Annular Widths Research Articles

Thermal-hydraulic characteristics of FLiBe in annuli with helical wire

A fully tight-fitting triangular wire coil is used to enhance heat transfer of FLiBe in the annular channel for the first time. The numerical simulations of annular channels with triangular wire coil were performed with three helical pitches, three annular widths, and one equilateral triangle corresponding to Reynolds number of 4000–10000. It is found that adding triangular wire coil generates a vortex and reduces significantly the lengths of flow and thermal entrance regions of the annular channel. The local Nusselt number for peripheral point is extremely non-uniform. The Nusselt number decreases with annular width and helical pitch increase. The Nusselt number of enhanced annular channel is 1.45–1.98 times that of smooth annular channel. The overall performance of annular channel with helical pitch of 15 mm and annular width of 10 mm is best by using the principle of entropy generation. However, by using the performance evaluation criteria, the annular channel with helical pitch of 15 mm and annular width of 19.4 mm is best when the Reynolds number is less than or equal to 5500. And the annular channel with helical pitch of 20 mm and annular width of 19.4 mm is best at the high Reynolds number.

Numerical study of the influence of annular width on the rotating detonation wave in a non-premixed combustor

A numerical study of three-dimensional non-premixed rotating detonation combustor is performed to investigate the effects of combustor annular width (w) on the formation process of rotating detonation wave, characteristics of flow field, propagating stability and propulsive performance of combustor. Fixing the outer diameter and changing the inner diameter, three annular widths (i.e., 5 mm, 8 mm and 12 mm) are investigated. The results show that annular width plays a crucial role to affect the behavior of rotating detonation wave. Dual-wave mode is obtained in case of w=5 mm while single-wave mode is obtained in cases of w=8 mm and 12 mm. Additionally, the highest height of rotating detonation wave is observed in case of w=8 mm, which leads to the largest pressure peak. Besides, great fluctuation of detonation wave height is observed in case of w=12 mm. Furthermore, accumulated pressure wave appears in each case and its intensity is an important factor for the generation of new detonation wave front. Moreover, as the increase of annular width, the thrust and specific impulse both decrease. The largest thrust and specific impulse are both obtained in case of w=5 mm, which are 79.16N and 2692.52 s respectively.

Detonation propagation limits in highly argon diluted acetylene-oxygen mixtures in channels

In this study, the velocity deficits and detonation limits of highly argon (70% and 85% vol.) diluted stoichiometric acetylene-oxygen mixtures in a small diameter circular tube (D=36mm) and three annular channel widths (w=2, 4.5 and 7mm) were investigated. The purpose is to explore the velocity behavior of stable mixture due to its different failure mechanism by investigating the scaling law between the hydraulic diameter (DH) of the circular tube and annular channels with the characteristic length scale (i.e., cell size λ and ZND induction zone length ΔI) of stable detonations. The experimental results suggest the detonation velocity decreases with the decreasing of initial pressure, when the initial pressure reaches a critical value corresponding the maximum velocity deficit, the detonation fails and the detonation limits are approached. The results indicate that the average maximum velocity deficit is approximately 14% of the corresponding CJ value VCJ for the highly argon diluted mixtures in the tubes with different geometries. The scaling between the hydraulic diameter and the cell size (DH/λ) or ZND induction zone length (DH/ΔI) gives an average estimation for the highly argon diluted mixtures at the detonation limits 0.66 and 15, respectively.

Experimental Performance Scaling of Rotating Detonation Engines Operated on Gaseous Fuels

Thrust stand testing is conducted on rotating detonation engines of various annular detonation channel widths and the impact of different scaling parameters on the performance of the devices is discussed. A focus is given in this work to identifying the relationship between the appropriate flow variables to facilitate comparison between geometries and the experimentally obtained specific impulses and specific thrusts, as well as an examination of stagnation pressure usage, through corrected thrust. A tradeoff space is found in the performance of the engines in which the fuel efficiency and effective usage of feed stagnation pressure can be exchanged. Operation on both hydrogen and gaseous hydrocarbon fuel is presented. A comparison is made between the resulting propulsive performances found for similar rotating detonation engine geometries.

Scale-Up Study on the Performance of Annular Pulsed Disc-and-Doughnut Columns

ABSTRACTBased on the previous work of the hydraulic, axial mixing, and mass-transfer performance of an annular pulsed disc-and-doughnut column (APDDC), the effects of scaling-up on its performance were studied in APDDCs with different annular space widths. The effect of scaling-up on holdup was discussed, and a more universal correlation was established. The effect of scaling-up on the Sauter drop diameter could be expressed by the previous obtained correlation. The effects of scaling-up on both the flooding holdup and throughput were mainly reflected in the correlation of the Sauter drop diameter. The equations of operating regimes were also independent of the scaling-up effect as demonstrated by experimental data. The scaling-up effects on the axial-dispersion coefficient of the continuous phase, the mass-transfer coefficient, and the height of a transfer unit were also discussed.

Methane–oxygen detonation characteristics near their propagation limits in ducts

In this study, the near-limit behavior of gaseous detonations in three methane–oxygen mixtures (CH4–2O2, CH4–1.5O2 and CH4–4O2) is investigated experimentally. A 36-mm diameter circular tube and three annular channel gaps (w=2mm, 4.5mm and 7mm) were used to look at the effect of different geometries on the detonation limits phenomenon. Photodiodes and smoked foils were employed to measure the time-of-arrival of the detonation wave and record the cellular detonation structure, respectively. As the detonation propagates within the limits, the velocity is steady with only a few percent deficit. By decreasing the initial pressure and, hence, reducing the sensitivity of the mixture, the detonation velocity deficit increases gradually. When the initial pressure is approaching the detonation limits, no steady detonation velocity can be realized, and failure occurs. With decreasing initial pressure, the smoked foil records indicate clearly that the cellular detonation evolves from a multi-headed to double-headed and eventually to a single-head spin as the limits approaches. The single-headed spinning detonations for CH4–2O2, CH4–1.5O2 and CH4–4O2 mixtures in the 36-mm round tube occurs from p0=3–7kPa, 4–10kPa and 7–16kPa, respectively. Using different annular channel widths, it is observed that the detonation velocity decreases as the channel gap is reduced.

Detonation Transition Around Cylindrical Reflector of Pulse Detonation Engine Initiator

To achieve reliable transmission of detonation waves to a pulse detonation engine combustor (detonation chamber), the authors propose a pulse detonation engine initiator that uses a cylindrical reflector downstream of a predetonator exit. The detonation wave propagates around the reflector to change the wave shape in three transition stages: from a planar detonation wave in the predetonator to an expanding cylindrical detonation wave, from the cylindrical wave to a planar toroidal detonation wave, and from the toroidal wave to a planar detonation wave in the detonation chamber. The cylindrical wave propagates along a cylindrical path between the reflector and front wall of the detonation chamber, and the toroidal wave propagates along an annular path between the reflector and sidewall of the detonation chamber. The purpose of this study was to examine the influence of the gap width of the annular path on the transition stages from cylindrical to toroidal and from toroidal to planar. A series of experiments that filled the entire test section with the driver gas mixture (stoichiometric hydrogen–oxygen mixture) showed that the expanding cylindrical detonation wave was sufficiently strong to survive the rarefaction waves from the corners of the reflector at all of the investigated annular gap widths (5, 10, 15, and 20 mm) and was transmitted to the planar toroidal wave successfully in all cases. When the strength of the cylindrical detonation wave was under a supercritical condition for diffraction at the reflector corner, the necessary filling distance for the driver gas was predicted well by the Whitham theory. A second series of experiments showed the influence of the annular gap width on the detonation transition from the planar toroidal detonation wave to the planar detonation wave. Two different types of detonation transitions termed “continuous transition” and “temporal quenching” were observed. The threshold value of for continuous transition is approximately four.

Hysteresis in annular impinging jets

Annular impinging air jets were investigated experimentally using the techniques of flow visualization, measurement of the stagnation pressure, and of the mass transfer by the naphthalene sublimation method. The bistability and hysteresis lead to two different alternative flow field patterns (A or B) under identical boundary conditions. The patterns differ in the size of the separated-flow recirculation regions: the pattern A is characterized by recirculation region (bubble) of separated flow quite small and located immediately under the nozzle centerbody. The heat/mass transfer distribution on the impingement wall is bell-shaped, with the maximum in the central stagnation point. On the other hand, the flowfield of the pattern B exhibits a large recirculation region of the separated flow, reaching up to the impingement wall, on which there is a stagnation circle. The entire space between the inner lip of the nozzle exit and the stagnation circle is filled with the recirculating fluid. The heat/mass transfer rate distributions reach the maximum on the stagnation circle. The bistability and hysteresis were found only at small Reynolds numbers, and at small annular nozzle slot widths. An extension of the nozzle centerbody promotes the tendency to the hysteretic behavior. An enhancement effect for the area-average heat/mass transfer coefficients for the pattern B over A has been quantified. The highest achieved increase was quite significant 32%.

Micromagnetic study of magnetization reversal in ferromagnetic nanorings

We present results of micromagnetic simulations of thin ferromagnetic rings undergoing magnetization reversal. This geometry is one of few examples in micromagnetics in which the transition states have been found analytically in a one-dimensional (1D) model. According to this model, at low fields and large ring sizes, the energetically preferred transition state is a localized magnetization fluctuation (instanton saddle). At high fields and small ring size, the preferred saddle state is a uniformly rotated magnetization (constant saddle). In the first part of this paper, we use numerical micromagnetic simulations to test these predictions of the 1D analytical model for more realistic situations including a variety of ring radii, annular widths and magnetic fields. The predicted activation energies for magnetization reversal are found to be in close agreement with numerical results even for rings with a large annular width where the 1D approximation would be expected to break down. We find that this approximation breaks down only when the ring's annular width exceeds its radius. In the second part, we present new metastable states found in the large radius limit and discuss how they provide a more complete understanding of the energy landscape of magnetic nanorings.

Cylindrically symmetric electrohydrodynamic patterning

Cylindrically symmetric structures such as concentric rings and rosettes arise out of thin polymeric films subjected to strong electric fields. Experiments that formed concentric rings and theory capable of explaining these and other cylindrical structures are presented. These rings represent an additional member of a class of structures, including pillars and holes, formed by electrohydrodynamic patterning of thin films, occasionally referred to as lithographically induced self-assembly. Fabrication of a set of concentric rings begins by spin coating a thin poly(methyl methacrylate) film onto a silicon wafer. A mask is superimposed parallel to the film leaving a similarly thin air gap. Electric fields, acting in opposition to surface tension, destabilize the free interface when raised above the glass transition temperature. Central pillars nucleate under small cylindrical protrusions patterned on the mask. Rings then emerge sequentially, with larger systems having as many as 10 fully formed rings. Ring-to-ring spacings and annular widths, typically on the order of a micron, are approximately constant within a concentric cluster. The formation rate is proportional to the viscosity and, consequently, has the expected Williams-Landel-Ferry dependence on temperature. In light of these developments we have undertaken a linear stability analysis in cylindrical coordinates to describe these rings and ringlike structures. The salient feature of this analysis is the use of perturbations that incorporate their radial dependence in terms of Bessel functions as opposed to the traditional sinusoids of Cartesian coordinates. The theory predicts approximately constant ring-to-ring spacings, constant annular widths, and growth rates that agree with experiment. A secondary instability is observed at higher temperatures, which causes the rings to segment into arcs or pillar arrays. The cylindrical theory may be generalized to describe hexagonal pillar/hole packing, gratings, and rosettes with the first being of particular importance given the ubiquitous observation of hexagonal packing. The perturbation analysis presented here is relevant to any system with cylindrical symmetry, for which the radial dependence can be described in terms of Bessel functions.

Appropriate Correlations for Assessing Expanded Tube-to-Tubesheet Joint Strength

This paper advocates correlating rolling torque or hydraulic expanding pressure with shear load strength instead of percent wall reduction as the criterion of expanding adequacy for thin-walled tubes. It compares magnitudes of measurements required to estimate wall reduction with cumulative hole-drilling and tube-manufacturing tolerances and graphically illustrates data scatter increase with ratio of tube diameter to wall thickness. As background it discusses expanding theory. It covers hydraulic, roller and hybrid expanding. It describes appropriate annular groove widths for hydraulic and hybrid expanding and concludes with comments on tube rolling procedure specifications and recommendations for two-stage and hybrid expanding.

Erpobdella (Dina) parva complex (Annelida: Hirudinea: Arhynchobdellida: Erpobdellidae): additional description of Erpobdella parva, E. dubia, and E. lahontana and taxonomic revision

Erpobdella (Dina) parva complex (E. parva, E. dubia, and E. lahontana) were analyzed by species distribution, pigmentation, size, number and position of the labial eyes, tubercle presence, annulus width, gonopore position, and male genital atrium shape. I concluded that E. parva and E. dubia are conspecific, with the priority name of E. parva. The Lahontan basin species (E. lahontana), was found to be distinct by gonopore position, and now includes two additional basins (Granite Springs and Winnemucca). Endemism occurred in the Relict Dace basin complex (Butte and Steptoe basins) with respect to one form of gonopore positions. The variation in pigmentation, eye position and number, gonopore position, and male genital organ shape were highly variable in the Great Basin. Examination of the annular widths did not give any guidance to the taxonomic problem of the genera of this complex. It was noted that the Erpobdella parva complex did have some similarities with Nephelopsis obscura.

Onset of flow instability and critical heat flux in thin horizontal annuli

The onset of flow instability (OFI) and critical heat flux (CHF) in heated thin horizontal annular flow passages cooled by subcooled water were investigated. For OFI, six different test sections, with inner diameter of 6.4 mm, annular gap widths of 0.724–1.001 mm, and heated lengths of 174–197 mm were used. The test parameter ranges were: 85– 1428 kg/m 2 s mass flux, 0.344–1.034 MPa exit pressure, 50–150 °C inlet temperature, 0.124– 1.0 MW/m 2 surface heat flux and 0–∞ inner-to-outer surface heat flux ratio. In addition, the effect of dissolved non-condensables on OFI was examined by performing similar experiments with degassed water and water saturated with air at test section inlet temperature and exit pressure. A total of 138 OFI test were run addressing important parametric trends. A theoretical model based on the solution of one-dimensional fluid conservation equations, which assumes no voidage upstream the onset of significant void (OSV) point, and accounts for thermodynamic non-equilibrium beyond the OSV point using an empirical quality profile fit, was shown to predict the conditions leading to OFI reasonably well. For CHF, the test section was an annulus with 6.45 and 7.77 mm inner and outer diameters, respectively (0.66 mm gap width), and an 18.5-cm long heated section. The experimental parameters investigated covered the following ranges: test section exit pressure: 0.344–1.034 MPa; coolant (water) mass flux: 100– 380 kg/m 2 s ; wall heat flux: 0.231– 1.068 MW/m 2 ; water inlet temperature: 30–65 °C. The measured CHF values were considerably lower than the expected CHF values for vertical test section configuration. In all the tests CHF occurred at relatively high equilibrium qualities, and was preceded by flow stratification which caused dry-out of the upper surface of the flow channel. The data were correlated in two ways: by introducing empirical correction multipliers into three widely used correlations for vertical channels; and based on the compensated distortions method.

Unstable Sloshing Vibration in a Thin Cylindrical Weir

Traveling unstable sloshing modes have been observed in experiments on the fluidstructure coupled system in a thin cylindrical weir. The unstable vibration was found to be classified into three types: i.e., traveling wave which travels along one circumference direction; traveling wave which travels in the clockwise and counterclockwise directions alternately; and stationary wave. Influences of the outer and the inner annular gap widths, the height of weir, the elastic rigidity of weir, and baffle plate on the characteristics of the unstable sloshing vibration were clarified.

Criticality Safety Calculations of ‘Poison Tube Tank’ Compared with Annular Tanks for Storing Fissile Solutions

A comparative study of the shielded area space required for storing fissile solution by the conventional annular tank and by poison tube tank is made. Poison tube tank is similar to commercial heat exchanger. The neutron poisons studied are gadolinium oxide and borax. Variation of multiplication factor for an array of annular tanks containing uranium nitrate or plutonium nitrate solutions are presented for annular widths of 10, 7.5 and 5 cm. It is concluded that for the given concentration, 5 cm annular width tanks are safe at a pitch distance of 120 and 90 cm for uranium and plutonium solutions respectively. Using these, as reference values, it i5 found that the shielded area saving for the poison tube tank is a factor of 12 and 8 for the given concentration of uranium and plutonium solutions respectively.

Criticality Safety Calculations of 'Poison Tube Tank' Compared with Annular Tanks for Storing Fissile Solutions.

A comparative study of the shielded area space required for storing fissile solution by the conventional annular tank and by poison tube tank is made. Poison tube tank is similar to commercial heat exchanger. The neutron poisons studied are gadolinium oxide and borax. Variation of multiplication factor for an array of annular tanks containing uranium nitrate or plutonium nitrate solutions are presented for annular widths of 10, 7.5 and 5cm. It is concluded that for the given concentration, 5cm annular width tanks are safe at a pitch distance of 120 and 90cm for uranium and plutonium solutions respectively. Using these, as reference values, it is found that the shielded area saving for the poison tube tank is a factor of 12 and 8 for the given concentration of uranium and plutonium solutions respectively.

STATIC CHARACTERISTICS OF CIRCULAR SYMMETRIC ANNULAR JOSEPHSON JUNCTION (Ⅰ)——THE QUASI-DIFFRACTION CHARACTERISTIC AND THE SELF-FIELD EFFECTS

The static Josephson effects in the circular symmetric annular junctions are discussed in this paper. When the annular widths are smaller than λJ, analytic solutions of magnetic field dependence of the maximum Josephson current Im, are obtained. Because the external magnetic field is nonuniform distributed in the junctions, the minima of lm are not equal to zero. These results exhibit a quasi-diffraction characteristic. When the annular widths are larger than λJ, the numerical solutions of the self-field equations are obtained, which show the magnetic field dependence of the maximun Josephson current Im and the current distribution. Due to the 2-dimensional character, the points of maximum of Im are not on the axis where the external field is equal to zero in the plot of Im vs. IH.

Diffusion distillation - A new separation process for azeotropic mixtures Part 1: Selectivity and transfer efficiency

A new separation process for azeotropic mixtures3̄diffusion distillation—is proposed. A liquid mixture is evaporated below the boiling temperature, diffuses through an inert gas gap and is recondensed. Hence the separation effect is not only based on the relative volatility of the components concerned but also on their diffusivity in the inert gas. In a wetted-wall column consisting of two concentric tubes, a significant separation effect can be achieved. Several experiments were carried out with a binary isopropanol-water mixture and a ternary isopropanol-water-methanol mixture at different evaporation and condensation temperatures, with different inert gases and annular widths in the wetted-wall column. The experimental results were well described by the vapour-liquid equilibrium and the Stefan-Maxwell equations, that is, by steady-state molecular diffusion.

On the flow in an annulus surrounding a whirling cylinder

When fluid in an annulus between two cylinders is set in motion by whirling movements of one or both of the cylinders, dynamic forces are imposed by the fluid on the cylinders. Knowledge of these forces is frequently important, indeed often critical, to the engineer designing rotor systems or journal bearings. Quite general solutions of the Navier-Stokes equations are presented for this problem and are limited only by restrictions on the amplitude of the whirl motion. From these solutions, the forces are derived under a wide variety of circumstances, including large and small annular widths, high and low Reynolds numbers and with and without a mean flow created by additional net rotation of one or both of the cylinders.

The radial transfer of mass and momentum in an axial fluid stream between coaxial rotating cylinders—I experimental measurements

The authors have made an experimental study of the fluid-mechanical and transport properties of a system in which a stream of air flows axially in the long annulus between a rotating cylinder and a stationary coaxial outer tube. The fluid mechanical investigations were aimed at elucidating the nature of the flow in, and the stability limits for, the laminar + vortex flow regime. Only a part of these results, relating to the criteria for transition between the laminar and laminar + vortex flow regimes, is outlined in this paper, which is divided into two parts. Part I deals chiefly with the measurement of mass and momentum transfer rates from the walls to the stream in the laminar + vortex regime. In Part II an apparent dissimilarity between these mass and momentum transfer results is discussed. Over the range of axial flows, rotor speeds and annular widths studied, it is concluded that the laminar/laminar + vortex transition is significantly affected by only two dimensionless groups, viz. Re z ≡ V zb/v and Ta ≡ V φ b/ν√ b/r 1‡. It appears that the latter group can advantageously replace previous, more complicated, formulations of the Taylor number in all fluid stability and transport correlations for rotating cylinder systems. Mass transfer rates were measured chiefly in a wetted wall apparatus, using water or n-heptane, and in a mercury-transfer system from a solid surface of silver amalgam. Four different values of b/r 1 were used in this set of experiments, whose results could be correlated over the entire range of experimental conditions (within the laminar + vortex regime) by an expression of the form Sh  f(Re z, Ta, Sc ). Rates of axial momentum transfer were determined from axial pressure-drop measurements. It appears that the results can be correlated over the whole range of flow conditions and for two different values of b/r 1 in terms of three dimensionless groups, viz. pressure drop enhancement over that for laminar flow, Re z and Ta; but there are indications that b/r 1 may enter additionally into the correlation.