Unsteady Reaction Research Articles

Studies on drug delivery from dissolving microneedle and its uptake in tissue

Abstract In this study, a mathematical model for transdermal drug administration using a dissolving conical microneedle (MN) is developed. The drug is absorbed in the blood compartment after passing through the skin’s viable epidermis. Reversible reaction kinetics then allow the drug to enter the tissue compartment. An unsteady reaction equation is used to predict the drug’s distribution into the skin following MN’s dissolution. An unsteady reaction mechanism is used to determine the concentrations in the blood and tissue compartments. Using the fourth-order Runge–Kutta method, the governing equations with their initial conditions are solved numerically. The MN is expected to dissolve completely in 108.1 s, according to the predicted results, while the concentration in the skin peaks at 94.4 s. The concentrations in the tissue and blood compartments, however, reach their respective maximal amounts later on. In this investigation, the impacts of the drug’s mass fraction, volume fraction, and penetration rate cannot be completely ruled out. A thorough sensitivity analysis has been carried out for some of the parameters involved. Our findings closely align with the findings reported in the literature.

VARIABLE THERMAL CONDUCTIVITY AND VISCOSITY EFFECTS ON THIN FILM FLOW OVER AN UNSTEADY POROUS STRETCHING SHEET

The aim of our current investigation is to examine, heat and mass transfer rates of thin liquid film, considering a permeable stretching sheet. In this study, we assume that the medium is porous, the sheet is unstable, and the flow is two-dimensional with nonlinear radiation, variable viscosity, and variable thermal conductivity. Similarity variables are used to convert partial differential equations (PDEs) into dimensionless ordinary differential equations (ODEs), which are solved numerically by Runge-Kutta-Fehlberg (RKF45) method along with shooting technique. MATLAB R2017a is used to illustrate the effects of unsteady, magnetic field, permeability, radiation, suction, temperature ratio, and chemical reaction parameter on velocity, temperature, and concentration profiles. Numerical values of physical quantities are provided in tabular form. The velocity profile tends to diminish as the permeability parameter increases. Our current results show great agreement with the previously reported investigations.

A dual-scale transport model of the porous ceria based on solar thermochemical cycle water splitting hydrogen production

Solar thermochemical cycle converts the full-spectrum range of solar irradiation into chemical energy, which can then be stored or transported, paving the route to achieving carbon neutrality. This cycle involves complex heat and mass transfer processes in porous media, and a relatively complete mathematical model has not been established yet. In particular, there is a lack of a heat and mass transfer model that couples particle scale and macro scale, which is of great theoretical value for the design of solar thermochemical cycle reactors. A comprehensive model combining transport of species in the porous bed (macroscopic) and the lattice oxygen within the particles (mesoscopic) is developed. Dilute mass transfer model and P-1 radiation model are adopted in this model. Based on the model validation by comparing it with existing experimental data, the unsteady heat and mass reaction process, and transportation in the solar thermochemical cycle are investigated under dual-scales. For the oxidation and reduction process respectively, the peak deviation between the model and the experiment is less than 2 s, which thoroughly validated our model. The additional results show that the local thermal and mass non-equilibrium effect and the parameters at the macro and micro scales have a significant influence on the temperature distribution and reaction kinetics. The oxidation process is faster than the reduction process, but the transport time scales in macroscopic (axial) and mesoscopic (radial) direction is of the same order. The increase of reactant concentration can effectively raise the peak value of the hydrogen production, but the product in the oxidation stage is not only limited by the oxidant concentration but also affected by the final state in the reduction stage.

Numerical investigation of non-premixed H2-Air rotating detonation combustor with different equivalence ratios

The initiation and propagation of rotating detonation waves (RDWs) are key techniques for the engineering application of rotating detonation engine (RDE). In this paper, the Naiver-Stokes equation of the three-dimensional unsteady reaction flow has been solved using one step hydrogen/air reaction mechanism to investigate the formation and propagation processes of RDWs under the non-premixed condition with different inlet equivalence ratios. The numerical results show that compared with RDWs generated by premixed propellants, less stable propagation processes and lower detonation fronts can be observed in RDWs generated under the non-premixed condition. It can be also concluded that the equivalence ratio has a great effect on the formation and propagation of RDWs. The formation time of stable RDWs increases when the ER changes from 1.0 to 1.3. However, under the conditions of the two ERs, similar re-initiation phenomena is observed in the evolution process of RDWs and a three-wave mode has been established.

Observations on Detonation Growth of Lead Azide at Microscale.

Lead azide (LA) is a commonly used primary explosive, the detonation growth of which is difficult to study because it is so sensitive and usually has a small charge size in applications. We used photon Doppler velocimetry (PDV) and calibrated polyvinylidene fluoride (PVDF) gauges to reveal the detonation growth in LA, which was pressed in the confinements with controlled heights. The particle-velocity profiles, output pressure, unsteady detonation velocity, reaction time, and reaction-zone width were obtained and analyzed. Three phases of detonation propagation of LA microcharges are discussed. The volume reactions occur at the beginning of detonation in LA microcharges without forming complete shock profiles. Then the shock front is fast with a slow chemistry reaction zone, which is compressed continuously between the height of 0.8 mm and 2.5 mm. Finally, the steady detonation is built at a height of 2.5 mm. The stable detonation velocity and CJ pressure are 4726 ± 8 m/s and 17.12 ± 0.22 GPa. Additionally, the stable reaction zone time and width are 44 ± 7 ns and 148 ± 11 μm. The detailed detonation process has not previously been quantified in such a small geometry.

The Effect of Inflow Distortion on the Rotordynamic Characteristics of a 1400-MW Reactor Coolant Pump Annular Seal

The annular seal between stator and rotor substantively acts as a bearing that affects the rotordynamic characteristic of the turbomachinery rotor system. The rotor wake turbulence in a canned motor Reactor Coolant Pump (RCP) will lead to inflow pressure distortion at the annular seal entrance, thus further affecting the seal rotordynamic characteristics and threatening the stable operation of RCP. In order to obtain the seal rotordynamic coefficients, a transient numerical method applies the mesh deformation technique to simulate the multiple-frequency elliptical rotor whirling orbit model. The transient solutions were proposed to solve the unsteady reaction forces of annular seals at five excitation frequencies for each case. The inflow pressure distortion patterns were simplified as harmonic functions, including two important influence parameters that are impeller blades number m and pressure fluctuation ratio λ. The numerical results showed that with nonuniform time-averaged pressure distribution at the entrance of the annular seal in Case 2, the inflow distortion significantly affects the seal rotordynamic coefficients, while the rotational spinning speed in Case 3 can weaken the time-averaged nonuniformity and accordingly make a dent in the influence. Increasing impeller blades number m and pressure fluctuation λ both result in a sharp diminution of the negative stiffness Keff, as well as an obvious increase in the effective damping Ceff, which will strengthen rotor misalignment and system stability. In addition, the larger impeller blades number m and higher pressure fluctuation λ will make the effective damping Ceff more independent of the whirling frequency. These results provide theoretical guidance for the operation safety of RCP.

EFFECTS OF COATING PROPERTIES ON CONTROLLED DELIVERY FROM AN EMBEDDED DRUG-ELUTING STENT: A SIMULATION STUDY

The present investigation deals with the effects of biodegradable, biodurable and polymer-free coating of a stent on the release mechanism of the drug in a porous medium. The Brinkman equations for the interstitial fluid, the unsteady convection-diffusion-reaction equation for the transport of free drug in the tissue and the unsteady reaction equations for the bound as well as the internalized drug have been considered. In the coating, the transport of drug has been modeled as a diffusion process. Effects of different percentages of the embedment, convection and various coating properties of the stent on the transport of free drug, its retention and the internalization of the bound drug have been studied. Immersed Boundary Method (IBM) in the staggered grid formulation (IBM-MAC) has been used to tackle numerically the system of nonlinear governing equations. Simulated results predict the fastest release of drug from a biodegradable coating, but the averaged concentrations of all drug forms do reach a quasi-steady state in case of a biodurable coating irrespective of the degrees of embedment. Moreover, for all embedment levels of the stent, a biodegradable coating is superior to that of biodurable and polymer-free coating in the presence/absence of convection for larger times, but this superiority is lost for smaller times. Unlike biodurable coating, it is also predicted that the more the embedment level does not necessarily imply the more the effectiveness of delivery for biodegradable and polymer-free coatings of a stent.

Unsteady homogeneous-heterogeneous reactions in MHD nanofluid mixed convection flow past a stagnation point of an impulsively rotating sphere

This paper establishes a mathematical analysis for describing the homogeneous and heterogeneous chemical reactions in the nearness of stagnation region of a sphere immersed in a single-phase nanofluid due to a Newtonian heating. The flow is resulted by an impulsively rotating sphere, and the nanofluid involves nanoparticles of Copper and Ferro. The available unsteady-states of the considered system are given in the case when the diffusion coefficients of both reactant and auto catalyst have the same size. The resulting non-linear dimensionless coupled partial differential equations in which governing the mixed convection flow have been tackled numerically via an implicit finite difference technique in combination with the quasi-linearization scheme. The similarities and differences in the behavior of physical pertinent fluid parameters have been elaborated and discussed graphically. It has been clarified that the nanofluid velocity and temperature profiles grow gradually by adding nanoparticles in the base fluid. Again it is noticed from present contribution that concentration of the nanofluid is decreases function by rising the strength of homogeneous and heterogeneous chemical reactions. Finally, numerical computations of the skin friction and heat transfer factors are presented.

Computational model of stent-based delivery from a half-embedded two-layered coating

An attempt is made in the present investigation to develop a computational model for the purpose of studying the effect of interstitial flow in the porous media on the distribution of drug eluted from a half-embedded drug-eluting stent and its retention in the presence of two-layered coating of the stent. The transport of free drug inside the coatings is considered as an unsteady diffusion process while that in the tissue as an unsteady convection–diffusion–reaction process. The bound drug is governed by an unsteady reaction process only. Immersed boundary method (IBM) in the staggered grid formulation, popularly known as marker and cell (MAC) method, has been leveraged to tackle numerically the governing equations. This model highlights the benefits of consideration of two-layered coating and does predict underlying mechanism for better efficacy by tweaking the kinetics parameters. Comparisons are also made with the results available for stent-based delivery.

A High-Order Compact (HOC) Implicit Difference Scheme and a Multigrid Method for Solving 3D Unsteady Reaction Diffusion Equations

A high-order compact (HOC) implicit difference scheme is proposed for solving three-dimensional (3D) unsteady reaction diffusion equations. To discretize the spatial second-order derivatives, the fourth-order compact difference operators are used, and the third- and fourth-order derivative terms, which appear in the truncation error term, are also discretized by the compact difference method. For the temporal discretization, the multistep backward Euler formula is used to obtain the fourth-order accuracy, which matches the spatial accuracy order. To accelerate the traditional relaxation methods, a multigrid method is employed, and the computational efficiency is greatly improved. Numerical experiments are carried out to validate the accuracy and efficiency of the present method.

Unsteady deflagration speed of an auto-ignitive dimethyl-ether (DME)/air mixture at stratified conditions

The propagation speed of an auto-ignitive dimethyl-ether (DME)/air mixture at elevated pressures and subjected to monochromatic temperature oscillations is numerically evaluated in a one-dimensional statistically stationary configuration using fully resolved numerical simulations with reduced kinetics and transport. Two sets of conditions with temperatures within and slightly above the negative temperature coefficient (NTC) regime are simulated to investigate the fundamental aspects of auto-ignition and flame propagation along with the transition from auto-ignitive deflagration to spontaneous propagation regimes under thermal stratification. Contrary to the standard laminar flame speed, the steady propagation speed of an auto-ignitive front is observed to scale proportionally to its level of upstream reactivity. It is shown that this interdependence is primarily influenced by the characteristic residence time and the homogeneous auto-ignition delay. Furthermore, the unsteady reaction front in either of the two cases responds distinctly to the imposed stratification. Specifically, the results in both cases show that the dynamic flame response depends on the mean temperature at the flame base Tb and the time-scale of thermal stratification. It is also found that, based on Tb and the propensity of the mixture to two-stage chemistry, the instantaneous peak propagation speed and the overall time taken to achieve that speed differs considerably. A displacement speed analysis is carried out to elucidate the underlying combustion modes that are responsible for such a variation in flame response.

Teaching Citations/Referencing: How do Chinese College Student Writers Respond?

This study investigated Chinese college students’ responses to the instruction of citations/referencing. Data included three focal students’ interviews, reflections and writing collected over one-semester-long English writing course at a Chinese university. Through qualitative content analyses, this study shows that the students embraced the importance of maintaining academic integrity, but their development in teaching contexts alternately interacted with multiple factors that hindered (e.g., generic demands) or facilitated (e.g., their perception of their progress) their actual practice of citations/referencing. In the process, teacher mediation was crucial in balancing their unsteady reactions.

Detection of unstable periodic orbits in mineralising geological systems.

Worldwide, mineral exploration is suffering from rising capital costs, due to the depletion of readily recoverable reserves and the need to discover and assess more inaccessible or geologically complex deposits. For gold exploration, this problem is particularly acute. We propose an innovative approach to mineral exploration and orebody characterisation, based on the analysis of geological core data as a spatial dynamical system, using the mathematical tools of dynamical system analysis. This approach is highly relevant for orogenic gold deposits, which-in contrast to systems formed at chemical equilibrium-exhibit many features of nonlinear dynamical systems, including episodic fluctuations on various length and time scales. Feedback relationships between thermo-chemical and deformation processes produce recurrent fluid temperatures and pressures and the deposition of vein-filling minerals such as pyrite and gold. We therefore relax the typical assumption of chemical equilibrium and analyse the underlying processes as aseismic, non-adiabatic, and inherent to a hydrothermal, nonlinear dynamical open-flow chemical reactor. These processes are approximated using the Gray-Scott model of reaction-diffusion as a complex toy system, which captures some of the features of the underlying mineralisation processes, including the spatiotemporal Turing patterns of unsteady chemical reactions. By use of this analysis, we demonstrate the capability of recurrence plots, recurrence power spectra, and recurrence time probabilities to detect underlying unstable periodic orbits as one sign of deterministic dynamics and their robustness for the analysis of data contaminated by noise. Recurrence plot based quantification is then applied to three mineral concentrations in the core data from the Sunrise Dam gold deposit in the Yilgarn region of Western Australia. Using a moving window, we reveal the episodic recurring low-dimensional dynamic structures and the period doubling route to instability with depth, embedded in and originating from higher-dimensional processes of the complex mineralisation system.

Effect of Stoichiometric Ratio on Char-Nitrogen Conversion under High-Temperature Entrained Flow Combustion Conditions

The char-O2 reaction under high-temperature entrained flow conditions was investigated experimentally and numerically to study the effect of the stoichiometric ratio (SR) on char-nitrogen conversion. A numerical model was established based on the unsteady convection-diffusion and reaction equations, in which Stefan flow was considered. The change of specific surface area during char combustion was considered by locally using the Random Pore Model (RPM). The intrinsic reactivity parameters of char-O2 and char-NO reactions were measured using a thermal gravimetric analyzer (TGA) and drop tube furnace (DTF), respectively. The numerical results indicated that the predictions of carbon conversion and NO release agreed well with experimental data. Based on numerical results, for a given carbon conversion, with the decrease of SR, the kinetics-diffusion controlled char-O2 reaction is pushed toward the kinetics-controlled regime, leading to more significant local carbon conversion and thus more developed pore structure inside char particles, including higher porosity and larger specific surface area. Therefore, the oxidation of char-nitrogen occurs more deeply inside char particles and NO release prefers to diffuse toward the particle center, rather than outward. The larger specific surface area deep inside the char particle also gives rise to the total char-NO reaction rate at a lower SR. As a result, the total fractional conversion of char-N to NO decreases as SR decreases under high-temperature entrained flow combustion conditions.

Modelling Time-dependent Release Kinetics in Stent-based Delivery

The present study deals with a computational model of the transport and retention of drug within the arterial wall eluted from a drug-eluting stent, to enhance our understanding of the performance of this device. We considered a two-species model (free and bound) incorporating a reversible reaction to describe drug interactions with the constituents of the arterial wall. An axisymmetric model of drug delivery from a pair of stent struts has been developed, where the transport of free drug is modelled as an unsteady reaction-diffusion process, while the bound drug, assuming complete immobilization in the tissue, is modeled as an unsteady reaction process. The model also took into account a second-order binding process that describes a saturating reversible binding and time-dependent release kinetics of the drug-eluting stent. Considering that diffusion takes place over a tortuous path in a porous media, the effects of porosity and tortuosity on diffusion cannot be ruled out from this present investigation.

Effect of Interstitial Fluid Flow on Drug-Coated Balloon Delivery in a Patient-Specific Arterial Vessel with Heterogeneous Tissue Composition: A Simulation Study.

Angioplasty with drug-coated balloons (DCBs) using excipients as drug carriers is emerging as a potentially viable strategy demonstrating clinical efficacy and proposing additional compliance for the treatment of obstructive vascular diseases. An attempt is made to develop an improved computational model where attention has been paid to the effect of interstitial flow, that is, plasma convection and internalization of bound drug. The present model is capable of capturing the phenomena of the transport of free drug and its retention, and also the internalization of drug in the process of endocytosis to atherosclerotic vessel of heterogeneous tissue composition comprising of healthy tissue, as well as regions of fibrous cap, fibro-fatty, calcified and necrotic core lesions. Image processing based on an unsupervised clustering technique is used for color-based segmentation of a patient-specific longitudinal image of atherosclerotic vessel obtained from intravascular ultrasound derived virtual histology. As the residence time of drug in a stent-based delivery within the arterial tissue is strongly influenced by convective forces, effect of interstitial fluid flow in case of DCB delivery can not be ruled out, and has been investigated by modeling it through unsteady Navier-Stokes equations. Transport of free drug is modeled by considering unsteady advection-reaction-diffusion process, while the bound drug, assuming completely immobilized in the tissue, by unsteady reaction process. The model also takes into account the internalization of drug through the process of endocytosis which gets degraded by the lysosomes and finally recycled into the extracellular fluid. All the governing equations representing the flow of interstitial fluid, the transport of free drug, the metabolization of free drug into bound phase and the process of internalization along with the physiologically realistic boundary and initial conditions are solved numerically using marker and cell method satisfying necessary stability criteria. Simulated results obtained predict that faster drug transfer promotes rapid saturation of binding sites despite perivascular wash out and the concentrations of all drug forms are modulated by the presence of interstitial flow. Such premier attempt of its kind would certainly be of great help in the optimization of therapeutic efficacy of drug.

Polyimide membrane system for tetrafluoroethylene recovery: Industrial plant, optimal operation and economic analysis

Abstract This is a continuation of the development of polyimide membrane for tetrafluoroethylene (TFE) recovery [Sep.Purif.Technol.124(2014)89–98]. A industrial membrane plant has been designed and constructed after cryogenic acetone absorption to enhance TFE recovery for the first time. Specific to the 10-kt/a TFE synthesis plants, the nominal process capacity is set as 20 Nm3/h. The vacuum mode (−0.08 MPaG) is executed for the permeate side to increase the feed/permeate pressure ratio and enhance separation performance under low feed pressure. In response to the drastic fluctuation in feed flow rate and composition, owing to the unsteady cracking reaction, the adaptive control of membrane area is customized technically for the operation strategy. In detail, one or two modules can be shut down by the automatic response while the stream into membrane plant is lower in flow rate or higher in TFE content, and vice versa. In comparison to the ordinary mode under inflexible operation, whose TFE recovery ratio is about 84.5%, the adaptive control strategy behaved better with the recovery ratio up to 87.6%. According to the three-month running result, the membrane recovery plant with adaptive membrane area control can efficiently decrease TFE loss from 130 to 17 t/a, and the annual gross profit was about 1.264 million US dollar, with the payback period less than 4 months. It is undoubted that the polyimide membrane plant is practicable and cost-efficient for TFE recovery in the synthesis.

Synthesis and characterization of Ti/Al reactive multilayer films with various molar ratios

This study describes the fabrication and characterization of the bimetallic Ti/Al reactive multilayer thin films. Reactive films with different molar ratios of Ti and Al (mTi/nAl, m=1; n=1, 2, 3) were fabricated by using Magnetron Sputter Ion Plating (MSIP) method. Differential scanning calorimetry (DSC) was used to analyze the phase change temperature and heat flow. The heat of formation of different phases in Ti-Al system was also calculated by using thermodynamic simulation. Here, reaction heat depends on types of phases form. Microstructural features of the as-deposited Ti/Al reactive multilayer films show columnar microstructures with coherent multilayers. After an electrical ignition, all foils exhibit self-propagating reaction. The experimental result shows that 1Ti/1Al reactive foils have higher reaction velocity of 2.6±0.6m/s compared to other molar ratios at 9V electrical ignition. Here, 1Ti/1Al and 1Ti/2Al reactive films show only steady reaction propagation, whereas 1Ti/3Al films exhibit both steady and unsteady reaction propagation. The microstructural characterization exhibits two types of diffusion modes of reactants mixing in the periodic layers and grain boundary, which assure even faster diffusion during self-propagating reaction.

Self-propagating exothermic reaction analysis in Ti/Al reactive films using experiments and computational fluid dynamics simulation

This study describes the self-propagating exothermic reaction in Ti/Al reactive multilayer foils by using experiments and computational fluid dynamics simulation. The Ti/Al foils with different molar ratios of 1Ti/1Al, 1Ti/2Al and 1Ti/3Al were fabricated by magnetron sputtering method. Microstructural characteristics of the unreacted and reacted foils were analyzed by using electronic and atomic force microscopes. After an electrical ignition, the influence of ignition potentials on reaction propagation has been experimentally investigated. The reaction front propagates with a velocity of minimum 0.68±0.4m/s and maximum 2.57±0.6m/s depending on the input ignition potentials and the chemical compositions. Here, the 1Ti/3Al reactive foil exhibits both steady state and unsteady wavelike reaction propagation. Moreover, the numerical computational fluid dynamics (CFD) simulation shows the time dependent temperature flow and atomic mixing in a nanoscale reaction zone. The CFD simulation also indicates the potentiality for simulating exothermic reaction in the nanoscale Ti/Al foil.

Effect of flow pulsatility and time-dependent release kinetics on stent-based delivery through atherosclerotic plaque

The present work is concerned with an investigation of the effect of flow pulsatility and time-dependent release kinetics on the transport and retention of drug in a stent-based delivery through atherosclerotic plaque. Keeping the relevance of the physiological situation in view, an axi-symmetric two-dimensional model of drug delivery from a strut has been developed in cylindrical polar coordinate system. Here, the luminal drug transport is considered as an unsteady convection–diffusion process, while the drug transport within the arterial tissue is considered as a two-species model (both free and bound), capable of predicting the delivery of drug and its retention in the arterial tissue, in which the specific binding site action is modelled using a reversible chemical reaction. The transport of free drug is modelled as an unsteady convection–diffusion–reaction process, while the bound drug, assuming completely immobilised in the tissue, as an unsteady reaction process. An explicit finite-difference scheme in staggered grids has been developed to tackle numerically the governing equations of motion in order to compute the physiologically significant quantities with desired degree of accuracy. Model results predicted that, unlike within the tissue, the averaged free drug concentration increases with the increase of Re in the lumen and also the averaged concentrations of both drug forms decrease with increasing plaque thickness. Moreover, higher Peclet number in the tissue and coating results in lowering down the averaged concentrations of free as well as bound drug in the tissue and lumen. The present results predict that the presence of plaque dramatically affects the local pharmacokinetics and provides higher resistance to drug transfer in the arterial wall. The averaged concentration of free drug increases in the lumen due to flow pulsatility while a reverse trend is observed in the tissue. Additionally, modelling results also indicate that time-dependent release of drug in stent-based delivery diminishes the averaged concentration of both drug forms as compared to constant release kinetics. All the above observations are consistent with several existing results in the literature.