Channel Height Research Articles

Heat and mass transfer of water flow in graphene nanochannels: effect of pressure and interfacial interaction.

The low-resistance transport of water within graphene nanochannels makes it promising for electronic cooling applications. However, how the water pressure and the water-graphene interaction strength affect the flow field and the thermal transfer, including velocity slip, friction coefficient, Nusselt number, temperature slip, interfacial thermal resistance, and variation of physical properties, is still not clearly understood. In this paper, we employ molecular dynamics (MD) simulations to investigate qualitatively the heat transfer of water flow in graphene nanochannels. Our results reveal that the water peak density near the wall increases approximately linearly with water pressure and water-graphene interaction strength. The water peak density near the water-graphene interface is a key factor in regulating interfacial flow and heat transfer characteristics. Under constant inlet temperature, the relationship between velocity slip length and peak density follows a consistent power function, simply modifying the pressure or the interaction strength doesn't bring a specific effect. The Nusselt number and interfacial thermal resistance are not solely dependent on water peak density; at the same water peak density, increasing interaction strength results in lower interfacial thermal resistance compared to increasing pressure. Increasing pressure improves both interfacial heat transfer and internal heat transfer of water. Furthermore, the convection heat transfer coefficient increases approximately linearly with flow resistance when pressure and interaction strength vary moderately. Finally, we notice that pressure and interaction strength hardly affect the variation range of viscosity and thermal conductivity at a channel height of 10-12 nm. These qualitative insights could lead to the development of more efficient cooling systems for electronic devices.

Manipulation of Microparticles in Optofluidic Devices Fabricated by Femtosecond Laser Micromachining

This study reports the fabrication of three-dimensional microfluidic channels in fused silica, using femtosecond laser micromachining, to achieve two-dimensional hydrodynamic flow focusing in either the horizontal or the vertical directions. Spatial focusing of 3 μm polystyrene particles was successfully demonstrated, showing the ability of the fabricated devices to confine microparticles within a 6 μm layer over a channel width of 420 μm and within a 5 μm layer over a channel height of 260 μm. Integration of laser-direct written optical waveguides inside a microfluidic chip and orthogonal to the channel also enabled the implementation of a dual-beam optical trap, with trapping of polystyrene microparticles using a 1550 nm beam being demonstrated.

Improvement of Thermal Characteristics and On-Current in Vertically Stacked Nanosheet FET by Parasitic Channel Height Engineering

Improvement of Thermal Characteristics and On-Current in Vertically Stacked Nanosheet FET by Parasitic Channel Height Engineering

Two-phase imbibition of water-oil displacement in silica nanochannels

Classical imbibition theory cannot describe imbibition flow dynamics in nanoporous media in tight reservoirs and other industrial applications. In this work, the two-phase imbibition flow of water-oil displacement in silica nanochannels is studied by using molecular dynamics simulations. Interestingly, it is found that the two-phase imbibition length is shown to increase linearly with increasing time, which is inconsistent with the variation of single-phase imbibition length. The imbibition rate increases with the increase of external driving force, and the large channel height can accelerate the two-phase imbibition flow. As the increase of chain length of alkane molecules, the imbibition rate reduces gradually. The more oil-wet the silica surface is, the slower the imbibition rate is. Furthermore, we derive a theoretical model to describe the two-phase imbibition flow of water-oil displacement in nanochannels by considering the static force equilibrium of external driving forces, capillary forces, and viscous forces of the water and oil phases. The theoretical model can well describe the two-phase imbibition flow under various conditions using the pre-calculated oil-water interfacial tension, the viscosity of fluids, and the three-phase contact angle. This study will enrich the theoretical understanding of oil-water two-phase flow at nanoscale.

Fibrous catalyst flexibility as a source of mass transfer intensification

The work is devoted to computational fluid dynamics (CFD) simulation of structured cartridges with glass-fiber catalysts (GFCs). The CFD simulation of the model reaction of toluene deep oxidation at 350 °C in the cartridge with corrugated and plain structuring meshes (channel height 5.7 mm, width 8.0 mm, GFC length 44 mm) and sateen-type Pt-based GFC have confirmed the earlier formulated hypothesis, connecting their unusually high mass transfer efficiency with flexibility of GFCs, when the higher apparent transfer coefficient is provided by additional fluid flow in the back-side space of the textile thus leading to the rise of effective external contact surface area. The current study has revealed that relative effective surface area may vary from 1, equivalent to front-sided GFC contact only at low gas speed and up to 1.7 at higher velocities demonstrating that back-side flow may be quite significant even at quite moderate gas velocities (below 0.5 m/sec). The obtained knowledge may open the new approaches for development and optimization of structured GFCs.

Flow and Heat Transfer Performance of Liquid Metal in Mini-Channel and Verification of Geometric Parameter Optimization

With the rapid development of the electronics industry, the power of devices continues to rise, and seeking more efficient cooling technologies has become a key challenge in various applied scenarios. This study contributes to a novel and efficient heat dissipation method for chips employing liquid metal as a coolant. In this paper, the flow and heat transfer performance of a novel liquid metal (Ga61In25Sn13Zn1) in a mini-channel heat sink is conducted. Using pressure difference, pump power, and total thermal resistance as object parameters, a comprehensive optimization about Hp (channel height), Wc (channel width), Ww (wall thickness), and tb (base thickness) is presented. The optimized parameter combination is Hp = 7 mm, Wc = 0.6 mm, Ww = 0.4 mm, and tb = 0.2 mm. Furthermore, all of the optimization parameters are verified through the design method of orthogonal experiments.

Multi-objective optimization design and sensitivity analysis of proton exchange membrane electrolytic cell

Proton exchange membrane electrolytic cells (PEMECs) are considered cleaner energy-conversion devices with potential commercial applications in hydrogen production. However, the heat- and mass-transfer characteristics inside the cell remain unclear, and its performance should be optimized to achieve future commercial applications. A three-dimensional multi-physical field-coupling model is developed for the PEMEC. Subsequently, a multi-objective optimization design is used to optimize the structural and operational parameters of the cell based on a neural-network regression model. The results of the sensitivity and response-surface analyses indicate that the initial working temperature has the most significant impact on the reaction temperature, the current density is increased by up to 65.22% under the coupling effects of temperature and flow rate. The cell performance is also enhanced by the increased channel width and depth, although this is accompanied by limitations. Under the combined effects of increasing the working temperature and channel width or height, the current density can be increased by a maximum of 65.22% and 38.61%, respectively. The improvement in cell performance requires a trade-off between improved mass and heat transfer and larger pressure drop, and the optimal design achieves this trade-off. In optimal design, the current density is improved by up to 16.56%, the oxygen mass fraction of the catalytic layer is reduced by 40.90% compared with the original design, accompanied by a reasonable pressure drop and uniform temperature distribution. This study provides a novel perspective for the optimization of the PEMEC to promote its commercial application, offering a reference for further advanced optimization of cell performance and reducing trial-and-error costs. It is also expected to have practical applications in energy conservation and sustainable development.

Cold Plate Cooling Investigation for High-Speed Electronic Interconnect

Power dissipation of quad small form-factor pluggable-double density (QSFP-DD) interconnect system is increasing almost linearly with respect to the data speed. Fan cooling solution which was conventionally used for QSFP-DD is no longer adequate to meet the demanding thermal management requirements. In the current work, water-cooled cold plate thermal management solution was proposed for QSFP-DD. The study starts with experiments for 1 × 3 QSFP-DD system, in which temperature rise, pressure drop, flow rate, and power dissipation are measured and compared with simulation results. Subsequently, case studies were carried out in simulations to investigate the effects of cold plate number of channels, channel height, and pedestal area on the thermal-hydraulic performance of the cold plate. An empirical Nusselt number correlation was proposed to predict the thermal performance of water-cooled cold plate for QSFP-DD applications. The study reveals that number of channels plays the most significant role in heat transfer performance while channel height plays the most significant role in pressure drop of the cold plate.

Nanoscale Meniscus Dynamics in Evaporating Thin Films: Insights from Molecular Dynamics Simulations.

Evaporation studies are focused on unraveling heat transfer and flow dynamics near the solid-liquid-vapor contact line, particularly focusing on the meniscus, which encompasses the nonevaporating adsorbed layer, thin-film, and bulk meniscus regions. Continuum models assume that there are no evaporating adsorbed layers due to the strong intermolecular forces. However, recent molecular dynamics (MD) simulations have unveiled the significant role of adsorbed layers in thin-film evaporation. Leveraging a recently published energy-based interface detection method, the current study presents nonequilibrium MD simulation results for thin-film evaporation from a phase-change-driven nanopump using liquid argon confined between parallel platinum plates. Notably, unlike the transient simulations often encountered in the literature, the simulation system achieves a statistically steady transport. In this context, we showcase the shapes of the evaporating menisci for two distinct channel heights, 8 and 16 nm, and elucidate the underlying flow physics through velocity vectors and temperature contours. This comprehensive investigation advances our understanding of thin-film evaporation and its mechanisms, offering insights that span from nanoscale phenomena to broader thermal management applications.

Sensitivity of acoustofluidic particle manipulation to microchannel height in standing surface acoustic wave-based microfluidic devices

In this paper, the flow and particle trajectories, induced by standing surface acoustic waves (SSAWs) in a poly-dimethylsiloxane microchannel, are investigated by establishing a two-dimensional cross-sectional model with the finite element method and improved boundary conditions. Extensive parametric studies are conducted regarding the channel height, ranging from 0.2 to 4.0 times the spacing of the repetitive vertical interference pattern, to investigate its influences on the flow field and microparticle aggregation. The first-order flow field is found to be related to the channel height, exhibiting a periodic spatial distribution and oscillatory variation in its amplitude as the height changes. We theoretically analyze the propagation mechanism of the acoustic waves in the vertical direction and thus determine the periodicity of the wave interference pattern. Furthermore, we find that the speed of the particle aggregation is a function of the channel height, so the channel height can be optimized to maximize the strength of the first-order flow field and thus minimize the time of particle aggregation. The optimum heights can reduce the aggregation time by up to 76%. In addition, the acoustophoretic motions of microparticles exhibit a spatially dependent pattern when the channel height becomes larger than a quarter of the wavelength of the SAW, which can be explained by the change in the ratio between the radiation force and the streaming drag force from position to position. Our findings provide guidelines to the design and optimization of SSAW-based acoustofluidic devices.

On flow disturbances caused by pressure taps in highly elastic flows around a microfluidic cylinder

The objective of this work is to characterize the onset of laterally asymmetric flow of viscoelastic solutions around a confined microfluidic cylinder, which was encountered in a recent study [Rodrigues et al., J. Non-Newtonian Fluid Mech. 289, 104406 (2020)]. To this end, two non-Newtonian fluids were employed in the same micro-geometry. Two microchannels were studied, both with a cylinder of diameter 75 μm, aspect ratio (channel height over width) of 0.37, and blockage ratio (cylinder diameter over channel width) of 0.28, differing only on the width of the pressure taps, located 500 μm up- and downstream from the respective cylinder face, on opposing walls. The working fluids consist of two poly(ethylene oxide) solutions: an elastic weakly shear-thinning fluid and an elastic shear-thinning fluid. Micro-Particle Image Velocimetry and streak imaging techniques were used to evaluate the flow over a Weissenberg number range 100≤Wi≤500, while maintaining a low Reynolds number, Re < 1. The elastic shear-thinning solution showed laterally asymmetric flow past the cylinder with both pressure tap designs, while with the weakly shear-thinning solution asymmetric flow was only observed with the wider pressure tap intake. In both cases, the fluids preferentially chose the cylinder/wall gap opposing the upstream pressure tap, which was found to influence the flow greatly, seemingly associated with time-dependent flow and possibly the lateral flow asymmetry itself. This work brings to light the necessary compromise between optimal pressure tap design for quality pressure measurements and minimal flow interference, due to the increased susceptibility of elastic microfluidic flows to flow perturbations.

Comparative study on the performance of a solar photocatalytic Trombe wall with different types of collectors: Copper/Thermoelectric/PV plate

Solar photocatalytic Trombe wall (SPTW) is a technology that can help to improve the indoor environmental quality and simultaneously enhance solar energy utilization, which receives lots of attention recently. Nevertheless, current research predominantly emphasizes the heat and mass transfer analysis of a single envelope system, with limited exploration of energy and performance comparison of multi-physics systems. For this, flow, heat and mass transfer analysis of SPTW with four different types of collectors: SPTW with copper plate, SPTW with photovoltaic plate (PV-SPTW), SPTW with thermoelectric plate (TE-SPTW) and SPTW with hybrid PV-TE plate (PV-TE-SPTW), are theoretically proposed to reveal the energy flow distributions among these systems. Based on the coupled analysis, parameter characteristics have been conducted to investigate the effects of solar radiation intensity, system channel width and system channel height together with ambient temperature on the heating Q, purification CADR and power generation Z performance of various SPTWs. Furthermore, a comparison of the energy and exergy performance among these four systems is clarified through the energy, exergy flow and equivalent energy saving. From the obtained results, it can be indicated that the reverse flow has a strong impact on the performance of natural ventilation and further varies the indexes of Q, CADR and Z when the solar radiation intensity is inferior to a threshold value from 100 to 300 W/m2. With the changing channel height and width, the maximum Q and CADR are both appears in TE-SPTW system, respectively reaching by 4.64 MJ/h and 136.56 m3/h as the structure of 0.05 m channel width and 3.0 m channel height, which show 39.7 % and 5.9 % respectively compared to the SPTW model. Conducting energy and exergy analyses on the four systems reveal their remarkable potential, sacrificing a minimal 0.032 MJ or less in purification performance in exchange for substantial energy savings of at least 0.314 MJ. This work will establish a foundational contribution to advancing energy-efficient building technologies.

Thermo-hydraulic performance evaluation of radial tree-branching microchannel heat sinks for electronic cooling applications

The fractal-like tree-branching microchannel heat sink (TB-MCHS) can be a potential solution for the cooling of electronic devices due to its ability to dissipate high heat flux and maintain uniform temperature distribution on the chip surface while requiring lower pumping power compared with the conventional TB-MCHS with constant channel depth. This work proposed a novel design of radial TB-MCHS with continuously varying, either increasing or decreasing, channel depth from the centre inlet to the radial outlet, resulting in different fluid volumes in channels compared to the conventional constant channel height TB-MCHS. A three-dimensional numerical model is developed and validated with in-house experimental results to study the thermo-hydrodynamic performance and entropy generation of the TB-MCHSs. The results reveal that the proposed designs of TB-MCHS with different fluid volumes negligibly enhance the heat transfer coefficient and significantly reduce the pressure drop and pumping power requirement compared to that of constant fluid volumes. The TB-MCHS with continuously increasing channel depth (TB-MCHS-DIV) exhibits the lowest pressure drop, thermal resistance, substrate temperature, and entropy generation among all the designs of TB-MCHS considered. However, the TB-MCHS-DIV shows the lowest convective heat transfer coefficient and higher coefficient of performance (COP) and heat removal rate than the TB-MCHS-ST. Further, it is found that the TB-MCHS-DIV can be performed better than TB-MCHS-CON and TB-MCHS-ST at constant pumping power.

Energy, economic, and environmental analysis of converging air-based photovoltaic-thermal (air/PV-T) systems: A yearly benchmarking

Two converging channel configurations of photovoltaic-thermal (PV-T) systems, i.e., inlet and outlet at different sides (Case 1) and the inlet at the middle and outlets at the sides (Case 2), are investigated numerically. The results reveal that Case 1 features a nearly uniform and lower temperature distribution (up to 7 °C) for practical air flows, and the appropriate convergence ratio is 2:1 (inlet to outlet channel height) for which the PV surface temperature is lower by 8 °C than that of a similar conventional collector. Meanwhile, energy analyses based on the so called ‘rate of extra energy gain per PV surface area, E˙Ac (W/m2)’ show that the air mass-flow rate has an optimal limit of ∼0.1 kg/m2. s for a typical conventional power plant conversion factor (PPCf) of 0.3–0.4. At this limit, up to 500 W extra power per unit area of PV module can be gained by changing a PV to its proposed counterpart PV-T system reducing the levelized cost of energy (LCOE) by an order of magnitude for the PV-T (0.02–0.054 $/kWh) compared to a standalone PV (0.037–0.146 $/kWh). Such PV-Ts show 10% and 150% net CO2 mitigation in comparison to the conventional PV-Ts and the corresponding standalone PVs.

Experimental analysis and multi-objective optimization of flame dynamics and combustion performance in methane-fueled slit-type combustors

Achieving a high wall temperature and wide flame stability limit is of extreme significance for the maximization of power generation in hydrocarbon fuel-driven micro-thermophotovoltaic and thermoelectric devices. For this, detailed experimental investigations are conducted on variable channel height combustors, with the emphasis on analyzing the flame dynamics and thermal performance. A preliminary understanding of the multiple flame morphologies in the combustors is developed by varying the input working conditions and wall materials. The basic types of unstable flames are clarified. The pinch-off phenomenon of a weak flame in a slit combustor is observed for the first time, and the cell tends to decrease and emerge with an increase in the flow rate. The combustible range is shown to be extended with increasing the channel height, while it exhibits a non-monotonic changing trend with the combustor thermal conductivity which highlights the importance of the balance between heat transfer due to the flame-wall coupling and heat losses. Furthermore, a Kriging-NSGA-II optimization model is also developed to obtain the optimal characteristics in terms of radiation efficiency, standard deviation and volume power density. A Pareto-optimal front solution is determined to identify three distinct regions of thermal conductivities, which is crucially useful in guiding the design of practical micro-power systems based on different working requirements.

Capillary-driven microfluidic device integrating recombinase polymerase amplification for human papillomavirus detection

Nucleic acid amplification tests (NAATs) have been widely used in clinical diagnosis. However, current NAATs lack of user-friendly operation, require expensive systems and rely on laboratory-based methods. Here, we present low-cost and easy-to-fabricate capillary-driven microfluidic device for detecting DNA targeting of Human papillomavirus (HPV) type 16. The combination of recombinase polymerase amplification (RPA), reagent mixing, and detection of amplified DNA can be completed in a single microfluidic platform. We made the microfluidic circuit by stacking layers of transparent film and double-sided adhesive film, creating capillary in the gap between two layers of transparent film that drives the flow. The different channel height in each part of microfluidic channel creates push valve that allowed sequential fluid flow to be stopped for RPA reaction for 30 min. Users open the valve by pressing it to transfer RPA product into the detection system and automatically mix with SYBR Green I for fluorometric readout. The microfluidic device could detect HPV 16 DNA within the range of 0.002–1.0 ng/μL, with a detection limit of 0.24 pg/μL. The device demonstrated great performance when applied to clinical sample. The innovative device design transforms NAATs development for real-world point-of-care use through streamlined single-device operation.

Effects of channel height on the combustion efficiency and thermal performances of a micro Swiss-roll combustor

Micro combustors can be used as the heat sources for micro power generation systems. Recently, we developed a micro Swiss-roll combustor for non-premixed fuel and air. In the current work, the impacts of channel height (H2 = 7.5, 7.0, 6.5, and 6.0 mm) on the combustion efficiency of CH4, and the surface radiation efficiency of this combustor were numerically investigated under constant flow rates of the fuel and air. The results showed that both combustion efficiency and radiation efficiency varied non-monotonically with the decrease of channel height. The optimal channel height was found to be 6.5 mm under which a highest combustion efficiency of 99.05 % and a highest radiation efficiency of 27.03 % were obtained. These variation rules were theoretically analyzed in terms of the heat recirculation effect on fresh reactants, the buoyancy effect on flame structure and chemical reaction, and the non-dimensional residence time of the reactants (DaL). It was unraveled that the combustor with H2 = 6.5 mm achieved a best heat recirculation effect. In addition, the buoyancy effect was weakened as the channel height was decreased, resulting in a more symmetric flame structure in the vertical direction and a more intense chemical reaction. Furthermore, when the channel height descended from H2 = 7.5 mm to 6.5 mm, the region of DaL > 1 was broadened due to the intensified reaction, but narrowed as the channel height was further reduced to 6.0 mm due to increased flow velocity. The combined actions of these factors led to non-monotonic variations of combustion efficiency and radiation efficiency. The discoveries of the present study provided a guideline for optimal design of such kind of micro Swiss-roll combustors.

Thermal Resistance of Heated Superhydrophobic Channels With Thermocapillary Stress

Abstract A pressure-driven channel flow between a longitudinally ridged superhydrophobic surface (SHS) and solid wall is studied, where a constant heat flux enters the channel from either the SHS or solid wall. First, a model is developed which neglects thermocapillary stresses (TCS) in the transverse direction. The caloric, convective, and total thermal resistance are evaluated, and their dependence on the shape of the liquid–gas interface (meniscus), gas ridge width, texture period, channel height, streamwise TCS, Péclet number, and channel length is established. The caloric resistance is minimized with menisci that protrude into the gas cavity, large slip fractions, small channel heights, and small streamwise TCSs. When heating from the SHS, the convective resistance increases, and therefore, a design compromise exists between caloric and convective resistances. However, when heating from the solid wall, the convective resistance remains the same and SHSs that minimize caloric resistance are optimal. We investigate both water and Galinstan for microchannel applications and find that both configurations can have a lower total thermal resistance than a smooth-walled channel. Heating from the solid wall is shown to always have the lowest total thermal resistance. Numerical simulations are used to analyze the effect of transverse TCSs. Our model captures much of the physics in heated superhydrophobic channels but is computationally inexpensive when compared to the numerical simulations.

Effect of pressure on slip length of supercritical water flow in graphene nanochannels

Flow transport of supercritical fluids (SFs) in nanopores widely occurs in shale and deep geothermal systems. Pressure is an important parameter for describing SFs, but the effect of pressure on the flow and slip behaviors of SFs in nanochannels is unknown. In this study, molecular dynamics simulations of the Poiseuille flow of supercritical water (SCW) in graphene nanochannels are performed by applying a driving force to water molecules between two walls. Fluid pressure is realized by varying water density. The effect of pressure on the slip length under different surface wettability is investigated. The results show that the slip length nonmonotonically varies with the pressure, which is divided into three regions according to the three-regime-model of SFs. For the gas phase and gas-like SCW, the slip length gradually decreases with increasing pressure. For two-phase-like SCW, the slip length increases with pressure. For liquid-like (LL) SCW, the slip length decreases with increasing pressure. The slip length nonmonotonically varies due to the nonlinear variation of the fluid viscosity and interfacial friction with pressure. For LL SCW, the slip length is linearly related to the inverse of the density ratio, and the effect of pressure and wettability on the slip length is evaluated using the density ratio and energy barrier. Furthermore, the relationship between the slip length and pressure is verified for application to a wide range of wettability, different channel heights, and different fluid types and solid wall types. This study deepens the understanding of the microscale flow theory under supercritical conditions.

Large Eddy Simulation of a Low-Pressure Turbine Cascade with Turbulent End Wall Boundary Layers

We present results of implicit large eddy simulation (LES) and different Reynolds-averaged Navier–Stokes (RANS) models of the MTU 161 low pressure turbine at an exit Reynolds number of 90000\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$90\\,000$$\\end{document} and exit Mach number of 0.6. The LES results are based on a high-order discontinuous Galerkin method and the RANS is computed using a classical finite-volume approach. The paper discusses the steps taken to create realistic inflow boundary conditions in terms of end wall boundary layer thickness and freestream turbulence intensity. This is achieved by tailoring the input distribution of total pressure and temperature, Reynolds stresses and turbulence length scale to a Fourier series based synthetic turbulence generator. With this procedure, excellent agreement with the experiment can be achieved in terms of blade loading at midspan and wake total pressure losses at midspan and over the channel height. Based on the validated setup, we focus on the discussion of secondary flow structures emerging due to the interaction of the incoming boundary layer and the turbine blade and compare the LES to two commonly used RANS models. Since we are able to create consistent setups for both LES and RANS, all discrepancies can be directly attributed to physical modelling problems. We show that both a linear eddy viscosity model and a differential Reynolds stress model coupled with a state-of-the-art correlation-based transition model fail, in this case, to predict the separation induced transition process around midspan. Moreover, their prediction of secondary flow losses leaves room for improvement as shown by a detailed discussion of turbulence kinetic energy and anisotropy fields.