Non-circular Geometry Research Articles

ICONE15-10272 Critical Heat Flux correlation for thin rectangular channels

The effect of heated length on Critical heat flux (CHF) in thin rectangular channels was studied based on CHF data obtained under atmospheric pressure. CHF in small channels has been widely studied in the past decades but most of the studies are related to CHF in round tubes. Although basic mechanisms of burnout in thin rectangular channels are similar to tubes, applicability of CHF correlations for tubes to rectangular channels are questionable since CHF in rectangular channels are affected by the existence of non-heated walls and the non-circular geometry of channel circumference. Several studies of CHF in thin rectangular channels have been reported in relation to thermal hydraulic design of research reactors and neutron source targets and CHF correlations have been proposed, but the studies mostly focus on CHFs under geometrical conditions of the application of interest. In his study, existing CHF data obtained in thin rectangular channels were collected and the effect of heated length on CHF was examined. Existing CHF correlations were verified with positive quality flow CHF data but none of the correlations successfully reproduced the CHF for a wide range of heated length. A new CHF correlation for quality region applicable to a wide range of heated length was developed based on the collected data.

Critical flux and rejection behaviour of non-circular-channelled membranes: Influence of some operating conditions

The use of non-circular-channelled membranes and their performance in microfiltration as a pretreatment for desalination for wastewater reuse has been investigated. The membrane performance is appraised with reference to (i) fouling propensity, i.e. critical flux and (ii) removal of organics in terms of solid retention efficiency. Critical flux determinations were carried out, employing the step by step technique, under various operating conditions (Reynolds number, pore sizes and pH). The effluent from a synthetic activated sludge production process was used. Increasing the pore size appears to increase the critical flux as does increased pH. It is shown that the non-circular geometry gives rise to a very high critical flux under turbulent flow conditions whilst the opposite effect is observed under laminar flow conditions. Solid retention efficiencies of up to 92% are achieved which decrease with increment in pore size. It seems that a larger pore size allows the transmission of smaller particles accounting for the lower rejection seen. All observations are explained in terms of the hydrodynamics and particle interactions involved within the membrane filtration system.

Electrokinetic characterisation of cleaned non-circular multi-channelled membranes

The possibility of using the electroviscous effect to measure the zeta potential of previously fouled non-circular multi-channelled membranes is assessed. Measurements of the streaming potential and permeate flow reduction arising from the electroviscous effect were carried out at various electrolyte (KCl) concentrations and pH. Under similar conditions both techniques identify the isoelectric point although the magnitude of the zeta potentials found from the electroviscous measurements is greater than those found from streaming potential measurements even when surface conductivity has been taken into consideration through the inclusion of correction factors. Differences between the two methods increase when comparison is made between virgin and used membranes. These differences are attributed to inadequacies in the electrokinetic flow theory used to convert electroviscous ratios to zeta potentials. Furthermore, the non-circular multi-channel geometry may introduce some discrepancies in calculations using electroviscous theory. The results suggest that the electroviscous technique is suitable for the determination of the isoelectric point but not the complete characterisation of used membranes.

Failure Initiation and Progression in Internally-Pressurized Elliptical Composite Cylinders

The article addresses the influences of the noncircular geometry and material orthotropy on the predictions of initial material failure, and the extent and spatial distribution of subsequent failures, within the walls of internally-pressurized laminated composite cylinders with elliptical cross sections. The work is focused on the potential for leakage through the cylinder walls due to the accumulation of matrix cracking and fiber breakage within the layers. The predictions are based on numerical analyses using the finite-element code STAGS. Three different graphite-epoxy lamination sequences, a quasi-isotropic, a circumferentially stiff, and an axially stiff lamination sequence, are considered to investigate the effects of material orthotropy. Three different failure criteria and several material degradation schemes are considered to determine the sensitivity of the predictions to these steps in the analyses. It is shown that the spatial location of and pressure level to produce the initial failure, which occurs in the form of matrix cracking, is highly dependent on the lamination sequence. The pattern of failure accumulation due to increased pressure levels following initial failure is also dependent on the lamination sequence. No leakage is predicted for pressures to the level to produce first fiber failure. However, leakage is predicted for just a slight increase in pressure beyond that pressure, and the leakage pressure level and location are virtually independent of lamination sequence. The location of the first matrix failure and the spatial pattern of subsequent failures are not overly sensitive to the failure criterion used or the details of material degradation scheme employed. The pressure levels to produce the first matrix and first fiber failures are dependent on the failure criterion used.

Critical flux determination of non-circular multi-channel ceramic membranes using TiO 2 suspensions

The step by step technique was used to determine the critical flux of non-circular multi-channel ceramic membranes under various operating conditions (crossflow velocities, surfactant content, coagulant content, valency, pH and ionic strength) using TiO 2 suspensions. It is shown that the non-circular geometry gives rise to a very high critical flux under turbulent flow conditions whilst the opposite effect is observed under laminar flow conditions. The impact of surfactant and coagulant additions on critical flux is also considered. Addition of surfactant decreases the critical flux whilst in contrast, coagulant addition results in a higher critical flux. All the various observations are explained in terms of the hydrodynamics and particle interactions involved within the filtration system.

Double-clad Yb3+-doped fiber lasers with virtually non-circular cladding geometry

We report on cladding pumped Yb3+-doped fiber lasers with novel virtual-non-circular cladding geometries. The geometry of the inner-cladding is in fact circular with D-shaped holes, that form virtual-D (one hole), and virtual-square (four holes) structures for the inner-cladding. The laser performance of this structure is assessed at several wavelengths and lengths, showing efficiencies as high as those obtained with other geometries for the inner-cladding.

Plasma shape and finite β effects on stability thresholds of the ion temperature gradient modes

The stability of electromagnetic ion temperature gradient driven modes with emphasis on the lower and upper stability thresholds is investigated by a collisionless magnetized plasma in both circular and noncircular geometry. The stability properties are discussed and the results are compared for finite β effects, arbitrary elongation, and Shafranov shift rate parameters. It has been found that the lower stability thresholds are weakly dependent on the (combined) effects of Shafranov shift rate, finite β, and elongation whereas the second (upper) stability regime shows substantial dependence depending on parameter regimes.

Analysis, Design, and Modeling of a Rotary Vane Engine (RVE)

This paper introduces a mathematical model to analyze the dynamic behavior of a novel rotary vane engine (RVE). The RVE can be considered to have a number of advantages when compared to a majority of other reciprocating engine types. The advantages are found in the simple structure and the small number of moving parts. In this paper the geometrical structure and dynamical behavior of engines with a different number of work units is considered in detail. This has been examined through a study of torque transmission with a particular reference to how this is affected by the noncircular geometry of gear pitch curves. Using the Coulomb friction model, consideration has been given to the mechanical power loss due to friction in different parts of the engine, which must be taken into account. The study also proposes a possible method for balancing of asymmetric cogwheels. The analysis concludes that by using an appropriate design and arrangement of cogwheels and all moving parts, vibration can be attenuated due to impulsive gas torque.

Dynamics and structure of transitional buoyant jet diffusion flames with side-wall effects

Transitional buoyant jet diffusion flames are investigated by 3D spatial direct numerical simulations (DNS). To examine the fuel source and sidewall geometry effects, a comparative study has been performed with four computational cases: a free-square and a free-round case in an open boundary domain, and a corner-square and a corner-round case in a half-confined domain. A grid system of 192 × 192 × 288 nodes is employed to obtain the near field flame structure. Due to the absolute buoyancy instability and the geometry effects, the jet diffusion flames display unsteady dynamic behavior. The free-square case is more vortical and entrains more efficiently than the free-round case because of the self-induced vortex deformation associated with the noncircular source geometry. The sidewall effects are found to be significant on the flame structure because of the wall-induced vortex deformation. Compared with the free cases, the corner cases have a stronger tendency of transition to turbulence. The corner-round case has a much stronger wall-induced vortex deformation than the corner-square case. Although the corner cases are half-confined in the cross-streamwise directions, entrainment of the corner cases is not lower than that of the free cases due to the wall-induced vortex deformation. In contrast with the comparison between the free-round and free-square cases, entrainment of the corner-round case is higher than that of the corner-square case.

On the stabilization of ion temperature gradient modes by finite beta in noncircular toroidal devices

The stability of electromagnetic ion temperature gradient driven (ITG) modes is investigated using an advanced fluid model allowing for arbitrary elongation (κ) and finite beta effects and Shafranov shift. The ITG eigenvalue equation is solved numerically and the stability properties are discussed and compared in both the circular and noncircular geometry. It is found that elongation and finite beta effects reduce the growth rates and that the ITG mode is stabilized for large beta values as elongation is increased. Elongation increases the beta threshold of the ITG mode with larger growth rates as compared with circular geometry.

Comparison of the Flame Characteristics of Turbulent Circular and Elliptic Jets in a Crossflow

This study was directed to understand the coupling effects of the noncircular geometry of the burner and a crossflow on the combustion of gas jets. This paper compares the characteristics of turbulent propane jet flames from circular (diameter=0.45 cm) and elliptic (major axis/minor axis=3) burners of equivalent exit area in a crossflow. The elliptic burner was oriented with its major axis or minor axis aligned with the crossflow. Experiments were conducted in a wind tunnel provided with optical and probe access and capable of wind speeds up to 12.5 m/s. The burners were fabricated with metal tubes. Instrumentation included a Pt-Pt/13% Rh thermocouple, a quartz-probe gas sampling system, chemiluminescent and nondispersive infrared analyzers, a video-recorder, and a computer data acquisition system. The measurements consisted of the upper and lower limits of jet velocity for a stable flame, flame configuration, and visible length. Flame structure data including temperature profiles and concentration profiles of CO2,O2, CO, and NO were obtained in a two-zone flame configuration (at jet to crossflow momentum flux ratio=0.11), where a planar recirculation exists in the wake of the burner tube followed by an axisymmetric tail. The relative emission indicators of CO and NO were estimated from the composition data. Results show that the upper and lower limits of the fuel jet velocity increase with the crossflow velocity for all burners, and the rate of increase is highest for the elliptic burner with its minor axis aligned with the crossflow. That burner configuration also produces the longest flame. The relative emission indicators show that the CO production is lower and NO production is higher with elliptic burners than with circular burners in crossflow.

Intravascular ultrasound scan evaluation of the obstructed vein

Purpose: The purpose of this study was the comparison of intravascular ultrasound scanning (IVUS) with transfemoral venography in the assessment of chronic iliac vein obstruction. Methods: IVUS and standard, single-plane, transfemoral venography were performed in 304 consecutive limbs during balloon dilation and stenting of an obstructed iliac venous segment. The appearance of the obstruction was described, and the degree of stenosis (maximal diameter reduction) was estimated with venography and IVUS. The stenotic area was derived with diameter calculations (πr2) and also was measured with the built-in software of the IVUS apparatus before and after dilation and stenting in 173 limbs. Preoperative hand/foot differential pressure and preoperative dorsal foot venous and intraoperative transfemoral hyperemia-induced pressure elevations after intra-arterial injection of papaverine hydrochloride were measured. Results: With IVUS, fine intraluminal and mural details were detected (eg, trabeculation, frozen valves, mural thickness, and outside compression) that were not seen with venography. The median stenosis (with diameter reduction) on venographic results was 50% (range, 0 to 100%) and on IVUS results was 80% (range, 25% to 100%). In a comparison with IVUS as the standard, venography had poor sensitivity (45%) and negative predictive value (49%) in the detection of a venous area stenosis of >70%. The actual stenotic area was more severe when measured directly with IVUS (0.31 cm2; range, 0 to 1.68 cm2) versus derived (0.36 cm2; range, 0 to 3.08 cm2; P <.001), probably as a result of the noncircular lumen geometry of the stenosis. No correlation was found between any of the preoperative or intraoperative pressure measurements and degree of stenosis with or without collaterals. When collaterals were present, a more severe stenosis (median, 85%; range, 25% to 100%) was observed (versus a 70% stenosis in the absence of collaterals; range, 30% to 99%; P <.001), along with actual stenotic area (with collaterals: median, 0.24 cm2; range, 0 to 1.18 cm2; without collaterals: median, 0.45 cm2; range, 0.02 to 1.68 cm2; P <.01) and a higher rate of hyperemia-induced pressure gradient (≥2 mm Hg; with collaterals, 34%; without collaterals, 11%; P <.05). Conclusion: Venous IVUS appears to be superior to single-plane venography for the morphologic diagnosis of iliac venous outflow obstruction and is an invaluable assistance in the accurate placement of venous stents after venoplasty. No preoperative or intraoperative pressure test appears to adequately measure the hemodynamic significance of the stenosis. In lieu of adequate hemodynamic tests, IVUS determination of morphologically significant stenosis appears to be presently the best available method for the diagnosis of clinically important chronic iliac vein obstruction. Collateral formation should perhaps be looked on as an indicator of a more severe stenosis, although significant obstruction may exist with no collateral formation. (J Vasc Surg 2002;35:694-700.)

Direct Numerical Simulation of Transitional Noncircular Buoyant Reactive Jets

The near field dynamics of transitional buoyant reactive jets established on noncircular geometries, including a rectangular nozzle with an aspect ratio of 2:1 and a square nozzle with the same cross-sectional area, are investigated by three-dimensional spatial direct numerical simulations. Without applying external perturbations at the inflow boundary, large vortical structures develop naturally in the flow field due to buoyancy effects. Simulation results and analysis describe the details and clarify mechanisms of vortex dynamics of the noncircular buoyant reactive jets. The interaction between density gradients and gravity initiates the flow vorticity. Among the major vorticity transport terms, the gravitational term mainly promotes flow vorticity in the cross-streamwise direction. For the baroclinic torque, it can either create or destroy flow vorticity depending on the local flow structure. The vortex stretching term has different effects on the streamwise and cross-streamwise vorticity. Streamwise vorticity is mainly created by vortex stretching, while this term can either create or destroy cross-streamwise vorticity. Under the coupling effects of buoyancy and noncircular nozzle geometry, three-dimensional vortex interactions lead to the transitional behavior of the reactive jets. Simulations also show that the rectangular jet is more vortical than the square jet. The rectangular jet has a stronger tendency of transition to turbulence at the downstream due to the aspect ratio effect. Mean flow property calculations show that the rectangular buoyant reactive jet has a higher entrainment rate than its square counterpart.

Mixing in frozen and time-periodic two-dimensional vortical flows

In the first part of this paper, we investigate passive scalar or tracer advection–diffusion in frozen, two-dimensional, non-circular symmetric vortices. We develop an asymptotic description of the scalar field in a time range 1 [Lt ] t/T [Lt ] Pe1/3, where T is the formation time of the spiral in the vortex and Pe is a Péclet number, assumed much larger than 1. We derive the leading-order decay of the scalar variance E(t) for a singular non-circular streamline geometry,The variance decay is solely determined by a geometrical parameter μ and the exponent β describing the behaviour of the closed streamline periods. We develop a method to predict, in principle, the variance decay from snapshots of the advected scalar field by reconstructing the streamlines and their period from just two snapshots of the advected scalar field.In the second part of the paper, we investigate variance decay in a periodically moving singular vortex. We identify three different regions (core, chaotic and KAM-tori). We find fast mixing in the chaotic region and investigate a conjecture about mixing in the KAM-tori region. The conjecture enables us to use the results from the first section and relates the Kolmogorov capacity, or box-counting dimension, of the advected scalar to the decay of the scalar variance. We check our theoretical predictions against a numerical simulation of advection–diffusion of scalar in such a flow.

Effects of cross-sectional elongation on the resistive edge modes

Resistive edge modes in a shifted noncircular tokamak geometry are investigated in the electrostatic limit. The reduced Braghinskii equations are used as a model for the electrons and an advanced fluid model for the ions. An eigenvalue problem is derived from these equations which is solved numerically. It is found that the resistive ballooning modes are stabilized by plasma elongation for peaked density profiles. In addition, it is found that the resistive ηi-modes (ηi=Ln/LTi) may be either stabilized or destabilized by elongation depending on the collision frequency.

Internally pressurized elliptical composite cylinders

Presented is the development of a semi-analytical solution based on minimization of the total potential energy, the Rayleigh–Ritz procedure, and the Kantorovich method, to study the response of elliptical composite cylinders to internal pressure. Using the solution, the response of a quasi-isotropic elliptical cylinder is compared with the response of a quasi-isotropic circular cylinder to study the effects of noncircular geometry. The distinguishing features of the response of an ellipse are the inward normal displacement at the ends of the major diameter that occur despite the outward force of the internal pressure, the presence of circumferential displacements, and the presence of inplane shear strains. These effects lead to spatial variations, including sign reversals, of a number of displacement, strain, and curvature responses. To study the influence of material orthotropy, the responses of axially stiff and circumferentially stiff elliptical cylinders are also examined. It is shown that in some instances material orthotropy can be used to mitigate the influence of the elliptical geometry, and make particular responses look like those of a circular cylinder.

Pseudospectral Orbit Simulation of Nonideal Gas-Lubricated Journal Bearings for Microfabricated Turbomachines

A journal bearing simulation tool developed to aid the design of the MIT microturbo-machine bearings is described. This tool uses an orbit method with a pseudospectral technique for treating the Reynolds equation. Comparison is made to various published data. Two types of stability chart are presented and their application to turbo-machine bearing design is discussed. Simulations of imbalance, noncircular geometry, and nonuniform pressures at the bearing ends are also demonstrated.

Experimental study of the effect of non-circular flow geometry on the critical heat flux

An experimental study of the effect of flow geometry (circular, rectangular, triangular, and dumb-bell shaped) on the critical heat flux (CHF) was performed using R-134a as a coolant. The CHF is affected by the following geometric parameters: hydraulic-equivalent diameter, heated length, gap size, channel shape, and curvature. It may also be affected by the thermal conductivity of the wall material and wall thickness. The effect of flow geometry on CHF is influenced by flow parameters. The effect of these parameters on CHF was examined, and recommendations for predicting the CHF in non-circular geometries have been made.

Thermal Residual Stress Analyses of Nextel 440/Aluminum Composite

Finite element analyses were performed to evaluate the residual stresses developed during the fabrication process of the Nextel 440/A1-6061 composite. The purpose of the analyses was to find the effect of fiber orientation and fiber spacing on processing-induced thermal residual stresses in the metal matrix composite with noncircular fibers' cross section. It was found that high compressive stresses developed between two fibers that increased as the two fibers approached each other. However, significant tensile stresses developed between fibers once the two fibers touched each other. The transition from compression to tension is explained by considering the force balance between contacting fibers. The normal stress on the fiber/matrix interface was higher when the major axis of two noncircular fibers' cross section was oriented at 45° to each other, than it was when the major axis was oriented at 0° to each other. The normal residual stress at the fiber/matrix interface for noncircular fiber geometry was higher compared to what would have developed using round fibers.

Optical fiber structures studied by a tapping-mode scanning near-field optical microscope

A tapping-mode scanning near-field optical microscope system with a bent optical fiber probe has been successfully developed to study the near-field optics at fiber end faces. Topographic and near-field intensity images of fiber end faces were obtained simultaneously. A brief and gentle etching process on fiber end faces in a saturated solution of ammonium bifluoride was able to reveal the structures of fibers. The topographic features generated by different local chemistry is closely related to the local doping concentration and the distribution of refractive index. The correlation between fiber structures and their near-field intensity profiles of propagating modes can be acquired directly by this novel method. Fibers of step-index, graded-index, single-mode, multimode, and circular, or noncircular geometry were imaged with a near-field resolution.