Air Core Diameter Research Articles

Study on the dynamic response of a pressure swirl injector to ramp variation of mass flow rate

The response of a pressure swirl injector to ramp variation of mass flow rate is numerically and experimentally investigated. Numerical simulation based on volume of fluid method was conducted to obtain the response of the two-phase flow inside the injector. In experiments, ramp variations of mass flow rate with different slope were introduced into the swirl injector through a throttleable cavitating venturi nozzle. Backlit photography technique and laser diffraction technique were adopted to capture the spray and measure the size distribution of droplets respectively. The relationships between mass flow rate at the injector inlet and transient parameters (such as mass flow rate, air core diameter, axial and tangential velocities) at the injector exit were analyzed. Besides, the effects of throttling rate were discussed. The results indicate that the mass flow rate, air core diameter, velocities at the injector exit, spray angle and the sauter mean diameter vary almost linearly with mass flow rate at the inlet. It is found that the responses of inner flow, spray and size distribution of droplets are delayed. Larger throttling rate results in larger time lag. The numerical results show that the inner flow response quickly and the time delay is small when compared with the external flow. Phenomena, such as anti-regulation and overshoot of the film thickness at the exit, are observed. The anti-regulation was thought to be related with the anti-regulation of combustion which was observed in hot tests.

Transition from spray to roping in hydrocyclones

Although roping in hydrocyclones is a problem that has been studied by many researchers, we do not yet have a theory that relates all the variables involved. Several experimental works with different approaches such as mechanical energy balance, work on hydrocyclones with water only and hydrocyclone air core measurements with different instruments in the laboratory and plant have attempted to explain the roping phenomenon. They have addressed design variables such as apex to vortex diameters and different cone angles as well as operating variables such as inlet solid concentration, particle size, pressure and overflow and underflow flow rates and concentrations in order to understand their effect on the roping condition. In this paper we intend to verify some of the conclusions of these studies and establish inlet pressure and particle diameter as the variables that, in combination, lead from spray to roping. We present a computational model using Computational Fluid Dynamics (CFD) to study a 75-mm laboratory hydrocyclone operating at a variable flow rate and with three different particle diameters. The Reynolds Stress Model and Eulerian multiphase model were used to model the turbulence and interaction of phases, respectively. The solid particles are described with the kinetic theory of granular flows (KTGF). Physical coherence and accuracy were compared with experimental data, where errors are within the expected range for an engineering prediction. The results indicate that transition from spray to roping generates an increase in the inlet flow pressure and/or particle diameter at a constant solid concentration in the feed flow. For 20-µm-diameter particles an increase results in a decrease in the discharge angle, although it always has a spray shape, for 34-µm diameters increasing the inlet pressure generates a semi-rope discharge and for 70-µm-diameter particles a small increase in the inlet pressure generates a roping condition. Transition to roping is characterized by a decrease in the air core diameter and discharge angle due to slow rotational velocity when particle diameter increases or higher accumulation of the solid fraction in the apex when inlet pressure increases. The passage from spray to roping occurs with a change in the frequency spectrum of the pressure oscillations in the walls of the hydrocyclone, with high amplitudes at low frequencies for spray discharge and noisy signals in all spectrums and damped low frequencies for roping.

Multiphase CFD Modelling of Mineral Separators Performance: Validation Against Tomography Data

This paper aims on application of computational fluid dynamic (CFD) modelling on two-phase and multiphase flows in 75 mm conventional hydrocyclone, 350 mm dense medium cyclone (DMC) and 100 mm diameter laboratory column flotation. Along with classification experiments, a dual-planar high speed electrical resistance tomography (ERT) is used to measure the internal flow dynamics in situ in 75 mm hydrocyclone and column flotation at different operating conditions. Experiments are carried out in 75 mm hydrocyclone for different operational parameters and feed solids content varying in the range of 0–30 wt%. Results are presented in terms of the air core diameter and solids volume fraction contours using Maxwell’s conductivity equation. It is confirmed by ERT that the air core occupied area and wall solids conductivity levels decreases with increasing the feed solids concentration. Similarly, ERT is used to estimate the mean gas hold-up and radial distribution in order to understand the hydrodynamics of the column flotation. The effect of air superficial velocity and liquid flow rate on gas hold-up is discussed. Algebraic slip mixture model modified with lift, drag and rheology is used to predict the air core size, the solid concentration distribution and classification performance for the hydrocyclone. Validation of air core size and mean solid volume fractions by ERT measurements with the CFD simulations is attempted. The CFD predicted air core radius and medium segregation in the DMC is validated against literature based gamma ray tomography data. Further discrete phase model (DPM) is used to track and measure the residence times of different size and density coal particles. CFD predicted gas hold-up in the column flotation is also validated against in-house measured ERT and imaging data.

Development of a simulation analysis environment for ventilated slab systems

In this paper, a new simulation environment is presented to evaluate the energy performance of ventilated slab systems in multi-floor buildings. The simulation environment combines a transient two-dimensional finite difference solution of a ventilated slab system comprising slab-wall joints with thermal network model for indoor spaces and associated exterior walls. The developed simulation environment can assess the impact of thermal bridging effects on both heating and cooling building thermal loads. First, the predictions of the developed simulation environment are verified against those obtained from a detailed whole-building energy simulation tool when thermal bridging effects are neglected. Then, a series of parametric analyses are performed to determine the performance of ventilated slab systems under various design and operating conditions considering the thermal bridging effects. It is found that the energy performance of ventilated slab systems and thermal bridging impact depend on a wide range of factors including size of the slab, supply air inlet temperature, air mass flow rate, core diameter, core pitch, and depth of hollow cores. In particular, it is found that the thermal bridge affects significantly the energy performance of ventilated slab systems and can increase both heating and cooling energy consumptions by 17% and 11%, respectively.

PIV measurements of air-core intake vortices

Particle Image Velocimetry (PIV) measurements were conducted in a large-scale hydraulic model to obtain the horizontal velocity field of intake vortices with considerable air entrainment. Vortex wandering and air-core diameter were determined by object detection using PIV images and velocity vector maps. This allowed for the calculation of ensemble- and azimuthally-averaged velocity fields. The results of the horizontal velocity field around the vortex, whose circulation affects a wide area, confirm the applicability of the analytic solution of the 2D Navier–Stokes equation for a potential (free) vortex. The setup and the improvement of the PIV measurements and their limitations are presented. For example, a completely resolved velocity field close to the air-core did not exist due to reflections at the interface between air and water. Vortex characteristics like vorticity and circulation are derived from the velocity fields and turbulence is shown by representing in-plane Turbulent Kinetic Energy (TKE).

Characteristics of bent terahertz antiresonant reflecting pipe waveguides

Bending characteristics of the terahertz (THz) pipe waveguides are numerically investigated. Numerical results reveal that the inherent periodic feature of the loss spectrum, resulting from the antiresonant reflection guiding mechanism, is nearly unaffected under bending. However, attenuation constant of the fundamental (HE(11)) mode becomes polarization dependent for the bent pipe waveguide, and the polarization perpendicular to the bending plane experiences less bending losses. Moreover, unlike the straight case where a larger air-core diameter leads to a smaller attenuation constant, increasing core diameter of the bent pipe waveguide is unable to reduce attenuation constant effectively if the propagation mode is a whispering gallery mode. Finally, behavior of the bent pipe waveguide connected to a straight one is also examined in this work.

An Experimental Investigation on the Effect of Various Swirl Atomizer Orifice Geometries on the Air Core Diameter

Liquid atomization is a process of changing the liquid into small droplets. There are many applications which are related to liquid atomization including fuel injection in combustion systems and also in agricultural sprays. In pressure swirl atomizer, the liquid is injected into the atomizer through tangential port and a swirling motion is formed inside the swirl chamber. In high strength of swirling motion, an air core will be visible inside the atomizer. The liquid is then discharged from the orifice to form a spray which breaks up the liquid into small droplets. The objective of this research is to investigate the effect of various orifice geometries on the air core diameter. The injection pressure was varied in the range of 2 to 8 bar and water was used as the working fluid. Experiment data shows that the air core diameter increases as the injection pressure increased, regardless the discharge orifice diameter and discharge orifice length. It also found that the air core diameter increases as the discharge orifice length decreases and the discharge orifice diameter increases.

Comparative study of fundamental properties of honey comb photonic crystal fiber at 1.55µm wavelength

Fundamental properties such as mode field distribution, real effective refractive index, imaginary effective refractive index, confinement loss of two new kinds of honeycomb photonic crystal fibers are successfully studied by using Full-Vectorial Finite element method (FV-FEM). Low confinement loss 0.1×10-4dB/km is achieved at wavelength 1.55µm in hollow core honey comb PCF by removing 6-air holes in cladding region with air hole diameter 1.38µm in cladding region, pitch 2.3µm and air core diameter 0.2µm.

Air-core modelling for hydrocyclones operating with solids

The air core is one of the most important internal structures in the flow field of hydrocyclones. One difficulty in predicting the flow split in cyclones is the inability to specify the form and location of the air-core surface. In the usual cyclone models the interface that bounds the air-core is described by a fixed cylindrical surface, which greatly simplifies the problem. Most of the available air core models suit only hydrocyclones operating with water or very dilute slurries. An attempt has been made in this study to develop a semi-empirical model, capable of predicting the air core diameter correctly for hydrocyclones under different operating conditions. The model considers the effect of axial length, underflow and overflow dimensions, feed solids concentration and other significant operating conditions on predict air core size. The predictions are compared with data sets available in the literature. It was observed that the air core diameter reduces with increase in the feed solids concentration and with decrease in height from the roof of the cyclone towards the apex zone.

Flow conditions in the air core of the hydrocyclone

The geometry and movement of the air core are sensitive indicators of the operational state of hydrocyclones. Therefore, an increased knowledge of the air core behaviour is desirable. Measurements of the air-core diameter in transparent hydrocyclones operated with a suspension of glass spheres are presented. The experiments indicate the dependencies between feed-solids content, air-core diameter, and spray angle of the underflow. Additionally, the air flow inside the air core in a transparent 50 mm water cyclone was experimentally investigated. Visualization techniques show the whirling motion of the air-core flow that is driven by the rotating liquid boundary of the air core. The radial distribution of the tangential air velocity was determined. The mean axial air velocity was shown to be dependent on the pressure drop and the hydrocyclone geometry. At the overflow discharge the air core is dispersed, and forms an aero suspension. Thus, the paper presents new aspects of the complex multi-phase flow in the hydrocyclone considering the gaseous phase.

Identification and prediction of air core diameter in a hydrocyclone by a novel online sensor based on digital signal processing technique

A hydrocyclone is a particle separation device widely used in chemical and allied process industries in which a particle-fluid mixture is injected tangentially creating a strong swirling, recirculation flow. The particle separation efficiency increases by suppressing the air core, so online prediction of air core formation has significant importance in the industrial operations. Performance of hydrocyclone is greatly influenced by shape and size of air core. A novel type of senor technique has been developed to identify and predict the air core diameter from online live data using data acquisition card. The true signal amplitudes change as a function of the time was used with noise interruption for random changes in amplitude. Noises are eliminated by using moving average technique. The slope of the curve is continuously tracked to determine sudden or abrupt change and indicates the formation of air core. It has been observed that a strong air core of diameter 0.95 cm to 1.2 cm was formed during experimentation and matched with predicted values over an entire flow regime. The experimental and finally an alarm is designed which gives alerts once air core is formed and calculates air core diameter. For calculating air core diameter a polynomial equation is fitted between pressure difference and the pressure transmitter reading. A simple moving average with a smooth width of 10 was used for prediction of air core. Experimental results indicate that the digital signal sensor techniques identify the air core and measure air core diameter very accurately and can be used in many mining and mineral based chemical and allied process industries.

Computational fluid dynamics prediction of the air-core in hydrocyclones

The air-core diameter was successfully predicted by large eddy simulation and volume of fluid models. The dynamics of the flow for 75- and 250-mm hydrocyclones was evaluated in 11 cases in which the spigot diameter, conical length, vortex finder diameter, diameter of the hydrocyclone and viscosity of the fluid were changed. FLUENT™ 6.0 code was used to compute the dynamics of the flow. The air-core profiles demonstrated the accuracy of the simulations. A reduction in the spigot increases tangential velocity and hence air-core diameter is increased. An increase in hydrocyclone diameter also increases air-core diameter, while the vortex finder or an increase in fluid viscosity inversely influences it. The vortex finder influences inversely air-core diameter. The increase in viscosity of the fluid reduces air-core diameter. The computational time required to reach the solution increases as the domain size increases. The LES model cannot be used in everyday cases to optimise hydrocyclones, but it is an outstanding tool to study new designs.

AN APPRAISAL OF SWIRL ATOMIZER INVISCID FLOW ANALYSIS, PART 2: INVISCID SPRAY CONE ANGLE ANALYSIS AND COMPARISON OF INVISCID METHODS WITH EXPERIMENTAL RESULTS FOR DISCHARGE COEFFICIENT, AIR CORE RADIUS, AND SPRAY CONE ANGLE

Several methods of deriving an expression for the spray cone half-angle of a swirl atomizer are presented. Expressions found in the literature, based on the inviscid theory, together with suggestions for improvements, are given. The theoretically derived functions for air core diameter and discharge coefficient as functions of atomizer constant that were presented in Part 1 of this article series are charted and compared with experimental results from the literature. Also, the various functions for spray cone half-angle, as a function of the atomizer constant, are charted and compared with the experimental results from the literature. The fit of these theoretical functions with the experimental results are discussed in terms of the flow physics and the limitations of the inviscid theory. This comparison leads to the conclusion that the inviscid theory may be of benefit in the basic understanding of the flow physics of swirl atomizer internal flow. However, it is of limited value for detailed understanding of the complicated flow regime as it does not consider variations in the supply pressure, viscosity, turbulence, wall effects, or fluid-gas interactions. It does allow ballpark estimates for flow rate and for air core size.

Effect of geometric parameters on the liquid film thickness and air core formation in a swirl injector

Many theoretical and experimental studies have been conducted to investigate elements of swirl injector hydrodynamics, such as variations in liquid film thickness or air core diameter. From these studies, some theoretical relationships have been established through an approximate analytical solution of flow hydrodynamics in a swirl nozzle. However, experimental studies on elements such as the measurement of liquid film thickness have not produced conclusive results. In a swirl injector, the atomization process is significantly influenced by the liquid film thickness. Thus, it is possible to investigate the effects of various geometric parameters on spray characteristics through the measurement of liquid film thickness. We used a specially designed injector based on the electrical conductance method to measure the liquid film thickness accurately. The liquid film thickness was measured through precise calibration, and the accuracy of this measurement was demonstrated in comparison with previous theories and experiments. From these results, we present an empirical relation for the liquid film thickness by adding orifice length to an existing analytical equation. The variations and stability of the air core were also examined by visualizing the formation of the air core in the swirl chamber with a high-speed camera system. This study confirms that air core shape and liquid film thickness are directly related. Thus, study of the fluctuations of liquid film thickness under various geometric conditions can be applied to the analysis of internal flow.

Air core formation in the hydrocyclone

Air core formation has been investigated in hydrocyclones operated with clear water and a lucid suspension of glass balls. Hydrocyclones form a central air core which extends over the complete hydrocyclone length. Air is sucked in the core at the underflow discharge. The air core diameter can be determined balancing the positive pressure gradient and the centrifugal force in the rotational flow field. In dense flow separation (high feed solids content) the air core in the conical part of the hydrocyclone is suppressed. The hydrocyclone operates as it is air sealed because the solids are discharged trough the underflow as a rope. Then, air can be introduced to the hydrocyclone only on the feed side. In practice, feed suspension always contains more or less dissolved or dispersed air. Observations in a transparent hydrocyclone show that dissolved gas is released due to the pressure drop inside the hydrocyclone. The generated micro bubbles grow by coalescence and move in the centrifugal field toward the centre, where an air core is formed.

Nonlinear Breakup Model for a Liquid Sheet Emanating From a Pressure-Swirl Atomizer

Predictions of breakup length of a liquid sheet emanating from a pressure-swirl (simplex) fuel atomizer have been carried out by computationally modeling the two-phase flow in the atomizer coupled with a nonlinear analysis of instability of the liquid sheet. The volume-of-fluid (VOF) method has been employed to study the flow field inside the pressure-swirl atomizer. A nonlinear instability model has been developed using a perturbation expansion technique with the initial amplitude of the disturbance as the perturbation parameter to determine the sheet instability and breakup. The results for sheet thickness and velocities from the internal flow solutions are used as input in the nonlinear instability model. Computational results for internal flow are validated by comparing film thickness at exit, spray angle, and discharge coefficient with available experimental data. The predictions of breakup length show a good agreement with semiempirical correlations and available experimental measurements. The effect of elevated ambient pressure on the atomizer internal flow field and sheet breakup is investigated. A decrease in air core diameter is obtained at higher ambient pressure due to increased liquid-air momentum transport. Shorter breakup lengths are obtained at elevated air pressure. The coupled internal flow simulation and sheet instability analysis provides a comprehensive approach to modeling sheet breakup from a pressure-swirl atomizer.

CFD validation for flyash particle classification in hydrocyclones

The investigation pertains to establishing a simulation methodology for understanding the flyash classification characteristics of a 76 and 50 mm diameter hydrocyclone where the work was carried out using commercially available CFD software. Comparative results on the simulated and experimental water throughput, split values are presented. Results indicted that there is a good match in water split between the experimental and simulated values with error values below 10% at different hydrocyclone designs. Further a discussion is made on the flow features at comparable ratio of cyclone diameter to spigot opening in the 76 and 50 mm designs. The vertical core region around the cyclone axis having static pressure equal to or below the atmospheric pressure is examined to be increasing in diameter from bottom of the spigot opening till the interface where vortex finder joins the main cylindrical cyclone body and remains more or less similar at the vortex finder outlet. The diameter of this zone at the spigot outlet is 0.6 and 5.4 mm at 3.2 and 9.4 mm spigot openings in case of 50 mm diameter hydrocyclone. The diameter of the core at spigot outlet is found to be around 9.2 and 11 mm at 15 and 20 mm for spigot openings in case of 76 mm diameter hydrocyclone. Classification of flyash particulates is simulated through discrete phase modeling using particles injection technique and the simulated results are further validated with suitably performed experiments. With 50 mm diameter hydrocyclone, reasonable predictions are observed at 9.4 mm spigot opening. Considerable deviation in particle distribution points with this hydrocyclone is observed at narrowest spigot diameter of 3.2 mm. The simulated values of d 50 in case of 50 mm diameter hydrocyclone are 8 and 10 μm at 9.4 and 3.2 mm diameter spigot openings. Better predictions are obtained with 76 mm diameter hydrocyclone at both 10 and 15 mm diameter spigot openings. Similarly, the simulated d 50 values are 14 and 20 μm at 15 and 10 mm diameter hydrocyclones. Possible reasons for deviations in the results relating the spigot opening, solids concentration at the underflow and in turn role of slurry viscosity on the air core diameter are proposed.

Numerical simulation of magnetite segregation in a dense medium cyclone

Numerical simulations of turbulent driven flow in a dense medium cyclone with magnetite medium have been conducted using Fluent. The predicted air core shape and diameter were found to be close to the experimental results measured by gamma ray tomography. It is possible that the Large eddy simulation (LES) turbulence model with Mixture multi-phase model can be used to predict the air/slurry interface accurately although the LES may need a finer grid. Multi-phase simulations (air/water/medium) are showing appropriate medium segregation effects but are over-predicting the level of segregation compared to that measured by gamma-ray tomography in particular with over prediction of medium concentrations near the wall. Further, investigated the accurate prediction of axial segregation of magnetite using the LES turbulence model together with the multi-phase mixture model and viscosity corrections according to the feed particle loading factor. Addition of lift forces and viscosity correction improved the predictions especially near the wall. Predicted density profiles are very close to gamma ray tomography data showing a clear density drop near the wall. The effect of size distribution of the magnetite has been fully studied. It is interesting to note that the ultra-fine magnetite sizes (i.e. 2 and 7 μm) are distributed uniformly throughout the cyclone. As the size of magnetite increases, more segregation of magnetite occurs close to the wall. The cut-density ( d 50) of the magnetite segregation is 32 μm, which is expected with superfine magnetite feed size distribution. At higher feed densities the agreement between the [Dungilson, 1999; Wood, J.C., 1990. A performance model for coal-washing dense medium cyclones, Ph.D. Thesis, JKMRC, University of Queensland] correlations and the CFD are reasonably good, but the overflow density is lower than the model predictions. It is believed that the excessive underflow volumetric flow rates are responsible for under prediction of the overflow density.

Large eddy simulation of hydrocyclone—prediction of air-core diameter and shape

Hydrocyclones are widely used in the mining and chemical industries. An attempt has been made in this study, to develop a CFD (computational fluid dynamics) model, which is capable of predicting the flow patterns inside the hydrocyclone, including accurate prediction of flow split as well as the size of the air-core. The flow velocities and air-core diameters are predicted by DRSM (differential Reynolds stress model) and LES (large eddy simulations) models were compared to experimental results. The predicted water splits and air-core diameter with LES and RSM turbulence models along with VOF (volume of fluid) model for the air phase, through the outlets for various inlet pressures were also analyzed. The LES turbulence model led to an improved turbulence field prediction and thereby to more accurate prediction of pressure and velocity fields. This improvement was distinctive for the axial profile of pressure, indicating that air-core development is principally a transport effect rather than a pressure effect. An increase in feed flow rate will increase the air-core diameter by increasing the centrifugal force on fluid elements, which in turn increases the tangential velocity component, thereby lowering the pressure at the hydrocyclone axis near the apex. It has been observed that when the spigot diameter increased, the air-core diameter increased, since the tangential velocity increases when underflow diameter increases. As expected, a rise in the viscosity of liquid reduces the air-core diameter at constant feed velocity, by lowering the pressure drop over the cyclone. This LES turbulent multi-phase CFD model, once extended to particulate phase flow in hydrocyclone, would be a useful tool for studying design dimensions. More importantly, alternative geometries may be examined swiftly.

Properties of index-guided PCF with air-core

In the paper, index-guided triangular PCF with air-core is introduced which guides light by total internal reflection (TIR) when the air-core is smaller than the air holes in cladding. Properties of dispersion and leakage loss are investigated using finite-element method. The total dispersion curve gets more flattened and leakage loss increases significantly as the air-core diameter increases. High birefringence PCF with elliptical air-core is put forward and the birefringence increases with the ellipticity of the air-core.