Twisted Duct Research Articles

Laminar Flow Heat Transfer Studies in a Twisted Square Duct for Constant Wall Temperature

Abstract: Three dimensional analysis of steady fully developed laminar flow inside twisted duct of square cross section flow area is studied for Reynolds number range of 100 – 2000 using a commercially available software. Twist ratio used are 2.5, 5, 10 and 20. Heat transfer results are generated for a uniform wall temperature case for Prandtl number range of 0.7 to 20. Maximum values for the product of friction factor and Reynolds number is observed for a twist ratio of 2.5 and Reynold number of 2000. Maximum Nusselt number is observed for the same values along with Prandtl number of 20. Correlations for friction factor and Nusselt number are developed involving swirl parameter. Local distribution of friction factor ratio and Nusselt number across a cross-section is presented. Heat transfer enhancement for Reynold number of 2000 and Prandtl number of 0.7 for twist ratio of 2.5, 5, 10, and 20 are 33 %, 18 %, 10 % and 7 % respectively. The results are significant because it will contribute to development of compact heat exchanger.

Influence of applying magnetic field on thermal and hydraulic characteristics of Fe3O4/water magnetic nanofluid moving in a twisted duct

Influence of applying magnetic field on thermal and hydraulic characteristics of Fe3O4/water magnetic nanofluid moving in a twisted duct

Numerical Study on Thermal Performance of Water Flow in a Twisted Duct Heat Exchanger

This paper presents a numerical study of heat transfer through a downstream annulus using water as the working fluid within the laminar flow region. The annulus consisted of an outer twisted square duct and an inner circular pipe. A three-dimensional formulation was used to solve the Navier-Stokes equations numerically for the laminar flow system with a low Reynolds number. Three parameters were used in the numerical simulation: the length of the twisted square (<i>a</i>: <i>6.6</i>, <i>8.2 10.2, 12.6 mm</i>) the inner diameter of the inner circular pipe (<i>d</i>: <i>19, 21, 23 and 25 mm</i>); and the twist angle (θ: <i>0</i>° (smooth), <i>45</i>°, <i>60</i>°, <i>and 90</i>°). Numerical calculations were conducted on sixteen twisted square duct heat exchangers, with water flowing within a Reynolds number range of <i>220</i> – <i>1100</i>. The results were illustrated as a profile of the thermal enhancement factor, the friction factor and the Nusselt number. The results show that the twisted outer duct of the heat exchanger can create a swirl flow along the length of the heat exchanger. It also caused a boundary layer separation-reattachment on the wall of the inner pipe. Moreover, an increase in the twist angle increased the Nusselt number by <i>20</i> %, and the friction factor was also increased as the annular gap of the heat exchanger decreased.

IMPACT OF TWISTED DUCTS WITH DIFFERENT TWIST RATIOS ON HEAT TRANSFER AND FLUID CHARACTERISTICS OF NiO/WATER NANOFLUID FLOW UNDER MAGNETIC FIELD EFFECT

Laminar forced convection of NiO/water nanofluid in a twisted square duct has been investigated numerically under the effect of an external magnetic field (B = 0 G, 450 G, and 550 G) in the Reynolds number range of 500 ≤ Re ≤ 2000. Four different twist ratios (D/L = 0.0, 1.0, 1.5, and 2.0) of the square duct have also been examined. The nanoparticle volume fractions (NPVF) of NiO/water nanofluid have been selected between 0.6 vol.% and 2.5 vol.%. The effects of the magnitude of the magnetic field (MF), twist ratio, Reynolds number, and nanoparticle volume fraction on thermohydraulic performance have been examined by using the homogeneous model in the numerical analyses. The results of the numerical computations have been reported with average Nusselt number Nu, pressure loss, average Darcy friction factor f, and performance evaluation criterion (PEC). The highest heat transfer increment by 20% has been achieved at D/L = 2.0 with the highest nanoparticle volume fraction, namely, 2.5 vol.% NiO/water nanofluid, compared to the case of distilled water (DW) flowing in a plain duct (PD). In addition, it was determined that the magnetic field effect increases the convective heat transfer in the twisted duct with D/L = 2.0 up to 35% compared to the cases in the absence of a magnetic field at D/L = 2.0. Among all cases, the highest PEC has been obtained with 2.5 vol.% NiO/water nanofluid flowing in the twisted duct with the twist ratio D/L = 2.0.

Heat transfer and fluid flow characteristics within non-circular duct

Heat transfer and the characteristics of a fluid flow of water flows in the twisted square duct are studied numerically using commercially available software. Numerical investigations of fully-developed laminar fluid flow within duct was performed for water with constant wall temperature boundary conditions, twist ratios of 16.5, 11.5, and 7.5, Reynolds number 100-6000, Prandtl numbers 8 and 10. The numerical results of friction factor and Nusselt number found in this present study were compared with the results obtained from the correlations for the same working fluid such as water. Nusselt number in the twisted square duct is higher than the result found by correlation together with a lower friction factor. Percentage increase in average Nusselt number found for twist ratios 16.5, 11.5, and 7.5 compared to the theoretical values was 25.56, 33.21, and 37.73 The heat transfer coefficients of the square twisted duct are increased with a lower twist ratio and higher Prandtl number.

Study of heat transfer enhancement through the non-circular ducts using three dimensional numerical investigations and their comparisons

The present study numerically surveyed fluid characteristics such as friction factor and heat transfer through the different geometry of twisted ducts. A three-dimensional analysis is performed for the Reynolds number of 100-50000 for the air that acts as a working fluid and a uniform wall temperature boundary case is studied numerically throughout the simulation of ducts. Numerical results of this present study are validated against the experimental results. Heat transfer improvements are made over the hexagonally twisted channels, straight square duct and clockwise-counterclockwise (CW-CCW) twisted square channels. Twist ratios of 7 · 5, 11 · 5 and 16 · 5 are considered for the current simulations. Laminar flow to turbulent flow regimes over the ducts are briefly presented and discussed. Results show enhancement in heat transfer through the CW-CCW, hexagonal and straight square ducts(SSD) for the laminar as well as for turbulent regimes are figured out and perfectly described. It is seen that in laminar regime, twisted CW-CCW square duct performed well than twisted square and hexagonal duct. A comparison of friction coefficients are also established for both the channels. Less friction factor is observed for twisted CW-CCW duct compared to other duct.

Numerical analysis of heat transfer and fluid flow through the twisted square duct (TSD): Nanofluid as working fluid

Friction factor and heat transfer characteristics of Al2O3/water nano-particle inside the twisted square duct (TSD) using water as base fluid numerically investigated. A uniform heat flux applied on the external walls. Three-dimensional (3D) analysis of steady fully developed laminar flow inside the twisted duct of square cross-section flow area is carried out for Reynolds number and nanoparticle concentration range of 600–2000 and 0.5 %–1 % using the commercially available software. Heat transfer and friction factor characteristics of fluid flowing through twisted square duct using water/Al2O3 as normal fluid are compared with the results of the straight square duct using water as working fluid. The numerical study is conducted for a uniform wall temperature case, twist ratio of 7.5, 11.5 and 16.5, for Prandtl number range of 8 to 10. One of the key outcomes of this work is up to a limiting concentration of nanoparticle in the base fluid beyond which it decreases. It is also found that the Nusselt number increases by decreasing the twist ratio (tr) and increasing the Prandtl number. However, in the case of friction factor (f), a regular decrease in friction factor is observed with increasing Reynolds number (Re).

Numerical Analysis of Heat Transfer and Fluid Flow through Twisted Hexagonal and Square Duct and their Comparisons

The purpose of this study was to investigate the heat transfer and friction factor characteristics for hexagonal twisted duct. Numerical simulations were used to study the swirling flow inside a twisted duct of hexagonal cross section cooled by using counter flow in a shell and tube arrangement. Nusselt number and friction factor was calculated for a wide range of Reynolds number (Re) ranging from 1,000-1, 00,000. Laminar to turbulent transition point was identified. The results of twisted hexagonal duct were compared to that of twisted square duct. Results revealed that in the laminar regime, Nusselt number (NU) for hexagonal twisted duct is greater than that of square twisted duct, whereas in the turbulent region the corresponding Nusselt number for square twisted duct was greater than its hexagonal counterpart. However, in case of friction factor, a regular decrease in friction factor observed with increasing Reynolds number. The survey concluded that hexagonal ducts can be used in laminar flow regime and square ducts for turbulent flow regime. The impacts of the obtained results are practically applicable in power plant, aerospace, biomedical purposes.

Heat Transfer and Friction Factor Characteristics in Twisted Square Ducts

The present work experimental and numerical investigations have been carried out to study friction factor and heat transfer characteristics of twisted square ducts. Experiments were conducted for different velocities of air under constant heat flux condition, twist ratio is equal to 6.12 and also Reynolds number varied from 8000 to 30000. The experimental analysis has been carried out and the experimentation is completely based upon design of experiments to get the optimum heat transfer rate and minimize pressure drop. The results of friction factor, Nusselt number and thermal performance factor are presented. Twisted square duct shows relatively higher heat transfer and optimum pressure drop compared to plain square duct. The experimental results shows that Nusselt number for the twisted square duct is about 1.89 times above that for the plain square duct while friction factor is 2.29 times higher (f/fo= 2.29). Thermal performance (enhancement ratio) of the twisted square duct is 1.41. In this work Numeric simulations were calculated by using the CFD software package ANSYS 18.2 FLUENT has been used. Heat transfer and numerical flow behaviors such as temperature, friction factor contours of the are also reported. Hence it is concluded that square duct with twist ratio 6.12 gives highest heat transfer and thermal performance due to lowest friction loss indicating promising device of the twisted square duct.

Laminar flow heat transfer studies in a twisted square duct for constant wall heat flux boundary condition

The problem of fluid flow and heat transfer was studied for flow inside twisted duct of square cross-section. Three-dimensional numerical solutions were obtained for steady fully developed laminar flow and for uniform wall heat flux boundary conditions using commercially available software. Reynolds number range considered was 100–3000. Twist ratio used are 2.5, 5, 10 and 20. Fluids considered are in Prandtl number range of 0.7–20. Product of friction factor and Reynolds number is found to be a function of Reynolds number and maximum values are observed for a twist ratio of 2.5 and Reynolds number of 3000. Maximum Nusselt number is observed for the same values along with Prandtl number of 20. Correlations for friction factor and Nusselt number are developed involving swirl parameter. Local distribution of friction factor ratio and Nusselt number across a cross-section is presented. Based on constant pumping power criteria, enhancement factor is defined to compare twisted ducts with straight ducts. Selection of twisted square duct is presented in terms of enhancement factor. It is found that twisted duct performs well in the laminar region for range of parameters studied. Heat transfer enhancement for Reynolds number of 3000 and Prandtl number of 0.7 for twist ratio of 2.5, 5, 10, and 20 is 20%, 17.8%, 16.1% and 13.7%, respectively. The results are significant because it will contribute to development of energy efficient compact size heat exchangers.

Experimental and numerical study of fluid flow and heat transfer in an annulus of inner twisted square duct and outer circular pipe

Heat transfer and friction factor characteristics of air flow in an annulus formed by an inner twisted square duct and an outer circular pipe is studied experimentally for Reynolds number range of 400–60,000. Experiments were conducted with air as working fluid. A uniform wall temperature at the inner wall of annulus was maintained while the outer pipe was kept insulated. Twist ratios (ratio of the pitch to outer width of twisted square duct) of 10.6 and 15 were used in the experiments. The transitional Reynolds number for laminar flow to turbulent flow was identified as 3000. Results were compared with the flow inside the annulus of an equivalent straight square duct in a circular pipe for identical pumping power. Results showed considerable enhancement in the heat transfer and pressure drop in both the laminar and turbulent flow regimes. The influence of the annulus parameter on the friction factor and heat transfer was also studied by varying the outer pipe diameter. At a given twist ratio, higher values of the friction factor and the Nusselt number were observed for smaller annulus parameters (ratio of cross section area of square twisted duct to cross section area of circular pipe). In order to extend the experimental results to a larger range of fluids, a numerical analysis was undertaken for fully developed flow inside the annulus for the experimental twist ratios. The parameters range covered in the simulations were Reynolds numbers of 100–100,000 and Prandtl numbers of 0.7–20. The local velocity distribution across the annulus cross-section were analysed. Empirical correlations for friction factor and Nusselt number are provided separately for laminar and turbulent regimes. Guidelines for selection of twisted square duct are provided in terms of Reynolds number and enhancement factor. The results are significant because it will contribute to the development of compact double pipe heat exchangers.

Experimental and numerical study of fluid flow and heat transfer in a twisted square duct

Heat transfer and friction factor characteristics of air flow inside twisted square duct are studied experimentally and through three-dimensional numerical simulations. Experiments were conducted for air with uniform wall temperature boundary condition, twist ratio of 11.5 and 16.5, and Reynolds number 600–70,000. Laminar to turbulent flow transition point was identified. The results show considerable enhancement in heat transfer and pressure drop in both laminar and also in turbulent flow regimes till Reynolds number of 9500. Twist ratio of 11.5 shows relatively higher heat transfer and pressure drop compared to straight square duct. Three-dimensional analysis of steady fully-developed laminar flow inside twisted duct of square cross section flow area is carried out for Reynolds number range of 100–100,000 using commercially available software. The numerical study is conducted for a uniform wall temperature case, twist ratio of 2.5, 5, 10 and 20, for Prandtl number range of 0.7 to 20. The maximum value for product of friction factor and Reynolds number is observed for a twist ratio of 2.5 and a Reynolds number of 3000. The maximum Nusselt number is observed for the same values for Prandtl number of 20. Correlations for friction factor and Nusselt number are developed involving swirl parameter in the laminar flow regime. Correlations are also provided for the entire range of turbulent regime. Local distribution of friction factor ratio and Nusselt number across a cross-section is presented. Based on constant pumping power criteria, enhancement factor is defined to compare twisted ducts with straight ducts. Selections of twisted square duct are presented in terms of enhancement factor. It is found that twisted duct performs well in laminar and also to some extent in turbulent flow regime due to strong presence of secondary flow. It is recommended to use twisted square duct in laminar flow regime for entire range of Prandtl number studied. Maximum enhancements factor of 10.5 is obtained with twist ratio of 2.5, Prandtl number of 20 and Reynolds number of 3000. Guidelines are provided for selection of twisted square duct in terms of Reynolds number and Prandtl number. Comparison with twisted elliptic tube and twisted tape are discussed. These results would help in the design and development of compact heat exchanger.

Mixing by Time-Dependent Orbits in Spatiotemporal Chaotic Advection

The simultaneous effects of flow pulsation and geometrical perturbation on laminar mixing in curved ducts have been numerically studied by three different metrics: analysis of the secondary flow patterns, Lyapunov exponents and vorticity vector analysis. The mixer that creates the flow pulsation and geometrical perturbations in these simulations is a twisted duct consisting of three bends; the angle between the curvature planes of successive bends is 90 deg. Both steady and pulsating flows are considered. In the steady case, analysis of secondary flow patterns showed that homoclinic connections appear and become prominent at large Reynolds numbers. In the pulsatile flow, homoclinic and heteroclinic connections appear by increasing β, the ratio of the peak oscillatory velocity component of the mean flow velocity. Moreover, sharp variations in the secondary flow structure are observed over an oscillation cycle for high values of β. These variations are reduced and the homoclinic connections disappear at high Womersley numbers. We show that small and moderate values of the Womersley number (6 ≤ α ≤ 10) and high values of velocity amplitude ratio (β ≥ 2) provide a better mixing than that in the steady flow. These results correlate closely with those obtained using two other metrics, analysis of the Lyapunov exponents and vorticity vector. It is shown that the increase in the Lyapunov exponents, and thus mixing enhancement, is due to the formation of homoclinic and heteroclinic connections.

Mass transfer and mixing by pulsatile three-dimensional chaotic flow in alternating curved pipes

We present an experimental and numerical study of three-dimensional pulsatile flow in a twisted pipe in order to show the effects of chaotic advection on mixing in this flow configuration. The numerical study is done by CFD code with a pulsatile velocity field imposed as an inlet condition. The experimental setup is composed principally of a “Scotch-yoke” pulsatile flow generator and a twisted duct. The twisted duct consists of six 90° bends of circular cross-section; the plane of curvature of each bend is at 90° to that of its neighbors. The secondary flow, generated by centrifugal force, the pulsating velocity field and also due to the change in curvature plane, leads to irregular fluid particle trajectories. Velocity measurements were made for a range of stationary Reynolds numbers (300 ⩽ Re st ⩽ 1200) and frequency parameters (1 ⩽ α < 20) and for two velocity-amplitude ratios ( β); agreement between the numerical and experimental results is satisfactory. In the first bend, for certain control parameter values, the secondary flow becomes more complex due to the pulsation, and in some cases Lyne instability and a siphon phenomenon appear. However, in the other bends, one passes from four cells (Lyne instability) in the first bend with two cells in the other bends. The numerical and experimental study revealed modifications in the trajectories’ evolution due to pulsation. The superposition of an oscillating flow on a stationary curved-pipe flow, in some cases, causes the destruction of the trapping zones (KAM structures). The number of regular zones that disappear with an increase in the number of bends decrease with pulsation frequency and velocity-amplitude ratio. Both these phenomena contribute to the mixing and mass transfer enhancement in the flow.

Numerical simulation of steady flow fields in coiled flow inverter

Flatter velocity profiles and more uniform thermal environments are extremely desirous factors for improved performance in flow reactors and heat exchangers. One means of achieving it in laminar flow systems is to use mixers and flow inverters. In the present study a new device is introduced based on the flow inversion by changing the direction of centrifugal force in helically coiled tubes. The objective of the present study is to characterize flow development and temperature fields in the proposed device made up from the configurations of bent coils. The main mechanism generating the flow is the production of spatially chaotic path by changing the direction of flow using a 90° bend in helical coils (alternating Dean flow). If the direction of centrifugal force is rotated by any angle, the plane of vortex formation also rotates with the same angle. Thus in helical flow a 90° shift in the direction of centrifugal force cause a complete flow inversion. Complete flow fields and thermal fields in helical coil and bent coil configuration were studied using computational fluid dynamics software (FLUENT 6.0). The three-dimensional governing equations for momentum and energy under the laminar flow conditions were solved with a control-volume finite difference method (CVFDM) with second-order accuracy. The flow pattern obtained for the helical coil was in good agreement to those observed by the previous investigators [S.W. Jones, O.M. Thomas, H. Aref, Chaotic advection by laminar flow in twisted pipe. J. Fluid Mech. 209 (1989) 335–357; Ch. Duchene, H. Peerhossaini, P.J. Michard, On the velocity field and tracer patterns in a twisted duct flow. Phys. Fluids 7 (1995) 1307–1317]. The comparison of the flow fields and temperature fields in the helical tube and bent coil configuration are discussed. The bent coil configuration shows a 20–30% enhancement in the heat transfer due to chaotic mixing while relative pressure drop is 5–6%. The results of the present study can be used to model transport processes for developing flows in curved tubes such as chromatographic columns (less axial dispersion [A.K. Saxena, K.D.P. Nigam, Coiled configuration for flow inversion and its effect on residence time distribution. AIChE J. 30 (1984) 363–368]), Chemical reactors (narrower RTD), heat transfer devices, and some biomedical devices.

NUMERICAL INVESTIGATION OF FLOW AND HEAT TRANSFER IN TWISTED CIRCULAR-SECTOR DUCTS

Flow and heat transfer in a twisted circular-sector duct are analyzed numerically for steady, fully developed, and incompressible laminar flow with a uniform-wall-temperature boundary condition. A rotating coordinate system is employed to account for the duct twist. The friction factors and Nusselt numbers are predicted for duct sector angles ranging from 15° to 90°, Reynolds numbers ranging from 1 to 1,000, and Prandtl numbers ranging from 1 to 100. Results show significant influence of duct twist on both friction factors and Nusselt numbers, particularly at large values of Reynolds and Prandtl numbers. Accurate correlations are developed to predict the friction factors and Nusselt numbers for the entire range of geometric and operating conditions studied.

Experimental study of chaotic advection regime in a twisted duct flow

The generation of Lagrangian chaos has been studied experimentally in a twisted duct flow, a configuration representing a three-dimensional steady open flow in which various signatures of Lagrangian chaos are documented. The twisted duct consists of four 90 ∘ bends of square cross-section; the plane of curvature of each bend is at 90 ∘ to that of its neighbors. Dean roll-cells, generated by centrifugal forces and the geometrical perturbation due to the change in curvature plane, are the source of the irregular trajectories of the fluid particles. The Eulerian nature of the flow was investigated using a laser Doppler velocimeter (LDV). From the Eulerian point of view, the flow is completely regular. We show by laser-induced fluorescence (LIF) visualization that many characteristics of a Lagrangian chaotic system are present in this flow: − strong stretching and folding of material lines and surfaces; − sensitivity to initial conditions; − exponential growth of stretching in some flow regions. It is also shown that in some regions of the flow stretchings grow linearly with space, indicating non-chaotic behavior. Due to the chaotic nature of the flow, an equalization of the fly-time of fluid particles was observed during their passage through the twisted duct.

First- and higher-order effects of curvature and torsion on the flow in a helical rectangular duct

A series expansion method is employed to determine the first-order terms in curvature ε and torsion η of fully developed laminar flow in helical square ducts and in helical rectangular ducts of aspect ratio two. The first-order solutions are compared to solutions of the full governing equations. For toroidal square ducts with zero pitch, the first-order solution is fairly accurate for Dean numbers,De=Reε1/2, up to about 20, and for straight twisted square ducts the first-order solution is accurate for Germano numbers,Gn= ηRe, up to at least 50 whereReis the Reynolds number. Important conclusions are that the flow in a helical duct with a finite pitch or torsion to the first order (i.e. with higher-order terms in ε and η neglected) is obtained as a superposition of the flow in a toroidal duct with zero pitch and a straight twisted duct; that the secondary flow in helical non-circular ducts for sufficiently smallReis dominated by torsion effects; and that for increasingRe, the secondary flow eventually is dominated by effects due to curvature. Torsion has a stronger impact on the flow for aspect ratios greater than one. A characteristic combined higher-order effect of curvature and torsion is an enlargement of the lower vortex of the secondary flow at the expense of the upper vortex, and also a shift of the maximum axial flow towards the upper wall. For higher Reynolds numbers, bifurcation phenomena appear. The extent of a few solution branches for helical ducts with finite pitch or torsion is determined. For ducts with small torsion it is found that the extent of the stable solution branches is affected little by torsion. Physical velocity components are employed to describe the flow. The contravariant components are found useful when describing the convective transport in the duct.

On the velocity field and tracer patterns in a twisted duct flow

The laminar flow in a twisted square duct is studied numerically to analyze the formation and reorganization of Dean roll cells under abrupt curvature change. The flow geometry consists of four 90° bends, the curvature plane of each making a 90° angle with that of its neighbor. Straight ducts are inserted between successive bends. The flow geometry studied here is a basic element of a twisted duct flow in which chaotic advection has been observed [Jones, Thomas, and Aref, J. Fluid Mech. 209, 335 (1989)]. Steady incompressible full Navier–Stokes equations have been solved by an approximate factorization technique. Vector plots of the secondary flow (Dean roll cells) show upstream diffusion of curvature effects. The upstream propagation is weak, however, and the Dean roll cells adopt the rotation protocol of the bend immediately at its entrance. Farther downstream, roll cells continue to develop and reach an axially invariant state before leaving the bend. A passive tracer was also introduced at the center of the entrance to the twisted duct. Tracer distribution was followed in the downstream direction and was visualized every 30°. Comparison of the secondary flow velocity vector plots and the corresponding tracer patterns shows that the tracer distribution lags behind the velocity field in adjusting to changes in curvature. Tracer patterns show strong stretching and folding, which lead to mixing enhancement.

Laminar flow in twisted ducts

Fully developed flow of an incompressible Newtonian fluid through a duct in which the orientation of the cross section is twisted about an axis parallel to an imposed pressure gradient is analyzed here with the aid of the penalty/Galerkin/finite element method. When the axis of twist is located within the duct, flow approaches limits at low and high torsion, the spatial frequency τ by which the duct is twisted. For small torsion, flow is nearly rectilinear and solutions approach previous asymptotic results for an elliptical cross section. For large torsion, flow exhibits an internal layer structure: a rotating circular-cylinder core with a nearly parabolic axial velocity profile, an internal layer of thickness τ−1 along the perimeter of the largest circular cylinder that can be inscribed in the duct, and nearly quiescent flow outside of the circular cylinder. The maximum rate of swirl in the core of a square duct is found to be at moderate torsion. The primary effect of inertia is an increase in pressure with distance from the axis, due to centrifugal acceleration. When the duct is offset from the axis of twist, inertia leads to one, two, or three primary vortices without apparent bifurcation of steady states, although stability of steady flows is lost beyond detected Hopf points.