Non-uniform Height Research Articles

Investigating Enhanced Convection Heat Transfer in 3D Micro-Ribbed Tubes Using Inverse Problem Techniques

The improved heat dissipation observed in 3D micro-ribbed tubes is primarily influenced by the intricate interplay of multiple structural parameters. Nevertheless, research into the coupling mechanisms of these multi-structural parameters remains constrained by the absence of effective methodology in numerical solutions. In the present work, a new 3D micro-rib structure based on discrete adjoint method is established. Firstly, the research examines the interplay of different parameters (such as arrangement, relative roughness height, angle of attack, and circumferential rows) on the thermo-hydraulic performance. It is noted that the heat transfer efficiency is notably impacted by the relative roughness height. And the arrangement methodology dictates the optimal positioning for heat transfer efficiency. An increase in the number of circumferential rows enhances fluid mixing, while the angle of attack plays a crucial role in the formation of longitudinal vortices. Secondly, the coupling optimization technique is employed to obtain the optimal structure featuring non-uniform relative roughness height by the developed numerical solution. Overall, in comparison to the smooth tube, the optimized ribbed tube exhibits a remarkable 64.9% enhancement in performance evaluation criteria. Finally, a notable enhancement of 10.65–22.78% is observed when comparing with the prevailing micro-rib structures.

Similarity of the low-occurrence wind profiles within urban turbulent boundary layers of uniform and non-uniform height block arrays

Within urban boundary layers, the safety of pedestrians is markedly affected by wind speed, particularly in urban areas. The characteristics of turbulence above the canopy layers can lead to unpredictable changes in wind speed at the pedestrian level. Therefore, this study analyzes low-occurrence wind speed phenomena above canopy heights for uniform and nonuniform block configurations using wind tunnel experiments (WTE) to understand the background turbulence characteristics which the wind profile is generated by the boundary layer above the canopy. The urban canopy arrays were reproduced using solid blocks arranged in 30 rows in a streamwise direction with a packing density of 25 % at three different heights. An x-type hot-wire anemometer was used to measure the streamwise and vertical velocity components. The low-occurrence values were classified based on wind speed percentiles of 0.1 %, 1.0 %, 99.0 %, and 99.9 % wind speeds. The results demonstrated that above the canopy, there were minor influences of block height variations on the low occurrence factor. The peak factor demonstrated a comparable value between the uniform and nonuniform cases, regardless of the block arrangement. Statistical models based on the Weibull distribution and Gram–Charlier series demonstrating good agreement with the WTE data in predicting the low occurrence and peak factors. This study found that variations in block height have a minor influence on the low occurrence and peak factors within the turbulent boundary layers, implying that we can separate the effect of background turbulence from the local turbulent generation within the urban canopy layers.

Experimental study on mechanical dewatering and displacement washing of filter cakes with inhomogeneous cake geometry

The influence of cake geometry on mechanical gas dewatering and displacement washing of filter cakes is investigated experimentally. For this purpose, five cake geometries are considered. The results show that the dewatering kinetics are independent of cake geometry within the limits of measurement accuracy. Further investigations demonstrate that an inhomogeneous cake geometry negatively influences displacement washing, which can be quantified in a wash diagram. Compared to flat filter cakes used as reference, non-uniform cake heights increase the needed amount of wash liquid to achieve the same cake purity in the dispersion and diffusion regime. This research underlines the importance of ensuring that filter cakes have homogeneous properties to maintain sustainability of industrial filtration processes.

A systematic and precise study of deformed Zr52.5Cu17.9Ni14.6Al10Ti5 BMGs for notable shear bands towards better global plasticity

Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit 105) bulk metallic glasses with varying deformation strains (1.6, 4.5, 6.6, 8.7, 10.8, and 12.5 %) were subjected to compressive deformation. The evolution of shear bands was systematically and mathematically conducted to investigate the impacts of largest plastic strains. It was observed that the average length of shear steps per unit area of surface increased with increasing plastic strain. The protruded areas exhibited non-uniform heights, which appeared to gradually extend as the plastic strain was increased. Mathematical formulation was employed to calculate the contribution of individual shear bands to the total strain, aiming to explore the microstructure and mechanical features of BMGs. This approach provided insights into the relationship between atomic-scale structure, local dynamics, and macroscopic mechanical behavior. Moreover, the contribution of shear bands to normal strain along with the assessment of congruent results for normal strain originating from shear bands were evaluated for attaining better global plasticity.

Manufacturing seamless three-dimensional woven preforms with complex shapes based on a new weaving technology

A new weaving technology using a modified z-binder interlacement system was designed to demonstrate its potential for the effective, continuous, efficient, and rapid manufacturing of various three-dimensional (3D) woven structures. First, three representative 3D woven preforms were fabricated. Then, epoxy resin was transferred to a preform. The manufactured 3D woven textile-reinforced composites were investigated using micro-CT analysis, tensile tests, and bending tests to study the effect of the z-binder interlacing on the structure. Furthermore, a design rule was established that could seamlessly create complex 3D woven structures with non-uniform heights in the z-direction, such as boxes, bowls, and pyramids, demonstrating that the seamless 3D woven preform of the complex shape can be fabricated with structural integrity.

Introducing abrasive wear into undeformed chip thickness modeling with improved grain kinematics in belt grinding

The undeformed chip thickness (UCT) is an essential indicator in understanding the belt grinding mechanism, especially in relation to surface roughness. However, abrasive wear has not been involved in the above modeling studies, even though it always influences the grinding process. In this paper, the evolving grain height distribution, which is governed by the accumulative active grain group, is first quantified using a gamma distribution. Then, based on the geometric analysis of the compliant contact area, improved grain kinematics considering non-uniform grain height is proposed. Finally, based on the above findings, an improved undeformed chip thickness model is established and experimentally validated. The calculated results based on the model show that undeformed chip thickness value approximately obeys a normal distribution at various wear stages unlike a Rayleigh distribution in rigid wheel grinding. The mean and variance of the distribution generally decrease with increasing grinding time, which is supported by the transformation from spiral chips to fragmented chips. Surface prediction is performed to help experimentally verify the reliability of the proposed model, where results illustrate that the predicted and measured roughness exhibits a similar decreasing trend with increasing active grain percentage and grinding time.

Aerothermal optimization of turbine cascade squealer tip with non-uniform squealer height

The squealer tip has significant influence on both the aerodynamic and heat transfer characteristics of the high-pressure turbine blade. However, due to the complexity of parameterization and meshing of the squealer and the complicated flow structure within the over-tip region, the existing squealer designs in the open literature have constant squealer heights. In this paper, the design space to the squealer height with non-uniform squealer height is extended and the new flow features it may bring are investigated. A parameterization system specifically designed for the non-uniform squealer height using five control parameters is implemented to automatically generate the geometry and hybrid meshes. Combining it with the multi-objective optimization system using genetic algorithms, a transonic turbine cascade squealer tip is optimized employing Reynolds-averaged Navier–Stokes k–ω shear stress transport model. The main objective of this study is to obtain a squealer configuration with the lowest total pressure loss coefficient and heat transfer coefficient. The optimum configuration with non-uniform squealer height achieves improvements in both the aerodynamic efficiency and the heat transfer performance, relative to the baseline conventional squealer tip geometry with the constant squealer height. Additionally, this work demonstrates that a flow structure in which the main flow forms a “blanket” below the leakage flow in the squealer is beneficial for aerothermal performance, especially reducing heat transfer losses, which provides valuable insight into the squealer tip design of advanced high-pressure turbines.

Elliptical Quantum Rings with Variable Heights and under Spin–Orbit Interactions

We investigate the electronic properties of a semiconductor quantum ring with an elliptical shape and non-uniform height, allowing for distributed quantum-dot-like bulges along its perimeter. The adiabatic approximation and the finite element method are combined to calculate the allowed electron states in the structure under the effective mass approximation, considering the contributions from Rashba and Dresselahaus spin–orbit interactions and the Zeeman effect in the presence of an applied magnetic field. We discuss the features of the calculated spectra for two different ring geometries: a symmetric one with four dot-like bulges, and an asymmetric one with three hilled protuberances. The information about those states allows us to evaluate the linear optical absorption response associated with interlevel transitions between the ground and lowest excited states. This phenomenon takes place at resonant energies of only a few milielectronvolts. It is observed that spin–orbit interactions tend to quench this response under zero-field conditions in the case of symmetric confinement.

Nonuniform height endwall fence optimization of a low-pressure turbine cascade

As the lift of turbine increases, the secondary flow in the endwall region seriously restricts improvement of the aerodynamic performance of low-pressure turbine. The particle swarm optimization is improved to pursue stronger optimization ability. An optimization platform is built that combines Bayesian optimization (BO), the eXtreme Gradient Boosting (XGBoost) algorithm and improved chaos particle swarm optimization. The test results show that the surrogate model optimization method proposed in this paper has more prominent application potential in the field of aerodynamic optimization. The above optimization method is used for the optimization design of the nonuniform height endwall fence (NHEF) of a high-lift low-pressure turbine cascade to achieve an efficient secondary flow control effect. The NURBS curve is used to realize the parameterization of the NHEF. The secondary flow control mechanism of traditional fences and NHEF has been compared and analysed in detail. The numerical results show that the NHEF can effectively prevent the cross migration of the boundary layer, and the fence vortex induced by the fence can suppress the development of the passage vortex. The total pressure loss at the outlet of the cascade installed NHEF is reduced by 14.82%. The secondary flow control effect of the NHEF is better than that of a traditional fence. The feature importance analysis results of input variables show that the pitchwise position of the fence has the most obvious influence on the loss of the cascade. In addition, based on the results of feature importance, a 53% length reduction fence is designed to reduce the extra weight brought by the fence. The design achieves the effect of comprehensively considering the weight and aerodynamic performance.

Influence of operating conditions on in-tube vapour condensation heat transfer characteristics in the presence of noncondensable gases at high pressure

The in-tube vapour condensation heat characteristics in the presence of noncondensable gas at high pressures are experimentally validated with an error of 20%. Numerical simulations are then conducted to explore heat transfer characteristics at different operating pressures, wall surface temperatures and mass flow rates. The flow and heat transfer characteristics are thoroughly discussed and the buoyancy effects induce a secondary circulation altering flow patterns, changing the water film and heat transfer coefficient distribution. In addition, increasing operating pressures is found to reduce the wall heat transfer rate. Even though the flow pattern is significantly different caused by the increased gas mixture density, the total condensate flow rate is approximately the same at different operating pressures due to the same inlet conditions and temperature difference. The difference in the wall heat transfer rate is attributed to the change of latent heat of vapourisation and falling film at different pressures and is evident at high pressures. At the high temperature difference, a nonuniform film height towards the bottom is emerged, sweeping the condensate upward. Similarly, increasing the mass flow rate can also increase the total wall heat transfer rate and total condensate flow rate.

Airflow Mitigation and Pollutant Purification in an Idealized Urban Street Canyon with Wind Driven Natural Ventilation: Cooperating and Opposing Effects of Roadside Tree Plantings and Non-uniform Building Heights

A refined CFD research was reported, concerning on the individual characteristics of different roadside tree planting and street canyon with non-uniformity of buildings. In this work, airflow field and pollutant dispersion were simulated by the use of a full three-dimensional simulations. Ventilation air exchange rate (ACH) and net escape velocity (NEV) were, respectively applied to assess ventilation exchange and pollutant dispersion in the street canyons. Numerical results show that the airflow patterns within the street canyon varies significantly with canopy leaf area density (LAD) and trunk height (Htb), whereas the distribution of pollutants at pedestrian level in the street canyon is mainly determined by the symmetrical inverse vortex structure of the street canyon. Promotions of LAD and Htb of the road trees simultaneously reduce the wind velocity, ACH and NEV of the street canyon, deteriorating the ventilation exchange conditions as well as the pollutant dispersion performance. As the value of the Htb becomes larger, NEV of the step-up canyon is approximately approaching to that of the step-down canyon. In addition, as the value of standard deviation (σH) of the street canyon increases, the vortex structure of the canyon disappears and promotes the diffusion of pollutants inside canyons. As σH further increases, ACH of the step-down canyon also boosts, indicating better ventilation performance of street canyon, whereas the step-up canyon only tops the best ventilation performance at the value of σH= 33.3%. The best dispersion of pollutants is observed at σH= 16.7% in the step-down canyon while in the step-up canyon the dispersion of pollutants becomes better continuously with increasing σH. In particular, as σH= 66.7% was maintained, the dispersion of pollutants is better in the step-up canyon compared to the street canyon without trees. At the same time, the difference between the NEV of the step-down canyon and the step-up canyon increases as σH increases, which indicates that the difference in pollutant dispersion performance between adjacent street canyons becomes greater. This research provides a theoretical reference for urban design and vegetation planning in urban areas to improve the habitat.

Effect of Micropillar Array Morphology on Liquid Propagation Coefficient Enhancement.

Roughness on hydrophilic surfaces allows for fast propagation of liquids. In this paper, the hypothesis is tested which theorizes that pillar array structures with nonuniform pillar height levels can enhance wicking rates. In this work, within a unit cell, nonuniform micropillars were arranged with one pillar at constant height, while other shorter pillars were varied in height to study these nonuniform effects. Subsequently, a new microfabrication technique was developed to fabricate a nonuniform pillar array surface. Capillary rising-rate experiments were conducted with water, decane, and ethylene glycol as working liquids to determine the behavior of propagation coefficients that were dependent on pillar morphology. It is found that a nonuniform pillar height structure leads to a separation of layers in the liquid spreading process and the propagation coefficient increases with declining micropillar height for all liquids tested. This indicated a significant enhancement of wicking rates compared to uniform pillar arrays. A theoretical model was subsequently developed to explain and predict the enhancement effect by considering capillary force and viscous resistance of nonuniform pillar structures. The insights and implications from this model thus advance our understanding of the physics of the wicking process and can inform the design of pillar structures with an enhanced wicking propagation coefficient.

Indications of a more complex DNA toroid formation.

The compaction of DNA in sperm is at semi-crystalline levels, reaching nearly the physical condensation limit. This incredible compaction is carried out by a positively charged condensing agent, the protein protamine. Many protamines bind to the negatively charged DNA backbone, each creating a bend that eventually forms a loop. Our goal is to determine how protamine compacts the DNA even further to form toroids. Using atomic force microscopy (AFM), we imaged samples of varying lengths of DNA (1-48 kbp) under varying concentrations of protamine (0.1-3 μM). This allowed us to isolate folding states in the toroid formation pathway. It is thought that toroids are formed loop by loop and create a hexagonally packed toroid. However, we observed structures with many unstacked loops that look like flowers, vertical stacks of several loops, rod-like structures with approximately the thickness of a toroid, and toroids with nonuniform diameter and height. These structures suggest that the DNA toroid formation pathway could be more complex than we first thought. Could there be multiple pathways for the formation of toroids? Here we will present our progress toward determining toroid formation mechanisms. Understanding the physics of DNA compaction into a toroid may have interesting nanoengineering applications.

Enhanced phonon resonance by non-uniform surface nanopillars in Si nanowires

Based on the wave nature of phonons, the lattice thermal conductivity of nanowires can be regulated by phonon resonators such as surface nanopillars. In this study, the impact of the height and distance of the resonant nanopillars on the thermal conductivity of Si nanowires were investigated. The results calculated from the molecular dynamics simulations show that with increasing height of the nanopillars, the corresponding thermal conductivity decreases. We found that the nanopillars with non-uniform heights induce stronger phonon resonance effect and can further suppress the thermal conductivity, as compared to the models with nanopillars of uniform heights. On the other hand, changing the distance between two adjacent nanopillars does not obviously affect the phonon propagation. Phonon dispersion relations show the formation of flat bands, which confirms the occurrence of phonon resonances due to the nanopillars. When the heights of the nanopillars become non-uniform, more flat bands were generated, and the phonon group velocity further decreased. Analyses of phonon participation ratio indicate stronger phonon localization induced by the disorder stemming from the non-uniform nanopillar height. Our investigation yields new insights into the physical mechanisms governing heat transport in nanowires with phonon resonators, and thus opens potential new routes to the design of thermoelectric devices.

Reaching homogeneous field emission current from clusters of emitters with nonuniform heights

In field electron emission from pointed structures in a cluster or in an array, electrostatic depolarization due to neighboring emitters diminishes the local field enhancement factor (FEF). This effect can limit the maximum macroscopic emission current from devices. If emitters in a cluster are regularly spaced and uniformly tall, the local FEF will always present significantly larger values at the edges, making most of the cluster ineffective. In this work, we explore conducting emitters in a cluster that are not uniformly tall and numerically calculate the local electrostatic field and the macroscopic emitted current over classical emitter’s surfaces using the Murphy–Good emission model. Our results show the conditions to homogenize and, therefore, optimize the emission current extractable from the cluster.

Air exchange rate and pollutant dispersion inside compact urban street canyons with combined wind and thermal driven natural ventilations: Effects of non-uniform building heights and unstable thermal stratifications

In the present work, a delicate CFD research of a multi-street canyon model with varying thermal stratifications and non-uniformities of buildings was conducted to investigate the street ventilation and pollutant dispersion between the compact urban blocks. Non-isothermal turbulent wind flow, temperature field and pollutant dispersion in a two-dimensional computational domain were solved by the Renormalization Group (RNG) k-ε turbulence model along with the enhanced wall treatment. Present numerical results indicated that the variation of ground heating intensity has a significant influence on the airflow pattern in the step-down case, and the distribution of pollutants in the street canyons mainly depends on the variation of the upper clockwise vortex. The canyon ventilation performance became better as the unstable thermal stratification strengthened. Similarly, the increase of ground heating intensity could reduce ADF (atmospheric dispersion factor) in the step-down case and ADF became the lowest when Ri = −3.92 was maintained. Additionally, the increase of building unevenness further complicated the canyon airflow structure, which aggravated the pollution of the canyon. In the step-down configuration, as the standard deviation of adjacent building height gradually increases, canyon ventilation could be further enhanced. For the step-up configuration, the best ventilation performance was found at σH = 16.7 %. ADF of adjacent canyons also varied greatly. When σH = 33.3 % was maintained, the peak and bottom values of ADF were discovered in the step-up and step-down cases, respectively. Present research has provided a theoretical reference for guiding urban design and improve living environment in modern compact cities.

Air Exchange Rate and Pollutant Dispersion Inside Compact Urban Street Canyons with Combined Wind and Thermal Driven Natural Ventilations: Effects of Non-Uniform Building Heights and Unstable Thermal Stratifications

Air Exchange Rate and Pollutant Dispersion Inside Compact Urban Street Canyons with Combined Wind and Thermal Driven Natural Ventilations: Effects of Non-Uniform Building Heights and Unstable Thermal Stratifications

Rotating tip clearance asymmetry and role of endwall injection in stability desensitization of a transonic fan

The current study is aimed at understanding the effect of rotating tip clearance asymmetry on the operability and performance of a transonic compressor. Another objective of this investigation is to determine the influence of tip injection on reducing the detrimental effects of clearance asymmetry. Three dimensional unsteady Reynolds-averaged Navier–stokes simulations have been performed from choke to stall for different arrangements of non-uniform blade heights in a transonic fan. Furthermore, numerical computations have been conducted with endwall injection of air. The numerical results have been validated against experimental data. Results show that having the same mean tip clearance, the asymmetric compressor is less stable than the axisymmetric configuration. However, the peak pressure rise is found to be almost linearly correlated to the mean tip clearance for both the axisymmetric and asymmetric compressors. It is found that tip injection can desensitize the compressor to the tip clearance asymmetry. Results further reveal that tip clearance asymmetry does not change the compressor path to instability. However, endwall injection is found to be able to change the compressor stalling mode. Investigations concerning rotating non-uniformity (caused by non-uniform blade heights) are very few in open literature. The obtained results can assist in predicting the effect of rotating tip clearance asymmetry on the stability and performance of high-speed compressor rotors. Furthermore, the results uncover how tip injection can desensitize the compressor stability and affect its path into instability, which is one of the most important questions in the turbomachinery world.

Buoyancy effects on the flows around flat and steep street canyons in simplified urban settings subject to a neutral approaching boundary layer: Wind tunnel PIV measurements

The present wind tunnel particle image velocimetry (PIV) measurements document flows around flat and steep street canyons subject to thermal conditions at different levels, ranging from the Richardson number of 0.31 to 2.07. A steepness ratio, that is, the ratio of windward and leeward building heights, is proposed to characterise the geometrical influence of street canyons surrounded by buildings of non-uniform height. To study the thermal effects of building façades and ground on surrounding flow, surfaces of building models and the ground between them are heated up and maintained at three different temperatures to induce buoyant flows of different strength. The transition of the canyon flow from the typical rooftop shear-layer driven vortex to the buoyant plume type of flow is clearly revealed from the measurement results, which enhances the air removal that takes place at the roof-level of the two canyons. However, due to the different steepness of the canyons, the air removal rate from the steep canyon of a steepness ratio 2.52 is approximately 50% of that from the flat canyon with a steepness ratio of 1.53 in the buoyant plume-driven case because the downward flush flow along the windward façade suppresses the ascending plumes in the steep canyon. At the pedestrian level, the wind field is jointly dominated by the interplay between canyon-wide vortical flow and the buoyant plume rising ascending from the ground. The dynamics of non-isothermal flow in flat and steep canyons are revealed in detail, the implication of which is that the steepness of street canyons has to be considered in urban morphology planning, as well as in simplified geometrical representations of street canyons and in simplified urban canopy models.

Strengthening Thermal Stability in V2O5-ZnO-BaO-B2O3-M(PO3)n Glass System (M = Al, Mg) for Laser Sealing Applications

Today, the most common way of laser sealing is using a glass frit paste and screen printer. Laser sealing using glass frit paste has some problems, such as pores, nonuniform height, imperfect hermetic sealing, etc. In order to overcome these problems, sealing using fiber types of sealant is attractive for packaging devices. In this work, (70-x)V2O5-5ZnO-22BaO-3B2O3-xM(PO3)n glasses (mol%) incorporated with xM(PO3)n concentration (where M = Mg, Al, n = 2, 3, respectively) were fabricated and their thermal, thermomechanical, and structural properties were investigated. Most importantly, for this type of sealing, the glass should have a thermal stability (ΔT) of ≥80 °C and the coefficient of thermal expansion (CTE) between the glass and panel should be 1.0 ppm/°C. The highest thermal stability ΔT of the order of 93.2 °C and 112.9 °C was obtained for the 15 mol% of Mg(PO3)2 and Al(PO3)3 doped glasses, respectively. This reveals that the bond strength and connectivity is more strongly improved by trivalent Al(PO3)3. The CTE of a (70-x)V2O5-5ZnO-22BaO-3B2O3-xAl(PO3)3 glass system (mol%) (where x = 5–15, mol%) is in the range of 9.5–15.5 (×10−6/K), which is comparable with the CTE (9–10 (×10−6/K)) of commercial DSSC glass panels. Based on the results, the studied glass systems are considered to be suitable for laser sealing using fiber types of sealant.