Sudden Expansion Channel Research Articles

Turbulent models of shallow-water equations-based smoothed particle hydrodynamics

The depth-averaged models such as those based on the shallow water equations (SWEs) are commonly used to simulate the large-scale flows with engineering importance. The smoothed particle hydrodynamics (SPH) approach has been documented to solve the SWEs due to its mesh-free superiority in treating the free surfaces and wet-dry boundaries. However, nearly all SWE-SPH models were developed without a turbulent model, which seriously limited the model applications where the flows are complex and where the turbulent parameters are explicitly needed. For the first time, this paper includes a depth-averaged turbulent k̂-ε̂ model in the SWE-SPH solver, making the model more capable of treating the turbulent flows in the practical field. For comparison purpose, a sub-particle-scale turbulent model widely adopted in three-dimensional (3D) SPH was also included in the present SWE-SPH scheme. To evaluate the performance of the two proposed turbulent SWE-SPH models, various open channel flows of increasing complexity were simulated, and the SPH computations were compared with the reported data in the literature. Through the analysis of results for a rough riverbed, L-shaped and sudden expansion channels, it is demonstrated that the present turbulent SWE-SPH models are equipped with good robustness and accuracy in capturing the shallow water turbulent dynamics, with the potential to be used in practical river and coastal flows. In summary, there are two distinct novelties in the proposed work. First, the mesh-free numerical modeling technique SPH is used to solve the shallow water equations, which enable the model to work in large engineering field through simple and effective tracking of free surfaces and wet-dry boundaries. Second, the proposed research expands the shallow water SPH modeling technique by including robust turbulence simulation capacity. The newly developed model can address more challenging engineering scenarios such as the sediment and pollutant transports when the flow turbulence plays an important role and where the turbulent parameters are explicitly required in the relevant transport equations.

Investigation on flame behaviors in the mesoscale channel of one sudden-expansion step during the preliminary stage of ignition

The dynamic flame behaviors at the preliminary stage of ignition in the mesoscale sudden-expansion channel are numerically investigated in this study under the isothermal condition (300 K) at walls by using a two-dimensional model without symmetric assumption at the centerline. It is found that the flashback velocity is dominated by the upstream channel height; nevertheless, the blowoff velocity is determined by not only the downstream channel height but also the flow recirculation behind the sudden-expansion steps. As the expansion ratio is sufficiently large, the flame could exist within a substantially wider range of inlet flow velocity. In addition, four types of flame behaviors are found at the expansion ratio of 2: (I) steady convex flame, (II) steady concave flame, (III) simple oscillating flame, and (IV) complex oscillating flame. Above the flashback velocity, the convex flame exists steadily. With the increase in flow velocity, the flame becomes concave to the upstream and is stabilized by the wall-quenching effect of sudden-expansion steps. If the flow velocity is further increased, the flame becomes unstable and oscillates periodically (simple oscillating flame) due to the interaction of flame and the symmetric flow field in the sudden-expansion channel, while the occurrence of complex oscillating flame at high flow velocities is attributed to the asymmetric flow pattern. The frequency of oscillating flames decreases with the increase in flow velocity.

A critical analysis and prediction of heat transfer and flow characteristics in the sudden expansion channel with square block using machine learning

In this article, the numerical simulation of a 2D flow in an SEC (Sudden expansion channel) with a square block as a VG (Vortex generator) is performed using FVM (Finite volume method) in Ansys FLUENT 2022 R1. A square block of 5x5 mm is deployed in the SEC having ER = 2 (Expansion ratio). The impact of vortex dynamics on heat interaction amplification and fluid flow attributes are observed for various values of Re ranging from 117 to 583. The vortex dynamics for q = 500, 750, and 1000 W/m2 have been thoroughly investigated for different positions of the square block. The working fluid is water that flows in an SEC with a square block at the step, 10, 20, 30, and 40 mm. Machine learning algorithms were utilized to forecast the most advantageous conditions of input variables, including block position, heat flux (q), and Re number, in order to achieve the highest possible overall performance factor (OPF). It is predicted that the maximum OPF is achieved for Re = 640 when the block is placed 30 mm from the step on the bottom wall.

Nonlinear Phenomena of Fluid Flow in a Bioinspired Two-Dimensional Geometric Symmetric Channel with Sudden Expansion and Contraction

Inspired by the airway for phonation, fluid flow in an idealized model within a sudden expansion and contraction channel with a geometrically symmetric structure is investigated, and the nonlinear behaviors of the flow therein are explored via numerical simulations. Numerical simulation results show that, as the Reynolds number (Re = U0H/ν) increases, the numerical solution undergoes a pitchfork bifurcation, an inverse pitchfork bifurcation and a Hopf bifurcation. There are symmetric solutions, asymmetric solutions and oscillatory solutions for flows. When the sudden expansion ratio (Er) = 6.00, aspect ratio (Ar) = 1.78 and Re ≤ Rec1 (≈185), the numerical solution is unique, symmetric and stable. When Rec1 < Re ≤ Rec2 (≈213), two stable asymmetric solutions and one symmetric unstable solution are reached. When Rec2 < Re ≤ Rec3 (≈355), the number of numerical solution returns one, which is stable and symmetric. When Re > Rec3, the numerical solution is oscillatory. With increasing Re, the numerical solution develops from periodic and multiple periodic solutions to chaos. The critical Reynolds numbers (Rec1, Rec2 and Rec3) and the maximum return velocity, at which reflux occurs in the channel, change significantly under conditions with different geometry. In this paper, the variation rules of Rec1, Rec2 and Rec3 are investigated, as well as the maximum return velocity with the sudden expansion ratio Er and the aspect ratio Ar.

Study on mixing characteristics of multichamber static mixer

AbstractThis study presents a specialized mixer design for gas burners aimed at improving the efficiency of phase mixing while minimizing system resistance and energy consumption. The proposed design utilizes a multi‐cavity mixer employing both radial and axial mixing techniques, eliminating the need for additional power supply. Full premixed combustion of small gas‐fired industrial boiler has the function of “inhibiting” NOx emission from the source, and good mixing of fuel and combustion supporting air in the early stage is the primary prerequisite for realizing full premixed combustion denitration. In this article, a radial multichamber static mixer is designed following the characteristics of full premixed mixing in gas‐fired industrial boilers. The results demonstrate that the maximum pressure drop of the static mixer is 806 Pa; for gas and air branch pipes, the resistance coefficient f decreases rapidly with the increase in Reynolds number; when Re ≥ 1.2 × 103, the resistance coefficient in the air branch pipe decreases slowly; when Re ≥ 4.5 × 103, the resistance coefficient of gas branch pipe decreases slowly. Additionally, the maximum calculation error of symmetrical gas branch pipe is 9.1%. The static mixer's inlet exhibits a converted velocity of 24 m/s, and the outlet demonstrates an airflow velocity of 23.2 m/s. As a result, a kinetic energy loss of 6.5% is observed. The static mixing chamber makes the airflow rotate and causes different gases to shear and mix. The mixing channel has the function of correcting the airflow deviation, especially for the sudden expansion section. Generally, the two gases can be mixed evenly at the outlet of the mixer. The standard k‐e model and realizable k‐e model are employed to simulate the sudden expansion channel, and indicate that the standard k‐e has a wider range of influence. Further investigation is recommended to better comprehend and optimize this particular area of influence.

Driving bifurcating parametrized nonlinear PDEs by optimal control strategies: application to Navier–Stokes equations with model order reduction

This work deals with optimal control problems as a strategy to drive bifurcating solution of nonlinear parametrized partial differential equations towards a desired branch. Indeed, for these governing equations, multiple solution configurations can arise from the same parametric instance. We thus aim at describing how optimal control allows to change the solution profile and the stability of state solution branches. First of all, a general framework for nonlinear optimal control problem is presented in order to reconstruct each branch of optimal solutions, discussing in detail the stability properties of the obtained controlled solutions. Then, we apply the proposed framework to several optimal control problems governed by bifurcating Navier–Stokes equations in a sudden-expansion channel, describing the qualitative and quantitative effect of the control over a pitchfork bifurcation, and commenting in detail the stability eigenvalue analysis of the controlled state. Finally, we propose reduced order modeling as a tool to efficiently and reliably solve parametric stability analysis of such optimal control systems, which can be challenging to perform with standard discretization techniques such as Finite Element Method.

Newtonian hydro-thermal fluid fl 1 ow phenomena through a sudden 2 expansion channel with or without baffles

This work aims to study the different characteristics of Newtonian fluid flow and heat transfer througha 1:3 sudden expansion channel with or without plane baffles using the finite volume method. The flowis assumed viscous, incompressible, steady, and laminar. The different characteristics of hydro-thermalfluid flow phenomena have been studied for 𝑅𝑒 ∈ [0.1 − 200] to demonstrate the influence of thepresence of baffles. The profiles of velocity, pressure, skin friction coefficient, friction factor, averageNusselt number, and pumping power have been examined for both the cases of the presence and absenceof baffles. It has been observed that the hydro-thermal characteristics become more pronounced withthe increase in the number of baffles. In the case of three baffles of equal length, at 𝑅𝑒 = 200 it iscalculated that if the thickness of the baffles is same and equal to 10% of the length, then the value of Nu becomes approximately 1.2 times of that in the absence of baffle. For the same value of Re, ithas been found that for the presence of one baffle of length equal to the width of the inlet section of thechannel and of thickness 10% of the same, the value of 𝑁𝑢􀯔􀯩 becomes 1.11 times of that in the absenceof baffle, while in the case of three baffles of equal length and of equal thickness like the abovementioned case of one baffle, the value of 𝑁𝑢􀯔􀯩 becomes 1.25 times of that in absence of baffle. It hasalso been revealed that the enhancement of thermal phenomena increases with the increase inthe baffle’s height.

Bifurcation Analysis of Two-Dimensional Laminar Flow Through Sudden Expansion Channel

In this work, bifurcation characteristics of unsteady, viscous, Newtonian laminar flow in two-dimensional sudden expansion and sudden contraction-expansion channels have been studied for different values of expansion ratio. The governing equations have been solved using finite volume method and FLUENT software has been employed to visualize the simulation results. Three different mesh studies have been performed to calculate critical Reynolds number (Recr) for different types of bifurcation phenomena. It is found that Recr decreases with the increase in expansion ratio (ER).

Hydrothermal analysis of water-Al2O3 nanofluid flow through a sudden expansion channel with intermediate step

This work presents the dynamic behavior of the laminar nanofluid hydrothermal flow through a sudden expansion channel with the variations in the normalized length, height, and pitch to width ratio of the intermediate step. The governing equations have been discretized by means of the finite volume method, and the SIMPLE algorithm has been applied to solve the system of algebraic equations. The effect of variations in the Reynolds number (Re), normalized height (B/C), normalized length (A/C), and step pitch-width ratio (A/B) on the enhancement of heat transfer have been studied. At various values of A/C, A/B and B/C, this work indicates that the average Nusselt number (Nuavg), thermal resistance factor (R), and performance number (PN) increase with the increase in Re. In the case of the hydrothermal flow phenomena, the configuration associated with B/C provides better results than the configurations associated with A/B and A/C. As an important outcome, we see that the presence of intermediate step enhances the average Nusselt number, and thermal resistance factor. This will become very much helpful for various realistic circumstances for triggering additional thermo-mechanical loads on the surface.

Effect of Asymmetry of Channels on Flows in Parallel Plates with a Sudden Expansion

In order to quantitatively grasp the influence of asymmetry of a channel, flow in an eccentric sudden expansion channel, in which the channel centers are different on the upstream and downstream sides, was calculated to be less than the Reynolds number of 400, where the expansion rate was 2. The asymmetry of a channel is expressed by an eccentricity S, where a symmetric expansion channel is S = 0 and a channel with one side step is S = 1. Both flows firstly reattached on the wall located on the short and long side of a sudden expansion and were observed in the range of S ≤ 0.2, although only the former was seen in the range of S > 0.2. The critical Reynolds number of the multiple solutions increases parabolically to S. At least two separation vortices occur, and the third separation vortex is generated in both solutions above the critical Reynolds number of the third vortex. The length of an entrance region increases linearly to the Reynolds number and slightly with the increase in S. The pressure drop coefficient is proportional to the power of the Reynolds number and increases with S.

Fiber-Concentration Distributions of Papermaking Pulp Suspension Flowing in a Channel with a Sudden Expansion

In this study, we analyze the flow characteristics of pulp suspensions in a sudden expansion channel, which is used as a tube bank in papermaking machines. Flow visualization using a light section method and measurements of time averaged and fluctuating fiber concentrations, C<SUB>a</SUB> and C’, and pressure were performed. The experiments were carried out on pulp suspensions with a bulk fiber concentration (C<SUB>s</SUB>) of 0 2.0 wt% in a channel with an expansion ratio of β = 2 at varied average velocities (U<SUB>a</SUB>) of 0.04-6.6 m/s. We examined the effect of flow velocity on the changes in concentration and pressure distributions along the channel axis The distribution of the pulp fiber concentration behind the sudden expansion significantly changes depending on the flow state in the upstream channel before the expansion. In particular, for low and moderate velocities, the flow field is characterized in the main flow region consisting of flocculated plug fibers and a recirculation region with a low concentration near the expansion corner by a type of streamline originated from the thin water layer in the upstream channel. The distribution of C<SUB>a</SUB> shows a tongue shaped structure in the central part of the cross section and C’ is large near the annulus streamline As the velocity is further increased, the flocculated fibers gradually disperse and the distributions of C<SUB>a</SUB> and C’ become almost uniform. In addition, the uniformity of fiber concentration does not change much in the section downstream from a distance 10 times the step height from the sudden expansion plane.

THERMAL ENHANCEMENT AND ENTROPY GENERATION OF LAMINAR WATER-AL2O3 NANOFLUID FLOW THROUGH A SUDDEN EXPANSION CHANNEL WITH BELL-SHAPED SURFACE

THERMAL ENHANCEMENT AND ENTROPY GENERATION OF LAMINAR WATER-AL2O3 NANOFLUID FLOW THROUGH A SUDDEN EXPANSION CHANNEL WITH BELL-SHAPED SURFACE

Simulation of blood flow in a sudden expansion channel and a coronary artery

In this paper, we numerically simulate the flow of blood in two benchmark problems: the flow in a sudden expansion channel and the flow through an idealized curved coronary artery with pulsatile inlet velocity. Blood is modeled as a suspension (a non-linear complex fluid) and the movement of the red blood cell (RBCs) is modeled by using a concentration flux equation. The viscosity of blood is obtained from experimental data. In the sudden expansion flow, the predicted velocity profiles for two different Reynolds numbers (based on the inlet velocity) agree well with the available experiments; furthermore, the numerical results also show that after the sudden expansion there exists a RBCs depletion region. For the second problem, the idealized curved coronary artery, it is found that the RBCs move towards and concentrate near the inner surface where the viscosity is higher and the shear stress is lower; this phenomenon may be related to the atherosclerotic plaque formation which usually occurs on the inside surface of the arteries.

A new method of acquiring perquisites of recirculation and vortex flow in sudden expansion solar water collector using vortex generator to augment heat transfer

The present study focuses on the heat transfer enhancement in sudden expansion channel (SEC) which is to be employed as solar water collector using vortex generator (VG) at different angles of attack. Expansion ratio (ER) of SEC is ascribed as 2:1. The analysis of heat transfer enhancement is carried out for various aspect ratios (AR) of ∞, 8 and 6 of SEC. The angle of attack (β) of VG is varied from 30° to 90° by an increment of 15° in each case. The results of this study are compared with that of simple rectangular channel (SRC) with VG. The flow is assumed to be laminar, steady and incompressible in the Reynolds (Re) number range investigated. Finite volume based commercial computational fluid dynamics (CFD) code FLUENT 16.2 has been used for solving three dimensional governing equations. The investigation shows that VGs are very effective in SEC than in SRC in enhancing the heat transfer against pressure drop at all angles of attack. Heat transfer rate increases with increase in AR of SEC while the change in friction factor with AR of SEC is negligible except at higher angle of attack. However with increasing angle of attack of VG, friction factor is on the rise. The results reveal that the enhancement of heat transfer rate is due to the combined effect of longitudinal vortex flow and flow separation. VG at β = 30° and 60° is found to have an optimal heat transfer and fluid flow characteristics in SEC.

Flow characteristic of magneto-viscoelastic fluid in sudden expansion and contraction channel

Flow characteristic of magneto-viscoelastic fluid in sudden expansion and contraction channel

Magneto Viscoelastic Fluid flowing in Sudden Contraction and Sudden Expansion Channel with Magnetic Field

Magneto Viscoelastic Fluid flowing in Sudden Contraction and Sudden Expansion Channel with Magnetic Field

The effect of pressure-oscillation on the suspension flowing through a sudden contraction-sudden expansion channel

The effect of pressure-oscillation on the suspension flowing through a sudden contraction-sudden expansion channel

A new method of enhancing heat transfer in sudden expansion channel using vortex generators with toe-out and toe-in configurations by acquiring perquisites of recirculation and secondary vortex flow

The relative positions of leading edges and trailing edges of rectangular vortex generators (VG) in the winglet form play a crucial role in modifying thermal and hydraulic boundary layers thereby affecting the heat transfer augmentation and pressure drop in a sudden expansion channel having expansion ratio (ER) 2:1. A numerical simulation was carried out for solving the momentum and energy equations of a three dimensional vortex- laminar flow (Reynolds number < 190) with a finite volume method based commercial code FLUENT 16.2. Reynolds number, angle of attack and VG configuration have been considered as influential parameters affecting the thermo-hydraulic performance of the sudden expansion channel due to the combined effect of primary recirculation and secondary vortex flow. The heat transfer enhancement along with corresponding pressure drop penalty was compared for the toe-out and toe-in configurations at various Reynolds (Re) numbers and angle of attacks (β). Eventually irrespective of configurations, rectangular winglet VGs are found to have better overall performance at β = 30° and β = 45° in both toe-out and toe-in configurations due to strong secondary vortex flow.

Laminar Forced Convection of Various Nanofluids in Sudden Expansion Channels Under Constant Heat Flux: A CFD Study

In the present work, forced convection heat transfer was investigated numerically for the fully developed fluid flow of incompressible viscous laminar flow under the constant wall heat flux in sudden expansion channels. Various fluids were used with different concentration of nanoparticle such as Al2O3, TiO2, ZnO, CuO, SiO2. These nanoparticles were dispersed with the range of 0.5–2% volume concentrations in pure water to form stable suspensions of nanofluids. The flow assumed to be uniform in the channel inlet and numerical computations were performed for the fully developed laminar flow conditions. Ansys Fluent 16.1 code, based on finite volume approach was used to calculate the governing continuity, momentum and energy equations. Effect of the Re numbers (100[Formula: see text]Re[Formula: see text]500), nanoparticle volume concentration (0.5%[Formula: see text]2.0%) and nanofluid type on the flow and heat transfer characteristics such as convective heat transfer coefficient, Nu number, friction factor and pressure drop has been investigated. Al2O3/water nanofluid was found better when compared the other references working fluids by means of heat transfer enhancement.

Simulation of Compressible and Incompressible Flows Through Planar and Axisymmetric Abrupt Expansions

In this paper, compressible and incompressible flows through planar and axisymmetric sudden expansion channels are investigated numerically. Both laminar and turbulent flows are taken into consideration. Proper preconditioning in conjunction with a second-order accurate advection upstream splitting method (AUSM+-up) is employed. General equations for the loss coefficient and pressure ratio as a function of expansion ratio, Reynolds number, and the inlet Mach number are obtained. It is found that the reattachment length increases by increasing the Reynolds number. Changing the flow regime to turbulent results in a decreased reattachment length. Reattachment length increases slightly with a further increase in Reynolds number. At a given inlet Mach number, the maximum value of the ratio of the reattachment length to step height occurs at the expansion ratio of about two. Moreover, the pressure loss coefficient is a monotonic increasing function of expansion ratio and increases drastically by increasing Mach number. Increasing inlet Mach number from 0.1 to 0.2 results in an increase in pressure loss coefficient by less than 5%. However, increasing inlet Mach number from 0.4 to 0.6 results in an increase in loss coefficient by 70–100%, depending on the expansion ratio. It is revealed that increasing Reynolds number beyond a critical value results in the loss of symmetry for planar expansions. Critical Reynolds numbers change adversely to expansion ratio. The flow regains symmetry when the flow becomes turbulent. Similar bifurcating phenomena are observed beyond a certain Reynolds number in the turbulent regime.