Straight Duct Research Articles

MATHEMATICAL MODELLING ELECTRICALLY DRIVEN FREE SHEAR FLOWS IN A DUCT UNDER UNIFORM MAGNETIC FIELD

We consider a mathematical model of two-dimensional electrically driven laminar free shear flows in a straight duct under action of an applied uniform homogeneous magnetic field. The mathematical approach is based on studies by J.C.R. Hunt and W.E. Williams [10], Yu. Kolesnikov and H. Kalis [22,23]. We solve the system of stationary partial differential equations (PDEs) with two unknown functions of velocity U and induced magnetic field H. The flows are generated as a result of the interaction of injected electric current in liquid and the applied field using one or two couples of linear electrodes located on duct walls: three cases are considered. In dependence on direction of current injection and uniform magnetic field, the flows between the end walls are realized. Distributions of velocities and induced magnetic fields, electric current density in dependence on the Hartmann number Ha are studied. The solution of this problem is obtained analytically and numerically, using the Fourier series method and Matlab.

Large-Eddy vs. Reynolds-Averaged Navier–Stokes Simulations of Flow and Heat Transfer in a U-Duct with Unsteady Flow Separation

Large-eddy simulation (LES) and Reynolds-Averaged Navier–Stokes (RANS) equations were used to study incompressible flow and heat transfer in a U-duct with a high-aspect-ratio trapezoidal cross section. For the LES, the WALE subgrid-scale model was employed, and its inflow boundary condition was provided by a concurrent LES of incompressible fully-developed flow in a straight duct with the same cross section and flow conditions as the U-duct. LES results are presented for turbulent kinetic energy, Reynolds stresses, pressure–strain rate, turbulent diffusion, turbulent transport, and velocity–temperature correlations, with a focus on how they are affected by the U-turn region of the U-duct. The LES results were also used to assess three commonly used RANS models: the realizable k-ε with the two-layer model in the near-wall region, the two-equation shear-stress transport model, and the seven-equation stress-omega Reynolds stress model. Results obtained show steady and unsteady RANS to incorrectly predict the effects of unsteady flow separation. The results obtained also identified the terms in the RANS models that need to be modified and suggested how turbulent diffusion should be modeled when there is unsteady flow separation.

Periodic bursting cycles on the edge to square duct turbulence

The long-time behaviour of trajectories embedded in the edge manifold between the laminar and turbulent basins of attraction is investigated in order to identify simple invariant solutions to the Navier-Stokes equations in straight ducts with square cross-section. With the aid of the iterative bisection procedure of Toh and Itano (J. Fluid Mech., vol. 481, 2003, pp. 67-76), we detect three travelling waves that represent attracting ‘edge states’ within the separatrix and saddles w.r.t. the full state space. All three travelling waves are presumably connected to solutions known in the community. In other situations, the edge states to which trajectories are attracted seem to be invariant periodic cycles that feature a rich dynamics: Along each cycle, the flow oscillates periodically between two flow states, in each of which streamwise vorticity is predominantly residing near one set of parallel walls, while the vortical activity associated to the other pair of opposing walls is markedly reduced. This ‘switching’ dynamics is accompanied by intermittent bursting-like high-dissipation excursions. While we cannot rigorously prove that the identified periodic cycle is a periodic orbit rather than a finite-precision approximation to a heteroclinic cycle, sound arguments for the former supposition are given. Interestingly, the different phases of the periodic cycle qualitatively resemble a similar ‘switching’ behaviour observed in (chaotic) marginal turbulence, which suggests that the found periodic cycles could help understanding the characteristic dynamics of duct turbulence at low Reynolds numbers.

Experimental investigation of turbulent flow through a square-sectioned duct and 90ᵒ square elbow by using circular turbulator

The ducting system is made up of 90° elbows and a few other fittings and accessories in addition to straight ducts. The friction loss, separation loss, and secondary flow loss are the main causes of the pressure decrease in an elbow, and they all increase with the presence of elbows. Utilizing too much energy to propel the flow is a cost associated with pressure loss. The aim of this research is to investigate turbulent flow via 90° square elbows and a square-sectioned duct through experimental methods by adding a circular turbulator (CT) close to the elbow wall's inner radius. The Reynolds numbers (ReDh) used in this research are 1.6×104, 4.8×104, and 9.5×104, with average flow velocities of 2 m/s, 6 m/s, and 12 m/s. Circular turbulators are added to the inner walls with angular positions (α) of 5°, 10°, 15°, and 20°. The results showed that the turbulence intensity increased toward the inner radius wall of the elbow in ducting with CT. For this study, the flow within the ducting with the CT is generally more turbulent than the flow inside the ducting without the CT. By including CT, it was possible to reduce the overall pressure loss in ducting with an elbow. The pressure reduction at two ReDh values (1.6×104 and 4.8×104) was only lessened by CT positioned at α=10°, 15°, and 20° out of the four CT placements. Conversely, CT positioned at α=5° can only effectively mitigate pressure decrease at ReDh=1.6×104. CT installation cannot lessen the pressure drop that happens in the ducting at a value of ReDh=9.5×104. CT placement at the three α values often helps to lower the pressure drop in the ducting. Using the CT, positioned at α=15°, is the greatest strategy to reduce the overall pressure decrease

Heat transfer characteristics of pulsating flow in a straight channel with rhombic-shaped expanding chamber: A numerical study

ABSTRACT The combination of passive and active heat transfer recovery techniques in channel flows has significant potential in terms of increasing thermal performance. Therefore, this study focused on the numerical examination of the flow and thermal behavior of pulsating flow in a straight duct containing a rhombus chamber. In the context of the study, iterations were solved using the finite volume method (FVM). The study was carried out for different pulsating amplitudes (A: 0.2, 0.4, 0.6, and 0.8), Strouhal numbers (St: 1, 2, 3, 4), and Reynolds numbers (200 ≤ Re ≤ 800). The surfaces other than the adiabatic lengths at the inlet and outlet of the duct were kept constant at Tw = 360 K. Results were compared with the results of the steady flow case. The effects of pulsating velocity and oscillatory parameters on Nusselt number, pressure drop, and performance factor were discussed. Velocity and temperature images were presented for different Reynolds numbers and pulsating components in the channel. The findings revealed that although the pulsating parameters significantly enhanced the heat transfer at increasing Reynolds numbers, they had a fairly low effect on heat transfer at the same Reynolds number. It was observed that at Re = 800, the Nusselt number formed a peak at St = 3 for all tested pulsating amplitudes. For the parameters of Re = 800, A = 0.6, and St = 2, heat transfer and performance factor in pulsating flow increased by 8.96 and 8.22 times, respectively, compared to steady flow conditions.

Multivariate Peristalsis in a Straight Rectangular Duct for Carreau Fluids

Peristaltic flow in a straight rectangular duct is examined imposed by contraction pulses implemented by pairs of horizontal cylindrical segments with their axes perpendicular to the flow direction. The wave propagation speed is considered in such a range that triggers a laminar fluid motion. The setting is analyzed over a set of variables which includes the propagation speed, the relative occlusion, the modality of the squeezing pulse profile and the Carreau power index. The numerical solution of the equations of motion on Cartesian meshes is grounded in the immersed boundary method. An increase in the peristaltic pulse modality leads to the reduction in the shear rate levels on the central tube axis and to the movement of the peristaltic characteristics to higher pressure values. The effect of the no slip side walls (NSSWs) is elucidated by the collation with relevant results for the flow field produced under the same assumptions though with slip side walls (SSWs). Shear thinning behavior exhibits a significantly larger effect on transport efficiency for the NSSWs duct than on the SSWs duct.

Self-ordering and organization of a staggered oblate particle pair in three-dimensional square ducts

We numerically investigate the formation and ordering of staggered oblate particle pairs in three-dimensional straight ducts with a square cross section. The lattice Boltzmann method is employed to simulate rigid particle pairs in a Newtonian liquid. The effects of initial axial spacing, Reynolds number, blockage ratio, and particle aspect ratio on the formation process, migration behavior, and interparticle spacing are explored in detail. Current results indicate that the process from initial to final steady state can be divided into two stages. The first stage is rapid migration from initial positions toward equilibrium positions under shear-induced lift force and wall-induced repulsive force. The second stage is the slow self-assembly of stable particle pairs in the axial direction due to the interparticle interaction. Interestingly, initial axial spacing significantly affects the formation process of particle pairs but does not affect the final steady state. It is found that the equilibrium positions of staggered particle pairs move slightly toward the duct walls, and the axial spacing increases with increasing Reynolds number or particle aspect ratio, or decreasing blockage ratio. For a staggered particle pair, the second particle will occupy the eddy center induced by the first focusing particle. Based on the existing data, a correlation is put forward to predict the axial interparticle spacing of staggered oblate particle pairs in duct flows. The present results may give insights into manipulating and comprehending non-spherical particle dynamics in microfluidic applications.

Asymmetry of oblique shock train and flow control

An oblique shock train generally forms an asymmetric structure in a Mach-2.7 flow field within a duct. To study the flow structure and interaction between oblique shock trains and upstream shocks, a ramp with equal width was installed inside a Mach-2.7 straight duct to generate an incident shock and an oblique shock train interaction. A Schlieren system, transient pressure measurements and particle image velocimetry were used to capture quantitative and qualitative shock structure information. Results show that the asymmetric separation deflection of the oblique shock train occurs randomly in the symmetrical straight duct. The separation deflection of the oblique shock train was steady with upstream shock interactions. Under backpressure conditions, the rate of movement of the oblique shock train increases rapidly when it passes through the separation regions generated by the ramp, and the deflection direction of the asymmetric separation may switch. Based on the characteristics of the oblique shock train and upstream shock interaction, a flow control method was used to generate asymmetric upstream flow conditions, providing active control of the oblique shock train deflection direction.

Analysis of Noise Level with Convergent and Divergent Shape of Muffler and its Impact on Noise Pollution

Noise pollution is one of the key hazards that decreases people's quality of life globally. Due to the rapid development of technology, industrialization, urbanization, and transportation networks, noise pollution has reached serious levels in recent years. Sound transmission loss, the attenuation of sound as it passes through diverse objects and environments, is crucial in lowering noise pollution and preserving the acoustic integrity of ecosystems. Due to de-throttling tactics implemented to optimize cylinder fill, the intake system of internal combustion engines is the primary noise source while the vehicle is at low speeds and high loads. Therefore, achieving high acoustic performance of intake systems constitutes a critical issue in complying with the increasingly stringent European regulations for overall noise emission. The Transmission Loss computation is typically the most used technique for describing a system's acoustic performances. This work provided an overview of sound transmission loss and discussed its substantial environmental effects. It investigates the fundamentals of sound transmission, the factors that influence it, and the environmental impacts of sound attenuation. This paper proposed a comparison of three designs like straight duct (muffler), convergent and divergent design of muffler. The main aim is to observe sound transmission loss and compare the noise level with three muffler configurations with the same gas volume. High transmission loss indicates more noise reduction in muffler design, which will help reduce environmental noise pollution.

Design of sinusoidal-shaped inlet duct for acoustic mode modulation noise reduction of cooling fan

The noise reduction method of installing a sinusoidal-shaped inlet duct on a cooling fan is analyzed theoretically and experimentally in terms of the acoustic mode modulation. Based on Lowson's point force model, the correlation between the free field noise and acoustic mode of the fan rotor and the unsteady forces on the rotor blade surface is established. The azimuthal acoustic mode is demonstrated to partially reflect the composition and intensity of the acoustic source. The theoretical variation in azimuthal acoustic mode indicates that it is possible to completely cancel each mode simultaneously when the phase differences of forces of each order in the secondary source are controlled. A sinusoidal-shaped inlet duct noise reduction structure controlled by a parameter equation is proposed, and the preferred parameter selection is given by the far-field noise measurements. The results of azimuthal mode decomposition exhibit a similar trend to the theoretical analysis. Compared with the straight duct, the chosen sinusoidal-shaped inlet duct achieved greater noise reduction at the blade passing frequency and its first harmonic. The average total sound pressure level in the far field was 6.0 dBA lower than that of the prototype fan, and 1.4 dBA lower than that of the straight duct model.

Parameter estimation for unsteady MHD oscillatory free convective flow of generalized second grade fluid with Hall effects and thermal radiation effects

This work first establishes an unsteady magnetohydrodynamic (MHD) oscillatory free convection flow model of the generalized second grade fluid with Hall heat and mass transfer effect in a straight rectangular duct. A fast second-order spectral method with fractional backward difference formula (FBDF) is proposed to obtain the numerical solution of the established model. The distribution of the velocity and the temperature fields is discussed, and the effect of each parameter on the velocity and temperature fields is shown through graphical experiments. Moreover, in order to avoid the problems of slow computational speed and low accuracy of traditional numerical methods, new fractional physics-informed neural networks (PINNs) are proposed for the multi-parameter estimation of this model. At the same time, a comparative study of our method with the Bayesian method is presented. Experimental results show that fractional PINNs is more effective for multi-parameter estimation problems and get better accuracy at the same number of parameters.

Influence of vent duct bend angle on inner dust explosion

A visualized tank was applied to carry out explosion venting experiments to investigate the effect of the bend angle of the vent duct on the venting dynamics process during the dust explosion. By changing the bend angle of the duct and comparing the conventional ductless and straight duct venting, the distribution law of venting overpressure and the development process of flame shape in the duct and container were analysed. There are typically three-peaked features in the duct caused by film-breaking shock waves, secondary explosions, and continuous combustion of the flame in the container. The secondary explosion strength in the duct increases with the bend angle, which exacerbates the severity of the explosion compared to conventional ductless or straight duct explosions. In the case of bend venting, as the bend angle of the duct increases, the Pmax and the (dP/dt)max all show an increasing trend in the container, and the Pred,max difference on both sides of the bend is positively correlated with the bend angle. In addition, with an increase in the bend angle, the total reaction time in the container gradually decreased, namely, the flame duration shortened.

Large eddy simulation of supersonic flow in ducts with complex cross-sections

Large Eddy Simulation (LES) has been employed for the investigation of supersonic flow characteristics in five ducts with varying cross-sectional geometries. The numerical results reveal that flow channel configurations exert a considerable influence on the mainstream flow and the near-wall flow behavior. In contrast to straight ducts, square-to-circular and rectangular-to-circular ducts exhibit thicker boundary layers and a greater presence of vortex structures. Given the same inlet area, rectangular-to-circular ducts lead to higher flow drag force and total pressure loss than square-to-circular ducts. Characterized by the substantial flow separation and shock waves, the “S-shaped duct shows significant vertically-asymmetric characteristics.

A variable gradient descent shape optimization method for transition tee resistance reduction

In this paper, a local component optimization method is proposed for reducing resistance and consumption in building transmission and distribution systems. The gradient descent method is used to investigate energy dissipation minimization constrained by the Navier‒Stokes equation with nonlinear boundary conditions, and the optimality conditions of the shape are derived from the shape optimization problem using the function space parameterization method. The shape of the upper wall of the transition tee is iteratively optimized until the optimal shape with the minimum energy dissipation is obtained. The derivative of the free boundary shape of the transition tee and the objective function are proposed, and the normalized wall height of the transition tee with different area ratios are provided; these results can enable the adaptation of various sizes of transition tees in engineering to achieve air supply conditions that meet the engineering requirements. In this paper, the flow field and pressure loss of the optimized and conventional tees are compared, and the optimized tee performance improvement and energy saving potential are evaluated. The optimized transition tee has a resistance reduction rate of 6∼99% for the straight duct and 2–98% for the branch duct under different area ratios and flow ratios. Our method has the advantages of being more accurate, flexible and general and provides a novel method for the resistance optimization design of building transmission and distribution systems.

Study of particle dynamic resuspension in a recirculation flow using large-eddy simulations

Particle resuspension in the recirculation flow is a significant phenomenon in many industrial applications. This paper elaborated on particle resuspension including incipient motion, wall migration, and re-entrainment in the recirculation vortices of periodic semicircular obstructions, wherein particle incipient motion is predicted by the Rock'n'Roll model while the coupling between fluid and particles is calculated by Large-Eddy Simulation (LES) and Euler-Lagrange method. The results show that an upward instantaneous flow and enough friction velocity are essential for particle resuspension. Small particles are easily re-entrained into the primary vortex and then entered into the mainstream flow. In addition, particles entering the primary vortex of the recirculation region may return to the wall to produce the recycle resuspension. Compared with straight duct, the recirculation flow on particle resuspension greatly reduces the fraction of short-term resuspension particles, but the inhibition ability gradually decreases with time elapsing.

Large eddy simulation of fuel-air mixing process in a convergent-divergent duct spray under non-vaporizing conditions

The spray mixing process can be improved via straight (ST) duct fuel injection, but there is a risk of potential heat transfer loss due to prolonged spray tip penetration (STP) and spray impingement. We proposed a convergent-divergent (CD) duct spray to produce an acceptable STP along with improved spray air entrainment and spray cone angle (SCA), which is validated in optical spray experiments. How the CD duct enhances the fuel–air mixing and why the spray left–right swing phenomenon occurs near the outlet of the CD duct is still unknown. This study focuses on the spray mixing process inside CD and ST ducts using large eddy simulations. Results showed that the larger vortex cluster is formed in the divergent outlet with the large diameter of 4.5 mm and 6 mm, named CD4.5 and CD6 ducts, which leads to the unstable fluctuations of the separated shear flow and a large number of vortex shedding from the inner wall of the CD duct outlet to promote the radial fuel–air mixing with the left–right swing phenomenon, which further enlarges the SCA and appropriately shortens the STP with reduced spray energy. Compare to free spray, there are two turbulence kinetic energy (TKE) peaks in the spray axial centerline, indicating that duct spray brings extra disturbance to spray and is conducive to promoting the fuel–air mixing process. The strongest TKE region appears near the outlet of CD4.5 and CD6 duct sprays that are consistent with the left–right spray swing motion phenomenon.

Three-dimensional/four-dimensional spatiotemporal image correlation morphology of the ductus arteriosus in fetuses with pulmonary atresia undergoing neonatal ductal stenting.

The value of prenatal identification of morphology of ductus arteriosus in fetuses with congenital heart defects (CHD) with pulmonary atresia and duct-dependent pulmonary circulation (DDPC) in planning neonatal ductal stenting procedure is untested. The aim of the study is to analyze the utility of three-dimensional/four-dimensional (3D/4D) spatiotemporal image correlation (STIC) fetal echocardiography in delineating the morphology of ductus arteriosus in fetuses with DDPC undergoing neonatal ductal stenting. In this retrospective study (2017-22), prenatal imaging of pulmonary artery (PA) anatomy, aortic arch sidedness, and morphology of ductus arteriosus (ductal origin was classified as vertical/horizontal and ductal course as tortuous/straight) was done using 3D/4D STIC imaging and volume datasets. Prenatal findings were correlated with angiographic findings during stenting and the degree of agreement was calculated. We included 27 fetuses with a prenatal diagnosis of CHD with DDPC who underwent neonatal ductal stenting. The accuracy of prenatal assessment of PA anatomy, branch PA stenosis, and arch sidedness was 100%, 92.6%, and 88.9%, respectively. The accuracy of prenatal assessment of ductal origin and course, compared with angiography, was 85.2% and 88.9%, respectively. Prenatal imaging had a diagnostic accuracy of 100% for vertical straight and horizontal tortuous ducts, 84.6% for vertical tortuous, and 67% for horizontal straight ducts. Duct stenting was successful in 25 (92.6%) babies; two died after the procedure from stent occlusion. Fetal echocardiography using 3D/4D STIC imaging enables accurate delineation of the morphology of ductus arteriosus in fetuses with DDPC, thereby aiding parental counseling and planning neonatal ductal stenting.

On–off pumping for drag reduction in a turbulent channel flow

We show that the energy required by a turbulent flow to displace a given amount of fluid through a straight duct in a given time interval can be reduced by modulating in time the pumping power. The control strategy is hybrid: it is passive, as it requires neither a control system nor control energy, but it manipulates how pumping energy is delivered to the system (as in active techniques) to increase the pumping efficiency. Our control employs a temporally periodic pumping pattern, where a short and intense acceleration (in which the pumping system is on) followed by a longer deceleration (in which the pumping system is off) makes the flow alternately visit a quasi-laminar and a turbulent state. The computational study is for a plane channel flow, and employs direct numerical simulations, which present specific computational challenges, for example the highly varying instantaneous value of the Reynolds number, and the importance of discretisation effects. Particular care is devoted to a meaningful definition of drag reduction in the present context. The ability of the forcing to yield significant savings is demonstrated. Since only a small portion of the parameter space is investigated, the best performance of the control technique remains to be assessed.

Noise reduction and power generation in blower duct using Helmholtz resonator with piezoelectric diaphragm

Ventilation duct systems are indispensable for air purification and room temperature management in industrial and commercial buildings. The noise generated by blowers for ventilating spaces should be as low as possible. In addition, to power the wireless nodes for monitoring the ventilation conditions such as temperature and pressure at any positions in all the ducts, electric power is preferred to be generated in situ without batteries and wired power supply. In this study, to demonstrate simultaneous noise reduction and in situ power generation using a Helmholtz resonator with a piezoelectric diaphragm in a straight circular pipe duct, the frequency properties of the sound pressure level (SPL) with/without the piezoelectric diaphragm and its generated voltage were measured using an in-house apparatus simulating an industrial blower duct. The experimental results show that the Helmholtz resonator with the piezoelectric diaphragm has a noise reduction performance similar to that without the piezoelectric diaphragm. Power generation was improved not only by impedance matching between the inner and outer resistance but also by the frequency resonance between the frequency of pressure fluctuation in the Helmholtz resonator and the natural frequency of the piezoelectric diaphragm. The Helmholtz resonator with the piezoelectric diaphragm achieved a noise reduction of 10 dB from an SPL of 103 dB and power generation of 12 μW at an SPL of 93 dB.

Smoothed Particle Hydrodynamics-Based Study of 3D Confined Microflows

In this study, we investigate the performance of the smoothed particle hydrodynamics (SPH) method regarding the computation of confined flows in microchannels. Modeling and numerical simulation with SPH involve the representation of flowing matter as distinct mass points, leading to particle discretization of the Navier–Stokes equations. The computational methodology exhibits similarities with other well-established particle methods, such as molecular dynamics (MD), dissipative particle dynamics (DPD), and smooth dissipative particle dynamics (SDPD). SPH has been extensively tested in the simulation of free-surface flows. However, studies on the performance of the method in internal flow computations are limited. In this work, we study flows in microchannels of variable cross-sections with a weakly compressible SPH formulation. After preliminary studies of flows in straight constant cross-section ducts, we focus on channels with sudden expansion and/or contraction. Flow models based on periodic or various inlet/outlet boundary conditions and their implementations are discussed in the context of 2D and 3D simulations. Numerical experiments are conducted to evaluate the accuracy of the method in terms of flowrate, velocity profiles, and wall shear stress. The relation between f and Re for constant cross-section channels is computed with excellent accuracy. SPH captured the flow characteristics and achieved very good accuracy. Compressibility effects due to the weakly compressible smoothed particle hydrodynamics (WCSPH) formulation are negligible for the flows considered. Several typical difficulties and pitfalls in the application of the SPH method in closed conduits are highlighted as well as some of the immediate needs for the method’s improvement.