Step Flow Research Articles

Structural, electrical, morphological, and interfacial characteristics of lattice-matched InAlN/GaN HEMT structure on SiC substrate

This work highlights the influence of surface properties, on the characteristics of InAlN/GaN based high electron mobility transistor (HEMT) structures grown on the SiC substrate by metalorganic vapor phase epitaxy. The growth parameters, i.e., reactor pressure and V/III ratio were tuned to improve the morphological and two-dimensional electron gas (2DEG) characteristics of the HEMT structure. It was found that V/III ratio plays a significant role in improving surface morphology and 2DEG properties without altering average indium composition. It was also found that 2DEG properties are highly sensitive to surface morphology and its features. The step flow smooth surface morphology with very low surface and interface roughness was observed in optimized lattice-matched InAlN/GaN HEMT structures. The sheet resistance of ∼170 Ω/sq with good 2DEG concentration (∼2.4 × 1013 cm−2) and 2DEG mobility (∼1500 cm2/V s) was achieved in the optimized lattice-matched InAlN/GaN HEMT structure. A comparison between different barrier-based HEMT structures, i.e., lattice-matched InAlN/GaN and strained AlGaN/GaN, was also discussed. Their structural, electrical, morphological, and interfacial characteristics were compared.

An optimisation–based domain–decomposition reduced order model for the incompressible Navier-Stokes equations

The aim of this work is to present a model reduction technique in the framework of optimal control problems for partial differential equations. We combine two approaches used for reducing the computational cost of the mathematical numerical models: domain–decomposition (DD) methods and reduced–order modelling (ROM). In particular, we consider an optimisation–based domain–decomposition algorithm for the parameter–dependent stationary incompressible Navier–Stokes equations. Firstly, the problem is described on the subdomains coupled at the interface and solved through an optimal control problem, which leads to the complete separation of the subdomain problems in the DD method. On top of that, a reduced model for the obtained optimal–control problem is built; the procedure is based on the Proper Orthogonal Decomposition technique and a further Galerkin projection. The presented methodology is tested on two fluid dynamics benchmarks: the stationary backward–facing step and lid-driven cavity flow. The numerical tests show a significant reduction of the computational costs in terms of both the problem dimensions and the number of optimisation iterations in the domain–decomposition algorithm.

Numerical study on the performance of a novel three-dimensional stepped wavy flow field in PEMFC

The design of the flow field plate plays a crucial role in the performance of proton exchange membrane fuel cell (PEMFC). In this study, a novel three-dimensional stepped wavy flow field (SWFF) is developed to enhance the output performance of PEMFC. The effects of geometric parameters of this novel flow field, included the total stepped length (S) and depth (H), the ratio of the sub-stepped length (s1:s2 and s1:s3), and the ratio of the sub-stepped depth (h1:h2), on the transport and distribution characteristics of oxygen and water in PEMFC are numerically investigated. The results demonstrated that the smaller stepped total length and larger stepped total depth result in better cell performance. In addition, the output performance of PEMFC increases with the rise of the ratio of s1:s2, and the decrease of the ratio of s1:s3 under the optimal total stepped length and depth. It is also worth noting that the output performance of PEMFC first increases and then decreases with the rise of the ratio of h1:h2. The optimal values for S, H, s1, s2, s3, h1 and h2 are 8 mm, 0.8 mm, 1.6 mm, 1.6 mm, 1.6 mm, 0.4 mm and 0.4 mm, respectively. Furthermore, when compared to the rectangular baffle flow field, wavy flow field, and parallel flow field, the 3D stepped wavy flow field demonstrated a significant increase in the maximum net output power with improvements of 4.8 %, 11.3 %, and 27.9 %, respectively.

NUMERICAL MODELING OF BACKWARD-FACING STEP FLOW VIA COMPUTATIONAL FLUID DYNAMICS

As a fundamental case for problems of fluid mechanics, examination of flow separation and its reattachment is important for engineering applications. Considering the significance of the subject, backward-facing step flow has been modeled via Computational Fluid Dynamics (CFD) based on an experimental study previously done at Re = 5000. Steady simulations have been conducted by k-ε Renormalization Group (RNG) considering the same flow conditions of the reference study. Pressure distributions, streamwise and cross-stream velocity components, velocity magnitude values with streamline patterns and turbulence kinetic energy values have been presented by using contour graphics. Furthermore, the stations for pressure distributions, velocity profiles for streamwise components and turbulence kinetic energy values have been defined for evolution of related data. Lower pressure zone for the wake region of the backward-facing step has been attained due to flow separation. Separation of the upstream boundary layer has been seen and it became a curved one. Moreover, turbulence level of the step wake has been obtained as higher than those of any other points. Transition to core flow has been attained at y* = 1.1 that is above the step height. Flow oscillations have been observed for x* ≥ 2 and y* ≤ 1 since the fluctuations for these values were effective in the wake region. To sum up, the dimensionless reattachment length has been numerically obtained as 5.92 which is very good agreement with the experimental results at same Reynolds number. The deviation from the reference results is from 0.34 % to 1.33 %.

Numerical and experimental investigation of 3D flow field bipolar plates for PEMFCs by metal 3D printing

The bipolar plate is one of the core components of a proton exchange membrane fuel cell (PEMFC). Its flow field structure directly affects the drainage and heat dissipation performance of the cell, as well as the transmission and distribution of the reaction gas. Compared with conventional 2D flow fields, novel 3D flow fields can provide superior gas–liquid mass transfer performance, significantly improving the performance of PEMFCs. However, traditional manufacturing processes fail to meet the associated molding needs, due to the complex structure of such plates. This research aims to investigate the metal 3D printing process, mass transfer law, and efficiency of 3D flow field bipolar plates, focusing on the influence of structural parameters on the gas–liquid mass transfer performance, by combining CFD multi-phase flow numerical simulations and experiments. The results indicate that the use of a 3D stepped flow field enhances the convection effect in the vertical direction due to the increase in Z-directional inflow, thus promoting oxygen transfer and reducing the accumulation of water. In particular, better gas–liquid mass transfer performance is achieved when the number of steps is 4. At 0.55 V, the measured power density of the Stepped-IV cell reached as high as 6727 W/m2, which is about 26 % higher than the conventional parallel flow field. This study provides an important technical and theoretical reference for the design and preparation of novel 3D complex flow fields.

Study Reviews Two Decades of Surveillance Using Distributed Acoustic Sensing

_ This article, written by JPT Technology Editor Chris Carpenter, contains highlights of paper SPE 200088, “Downhole Monitoring Using Distributed Acoustic Sensing: Fundamentals and Two Decades of Deployment in Oil and Gas Industries,” by Mohammad Soroush and Mohammad Mohammadtabar, RGL Reservoir Management and University of Alberta, and Morteza Roostaei, RGL Reservoir Management, et al. The paper has not been peer reviewed. _ Distributed acoustic sensing (DAS) through fiber optics has been deployed in downhole monitoring for over 2 decades. The complete paper reviews the basics of DAS, fiber types, installation methods, types of recorded data, data processing, historical development, current applications, and limitations of the technology, providing a concise review using several field cases from more than 200 published SPE papers and journal databases. Because this synopsis cannot retain these many paper references or their overview, readers are encouraged to access the complete paper on OnePetro. DAS Fundamentals DAS Units. DAS systems consist of an interrogator that includes a laser transmitter and detector, a processing unit, and a distributed sensing fiber. Laser pulses are sent periodically into the fiber that is installed in the medium. The detector records backscattered response vs. time along the fiber. Because an acoustic field exerts pressure on the fiber it surrounds, some strain is induced on the fiber. An interrogator is sensitive to this strain between two points of fiber separated by gauge length. Multi- and Single-Mode Fibers. Generally, multimode fibers are used for distributed temperature sensing (DTS) and single-mode fibers are used for DAS. Multimode fibers have a larger-diameter core through which multiple modes of light can pass, while, in single mode, only one mode of light passes through the core. Attenuation in single-mode fiber is lower; therefore, single mode is suitable for long distances. On the other hand, multimode fiber can pass more data through the fiber and has higher attenuation. Data Type and Processing. Previous authors described how to reduce, convert, and transmit DAS recordings as data acquisition accumulates over time at the site without losing significant information. They also established three methods of data reviewing: local, remote display (from a remote server), and post-job integration (displaying all available data). A work flow of main steps for using DAS and DTS includes data gathering, auxiliary data, quality control and assurance, data management, and determination of interpretation options. Live data can be made available in permanent installations, while analytics libraries can be integrated into cloud-based DAS system. This will result in (nearly) real-time decision-making and easy-to-use forms of data. Multicomponent DAS. Because DAS systems are more sensitive in an axial direction, to estimate strain tensor, a multicomponent DAS was proposed with two approaches: multiple parallel or helix optical fiber. Another configuration was proposed with five helical fibers with equal space and one straight fiber for shorter wavelengths, addressing a limitation of previous methods.

Lattice Boltzmann modeling of backward-facing step flow controlled by a synthetic jet

Lattice Boltzmann modeling of backward-facing step flow controlled by a synthetic jet

Development of a multi-scale mathematical model for green hydrogen production via biogas steam reforming process

In this work, a dynamic 2D heterogeneous model for catalytic hydrogen production by biogas steam reforming process was developed, considering both the macroscale and the microscale of the system. The partial differential equations (which describe mass and energy balances) of the mathematical model, have been converted into total differential equations, by spatial discretization and implemented in Matlab/Simulink for process simulation. Based on simulation results, the profiles for mass and concentration of the reactants and products were obtained. The developed model has been validated with available literature data. For the conversion of methane, it predicted 72.43% and for the hydrogen percentage on the dry basis, it gave the value of 72.72%. In this study, a special emphasis was placed on highlighting the influence of a high content of carbon dioxide in biogas on the size of the reactor. The developed model is used to study the effects of step and ramp flow inputs disturbances for the methane conversion and hydrogen percentage on the dry basis and to observe how the reactor design parameters change when the CO2 concentration in the feed flow increases. Furthermore, the developed model for biogas reforming was integrated in a green hydrogen plant with CO2 capture feature for overall performance evaluation (e.g., energy efficiency, CO2 capture rate etc.). The decarbonized green hydrogen plant has promising performance indicators (high overall energy efficiency of 54.5% and a high CO2 capture rate 70%).

A novel technique of heat transfer enhancement in backward-facing step flow using a flapping vortex generator

A novel technique of heat transfer enhancement in backward-facing step flow using a flapping vortex generator

Influence of Pulse Atomic-Layer Epitaxy (PALE) AlN Buffer Layer on Quality of MOCVD Grown GaN on Si(111) Substrate.

A crack-free GaN-on-Si with a high capability to grow a thick GaN layer is important for the performance of optoelectronic devices. Regrettably, a large lattice and thermal mismatch degraded the quality of GaN-on-Si. Therefore, we introduce the pulse atomic-layer epitaxy (PALE) AlN buffer layer to overcome the surface crack issues and improve the quality of GaN epilayers. The effect of PALE AlN buffer layer with 0, 35, 70, and 140 cycle numbers towards the quality of grown GaN on Si(111) substrate is studied. All samples have been observed using AFM, FESEM, XRD and Raman spectroscopy. AFM imaging revealed a fully-coalesced and clear step flow GaN layer with the lowest RMS roughness of 0.606 nm. FESEM surface morphology shows a crack occurs on a sample without AlN PALE (0 pulse cycle) while crack-free GaN is obtained for samples with AlN PALE layer (35, 70 and 140 cycles). It was observed by XRD that the AlN PALE pulse cycle number greatly affects the structural properties of the top GaN layer where the lowest x-ray rocking curve for (002) and (102) achieved at 70 cycles, indicating the reduction of threading dislocations density in the growth structure. Further, the transition of the GaN E2 (high) from the Raman spectroscopy revealed a tensile strain in the GaN epilayers reduces with the increasing number of pulse cycles.

Reduction of dislocation density in single crystal diamond by Ni-assisted selective etching and CVD regrowth

The development of large-area diamond films with low dislocation density plays a key role in diamond-based devices. In this study, we present a straightforward and reasonably priced Ni-assisted etching and re-growth method to decrease the dislocation density of diamond. By adjusting the annealing temperature, a high-density inverted pyramid and/or trapezoidal pit can be effectively exposed because the interaction between carbon and Ni metal. The Ni tends to dissolve the diamond surrounding the dislocation at high temperature while maintaining the surface's flatness. The precipitated graphite layer is removed using wet etching, and the step flow mode is then restored via the re-growth procedure. It shows that the dislocations located beneath the etching pits are partially annihilated during the lateral coalescence, resulting in a decrease in dislocation density.

LCA Case Study of Ceramic Tableware: Ecodesign Aspects of Ceramics Production from Ancient Technology to Present Factory

Ceramic tableware, as one of the products of daily use, can be produced in multiple ways. However, they all consume primary raw materials for manufacturing and energy for firing and generate emissions and waste. To assess how different types of production cause an environmental impact, five scenarios were compared using the Life Cycle Assessment (LCA). In the first step, the energy and material flows from the following scenarios were analysed: factory (FAC), slip casting (PSC), pottery wheel (PW), high energy rate formation technique (HERF) and experimental reconstruction of the ancient technique (ANC). The results related to 1 kg of ceramics were interpreted using the perspective of ecodesign to discuss key parameters that affect environmental impacts. Considering the results in the Climate Change category, the FAC production of 1 kg of ceramics reached 3.64 kg CO2 eq. but the highest impact in this category was caused by low energy efficiency in the specific case of the PSC scenario (8.93 kg CO2 eq.). On the one hand, this paper contributes to the discussion of environmentally friendly tableware, but also serves as a case study that describes the implementation of ecodesign in the ceramic tableware industry as an important field of industrial production in the specific context of the Czech Republic, as a state, which has significant deposits of kaolin.

Epitaxial Growth of Boron Carbide on 4H-SiC

In this work, the successful heteroepitaxial growth of boron carbide (BxC) on 4HSiC(0001) 4° off substrate using chemical vapor deposition (CVD) is reported. Towards this end, a two-step procedure was developed, involving the 4H-SiC substrate boridation under BCl3 precursor at 1200°C, followed by conventional CVD under BCl3 + C3H8 at 1600°C. Such a procedure allowed obtaining reproducibly monocrystalline (0001) oriented films of BxC with a step flow morphology at a growth rate of 1.9 μm/h. Without the boridation step, the layers are systematically polycrystalline. The study of the epitaxial growth mechanism shows that a monocrystalline BxC layer is formed after boridation but covered with a B-and Si-containing amorphous layer. Upon heating up to 1600°C, under pure H2 atmosphere, the amorphous layer was converted into epitaxial BxC and transient surface SiBx and Si crystallites. These crystallites disappear upon CVD growth.

Coherent structures in the turbulent stepped cylinder flow at [formula omitted

The coherent structures arising in the turbulent flow around a three-dimensional stepped (or step) cylinder are studied through direct numerical simulation. This geometry is widespread in many applications and the junction substantially modifies the wake behaviour, generating three main cells. The mechanisms of vortex connections on the junction are difficult to be captured and interpreted. We thus use a high-order spectral-element methodology (SEM), and the adaptive mesh refinement technique (AMR) to adequately resolve each region of the domain, capturing the smallest turbulent scales. In this way, we can analyse the vortical interactions on the junction via the λ2-criterion and understand the evolution of the train of hairpins, which appears only when the cylinder shear layer gets unstable. Together with the hairpins, four horseshoe and edge vortices coexist on the flat junction surface. A complete picture of the vortices’ evolution in time is provided. To extract the large-scale, and most energetic, structures in the wake we perform a three-dimensional proper orthogonal decomposition (POD) of the flow. The first six POD modes correspond to three travelling modes which identify the large (L), the small (S) and the modulation (N) cells. The ReD trend shows that these cells persist at higher Reynolds numbers with a larger separation between the vortex shedding frequencies fN and fL. At the same time, the downwash POD mode gets less strong with a more intense and localised modulation region which affects a more extended portion of the large cylinder wake.

Investigation of carbon incorporation in laser-assisted MOCVD of GaN

Background carbon (C) impurity incorporation in metalorganic chemical vapor deposition (MOCVD) grown gallium nitride (GaN) represents one of the major issues in further improving GaN vertical power device performance. This work presents a laser-assisted MOCVD (LA-MOCVD) technique to address the high-C issue in MOCVD homoepitaxial GaN under different growth rate (Rg) regimes and studies the correlations between [C] and Rg. [C] in LA-MOCVD GaN is reduced by 50%–90% as compared to the conventional MOCVD GaN for a wide growth rate range between 1 and 16 μm/h. A mass-transport based model is developed to understand the C incorporation at different Rg regimes. The results obtained from the developed model are in good agreement with experimental data. The model further reveals that LA-MOCVD effectively suppresses C incorporation by reducing the active C species in the gas phase. Moreover, high step velocity in step flow growth mode can facilitate C incorporation at fast Rg, exhibiting steeper C increase. The theoretical model indicates that [C] can be suppressed below 1016 cm−3 with a fast growth rate (Rg) of 10 μm/h by utilizing higher power LA-MOCVD and freestanding GaN substrates with larger off-cut angles.

Step-bunching Instability of Growing Interfaces between Ice and Supercooled Water

We review our recent study on step-bunching instability (SBI) of ice melt growth, one of the interfacial instabilities driven by self-organization of elementary step flow associated with crystal growth dynamics. With the aid of an advanced optical microscope and its combination with a two-beam interferometer, we realized quantitative in situ observations of the spatiotemporal dynamics of the SBI. This enables us to examine the origin of the SBI at the level of the step–step interaction. Our finding offers a significant clue to understanding the general mechanism of melt growth beyond ice-crystal growth, inseparably involving several broad research fields, including cryobiological, geophysical, and material branches.

Modeling Catalyst-Free Growth of III-V Nanowires: Empirical and Rigorous Approaches

Catalyst-free growth of III-V and III-nitride nanowires (NWs) by the self-induced nucleation mechanism or selective area growth (SAG) on different substrates, including Si, show great promise for monolithic integration of III-V optoelectronics with Si electronic platform. The morphological design of NW ensembles requires advanced growth modeling, which is much less developed for catalyst-free NWs compared to vapor-liquid-solid (VLS) NWs of the same materials. Herein, we present an empirical approach for modeling simultaneous axial and radial growths of untapered catalyst-free III-V NWs and compare it to the rigorous approach based on the stationary diffusion equations for different populations of group III adatoms. We study in detail the step flow occurring simultaneously on the NW sidewalls and top and derive the general laws governing the evolution of NW length and radius versus the growth parameters. The rigorous approach is reduced to the empirical equations in particular cases. A good correlation of the model with the data on the growth kinetics of SAG GaAs NWs and self-induced GaN NWs obtained by different epitaxy techniques is demonstrated. Overall, the developed theory provides a basis for the growth modeling of catalyst-free NWs and can be further extended to more complex NW morphologies.

Structural optimization to improve the dynamic performance of turbine flowmeters

It is of great significance to study the dynamic performance of flowmeters to improve their measurement accuracy in complex working conditions. The dynamic performance of the turbine flowmeter is studied by experiments and numerical simulations under step flow and pulsatile flow conditions. The results suggest that the mass and length of the impeller, the number of blades, the blade angle, and the tip radius of the impeller are the main factors affecting the dynamic performance of the turbine flowmeter. An optimized scheme is proposed based on the analysis results with the feasibility verified by CFD simulations and experiments. The results show that the dynamic performance of the turbine flowmeter is greatly improved after structural optimization. The step response time constant of the optimized model is reduced to 50 % of that of the original model, and the cutoff frequency of the amplitude-frequency characteristic is increased by 87.7 % under pulsatile flow conditions. The overall measurement errors for the pulsatile flow experiments are within 2 %.

InGaN surface morphology evolution investigated by atomic force microscope with power spectral density analysis

The morphology of thin InGaN layers grown at varying temperature by metalorganic chemical vapor deposition (MOCVD) has been studied by atomic force microscope (AFM). The surface morphology undergoes a transition process from three-dimensional (3D) island growth mode to two-dimensional (2D) nucleated mound and hillock growth mode and finally to layer-by-layer step-meandering and step flow growth mode. The changes in morphology were analyzed from a thermodynamic (mostly based on the BCF theory of crystal growth by Burton, Cabrera and Frank) and kinetic (mostly the ESB theory based on the Ehrlich-Schwöbel barrier) perspective. Power spectral density (PSD) analysis of AFM surface images is used to determine the dominant smoothing mechanisms that contribute to the feature of surface morphology of InGaN, and validate our analysis based on BCF and ESB theories.

Using pressure-driven flow systems to evaluate laser speckle contrast imaging.

Microfluidic flow phantom studies are commonly used for characterizing the performance of laser speckle contrast imaging (LSCI) instruments. The selection of the flow control system is critical for the reliable generation of flow during testing. The majority of recent LSCI studies using microfluidics used syringe pumps for flow control. We quantified the uncertainty in flow generation for a syringe pump and a pressure-regulated flow system. We then assessed the performance of both LSCI and multi-exposure speckle imaging (MESI) using the pressure-regulated flow system across a range of flow speeds. The syringe pump and pressure-regulated flow systems were evaluated during stepped flow profile experiments in a microfluidic device using an inline flow sensor. The uncertainty associated with each flow system was calculated and used to determine the reliability for instrument testing. The pressure-regulated flow system was then used to characterize the relative performance of LSCI and MESI during stepped flow profile experiments while using the inline flow sensor as reference. The pressure-regulated flow system produced much more stable and reproducible flow outputs compared to the syringe pump. The expanded uncertainty for the syringe pump was 8 to higher than that of the pressure-regulated flow system across the tested flow speeds. Using the pressure-regulated flow system, MESI outperformed single-exposure LSCI at all flow speeds and closely mirrored the flow sensor measurements, with average errors of and , respectively. Pressure-regulated flow systems should be used instead of syringe pumps when assessing the performance of flow measurement techniques with microfluidic studies. MESI offers more accurate relative flow measurements than traditional LSCI across a wide range of flow speeds.