Central Air Core Research Articles

High birefringence in elliptical hollow optical fiber

We propose a novel design of highly birefringent optical fiber composed of a central elliptical air hole, a circumferential elliptical ring core, and a circular cladding. The proposed waveguide structure is predicted to produce a linear birefringence higher by an order of magnitude than the solid elliptical core fiber. The large index contrast between the central air and germanosilica elliptical ring core is mainly attributed to the high birefringence and its characteristics are theoretically analyzed in terms of its waveguide parameters.

Initiation of air core in a simplex nozzle and the effects of operating and geometrical parameters on its shape and size

Experimental investigations have been carried out to recognize the phenomenon of initiation of air core in a simplex type swirl spray pressure nozzle and to determine the influences of nozzle geometry and nozzle flow on the size of the fully developed air core. It has been recognized that below a certain Reynolds number at inlet to the nozzle, liquid flows full through the nozzle without the formation of an air core, while, above a certain Reynolds number at inlet to the nozzle, the formation of a fully developed central air core of cylindrical shape takes place in the nozzle. A transition flow with the development of a central air core takes place within the nozzle in between the two limiting inlet Reynolds numbers. These two limiting Reynolds numbers Re L1 and Re L2 are found to be an inverse function of the ratio of orifice to swirl chamber diameter ( D o/ D s) and a direct function of the ratio of tangential entry port to swirl chamber diameter ( D p/ D s). It has been observed that the fully developed central air core is cylindrical in shape with a sudden bulging in diameter at the entrance to the orifice. The air core diameter in both converging and orifice sections of the nozzle increases with an increase in the inlet Reynolds number ( Re) at its lower range, while it becomes independent with Re at its higher range. The ratio of air core to orifice diameter (da 2/ D o) increases with an increase in D o/ D s or swirl chamber cone angle ( α), and a decrease in D p/ D s or L o/ D s (the ratio of the orifice length to swirl chamber diameter). The ratio of air core diameter in the orifice to air core diameter in the swirl chamber of the nozzle (da 2/da 1) is an inverse function of D o/ D s but a direct function of D p/ D s. Empirical relations of the ratios of air core to orifice diameter, da 1/ D o and da 2/ D o, with the pertinent dimensionless controlling parameters have been established.

Numerical prediction of air core diameter, coefficient of discharge and spray cone angle of a swirl spray pressure nozzle

Abstract Theoretical predictions of air core diameter, coefficient of discharge and spray cone angle of a swirl spray pressure nozzle have been made from numerical computations of flow within the nozzle. The diameter of central air core that stabilizes inside the nozzle has been predicted from a hydrodynamic situation that yields the minimum resistance to the liquid flow in the nozzle at given operating conditions. The coefficient of discharge and spray cone angle have been evaluated from the distributions of the different velocity components of liquid flow at the nozzle orifice. It has been observed that the coefficient of discharge Cd decreases, while the air core diameter da and spray cone angle ψ increase with the increase in nozzle flow in its lower range. However, all these parameters Cd, da and ψ finally become independent of nozzle flow. Predicted values of da, Cd and ψ for different geometrical dimensions of the nozzle have been compared with the empirical data available in the literature.

Numerical prediction of the liquid flow within a hydrocyclone

The three-dimensional flow fields with a central air core in a hydrocyclone are numerically simulated using a k– ε turbulence model. The model constants C 1, C 2 and C μ are modified because of the anisotropic character of the turbulent viscosity in the hydrocyclone. Experiments show that the predicted velocity profiles agree well with data measured by laser Doppler anemometry (LDA). Based on this prediction, the flow field and the pressure field as well as the distribution of the rate of dissipation of the turbulent energy are discussed. The study shows that an important approach towards reducing energy dissipation in a hydrocyclone is to improve the flow pattern in the cylindrical part.

Performance characteristics within a modified hydrocyclone

It has been reported by Xu et al (1990) that the suppression of the axial air core within a hydrocyclone could improve process performance. This work presents the results of an extensive series of experiments to test this postulation by measuring the classification and separation efficiency of conventional hydrocyclones compared with modified hydrocyclones, in which the air core is replaced by an axial steel rod insert. The influence of rods of various dimensions on a number of design parameters (e.g. pressure drop, volumetric flowrate and cut size) is reported. The insertion of a 3 mm diameter 377 mm long rod into the hydrocyclone, 8 mm vortex finder diameter and 5 mm spigot diameter, to remove the central air core resulted in a maximum increase of 44% in the volumetric throughput for the same pressure drop. But in all cases the sharpness of classification, fineness of cut-size and overall separation efficiency was inferior. Since the insertion of a rod into the hydrocyclone does not appear to improve the separation efficiency other approaches, e.g. hydrocyclone networks, must be employed.

Modelling turbulent flow within a small-diameter hydrocyclone

A revised approach to modelling turbulent flow in small-diameter (10–44 mm) hydrocyclones is presented and tested quantitatively against experimental measurements. Some of the limitations inherent in the existing approaches, such as the anisotropy of turbulent viscosity and the non-linear interaction between mean vorticity and mean strain rate have been overcome utilising a k—ε model coupled with equations for calculating normal components of Reynolds stresses. The model predicts correctly the behaviour of a conventional water-fed hydrocyclone of various geometries, and has also been used to examine how the flow field can be modified by the insertion of a solid rod in place of the central air core in order to sharpen the classification performance of a hydrocyclone treating particulate suspensions.

Mechanisms of nozzle type reactor operation with gas removal

A nozzle type reactor with provision for gas removal has been studied for the neutralisation of dodecylbenzene sulphonic acid with sodium carbonate solution. Gas was removed via a central air core so that the product was essentially foam-free. The rate of mixing and the chemical reaction between dodecylbenzene sulphonic acid and sodium carbonate solution, were observed to be much faster than the liquid residence time so complete conversion into the product was achieved within seconds. Some gas bubbles were entrained in the product: however these were negligible compared with the bulk of the gas bubbles expelled at the top of the reactor. The criteria for gas removal was that the time for bubbles to travel to the air core under the prevailing centripetal force was less than the liquid phase residence time in the reactor. Mathematical models were therefore proposed to predict the equilibrium bubble diameter and the time for bubble travel to the core. Predicted times were found to be very much less than the average residence time.

Second-Law Analysis of Heat Transfer in Swirling Flow Through a Cylindrical Duct

A second-law analysis is made on a swirling flow in a cylindrical duct with constant wall temperature. A purely tangential entry of the fluid is considered and a simplified model, consisting of a central air core enclosed by a potential, free vortex region and a boundary layer, is assumed. The approximate hydrodynamic boundary layer equations, and the continuity equation, are set up and solved numerically for the velocity gradients in the boundary layer. Similarly, the temperature gradients within the thermal boundary layer are obtained from the energy equation. The local Nusselt number and rate of entropy generation are calculated and used to evaluate the rate of heat transfer and loss of available energy, respectively. A merit function, defined as the ratio of exergy transferred to the sum of exergy transferred and exergy destroyed, is evaluated for various values of Reynolds number, based on the inlet tangential velocity, and conclusions are drawn about the influence of inlet swirl on irreversibility.

Nonlinear azimuthal waves in a centrifuge

Azimuthal wave motions in a liquid which partially fills a cylinder (centrifuge) rapidly rotating about a horizontal axis are discussed in this paper. Under the action of centrifugal force the liquid is pressed to the wall of the cylinder and moves together with it about the central air core. The vibrations of the free surface which arise are called centrifugal waves [1]. The difficulties of their theoretical investigation are related to the nonlinearity both of the basic equations and also of the boundary condition for the pressure on the free surface; therefore they have previously been studied only by linear methods [1, 2]. Nonlinear azimuthal waves in a centrifuge with an infinite radius of the rotating cylinder are analytically described below. The waves found are an analog of Gerstner trochoidal waves on a cylindrical surface. An approximate solution for a centrifuge with a finite outer radius is constructed by matching the waves obtained to the known linear ones.