Effects Of Radius Ratio Research Articles

Effect of radius ratio on the sheared annular centrifugal turbulent convection

Effect of radius ratio on the sheared annular centrifugal turbulent convection

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.

Study on the Coalescence-Induced Jumping of Droplets with Different Radii on Superhydrophobic Surface

The phenomenon of droplet coalescence and jumping has received increasing attention due to its potential applications in the fields of condensation heat transfer and surface self-cleaning. Basic research on the process and mechanism of coalescence-induced droplet jumping has been carried out, and some universal laws have been established. However, it is found that the focus of these studies is based on two identical droplets, and the coalescence-induced jumping with different radii is rarely investigated, which is commonly encountered in nature. Therefore, it is essential to proceed with the research of coalescence and jumping of droplets with unequal radii. In this paper, molecular dynamics (MD) simulations are performed to reveal the effects of radius ratio and radius of small droplets on jumping velocity. The results show that as the increasing of radius ratio with an unchanged small droplet radius of 8.1 nm, the jumping velocity increases then decreases, which indicates there is an optimal radius ratio to maximize the jumping velocity. Additionally, it is found that if the small droplet radius is changed, the critical radius ratio for characterizing whether the coalesced droplet jumping increases with increasing the small droplet radius. Furthermore, according to energy conservation, the conversion efficiency of energy is discussed. The results show that when the radius ratio is greater than 1.3 with three different small droplet radii, the energy conversion efficiency rapidly decreases to below 1.0%; and the critical radius ratios are consistent with the result obtained from the velocity analysis. This work broadens the understanding of the more general phenomenon of coalescence-induced droplet jumping and can better guide industrial applications.

Thermally Driven Continuous Rolling of a Thick-Walled Cylindrical Rod.

Self-sustained motion can take advantage of direct energy extraction from a steady external environment to maintain its own motion, and has potential applications in energy harvesting, robotic motion, and transportation. Recent experiments have found that a thermally responsive rod can perform self-sustained rolling on a flat hot plate with an angular velocity determined by the competition between the thermal driving moment and the friction moment. A rod with a hollow cross section tends to greatly reduce the frictional resistance, while promising improvements in thermal conversion efficiency. In this paper, through deriving the equilibrium equations for steady-state self-sustained rolling of the thick-walled cylindrical rod, estimating the temperature field on the rod cross-section, and solving the analytical solution of the thermally induced driving moment, the dynamic behavior of the thermally driven self-sustained rolling of the thick-walled cylindrical rod is theoretically investigated. In addition, we investigate in detail the effects of radius ratio, heat transfer coefficient, heat flux, contact angle, thermal expansion coefficient, and sliding friction coefficient on the angular velocity of the self-sustained rolling of the thick-walled cylindrical rod to obtain the optimal ratio of internal and external radius. The results are instructive for the application of thick-walled cylindrical rods in the fields of waste heat harvesters and soft robotics.

Dynamic theory of composite anisogrid lattice conical shells with nonconstant stiffness and density

Composite anisogrid lattice conical shells have been applied in rocket adapters and the fundamental frequency is the basic design parameter. Free vibrations of three types composite anisogrid lattice conical shells under arbitrary boundary conditions are considered in the paper to theoretically evaluate the fundamental frequency. The lattice conical shell is modelled as continuous conical shell with nonconstant stiffness and density by equivalent continuum method. Employing Fourier decomposition and power series method, the governing equations of motion based on Donnell-type thin shell theory are converted into the recurrence relations through identity transformation. Displacement and stress resultants expressions containing the unknown frequency and initial terms are obtained through the recurrence relations, respectively. Compared with the previous and numerical results, the analytical solution is remarkably accurate and efficient in dealing with lattice conical shells with variable cross-sectional radius, nonconstant stiffness and density for different number of circumferential waves and geometric parameters. The effect of radius ratio, height of cone and thickness of ribs on the value of frequencies is studied using a parametric analysis. It is shown that the fundamental frequency increase with the thickness of rib and decrease with increasing height of cone.

Electrohydrodynamic enhancement of phase change material melting in cylindrical annuli under microgravity

Latent heat thermal energy storage has been recommended as an effective technology for the thermal management system of space exploration for its excellent ability to store thermal energy. However, the low thermal conductivity of phase change material seriously weakens the heat charging and discharging rates of the system. This paper introduces electrohydrodynamic, a popular active heat transfer enhancement technology, to enhance the phase change material melting in a shell-tube latent heat storage system under microgravity using the lattice Boltzmann method. Different from some previous works, this work mainly focuses on the combined effects of the electric Rayleigh number T and different gravity conditions on the melting performance. Results indicate that the electrohydrodynamic technique always shows good performance in accelerating the melting process, and especially, it is possible to save up to 90% of the charging time in some cases. In addition, the effect of eccentricity on the charging process is investigated, and it is found that although the concentric annulus is always the optimal configuration under no-gravity conditions, the optimal eccentric position of the internal tube largely depends on the electric Rayleigh number if one takes into account the gravity effects. Moreover, the effect of radius ratio Γ on the heat storage efficiency is also evaluated under no-gravity condition. The present work illustrates that electrohydrodynamic is an effective method for enhancing phase change material melting even under microgravity. • The combined effect of Coulomb force and gravity on PCM melting is studied. • The transient charging process of PCM melting under an electric field is presented. • A maximum of over 90% melting time saving can be observed under an electric field. • The optimal eccentric position relies on the gravity condition.

Effects of radius ratio on annular centrifugal Rayleigh–Bénard convection

We report on a three-dimensional direct numerical simulation study of flow structure and heat transport in the annular centrifugal Rayleigh–Bénard convection (ACRBC) system, with cold inner and hot outer cylinders corotating axially, for the Rayleigh number range $Ra \in [{10^6},{10^8}]$ and radius ratio range $\eta = {R_i}/{R_o} \in [0.3,0.9]$ ( $R_i$ and $R_o$ are the radius of the inner and outer cylinders, respectively). This study focuses on the dependence of flow dynamics, heat transport and asymmetric mean temperature fields on the radius ratio $\eta$ . For the inverse Rossby number $Ro^{-1} = 1$ , as the Coriolis force balances inertial force, the flow is in the inertial regime. The mechanisms of zonal flow revolving in the prograde direction in this regime are attributed to the asymmetric movements of plumes and the different curvatures of the cylinders. The number of roll pairs is smaller than the circular roll hypothesis as the convection rolls are probably elongated by zonal flow. The physical mechanism of zonal flow is verified by the dependence of the drift frequency of the large-scale circulation (LSC) rolls and the space- and time-averaged azimuthal velocity on $\eta$ . The larger $\eta$ is, the weaker the zonal flow becomes. We show that the heat transport efficiency increases with $\eta$ . It is also found that the bulk temperature deviates from the arithmetic mean temperature and the deviation increases as $\eta$ decreases. This effect can be explained by a simple model that accounts for the curvature effects and the radially dependent centrifugal force in ACRBC.

Direct numerical simulation of turbulent heat transfer in concentric annular pipe flows

The effect of radius ratio on turbulent convective heat transfer within a concentric annular pipe has been studied using direct numerical simulation. Four radius ratios (Ri/Ro = 0.1–0.7) have been compared at a fixed Reynolds number, where Ri and Ro denote the radii of the inner and outer pipes, respectively. The statistical moments of the temperature field, budget balances of the temperature variance and turbulent heat fluxes, and turbulence structures that dominate the heat transfer process have been thoroughly studied in both physical and spectral spaces. It is observed that the radius ratio has a significant impact on the Nusselt numbers and skin friction coefficients of the inner and outer cylinder walls, and on the interaction of thermal boundary layers developed over these two curved walls. Owing to the curvature difference between the two cylinder surfaces, the thermal boundary layer developed over the outer cylinder wall is thicker than that over the inner cylinder wall. Also, turbulent heat transfer is more intense on the outer cylinder side than on the inner cylinder side. As the radius ratio decreases, the difference in turbulence statistics between the inner and outer cylinder sides becomes increasingly pronounced. It is also observed that both axial and azimuthal characteristic length scales of the most energetic turbulent thermal structures are larger on the inner cylinder side than on the outer cylinder side.

Experimental Study of Heat Transfer Through a Rotating Heat Exchanger.(Dept.M)

This paper presents an experimental study of laminar forced convection heat transfer for hot water flows through annular tube. The test section consisted of two annular spaces the inner of which for the tested hot water while the outer for the cooling water flow. The inner tube of the annulus is rotated with rotational speeds which varied from 350 to 1580 r.p.m. to give rotational Reynolds number in the range (200 ≤ Rew ≤ 1750). The outer tube of the annulus have a constant diameter of 54.5mm but the diameter of inner tube of the annulus is equal to 21, 26.5 and 33.5 mm so as to give diameter ratios of 0.62, 0.487 and 0.386. Hot water flowed axially through the annular space with velocities that ranged from (0.0042 to 0.52m/s), to give axial Reynolds numbers in the range (130 ≤ Re ≤ 2300), to cover the laminar flow regime. The cold water that flowed through the outer annulus was cooled by a refrigerating circuit. In the present work the effects of radius ratio, axial Reynolds number and rotational Reynolds number on the heat transfer were investigated. The results show that. The rate of heat transfer of the rotational annular tubes is higher than that of the stationary one of the same radius ratio and axial Reynolds numbers. An increase of as much as 200% in the heat transfer is observed for annular tube of radius ratio equal to 0.62, Reynolds number of 1951 rotational Reynolds number of 1744 and rotational speed of 1580 r.p.m respectively, a decreases in heat transfer as much as 36% in the heat transfer is reported for annular tube of radius ratio (B) of 0.386, Reynolds number of 1100, and rotational Reynolds number of 205 and rotational speed of 350 r.p.m respectively. Two correlations were established between Nusselt number and axial Reynolds number. Rotational Reynolds number and radius ratio of the annulus for both stationary and rotational annular tubes.

Effects of radius ratio on turbulent concentric annular pipe flow and structures

The effects of radius ratio on fully-developed turbulent concentric annular pipe flow and structures are investigated using direct numerical simulations (DNS). In order to study the radius ratio effects on the flow, four radius ratios of a concentric annular pipe (for Ri∕Ro=0.1–0.7) are compared at a fixed Reynolds number. Here, Ri and Ro are the radii of the inner and outer pipes, respectively. It is observed that the radius ratio influences the interaction of two boundary layers developed over the inner and outer cylinder surfaces, and has a significant impact on the turbulent flow structures and dynamics. The characteristics of the flow field are investigated in both physical and spectral spaces, which include the analyses of the first- and second-order statistical moments, budget balance of the transport equation of Reynolds shear stress, two-point correlation coefficients, and premultiplied spectra of velocity and vorticity fluctuations.

On Investigation of Liquid Sloshing in Cylindrical Tanks With Single and Multiply Connected Domains Using Isogeometric Boundary Element Method

Abstract This paper explores an isogeometric boundary element method (IGA-BEM) for sloshing problems in cylindrical tanks with single and multiply connected domains. Instead of the Lagrange basis functions used in the standard BEM, the nonuniform rational B-splines (NURBS) basis functions are introduced to approximate the geometries of the problem boundaries and the unknown variables. Compared with the Lagrange basis functions, NURBS basis functions can accurately reconstruct the geometric boundary of analysis domain with almost no error, and all the data information for NURBS basis functions can be directly obtained from the computer-aided design (cad) or computer-aided engineering (cae) commercial software, which implies the modeling process of IGA-BEM is more simple than that of the standard BEM. NURBS makes it possible for the IGA-BEM to realize the seamless connection between cad and cae software with relative higher calculation accuracy than the standard BEM. Based on the weighted residual method as well as the divergence theorem, the IGA-BEM is developed for the single and multiply connected domains, whose boundaries are separately defined in the parameter space by different knot vectors. The natural sloshing frequencies of the liquid sloshing in a circular cylindrical tank with a coaxial or an off-center circular pipe, an elliptical cylindrical tank with an elliptical pipe, a circular cylindrical tank with multiple pipes are estimated with the introduced method by assuming an ideal (inviscid and incompressible) liquid, irrotational small-amplitude sloshing, and the linear free-surface condition. The comparison between the results obtained by the proposed method and those in the existing literatures shows very good agreements, which verifies the proposed model well. Meanwhile, the effects of radius ratio, liquid depth, number, and location of internal pipe (pipes) on the natural sloshing frequency and sloshing mode are analyzed carefully, and some conclusions are outlined finally.

Study of Heat Transfer through Radially Buffled Rotating Annulus

This paper presents an experimental study of laminar forced convection heat transfer for cold water flows through annular tube. The test section consisted of two annular spaces the inner of which for the tested cold water while the outer for the heating water flow that heated in a tank by electrical heaters. The inner tube of the annulus is rotated with rotational speeds which varied from 175 to 650 p.m. The outer tube of the annulus have a constant diameter of 54.5mm but the diameter of inner tube of the annulus is equal to 21, 26.7 and 33.5 mm so as to give diameter ratios of 0.62, 0.49 and 0.386. Variable numbers of semi-circle of buffles are used circumferentially around the rotating tube. The diameter of buffle is equal to 52 mm and 2 mm thickness. Cold water flowed axially through the annular space with velocities that ranged from (0.0192 to 0.0666 m/s), to give axial Reynolds numbers in the range (612 ≤ Re ≤ 2175), to cover the laminar flow regime. In the present work the effects of radius ratio, axial Reynolds number and rotational Reynolds number on the heat transfer are investigated. The results show that, the rate of heat transfer of the rotational annular tubes is higher than that of the stationary one of the same radius ratio and axial Reynolds numbers, and the rate of heat transfer of rotational annular tubes with baffles higher than rotational annular tubes without baffles of the same radius ratio and axial Reynolds numbers. 1- Rotation without buffle, Produces a maximum increase in Nusselt number of the order of 79% at Rea= 2080, Reω= 21545 and rotational speed= 480 r.p.m, (increase of only 11% in Nusselt number at Rea=646.2, Reω=7808 ,( rotational speed of 175 r.p.m), compared to the results of the stationary annular tube of the same radius ratio . 2- Rotation with buffle, Produces a maximum increase in Nusselt number of 200% at Rea= 2175, Reω= 21884 and rotational speed= 480 r.p.m, and increase of only 84% in Nusselt number at Rea=634, Reω=21778, ( rotational speed of 175 r.p.m) compared to the results of the stationary annular tube of the same radius ratio . Two correlations are made to describe the relation between Nusselt numbers and axial Reynolds number, diameter ratio, pitch to diameter ratio P/D and rotational Reynolds number and Reynolds number for stationary, rotating and buffled rotating annuli.

On non-axisymmetric flow structures of graphene suspensions in Taylor–Couette reactors

The quality of graphene sheets significantly depends on the degree of oxidation of graphite and the methods used for synthesis. Therefore, selecting an eco-friendly and cost-effective process is an important step in order to increase the oxidation level. The latest studies show that Taylor–Couette reactors are one of the best options to improve the oxidation level of graphite. Graphene suspensions show shear-thinning behavior, and the emergent flow structures in TC flows significantly influence the oxidation degree. In this study, we investigated the flow patterns of shear-thinning fluids in a TC reactor. The effect of radius ratio, power-law index and the rotating direction of the cylinders on the flow patterns and their critical values is studied experimentally in a Taylor–Couette flow that occurred between concentric cylinders. The Reynolds numbers defined with the wall shear viscosities (Rei and Reo) are used for evaluating the critical conditions of various flow structures. The results demonstrate that fluid properties and radius ratio may have significant destabilization effects in forming non-axisymmetric flow patterns and change their critical values. The characteristics of various flow regimes are altered substantially with increasing inner cylinder speed. A strong influence of the rotation direction of the outer cylinder on flow structures and their critical Reynolds numbers has also been revealed in this study. The obtained results also provide a deeper understanding of fluid–suspension interactions in TC reactors. These new findings will help in designing and developing more efficient TC reactors to be used in synthesizing high-quality graphene products.

A numerical analysis to study the effect of radius ratio and attachment angle on hybrid hydrokinetic turbine performance

The performance of a hybrid hydrokinetic turbine rotor under different operating conditions is investigated in this study. The effects of radius ratio and attachment angle on the power coefficient, self-starting capability and flow behavior around the turbine were simulated using commercially available software, ANSYS, with FLUENT as the solver. Realizable k-ε turbulence model has been used to solve the unsteady Reynolds-Averaged Navier-Stokes (URANS) equations. We compared the performance of a hybrid hydrokinetic turbine with the identically designed hybrid wind turbine under similar input power conditions. The hydrokinetic turbine rotor showed better performance than that of wind turbine rotor. The maximum values of coefficient of power (CP) and torque (CT) occur at attachment angles of 30° and 60° for a radius ratio of 0.2. At an attachment angle of 90°, fluctuations in CT were minimized.

Measurements of velocity fields in 90° elbow and the effect of radius ratio

Measurements of velocity fields in 90° elbow and the effect of radius ratio

Assessment and optimization of hydrothermal characteristics for a non-Newtonian nanofluid flow within miniaturized concentric-tube heat exchanger considering designer’s viewpoint

Flow and convective heat transfer of a non-Newtonian nanofluid containing Cu nanoparticles in the annuli are investigated. The base fluid is solution of 0.4wt% Carboxymethyl Cellulose (CMC) in water, having pseudo-plastic behavior. Increasing the concentration and reducing the particle size enhance the pressure drop of the nanofluid flow, and also the convective heat transfer coefficient on both inner and outer walls of the annulus. Meanwhile by narrowing the annulus, the pressure drop intensifies and the convective heat transfer coefficient of the outer wall improves, while that of the inner wall decreases. The influence of the concentration changing on the pressure drop is more significant than the effects of radius ratio and particle size. Using the data obtained from the numerical simulations, an Artificial Neural Network (ANN) model is developed to predict the convective heat transfer coefficients in both wall, and the pressure drop in terms of radius ratio, volume concentration, and particle size. In addition, the Genetic Algorithm (GA) coupled with compromise programming technique is employed for optimization in order to find the optimal cases with maximum heat transfer and minimum pressure drop considering designer’s viewpoint. Eventually, the optimum states are suggested and discussed under the various conditions.

Steady deformation characteristics of double emulsion droplet in shear flow

The manipulation of double emulsion droplet via shear flow field is widely encountered in microfluidic devices. However, the interface evolution and hydrodynamics behavior of double emulsion droplet in shear flow is less understood till now. In this paper, a theoretical model of double emulsion droplet in a Couette flow device is developed and numerically analyzed to characterize the interface behavior of incompressible double emulsion droplet, which is also verified by a visualization experiment. Based on this model, the mechanisms underlying the steady deformation of double emulsion droplet under shear are investigated, and the effects of radius ratio of inner droplet to the outer one and viscosities of working fluids on the steady deformation are discussed. The results indicate that the steady deformation of double emulsion droplet in the shear increases with the rise in capillary number, and the deformation resistance of inner droplet is larger than that of the outer droplet. With increasing the radius ratio of inner droplet to the outer one, the interaction between the inner and outer droplets becomes great and thus the deformation degree of the inner droplet is increased. In addition, the effect of big deformation resistance by the inner droplet tends to be obvious, leading to decreasing the deformation degree of outer droplet. The viscosities of both inner and outer droplets provide a resistance for the deformation of double emulsion droplet. With the rises in viscosities of inner and outer droplets, the deformation degree of integral double emulsion droplet decreases. In addition, the effect of outer droplet viscosity on the steady deformation is more obvious than that of the inner droplet.

Heat Transfer in Enclosure of Composite Material with Fibers Arranged in Various Geometrical Arrays

paper presents a numerical investigation for natural convection of air in a three dimensional inclined annulus enclosure. This study wills exam the effect of radius ratio of an annulus made from graphite/epoxy laminated composite material on heat transfer taking two types of optimization of effective thermal conductivity in consideration: minimization and maximization of thermal conductivity. The annulus enclosure is filled with porous media between two concentric cylinders with 12 fins attached to the inner cylinder. Two cases are taken for the inclination angle of the annulus: horizontal and vertical annulus. The system is under steady state condition and constant walls temperature boundary condition. The parameters affected on the system are modified Rayleigh number (10 ≤Ra * ≤ 500), the annulus inclination angle δ (0 o and 90 o ) and the radius ratio Rr= (RI/RO)=0.2, 0.3, 0.4 and 0.5. For all parameters, results showed that Nusselt number decrease with the decrease of the radius ratio Rr (which means larger gap) for the outer cold cylinder. the average Nu number increases with an increase in modified Rayleigh number and decrease with the increase of δ for high values of Ra * , but hardly affected by δ for low values of Ra * . The deviation between the average Nu for the maximization and minimization of the thermal conductivity is equal to 5.1% for horizontal annulus δ=0 o and 10% for vertical annulus δ=90 o . Local Nu increases with the length of the cylinder and the effect of the fins attached to the inner cylinder is more significant for the horizontal cylinder because of its hindering effect. A correlation for the average Nusselt number in terms of Ra * and δ, has been developed for the outer cold cylinder. General Terms Cp: Specific heat at constant pressure (kJ/kg o C), g: Acceleration due to gravity (m/s 2 ), Hf: Fin length (m) kf: Thermal conductivity of the fluid (W/m K), ks: Thermal conductivity of the solid (W/m K), keff.: Effective thermal conductivity of the porous media (W/m K), K: Permeability (m 2 ), l: cylinder length (m), L: Dimensionless cylinder length, NuLocal 1 : Local Nusselt number on the inner surface, NuLocal 2: Local Nusselt number on the outer surface, Nu 1: Mean Nusselt number on the inner surface, Nu 2: Mean Nusselt number on the outer surface, p: Pressure (N/m2), q: Local heat flux (m), r: Radial coordinate (m), R: Dimensionless radial coordinate, Ra*: Modified Rayleigh number, Rr: Radius ratio, S: fin pitch (m), T: Temperature (K), ur,uϕ, uz: velocity component in r,ϕ and z - direction (m/s), Ur, Uϕ, Uz: Dimensionless velocity component in R, ϕ and Z direction, x, y, z: Cartesian coordinate system (m), Z: Dimensionless axial coordinate, αf : Fluid thermal diffusivity (m 2 /s), αs : Solid thermal diffusivity, αeff. : Effective thermal diffusivity (m 2 /s),

A mechanical model of overnight hair curling.

Based on the observation of overnight hair curling procedure, we establish a mechanical model to describe the temporary wave formation of straight hair (initial curvature is zero), which incorporates the contact between hair and hair roller. Systematic studies are carried out to explore the effects of radius ratio between hair and hair roller, hair's average axial strain, creep time, Poisson's ratio and gravity on the curl retention. The variation of curl retention with respect to time obtained from our numerical model is validated by a simple theoretical model and by overnight curling experiments on hair samples. The results of simulation show that overnight hair curling is suitable to create a wavy hairstyle within about 7 hours, while the combined usage with hair fixatives enables a wavy hairstyle with desired curvature that lasts for a day or more.