Gas heating in plasma microcavities: surface effects

Paper presented at the 8th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, Mauritius, 11-13 July, 2011.Interest in microcavities containing plasmas is growing rapidly due to the number of high-technology applications: such as active flow control, spacecraft propulsion, controlled combustion and pollution control. The present paper discusses a self-consistent model for gas and charged and species dynamics in atmospheric microcavities. A self-consistent and timedependant model is described and applied with emphasis on terms involved in the close coupling among the fluid, the charged species and the electric field. The microplasmas are studied from an initial cloud until the stages of charged particle over-amplification and breakdown in small-spaces, where transients are particularly important. The importance of surface effects (namely secondary emission of charged particles from the electrodes) is compared in terms of spatial and temporal evolution of the plasma and fluid dynamics. Heating effects and gas depletion initiation are observed, highlighting the close interaction between neutral gas and charged species in governing the evolution of the microplasma.pm201

Numerical Modeling of Neutral and Charged Particles Within a Gridded Ion Thruster

Introduction The high specific impulse of ion thrusters and their reliability make them well suited for near-Earth and deep space missions. However, due to the complexity of transport of neutral and charged particles in the different component of these devices, the detailed properties of the ion thruster are not fully understood. Numerical simulations could be used to yield an understanding of the physical phenomena involved in thruster operation and thus help in optimizing and achieving ultra-high specific impulse ion thrusters. The external plume has been extensively studied Neutral simulation The first part using a 3D Navier-Stokes continuum code, adapted so as to cater for the pressure differences associated with the flow, has been made previously for the neutral flow within the hollow cathode These calculations have provided information for the upstream boundary conditions for rarified computations using particle methods for the remainder of the thruster internal flow. Modeling of the post-hollow cathode regions The post-hollow cathode regions where the pressure is observed to be several orders of magnitude lower than that in the hollow cathode is now 2 investigated. As it is reported in the literature, a particle code is required for the discharge chamber due to the relatively low densities of particles Description of the DSMC method The DSMC method is based on using simulator particles that mimic the behaviour of and statistically represent the very larger number of real molecules/atoms of which the fluid is comprised. Computations advance in real time but the movement of the simulators is decoupled from their collisions by modeling each of the processes alternately and independently over small, equal time increments. Candidates for inter-molecular collisions are selected from neighbouring particles in a probabilistic manner by comparing a random number with the collision probability of each pair. The outcome of each collision is determined using appropriate models, stochastic ones being the most commonly currently in use. To perform the computations, physical space is discretized into cells of the same order of magnitude as the local mean free path. The Imperial College DSMC program is a modular object-oriented code with reuse capabilities which has been adapted to take into account neutral and charged particles DSMC modeling of the neutral flow DSMC is therefore well adapted to the posthollow cathode regions studied hereafter as the medium is rarefied. The first simulation describes the flow of neutral particles in the thruster and corresponds to the initiation of the discharge. The continuum code of the neutral flow provides boundary conditions at the hollow cathode exhaust for the particle modeling of the main chamber. The simulation considers the neutral particles in the ion thruster starting in a zone approximately 5 mm from the hollow cathode exit plane. The simulation has been performed with the input gas parameters from the continuum code (T = 520K; Number density = 2.46 10 22 m -3 ; Freestream speed ratio = 0.12). The variable soft sphere collision model Figure 2: Velocity magnitude (m/s) of xenon. It should be noted that both collision and plasma effects are important in the thruster. The 3 neutral gas is of importance and can be closely coupled to the evolution of a discharge due to the leading part it can play in ionization, joule heating, constriction etc. The second part of the paper will deal with the self-consistent fully kinetic simulation of the neutral Xenon, electrons, ions and the resulting space charge in the main chamber of the ion thruster. The current axi-symmetric DSMC simulation is applied to initial conditions corresponding to electron and ion clouds (number density 10 16 m -3 ) [15] plus xenon neutral gas in the post-hollow cathode region (more precisely after the keeper electrode and after the potential hill, but before the baffle disc). It should be noted that previous computations [16] using a lower initial density of particles gave the same general physical results in the plasma bulk as those presented in the present paper. Due to its weak spatial extension, the sheath is not directly considered -however the effects of a sheath are taken into account at the metallic parts and the anode, by considering the absorption/repulsion of particles at the boundary conditions. Ionization processes are known to be of paramount importance in the operation of the ion thruster. Thus, ionization is explicitly taken into consideration in the DSMC simulation at a collisional level implying no macroscopic averaging of the phenomena -this is a strong point with respect to fluid models where only global parameters can be implemented. The theory predicts that the ionization cross section will vary according to an expression in the form of A feature of this calculation is to relax the time restriction (due to the high mobility of electrons compared to ions) of the calculation time step by grouping electrons. This technique successfully tested in a particle simulation of the ion extraction system The applied magnetic field is also implemente

Water Dynamics at Protein Interfaces: Ultrafast Optical Kerr Effect Study

The behavior of water molecules surrounding a protein can have an important bearing on its structure and function. Consequently, a great deal of attention has been focused on changes in the relaxation dynamics of water when it is located at the protein surface. Here we use the ultrafast optical Kerr effect to study the H-bond structure and dynamics of aqueous solutions of proteins. Measurements are made for three proteins as a function of concentration. We find that the water dynamics in the first solvation layer of the proteins are slowed by up to a factor of 8 in comparison to those in bulk water. The most marked slowdown was observed for the most hydrophilic protein studied, bovine serum albumin, whereas the most hydrophobic protein, trypsin, had a slightly smaller effect. The terahertz Raman spectra of these protein solutions resemble those of pure water up to 5 wt % of protein, above which a new feature appears at 80 cm–1, which is assigned to a bending of the protein amide chain

Temperature dependence of protein dynamics simulated with three different water models

The effect of variation of the water model on the temperature dependence of protein and hydration water dynamics is examined by performing molecular dynamics simulations of myoglobin with the TIP3P, TIP4P, and TIP5P water models and the CHARMM protein force field at temperatures between 20 and 300 K. The atomic mean-square displacements, solvent reorientational relaxation times, pair angular correlations between surface water molecules, and time-averaged structures of the protein are all found to be similar, and the protein dynamical transition is described almost indistinguishably for the three water potentials. The results provide evidence that for some purposes changing the water model in protein simulations without a loss of accuracy may be possible

Carboxyl Group Enhanced CO Tolerant GO Supported Pt Catalysts: DFT and Electrochemical Analysis

The effect of residual oxygen species in as-prepared Pt nanoparticle on partially reduced graphene oxide (Pt/PRGO) and partially reduced carboxylated-GO (Pt/PR­(GO–COOH)) supports was investigated using electrochemical CO stripping and density functional theory (DFT) analysis. Pt/PRGO and Pt/PR­(GO–COOH) revealed a clear negative shift in CO-stripping onset potential compared to commercial Pt/carbon black. DFT analysis confirmed that the presence of a −COOH group provides the most resistance for CO adsorption. This CO-Pt binding energy is significantly lower than that observed in the presence of an −OH group, which is the most abundant oxygen group in carbon supports. The Pt-CO dissociation energies (on a 42-atom graphene sheet) in the presence of various oxygen groups, in descending order, were OH > CO ≈ C–O–C > COOH. Although single-bonded carbon–oxygen groups (−OH and C–O–C) are more abundant on the GO basal plane and play an important role in Pt nanoparticle nucleation and distribution on graphene sheets, the double-bonded carbon–oxygen (CO and COOH) groups are more abundant residual species post Pt nanoparticle growth and play a vital role in enhancing CO tolerance