Mahuthannan, Ariff Magdoom; Lacoste, Deanna; Damazo, Jason; Kwon, Eddie; Roberts, William L.(55th AIAA Aerospace Sciences Meeting, American Institute of Aeronautics and Astronautics (AIAA), 2017-01-05)[Conference Paper]
Understanding flame quenching for different conditions is necessary to develop safety devices like flame arrestors. In practical applications, the speed of a deflagration in the lab-fixed reference frame will be a strong function of the geometry through which the deflagration propagates. This study reports on the effect of the flame speed, at the entrance of a quenching section, on the quenching distance. A 2D rectangular channel joining two main spherical vessels is considered for studying this effect. Two different velocity regimes are investigated and referred to as configurations A, and B. For configuration A, the velocity of the flame is 20 m/s, while it is about 100 m/s for configuration B. Methane-air stoichiometric mixtures at 1 bar and 298 K are used. Simultaneous dynamic pressure measurements along with schlieren imaging are used to analyze the quenching of the flame. Risk assessment of re-ignition is also reported and analyzed.
Sesha Giri, Krishna; Lacoste, Deanna; Damazo, Jason; Kwon, Eddie; Roberts, William L.(55th AIAA Aerospace Sciences Meeting, American Institute of Aeronautics and Astronautics (AIAA), 2017-01-05)[Conference Paper]
Flame quenching by heat loss to a surface continues to be an active area of combustion research. Close wall temperature measurements in an isothermal wall-stabilized flame are reported in this work. Conventional N-vibrational Coherent Anti-Stokes Raman Scattering (CARS) thermometry as close as 275 μm to a convex wall cooled with water has been carried out. The standard deviation of mean temperatures is observed to be ~6.5% for high temperatures (>2000K) and ~14% in the lower range (<500K). Methane/air and ethylene/air stoichiometric flames for various global strain rates based on exit bulk velocities are plotted and compared. CH* chemiluminescence is employed to determine the flame location relative to the wall. Flame locations are shown to move closer to the wall with increasing strain rates in addition to higher near-wall temperatures. Peak temperatures for ethylene are considerably higher (~250-300K) than peak temperatures for methane. Preheat zone profiles are similar for different strain rates across fuels. This work demonstrates close wall precise temperature measurments using CARS.
Carpenter, Mark H.; Parsani, Matteo; Nielsen, Eric J.; Fisher, Travis C.(54th AIAA Aerospace Sciences Meeting, American Institute of Aeronautics and Astronautics (AIAA), 2016-01-02)[Conference Paper]
Nonlinearly stable finite element methods of arbitrary type and order, are currently unavailable for discretizations of the compressible Navier-Stokes equations. Summation-by-parts (SBP) entropy stability analysis provides a means of constructing nonlinearly stable discrete operators of arbitrary order, but is currently
limited to simple element types. Herein, recent progress is reported, on developing entropy-stable (SS) discontinuous spectral collocation formulations for hexahedral elements. Two complementary efforts are discussed. The first effort generalizes previous SS spectral collocation work to extend the applicable set of points from tensor product, Legendre-Gauss-Lobatto (LGL) to tensor product Legendre-Gauss (LG) points. The LG
and LGL point formulations are compared on a series of test problems. Both the LGL and LG operators are of comparable efficiency and robustness, as is demonstrated using test problems for which conventional FEM techniques suffer instability. The second effort extends previous work on entropy stability to include p-refinement at nonconforming interfaces. A generalization of existing entropy stability theory is required to
accommodate the nuances of fully multidimensional SBP operators. The entropy stability of the compressible Euler equations on nonconforming interfaces is demonstrated using the newly developed LG operators
and multidimensional interface interpolation operators. Preliminary studies suggest design order accuracy at
Matlock, Taylor; Gildea, Stephen; Hu, Fuzhou; Becker, Nicholas Michael; Lozano, Paulo; Martinez-Sanchez, Manuel(46th AIAA/ASME/SAE/ASEE Joint Propulsion Conference & Exhibit, American Institute of Aeronautics and Astronautics (AIAA), 2012-11-15)[Conference Paper]
The Diverging Cusped-Field Thruster (DCFT) uses three permanent ring magnets of
alternating polarity to create a unique magnetic topology intended to reduce plasma losses
to the discharge chamber surfaces. The magnetic field strength within the DCFT discharge
chamber (up to 4 kG on axis) is much higher than in thrusters of similar geometry, which is
believed to be a driving factor in the high measured anode efficiencies. The field strength
in the near plume region is large as well, which may bear on the high beam divergences
measured, with peaks in ion current found at angles of around 30-35 from the thruster
axis. Characterization of the DCFT has heretofore involved only one magnetic topology.
It is then the purpose of this study to investigate changes to the near-field plume caused by
altering the shape and strength of the magnetic field. A thick magnetic collar, encircling
the thruster body, is used to lower the field strength outside of the discharge chamber
and thus lessen any effects caused by the external field. Changes in the thruster plume
with field topology are monitored by the use of normal Langmuir and emissive probes
interrogating the near-field plasma. Results are related to other observations that suggest
a unified conceptual framework for the important near-exit region of the thruster.
Luong, Minh Bau; Hernandez Perez, Francisco E.; Sow, Aliou; Im, Hong G.(AIAA Scitech 2019 Forum, American Institute of Aeronautics and Astronautics (AIAA), 2019-01-06)[Conference Paper]
The objective of the present study is to establish a theoretical prediction of the autoignition behavior of a reactant mixture for a given initial bulk mixture condition. The ignition regime criterion proposed by Im and coworkers based on the Sankaran number (Sa), which is a ratio of the laminar flame speed to the spontaneous ignition front speed, is extended to account for both temperature and equivalence ratio fluctuations. The extended ignition criterion is then applied to predict the autoignition characteristics of dimethyl ether (DME)/air mixtures and validated by two-dimensional direct numerical simulations (DNS). The response of the ignition mode of DME/air mixtures to three initial mean temperatures of 770, 900 K, and 1045 K lying within/outside the NTC regime, two levels of temperature and concentration fluctuations at a pressure of 30 atm and equivalence ratio of 0.5 is systematically investigated. The statistical analysis is performed, and a newly developed criterion –the volumetric fraction of Sa < 1.0, FSa,S, is proposed as a deterministic criterion to quantify the fraction of heat release attributed to strong ignition. It is found that the strong and weak ignition modes are well captured by the predicted Sa number and FSa,S regardless of different initial mean temperatures and the levels of mixture fluctuations and correlations. Sap and FSa,S demonstrated under a wide range of initial conditions as a reliable criterion in determining a priori the ignition modes and the combustion intensity.
Abu Jbara, Khaled F.; Sundaramoorthi, Ganesh; Claudel, Christian(Journal of Guidance, Control, and Dynamics, American Institute of Aeronautics and Astronautics (AIAA), 2018-06-30)[Article]
This work presents a novel real-time algorithm for runway detection and tracking applied to unmanned aerial vehicles (UAVs). The algorithm relies on a combination of segmentation-based region competition and minimization of a particular energy function to detect and identify the runway edges from streaming video data. The resulting video-based runway position estimates can be updated using a Kalman filter (KF) that integrates additional kinematic estimates such as position and attitude angles, derived from video, inertial measurement unit data, or positioning data. This allows a more robust tracking of the runway under turbulence. The performance of the proposed lane detection and tracking scheme is illustrated on various experimental UAV flights conducted by the Saudi Aerospace Research Center (KACST), by the University of Texas, Austin, and on simulated landing videos obtained from a flight simulator. Results show an accurate tracking of the runway edges during the landing phase, under various lighting conditions, even in the presence of roads, taxiways, and other obstacles. This suggests that the positional estimates derived from the video data can significantly improve the guidance of the UAV during takeoff and landing phases.
Sim, Jaeheon; Badra, Jihad A.; Im, Hong G.(54th AIAA Aerospace Sciences Meeting, American Institute of Aeronautics and Astronautics (AIAA), 2016-01-02)[Conference Paper]
Linear instability sheet atomization (LISA) breakup model has been widely used for
modeling hollow-cone spray. However, the model was originally developed for inwardlyopening
pressure-swirl injectors by assuming toroidal ligament breakups. Therefore, LISA
model is not suitable for simulating outwardly opening injectors having string-like
structures at wide spray angles. Furthermore, the varying area and shape of the annular
nozzle exit makes the modeling difficult. In this study, a new spray modeling was proposed
for outwardly opening hollow-cone injector. The injection velocities are computed from the
given mas flow rate and injection pressure regardless of ambiguous nozzle exit geometries.
The modified Kelvin-Helmholtz and Rayleigh-Taylor (KH-RT) breakup model is used with
adjusted initial Sauter mean diameter (SMD) for modeling breakup of string-like liquid film
spray. Liquid spray injection was modeled using Lagrangian discrete parcel method within
the framework of commercial CFD software CONVERGE, and the detailed model was
implemented by user defined functions. It was found that the new model predicted the liquid
penetration length and local SMD accurately for various fuels and chamber conditions.
Cutler, Andrew D.; Cantu, Luca M.L.; Gallo, Emanuela C.A.; Magnotti, Gaetano; Rockwell, Robert D.; Goyne, Christopher P.(AIAA Journal, American Institute of Aeronautics and Astronautics (AIAA), 2018-12-14)[Article]
Dual-pump coherent anti-Stokes Raman spectroscopy (CARS) was used to measure the mole fractions of major species as well as the rotational and vibrational temperatures of molecular nitrogen in a hydrogenfueled dual-mode scramjet flowpath operated in the
Sim, Jaeheon; Im, Hong G.; Chung, Suk Ho(53rd AIAA Aerospace Sciences Meeting, American Institute of Aeronautics and Astronautics (AIAA), 2015-01-03)[Conference Paper]
The vaporization characteristics of water-oil emulsion droplets are investigated by high
fidelity computational simulations. One of the key objectives is to identify the physical
mechanism for the experimentally observed behavior that the component in the dispersed
micro-droplets always vaporizes first, for both oil-in-water and water-in-oil emulsion
droplets. The mechanism of this phenomenon has not been clearly understood. In this study,
an Eulerian-Lagrangian method was implemented with a temperature-dependent surface
tension model and a dynamic adaptive mesh refinement in order to effectively capture the
thermo-capillary effect of a micro-droplet in an emulsion droplet efficiently. It is found that
the temperature difference in an emulsion droplet creates a surface tension gradient along
the micro-droplet surface, inducing surface movement. Subsequently, the outer shear flow
and internal flow circulation inside the droplet, referred to as the Marangoni convection, are
created. The present study confirms that the Marangoni effect can be sufficiently large to
drive the micro-droplets to the emulsion droplet surface at higher temperature, for both
water-in-oil and oil-and-water emulsion droplets. A further parametric study with different
micro-droplet sizes and temperature gradients demonstrates that larger micro-droplets
move faster with larger temperature gradient. The oil micro-droplet in oil-in-water emulsion
droplets moves faster due to large temperature gradients by smaller thermal conductivity.
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