Zigzag Channels Research Articles

Topological Strain-Induced Regioselective Linker Elimination in a Chiral Zr(IV)-Based Metal-Organic Framework

Summary Zr-carboxylate metal-organic frameworks (MOFs) are structurally robust materials, in part due to their strong coordination bonds. The regioselective Zr–O bond cleavage and formation between 3D architectures are thus challenging and are heretofore unexplored. In this work, by introducing highly flexible 18-crown-6-ether functionalities into a homochiral Zr-MOF, we report an unprecedented topology transition in which a 4,10-connected framework undergoes a rapid solid-state transition into a thermodynamically more stable 4,8-connected analog by a regioselective-linker-elimination under ambient conditions. The transition process was unambiguously unraveled by single-crystal and powder X-ray diffraction studies, and we proposed a possible transition mechanism based on various control experiments and theoretical calculations. The excellent chemical stability and substantially expanded porosity and pore apertures endowed the transformed chiral MOF with an exceptional capacity for the enantioadsorptive and solid-phase extractive separation of the racemic drug molecule of lansoprazole with 98% ee and 93% ee, respectively.

Numerical study of subfreezing temperature cold start of proton exchange membrane fuel cells with zigzag-channeled flow field

An automotive proton exchange membrane fuel cell (PEMFC) engine needs to be capable of surviving from sub-freezing temperatures. Cold starting up a PEMFC engine presents great technical challenge due mainly to the intra-electrode ice formation that is closely related to the involved multi-physical transport. The flow field configuration to some extent dictates the reactants and product transport and is thus of important influence on PEMFC cold start performance. The present work carefully analyzes the cold start process of PEMFCs with zigzag-channeled flow field (ZZFF). A transient three-dimensional numerical model is established and experimentally validated first, and then we make comparisons of cold-start performance between zigzag and straight channel flow-field PEMFCs to reveal the effects of ZZFF. It is found that the ZZFF better distributes the reactants/product and current density in the PEMFC as it enhances the transport along the flow direction in-between the under-land and under-channel regions inside the cell. The process of cold starting the ZZFF PEMFC is found to generate slightly less heat, which is not good to cold start. Therefore, the ZZFF enhances the survivability of PEMFC engine cold starting from subfreezing temperatures, whereas it slightly degrades the quick self-startup ability of PEMFC engine. Case study with PEMFCs of more zigzagged flow fields further corroborates the effects of ZZFF configuration on PEMFC cold start performance.

Impact of non-Newtonian fluid behavior on hydrodynamics and mass transfer in spacer-filled channels

Spiral wound membrane modules operate as filtration modules in the food, beverage, and pharmaceutical industries. As many fluids are non-Newtonian or become non-Newtonian during concentration, it is essential to gain an understanding of the effect of a shear-rate dependent viscosity. This computational work examines the impact of the fluid rheology and the Reynolds number on the hydrodynamics and the mass transfer in zigzag and cavity spacer-filled channels under laminar flow conditions. To describe the rheology of the working fluid, we utilized the well-known power-law model. The power-law index values corresponded to 0.7, 1, and 1.3, representing shear-thinning, Newtonian, and shear-thickening behavior, respectively. The Reynolds number was varied up to 160 whereas the Schmidt number was kept constant at 500. A standard convection–diffusion equation with a constant wall concentration was employed to describe the mass transfer. In agreement with previous works on Newtonian fluids, the zigzag spacer was found to be the most promising spacer type, because of the enhanced mass transfer in the reattachment regions. Whereas the mass transfer was only slightly affected by the rheology, the shear thinning fluid produced the smallest pressure drop. As a direct consequence of the positive effect on the pressure drop, the spacer efficiency was highest in all configurations. The results of this first numerical investigation in the context of non-Newtonian fluid flow through spacer-filled channels emphasize the importance of taking into account a varying fluid viscosity in the design and operation of the filtration process.

Experimental investigation on heat transfer characteristics of flow boiling in zigzag channels of printed circuit heat exchangers

To optimize printed circuit heat exchangers, the experimental heat transfer characteristics of two-phase flow boiling in zigzag channels of PCHE were investigated. The experimental conditions cover mass fluxes of 100~400 kg /(m²•s), vapor qualities of 0.1~1.0, heat fluxes of 1.5~12.5 kW/m², and saturation pressures of 0.15~0.35 MPa. The research results show that, the heat transfer coefficient initially increases and then decreases as the vapor quality increases, representing a maximum at vapor quality of 0.85~0.9; as the heat flux increases, the heat transfer coefficient increases at a vapor quality < 0.35; the critical conditions of slug-annular transition and annular-dryout transition are corresponding to the void fractions of 0.96 and 0.994; as the saturation pressure decreases from 0.35 to 0.15 MPa, the heat transfer coefficient is intensified by 16.7%. A new correlation of two-phase heat transfer coefficients for flow boiling in zigzag channels was developed with a deviation within ±25%.

Label-free Separation of Circulating Tumor Cells Using a Self-Amplified Inertial Focusing (SAIF) Microfluidic Chip.

Circulating tumor cells (CTCs) are rare cells existing in the bloodstream with a relatively low number, which facilitate as a predictor of cancer progress. However, it is difficult to obtain highly purified intact CTCs with desired viability due to the low percentage of CTCs among blood cells. In this work, we demonstrate a novel self-amplified inertial focused (SAIF) microfluidic chip that enables size-based, high-throughput, label-free separation of CTCs from a patient's blood. The SAIF chip introduced in this study demonstrated the feasibility of an extremely narrow zigzag channel (with 40 μm channel width) connected with two expansion regions to effectively separate different-sized cells with amplified separation distance. The chip performance was optimized with different-sized polystyrene (PS) particles and blood cells spiked with three different types of cancer cells. The separation efficiencies for blood cells and spiked cancer cells are higher than 80%. Recovery rates of cancer cells were tested by spiking 1500 lung cancer cells (A549), breast cancer cells (MCF-7), and cervical cancer cells (HeLa) separately to 3 mL 0.09% saline with 3 × 106 white blood cells (WBCs). The recovery rates for larger cells (MCF-7 and HeLa) were 79.1 and 85.4%, respectively. Viabilities of the cells harvested from outlets were all higher than 97% after culturing for 24, 48, and 72 h. The SAIF chip performance was further confirmed using the real clinical patient blood samples from four lung cancer patients. Theoretical force balance analysis in physics, computational simulations, and experimental observations indicate that the SAIF chip is simple but effective, and high-throughput separation CTCs can be readily achieved without complex structures.

Transport of finite size self-propelled particles confined in a [formula omitted] zigzag channel with Gaussian colored noise

The transport of finite size self-propelled Brownian particles confined in a 2D zigzag channel with colored noise is investigated. The noises(noise parallel to x-axis and y-axis), the asymmetry parameter Δk, the ratio f(ratio of the particle radius and the bottleneck half width), the self-propelled speed v0 have joint effect on the particles. The average velocity 〈V〉 and diffusion coefficient D of self-propelled particles are significantly different from passive particles. 〈V〉 and D exhibit complicated behavior with increasing asymmetry parameter Δk.

Labyrinthine Structure with Subwavelength and Broadband Sound Insulation

In this text, the combination of spiral structure and zigzag channels is introduced to design labyrinthine structures, in which sound waves can propagate alternately in the clockwise and counterclockwise directions. Finite element method and S-parameter retrieval method are used to calculate band structures, effective parameters, and transmission properties of the structures. The influences of different structural parameters on their acoustic properties are also studied. These results show labyrinthine structures have multiple bandgaps in the range of 0 Hz–1000 Hz, and the proportion of bandgaps exceeds 33%, which indicates labyrinthine structures have good broadband properties. The normalized frequency of the lowest bandgaps is far smaller than 1, which indicates the structures take good control of sound waves on subwavelength scale. Combining units with different structural parameters can achieve better sound insulation. This research provides a new kind of space-coiling structure for low-frequency and broadband sound waves control, which have excellent application prospects.

Local Flow and Heat Transfer of Supercritical CO2 in Semicircular Zigzag Channels of Printed Circuit Heat Exchanger during Cooling

Understanding and predictions of the local thermal-hydraulic performance of supercritical CO2 in channels of printed circuit heat exchanger geometries are of great importance to the design and optimization of the pre-cooler and recuperators in supercritical CO2 cycle. The local flow and heat transfer of supercritical CO2 in horizontal semicircular zigzag channel during cooling near the critical or pseudo-critical point have been numerically investigated in this paper. The effects of thermophysical property variations, mass flux, and the channel geometrical parameters on the flow and heat transfer are discussed. It is found that the extrude angle in the zigzag channel impedes the bulk flow and generates secondary flow, which induces asymmetric flow and temperature distribution, and enhances fluid mixing and mitigates the temperature stratification. A dimensionless secondary flow velocity is adopted to evaluate the influence of secondary flow on the heat transfer, which is more enhanced for cases with relatively large mass flux and in channels with relatively large inclined angle and small pitch length, however, the pressure drop increases meanwhile. Finally, pitch-averaged local flow and heat transfer correlations have been developed based on the numerical results and mechanism analysis accounting for the thermophysical property variations and the channel geometrical parameters.

Optimization and heat transfer correlations development of zigzag channel printed circuit heat exchangers with helium fluids at high temperature

The printed circuit heat exchanger (PCHE) is an important candidate to be used as intermediate heat exchanger (IHX) in a High Temperature Gas-cooled Reactor (HTGR) plant due to its advantages in terms of safety, heat transfer and compactness. In this work, an analysis of the thermal-hydraulic performance of the zigzag channel PCHE within wide parameter ranges was conducted using computational fluid dynamics (CFD) techniques. The Nusselt number and the Fanning friction factor obtained from simulation were validated using experimental data available. To consider the interaction among parameters with a reduced computing time, the Taguchi method was used, which reduces the quantity of analyzed geometric designs. Optimized designs were proposed to reach the highest Nusselt number and lowest friction losses using the signal to noise ratio as objective function. It was found that the zigzag angle has contribution ratios around 50% and 70% on the values of pressure drop and heat transfer, respectively. Although the zigzag phase-shift was the parameter of least influence, its contribution ratio was close to 10% in the turbulent regime, which is comparable to the zigzag pitch length and bend radius contributions. As the most important contribution, Nusselt number and Fanning friction factor correlations were proposed as functions of the Reynolds number and the geometrical parameters. The new correlations were found to be more accurate as compared to existing ones in the literature.

Thermo-hydraulic analysis of compact heat exchanger for a simple recuperated sCO2 Brayton cycle

Supercritical carbon dioxide (sCO2) Brayton cycle-based power plants are being extensively explored as viable alternatives to traditional Rankine based steam power plants. Higher temperatures at the turbine exit in a sCO2 cycle provide better opportunities for heat recuperation, thus improving overall cycle efficiency. Among the various heat exchanger configurations available, compact microchannel heat exchangers commonly known as Printed Circuit Heat Exchangers (PCHE's) are typically used in sCO2 power plants. In the current work, a hybrid approach comprising of a Thermal Resistance Network (TRN) model coupled with a CFD model for estimating local heat transfer and pressure drops is presented for a PCHE core with straight and zigzag channel configurations. Full-scale TRN model is used to optimize the overall stack dimensions based on minimum rate of heat loss from the external surfaces of the PCHE core. The TRN model accounts for the thermo-physical property variations of sCO2 along the channel length to effectively capture the channel pressure drop and heat transfer. This is achieved by discretizing the heat transfer domain comprising of alternatively stacked hot and cold streams into sub-heat exchangers to account for variations in thermophysical properties while calculating the nodal friction factors and local heat transfer coefficients. CFD simulations are performed for a full length of a single stack of hot and cold fluid streams to arrive at corrected heat transfer and pressure drop correlations. Thermo-hydraulic analysis is performed for a range of channel hydraulic diameters and channel mass flow rates for both straight and zig-zag configurations to deduce optimum stack dimensions. The efficacy of the hybrid model is demonstrated with a case study of a counterflow recuperator used in a simple recuperated 1 MW sCO2 Brayton power plant.

DFT Study of Si/Al Ratio and Confinement Effects on the Energetics and Vibrational Properties of some Aza-Aromatic Molecules Adsorbed on H-ZSM-5 Zeolite

The Si/Al ratio and confinement effects of zeolite framework on energetics and vibrational frequencies of pyridine and 4,4′-bipyridine adsorbed on Brønsted acid sites in the straight channel of H-ZSM-5 are investigated by DFT calculations at the B3LYP and M06-2X+D3 levels. The straight channel of H-ZSM-5 is simulated by a cluster of 32 tetrahedral centers covering the intersection between straight and zigzag channels. Pyridine and 4,4′-bipyridine adsorption at two different sites in the intersection (open region) and/or in the narrow region situated between two intersections (closed region) is studied. For two Si/Al ratios (31, 15), the ion pair complexes formed by proton transfer upon pyridine and 4,4′-bipyridine adsorption in the open region and for the first time in the closed region are characterized. Our results indicate: (i) the stability for all adsorption complexes is essentially governed by the dispersive van der Waals interactions and the open region is energetically more favorable than the closed region owing to the predominance of the dispersive interactions over the steric constraints exerted by the confinement effects; (ii) as the Al centers are sufficiently spaced apart, Si/Al ratio does not influence pyridine adsorption energy, but significantly affects the adsorption energies and the relative stability of 4,4′-bipyridine complexes; (iii) neither Si/Al ratio nor confinement significantly influence pyridine and 4,4′-bipyridine vibrational frequencies within their complexes.

Numerical investigation of thermal and hydraulic characteristics of sCO2-water printed circuit heat exchangers with zigzag channels

Since the precooler and the recuperator are the largest components of a supercritical carbon dioxide Brayton cycle, their design can substantially affect the performance and size of the whole system. Although the design of a precooler with zigzag channel geometry as an alternative to straight channels can reduce its size significantly, the applicability of available correlations (e.g. 0.7 < Pr<2.2) for the zigzag channel geometries is limited to the operational range of recuperators only. The current study, therefore, aims to develop correlations and understating of the complex flow and heat transfer characteristics in the zigzag channel printed circuit heat exchangers (PCHEs) operating under precooler conditions (2.2<Pr<13) of supercritical carbon dioxide Brayton cycle(sCO2-BC). Thermal and hydraulic characteristics of the PCHEs are computed numerically for a wide range of Reynolds numbers (5000<Re<7000) and Prandtl number (2.2<Pr<13). Also, a new data reduction method based on segmental averaged values has been proposed to handle adverse variations in the thermophysical properties of CO2 under precooler conditions. To ensure accurate evaluations, steep variations in the thermophysical properties of CO2 are implemented by supplying high-resolution real gas (RGP) property tables. Results suggest that thermal and hydraulic characteristics associated with zigzag channel vary substantially along the length of heat exchanger thus the conventional data reduction methods based on the channel average values cannot be used for true evaluations. Instead, segmental average values are used to develop pressure drop and heat transfer correlations for a broader range of Reynolds number and Prandtl number. The proposed correlations should be useful in the design of compact heat exchanger systems using zigzag channels for a wider range of cooling loads.

Introducing new designs of minichannel cold plates for the cooling of Lithium-ion batteries

Seven different new designs of minichannel cold plates (MCPs) are proposed in this work for the cooling of Li-ion battery modules discharging at high C-rates. As a proof of concept, detailed numerical investigations are carried out on the seven designs, considering a prismatic battery module consisting of five 45 Ah Li-ion batteries discharging at 3C rates. The aim is to figure out the role of profile of channels on the cooling performance of the MCPs. The thermal and hydraulic performances of each of the designs are studied systematically in detail and compared with that of an MCP having straight rectangular channels. All the designs proposed results in a better performance than the basic straight rectangular channels and are capable of cooling the batteries to below 40 °C under high discharging conditions. Among the designs, MCPs with zig-zag and circular slot channels exhibit an exceptional cooling performance.

A new Tl4.86Fe0.82Hf1.18(MoO4)6 ternary molybdate: crystal structure and properties.

Single crystals of Tl4.86Fe0.82Hf1.18(MoO4)6 [a = b = 10.5550 (3), c = 37.7824 (9) Å, γ = 120°] are obtained by the self-flux method in the Tl2MoO4-Fe2(MoO4)3-Hf(MoO4)2 system. On the differential scanning calorimetry curve in the temperature range 320-350 K and at T ∼ 690 K, endothermic peaks are observed. The second harmonic generation test shows an excess of the signal of the quartz standard by almost three times at room temperature. In the range 320-340 K its intensity decreases by almost three times and at T ∼ 700 K it drops to zero. In the same interval, the temperature dependences of the unit-cell parameters and volume show stepwise changes. According to the X-ray diffraction data, the crystal structure consists of nonpolar and polar domains with different local symmetries. The structure is a three-dimensional framework consisting of alternating (Hf,Fe)O6 octahedra connected by MoO4 tetrahedra. Hf and Fe atoms occupy mixed Hf/Fe positions with different probabilities: 0.77:0.23, 0.50:0.50 and 0.32:0.68. Tl cations are located inside the framework in zigzag channels extended along the a and b axes. The thallium arrangement is disordered, i.e. it involves additional positions and vacancies. The complex crystal structure has been solved using the nonstandard space group R1, taking into account the local symmetry R3c for the Mo atoms and mixed Hf/Fe positions mainly occupied by Hf atoms. The possible paths of ion transport are analyzed. The energy required to overcome the potential barrier between sites of Tl cations to migrate, which corresponds to the activation energy of conductivity, is estimated. The ion current is shown to be most probable in the ab plane.

Aharonov-Bohm Oscillations in Minimally Twisted Bilayer Graphene.

We investigate transport in the network of valley Hall states that emerges in minimally twisted bilayer graphene under interlayer bias. To this aim, we construct a scattering theory that captures the network physics. In the absence of forward scattering, symmetries constrain the network model to a single parameter that interpolates between one-dimensional chiral zigzag modes and pseudo-Landau levels. Moreover, we show how the coupling of zigzag modes affects magnetotransport. In particular, we find that scattering between parallel zigzag channels gives rise to Aharonov-Bohm oscillations that are robust against temperature, while coupling between zigzag modes propagating in different directions leads to Shubnikov-de Haas oscillations that are smeared out at finite temperature.

FDTD analysis of distribution line voltages induced by non-vertical lightning

This paper investigates lightning induced over-voltages on overhead lines considering “bent” and “computationally-generated” non-vertical lightning channels by using a finite-difference time-domain (FDTD) method. In the former case, combinations of vertical and inclined channels with different connecting heights are modeled to represent “bent” lightning. It is made clear that the peak voltage is significantly influenced by the lightning channel geometry under 100-m altitude when severe conditions of a 1/200-μs current and a lightning distance of 50 m are assumed. Induced voltages on the three-phase line show similar characteristics to those on the single conductor line. In the latter case, a lightning-like (zig-zag) channel is computationally generated by a probabilistic calculation based on an electric potential distribution in a three-dimensional space, and its induced voltage is compared with that by a simply-inclined channel. When average inclined angles under 100-m altitude in the computed channel are set to the angles for a simply inclined channel, the induced voltages show good agreement. The difference is less than 10% and it decreases as the earth resistivity increases. These results indicate that realistic non-vertical lightning can be represented by simply-inclined lightning by considering average inclined angles in low altitude.

Effect of printed circuit heat exchanger’s different designs on the performance of supercritical carbon dioxide Brayton cycle

Several fin configurations have been proposed in the literature to address the poor hydraulic performance associated with the PCHEs. However, the effect of the heat exchangers with proposed channel geometries on the performance of supercritical carbon dioxide(sCO2) power cycle is missing. In this context, the current study deals with the effects of different designs of the PCHEs varied by proposed channel configurations, heat exchanger’s effectiveness and design value of inlet Reynolds number on the performance of sCO2 power cycle. Moreover, a multi-object optimization study to find the best bargain between cycle’s efficiency and heat exchanger’s size is carried out using five different fin configurations (straight, zigzag, C-shaped, S-shaped, and airfoil fin channel configuration), heat exchanger’s effectiveness and inlet Reynolds number as a design variable. Results shows that enhancement in the hydraulic characteristics for a channel geometry that comes at the cost of thermal performance may not benefit the system’s efficiency. Optimization results suggest that C-shaped channel and zigzag channel geometries correspond to the cycle’s maximum efficiency and heat exchanger’s minimum size respectively. Optimization results further highlight that the comparison of channel geometries should be performed while in the setting of complete power generation cycle to account for all the variables involved.

Direct Crystallographic Observation of CO2 Captured in Zig Zag Channels of a Copper(I) Metal-Organic Framework.

Single crystal X-ray diffraction has been used to study the CO2 absorption sites in a microporous Cu-MOF, [CuI2(py-pzpypz)2(μ-CN)2]n (1) (where py-pzpypz = 4-(4-pyridyl)-2,5-dipyrazyl-pyridine), which features zigzag-shaped channels, at a range of CO2 pressures (1, 5, and 10 bar) and at two temperatures (240 and 298 K). Unlike the acetonitrile molecules in the as-synthesized MOF, 1·MeCN, the CO2 molecules in 1·nCO2 (n = 0.8, 0.7, 0.45) are preferentially centered on the vertices of each zig and zag, which allows for weak (azine) C-H···OCO interactions with the H atoms on the electron-deficient pyrazine and pyridine rings of the MOF.

Labyrinthine acoustic metastructures enabling broadband sound absorption and ventilation

There is growing interest in the development of path coiling-based labyrinthine acoustic metamaterials for realizing extraordinary acoustical properties such as low-to-mid frequency sound absorption. We present a subwavelength labyrinthine acoustic metastructure (≤3 cm) exhibiting a superior sound absorption with a high bandwidth (more than one octave in the range of 400–1400 Hz). The metastructure is orchestrated of multiple labyrinthine unit cells of different configurations in a hexagonal array, and broadband absorption has been achieved by the dissipation of incident propagating sound waves inside the labyrinthine zigzag channels. Furthermore, the unique design of the metastructure allows for simultaneous air circulation for facilitating natural ventilation and sound absorption. The proposed unique designs may find potential applications in architectural acoustics and noise shielding where simultaneous natural ventilation and noise mitigation are required.

Thermal performance of a zig-zag channel formed by two wavy fins mounted with vortex generators

Wavy fins and vortex generators can separately enhance the thermal performance of heat exchangers. A novel combination of wavy fins and vortex generators is proposed in the present study to optimize the thermal performance of heat exchangers. The effects of the longitudinal vortices and attack angle of the vortex generators on the thermal performance of the novel combination are numerically studied. The longitudinal vortices are strengthened and weakened when the fluid flows over the crests and troughs of the bottom wall, respectively. The heat transfer on the pressure sides of both top and bottom wall surfaces is significantly larger than that on the corresponding suction sides. There is an optimal attack angle of 60°, which can provide the best heat transfer performance with a maximum 31.5% increase in the Nusselt number with only a 23.4% increase in the friction factor. The maximum thermal performance factor can be up to 1.23 with the optimal attack angle. Correlations for Nu and f are reported for both laminar and turbulent flows.