1,721,073 research outputs found
A conceptual study of cavity aeroacoustics control using porous media inserts
No full textIn this study, an integrated flow simulation and aeroacoustics prediction methodology is applied to testing a sound control technique using porous inserts in an open cavity. Large eddy simulation (LES) combined with a three-dimensional Ffowcs Williams-Hawkings (FW-H) acoustic analogy is employed to predict the flow field, the acoustic sources and the sound radiation. The Darcy pressure - velocity law is applied to conceptually mimic the effect of porous media placed on the cavity floor and/or rear wall. Consequently, flow in the cavity could locally move in or out through these porous walls, depending on the local pressure differences. LES with "standard" subgrid-scale models for compressible flow is carried out to simulate the flow field covering the sound source and near fields, and the fully three-dimensional FW-H acoustic analogy is used to predict the sound field. The numerical results show that applying the conceptual porous media on cavity floor and/or rear wall could decrease the pressure fluctuations in the cavity and the sound pressure level in the far field. The amplitudes of the dominant oscillations (Rossiter modes) are suppressed and their frequencies are slightly modified. The dominant sound source is the transverse dipole term, which is significantly reduced due to the porous walls. As a result, the sound pressure in the far field is also suppressed. The preliminary study reveals that using porous-inserts is a promising technology for flow and sound radiation control
Effects of inert evaporating droplets on turbulent combustion
No full textDNS databases of turbulent reacting mixing layers laden with/without evaporating inert droplets are used to obtainthe statistics for conditional moment closure (CMC) and assess the droplet effects. The temporally developing mixinglayer has an initial Reynolds number of 1000 based on the vorticity thickness with more than 16 million Lagrangiandroplets traced. Conditional quantities such as singly-conditioned gas temperature hTg|?i, conditional variance of gastemperature hT??2g |?i and conditional co-variance between the mass fraction of fuel and gas temperature hY??f T??g |?i show the considerable droplet effects on such statistics. Comparison between the droplet-free and droplet-laden reacting mixing layer cases suggests that the first-order singly-conditioned CMC is sufficiently accurate for the former, while for the latter, more advanced CMC approaches such as doubly-conditioned or second-order CMCs become necessary due to extinction induced by droplets. The scalar dissipation rate doubly conditioned on the mixture fraction and normalized gas temperature h?|?, ?i exemplifies the differences between the droplet-free and droplet cases
Lattice boltzmann simulations of binary droplet collision by a multiphase multiple-relaxation-time model
No full textDirect numerical simulation of inert droplet effects on scaler dissipation rate in turbulant and non-reacting shear layers
No full textThree-dimensional direct numerical simulation has been performed to investigate the effects of inert evaporating droplets on scalar dissipation rate ? in temporally-developing turbulent reacting and non-reacting mixing layers with the Reynolds number based on the vorticity thickness up to 8000 and the number of traced Lagrangian droplets up to 107. The detailed instantaneous field analysis and ensemble-averaged statistics reveal complex interactions among combustion, droplet dynamics and evaporation, all of which have a considerable influence on ?. The presence of inert evaporating droplets promotes ? in both non-reacting and reacting mixing layers. In the latter, combustion reduces ?, so when combustion is suppressed by evaporating droplets, ? is enhanced. The transport equation of ? has been analyzed to investigate the various effects on ? in detail. The terms in the equation contain explicitly the evaporation rate and its spatial derivative, acting as a sink and a source for ?, respectively. On the whole, the net effect of the evaporation-rate terms is to promote ?. However, the production and dissipation terms are the dominant source and sink terms, respectively
Kinetic study on thermal decomposition of woods in oxidative environment
Full text linkThe purpose of this work is to gain knowledge on kinetics of biomass decomposition under oxidative atmospheres, mainly examining effect of heating rate on different biomass species. Two sets of experiments are carried out: the first set of experiments is thermal decomposition of four different wood particles, namely aspens, birch, oak and pine under an oxidative atmosphere and analysis with TGA; and the second set is to use large size samples of wood under different heat fluxes in a purpose-built furnace, where the temperature distribution, mass loss and ignition characteristics are recorded and analyzed by a data post-processing system. The experimental data is then used to develop a two-step reactions kinetic scheme with low and high temperature regions while the activation energy for the reactions of the species under different heating rates is calculated. It is found that the activation energy of the second stage reaction for the species with similar constituent fractions tends to converge to a similar value under the high heating rate.<br/
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