9,754 research outputs found
Computational Modeling and Analysis of Diesel-fuel Injection and Autoignition at Transcritical Conditions
The need for improved engine efficiencies has motivated the development of
high-pressure combustion systems, in which operating conditions achieve and
exceed critical conditions. Associated with these conditions are strong
variations in thermo-transport properties as the fluid undergoes phase
transition, and two-stage ignition with low-temperature combustion. Accurately
simulating these physical phenomena at real-fluid environments remains a
challenge. By addressing this issue, a high-fidelity LES-modeling framework is
developed to conduct simulations of transcritical fuel spray mixing and
auto-ignition at high-pressure conditions. The simulation is based on a
recently developed diffused interface method that solves the compressible
multi-species conservation equations along with a Peng-Robinson state equation
and real-fluid transport properties. LES analysis is performed for non-reacting
and reacting spray conditions targeting the ECN Spray A configuration at
chamber conditions with a pressure of 60 bar and temperatures between 900 K and
1200 K to investigate effects of the real-fluid environment and low-temperature
chemistry. Comparisons with measurements in terms of global spray parameters
(i.e., liquid and vapor penetration lengths) are shown to be in good agreement.
Analysis of the mixture fraction distributions in the dispersed spray region
demonstrates the accuracy in modelling the turbulent mixing behavior. Good
agreement of the ignition delay time and the lift-off length is obtained from
simulation results at different ambient temperature conditions and the
formation of intermediate species is captured by the simulations, indicating
that the presented numerical framework adequately reproduces the corresponding
low- and high-temperature ignition processes under high-pressure conditions,
which are relevant to realistic diesel-fuel injection systems.Comment: THIESEL 2018 Conference on Thermo- and Fluid Dynamic Processes in
Direct Injection Engine
Efficient time stepping for reactive turbulent simulations with stiff chemistry
A combination of a steady-state preserving operator splitting method and a
semi-implicit integration scheme is proposed for efficient time stepping in
simulations of unsteady reacting flows, such as turbulent flames, using
detailed chemical kinetic mechanisms. The operator splitting is based on the
Simpler balanced splitting method, which is constructed with improved stability
properties and reduced computational cost. The method is shown to be capable of
stable and accurate prediction of ignition and extinction for
reaction-diffusion systems near critical conditions. The ROK4E scheme is
designed for semi-implicit integration of spatially independent chemically
reacting systems. Being a Rosenbrock-Krylov method, ROK4E utilizes the low-rank
approximation of the Jacobian to reduce the cost for integrating the system of
ODEs that have relative few stiff components. The efficiency of the scheme is
further improved via the careful choice of coefficients to require three
right-hand-side evaluations over four stages. Combing these two methods,
efficient calculation is achieved for large-scale parallel simulations of
turbulent flames
Transcritical Mixing and Auto-Ignition of n-dodecane Liquid Fuel using a Diffused Interface Method
High-fidelity simulations of transcritical spray mixing and auto ignition in
a combustion chamber are performed at high pressure and temperature conditions
using a recently developed finite rate LES solver. The simulation framework is
based on a diffused-interface (DI) method that solves the compressible
multi-species conservation equations along with the Peng Robinson state
equation and real-fluid transport properties. A finite volume approach with
entropy stable scheme is employed to accurate simulate the non-linear real
fluid flow. LES analysis is performed for non-reacting and reacting spray
conditions targeting the ECN Spray A configuration at chamber conditions with a
pressure of 60 bar and temperatures between 800 K and 1200 K to investigate
effects of the real-fluid environment and low-temperature chemistry.
Comparisons with measurements in terms of global spray parameters and mixture
fraction distributions demonstrates the accuracy in modeling the turbulent
mixing behavior. Good overall agreement of the auto-ignition process is
obtained from simulation results at different ambient temperature conditions
and the formation of intermediate species is captured by the simulations,
indicating that the presented numerical framework adequately reproduces the
corresponding low-and-high-temperature ignition processes under high-pressure
conditions that are relevant to realistic diesel fuel injection systems.Comment: AIAA Propulsion and Energy Forum, July 9-11, 2018, Cincinnati, Ohio.
arXiv admin note: text overlap with arXiv:1809.08721, arXiv:1705.0723
Modeling and Simulation of Diesel Injection at Transcritical Conditions
The need for improved engine efficiencies has motivated the development of
high-pressure combustion systems, in which operating conditions achieve and
exceed critical conditions. Associated with these conditions are large
thermodynamic gradients and strong variations in transport properties as the
fluid undergoes mixing and phase transition. Accurately simulating these
real-fluid environments remains a main challenge. Different modeling approaches
have been employed, which can be categorized as diffused and sharp interface
methods. The objective of this study is to examine the diffused interface
method for simulating diesel-fuel injection at conditions related to the
supercritical regime. To this end, a recently developed compressible real-fluid
solver for transcritical conditions is employed. Simulations of an ECN-relevant
diesel-fuel injector are performed and predictions for instantaneous and
statistical flow-field results are compared against available measurements. It
is expected that results from this analysis will be useful in identifying
limitations of current modeling techniques and in improving physical and
numerical models for high-pressure injection systems.Comment: ILASS-Americas 201
Numerical methods to prevent pressure oscillations in transcritical flows
The accurate and robust simulation of transcritical real-fluid effects is
crucial for many engineering applications, such as fuel injection in internal
combustion engines, rocket engines and gas turbines. For example, in diesel
engines, the liquid fuel is injected into the ambient gas at a pressure that
exceeds its critical value, and the fuel jet will be heated to a supercritical
temperature before combustion takes place. This process is often referred to as
transcritical injection. The largest thermodynamic gradient in the
transcritical regime occurs as the fluid undergoes a liquid-like to a gas-like
transition when crossing the pseudo-boiling line (Yang 2000, Oschwald et al.
2006, Banuti 2015). The complex processes during transcritical injection are
still not well understood. Therefore, to provide insights into high-pressure
combustion systems, accurate and robust numerical simulation tools are required
for the characterization of supercritical and transcritical flows.Comment: Annual Research Briefs 2016, Center for Turbulence Research, Stanford
Universit
The Galactic Census of High- and Medium-mass Protostars. II. Luminosities and Evolutionary States of a Complete Sample of Dense Gas Clumps
(Abridged) The Census of High- and Medium-mass Protostars (CHaMP) is the
first large-scale (280 degree<l<300 degree, -4 degree<b<2 degree), unbiased,
sub-parsec resolution survey of Galactic molecular clumps and their embedded
stars. Barnes et al. (2011) presented the source catalog of ~300 clumps based
on HCO+(1-0) emission, used to estimate masses M. Here we use archival
mid-infrared to mm continuum data to construct spectral energy distributions.
Fitting two-temperature grey-body models, we derive bolometric luminosities, L.
We find the clumps have 10Lsun<L<1E6.5Lsun and 0.1<L/M<1E3, consistent with
theoretical expectations of a clump population that spans a range of
instantaneous star formation efficiencies from 0 to ~50%. We thus expect L/M to
be a useful, strongly-varying indicator of clump evolution during the star
cluster formation process. We find correlations of the ratio of warm to cold
component fluxes and of cold component temperature with L/M. We also find a
near linear relation between L/M and Spitzer-IRAC specific intensity (surface
brightness), which may thus also be useful as a star formation efficiency
indicator. The lower bound of the clump distribution suggests the star
formation efficiency per free-fall time is epsilon<0.2. We do not find strong
correlations of L/M with mass surface density, velocity dispersion or virial
parameter. We find a linear relation between L and L_{HCO+(1-0}}, although with
large scatter for any given individual clump. Fitting together with
extragalactic systems, the linear relation still holds, extending over 10
orders of magnitude in luminosity. The complete nature of the CHaMP survey over
a several kiloparsec-scale region allows us to derive a measurement at an
intermediate scale bridging those of individual clumps and whole galaxies.Comment: Revisions have been made. Accepted to Ap
Direct numerical simulations of turbulent channel flow under transcritical conditions
Turbulent flows under transcritical conditions are present in regenerative
cooling systems of rocker engines and extraction processes in chemical
engineering. The turbulent flows and the corresponding heat transfer phenomena
in these complex processes are still not well understood experimentally and
numerically. The objective of this work is to investigate the turbulent flows
under transcritical conditions using DNS of turbulent channel flows. A fully
compressible solver is used in conjunction with a Peng-Robinson real-fluid
equation of state to describe the transcritical flows. A channel flow with two
isothermal walls is simulated with one heated and one cooled boundary layers.
The grid resolution adopted in this study is slightly finer than that required
for DNS of incompressible channel flows. The simulations are conducted using
both fully (FC) and quasi-conservative (QC) schemes to assess their performance
for transcritical wall-bounded flows. The instantaneous flows and the
statistics are analyzed and compared with the canonical theories. It is found
that results from both FC and QC schemes qualitatively agree well with
noticeable difference near the top heated wall, where spurious oscillations in
velocity can be observed. Using the DNS data, we then examine the usefulness of
Townsend attached eddy hypothesis in the context of flows at transcritical
conditions. It is shown that the streamwise energy spectrum exhibits the
inverse wavenumber scaling and that the streamwise velocity structure function
follows a logarithmic scaling, thus providing support to the attached eddy
model at transcritical conditions.Comment: AIAA SciTech 2018, Kissimmee, F
Lyapunov exponent and Wasserstein metric as validation tools for assessing short-time dynamics and quantitative model evaluation of large-eddy simulation
In this work, methods for the evaluation of LES-quality and LES-accuracy are
presented, which include the Lyapunov exponent for the analysis of short-time
predictability of LES-calculation and the Wasserstein metric for the
quantitative assessment of simulation results. Both methods are derived and
evaluated in application to the Volvo test case. Both the non-reacting and
reacting cases are calculated. For the non- reacting cases, good agreement with
the experimental data is achieved by solvers at high numerical resolution. The
reacting cases are more challenging due to the small length scale of the flame
and the suppression of sinuous mode of absolute instability by the density
ratio. The analysis of the turbulent simulation data using the concept of the
Lyapunov exponent and the Wasserstein metric provides a more quantitative
approach to assess the mesh dependency of the simulation results. The
convergence of the Lyapunov exponent is shown to be a more sensitive and
stronger indication of mesh-independence. Though grid convergence for the
reacting cases cannot be reached with the chosen resolutions, the Lyapunov
exponents and the Wasserstein metric are shown to be capable of identifying
quantity-specific sensitivities with respect to the numerical resolution, while
requiring significantly less computational resources than acquiring profiles of
conventional turbulent statistics.Comment: 2018 AIAA Aerospace Sciences Meeting, Kissimmee, F
Application of Pareto-efficient combustion modeling framework to large eddy simulations of turbulent reacting flows
In the application of the combustion models based on low-dimensional
manifolds (for instance flamelet models) to large-eddy simulation (LES) of
reactive turbulent flows, the modeling simplifications of the combustion
process is a critical source of uncertainty in addition to those due to the
turbulent closure model and numerical discretization. The ability to
quantitatively assess this uncertainty in absence of the reference result is
vital to the reliable and predictive simulations of practical combustion
devices.In the present study, the Pareto-efficient combustion (PEC) framework
is extended to adaptive LES combustion simulations of turbulent flames. The key
component of the PEC framework is the so-called manifold drift term. Its
extension LES is proposed to make such assessment by examining the compliance
of a particular combustion model in describing a quantity of interest with
respect to the underlying flow field representation. With the focus on
improving predictions of CO emissions of flamelet-based combustion models, this
work employs PEC to augment the flamelet/progress variable (FPV) model through
local sub-model assignment of the finite-rate chemistry (FRC) model. To this
end, a series of LES-PEC calculations are performed on a piloted
partially-premixed dimethyl ether flame (DME-D), using a combination of FPV and
FRC models. The drift term is utilized in the PEC framework to the estimate the
model related error for quantities of interest. The PEC approach is
demonstrated to be capable of significantly improving the prediction of CO
emissions compared with the monolithic FPV simulation. The improved accuracy is
achieved by enriching the FPV model with FRC in regions where the lower-order
model is determined insufficient through the evaluation of drift terms
MVP-Workshop Contribution: Modeling of Volvo Bluff Flame Experiment and Comparison of Finite-Volume and Discontinuous-Galerkin Schemes
The Volvo burner features the canonical configuration of a bluff-body
stabilized premixed flame. This configuration was studied experimentally under
the Volvo Flygmotor AB program. Two cases are considered in this study: a
non-reacting case with an inlet flow speed of 16.6 m/s and a reacting case with
equilibrium ratio of 0.65 and inflow speed of 17.3 m/s. The characteristic
vortex shedding in the wake behind the bluff body is present in the
non-reacting case, while two oscillation modes are intermittently present in
the reacting case. A series of large-eddy simulations are performed on this
configuration using two solvers, one using a high-resolution finite-volume (FV)
scheme and the other featuring a high-order discontinuous-Galerkin (DG)
discretization. The FV calculations are conducted on hexahedral meshes with
three different resolution (4mm, 2mm, and 1mm). The DG calculations are
performed using two different polynomial orders on the same tetrahedral mesh.
For the non-reacting cases, good agreement with respect to the experimental
data is achieved by both solvers at high numerical resolution. The reacting
cases are calculated using a two-step global mechanism in combination with the
thickened-flame model. Reasonable agreement with experiments is obtained by
both solvers at higher resolution. Models for combustion-turbulence interaction
are necessary for the reacting case as it contains the length scale of the
flame, which is smaller than the grid resolution in all calculations. The
impact of such models on the flame stability and flow/flame dynamics is the
subject of future research
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