Duct Geometry Research Articles

Effect of convergent duct geometry on the energy extraction performance of tandem oscillating hydrofoils system

In the present study, a new hydrokinetic energy harvester concept based on the tandem oscillating hydrofoils configuration is numerically investigated. Unsteady turbulent two dimensional flow simulations were performed, by using the commercial finite volume computational fluid dynamics code FLUENT, to study the power extraction from tandem hydrofoils oscillating inside a convergent duct. A sinusoidal function was used to interpret the plunging/pitching motion of NACA 0015 hydrofoil. Computations were carried out for the different configurations of area ratios (AR) that is defined as the ratio of the upstream hydrofoil’s inlet cross section (A1) to the downstream hydrofoil’s inlet cross section (A2) ranging from 1.0 (without the convergent section) to AR = 2.6. The study revealed that the power extraction was enhanced by the increased incoming flow velocity enabled by the geometry of the convergent duct, which positively affected the total pressure as well as the hydrofoil-vortex interaction time and resulted the generation of larger vertical hydrodynamic force. Upon analysis of the obtained results, it is found that the energy extraction efficiency is improved, compared to that of the conventional tandem configuration where the hydrofoils move in an open flow space, and, as the area ratio (AR) increased, the amount of extracted power becomes higher in a proportion ranging from 4% to 80%. The concept was numerically tested for tandem and single configurations and for non-dimensional frequencies in the range of f∗=0.04 - 0.20. One interesting finding is that, due to some positive effect of hydrofoil-vortex interaction, and at a specified area ratio and frequency, the downstream hydrofoil efficiency exceeds the efficiency of a single hydrofoil located at exactly same position as the downstream hydrofoil in the tandem configuration. The frequency and the area ratio are found to be critical factors that determine whether the downstream hydrofoil-vortex interaction is favorable or unfavorable. An optimal area ratio/frequency combination results in a positive hydrofoil-vortex interaction effect and allows the downstream hydrofoil to contribute positively to the total power extraction as a single hydrofoil. Based on the present results, and for an optimal efficient energy harvester design, a new correlation is suggested in order to properly control the area ratio in accordance with the variation of the oscillating frequency leading to more power production and high performance of the oscillating tandem hydrofoils. It is anticipated that this new innovative technique will be applied to improve the existing energy harvester devices.

Experimental study and modelling of developing intermittent slug flow in inclined annular pipelines

Geometric arrangements known as flow patterns are typically observed when there are multiple immiscible fluids flowing through any control volume. The adequate modelling of the flow characteristic is important, especially in applications where flow visualization is not possible due to field constrains, like, for instance, in oil production. Intermittent slug flow occurs frequently in petroleum production systems and there is a lack of understanding of its characteristics in inclined annular geometries. Even though this is a frequent geometry encountered in the production system, there is lack of in-depth experimental studies about it. Moreover, existing models for calculating the slug frequency in vertical annular duct cannot predict the slug frequency in inclined annular duct. In this work, a dedicated experimental apparatus equipped with flow meters and a high-speed camera was built in order to investigate some characteristics of the slug flow in slightly inclined annulus, mainly the drift-flux parameters and the frequency of the slug. It is shown that modified drift-flux models for vertical annular flow can be used to calculate drift-flux parameters in inclined annular flow. In addition, a new model is proposed for calculating the slug frequency with improved accuracy when compared to the models available in the literature. The proposed model is suitable for industrial applications in the design of equipment in which such duct geometry is present.

Influence of Liquid Viscosity and Geometry on Vertical Gas/Liquid Two-Phase Annular-Duct Flow

Summary Two-phase flow in large annular ducts is very common in industrial applications. Nevertheless, many two-phase-flow phenomena in such a geometry have not been fully understood. This article addresses an investigation on the effects of duct geometry and liquid viscosity on vertical upward annular-duct two-phase flows. Compressed air and tap water or mineral oil were the working fluids. Three oil viscosity ranges were tested, from 30 to 400 mPa·s. A 10.0-m-long inclinable experimental setup was designed and built for the experiments conducted in two equivalent geometries. The first is made of a 95-mm-inner-diameter circular pipe (called the reference pipe), and the second is a concentric annular duct with 95-mm hydraulic diameter, the same diameter as the reference pipe. The radial geometries were similar to those observed in oil wells. Flow patterns, total pressure drop, and in-situ volumetric fractions were obtained. According to the results, the standard hydraulic diameter might not be appropriate for the modeling of vertical gas/liquid flow in large annular ducts.

Combined convection–radiation heat transfer from n-octane combusted gas passing through different geometrical ducts

The combined radiation–convection heat transfer occurs in many industrial devices; in many occasions, the significant role of radiation is neglected in engineering modeling. The current study is an attempt to address the importance of radiation heat transfer involvement in numerical computations. The investigation takes place in two heat transfer modes of with radiation (case_rad) and without radiation consideration (case_norad) for different duct geometries of base cylinder, small cylinder, divergent frustum cone, and convergent frustum cone. To account the radiation effect in the ducts, discrete transfer radiation method is applied, while a weighted sum of gray gases model for absorption coefficient modeling in non-homogenous participating media is appropriate for combustion of n-octane gas in ducts. The convergent duct structure shows the most radiative flux of − 1514.4 W m−2, whereas small cylinder demonstrates the lowest radiative flux of − 1033.14 W m−2. The overall heat flux of convergent configuration outperforms due to homogenous distribution of O2 and high oxygen mass fraction for combustion, while increasing the surface area in the flow direction aids the heat transfer improvement. The small cylinder is characterized with better flow dynamics (Vmax = 25.72 m s−1, TKEmax = 9.45 m2 s−2); however, its deficiency in heat transfer leads to high temperature along the duct (Tmax = 710 K).

CFD Analysis of Supersonic Combustion at Mach 2 with K-E Turbulence Model

This In this paper combustion of hydrogen under unsteady and supersonic conditions has been presented. The combustor having an injector for injecting fuel parallel to the base. The main flow is also parallel to the base. Modeling of this supersonic combustion has been done using K-ε model with Finite rate chemistry combustion model. In supersonic combustion time available is very short for proper mixing. Intensity of heat release, duct geometry, injection techniques, and mixture (fuel + oxidizer) quality and flow regime are the predominating parameter which decides the flame stabilization. This numerical result demonstrates that flamelet approach is suitable in supersonic flows. Combustion characteristics for injected fuel and supersonic air flow in combustor are presented in this paper. The result shows that the mixing quality increases flame speed almost linearly. The stagnation temperature reaches up to 2420 k in the combustor.

Process Development: Burr and Film Formation during High Speed Punching of Extruded PC Based Polymers

First investigations focus on the usage, processing and material properties of polycarbonate (PC) based materials used in cable duct production. Test coupons were taken from in-situ cable ducts including further additives generally used in industry. Different mechanical and optical analytical methods were performed. Significant differences in tensile properties of polycarbonate/ acrylonitrile butadiene styrene (PC/ABS) compared to mineral reinforced PC were observed. The hardness of mineral reinforced PC is significantly dependent on the geometry of the cable ducts. The fracture behavior and morphology of the PC/ABS fracture surface is directly related to the coupon temperature during Charpy impact testing. The process temperature influences the failure behavior during high impact processing such as high speed punching. Due to the lower impact strength of mineral reinforced PC less film and burr formation compared to PC/ABS are likely. However, the mineral distribution is not homogeneous and therefore subject to further investigation. This study aims at a better understanding of process properties of PC/ABS products, parameter selection, quality improvement and general understanding of underlying microstructural and surface properties.

Design of Wake Equalizing Ducts using RANSE-based SBDO

We propose a Simulation-Based Design Optimization (SBDO) approach for the design of an Energy Saving Device (ESD) based on the Wake Equalizing Duct (WED) concept. Pre-Ducts, like Wake Equalizing Ducts, reduce the wake losses, improve the propeller-hull interaction and generate an additional thrust. An integrated design approach, relying on a parametric description of the duct geometry of the WEDs and on a RANSE method, managed by a global convergence optimization algorithm, is developed to maximize the delivered thrust. The Japan Bulk Carrier (JBC) test case, for which model scale experimental data on the effectiveness of the WED are available in the literature, is considered as a baseline. Results obtained by using the adaptive, fully automated proposed design framework highlight significant improvements of the overall propulsive efficiency when the Pre-Duct design is tailored to the actual hull wake shape. In addition, off-design conditions are considered to verify the robustness of the proposed designs with respect to variations of the working point and discuss the opportunity of a robust optimization process.

Effect of acoustic treatment on fan flutter stability

This paper presents an investigation of the effect of acoustically treated wall on fan flutter stability. The analysis is based on a three-dimensional analytical model which captures the aerodynamic coupling between oscillating annular rotor and finite-length liner in subsonic flow. With the response function of the entire system constructed as a whole, the perturbed duct field in modal description is determined by a simultaneous solution of the rotor and liner responses. Whether or not the aeroelastic instability occurs under a given wall impedance condition is evaluated through the energy method. Numerical experiments are conducted in the parametric space of inter-blade phase angle, liner distribution and acoustic impedance. The results show that under different conditions the acoustically treated wall can take diverse effects on the fan flutter stability. In the presented cases where the rotor stability is considerably affected, it is found that the aerodynamic interaction between the rotor and the lined section modify the overall distribution of the aerodynamic loadings on blade surface rather than change their magnitudes significantly. Moreover, the aeroelastic response of the oscillating blades turns out to be very susceptive to the reactance variation when the resistance is small, further indicating the important role of the reflections on the liner surface in such aerodynamic interaction. The analysis in general implies that the actual impact of a lined wall on the fan flutter stability depends on the aerodynamic excitation condition mutually determined by duct geometry, rotor state and configurations of liner.

Effects of divergent angle on the flow behaviors in low speed wind accelerating ducts

Combined with the negative pressure zone outside the duct, a new method based on the converging-diverging tube using for speeding up the low-grade wind is proposed in this paper. Firstly, a 3D CFD model of the converging-diverging air duct is developed, and the validation of the numerical model is also shown by comparing the experimental and numerical data. Secondly, based on the method of velocity stratification analysis at the air duct throat, the effect of the outlet angle on the speed increase is studied, and the mechanism of the negative pressure zone on changes to the wind speed is also analyzed and discussed. The air duct geometry was optimized, and the simulation study results were verified through experimental tests with a prototype duct and a wind tunnel. The results show that air ducts can significantly focus and increase wind speeds, offering insight for the future use low-speed winds for electricity generation.

Experimental investigation of the noise radiated by a ducted air flow discharge though diaphragms and perforated plates

An experimental investigation of the noise radiated by a ducted high pressure flow discharge through diaphragms and perforated plates is carried out for a large range of subsonic and supersonic operating conditions (Nozzle Pressure Ratio (NPR) from 1.2 to 3.6). A parametric study of the geometrical parameters is also conducted to characterize their influence on the acoustic radiation. This covers configurations from single diaphragms to multi-perforated plates with variable hole diameters and arrangements that are placed inside a cylindrical duct. Compared with the free discharge analysed in a first part of the study (perforated plates placed directly at the output of the duct), the discharge into a duct, which is closer to the practical applications, generates strong acoustic modifications. As expected, the broadband noise is disturbed by strong modulations due to acoustic resonances in the output duct (longitudinal resonances and transversal duct modes). However, as in the free configuration, a strong effect of the plate geometries on the mixing noise is observed, allowing to adapt or reduce this source. In particular, the increase of the ratio between the perforation spacing and the perforation diameter allows reducing the maximum amplitude of the mixing noise. Compared to the free-field discharge, the Sound Pressure Level (SPL) in the ducted configuration is on average proportional to the 6-th power of the velocity instead of the 8-th power. Moreover, there are two dominant frequency humps in the sound spectra. The low frequency one is characterized by a constant Helmholtz number, suggesting that the sound is shaped by the duct geometry, whereas the high frequency one is characterized by a constant Strouhal number suggesting that the sound is directly generated by the flow. Finally, for supersonic operating points, the screech radiation appearing with diaphragms in the free configuration is suppressed when the output duct is added but new high amplitude and low frequency tones appear for the largest diaphragms and perforated plates. These lines are due to a coupling between normal shock oscillations and longitudinal resonances.

Investigation of the relationship between fractal and hydraulic properties of porous structures of the upper respiratory tract of some Arctic animals

The respiratory ducts of animals and humans are presented by curved tubes with complex geometries. The open areas in such structures are filled with moving air governed by a pressure drop between the inlet and outlet of the duct. The complex structures formed by thin walls and warmed by constant blood flow at the body temperatures T=36-39 C serve for fast and efficient warming of the inhaled air to the body temperature and its moistening up to 100% humidity. The Arctic animals possess the most efficient nasal ducts allowing the heating of the inhaled air from T=-30-60C to T=38-39 C during the duct with the length L=8-15 only. The detailed geometry of the nasal ducts of some Arctic animal has been studied on the computed tomograms (CT) scans of the heads of the animals found in the open databases and published in literature. The highly porous structures on some slices are formed by fractal-like divisions of the walls protruded into the nasal lumen. Since the fractal structures are characterized by their fractal dimensions D, the relationships between the hydrodynamic properties and fractal dimensions of the porous structures of the upper respiratory tract of some Arctic animals has been studied. The dimensions D of the cross sections of the tract have been calculated by the counting box method. The porosities of the samples, the tortuosity of the pores, and the equivalent hydraulic diameter Dh of the channel have been calculated. Sierpinski fractals of various types have been used as models of porous structures, for which the above listed parameters, as well as hydraulic resistance to a stationary flow, have also been computed. A number of statistical dependencies between the calculated parameters were revealed, but the absence of their correlations with D was shown. It was obtained, the structures with different porosities and hydraulic resistance Dh can have the same values ​​of D. Therefore, the choice of an adequate model based on only D value introduces significant errors in the calculations of air heating along the upper respiratory tract. The statistical dependences inherent in the natural samples studied can be obtained only on the basis of multifractal models in which the number and shape of the channels, as well as the scale of their decrease, change in a certain way at each generation.

Addressing Mismatch Between the Peripheral and Local Nusselt Number for Non-Axisymmetric Flow Conditions: Redefining the Mean Temperature

For non-axisymmetric conditions, the traditional fluid mean (bulk) temperature at a tube cross section, subjected to constant heat flux, can be higher than the temperature of a point on the wall. This results in a mismatch between the direction of applied heat flux and the temperature difference resulting in (misleading) negative peripheral heat transfer coefficient and Nusselt number (Nu). The resulting cross-sectional Nu (average of peripheral Nu values) differs from that calculated from average surface temperature and mean temperature. To address this gap in the literature, a detailed procedure is investigated based on a radial mean temperature of individual lines calculated from the lowest temperature spot in the cross-section to wall points, physically satisfying the mechanism of actual heat transfer from hot to cold spots. Each radial line’s mean temperature is used to calculate a peripheral Nu. These peripheral and average Nu are always positive and matches with the cross-sectional Nu that is calculated using the traditional method – single average cross-sectional temperature. This method is verified for different duct geometries including circular, square, and rectangular. The method also succeeds in the estimation of the peripheral and cross-sectional Nu values for eccentric annular ducts that are inherently non-axisymmetric and have negative Nu values according to the published literature.

Numerical analysis of a small ducted wind turbine for performance improvement

ABSTRACTIn this study, Computational Fluid Dynamics simulations are carried out to assess the effect of duct geometry on power output and flow characteristics for a custom-designed wind turbine. The results show that the elevation of power output for a wind turbine is strongly reliant upon the shape of the duct. While before reaching a certain tip speed ratio, ducting would actually decrease the power output level. It is also argued that the velocity recovery index can also be considered dependent upon to the duct geometry. While the record power coefficient for the bare turbine is calculated to be 0.46, a power coefficient of 0.78 is shown to be reachable by adding a duct, which indicates an increase of approximately 70%. The near wake flow was analysed and a relation between the back-pressure level and the calculated power output was verified. The study also presents a framework for sizing the optimum duct.

Experimental Investigation of Wall Wetting Effect on Hydrocarbon Emission in Internal Combustion Engine

In naturally aspirated Spark Ignition engine, about 60-80% of total unburned hydrocarbon emissions are produced during initial stage of vehicle operation under cold start or warm up condition. Wall wetting is predominant effect occurs in idle and part load conditions due to impingement or condensing of un-vaporized fuel droplets around the intake wall, combustion chamber liners and top of the piston. These deposits can cause incomplete combustion which will impact on increase in total hydrocarbon emissions. In this study, wall wetting parameters like fuel density, intake duct geometry, wall film thickness, wall film height, mixture preparation, fuel vaporization has been investigated theoretically by considering droplet evaporation and temperature model for cylinder wall film through mathematical equations. The main objective is to control equivalence ratio and to maintain surface temperature is the most effective way to reduce unburnt hydrocarbon emissions due to cold start wall wetting during steady state and transient conditions. This methodology was carried out on four stroke single cylinder Spark Ignition engine, where additives were used with gasoline fuel of different proportions which could intern reduce the intake and combustion chamber deposits during steady state and transient conditions. This experimental analysis was analyzed at different speed and load conditions. Based on these experimental results, Hydrocarbon emissions were reduced by nearly 40% in steady state and 30% in transient state. It is observed that, bi-fuel injection strategy can be implemented for injecting an ample amount of additive in to cylinder before compression stroke at 110 deg of crack angle during cold start, which it enhances the performance and emission characteristics furthermore.

High precision measurements of acoustic impedance: Applications to the voice and respiratory tract, musical instruments and their players

The acoustic impedance spectrum of the vocal tract and trachea is important in speech and singing. The operation of musical wind instruments depends on the impedance spectra of their bores, and sometimes also on the impedance spectra of the player’s vocal tract. Here, we describe two measurement techniques. The three microphone technique, calibrated with three non-resonant loads, achieves large dynamic range measurements of the frequencies, magnitudes, and bandwidths of resonant duct maxima and minima. The impedance minima are particularly important in flute family instruments and in the voice, where resonances correspond to minima at the lips. These measurements can also be used to estimate the duct geometry and acoustic properties at the distant end, which may be otherwise inaccessible. The capillary or current source technique has been used to measure impedance maxima inside the mouths of wind players during performance, e.g., where saxophonists tune their tracts to the note played, and for “ecological” measurements during speech and singing that give estimates of resonant frequencies that are rather better than those from formant estimation at high pitch. This technique has recently been updated to run in real-time and made available as open source software, with modest equipment requirements.The acoustic impedance spectrum of the vocal tract and trachea is important in speech and singing. The operation of musical wind instruments depends on the impedance spectra of their bores, and sometimes also on the impedance spectra of the player’s vocal tract. Here, we describe two measurement techniques. The three microphone technique, calibrated with three non-resonant loads, achieves large dynamic range measurements of the frequencies, magnitudes, and bandwidths of resonant duct maxima and minima. The impedance minima are particularly important in flute family instruments and in the voice, where resonances correspond to minima at the lips. These measurements can also be used to estimate the duct geometry and acoustic properties at the distant end, which may be otherwise inaccessible. The capillary or current source technique has been used to measure impedance maxima inside the mouths of wind players during performance, e.g., where saxophonists tune their tracts to the note played, and for “ecological...

Correlations for Convection in Hydrodynamically Developing Laminar Duct Flow

Abstract A generalized correlation for combined entry convection in ducts of arbitrary cross section has been developed. The correlation is constructed for the average Nusselt number using knowledge of fully developed transport constants. The general correlation reproduces the first principle solutions for the well-established round and parallel plate duct geometries to within ±5% for both constant temperature and constant heat flux wall conditions when Pr ≥ 0.7. A survey of the literature demonstrates that the new generalized correlation performs as well or better than existing correlations, which are expressed for specific geometries and wall conditions. The new correlation is generally in good agreement with the first principle solutions of less common duct geometries so long as the duct has a convective surface equal to the wetted perimeter. The new correlation is not recommended for ducts having small aspect ratios that pinch the flow when convection is prescribed by the H2 constant heat flux wall condition.

A study on thermohydraulic characteristics of fluid flow through microchannels

At present, miniaturization of the devices in order to make them for effective performance, reliability and ease at cost is the primary need of any nation. In this direction, the role of microchannels can play an important role in the further development of industrial growth. The increased demand of the microchips in the industrial areas has increased the number of transistors for the improved functionality which leads to the emission of the higher heat flux, which is already a big challenge in the electronic sector. In the present paper, a comprehensive review on microchannels has been done regarding single-phase and multi-phase studies. In the present paper, an intensive review of the thermal and hydraulic characteristics of fluid flow in microchannels at different hydraulic diameters and their effects on performance has been done. The effects of the various parameters such as the Reynolds number (Re), Nusselt number (Nu), friction factor (f), pressure drop (P), working fluid and cross-sectional geometry of duct, as well as the hydrodynamic and thermal aspects, has been also studied. It was concluded through the literature study that transition from laminar to turbulent is very much affected by channel cross-sectional geometry, aspect ratio, channel wall roughness and compressibility effects. Also, researchers only explored the laminar region for its pressure drop and heat transfer characteristics, while the turbulent flow regime is yet to be explored. It was observed that most of the correlation over-estimated the value of pressure drop in multi-phase flow.

Laminar flow heat transfer enhancement in square and rectangular channels having: (1) A wire-coil, axial and spiral corrugation combined with helical screw-tape with and without oblique teeth and a (2) spiral corrugation combined with twisted tapes with oblique teeth

Heat transfer enhancement is a vital area of research as it helps to design heat exchangers of reduced sizes and improved heat transfer rates. Thus, there is a pressing need for techniques which result in significantly increased heat transfer over and above increased pressure drop. In this paper, the benefits of different compound techniques using (1) wire-coil inserts and helical screw-tape inserts with oblique teeth, (2) wire-coil inserts and helical screw-tape inserts without oblique teeth, (3) spiral corrugation and helical screw-tape inserts without oblique teeth, (4) axial corrugation and helical screw-tape inserts without oblique teeth, and (5) spiral corrugation and twisted-tape inserts with oblique teeth were studied experimentally for laminar flow in a non-circular duct (aspect ratio = 1, 0.5 and 0.33). Servotherm medium oil with Prandtl number ranging from 430 to 530 was used as working fluid. The experiment was conducted for a uniform heat flux boundary condition. The effect of geometric parameters of the inserts such as wire-coil diameter, wire-coil helix angle, tooth angle and tooth horizontal length of the screw tape and the twisted tape, corrugation height and corrugation pitch, etc. on heat transfer and pressure drop characteristics was presented. The influence of duct geometry (aspect ratio) on Nusselt number and friction factor was also studied. Correlations for Nusselt number and friction factor were developed and a performance evaluation was done.

Multi-element ducts for ducted wind turbines: a numerical study

Abstract. Multi-element ducts are used to improve the aerodynamic performance of ducted wind turbines (DWTs). Steady-state, two-dimensional computational fluid dynamics (CFD) simulations are performed for a multi-element duct geometry consisting of a duct and a flap; the goal is to evaluate the effects on the aerodynamic performance of the radial gap length and the deflection angle of the flap. Solutions from inviscid and viscous flow calculations are compared. It is found that increasing the radial gap length results in an augmentation of the total thrust generated by the DWT, whereas a larger deflection angle has an opposite effect. Reasonable to good agreement is seen between the inviscid and viscous flow calculations, except for multi-element duct configurations characterized by large flap deflection angles. The viscous effects become stronger at large flap deflection angles, and the inviscid calculations are incapable of taking this phenomenon into account.

Study of fully-developed turbulent flow and heat transfer in a rotating wavy-walled duct

Finite volume based computations of fully developed flow and heat transfer through rotating wavy-walled ducts using large eddy simulation (LES) has been carried out. The shear improved Smagorinsky model (SISM) is employed to model the unresolved turbulent fluctuations. The wavy-walls of the duct are described by the sine function y=2asin2(πx/L). The geometrical ratios Hmin/Hmax and L/a are kept fixed to 0.2 and 8 respectively. The effect of rotation on the fluid flow and heat transfer characteristics are presented. All the computations are at a Reynolds number of 5000 and the rotation number is varied in the range, 0–0.5. It has been observed that the pressure drop across the wavy-walled duct varies with increasing rotation number. It is also found out that with rotation, the heat transfer increases on the trailing wall while the leading wall shows the opposite trend. Both Nusselt number and pressure drop for the wavy-walled duct is observed to be less than the corresponding values in ribbed duct geometry.