. The experiments were conducted at a fixed total fuel energy and engine speed, and charge preparation was varied by adjusting the global equivalence ratio between 0.28 and 0.35 at intake temperatures of 40°C and 60°C. With a premixed injection of isooctane (PRF100), and a single direct-injection of n-heptane (PRF0), fuel stratification was varied with start of injection (SOI) timing. Combustion phasing advanced as SOI was retarded between -140° and -35°, then retarded as injection timing was further retarded, indicating a potential shift in combustion regime. Peak gross efficiency was achieved between -60° and -45° SOI, and NOx emissions increased as SOI was retarded beyond -40°, peaking around -25° SOI. Optimal cases in terms of both gross efficiency and peak pressure rise rate (PPRR) were in the mid-range SOI timings centered about -50° SOI, while late SOI resulted in decreased gross efficiency,…
Datasets icon
Annotation icon