Large Radius Ratio Research Articles

Study on Turbulent Taylor-Couette Flow with High Reynolds Number and Large Radius Ratio in High-Speed Canned Motor Pump Internals

Abstract The Taylor-Couette flow, a prevalent flow pattern in fluid machinery. This study investigates turbulent Taylor-Couette flow within the internal flow of a shield pump through numerical simulations. The study encompasses a range of radius ratios (0.934 to 0.977) and inner cylinder frequencies (50 to 383 Hz). Our findings reveal a significant positive correlation between torque magnitude and radius ratio within the studied parameter range. Additionally, an exponential relationship between non-dimensional torque and Taylor number was identified through fitting. Moreover, the study elucidates that larger radius ratios lead to an earlier transition to the final turbulent region with increasing Taylor numbers. Furthermore, detailed analysis of the flow field vortex structure demonstrates the profound influence of radius ratio on the number of Taylor vortices.

Effect of radius ratio on the instabilities in a vaneless diffuser

The prominent features of the primary instabilities of a turbulent source flow between finite-span parallel rings with spatially modulated inflow conditions rotating at angular rate Ω are investigated. Two different two-dimensional instabilities that occur at large (αQ≳1.00) and low (αQ≲0.65) flow rates have been traced back to two different physical mechanisms related to jet-wake and mean-core flow, respectively. The effect of the radial aspect ratios Γ on the instability characteristics of the flow between parallel rings is also considered in this paper. Numerical URANS simulations are conducted for three different radial aspect ratios, i.e. Γ=1.25, 1.5, and 2, the corresponding cross-sectional aspect ratios Λ=2(Γ−1)×3.325, and two conceptually different inflow conditions. Their impact on the rotating instabilities in terms of instability mode number and instability frequency is the main focus of this study and is analyzed by fast Fourier transform (FFT) and wavelet analysis. Realistic inflow conditions with a leakage flow are found to overtake the instability for small radius ratios and affect the primary instabilities for large radius ratios.

Numerical investigation about radius ratio effect of helical tubes in the heat exchanger

Enormous research has been conducted on the efficient flow and heat transfer performance of helical tube. The helical tube with a large radius ratio (ratio of curvature radius to tube radius) could not be installed when the available space is limited, and the differences in their thermal hydraulic characteristics caused by different radius ratios are rarely studied. The present study aims to use a numerical method to quantify and visualize the radius ratio effect and to determine its mechanism. The result shows that the fitness curve of the theoretical flow field and the numerical result are bell-shaped. Helical tubes with lower radius ratio have a significant improvement in flow and heat transfer performance, while higher radius ratio brings lower circumferential nonuniformity and more heat transfer area. The torsional effect or bending effect is the dominant effect when the radius ratio is low or high, respectively. The promotion of vortex generation is the mechanism of the radius ratio effect. The current study could be used as a reference for the engineering applications of helical tubes, as well as to support further research on the basic mechanism of heat and mass transfer in helical tubes.

Fire plume characteristics of annular pool fire with different cylindrical obstacles in a ship engine room

A numerical study of oil pool fire with different cylindrical obstacles in a ship engine room was carried out. The characteristics of flame height, maximal temperature rise, and centerline axial velocity of fire plume were investigated and analyzed. The results showed that the flame height of the merged flame decreases with the increased radius ratio and obstacle height, while the plume maximal temperature rise and axial velocity decreases with a larger radius ratio. Under the broken flames, the flame height only decreases with a larger radius ratio, while the plume temperature rise and axial velocity decreases with the increase of both radius ratio and obstacle height. When the obstacle is smaller, the annular flames merge above the obstacle, while the annular broken flames surround the cylindrical obstacle when the obstacle is larger. And there is a competition between flame height enhancement effect and weakening impact when the flame could merge above obstacle. The empirical models of flame height, centerline temperature rise and axial velocity of annular pool fire with different cylindrical obstacles are obtained and validated by experimental test data. These results can offer an essential reference for fire prevention design and fire science research in the ship engine room.

TOI-1696: A Nearby M4 Dwarf with a 3 R ⊕ Planet in the Neptunian Desert

We present the discovery and validation of a temperate sub-Neptune around the nearby mid-M dwarf TIC 470381900 (TOI-1696), with a radius of 3.09 ± 0.11 R ⊕ and an orbital period of 2.5 days, using a combination of Transiting Exoplanets Survey Satellite (TESS) and follow-up observations using ground-based telescopes. Joint analysis of multiband photometry from TESS, Multicolor Simultaneous Camera for studying Atmospheres of Transiting exoplanets (MuSCAT), MuSCAT3, Sinistro, and KeplerCam confirmed the transit signal to be achromatic as well as refined the orbital ephemeris. High-resolution imaging with Gemini/’Alopeke and high-resolution spectroscopy with the Subaru InfraRed Doppler (IRD) confirmed that there are no stellar companions or background sources to the star. The spectroscopic observations with IRD and Infrared Telescope Facility SpeX were used to determine the stellar parameters, and it was found that the host star is an M4 dwarf with an effective temperature of T eff = 3185 ± 76 K and a metallicity of [Fe/H] = 0.336 ± 0.060 dex. The radial velocities measured from IRD set a 2σ upper limit on the planetary mass to be 48.8 M ⊕. The large radius ratio (R p/R ⋆ ∼ 0.1) and the relatively bright near-infrared magnitude (J = 12.2 mag) make this planet an attractive target for further follow-up observations. TOI-1696 b is one of the planets belonging to the Neptunian desert with the highest transmission spectroscopy metric discovered to date, making it an interesting candidate for atmospheric characterizations with JWST.

Dynamic analysis of particles in vertical curved 90° bends of a horizontal-vertical pneumatic conveying system based on POD and wavelet transform

The pneumatic system is frequently operated in the high air velocity region, which aggravates the power consumption and erosion of bend, and the dynamic analysis of particles in bends with different radius of curvature in a horizontal-vertical pneumatic conveying system is necessary. This experimental study focuses on the particle motion characteristic of bend on the horizontal-vertical pneumatic conveying in terms of on pressure drop, particle velocity, power spectral characteristics of particle fluctuation velocity, the energy distribution of the proper orthogonal decomposition (POD) modes, time coefficients of POD, and spatial mode of POD mode during flowing through bends. The results indicate that the particle rope is the large-scale motion of particles containing high energy, which dominates the motion of particles in the bend, and the suppression of small-scale motion leads to the low pressure drop in a large radius ratio of the bend.

Annulus eccentric analysis of the melting and solidification behavior in a horizontal tube-in-shell storage unit

Abstract Previously, optimal eccentric parameters were considered to be greatly overall size- dependent, but the inherent factor haven’t been determined. In this study, we suggest that areas above and below inner tube may be the core factor, based on this, “eccentric space ratio” is proposed to explore the thermal behavior of phase change material in different radius ratio models. The results indicate the melting period can be shortened by more than 40% between 4:1 and 14:1, as is generally valid for different radius ratios. Additionally, the solidification can be shortened consistently by more than 45% between 1:4 and 1:14 in large radius ratios models which breaks the previous consensus, but still extended for small radius ratios models. Remarkably, optimal eccentric range in solidification is symmetrical with that in melting. With these numerical and mechanistic analyses, we find that areas above and below inner tube can significantly influence natural convection and temperature distribution, while, as a simple approximation, “eccentric space ratio” is well employed to analyze the area effect irrespective of diameters. Further, this analysis strategy provides a robust optimal eccentric parameter range for phase change heat exchangers and can be widely applied in rapid latent heat storage.

Stall Mode Transformation in the Wide Vaneless Diffuser of Centrifugal Compressors

A review on the rotating stall in the vaneless diffuser of centrifugal compressors is presented showing that different stall modes characterized by different numbers of cells can be detected within the diffuser even if the operating condition remains unchanged. The interaction between the inlet perturbation and the stall cells near the diffuser outlet is supposed to be the trigger of the stall mode transformation. In order to determine if the inlet perturbation will interact with the downstream stall cells, a characteristic time analysis is proposed to estimate the characteristic time of the perturbation in radial and tangential directions. An additional theoretical model which focused on the development of the vaneless diffuser rotating stall is presented to determine the propagation velocity of the cells. The comparison between the characteristic time in two directions shows that one stall mode is able to evolve into another stall mode if a critical condition is met, and the stall mode transformation is more likely to start from a mode with a higher number of cells and is more likely to occur in the diffuser with a large radius ratio. Experimental results are also employed to validate the proposed critical condition, and good agreements are obtained.

The White Dwarf Opportunity: Robust Detections of Molecules in Earth-like Exoplanet Atmospheres with the James Webb Space Telescope

Abstract The near-term search for life beyond the solar system currently focuses on transiting planets orbiting small M dwarfs, and the challenges of detecting signs of life in their atmospheres. However, planets orbiting white dwarfs (WDs) would provide a unique opportunity to characterize rocky worlds. The discovery of the first transiting giant planet orbiting a WD, WD 1856+534, showed that planetary-mass objects can survive close-in orbits around WDs. The large radius ratio between WD planets and their host renders them exceptional targets for transmission spectroscopy. Here, we explore the molecular detectability and atmospheric characterization potential for a notional Earth-like planet, evolving in the habitable zone of WD 1856+534, with the James Webb Space Telescope (JWST). We establish that the atmospheric composition of such Earth-like planets orbiting WDs can be precisely retrieved with JWST. We demonstrate that robust >5σ detections of H2O and CO2 can be achieved in a five-transit reconnaissance program, while the biosignatures O3 + CH4 and CH4 + N2O can be detected to >4σ in as few as 25 transits. N2 and O2 can be detected to >5σ within 100 transits. Given the short transit duration of WD habitable zone planets (∼2 minutes for WD 1856+534), conclusive molecular detections can be achieved in a small or medium JWST transmission spectroscopy program. Rocky planets in the WD habitable zone therefore represent a promising opportunity to characterize terrestrial planet atmospheres and explore the possibility of a second genesis on these worlds.

Long-term thermal performance analysis of deep coaxial borehole heat exchanger based on field test

Geothermal energy is an abundant and clean energy source that is abundant and widely and naturally distributed in the earth. Deep coaxial borehole heat exchanger (DCBHE) can efficiently extract heat from rock-soil for building heating. And DCBHE systems have superior application performance to that of the traditional BHEs. However, the existing related researches are primarily focused on theory and modeling, via which the temperature characteristics and thermal processes cannot be truly reflected, thus field testing is necessary. In this study, the long-term thermal performance of a DCBHE is developed based on field test with a distributed optical fiber temperature sensor (DOFTS). The numerical model is validated by field test data while considering power failure. The effects of the heat load mode, flow rate, heat load, radius ratio and soil thermal conductivity on the fluid and soil are analyzed and discussed. Moreover, the thermal process in the formation is analyzed. It is demonstrated that the results of the numerical simulation fit well with the test data. The temporal and spatial distributions of the fluid temperature and thermal process can be affected by two heat load modes, for which thermal compensation should be applied. A higher flow rate, lower heat load and larger radius ratio can promote the outlet fluid temperature. The thermal equilibrium points in the formation will move downward with the increase of the soil thermal conductivity, and main impact scope can reach 40m. A higher flow rate and larger radius ratio will also obviously strengthen the pressure drop, and the power consumption will increase accordingly. Furthermore, two methods respectively called “Intercept” and “Guide” are proposed to reply the reverse heat conduction in shallow soil for DCBHE. The results can provide a reference for optimum design of DCBHEs for practical engineering.

Rate Decline Analysis for Limited-Entry Well in Abnormally High-Pressured Composite Naturally Fractured Gas Reservoirs

Most naturally fractured gas reservoirs in China exhibit strongly heterogeneous, abnormally high-pressured and, stress-sensitive behaviors. In this work, a semianalytical solution is developed to study the production performance for limited-entry well in composite naturally fractured formations. The pressure-dependent porosity and permeability, anisotropy and limited-entry characteristics are taken into consideration. Furthermore, conventional Warren-Root model is amended to accommodate for permeability anisotropy. Laplace and finite Fourier cosine transforms are used to solve the diffusivity equations. The model is verified on the basis of previous literature’s results and data of a field example from Moxi gas field in Southwest China. Through the parameters sensitivity analysis, the effects of prevailing factors on production performance are investigated. Results indicate that a large inner region radius and high mobility ratio can improve gas production rate in the early stage, while they also lead to a drastic decline of production rate in the late stage. Large permeability stress-dependent coefficient and low penetrated interval both have a negative impact on production rate. With its high efficiency and simplicity, this proposed approach can serve as a convenient tool to evaluate the behavior of partially penetrated production well in abnormally high-pressured composite naturally fractured gas reservoirs.

Sensitive Contrast-Enhanced Magnetic Resonance Imaging of Orthotopic and Metastatic Hepatic Tumors by Ultralow Doses of Zinc Ferrite Octapods

Contrast ability of magnetic nanoparticles (MNPs) is critical for the sensitive and accurate diagnosis of cancer with magnetic resonance imaging (MRI). However, the low sensitivity of current contrast agents (CAs) remains a major limitation to meet clinical needs. Shape and composition engineering are two important means for improving the contrast ability of MNPs. Herein, we report a facile method incorporating both approaches to manufacture zinc ferrite octapods with a large effective radius and various Zn doping ratios. The saturated magnetizations and T2 relaxivities of these engineered octapods both show an interesting trend with ascending zinc doping level. Particularly, the r2 value of ZnxFe3–xO4 (x = 0.44) octapods at 7.0 T is 989.1 mM–1s–1, and the highest T2 relaxivity ever reported, which is 10.2 times higher than the commercial sample, Feraheme. With the aid of this octapod sample, contrast-enhanced MRI (CE-MRI) for detecting orthotopic and metastatic hepatic tumors could be accomplished even a...

Unexpected trend for thermodynamic stability of Au@void@AgAu yolk-shell nanoparticles: A molecular dynamics study

In this study, Au@void@AgAu yolk-shell nanoparticles with different shell compositions were investigated by molecular dynamics simulation. Various Ag mole fractions in the shell of nanoclusters were considered. The results indicated that sizes of these nanoparticles play the key role in their thermodynamic stability, in such a way that the ratio of external radius to the thickness of the shell region should be decreased enough to reach a thermodynamic stability at room temperature. For small nanoparticles with large ratio of external radius to the thickness of the shell region, collapse of the shell leads to complete filling of the void space and creation of the more stable Au@AgAu core-shell or mixed nanoclusters. Hence, they are not stable at room temperature.

Coalescence-Induced Jumping of Two Unequal-Sized Nanodroplets.

Coalescence-induced self-propelled jumping of droplets on superhydrophobic surfaces has potential applications for condensation heat transfer enhancement, anti-icing, self-cleaning, antidew, and so forth. However, most of the previous studies focused on two identical droplets which are not commonly encountered in the nature. In this work, coalescence-induced jumping phenomena of two unequal-sized droplets on superhydrophobic surfaces were investigated theoretically and numerically. First, by introducing modified inertial-capillary velocity (uic*) and Ohnesorge number (Oh*) with consideration of radius ratio (r*) of two coalescing droplets, we proposed a generalized inertial-capillary scaling law for the jumping velocity of coalesced droplets, which is expected to be applicable for both two identical droplets and two unequal-sized droplets coalescing on superhydrophobic surfaces. Subsequently, we employed molecular dynamics simulations to investigate the coalescence-induced jumping process of two unequal-sized nanodroplets. Our simulations showed that the dimensionless jumping velocity (vj/uic*) well follows the generalized inertial-capillary scaling law with vj/uic* ≈ 0.127 in a specific Oh* range; however, it rapidly reduces and finally vanishes when the radius ratio of large droplet to small droplet is larger than a certain threshold value. Our simulations also revealed that nonjumping of two unequal-sized droplets with a very large radius ratio is due to that the larger droplet swallows the small one, so that the liquid bridge has no chance to impact the solid surface, and hence the "liquid bridge impacting substrate" mechanism fails in this circumstance.

Onset of Rayleigh–Bénard convection in cold water near its density maximum in vertical annular containers

A set of three-dimensional numerical simulations of Rayleigh–Benard convection in cold water near its density maximum in vertical annular containers is performed with the aim of determining the critical Rayleigh number at the onset of convection and the primary flow patterns for different geometric dimensions and density inversion parameters. The Prandtl number of cold water is about 11.57. The annular container is heated from below and cooled from above. The inner and outer sidewalls are considered to be perfectly adiabatic. The results obtained show that the critical Rayleigh number at the onset of convection increases with increase in the density inversion parameter and the radius ratio and with decrease in the aspect ratio. When the radius ratio is small, the flow patterns in vertical annular containers are similar to those in cylindrical containers. At large radius ratios the flow pattern is relatively simple, with several convective rolls observable along the azimuthal direction and similar with those characteristic of Rayleigh–Benard convection in the Boussinesq fluid. The stratified flow phenomenon is found to exist at moderate values of the density inversion parameter. The results are compared with those obtained in the Boussinesq fluid to reveal the effect of the density inversion parameter.

Effect of a porous layer on Newtonian and power-law fluids flows between rotating cylinders using lattice Boltzmann method

This study explores numerically the effect of a homogeneous porous layer on the Taylor–Couette flows in finite aspect ratio concentric annular ducts involving non-Newtonian fluids. The porous layer is attached to the outer cylinder which is kept at rest with the end walls, while the inner cylinder is rotating. The gray lattice Boltzmann method is used to obtain the flow field for fluids obeying to the power-law model. The purpose of this contribution is to analyze how the stability of the flow is affected when inserting the porous layer and using a non-Newtonian fluid. Thus, the combined effects of the inner cylinder Reynolds number (Re), the volume fraction of the solid (n s), the porous layer thickness (e) and the power-law index (n), on the flow characteristics are analyzed for a large radius ratio (η) and a finite aspect ratio (Γ). The results show that the presence of the porous layer has a stabilizing effect on the flow structure in the annulus. In fact, it delays the onset of the transition from Couette flow (C.F) to Taylor vortex flow. However, the flow tends to recover the C.F regime for the dilatants and Newtonian fluids as n s increases, while for the pseudo-plastic fluids, the flow shows another bifurcation beyond n s value approximately equal to 0.01.

Continuum and Kinetic Simulations of Heat Transfer Trough Rarefied Gas in Annular and Planar Geometries in the Slip Regime

Steady-state heat transfer through a rarefied gas confined between parallel plates or coaxial cylinders, whose surfaces are maintained at different temperatures, is investigated using the nonlinear Shakhov (S) model kinetic equation and Direct Simulation Monte Carlo (DSMC) technique in the slip regime. The profiles of heat flux and temperature are reported for different values of gas rarefaction parameter δ, ratios of hotter to cooler surface temperatures T, and inner to outer radii ratio R. The results of S-model kinetic equation and DSMC technique are compared to the numerical and analytical solutions of the Fourier equation subjected to the Lin and Willis temperature-jump boundary condition. The analytical expressions are derived for temperature and heat flux for both geometries with hotter and colder surfaces having different values of the thermal accommodation coefficient. The results of the comparison between the kinetic and continuum approaches showed that the Lin and Willis temperature-jump model accurately predicts heat flux and temperature profiles for small temperature ratio T=1.1 and large radius ratios R≥0.5; however, for large temperature ratio, a pronounced disagreement is observed.

Vertically-Aligned Single-Crystal Nanocone Arrays: Controlled Fabrication and Enhanced Field Emission.

Metal nanostructures with conical shape, vertical alignment, large ratio of cone height and curvature radius at the apex, controlled cone angle, and single-crystal structure are ideal candidates for enhancing field electron-emission efficiency with additional merits, such as good mechanical and thermal stability. However, fabrication of such nanostructures possessing all these features is challenging. Here, we report on the controlled fabrication of large scale, vertically aligned, and mechanically self-supported single-crystal Cu nanocones with controlled cone angle and enhanced field emission. The Cu nanocones were fabricated by ion-track templates in combination with electrochemical deposition. Their cone angle is controlled in the range from 0.3° to 6.2° by asymmetrically selective etching of the ion tracks and the minimum tip curvature diameter reaches down to 6 nm. The field emission measurements show that the turn-on electric field of the Cu nanocone field emitters can be as low as 1.9 V/μm at current density of 10 μA/cm(2) (a record low value for Cu nanostructures, to the best of our knowledge). The maximum field enhancement factor we measured was as large as 6068, indicating that the Cu nanocones are promising candidates for field emission applications.

Confined disordered strictly jammed binary sphere packings.

Disordered jammed packings under confinement have received considerably less attention than their bulk counterparts and yet arise in a variety of practical situations. In this work, we study binary sphere packings that are confined between two parallel hard planes and generalize the Torquato-Jiao (TJ) sequential linear programming algorithm [Phys. Rev. E 82, 061302 (2010)] to obtain putative maximally random jammed (MRJ) packings that are exactly isostatic with high fidelity over a large range of plane separation distances H, small to large sphere radius ratio α, and small sphere relative concentration x. We find that packing characteristics can be substantially different from their bulk analogs, which is due to what we term "confinement frustration." Rattlers in confined packings are generally more prevalent than those in their bulk counterparts. We observe that packing fraction, rattler fraction, and degree of disorder of MRJ packings generally increase with H, though exceptions exist. Discontinuities in the packing characteristics as H varies in the vicinity of certain values of H are due to associated discontinuous transitions between different jammed states. When the plane separation distance is on the order of two large-sphere diameters or less, the packings exhibit salient two-dimensional features; when the plane separation distance exceeds about 30 large-sphere diameters, the packings approach three-dimensional bulk packings. As the size contrast increases (as α decreases), the rattler fraction dramatically increases due to what we call "size-disparity" frustration. We find that at intermediate α and when x is about 0.5 (50-50 mixture), the disorder of packings is maximized, as measured by an order metric ψ that is based on the number density fluctuations in the direction perpendicular to the hard walls. We also apply the local volume-fraction variance σ(τ)(2)(R) to characterize confined packings and find that these packings possess essentially the same level of hyperuniformity as their bulk counterparts. Our findings are generally relevant to confined packings that arise in biology (e.g., structural color in birds and insects) and may have implications for the creation of high-density powders and improved battery designs.

Self-Propelled Droplet Removal from Hydrophobic Fiber-Based Coalescers.

Fiber-based coalescers are widely used to accumulate droplets from aerosols and emulsions, where the accumulated droplets are typically removed by gravity or shear. This Letter reports self-propelled removal of drops from a hydrophobic fiber, where the surface energy released upon drop coalescence overcomes the drop-fiber adhesion, producing spontaneous departure that would not occur on a flat substrate of the same contact angle. The self-removal takes place above a threshold drop-to-fiber radius ratio, and the departure speed is close to the capillary-inertial velocity at large radius ratios.