Duct Bend Research Articles

Integrated Outlet Guide Vane Design for an Aggressive S-Shaped Compressor Transition Duct

Within gas turbines the ability to design shorter aggressive S-shaped ducts is advantageous from a performance and weight saving perspective. However, current design philosophies tend to treat the S-shaped duct as an isolated component, neglecting the potential advantages of integrating the design with the upstream or downstream components. In this paper, such a design concept is numerically developed in which the upstream compressor outlet guide vanes are incorporated into the first bend of the S-shaped duct. Positioning the vane row within the first bend imparts a strong radial gradient to the pressure field within the vane passage. Tangential lean and axial sweep are employed such that the vane geometry is modified to exactly match the resulting inclined static pressure field. The integrated design is experimentally assessed and compared to a conventional nonintegrated design on a fully annular low speed test facility incorporating a single stage axial compressor. Several traverse planes are used to gather five-hole probe data which allow the flow structure to be examined through the rotor, outlet guide vane and within the transition ducts. The two designs employ almost identical duct geometry, but integration of the vane row reduces the system length by 21%. Due to successful matching of the static pressure field, the upstream influence of the integrated vane row is minimal and the rotor performance is unchanged. Similarly, the flow development within both S-shaped ducts is similar such that the circumferentially averaged profiles at duct exit are almost identical, and the operation of a downstream component would be unaffected. Overall system loss remains nominally unchanged despite the inclusion of lean and sweep and a reduction in system length. Finally, the numerical design predictions show good agreement with the experimental data thereby successfully validating the design process.

Experimental study of aerosol distribution and concentration variation through curved ducts

This paper presents an experimental investigation of an aerosol problem by adopting two methods. One approach utilizes scanning electron microscope to study the realistic aerosol deposition status in a typical duct bend. Through scanning electron microscope images and on-site observation, the aerosols tend to deposit and accumulate more on the outer bend wall and bend outlet wall. The other approach is to design systematic experiments to measure aerosol concentration variation and distribution through 90-degree curved ducts. This experimental method is simple for realistic applications. Measured results show that the average particle volume concentration diverges greatly after flowing through the bend. The loading ability decreases significantly for larger particles at bend outlet under higher Dean number.

Dust deposition in ventilation and air-conditioning duct bend flows

Particles carried by airflows in ventilation and air-conditioning systems have adverse effects on the quality of air in buildings and hence the health of building occupants. Gaining insight on particle deposition onto ventilation and air-conditioning duct bends is important for controlling pollutant dispersion. Based on the Reynolds stress transport model (RSM), this paper has taken into account the effects of drag, lift force, gravity, inertia force, turbulent diffusions, particle size and air velocity on the dimensionless deposition velocity of particles in smooth duct bends using fully developed velocity profiles. At two different air velocities of 3.0 m/s and 7.0 m/s, the aforementioned effects were predicted by Reynolds-averaged Navier–Stokes (RANS)-Lagrangian simulation on square shaped duct bends with different ways of placement. Preliminary results suggest that gravity and inertia force enhance the dimensionless deposition as dimensionless relaxation time rises. Change tendency of the dimensionless particle deposition velocity on different surfaces of bend duct agrees well with previous studies. As air velocity and particle diameter increase, a significant increase of particle deposition coefficient in the duct bends is observed. Particle deposition to intrados can be intensified by the combined action of gravity and inertia force in different direction.

How Particle Resuspension from Inner Surfaces of Ventilation Ducts Affects Indoor Air Quality—A Modeling Analysis

Dust particles deposited on the inner surfaces of the ventilation ducts can be resuspended by passing airflow. A physical-science-based model is developed to understand how particle resuspension affects the indoor air quality. This integrated model takes into consideration particle mass balance models for straight ventilation duct, duct bend, ventilation room, and air filter. The straight duct and room models have been validated using experimental data. With the integrated model, we find that in-duct resuspension of particles could lead to significant increase in exposure to airborne particles for indoor occupants. It is also found that indoor particle exposure is a linear function of dust mass loading. Greater ventilation rate, which means higher air speed above the dust particles, would lead to greater exposure ratio, while fresh air ratio has little influence. Possible control methods are discussed as well.

A computational investigation of particle distribution and deposition in a 90° bend incorporating a particle–wall model

This paper investigates the particle flow movement and deposition in a 90° bend after a straight duct, utilizing the Lagrangian particle-tracking model incorporated with a particle–wall collision model. Particle turbulent dispersion is introduced by employing the ‘eddy lifetime’ model, and particle deposition velocity in the bend is proposed by counting the number of deposited trajectories in a time period. The developed models are validated for both airflow and particle flow by previous experimental data. Particle distribution and deposition behavior at five size groups (1, 3, 5, 9, and 16 μm) are investigated. The simulation results show that, compared with traditional ‘Trap’ model, the particle–wall collision model postpones the emergence and slows the increase of the ‘particle free zone’ as the particle diameter increases. Particle deposition velocity in the duct bend is also generally predicted by the proposed estimation equation under the simulated conditions. This reveals that adopting the particle–wall collision model obtains a reasonable prediction of particle distribution and deposition in the duct bend. This work will benefit the understanding and application of microparticle flow in curved duct systems.

Modal-Analyse im Bogen- oder Ring-Kanal. Teil II: Ring-Kanal

Sound fields in ducts with ring-shaped cross-sections or in flat ducts with a circular bend of their axis can be represented as mode sums of modes in the cylindrical co-ordinate system in which the duct wall surfaces are co-ordinate surfcaces. The modal analysis of the sound fields in both types of ducts proceeds in parallel over wide parts; the numerical solutions, however, reveal significant differences. The case of the duct bend was treated in Part I of this paper; this part will be concerned with the ring-shaped duct. Several forms of developments of the characteristic equation for the radial wave numbers as mode solutions will be described; all of them intend to approximate the characteristic equation by a polynomial equation so that its solutions can be used as starters for a numerical solution of the complete charateristic equation, thus leading to sets of mode solutions. Finally, a set of modes from a flat duct with un-symmetrical lining, which are easily and safely determined by a continued-fraction expansion, are used as starters for modes of a ring-shaped channel with same width and lining. This works quite well under certain parameter conditions.

A large eddy simulation of an airfoil turbulent wake subjected to streamwise curvature

AbstractThis paper presents large eddy simulations (LES) of the curved wake of an airfoil. The wake was generated by placing a NACA0012 airfoil in a uniform stream of air, which is then subjected to an abrupt 90° curvature created by a duct bend. The trailing edge of the airfoil is one chord length upstream of the bend entry. The duct cross‐section measures 457 mm × 457 mm, and the bend has radius to height ratio of 1.17. The flow Reynolds number (1.02 × 105) is based on a mainstream velocity of 10 m/s and airfoil chord length 0.15 m. The sub‐grid scale models employed are the classical Smagorinsky, its dynamic variant and the dynamic kinetic energy transport. The performance of LES in depicting the experimental flow is assessed and compared with results predicted by the Reynolds stress model (RSM). The results show the advantages of LES over Reynolds‐averaged Navier–Stokes methods in predicting convex wall separation in strongly curved ducts on relatively coarse grids. Results from LES on a considerably finer near‐wall‐resolved grid lead to much improved comparison with the experimental data in the near wake, bettering predictions by RSM and LES on the coarse grid. Copyright © 2008 John Wiley & Sons, Ltd.

Dilute gas–solid two-phase flows in a curved [formula omitted] duct bend: CFD simulation with experimental validation

Computational fluid dynamics (CFD) simulations of dilute gas–solid flow through a curved 90 ∘ duct bend were performed. Non-uniform sized glass spheres with a mean diameter of 77 μ m were used as the dispersed phase. The curved bend is square-sectioned ( 150 mm × 150 mm ) and has a turning radius of 1.5 D ( D = duct hydraulic diameter). Turbulent flow quantities for Re = 100 , 000 were calculated based on a differential Reynolds stress model. The solids mass loading considered is 0.00206 and hence justifies the application of one-way coupling between gas and particles. A Lagrangian particle-tracking algorithm which takes into account the effect of shear-slip lift (SSL) force on particles and particle-wall interactions (PWIs) has been utilised to predict velocities of the dispersed phase. The predictions were compared against the experimental data measured using Laser–Doppler Anemometry (LDA). The study found that the predicted gas flow field has a strong influence over the predicted particle velocities. PWI model considerably affects the prediction of particle velocity and distribution of particles at the inner duct wall within the bend. Inclusion of the SSL force also helps the distribution of the particle tracks towards the duct centre in the vertical duct downstream of the bend. Within the bend, particle velocities near the inner wall have been grossly over-predicted in the simulation, especially at mid-bend. The present study thus highlights the importance of the predicted gas flow field, SSL force and particle-wall collisions to Lagrangian particle tracking.

Effect of ventilation duct as a particle filter

This paper discusses the effect of ventilation duct as a particle filter by modeling particle deposition in ventilation ducts, which is the reason that ventilation ducts could “filter” particles. An Eulerian model is employed to predict the particle deposition velocity onto the wall and floor from fully developed turbulent flow in ventilation ducts [Zhao B, Wu J. Modeling particle deposition from fully developed turbulent flow in ventilation duct. Atmospheric Environment 2006;40:457–66], while an empirical equation is proposed to predict the particle deposition velocity onto the ceiling combined with experimental data and, another empirical equation by McFarland et al. [Aerosol deposition in bends with turbulent flow. Environmental Science and Technology 1997;31:3371–7] is used for predicting the particle penetration through the bends, which are hard to analyze by theoretical method.Losses through ventilation duct are thus evaluated for the particle size range 0.01–100μm. Both straight duct (implies fully developed turbulent flow) and duct bend (implies developing turbulent flow) cases are analyzed. Then particle penetration through the supply duct of a swimming pool, including some straight ducts and bends, is analyzed as an application case. The results show that when particle diameter is larger than 5μm, the filtration of the duct might not be ignored as the obvious loss by deposition.

CFD simulation of dilute gas-solid two-phase flows with different solid size distributions in a curved $90^\circ $ duct bend

Computational fluid dynamics predictions of dilute gas-solid flow through a curved 90 ? duct bend are performed. Flows with two different size distributions of glass spheres having mean diameters 66 ?m and 77 ?m are considered. The curved bend is square-sectioned (150 mm?150 mm) and has a turning radius of 1.5 times the duct's hydraulic diameter. Turbulent flow quantities at Re = 15,000 are calculated based on a Differential Reynolds Stress Model, while a Lagrangian particle tracking model predicts solids velocities. The model makes use of a modified shear-slip lift force formula which is consistent with experimental observation for 0.18 ? Re p ? 8. The predictions are compared against experimental measurements taken using laser-Doppler Anemometry. The study indicates that the predicted gas-solid flow behaviour near the outer wall is strongly dependent upon particle size fractions. Prediction quality deteriorates near the inner wall of the bend where local solids concentration diminishes and this points to a major limitation in the Lagrangian particle tracking methodology. The measured particle velocities at the inner wall region is found to be insensitive to the particle size distribution.

Large Eddy Simulation of Flow and Heat Transfer in the 180‐Deg Bend Region of a Stationary Gas Turbine Blade Ribbed Internal Cooling Duct

Large eddy simulation of the 180 deg bend in a stationary ribbed duct is presented. The domain studied includes three ribs upstream of the bend region and three ribs downstream of the bend with an outflow extension added to the end, using a total of 8.4 million cells. Two cases are compared to each other: one includes a rib in the bend and the other does not. The friction factor, mean flow, turbulence, and heat transfer are compared in the two cases to help explain the benefits and disadvantages of the wide number of flow effects seen in the bend, including flow separation at the tip of the dividing wall, counter-rotating Dean vortices, high heat transfer at areas of flow impingement, and flow separation at the upstream and downstream corners of the bend. Mean flow results show a region of separated flow at the tip of the dividing region in the case with no rib in the bend, but no separation region is observed in the case with a rib. A pair of counter-rotating Dean vortices in the middle of the bend is observed in both cases. Turbulent kinetic energy profiles show a 30% increase in the midplane of the bend when the rib is added. High gradients of heat transfer augmentation are observed on the back wall and downstream outside wall, where mean flow impingement occurs. This heat transfer is increased with the presence of a rib. Including a rib in the bend increases the friction factor in the bend by 80%, and it increases the heat transfer augmentation by approximately 20%, resulting in a trade-off between pressure drop and heat transfer.

Compressible swirling flow through a significantly bent duct

The three-dimensional vortical flow of an inviscid compressible fluid through a bend in a slender duct is considered. The duct of simple cross-section gradually bends the flow through a substantial angle. The flow is motivated by industry and involves high speeds and short time intervals. The relative radius of curvature and the magnitude of the inertial forces are important factors here. Weakly and fully non-linear coupling between the streamwise and cross-sectional velocities is considered, and numerical results are derived for a duct of rectangular cross-section in each case, based on a fourth-order compact-differencing approach to solving the three-dimensional Euler equations. The governing equations are parabolic in the streamwise direction, enabling a forward-marching approach. The bend leads to significant growth in vorticity which mixes the streamwise velocity. The weakly non-linear study reveals a role for compressibility which is absent in the fully non-linear study when the cross-sectional area is constant. In both cases there is relatively little pressure loss, in contrast to viscous flows. Linear growth of the total streamwise vorticity in the bend is shown analytically to occur in keeping with the numerical solutions. Far downstream behaviours in finite bends and bends which continue indefinitely are investigated. The latter case leads to a more strongly non-linear flow structure far downstream due to continued evolution of the vorticity.

Particle deposition in industrial duct bends.

A study of particle deposition in industrial duct bends is presented. Particle deposition by size was measured by comparing particle size distributions upstream and downstream of bends that had geometries and flow conditions similar to those used in industrial ventilation. As the interior surface of the duct bend was greased to prevent particle bounce, the results are applicable to liquid drops and solid particles where duct walls are sticky. Factors investigated were: (i) flow Reynolds number (Re = 203 000, 36 000); (ii) particle Reynolds number (10 < Repinfinity < 200); (iii) particle Stokes number (0.08 < Stk < 16); (iv) bend angle (theta = 45 degrees, 90 degrees, 180 degrees ); (v) bend curvature ratio (1.7 < R0 < 12); (vi) orientation (horizontal-to-horizontal and horizontal-to-vertical); and (vii) construction technique (smooth, gored, segmented). Measured deposition was compared with models developed for bends in small diameter sampling lines (Re < 20 000; Repinfinity < 13). Whereas deposition measured in this work generally agreed with that estimated with models for particles <30 microm (Stk < 0.7), it was significantly lower than that estimated for larger particles. As the flow around larger particles became increasingly turbulent, the models progressively under-represented drag forces and over-estimated deposition. For particles >20 microm, deposition was slightly greater in the horizontal-to-horizontal orientation than in the horizontal-to-vertical orientation due to gravitational settling. Penetration was not a multiplicative function of bend angle as theory predicts, due to the developing nature of turbulent flow in bends. Deposition in a smooth bend was similar to that in a gored bend; however, a tight radius segmented bend (R0 = 1.7) exhibited much lower deposition. For more gradual bends (3 < R0 < 12), curvature ratio had negligible effect on deposition.

Large Eddy Simulation of the Anti-Erosion Characteristics of the Ribbed-Bend in Gas-Solid Flows

In order to find out the ribs with the best anti-erosion efficiency, the erosion effects in three 90° duct bend gas-solid flows with different ribs are investigated and compared with that in the bare bend. Three different kinds of ribs are studied, which have square cross section, isosceles right-angled triangle shape cross section and isosceles triangle shape cross section, respectively. The arrangement and the geometry dimensions of the ribs are the same. The gas phase flow field is solved by the large eddy simulation technique considering the interaction between the gas and the particles, whereas the particles are tracked by Lagrangian method. The results exhibit that the erosion rates of all the ribbed bends are greatly reduced due to the adding of the ribs compared with that of the bare bend. Among the three different kinds of ribs, the isosceles right-angled triangle ribs have the highest anti-erosion ability. The reason is that the particle impact velocity on the walls in the isosceles right-angled triangle ribbed bends is decreased to the utmost and the corresponding particle impact angle is altered adequately due to the special shape of the ribs. In addition, the rib erosion rate, the sidewall erosion rate and the bend wall erosion rate are calculated separately. It is found that the rib erosion rate is half of the wall erosion rate, and the sidewall erosion rate is so low that it can be omitted compared with the bend wall erosion rate.

Measurement of particle deposition in industrial ducts

A new method was developed to measure particle deposition in industrial ducts. Greased wire grids were used to capture particles in situ by impaction upstream and downstream of a duct bend. Particles were recovered from the grid using a hexane extraction procedure. Cumulative mass distributions of the recovered particles were determined with a sedimentation pipette; these distributions were then used to compute particle deposition by size. As these procedures were compatible with solid particles only, the interior surface of the bend was coated with grease to eliminate solid particle bounce so that results obtained here could be compared to published models for droplet deposition in bends. In a 20.3-cm-diameter, 90° bend at an air velocity of 27.4 m s −1 , deposition increased from approximately 35% at 15 μm to nearly 100% at 100 μm . These data generally agreed with models developed for small sampling tubes.

Swirl-flow effects in a duct bending through a substantial angle

The vortical three-dimensional flow of an inviscid incompressible fluid through a bend in a slender duct is investigated with nonlinear coupling between the centre-line and cross-sectional velocities. The bend, although gradual, turns the duct flow through a finite angle typically. The theoretical work is motivated directly by the industrial application to food sorting, involving the rapid firing of impulsive ejectors and sprinklers which combine short time scales and flow turbulence, and indirectly by other applications. The flow response depends largely on the relative radius of curvature in the bend along with the cross-sectional swirl and the shear in the perturbed streamwise velocity profile at the entrance. Solutions for the motion within and beyond the bend are determined computationally through a fourth-order compact scheme, for both rectangular and smooth cross-sections, and analytically for a general cross-section, under varying entrance conditions (especially for increasing swirl there, although a long bend for example also enhances swirl downstream). The total streamwise vorticity is found analytically to increase in direct proportion to the distance along the bend, whether the inertial effects are weak or strong. The smallness of the pressure loss also contrasts with that in viscous flows.

Transfer matrix of curved duct bends and sound attenuation in curved expansion chambers

An explicit formula of the transfer matrix is derived for evaluating the acoustic performance of the chamber including curved duct bends. Based on the derived transfer matrix, the reactive sound attenuation of the chamber can be easily calculated by using an empirical formula for estimating the normalized maximum radial pressure of the fundamental mode in the curved duct bend. Predicted results are compared with experimental ones and they show good agreements. It is found that the classical plane wave theory for the straight duct of equivalent length to the curved duct bend is not applicable if the normalized maximum pressure is greater than 1.3. In addition, the sound attenuation in curved expansion chamber is investigated by studying the effect of geometrical parameters. The derived transfer matrix for the curved duct bend can be easily incorporated as a basic module into any existing computer programs dealing with four-pole parameters for predicting the acoustic performance of HVAC and other silencing systems.

Flow and Heat Transfer in a Smooth U-Duct with and without Rotation

This study shows that when computing a compressible flow in a rotating duct of a given geometry and radial distance from the axis of rotation, the inlet Mach number must be specified in addition to the inlet Reynolds number, inlet rotation number, coolant-to-wall temperature ratio, and Prandtl number. This is because the inlet Mach number and other dimensionless parameters collectively fix the rotational speed, which strongly influences centrifugal buoyancy. This study also shows the nature of the three-dimensional flow induced by Coriolis force, centrifugal buoyancy, and a 180-deg bend in a U-shaped square duct with smooth walls for three rotation numbers (0, 0.24, and 0.48), and two Reynolds numbers (2.5 x 10 and 5 X 10). Key flow mechanisms that affect heat transfer are identified. The computed heat transfer coefficient on the leading and trailing faces of the rotating duct compares well with available experimental data. This computational study is based on the ensemble-averaged conservation equations of mass, momentum (compressible NavierStokes), and energy closed by a &-<o/shear stress transport model of turbulence that can be integrated to the wall; i.e., wall functions were not used. Solutions were generated by using a cell-centered finite volume method on a structured grid based on second-order accurate Roe differencing, and on a diagonalized alternating-direction implicit scheme with local time-stepping and Vcycle multigrid.

Flame propagation along 90° bend in an open duct

Experiments of flame propagation in a small, open square duct including a 90° bend were performed for a methane-air mixture using high-speed cine camera and schlieren techniques. In a slow flame propagation from open end to closed end, a blunt flame front shape is kept along the bend. However, in the case of closed-to open-end rapid propagation, from the bend the sharpened flame tip proceeds near the inner wall with succeeding flame front elongated towards the axial direction, and in serious cases transition to turbulent flame occurs. The latter propagation mode was examined in detail. The flame tip speed takes a large value in the bend. In the latter straight section, after a short deceleration period, the flame propagation speed (in axial direction) continuously increases for a while. The relation between the flame behavior and flow field ahead of the flame was investigated further by observing time line movement in the unburned mixture. From the similarity between the flame front shape and time line shape variations, it was revealed that the flame behavior around the bend is determined primarily by the nature of unburned gas flow driven toward the open end. The observed time line movements were compared further with those obtained by numerical simulation based on two-dimensional incompressible inviscid or viscous cold flow models. It was shown that around the bend, the mixture flow pattern and also features of the flame behavior are determined primarily by potential flow characteristics. It was suggested that the temporary flame speed increase at the bend is mainly due to large axial flow velocity of unburned mixture near the inner wall, while the increase of flame propagation speed at the downstream straight section is primarily due to the increase of the flame area.

Correlations between flow resistance and geometry in a model of the human nose.

The relationship between the pressure losses within the nasal airways and nasal geometry were studied in a 3:1 scale model. The geometry of the model was based on magnetic resonance images of the skull of a healthy male subject. Pressure measurements, flow visualization, and hot-wire anemometry studies were performed at flow rates that, in vivo, corresponded to flows of between 0.05 and 1.50 l/s. The influence of nasal congestion and the collapse of the external nares were examined by using modeling clay to simulate local constrictions in the cross section. A dimensionless analysis of the pressure losses within three sections of the airway revealed the influence of various anatomic dimensions on nasal resistance. The region of the exterior nose behaves as a contraction-expansion nozzle in which the pressure losses are a function of the smallest cross-sectional area. Losses in the interior nose resemble those associated with channel flow. The nasopharynx is modeled as a sharp bend in a circular duct. Good correspondence was found between the predicted and actual pressure losses in the model under conditions that stimulated local obstructions and congestion.