Detailed 3D modelling of mass transfer processes in two phase flows with dynamic interfaces

We developed a method that will enable us to determine mass transfer coefficients for a\ud large number of two phase flow conditions based on numerical simulation. A three-dimensional\ud direct numerical simulation based on the Front Tracking technique taking into account the mass\ud transfer process was chosen for this purpose. The dissolved species concentration in the liquid\ud phase is tracked using a scalar mass balance while the value of the concentration at the interface\ud is determined via an immersed boundary technique. In the present study, simulations are carried\ud out to investigate the effect of the bubble shape on the dissolved species concentration fiel

Detailed modeling of hydrodynamics mass transfer and chemical reactions in a bubble column using a discrete bubble model

A 3D discrete bubble model is adopted to investigate complex behavior involving hydrodynamics, mass transfer and chemical reactions in a gas¿liquid bubble column reactor. In this model a continuum description is adopted for the liquid phase and additionally each individual bubble is tracked in a Lagrangian framework, while accounting for bubble¿bubble and bubble¿wall interactions via an encounter model. The mass transfer rate is calculated for each individual bubble using a surface renewal model accounting for the instantaneous and local properties of the liquid phase in its vicinity. The distributions in space of chemical species residing in the liquid phase are computed from the coupled species balances considering the mass transfer from bubbles and reactions between the species. The model has been applied to simulate chemisorption of CO2 bubbles in NaOH solutions. Our results show that apart from hydrodynamics behavior, the model is able to predict the bubble size distribution as well as temporal and spatial variations of each chemical species involved

Method for predicting rotor free-wake positions and the resulting rotor blade airloads

Computer program has been designed and written to predict rotor free-wake positions and resulting rotor blade airloads without requiring time-consuming and tedious calculations. This program was written in FORTRAN IV for use on an IBM-360 computer

Numerical simulation of small bubble-big bubble-liquid three-phase flows

Numerical simulations of the small bubble-big bubble-liquid three phase heterogeneous flow\ud in a square cross-sectioned bubble column were carried out with the commercial CFD\ud package CFX-4.4 to explore the effect of superficial velocity and inlet dispersed phase\ud fractions on the flow patterns. The approach of Krishna et al. (2000) was adopted in the\ud Euler-Euler framework to numerically simulate the gas-liquid heterogeneous flow in bubble\ud columns. On basis of an earlier study (Zhang et al. 2005), the extended multiphase k - ε\ud turbulence model (Pfleger and Becker, 2001) was chosen to model the turbulent viscosity in\ud the liquid phase and implicitly account for the bubble-induced turbulence. The obtained\ud results suggest that, first of all, the extended multiphase k - ε turbulence model of Pfleger and\ud Becker (2001) is capable of capturing the dynamics of the heterogeneous flow. With\ud increasing superficial velocity, the dynamics of the flow, as well as the total gas hold-up\ud increases. It is observed that with increasing inlet phase fraction of the big bubbles, the total\ud gas holdup decreases while the dynamic nature of the flow increases, which indicates that the\ud small bubble phase mainly determines the total gas holdup while the big bubble phase\ud predominantly agitates the liquid

Mechanism for Spontaneous Growth of Nanopillar Arrays in Ultrathin Films Subject to a Thermal Gradient

Several groups have reported spontaneous formation of periodic pillar-like arrays in molten polymer nanofilms confined within closely spaced substrates maintained at different temperatures. These formations have been attributed to a radiation pressure instability caused by acoustic phonons. In this work, we demonstrate how variations in the thermocapillary stress along the nanofilm interface can produce significant periodic protrusions in any viscous film no matter how small the initial transverse thermal gradient. The linear stability analysis of the interface evolution equation explores an extreme limit of B\'{e}nard-Marangoni flow peculiar to films of nanoscale dimensions in which hydrostatic forces are altogether absent and deformation amplitudes are small in comparison to the pillar spacing. Finite element simulations of the full nonlinear equation are also used to examine the array pitch and growth rates beyond the linear regime. Inspection of the Lyapunov free energy as a function of time confirms that in contrast to typical cellular instabilities in macroscopically thick films, pillar-like elongations are energetically preferred in nanofilms. Provided there occurs no dewetting during film deformation, it is shown that fluid elongations continue to grow until contact with the cooler substrate is achieved. Identification of the mechanism responsible for this phenomenon may facilitate fabrication of extended arrays for nanoscale optical, photonic and biological applications.Comment: 20 pages, 9 figure

Formation of Nanopillar Arrays in Ultrathin Viscous Films: The Critical Role of Thermocapillary Stresses

Experiments by several groups during the past decade have shown that a molten polymer nanofilm subject to a large transverse thermal gradient undergoes spontaneous formation of periodic nanopillar arrays. The prevailing explanation is that coherent reflections of acoustic phonons within the film cause a periodic modulation of the radiation pressure which enhances pillar growth. By exploring a deformational instability of particular relevance to nanofilms, we demonstrate that thermocapillary forces play a crucial role in the formation process. Analytic and numerical predictions show good agreement with the pillar spacings obtained in experiment. Simulations of the interface equation further determine the rate of pillar growth of importance to technological applications.Comment: 5 pages, 4 figure

Antiferromagnetic Order in MnO Spherical Nanoparticles

We have performed unpolarized and polarized neutron diffraction experiments on monodisperse 8 nm and 13 nm antiferromagnetic MnO nanoparticles. For the 8 nm sample, the antiferromagnetic transition temperature $T_N$ (114 K) is suppressed compared to the bulk material (119 K) while for the 13 nm sample $T_N$ (120 K) is comparable to the bulk. The neutron diffraction data of the nanoparticles is well described using the bulk MnO magnetic structure but with a substantially reduced average magnetic moment of 4.2$\pm$0.3 $\mu_B$/Mn for the 8 nm sample and 3.9$\pm$0.2 $\mu_B$/Mn for the 13 nm sample. An analysis of the polarized neutron data on both samples shows that in an individual MnO nanoparticle about 80$$ of Mn ions order. These results can be explained by a structure in which the monodisperse nanoparticles studied here have a core that behaves similar to the bulk with a surface layer which does not contribute significantly to the magnetic order.Comment: 7 pages, 5 figure

Accomodation Management 1

Exam paper for second semeste