50,909 research outputs found
Prediction of combustion instability limit cycle oscillations by combining flame describing function simulations with a thermoacoustic network model
Accurate prediction of limit cycle oscillations resulting from combustion instability has been a long-standing challenge. The present work uses a coupled approach to predict the limit cycle characteristics of a combustor, developed at Cambridge University, for which experimental data are available (Balachandran, Ph.D. thesis, 2005). The combustor flame is bluff-body stabilised, turbulent and partially-premixed. The coupled approach combines Large Eddy Simulation (LES) in order to characterise the weakly non-linear response of the flame to acoustic perturbations (the Flame Describing Function (FDF)), with a low order thermoacoustic network model for capturing the acoustic wave behaviour. The LES utilises the open source Computational Fluid Dynamics (CFD) toolbox, OpenFOAM, with a low Mach number approximation for the flow-field and combustion modelled using the PaSR (Partially Stirred Reactor) model with a global one-step chemical reaction mechanism for ethylene/air. LES has not previously been applied to this partially-premixed flame, to our knowledge. Code validation against experimental data for unreacting and partially-premixed reacting flows without and with inlet velocity perturbations confirmed that both the qualitative flame dynamics and the quantitative response of the heat release rate were captured with very reasonable accuracy. The LES was then used to obtain the full FDF at conditions corresponding to combustion instability, using harmonic velocity forcing across six frequencies and four forcing amplitudes. The low order thermoacoustic network modelling tool used was the open source OSCILOS (http://www.oscilos.com). Validation of its use for limit cycle prediction was performed for a well-documented experimental configuration, for which both experimental FDF data and limit cycle data were available. The FDF data from the LES for the present test case was then imported into the OSCILOS geometry network and limit cycle oscillations of frequency 342 Hz and normalised velocity amplitude of 0.26 were predicted. These were in good agreement with the experimental values of 348 Hz and 0.21 respectively. This work thus confirms that a coupled numerical prediction of limit cycle behaviour is possible using an entirely open source numerical framework
Phonon nanocapacitor for storage and lasing of terahertz lattice waves
We introduce a novel ultra-compact nanocapacitor of coherent phonons formed
by high-finesse interference mirrors based on atomic-scale semiconductor
metamaterials. Our molecular dynamics simulations show that the nanocapacitor
stores THz monochromatic lattice waves, which can be used for phonon lasing -
the emission of coherent phonons. Either one- or two-color phonon lasing can be
realized depending on the geometry of the nanodevice. The two color regimes of
the capacitor originates from the distinct transmittance dependance on the
phonon wave packet incident angle for the two phonon polarizations at their
respective resonances. Phonon nanocapacitor can be charged by cooling the
sample equilibrated at room temperature or by the pump-probe technique. The
nanocapacitor can be discharged by applying tunable reversible strain,
resulting in the emission of coherent THz acoustic beams.Comment: 12 pages, 5 figure
Faithful qubit transmission against collective noise without ancillary qubits
We present a faithful qubit transmission scheme with linear optics against
collective noise, not resorting to ancillary qubits. Its set-up is composed of
three unbalanced polarization interferometers, based on a polarizing beam
splitter, a beam splitter and a half-wave plate, which makes this scheme more
feasible than others with present technology. The fidelity of successful
transmission is 1, independent of the parameters of the collective noise, and
the success probability for obtaining an uncorrupted state can be improved to
100% with some time delayers. Moreover, this scheme has some good applications
in one-way quantum communication for rejecting the errors caused by the
collective noise in quantum channel.Comment: 3 pages, 1 figur
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