17 research outputs found
A computational study of the effects of multiphase dynamics in catalytic upgrading of biomass pyrolysis vapor
Peer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/145281/1/aic16184.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/145281/2/aic16184_am.pd
A feasibility study on the use of low-dimensional simulations for database generation in adaptive chemistry approaches
LES/PDF approaches can be used for simulating challenging turbulent
combustion configurations with strong turbulence chemistry interactions.
Transported PDF methods are computationally expensive compared to flamelet-like
turbulent combustion models. The pre-partitioned adaptive chemistry (PPAC)
methodology was developed to address this cost differential. PPAC entails an
offline preprocessing stage, where a set of reduced models are generated
starting from an initial database of representative compositions. At runtime,
this set of reduced models are dynamically utilized during the reaction
fractional step leading to computational savings. We have recently combined
PPAC with in-situ adaptive tabulation (ISAT) to further reduce the
computational cost. We have shown that the combined method reduced the average
wall-clock time per time step of large-scale LES/particle PDF simulations of
turbulent combustion by 39\%. A key assumption in PPAC is that the initial
database used in the offline stage is representative of the compositions
encountered at runtime. In our previous study this assumption was trivially
satisfied as the initial database consisted of compositions extracted from the
turbulent combustion simulation itself. Consequently, a key open question
remains as to whether such databases can be generated without having access to
the turbulent combustion simulation. Towards answering this question, in the
current work, we explore whether the compositions for forming such a database
can be extracted from computationally-efficient low-dimensional simulations
such as 1D counterflow flames and partially stirred reactors. We show that a
database generated using compositions extracted from a partially stirred
reactor configuration leads to performance comparable to the optimal case,
wherein the database is comprised of compositions extracted directly from the
LES/PDF simulation itself
A conceptual model of the flame stabilization mechanisms for a lifted Diesel-type flame based on direct numerical simulation and experiments
This work presents an analysis of the stabilization of diffusion flames created by the injection of fuel into hot air, as found in Diesel engines. It is based on experimental observations and uses a dedicated Direct Numerical Simulation (DNS) approach to construct a numerical setup, which reproduces the ignition features obtained experimentally. The resulting DNS data are then used to classify and analyze the events that allow the flame to stabilize at a certain Lift-Off Length (LOL) from the fuel injector. Both DNS and experiments reveal that this stabilization is intermittent: flame elements first auto-ignite before being convected downstream until another sudden auto-ignition event occurs closer to the fuel injector. The flame topologies associated to such events are discussed in detail using the DNS results, and a conceptual model summarizing the observation made is proposed. Results show that the main flame stabilization mechanism is auto-ignition. However, multiple reaction zone topologies, such as triple flames, are also observed at the periphery of the fuel jet helping the flame to stabilize by filling high-temperature burnt gases reservoirs localized at the periphery, which trigger auto-ignitions
On the Validation of a One-Dimensional Biomass Pyrolysis Model Using Uncertainty Quantification
Predictive modeling tools have the
potential to accelerate the
development and deployment of biomass thermochemical conversion. Considerable
progress has been made in the modeling of biomass pyrolysis at the
particle level, where chemical kinetics and transport processes are
coupled. However, rigorous validation of the corresponding models
is challenging because of the considerable uncertainty in the values
of several biomass properties. Toward this end, we use the principles
of uncertainty quantification (UQ) for a rigorous analysis of the
validity of a commonly used one-dimensional wood pyrolysis model.
Uncertainty in the modeling parameters of the transport processes
is propagated to the simulation results of the pyrolysis model. The
model predictions are compared with several detailed experimental
measurements for pyrolysis of wood particles. The results show that
the uncertainty in the model predictions account for some of the discrepancies
with the experimental measurements, especially for the particle temperature
profiles and the gas phase species production rates. Experimental
targets are identified whose predictions cannot be improved by an
accurate knowledge of the transport model parameters and require further
improvements in the chemical kinetics model. The use of a systematic
optimization technique is also demonstrated to choose the optimal
values of uncertain model parameters
P-DRGEP: A novel methodology for the reduction of kinetics mechanisms for plasma-assisted combustion applications
Detailed kinetics mechanisms for plasma-assisted combustion contain numerous species and reactions that model the interplay between non-equilibrium plasma processes and hydrocarbon oxidation. While physically accurate and comprehensive, such detailed mechanisms are impractical for simulations of unsteady multi-dimensional plasma discharges and their effect on reactive mixtures in practical devices. In this work, we develop and apply a novel methodology for the reduction of large detailed plasma-assisted combustion mechanisms to smaller skeletal ones. The methodology extends the Directed Relation Graph with Error Propagation (DRGEP) approach in order to consider the energy branching characteristics of plasma discharges during the reduction. Ensuring tight error tolerances on the relative proportions of energy lost by the electrons to the various classes of impact processes (i.e. vibrational and electronic excitation, ionization, and impact dissociation) is key to preserving the correct discharge physics in the skeletal mechanism. To this end, new targets that include energy transfers are defined and incorporated in DRGEP. The performance of the novel framework, called P-DRGEP, is assessed for the simulation of ethylene-air ignition by nanosecond repetitive pulsed discharges at conditions relevant to supersonic combustion and flame holding in scramjet cavities, i.e. from 600 K to 1000 K, 0.5 atm, and equivalence ratios between 0.75 and 1.5. P-DRGEP is found to be greatly superior to the traditional reduction approach applied to plasma-assisted ignition in that it generates a smaller skeletal mechanism with significantly lower errors. For ethylene-air ignition at the target conditions, P-DRGEP generates a skeletal mechanism with 54 species and 236 reactions, resulting in a 84% computational speed-up for ignition simulations, while guaranteeing errors below 10% on the time required for ignition following the first pulse, 1% on the mean electron energy, between 4 and 35% on electron energy losses depending on the process, and 5% on the laminar flame speed.SCOPUS: ar.jinfo:eu-repo/semantics/publishe
Reduced Chemical Kinetics for the Modeling of TiO<sub>2</sub> Nanoparticle Synthesis in Flame Reactors
Flame synthesis represents a viable
technique for large-scale production
of titanium dioxide (TiO<sub>2</sub>) nanoparticles. A key ingredient
in the modeling of this process is the description of the chemical
kinetics, which include Ti oxidation, hydrocarbon fuel combustion,
and chlorination. While detailed chemical mechanisms have been developed
for predicting TiO<sub>2</sub> nanoparticle properties by West et
al. (e.g., <i>Combust. Flame</i> <b>2009</b>, <i>156</i>, 1764), their use in turbulent reacting flow simulations
is limited to very simple configurations or requires significant modeling
assumptions to bring their computational cost down to an acceptable
level. In this work, a reduced kinetic scheme describing the oxidation
of TiCl<sub>4</sub> in a methane flame is derived from and validated
against the predictions of a detailed mechanism from the literature.
The reduction procedure uses graph-based methods for unimportant kinetic
pathways elimination and quasi-steady-state species selection. Reduction
targets are chosen in accordance with previous modeling results that
showed the importance of temperature and overall concentration of
titanium-containing species in both nucleation and surface growth
rates. The resulting reduced scheme is thoroughly evaluated over a
wide range of conditions relevant to flame-based synthesis, and the
capability of the reduced model to adequately capture the process
dynamics at a much lower computational cost is demonstrated