Straight Duct Research Articles

Experimental and Numerical Studies of Pressure Drop Reduction in a 90° Square Elbow with the Addition of Circular Turbulators

Ducting system has two major components: straight ducts and fittings (sudden enlargement, sudden contraction, elbow, etc.). Pressure drop in the ducting system is caused mostly by friction loss, separation loss, and secondary flow. The aim of this study is to propose a method for reducing the pressure drop in the ducting system by installing a circular turbulator (CT) inside the 90° elbow. This research has been carried out experimentally and numerically with the test section consist of upstream duct, 90° square elbow with R/Dh=3, CT, and downstream duct. Experimental research has used a Pitot tube, an inclined manometer, and a pressure transducer. The research has been also conducted using turbulent simulation in 3D using the k-ɛ standard turbulence model. Reynolds number (ReDh) used has been 1.6×104, 4.8×104 and 9.5×104 respectively based on hydraulic diameter. CT has been added inside the 900 elbow with angular positions (α)=5°, 10°, 15°, 20°, respectively. The gap ratio and the diameter ratio of CT have been g/Dh=0.02 and d/Dh=0.1, respectively, where g is the gap between CT with the elbow wall and d is the diameter of CT. The results have showed that the addition of CT has been able to reduce the pressure drop on square duct with 90° elbow. The effect of CT at α = 15° and Reynolds number 4.8×104 can reduce the overall pressure drop of the ducting system, about 22.14 % compared to ducting without CT.

Numerical solution of oscillatory flow of Maxwell fluid in a rectangular straight duct

A numerical analysis is presented for the oscillatory flow of Maxwell fluid in a rectangular straight duct subjected to a simple harmonic periodic pressure gradient. The numerical solutions are obtained by a finite difference scheme method. The stability of this finite difference scheme method is discussed. The distributions of the velocity and phase difference are given numerically and graphically. The effects of the Reynolds number, relaxation time, and aspect ratio of the cross section on the oscillatory flow are investigated. The results show that when the relaxation time of the Maxwell model and the Reynolds number increase, the resonance phenomena for the distributions of the velocity and phase difference enhance.

AI methods for duct acoustics

Acoustic mode decomposition is commonly used to separate pressure wave components in flow ducts, and established methods relay on mathematical descriptions of the wave motion. In this work, a new approach to mode decomposition is presented which uses artificial intelligence (AI) to separate the acoustic mode content. A neural network is trained with data gained from a small set of numeric solutions of the Linearized Navier-Stokes equation in the frequency domain in a straight duct. The network is tested on relevant experimental data. Good agreement with the analytical method is demonstrated in two applications with different flow conditions. We conclude that using AI for mode decomposition is a promising alternative to the standard method as it is applicable to a much broader set of aero-acoustic problems.Acoustic mode decomposition is commonly used to separate pressure wave components in flow ducts, and established methods relay on mathematical descriptions of the wave motion. In this work, a new approach to mode decomposition is presented which uses artificial intelligence (AI) to separate the acoustic mode content. A neural network is trained with data gained from a small set of numeric solutions of the Linearized Navier-Stokes equation in the frequency domain in a straight duct. The network is tested on relevant experimental data. Good agreement with the analytical method is demonstrated in two applications with different flow conditions. We conclude that using AI for mode decomposition is a promising alternative to the standard method as it is applicable to a much broader set of aero-acoustic problems.

Techniques for airflow measurements to determine the real efficiency of heat recovery in ventilation systems

Heat recovery in ventilation is essential to reduce energy use and thus mitigate greenhouse gas emissions from the building sector. Heat recovery efficiency of at least 80 % in new buildings is required according to Norwegian standards. However, measurements show that the real heat recovery efficiency during operation is commonly 10-20 % lower. Measuring heat recovery efficiency in buildings is challenging, mainly due to difficulties measuring airflow rates close to the air handling unit (AHU). This study assesses the following duct airflow measurement techniques and equipment: pressure differential, velocity traversal technique, ultrasound and tracer gas. The pressure differential method can provide accurate flow rates and thus it is used as the reference measurement. However, it is not suitable for duct flow measurements due to its high pressure penalty and long straight duct requirement. Velocity traverse and tracer gas methods introduce less disturbance to the flow. Nevertheless, both methods require intensive labour work and cannot track quick changes of the airflow with time. The application of ultrasound to measure airflow is relatively novel and it can automatically measure constant and fluctuating airflows with low pressure drop and acceptable accuracy when the proper installation and minimum straight duct are provided.

Optimal damping for a bend containing a microperforated panel

Microperforated panels (MPPs) with a backing cavity can be used as liners on duct walls to damp acoustic waves. However, recent studies show that a MPP (without the backing cavity) placed inside a duct also could give rise to an effective damping. But only if there is an acoustic pressure gradient across the MPP, e.g., as in a duct bend. This offers a new way of designing MPP for control of duct acoustics and leads to a variety of potential applications. In particular, the widely used guide vanes in a bend might be replaced by MPPs which serve as an acoustic damping treatment, while at the same time maintain the aerodynamic performance. In this work, the optimal damping by a MPP in a bend will be investigated based on the so called Cremer impedance theory. A derivation of the governing equation associated with this optimal value will be made for this configuration and the procedure for solving the equation presented. The trajectory of the axial wavenumber in the complex domain is investigated as well as the resulting optimal damping and the Cremer impedance. The results will be compared with the corresponding Cremer solution for a straight duct. Finally, possibilities to realize the Cremer impedance required for the optimal damping in a bend is discussed.

Normal shock wave attenuation during propagation in ducts with grooves

Experimental investigations and numerical simulations of normal shock waves of different strengths propagating inside ducts with roughness are presented. The roughness is added in the form of grooves. Straight and branching ducts are considered in order to better explore the mechanisms causing attenuation of the shock and the physics behind the evolution of the complex wave patterns resulting from diffraction and reflection of the primary moving shock. A well-established finite-volume numerical method is used and further validated for several test cases relevant to this study. The computed results are compared with experimental measurements in ducts with grooves. Good agreement between high-resolution simulations and the experiment is obtained for the shock speeds and complex wave patterns created by the grooves. High-frequency response time histories of pressure at various locations were recorded in the experiments. The recorded pressure histories and shock strengths were found in fair agreement with the two-dimensional simulation results as long as the shock stays in the duct. Overall, the physics of the interactions of the moving shock and the diffracted and reflected waves with the grooves are adequately captured in the high-resolution simulations. Therefore, shocks propagating in ducts with different groove geometries have been simulated in order to identify the groove shape that diminishes shock strength.

Optimization of duct structure and analysis of its impact on temperature inside the shelter

The flow field of the air duct in a shelter was studied using the computational fluid dynamics (CFD) and experiment methods. The improvement methods of vortexes at the bends were analysed. It was found that the corner chamfer and the installation of guide plates have significant effect on improving the vortex. The optimized scheme expanded the inlet and outlet area of the air duct and chamfered the inner bends, then the uniformity of velocity at outlets on both sides of the straight duct was increased by 47% and 42% respectively. The duct was placed in the shelter for simulation. The results showed that the total air volume of the air duct has increased, so the cooling effect is better. The temperature distribution in the shelter is more uniform after optimization, especially in the front part due to the improvement of vortexes.

Effect of inertial lift on a spherical particle suspended in flow through a curved duct

We develop a model of the forces on a spherical particle suspended in flow through a curved duct under the assumption that the particle Reynolds number is small. This extends an asymptotic model of inertial lift force previously developed to study inertial migration in straight ducts. Of particular interest is the existence and location of stable equilibria within the cross-sectional plane towards which particles migrate. The Navier–Stokes equations determine the hydrodynamic forces acting on a particle. A leading-order model of the forces within the cross-sectional plane is obtained through the use of a rotating coordinate system and a perturbation expansion in the particle Reynolds number of the disturbance flow. We predict the behaviour of neutrally buoyant particles at low flow rates and examine the variation in focusing position with respect to particle size and bend radius, independent of the flow rate. In this regime, the lateral focusing position of particles approximately collapses with respect to a dimensionless parameter dependent on three length scales: specifically, the particle radius, duct height and duct bend radius. Additionally, a trapezoidal-shaped cross-section is considered in order to demonstrate how changes in the cross-section design influence the dynamics of particles.

Convergence analysis of inertial lift force estimates using the finite element method

We conduct a convergence analysis for the estimation of inertial lift force on a spherical particle suspended in flow through a straight square duct using the finite element method. Specifically, we consider the convergence of an inertial lift force approximation with respect to a range of factors including the truncation of the domain, the resolution of the tetrahedral mesh and the boundary conditions imposed at the (truncated) ends of the domain. Additionally, we compare estimates obtained via the Lorentz reciprocal theorem with those obtained via a direct integration of fluid stress over the particle surface. 
 
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Optimization of city gas network: a case study from Gujarat, India

A natural gas flow network consists of city gate station, district regulator, pipelines, compressors and valves. The performance of such a complex system cannot be studied by analysing individual constituents. Simultaneous performance of integrated system can be expressed using non-linear equations to simulate and forecast operational behaviour. The operational behaviour is subject to various alternative operational conditions also. Nodal analysis is performed to calculate the flow capacity of each network segment and the pressure at each node. Two conditions are evaluated, namely, keeping flow capacity unknown or by making nodal pressure primary unknown. The resulting set of governing equations are then solved simultaneously. Genetic algorithm method is applied to solve the desired target unknown. Compressor governs the energy supply required to transport gas from one end to another. The compressor equation is also incorporated in city gas network simulation. The simulations are performed by straight gas ducts and complex gas ducts. Boundary conditions are dependent on the form in which gas duct is fed. It is assumed that the gas duct takes gas from City Gate Station at x = 0 and supplies the gas to a consumer located at x = L according to a time variable curve demand Qn (L, t). The capacity of the compressor is also changed and it is observed that the demand is usually periodic in nature. Flow demand at individual node of a gas pipeline network is calculated. Pressures at various nodes are also calculated and matched with original pressure at nodes. Percentage deviation error suggest a range between 0 and 1.02%. The aim of this paper is to optimise certain parameters of a gas transmission pipeline network namely flow rate, mass, cost and pressure. This optimization is performed to gas transmission pipeline network of Gandhinagar, district, where Gujarat State Petronet Limited (GSPL) and Sabarmati Gas Limited (SGL) are the major players. The network which has been chosen for pipeline network consists of 10 CNG stations and 10 DRS.

Peculiarities of low-Reynolds-number supersonic flows in long microchannel

The characteristics of low-Reynolds-number supersonic flows in a long microchannel having a rectangular cross section are investigated computationally. The channel is composed of a Laval nozzle and a straight duct. The design Mach number of the nozzle is 2.0 and the Reynolds number calculated at the nozzle exit is 3100. The length of the straight duct is changed from 2 to 18 h, where h is the duct height. In the computations, the Navier–Stokes equations are numerically solved. The computational code is validated using the experimental data measured by the laser-induced fluorescence (LIF) technique. The computational results demonstrate that neither a normal shock wave nor a pseudo-shock wave, which corresponds to the starting shock wave in a supersonic wind tunnel, appears in microchannel flows. Namely, a low-Reynolds-number supersonic flow is created in a channel without the starting shock wave passing along the duct, although it has been believed that a supersonic internal flow should have been formed through the starting shock wave. In addition, it is found that the microchannel flow changes gradually its supersonic state with the channel length under an underexpanded condition, although a starting shock wave for high-Reynolds-number flows suddenly appears in a channel just as its length exceeds a certain specific length. Such unexpected phenomena originate from the peculiarity that the low-Reynolds-number flows can expand (accelerate) along a straight duct at supersonic speeds, although the high-Reynolds-number flows cannot.

Rib Turbulator Heat Transfer Enhancements at Very High Reynolds Numbers

High-pressure stage gas turbine blades feature serpentine passages where rib turbulators are installed to enhance heat transfer between the relatively colder air bled off from the compressor and the hot internal walls. Most of the prior studies have been restricted to Reynolds number of 90,000 and several studies have been carried out to determine geometrically optimized parameters for achieving high levels of heat transfer in this range of Reynolds number. However, for land-based power generation gas turbines, the Reynolds numbers are significantly high and vary between 105 and 106. The present study is targeted toward these high Reynolds numbers where traditional rib turbulator shapes and prescribed optimum geometrical parameters have been investigated experimentally. A steady-state liquid crystal thermography technique is employed for measurement of detailed heat transfer coefficient. Five different rib configurations, viz., 45 deg, V-shaped, inverse V-shaped, W-shaped, and M-shaped have been investigated for Reynolds numbers ranging from 150,000 to 400,000. The ribs were installed on two opposite walls of a straight duct with an aspect ratio of unity. For very high Reynolds numbers, the heat transfer enhancement levels for different rib shapes varied between 1.4 and 1.7 and the thermal hydraulic performance was found to be less than unity.

Convection Heat Transfer Enhancement in Square Cross-Section with Obstacle Using Nanofluids

The characteristics of heat transfer and pressure drop in turbulent flow of Al2O3 and CuO/water nanofluid that flowing in a horizontal straight square cross-section duct with obstacle has been studied experimentally and numerically. The experimental work is attempted by using a water and nanoparticles concentration of (φ = 0.5 to 2.0 % vol.) that flows in an aluminium square duct with side dimensions of a = 20 mm and length of L = 1000 mm. The obstacle located at the middle of the duct with different obstacle height to pipe side ratio varied as (h/a = 0.0, 0.25, 0.5 and 0.75). The water entered to the duct with inlet Reynolds number of turbulent flow of 3500, 4300, 5200 and 6800. Constant heat flux of 6500 W/m2 is applied upon the external walls of the duct. The numerical finite volume method with appropriate boundary conditions used with ANSYS-Fluent v.16.2 was employed to simulate the temperatures and velocity of the nanofluids. The experimental and numerical results showed that the average Nusselt number increases with increasing the obstacle height ratio and with using nanofluids as compared with pure water. Moreover, the Nusselt number increases with increasing the nanoparticles concentrations in the nanofluids. In addition, the results indicated that increasing in the obstacle height ratio and increasing the nanoparticles, concentration leads to increasing the friction factor and pressure drop. Finally, the numerical results give good indication to the presence of the recirculation cell zone behind the obstacle.

Developing laminar flow in curved semi-circular ducts

The hydrodynamic characteristics of laminar flow in a curved semi-circular duct have been investigated numerically. The duct is evolved around an axis parallel to the flat side of the semi-circle section so that the curved side is outside. This configuration is convenient in cooling of circular tanks in which the contact will be guaranteed around the entire circular tank. The effects of Reynolds number, ranged between 100 and 1500, and the curvature ratio, ranged between 1/20 and 1/8, on the hydrodynamic developing length and the friction factor have been investigated. Significant reduction of the fully developed hydrodynamic length is observed compared to that of straight semi-circular duct. In addition to changing the heat transfer area of the semi-circular duct compared to the circular one, the friction factor becomes less. New correlations for the fully developed hydrodynamic length and the mean friction factor as functions of Reynolds number, the curvature ratio and Dean number have been deduced and compared with those of straight and curved ducts.

Numerical simulation of electrically conducting jet flow in a straight duct under longitudinal homogeneous magnetic field

Spatial evolution of electrically conducting jet flow at a supercritical Reynolds number, entering a duct filled with the liquid of the same physical properties, is studied by direct numerical simulations for the case of a streamwise uniform magnetic field. In contrast to the case of a transverse field, here the jet mean velocity does not interact with the streamwise field, and only the turbulent fluctuations of the flow are influenced and suppressed by the field. In this case, the jet saves its energy and has a tendency to spread at much larger distances. Therefore, one interesting and important property of this setup is the flow stabilization, i.e., transition to turbulence can be largely delayed due to the stabilizing effect of the magnetic field. This occurs in the presence of moderate magnetic fields. At strong magnetic fields, the second instability evolves—the jet profile becomes unsteady due to the traveling waves, which propagate along the jet while not interacting with the field. These traveling waves are generated by the interaction of secondary radial flows and magnetic field.

Numerical laminar forced convection modelling in a ceramic and concrete solar collector with non-circular duct

The paper presents simulations of laminar forced convection in non-circular piping of flat solar collectors made of ceramics or concrete, which are characterized by a low thermal conduction coefficient. The cross-sections of piping in the shape of a regular polygonal, elliptic, superellipse (Lamé curve) and Cassini oval were used for the calculations. In order to perform the simulation, a simplified two-dimensional model of laminar forced convection in a straight axis duct was used and the issue of H2 (constant axial wall heat flux with uniform peripheral wall heat flux) for materials with low thermal conductivity was applied. The calculations were made using the boundary element method (BEM) in a calculation program written by Fortran by the author. In the work, the number of Poiseuille, Nusselt and dimensionless parameters for the evaluation of heat exchangers such as the area goodness factor and the volume goodness factor were determined in the function of characteristic geometrical parameters of the assumed cross-sectional shapes of the flat solar collector piping.

Post-transitional periodic flow in a straight square duct

Direct numerical simulation of incompressible turbulence in a straight square duct finds the post-transition flow evolving substantially, and for Reynolds numbers based on the friction velocity and duct hydraulic radius greater than 600 a two-structure secondary flow regime has been established, suggesting the coexistence of two distinct sources of mean streamwise vorticity. The nominal source terms in the equation for the mean streamwise vorticity involve turbulent variables only, that allow us to identify the dominant dynamical process that marks and/or sustains the transverse mean flow. Close to the corner a mean profile instability is dominant, while farther away turbulent streamwise vorticity intensification is broadly distributed near the duct walls. The instability-driven secondary velocity maximum on the duct diagonals scales with the friction velocity. There is limited scaling of turbulent intensities on the wall bisectors.

Three-dimensional fluid topology optimization for heat transfer

In this work, an in house topology optimization (TO) solver is developed to optimize a conjugate heat transfer problem: realizing more complex and efficient coolant systems by minimizing pressure losses and maximizing the heat transfer. The TO method consists in an idealized sedimentation process in which a design variable, in this case impermeability, is iteratively updated across the domain. The optimal solution is the solidified region uniquely defined by the final distribution of impermeability. Due to the geometrical complexity of the optimal solutions obtained, this design method is not always suitable for classic manufacturing methods (molding, stamping....) On the contrary, it can be thought as an approach to better and fully exploit the flexibility offered by additive manufacturing (AM), still often used on old and less efficient design techniques. In the present article, the proposed method is developed using a Lagrangian optimization approach to minimize stagnation pressure dissipation while maximizing heat transfer between fluid and solid region. An impermeability dependent thermal conductivity is included and a smoother operator is adopted to bound thermal diffusivity gradients across solid and fluid. Simulations are performed on a straight squared duct domain. The variability of the results is shown on the basis of different weights of the objective functions. The solver builds automatically three-dimensional structures enhancing the heat transfer level between the walls and the flow through the generation of pairs of counter rotating vortices. This is consistent to solution proposed in literature like v-shaped ribs, even if the geometry generated is more complex and more efficient. It is possible to define the desired level of heat transfer and losses and obtain the closest optimal solution. It is the first time that a conjugate heat transfer optimization problem, with these constraints, has been tackled with this approach for three-dimensional geometries.

Investigation on choking behavior of gas flow in microducts

In continuum regime, the large surface-to-volume ratio in microscale flow indicates stronger influence of boundary layer on internal flow, which is confirmed by the present study through quantitatively analyzing the in-duct choking and profile of boundary layer in a series of straight rectangular microducts with convergent entrances. The exit height and width of the microducts are 500 µm and 2500 µm. The number density distribution along the duct centerline is measured using a laser-induced fluorescence technique in underexpanded conditions for Reynolds numbers ranging from 745 to 6710. The experimental results show that an unexpected drop in number density emerges upstream of the duct exits. By numerically solving the 3-D Navier–Stokes equations, the computational results reveal that the build-up of boundary layer forms a virtual throat upstream of the duct exit, thus turns the straight duct into a convergent–divergent micronozzle. The location of Mach-number unity (choking) and the boundary-layer thickness are found affected by both duct configuration and Reynolds number at choking. In addition, location of the farthest in-duct choking from duct exit is found corresponding to a certain range of transition from laminar to turbulent at Re ∼ 2000. The 1-D analysis confirms that the in-duct choking phenomenon is related to the boundary-layer blockage rather than friction. The results of the present survey indicate the significance of reckoning boundary-layer blockage in micronozzle or microduct design.

Large-Eddy Simulations of turbulent flow through a heated square duct

The behavior of the flow through a straight square duct is examined using Large-Eddy Simulations. Both isothermal and non-isothermal conditions were considered, the latter generated with either the wall temperature or the wall heat flux held constant. Fully-developed conditions were attained by initializing the velocity field with perturbed streamwise streaks, and by employing a method for efficiently applying cyclic boundary conditions to the velocity field while minimizing aliasing errors. Results were obtained for two values of bulk Reynolds numbers, Reb=6,000 and 10,000. The numerical accuracy was checked via several alternative methods that included performing computations on grids with different resolutions, both with and without sub-grid scale models. The results were used to test some of the assumption underlying the use of RANS approaches to predict the flow and thermal fields. Of particular interest was the examination of the effects of the turbulence-driven secondary motions on the near-wall processes, especially on existence and extent of the thermal logarithmic law of the wall, and on departures from the Reynolds analogy. Their effect on the turbulent Prandtl number was quantified, and the implications on the use of Fourier's law to relate the turbulent heat fluxes to the gradients on mean temperature are discussed.