72 research outputs found
Spark-ignited kernel dynamics in fine ethanol sprays and their relations with minimum ignition energy
Spark ignition of ethanol droplet/vapor/air mixture is studied with a
Eulerian-Eulerian method and detailed chemical mechanism. The flame
kernel-droplet interaction is quantified with an evaporation completion front
(ECF). Two categories of spray flames can hence be defined based on the
relative location between the ECF and flame front, i.e., homogeneous and
heterogeneous spray flames. An element-based equivalence ratio (ER) at the
flame front (flame ER for short) is introduced to measure the gas composition
in evaporating sprays. For overall fuel-lean mixtures, quasi-stationary
spherical flame (QSSF) occurs due to lean flame ER and the composition at the
QSSF front is homogeneous. For overall fuel-rich two-phase mixtures,
re-ignition, after the spark-ignited kernel fails, is observed when the droplet
diameter is 15 {\mu}m for fuel sprays with both fuel-lean and fuel-rich
background gas. This is due to rich flame ER and/or strong evaporative heat
loss. Meanwhile, the kernel is born in a heterogeneous mixture and transition
into homogeneous state is found. For both overall lean and rich two-phase
mixtures, fuel droplets affect the ignitability and flame trajectories.
Moreover, ignition energy affects the flame ER and front distance at the early
stage of kernel development. Lastly, the minimum ignition energies (MIE) with
different gas and overall ERs are investigated
Numerical simulation of two-dimensional detonation propagation in partially pre-vaporized n-heptane sprays
In this paper, two dimensional detonation propagation in partially
prevaporized n-heptane sprays is studied by using Eulerian/Lagrangian methods.
The effects of droplet preevaporation on the detonation propagation are
investigated. The general features and detailed structures of two-phase
detonations are well captured with the present numerical methods. The results
show that the detonation propagation speed and detonation structures are
significantly affected by the preevaporated gas equivalence ratio. The
numerical soot foils are used to characterize the influence of preevaporated
gas equivalence ratio on the detonation propagation. Regular detonation
cellular structures are observed for large preevaporated gas equivalence
ratios, but when decreasing the preevaporated gas equivalence ratio, the
detonation cellular structures become much more unstable and the average cell
width also increases. It is also found that the preevaporated gas equivalence
ratio has little effects on the volume averaged heat release when the
detonation propagates stably. Moreover, the results also suggest that the
detonation can propagate in the two-phase heptane and air mixture without
preevaporation, but the detonation would be first quenched and then re-ignited
when the preevaporated gas equivalence ratio is small or equal to zero
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Extinction in turbulent swirling non-premixed flames
This thesis investigates the localized and global extinction in turbulent swirling non-premixed flames with Large Eddy Simulation (LES) and sub-grid scale Conditional Moment Closure (CMC) model. The first part of this thesis describes the derivations of the three dimensional conservative CMC governing equations and their finite volume discretization for unstructured mesh. The parallel performance of the newly developed CMC code is assessed. The runtime data coupling interface between the 3D-CMC and LES solvers is designed and the different solvers developed during the course of this research are detailed. The aerodynamics of two swirling non-reacting flows from the Sydney and Cambridge burners are first simulated. The main ow structures (e.g. the recirculating zones) in both cases are correctly predicted. The sensitivity analysis about the influences of turbulent inlet boundary, computational domain and mesh refinement on velocity statistics is conducted. This analysis acts as the preparatory investigation for the following flame simulations. The Sydney swirl diluted methane flame, SMA2, is then simulated for validating the LES/3D-CMC solvers. Excellent agreements are achieved in terms of velocity and mixture fraction statistics, averaged reactive scalars in both physical and mixture fraction space. The local extinction level from the increased central fuel velocity is reasonably predicted. At the experimental blow-off point, the LES/3D-CMC modelling does not obtain the occurrence of complete extinction, but severe extinction occurs at the flame base, qualitatively in line with experimental observations. Localized extinction features of a non-premixed methane flame in the Cambridge swirl burner are investigated and it is found that the occurrence of local extinction is typically manifested by low heat release rate and hydroxyl mass fraction, as well as low or medium temperature. It is also accompanied by high scalar dissipation rates. In mixture fraction space, the CMC cells undergoing local extinction have relatively wide scatter between inert and fully burning solutions. The PDFs of reactedness at the stoichiometric mixture fraction demonstrate some extent of bimodality, showing the events of local extinction and reignition and their relative occurrence frequency. Local extinction near the bluff body in the Cambridge swirl burner is also studied. The convective wall heat loss is included as a source term in the conditionally filtered total enthalpy equation. It shows a significant influence on the mean flame structures, directly linked to the changes of the conditional scalar dissipation near the wall. Furthermore, the degree of local extinction near the bluff body surface is intensified because of the wall heat loss. However, the wall heat loss shows a relatively small influence on the statistics of lift-off height. Finally, the blow-off conditions and dynamics in the Cambridge swirl burner are investigated. The blow-off critical air bulk velocity from LES/3D-CMC is over-predicted, greater than the experimental one by at most 25%. The predicted blow-off transient lasts finitely long duration quantified by the blow-off time, in good agreement with the experimental results. The reactive scalars in both physical and mixture fraction space demonstrate different transient behaviors during blow-off process. When the current swirling flame is close to blow-off, high-frequency and high-amplitude fluctuations of the conditionally filtered stoichiometric scalar dissipation rate on the iso-surfaces of the filtered stoichiometric mixture fraction are evident. The blow-off time from the computations is found to vary with different operating conditions
On the evolution of fuel droplet evaporation zone and its interaction with the flame front in ignition of spray flames
Evolution of fuel droplet evaporation zone and its interaction with the
propagating flame front are studied in this work. A general theory is developed
to describe the evolutions of flame propagation speed, flame temperature,
droplet evaporation onset and completion locations in ignition and propagation
of spherical flames. The influences of liquid droplet mass loading, heat
exchange coefficient (or evaporation rate) and Lewis number on spherical spray
flame ignition are studied. Two flame regimes are considered, i.e.,
heterogeneous and homogeneous flames, based on the mixture condition near the
flame front. The results indicate that the spray flame trajectories are
considerably affected by the ignition energy addition. The critical condition
for successful ignition for the fuel-rich mixture is coincidence of inner and
outer flame balls from igniting kernel and propagating flame. The flame balls
always exist in homogeneous mixtures, indicating that ignition failure and
critical successful events occur only in purely gaseous mixture. The fuel
droplets have limited effects on minimum ignition energy, which however
increases monotonically with the Lewis number. Moreover, flame kernel
originates from heterogeneous mixtures due to the initially dispersed droplets
near the spark. The evaporative heat loss in the burned and unburned zones of
homogeneous and heterogeneous spray flames is also evaluated, and the results
show that for the failed flame kernels, evaporative heat loss behind and before
the flame front first increases and then decreases. The evaporative heat loss
before the flame front generally increases, although non-monotonicity exists,
when the flame is successfully ignited and propagate outwardly. For
heterogeneous flames, the ratio of the heat loss from the burned zone to the
total one decreases as the flame expands
Transmission of hydrogen detonation across a curtain of dilute inert particles
Transmission of hydrogen detonation wave (DW) in an inert particle curtain is
simulated using the Eulerian-Lagrangian approach with gas-particle two-way
coupling. A detailed chemical mechanism is used for hydrogen detonative
combustion and parametric studies are conducted based on a two-dimensional
computational domain. A detonation map of propagation and extinction
corresponding to various particle sizes, concentrations, and curtain
thicknesses is plotted. It is shown that the critical curtain thickness
decreases considerably when the particle concentration is less than the
critical value. The effects of curtain thickness on the trajectories of peak
pressure, shock front speed, and heat release rate are examined. Three
propagation modes of the DW in particle curtain are found: detonation
transmission, partial extinction and detonation reinitiation, and detonation
extinction. The chemical explosive mode analysis confirms that a detonation
re-initiation event is caused by a re-initiation point with high pressure and
explosive propensity, resulting from transverse shock focusing. The influence
of particle dimeter and concentration, and curtain thickness on the DW are also
examined with peak pressure trajectories, shock speed, and interphase exchange
rates of energy and momentum. Furthermore, the evolutions of curtain
morphologies are analyzed by the particle velocity, volume fraction, Stokes
drag and Archimedes force. This analysis confirms the importance of the drag
force in influencing the change of curtain morphologies. Different curtain
evolution regimes are found: quasi-stationary regime, shrinkage regime,
constant-thickness regime, and expansion regime. Finally, the influences of the
curtain thickness on the characteristic time of curtain evolutions are studied
Direct detonation initiation in hydrogen/air mixture: effects of compositional gradient and hotspot condition
Two-dimensional simulations are conducted to investigate the direct
initiation of cylindrical detonation in hydrogen/air mixtures with detailed
chemistry. The effects of hotspot condition and mixture composition gradient on
detonation initiation are studied. Different hotspot pressure and composition
are first considered in the uniform mixture. It is found that detonation
initiation fails for low hotspot pressures and supercritical regime dominates
with high hotspot pressures. Detonation is directly initiated from the reactive
hotspot, whilst it is ignited somewhere beyond the nonreactive hotspots. Two
cell diverging patterns (i.e., abrupt and gradual) are identified and the
detailed mechanisms are analyzed. Moreover, cell coalescence occurs if many
irregular cells are generated initially, which promotes the local cell growing.
We also consider nonuniform detonable mixtures. The results show that the
initiated detonation experiences self-sustaining propagation, highly unstable
propagation, and extinction in mixtures with a linearly decreasing equivalence
ratio along the radial direction respectively, i.e., 1 to 0.9, 1 to 0.5 and 1
to 0. Moreover, the hydrodynamic structure analysis shows that, for the
self-sustaining detonations, the hydrodynamic thickness increases at the
overdriven stage, decreases as the cells are generated, and eventually become
almost constant at the cell diverging stage, within which the sonic plane shows
a sawtooth pattern. However, in the detonation extinction cases, the
hydrodynamic thickness continuously increases, and no sawtooth sonic plane can
be observed
Modelling particle collisions in moderately dense curtain impacted by an incident shock wave
The interactions between an incident shock and moderately dense particle
curtain are simulated with the Eulerian-Lagrangian method. A customized solver
based on OpenFOAM is extended with an improved drag model and collision model,
and then validated against two benchmark experiments. In this work, parametric
studies are performed considering different particle sizes, volume fractions,
and curtain thicknesses. It is found that smaller particle size and larger
volume fractions lead to stronger reflected shock and weaker transmitted shock.
Different expansion stages of the curtain fronts are also studied in detail.
Attention is paid to the particle collision effects on the curtain evolution
behaviours. According to our results, for the mono-dispersed particle curtain,
the collision effects on curtain front behaviors are small, even when the
initial particle volume fraction is as high as 20%. This is due to the positive
velocity gradient across the curtain after the shock wave passage, leading to
faster motion of downstream particles than the upstream ones and hence no
collision occurs. For the bi-dispersed particle curtain, the collision effects
become important in the mixing region of different-size particles. Collisions
decelerate small particles while accelerate large ones and cause velocity
scattering. Moreover, increasing the bi-dispersed curtain thickness leads to
multiple collision force peaks due to the local particle accumulations, which
is the result of the delayed separation of different particle groups. Our
results indicate that the collision model may be unnecessary to predict curtain
fronts in mono-dispersed particles, but in bi-dispersed particles, the
collision effects are important and therefore must be modelled
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