Wall Reaction Research Articles

Heuristic modeling of the doping efficiency in sputtered TCO layers

The increased demand on throughput and precision of PVD sputter processes promote the development of modeling tools describing relevant process mechanisms in detail and allowing for finding efficient optimization strategies. For the rarefied gas flow conditions typical in PVD processes, particle based simulation methods such as the Direct Simulation Monte Carlo (DSMC) method for transport of neutral particles and Particle-in-Cell Monte Carlo (PIC-MC) method for plasma discharges can be used.While these methods allow for determining the deposition profile, film stoichiometry as well as to get a first idea on the ion bombardment distribution, they do not draw further conclusions about the resulting film quality. In order to bridge the gap between transport and film growth simulation we present a heuristic model of the dopant incorporation during growth of doped TCO layers such as ZnO:Al. For such materials, a good doping efficiency can usually be obtained only within a certain parameter range of the reactive sputtering process.In the DSMC/PIC-MC simulation tool, we address this issue by introducing an extended wall reaction model. This model involves several coating material phases with dynamic surface fractions assigned to the mesh elements of the reactor geometry. We present a set of concurrent surface reaction mechanisms which reproduce the overall behavior of the reactive ZnO:Al deposition process and its doping efficiency in good qualitative agreement with experimental data.

A conservative, positivity preserving scheme for reactive solute transport problems in moving domains

We study the mathematical models and numerical schemes for reactive transport of a soluble substance in deformable media. The medium is a channel with compliant adsorbing walls. The solutes are dissolved in the fluid flowing through the channel. The fluid, which carries the solutes, is viscous and incompressible. The reactive process is described as a general physico-chemical process taking place on the compliant channel wall. The problem is modeled by a convection–diffusion adsorption–desorption equation in moving domains. We present a conservative, positivity preserving, high resolution ALE-FCT scheme for this problem in the presence of dominant transport processes and wall reactions on the moving wall. A Patankar type time discretization is presented, which provides conservative treatment of nonlinear reactive terms. We establish CFL-type constraints on the time step, and show the mass conservation of the time discretization scheme. Numerical simulations are performed to show validity of the schemes against effective models under various scenarios including linear adsorption–desorption, irreversible wall reaction, infinite adsorption kinetics, and nonlinear Langmuir kinetics. The grid convergence of the numerical scheme is studied for the case of fixed meshes and moving meshes in fixed domains. Finally, we simulate reactive transport in moving domains under linear and nonlinear chemical reactions at the wall, and show that the motion of the compliant channel wall enhances adsorption of the solute from the fluid to the channel wall. Consequences of this result are significant in the area of, e.g., nano-particle cancer drug delivery. Our result shows that periodic excitation of the cancerous tissue using, e.g., ultrasound, may enhance adsorption of cancer drugs carried by nano-particles via the human vasculature.

OH*-chemiluminescence during autoignition of hydrogen with air in a pressurised turbulent flow reactor

Autoignition of hydrogen in air was studied in a turbulent flow reactor using OH*-chemiluminescence. High-speed imaging was used to visualise the formation of auto-ignition kernels in the flow, and to analyse the conditions under which temporary stabilisation of the flame kernels occurred. The experiments were carried out at temperatures of 800-850 K, pressures of 0.8-1.2 MPa and an equivalence ratio of phi = 0.25. Measurements of the autoignition delays yielded values in the range of tau = 210-447 ms. The autoignition delay results indicated that, over the range of conditions studied, ignition delays reduced with decreasing pressure. This observation contradicted homogeneous gas-phase kinetic calculations, which predicted an increase in autoignition delay with decreasing pressure. If the kinetic model was altered to include surface reactions at the reactor walls, the calculations could be qualitatively reconciled with the experimental data, suggesting that wall reactions had a significant influence on autoignition delays. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved. (Less)

Secondary reactions of turbulent reacting flows over a film-cooled surface

Demand for greater engine efficiency and thrust-to-weight ratio has driven the production of aircraft engines with higher core temperatures and pressures. Such engines operate at higher fuel–air ratios, resulting in the potential for significant heat release and chemical reactions on a film-cooled surface. Currently, there is little basis for understanding the effects on aero-performance and durability due to such secondary reactions. In this paper, the chemically reactive turbulent film cooling over a surface for three cases of an inclined single and two parallel and compound-angle coolant holes is investigated by a Reynolds-averaged Navier–Stokes approach with the shear-stress transport (SST) turbulence model using OpenFoam. To take into account the secondary combustion resulting from the unburned fuels in the crossflow, a two-step reaction scheme was used for the combustion of propane. The relative increase in surface heat flux due to near wall reactions was investigated for film cooling with N2 and air injections. An eddy dissipation concept fast chemistry approach was used to account for the turbulence–chemistry interaction. Results demonstrate that reactions in the turbine cooling film can result in increased heat transferred to the surface. Failure to design for this effect could result in augmented heat transfer caused by the cooling scheme, and turbine life could be degraded substantially. The analysis suggests that high fuel–air ratio designs may have to consider changes to cooling strategies to accommodate secondary combustion.

Numerical investigation of biogas flameless combustion

The purpose of this investigation is to analyze combustion characteristics of biogas flameless mode based on clean technology development strategies. A three dimensional (3D) computational fluid dynamic (CFD) study has been performed to illustrate various priorities of biogas flameless combustion compared to the conventional mode. The effects of preheated temperature and wall temperature, reaction zone and pollutant formation are observed and the impacts of combustion and turbulence models on numerical results are discussed. Although preheated conventional combustion could be effective in terms of fuel consumption reduction, NOx formation increases. It has been found that biogas is not eligible to be applied in furnace heat up due to its low calorific value (LCV) and it is necessary to utilize a high calorific value fuel to preheat the furnace. The required enthalpy for biogas auto-ignition temperature is supplied by enthalpy of preheated oxidizer. In biogas flameless combustion, the mean temperature of the furnace is lower than traditional combustion throughout the chamber. Compared to the biogas flameless combustion with uniform temperature, very high and fluctuated temperatures are recorded in conventional combustion. Since high entropy generation intensifies irreversibility, exergy loss is higher in biogas conventional combustion compared to the biogas flameless regime. Entropy generation minimization in flameless mode is attributed to the uniform temperature inside the chamber.

Transient electrical double layer dynamics in a quiescent fluid

From the moment a liquid is put in contact with a solid, physico-chemical reactions occur at their interface. These reactions have been modeled over the years by preferential adsorption or corrosion. Whatever the model, they lead to a charge distribution called electrical double layer (EDL), formed by a charge layer at the solid wall, and a diffuse charge layer of opposite sign in the liquid. In this work we focus on the formation of the EDL when a plane wall is put in contact with a quiescent fluid. The governing equations and a semi-analytical solution are presented. A simpler solution can be found by assuming the instantaneous formation of the charge density profile. The semi-analytical solution presented in this work is compared to the former at different reaction rates leading to the conclusion that for fast enough wall reactions, the instantaneous charge density profile cannot be assumed.

Transient electrical double layer dynamics in a quiescent fluid

From the moment a liquid is put in contact with a solid, physico-chemical reactions occur at their interface. These reactions have been modeled over the years by preferential adsorption or corrosion. Whatever the model, they lead to a charge distribution called electrical double layer (EDL), formed by a charge layer at the solid wall, and a diffuse charge layer of opposite sign in the liquid. In this work we focus on the formation of the EDL when a plane wall is put in contact with a quiescent fluid. The governing equations and a semi-analytical solution are presented. A simpler solution can be found by assuming the instantaneous formation of the charge density profile. The semi-analytical solution presented in this work is compared to the former at different reaction rates leading to the conclusion that for fast enough wall reactions, the instantaneous charge density profile cannot be assumed.

Chlorine Decay under Steady and Unsteady-state Hydraulic Conditions

This paper describes a simulation framework for the scale-adaptive hydraulic and chlorine decay modelling under steady and unsteady-state flows. Bulk flow and pipe wall reaction coefficients are replaced with steady and unsteady-state reaction coefficients. An unsteady decay coefficient is defined which depends upon the absolute value of shear stress and the rate of change of shear stress for quasi-unsteady and unsteady-state flows. A preliminary experimental and analytical investigation was carried out in a water transmission main. The results were used to model monochloramine decay and these demonstrate that the dynamic hydraulic conditions have a significant impact on water quality deterioration and the rapid loss of disinfectant residual.

Sampling design for water distribution network chlorine decay calibration

This paper provides a novel approach to determining optimal sampling locations for chlorine decay model calibration. Three questions are investigated: (1) What is the minimum number of chlorine sample locations needed? (2) How many combinations of sampling locations are available? (3) What is the optimal location combination? To answer the first two questions, the mathematical expressions of the chlorine concentrations between any two sampling locations are developed and sampling point relationship matrices are generated, then a mixed integer programming (MIP) algorithm is developed. Once obtained, the solutions to the first two questions are used to calculate the chlorine decay wall reaction coefficients and sensitivity matrix of chlorine concentration to wall reaction coefficients; then, sampling location combinations achieved in the second question are sorted using a D-optimality algorithm. The model frame is demonstrated in a case study.

Investigation of Risks for Cerebral Embolism Associated With the Hemodynamics of Cardiopulmonary Bypass Cannula: A Numerical Model

Cerebral emboli originating in the ascending aorta are a major cause of noncardiac complications following cardiac surgery. The hemodynamics of the aortic cannula has been proven to play a significant role in emboli generation and distribution. The aim of the current study was to perform a thorough numerical investigation in order to examine the effect of the design and orientation of the cannula used during cardiopulmonary bypass on the risk to develop cerebral embolism. Hemodynamic analyses compared numerical models of 27 cases consisting of six different cannula orientations, four aortic anatomies, and three cannula designs. The cannula designs included a straight-tip (ST) cannula, a moderately curved tip cannula (TIP1 ), and a sharp-angle curved cannula (TIP2 ). Outcome measures included hemodynamic parameters such as emanating jet velocity, jet velocity drop, maximal shear stress, aortic wall reaction, emboli pathlines and distribution between upper and lower vessels, and stagnation regions. Based on these parameters, the risks for hemolysis, atheroembolism, and cerebral embolism were evaluated and compared. On one hand, the jet emerging from the ST cannula generated large wall-shear stress at the aortic wall; this may have triggered the erosion and distribution of embolic atheromatous debris from the aortic arch. On the other hand, it diverted more emboli from the clamp region to the descending aorta and thus reduced the risk for cerebral embolism. The TIP1 cannula demonstrated less shear stress on the aortic wall and diverted more emboli from the clamp region toward the upper vessels. The TIP2 cannula exhibited a stronger emanating jet, higher shear stress inside the cannula, and highly disturbed flow, which was more stagnant near the clamp region. Current findings support the significant impact of the cannula design and orientation on emboli generation and distribution. Specifically, the straight tip cannula demonstrated a reduced risk of cerebral embolism, which may be pivotal in the clinical setting.

Physical properties of Portuguese pinewood chemically modified

Maritime Pine softwood has low dimensional stability. Chemical modification is the easiest way to improve technological features of the softwood into a material with enhanced and less varied properties. In this work, physical properties of modified wood using four methods of modification were studied. Pure sapwood of the Maritime pine species (Pinus pinaster Ait.) was submitted at different chemical modification processes: 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU), N-methylol melamine (MMF), tetra-alkoxysilane (TEOS) and wax (amid WA and montan/lignite wax WL). The following physical properties were assessed experimentally: density and weight percent gain (WP), equilibrium moisture content (EMC), isotherms curves, anti-swelling efficiency (ASE), swelling strain and swelling coefficient. Significant changes in the material properties took place. All modification processes increased the density of modified wood in different extent: Modification with cell wall reaction (DMDHEU and MMF resin) were the most effective methods to achieve high dimensional stability (ASE) in parallel with low equilibrium moisture content (EMC). The lumen fill modification (TEOS and wax) had no effect in to different physical properties.

Development of a Method for Studying Biomechanical Properties of the Chest Wall Using Low-Frequency Elastic Waves

A laboratory hardware-software system (HSS) for studying local vibrational biomechanical properties of the chest and lungs was developed. The HSS includes a percussion vibrator with accelerometer, a second accelerometer, and a control block for delivering pulse input to the chest wall and detection of the wall reaction. The HSS provides controlled percussion of the chest wall and detection of acoustic reaction to these pulse inputs. Acceleration of the indenter of the percussion vibrator and acceleration of the second accelerometer adhered to the chest wall at a distance 2–10 cm from the site of percussion are measured. On the basis of the accelerations, we measured a frequency dependence of the modulus and phase of the local transfer function for vibration propagation in the chest wall of healthy volunteers. The group speed and the dependence of phase speed on frequency were estimated. The techniques for the measurement of local mechanical parameters of the chest wall using low-frequency elastic waves could be used for biomedical diagnosis of chest wall tissues.

Variation of inlet boundary conditions on the combustion characteristics of a typical cavity-based scramjet combustor

The influences of wall functions, reaction model, and wall Prandtl number on the reacting flow field characteristics of a typical cavity-based scramjet combustor have been investigated numerically, and the computed results have been compared with the available experimental data in the open literature. The computed results are in reasonable agreement with the experimental data, but the pressure downstream of the leading edge of the cavity is overpredicted. Meanwhile, the grid discrepancy has been analyzed by employing three different grid scales, namely the coarse, the moderate, and the refined grids. The obtained results show that the moderate grid may be employed with confidence to compute the reacting flow field of the scramjet combustor. The wall functions, reaction model, and wall Prandtl number have a large impact on the pressure distribution along the floor face of the cavity, and this may be due to the influence of the variance of the flow separation point on the top wall for the different cases investigated with different inlet boundary conditions. The static pressure along the floor face of the cavity decreases with an increase in the wall Prandtl number, and then increases when the wall Prandtl number is large enough, namely 1.2 in the current study. This points out that there exists an optimal wall Prandtl number for the prediction of the reacting flow field of the cavity-based scramjet combustor.

Acquired peri-stent evaginations in a second generation durable polymer drug eluting stent

Hypersensitivity reaction towards the stent polymer resulting in formation of evaginations has been detected predominantly after treatment by the first generation drug eluting stents. This case shows that despite the use of more biocompatible polymers in second generation drug eluting stents there will still be patients developing adverse vessel wall reactions consistent with a hypersensitivity reaction towards the implanted stent.

Coronary Edema Demonstrated by Cardiovascular Magnetic Resonance in Patients With Peri-Stent Inflammation and Aneurysm Formation After Treatment by Drug-Eluting Stents

Drug-eluting stents (DES) minimized the occurrence of in-stent restenosis but with a penalty of increased risk of late stent thrombosis.1 In patients with late stent thrombosis, adverse vessel wall reactions such as uncovered and malapposed stent struts, coronary aneurysms, and accelerated neoatherosclerosis2 have been identified. The stent polymer is suspected to induce an inflammatory response,3 resulting in vessel wall abnormalities. Identification of patients with an inflammatory response to DES might be essential in preventing late stent thrombosis. Inflammatory processes increase vessel wall permeability, leading to edema, and both native and peri-stent vessel wall inflammation may be associated with tissue edema. Cardiovascular magnetic resonance (CMR) performed with a T2-weighted short-τ inversion recovery sequence (T2-STIR) has recently been shown to identify localized coronary edema in the culprit artery of patients with acute myocardial infarction.4 Parameters for the ECG-triggered, navigator-gated, dark-blood, T2-STIR fast-spin-echo sequence were as follows: repetition time, 2 RR intervals; echo time, 100 milliseconds; echo train length, twenty 0.68×0.68×8-mm3 voxels; and 2 signal averages. Here we present 3 patients treated with DES (Cypher Select+, Cordis, Johnson & Johnson, Miami Lakes, FL) who demonstrate peri-stent edema as a possible marker of stent-induced coronary inflammation. A 41-year-old woman experienced very late stent thrombosis 15 months after treatment with DES in the proximal left anterior descending …

Interplay between channel and catalyst operating regimes in wall-coated microreactors

When mass transfer to the wall of a microchannel interacts with the nonlinear reaction–diffusion processes occurring inside the catalyst coating, several operating regimes are possible in the overall system behavior. The conditions for which these regimes occur and the transition criteria are derived here using theoretical analyses for mass transfer in a catalytic layer and in channel flow with a nonlinear wall reaction. The results are expressed in terms of the variables of operation (in temperature diagrams), inlet reference conditions and criteria which can be set as strict as desired (the effectiveness factor or the degree of mass transfer control). Two main behaviors are identified regarding the major source for transport limitation, for different values of internal and external “diffusion velocities”. The transition between them is identified and the implications on the strategies to achieve overall kinetic or mass transfer control are discussed. Our analysis allow us to map all regimes analytically (inclusively for nonlinear kinetics), whereas in previous literature, numerical evaluation is required even for the simpler case of first-order reactions. The more complex effects introduced by kinetic nonlinearities are highlighted, namely the influence of the reactant local distribution.

Sonochemical degradation of estradiols: Incidence of ultrasonic frequency

This paper presents an experimental study of the ultrasonic degradation of organic pollutants in terms of the effect of ultrasonic frequency (40–380–850–1000kHz). The removal efficiency of two endocrine disrupting compounds [17β-estradiol (E2) and 17α-ethinylestradiol (EE2)] is investigated using laboratory scale ultrasonic baths at low power intensities. Higher ultrasonic frequencies were found to be more effective for pollutant degradation with 850kHz the best: 9.0×10−1mg/kWh for E2 and 6.8×10−1mg/kWh for EE2 at initial concentrations of 1ppm. Additionally, the removal of p-nitrophenol was investigated under the same conditions, as a dosimetry reaction for estimating the hydroxyl radical production, key component in organic pollutant removal. In order to describe the overall phenomena occurring inside the reactor and to predict the apparent hydroxyl radical production, a simulation algorithm is proposed. It incorporates the solution of ODE systems that embodies bubble dynamics, heat and mass transfer through the bubble wall and chemical reactions in the gas–vapor phase for an initial bubble nuclei population. The experimental degradation measures of p-nitrophenol over the range of frequencies studied were found to be comparable with the model results of hydroxyl radicals produced during the sonication treatment.

Time-resolved diagnostics and kinetic modelling of the ignition transient of a H2 + 10% N2 square wave modulated hollow cathode discharge

Time-resolved emission spectroscopy and quadrupole mass spectrometry are used for the experimental diagnostics of a low pressure (8 Pa) modulated dc hollow cathode discharge of H2 + 10% N2. The time-dependent experimental results are reasonably accounted for by a zero-order kinetic model developed in our group and checked previously with steady-state measurements (Carrasco et al 2011 Phys. Chem. Chem. Phys. 13 19561), which is now validated under more stringent conditions. In addition to the discharge precursors, which are partly recycled in wall reactions, NH3 is produced in small but appreciable amounts. The slower evolution of NH3, as compared with H2 and N2, is traced back to the multistep surface reaction mechanism responsible for its production. An analysis of the time variation of optical emission measurements shows that excited NH(c 1Π) radicals are essentially formed in the electron-impact dissociation of NH3.

Symptomatic abnormal vessel wall reaction after implantation of the first generation sirolimus-eluting stent: A case series

Expansive vessel wall remodeling has been previously reported after implantation of drug-eluting stents. These abnormal vessel wall reactions (AVWR), though uncommon may be associated with serious clinical events. We report on a series of patients in whom symptoms developed despite patent stents. We report a series of 10 consecutive patients with evidence of AVWR on angiography and/or intravascular ultrasonography after implantation of sirolimus-eluting stents (age 39-90 years, 4 females) during a period of 4 years. All patients were symptomatic despite patent stents in coronary angiographies done because of persistent chest pain. Four patients subsequently developed very late stent thrombosis (VLST). These 4 patients were only on aspirin monotherapy prior to the VLST. Of the remaining 6 patients, 3 patients underwent coronary interventions (with or without stenting) while the remaining 3 patients were managed conservatively. All 6 were advised for life-long dual antiplatelet therapy. Though uncommon, AVWR might precede VLST. In our series, all patients were symptomatic despite patent stents. Additional studies are required to identify patients at risk and to determine the best treatment modality for this challenging new entity. Until further data become available, these patients should stay on dual antiplatelet therapy after an AVWR has been identified.

ChemInform Abstract: Li2Sr4Al2Ta2N8O, a Nitridoalumotantalate with BCT‐Zeolite Type Structure.

AbstractThe title compound is synthesized from Li3AlN2, Sr(NH2)2, LiN3, and Li as a flux (wall reaction with the Ta crucible, 900 °C, 24 h).