370 research outputs found
Near limit premixed flamelets in Hele-Shaw cells
No full textIn this paper we report a preliminary attempt to stabilize near limit premixed flamelets in Hele-Shaw cells. As a quasi-two-dimensional analog of flame balls the flamelet is sustained by diffusive transport alone with fuel supplied from the open ends of the Hele-Shaw cell and heat dissipated to the ambient environment through conduction; radiative heat losses from both phases are neglected. Following Spalding's "one-dimensional idealization" approximation we construct a 2-D model to account for the heat and mass transfer processes in both the gas and the solid phases with the gap height as a parameter controlling the heat exchange rate between the two phases. For each of the three kinds of wall materials considered two steady solution branches are obtained as a function of the gap height one corresponding to large flames and the other to small flames. The large flame branch is critically dependent on the boundary and is therefore of little physical value. Linear stability analysis shows that the small flame branch is unstable to random perturbations. 2-D time dependent numerical simulations indicate that a slightly perturbed steady state on the small flame branch either evolves into a single flamelet drifting to the boundary as a whole or splits into two drifting to the boundary along opposite directions. A partially open square Hele-Shaw combustor has been proposed and is shown to be able to support stabilized flamelets for a certain range of the degree of opening. (C) 2016 by The Combustion Institute. Published by Elsevier Inc
Hele-Shaw rheometry
No full textIn this paper, we describe a novel approach to determine the flow behavior index of a power-law fluid by means of a microfluidic device. The concept of this method is based on a mathematical analysis by Aronsson and Janfalk [Eur. J. Appl. Math. 3, 343–366 (1992)] of Hele-Shaw flow of power-law liquids. We implement this approach by driving a non-Newtonian fluid through a glass microfluidic chip with a 100:1 contraction. The flow in this chip satisfies the Hele-Shaw flow conditions in most of the device. Two conjugate p-Laplace equations describe the pressure and stream function in such flows. These equations depend on the flow behavior index, n. Therefore, by fitting the p-Laplace equation to the velocity field obtained from a micro particle image velocimetry measurement of the flow, the flow behavior index of the fluid in the chip can be determined. Because in practice, fluids rarely show perfectly inelastic power-law behavior, conditions under which the assumption of inelastic flow is valid were derived by analyzing Hele-Shaw flow of an Oldroyd-B fluid. The concept was tested using three different classes of model fluids, a Newtonian fluid, an inelastic power-law fluid, and a Boger fluid. In all three cases, satisfactory results were obtained, with values of n deviating at most 4% from values measured using conventional rheometry. The method presented here is expected to be potentially useful in online quality control in, for example, polymer or food processing.Process and EnergyMechanical, Maritime and Materials Engineerin
Silicon Surface Passivation by Mixed Aluminum Precursors in Al2O3 Atomic Layer Deposition
No full textDimethylaluminum chloride (DMACl) is a cost-effective aluminium precursor alternative to conventional trimethylaluminium (TMA) for Atomic Layer Deposited (ALD) Al2O3. The DMACl water process shows better passivation after high temperature firing when compared with conventional TMA water process. However, after low-temperature post-anneal its passivation quality is slightly worse than with TMA. Here we show that a mixed use of TMA and DMACl precursors in the ALD process results in better surface passivation both after 400 °C post-anneal and after an 800 °C firing step. The high-quality passivation results from the low interface defect density and high negative charge at the surface. Specifically, we investigate the role of chlorine in the ALD Al2O3 passivation by varying the TMA and DMACl pulse proportions.Peer reviewe
Effect of Different ALD Al2O3 Oxidants on the Surface Passivation of Black Silicon
No full textAbstractWe study how different oxidants in atomic layer deposition of aluminium oxide (ALD Al2O3) affect the surface passivation of black silicon. Here we show that processes using ozone cause higher fixed charge but surprisingly lead to lower lifetimes in black silicon samples as compared to water-based samples. In planar samples however, the best surface passivation is reached with O3-based processes. In case of water as oxidant, the planar wafers suffer from severe blistering and poorer surface passivation, while this seems to be the best process for black silicon. To find a reason for the lifetime differences we also study different Al2O3 stacks where both H2O and O3 are used as oxidants. In conclusion, surface texture seems to affect the optimal oxidant in the ALD process
Differential diffusive instabilities of miscible two-layer stratifications in porous media and Hele-Shaw cells
No full textIn porous media, a stratification of a given solution on top of another miscible solution can be buoyantly unstable because of an unstable density stratification or because of differential diffusive effects. The former is the well known Rayleigh–Taylor (RT) mechanism wherein the interface is destabilized by the denser solution overlying a less dense one in the gravity field. Whereas the latter is of particular interest in the field of oceanography, when the upper solution is less dense than the lower one with the lower component diffusing faster than the upper one, resulting in a double diffusive (DD) instability. Similarly, a diffusive-layer convection (DLC) instability has also been observed for a stable density stratification with the upper solute diffusing faster than the lower one. Though the literature on differential diffusion effects is pretty vast, very few studies have managed to establish a connection, both qualitatively and quantitatively, between numerical simulations and experimental observations, which is the basis of the present study. We report our findings in a broad parameter range where the instability mechanism could be triggered by an unstable density stratification or due to differential diffusive effects, or even both, resulting in mixed modes
An alternative derivation of ring-polymer molecular dynamics transition-state theory.
No full textIn a previous article [T. J. H. Hele and S. C. Althorpe, J. Chem. Phys. 138, 084108 (2013)], we showed that the t → 0+ limit of ring-polymer molecular dynamics (RPMD) rate-theory is also the t → 0+ limit of a new type of quantum flux-side time-correlation function, in which the dividing surfaces are invariant to imaginary-time translation; in other words, that RPMD transition-state theory (RMPD-TST) is a t → 0+ quantum transition-state theory (QTST). Recently, Jang and Voth [J. Chem. Phys. 144, 084110 (2016)] rederived this quantum t → 0+ limit and claimed that it gives instead the centroid-density approximation. Here we show that the t → 0+ limit derived by Jang and Voth is in fact RPMD-TST
A review of one-phase Hele-Shaw flows and a level-set method for nonstandard configurations
No full textThe classical model for studying one-phase Hele-Shaw flows is based on a highly nonlinear moving boundary problem with the fluid velocity related to pressure gradients via a Darcy-type law. In a standard configuration with the Hele-Shaw cell made up of two flat stationary plates, the pressure is harmonic. Therefore, conformal mapping techniques and boundary integral methods can be readily applied to study the key interfacial dynamics, including the Saffman–Taylor instability and viscous fingering patterns. As well as providing a brief review of these key issues, we present a flexible numerical scheme for studying both the standard and nonstandard Hele-Shaw flows. Our method consists of using a modified finite-difference stencil in conjunction with the level-set method to solve the governing equation for pressure on complicated domains and track the location of the moving boundary. Simulations show that our method is capable of reproducing the distinctive morphological features of the Saffman–Taylor instability on a uniform computational grid. By making straightforward adjustments, we show how our scheme can easily be adapted to solve for a wide variety of nonstandard configurations, including cases where the gap between the plates is linearly tapered, the plates are separated in time, and the entire Hele-Shaw cell is rotated at a given angular velocity.
doi:10.1017/S144618112100033
Arter på hele planeten, foren jer!
No full textAnmeldelse af Timothy Mortons "Humankind: Solidarity with nonhuman people" fra 2017
Arter på hele planeten, foren jer!
No full textAnmeldelse af Timothy Mortons "Humankind: Solidarity with nonhuman people" fra 2017
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