Flow In Helical Pipe Research Articles

Asymmetric fluid flow in helical pipes inspired by shark intestines

Unlike human intestines, which are long, hollow tubes, the intestines of sharks and rays contain interior helical structures surrounding a cylindrical hole. One function of these structures may be to create asymmetric flow, favoring passage of fluid down the digestive tract, from anterior to posterior. Here, we design and 3D print biomimetic models of shark intestines, in both rigid and deformable materials. We use the rigid models to test which physical parameters of the interior helices (the pitch, the hole radius, the tilt angle, and the number of turns) yield the largest flow asymmetries. These asymmetries exceed those of traditional Tesla valves, structures specifically designed to create flow asymmetry without any moving parts. When we print the biomimetic models in elastomeric materials so that flow can couple to the structure's shape, flow asymmetry is significantly amplified; it is sevenfold larger in deformable structures than in rigid structures. Last, we 3D-print deformable versions of the intestine of a dogfish shark, based on a tomogram of a biological sample. This biomimic produces flow asymmetry comparable to traditional Tesla valves. The ability to influence the direction of a flow through a structure has applications in biological tissues and artificial devices across many scales, from large industrial pipelines to small microfluidic devices.

Asymmetric fluid flow in helical pipes inspired by shark intestines

Asymmetric fluid flow in helical pipes inspired by shark intestines

Fluid Flow in Helically Coiled Pipes

Helically coiled pipes are widely used in many industrial and engineering applications because of their compactness, larger heat transfer area per unit volume and higher efficiency in heat and mass transfer compared to other pipe geometries. They are commonly encountered in heat exchangers, steam generators in power plants and chemical reactors. The most notable feature of flow in helical pipes is the secondary flow (i.e., the cross-sectional circulatory motion) caused by centrifugal forces due to the curvature. Other important features are the stabilization effects of turbulent flow and the higher Reynolds number at which the transition from a laminar to a turbulent state occurs compared to straight pipes. A survey of the open literature on helical pipe flows shows that a good deal of experimental and theoretical work has been conducted to derive appropriate correlations to predict frictional pressure losses under laminar and turbulent conditions as well as to study the dependence of the flow characteristics and heat transfer capabilities on the Reynolds number, the Nusselt number and the geometrical parameters of the helical pipe. Despite the progress made so far in understanding the flow and heat transfer characteristics of helical pipe flow, there is still much work to be completed to address the more complex problem of multiphase flows and the impact of pipe deformation and corrugation on single- and multiphase flow. The aim of this paper is to provide a review on the state-of-the-art experimental and theoretical research concerning the flow in helically coiled pipes.

Existence of dual solutions and three-dimensional instability in helical pipe flow

Three-dimensional (3D) direct numerical simulations (DNS) of the viscous incompressible fluid flow through a helical pipe with circular cross section were performed. The flow is governed by three parameters: the Dean number (or the Reynolds number), curvature, and torsion. First, we obtained steady solutions by steady 3D calculations, where dual solutions were found, one was uniform in the pipe axial direction and the other varied very slowly, if torsion exceeded a critical value. Then, the instability of the steady solutions obtained was studied by unsteady 3D calculations. We obtained critical Reynolds numbers of steady to unsteady transition by observing the behaviors of the unsteady solutions. The present results of the critical Reynolds number nearly agreed with those by the 2D linear stability analysis (Yamamoto et al. [9]) except for the lowest critical Reynolds number region, where the present study gave the critical Reynolds number much less than that obtained by the 2D linear stability analysis. We found the vortical structures in the form of a standing wave slightly above the marginal instability state, which is a trigger of explosive 3D instability.

Effect of viscosity on entropy generation for laminar flow in helical pipes

Entropy generation for fully developed laminar flow in a helical pipe carrying high viscous fluid under constant temperature boundary conditions is investigated analytically. This work focuses on geometrical, fluid, and thermal aspects and their influence on irreversibilities in helical coils. The effect of viscosity on the irreversibilities and its influence on the operating parameters of the helical coil are studied with the second law of thermodynamics. The most commonly used relationships for estimating viscosity change due to temperature are selected for analysis. The entropy generation and avoidable exergy destruction in each case are presented. Bejan number is plotted for varying viscosities under different wall temperatures for both heat transfer to and from the fluid. The thermodynamic potential of improvement based on avoidable and unavoidable exergy destruction concepts showed that the potential of improvement for heating and the cooling condition is considerable for a given operating condition in helical tubes. The selected model for estimating viscosity influences the optimum operating wall temperature, thereby giving an insight into a selection of a proper viscosity model. The optimum helical number is not affected by fluid properties and wall temperature. The heat transfer to pumping ratio is evaluated and it is found that the optimal value is influenced by the change in viscosity.

Instability of steady flows in helical pipes

A parametric numerical study of three-dimensional instability of steady flows in a helical pipe of arbitrary curvature and torsion is carried out. The stability results are reported as dependences of the critical Reynolds number, critical wave number, and the critical frequency on the dimensionless pipe curvature and torsion. Patterns of several most unstable modes are reported and classified. Different routes to instability by viscous and inviscid mechanisms are discussed.

Pressure-driven flows in helical pipes: bounds on flow rate and friction factor

Abstract

A comparative study on instability of steady flows in helical pipes

A computational study of three-dimensional instability of steady flows in a helical pipe of arbitrary curvature and torsion is carried out for the first time. The problem is formulated in Germano coordinates in two equivalent but different forms of the momentum equations so that results obtained using both formulations cross verify each other. An additional formulation in the cylindrical coordinates is applied for a limiting case of the toroidal pipe. The calculations are performed by the finite volume and finite difference methods. Grid independence of the results is established for both steady flows, the eigenvalues associated with the linear stability problem, and the critical parameters. The calculated steady flows agree well with experimental measurements and previous numerical results. The computed critical Reynolds numbers corresponding to the onset of oscillatory instability agree well with the most recent experimental results, but disagree with the earlier ones. Novel results related to the parametric stability study are reported.

Effect of Torsion on the Mixing of Helical Pipe Flow

Effect of Torsion on the Mixing of Helical Pipe Flow

Numerical Study of Gas–Liquid Two-Phase Flow Regimes for Upward Flow in a Helical Pipe

This study presents numerical investigations of air–water two-phase flows for upward flow in a vertical helical pipe. The numerical simulations were done in accordance with the experimental work of...

On the mixing enhancement in concentrated non-colloidal neutrally buoyant suspensions of rigid particles using helical coiled and chaotic twisted pipes: A numerical investigation

Nevertheless the huge amount of numerical and experimental works on non-colloidal suspension flows, most studies in the literature were limited to the dynamics in different geometries like in channels and pipes. To author's knowledge, mixing enhancement quantification and suspension mixing behavior in static mixers has been rarely studied before. In the present contribution, the mixing enhancement in isothermal monodisperse neutrally buoyant non-Brownian suspensions is investigated through transient 3D CFD simulations. It is quantified for suspension Reynolds (ReS) numbers varying between 50 and 150, for spheres’ diameters (dp) between 25 and 400 μm and for initial volume fractions of particles (ϕ) between 0.25 and 0.45. The influences of dp, ReS and ϕ on the overall mixing enhancement and on pressure drop are quantified for a suspension flow in a chaotic and helical separate pipes (static mixers of equal lengths). It is found that for all cases, the chaotic flow configuration has better mixing performances than the helical pipe flow with almost no significant increase in pressure losses. Moreover, good mixing quality is observed in both static mixers (better at larger particles) where the mixing index ranges between 10 and 75% relative to the initial unmixed particles leaving the straight pipe.

Numerical Evaluation of Heat Transfer and Entropy Generation of Helical Tubes with Various Cross-sections under Constant Heat Flux Condition

The presence of curvature-induced secondary flow in helical pipe which create complex transport phenomena and higher transfer rate has attracted significant attention from both academic and industry. Flow behavior and transport processes in helical tube have been intensively investigated. Nevertheless, most studies were focused on the performance based on first law of thermodynamics with limited studies concerning the performance based on second law of thermodynamics. The objective of this study is to investigate the heat transfer performance of helical tube according to both first and second law. The heat transfer rate and entropy generation of helical tubes with various cross-sections, i.e. circular, ellipse and square, subjected to constant wall heat flux conditions are numerically evaluated by utilizing computational fluid dynamics (CFD) approach. Their performances are compared to those of straight tube with identical cross-section. The results indicate that helical tube provides higher heat transfer at the cost of higher pressure. Moreover, it was found that entropy generation in helical tubes is considerably lower as compared to that in straight tube. Among the studied cross-sections, square has the highest heat transfer albeit having the highest pressure drop and entropy generation for both straight and helical tubes.

Numerical Study of Turbulent Helical Pipe Flow With Comparison to the Experimental Results

Turbulent flow through helical pipes with circular cross section is numerically investigated comparing with the experimental results obtained by our team. Numerical calculations are carried out for two helical circular pipes having different pitches and the same nondimensional curvature δ (=0.1) over a wide range of the Reynolds number from 3000 to 21,000 for torsion parameter β (=torsion /2δ = 0.02 and 0.45). We numerically obtained the secondary flow, the axial flow and the intensity of the turbulent kinetic energy by use of three turbulence models incorporated in OpenFOAM. We found that the change to fully developed turbulence is identified by comparing experimental data with the results of numerical simulations using turbulence models. We also found that renormalization group (RNG) k−ε turbulence model can predict excellently the fully developed turbulent flow with comparison to the experimental data. It is found that the momentum transfer due to turbulence dominates the secondary flow pattern of the turbulent helical pipe flow. It is interesting that torsion effect is more remarkable for turbulent flows than laminar flows.

Effect of Torsion on the Friction Factor of Helical Pipe Flow

Three-dimensional direct numerical simulations of a viscous incompressible fluid flow through a helical pipe with a circular cross section were conducted for three Reynolds numbers, Re (= 80, 300, and 1000), and two nondimensional curvatures, δ (= 0.1 and 0.05), over a wide range of torsion parameters, β (= nondimensional torsion/\(\sqrt{2\delta } \)), from 0.02 to 2.8. Well-developed axially invariant regions were obtained where the friction factors were calculated, in good agreement with the experimental data obtained by Yamamoto et al. [Fluid Dyn. Res. 16, 237 (1995)]. It was found that the friction factor sharply increases as β increases from zero, then decreases after taking a maximum, and finally slowly approaches that of a straight pipe when β tends to infinity. It is interesting that a peak of the friction factor exists in the region 0.2 ≤ β ≤ 0.3 for all the Reynolds numbers and curvatures studied in the present paper, which manifests the importance of the torsion parameter in helical pipe flow.

Laminar forced convective heat transfer in helical pipe flow

Laminar forced convective heat transfer in a helical pipe with circular cross section subjected to wall heating was investigated numerically by three-dimensional (3D) direct numerical simulations (DNS) comparing with the experimental data obtained by Shatat (2010). The study was performed for three Prandtl numbers, Pr = 8.5, 7.5 and 4.02, over the wide range of torsion. In 3D steady calculations, we found the appearance of fully-developed axially invariant flow regions, where the averaged Nusselt number (averaged over the peripheral of the pipe cross section) were calculated, being in good agreement with the experimental data. Because of the effect of torsion on the heat transfer characteristics, the averaged Nusselt number exhibits repetition of decrease and increases as torsion increases from zero for all Reynolds numbers. It was found that there exists two maximums and two minimums of the averaged Nusselt number. It is interesting that the global minimum of the Nusselt number occurs at β≅0.1 and the global maximum at β≅0.55.

Two-phase slug flows in helical pipes: Slug frequency alterations and helicity fluctuations

Air-water numerical simulations in the slug flow regime have been performed in horizontal helical pipes and the effects of geometries on the flow regime have been investigated. Depending on the length of the helix, outlet slug frequencies have been reduced with various degrees of efficiency. Correlations between mean tangential velocity and helicity density fluctuations have been identified and investigated qualitatively. These flow fields show smaller time scales than those obtained in volume fractions fluctuations. Shifts observed in the tangential velocity and mean helicity fluctuations to smaller time scales (high frequencies) are associated with regime transitions. For a slug flow undergoing a continuous transition to the annular flow regime, it is shown that the presence of slower (low frequencies) helicity fluctuations is attributed to the variations in the axial velocity. Finally, the analysis of the helicity at gas-liquid interface confirms the presence of the mixing zone at the slug front.

High Regression Rate Hybrid Rocket Fuel Grains with Helical Port Structures

Results from a development campaign, where modern additive manufacturing methods are used to fabricate hybrid rocket fuel grains with embedded helical ports, are presented. The fuel grains were constructed from acrlyonitrile butadiene styrene using commercially available three-dimensional printer feedstock material. Gaseous oxygen is used as the oxidizer for this test campaign. When compared to cylindrical fuel ports, significant increases in fuel regression rates were observed, and these increases in regression rate diminished with time as the helical fuel port burns to become progressively more cylindrical. Comparisons to the helical pipe flow skin friction correlation developed by Mishra and Gupta indicate that increased skin friction only partially accounts for the increased regression rate. The Mishra–Gupta correlation underpredicts the regression rate amplification early in the fuel burn and overpredicts that value late in the burn. The authors conclude that radial wall-blowing due to fuel pyrolysis is suppressed by centrifugal forces introduced by the helical flow and compresses the boundary-layer flame zone closer to the fuel port wall. Thus, convective heat transfer is significantly enhanced, and that effect also contributes significantly to the observed regression rate increase. This effect diminishes with time and is not modeled by the Mishra–Gupta correlation.

On the helical pipe flow with a pressure-dependent viscosity

We address the flow of incompressible fluid with a pressure-dependent viscosity through a pipe with helical shape. The viscosity-pressure relation is defined by the Barus law. The thickness of the pipe and the helix step are assumed to be of the same order and considered as the small parameter. After transforming the starting problem, we compute the asymptotic solution using curvilinear coordinates and standard perturbation technique. The solution is provided in the explicit form clearly showing the influence of viscosity-pressure dependence and pipe's geometry on the effective flow.

Analyses of single- and two-phase flow pressure drops in helical pipes using a modified RELAP5 code

Thermal fluid dynamics analyses on single- and two-phase flows in helical pipes of steam generators to be used in Generation III and IV nuclear reactors have been performed. The study concerned with experimental activities as well code simulations carried out in the framework of a collaboration between the Department of Energetics of the Polytechnic of Torino and the Department of Energy of the University of Palermo. The goal was the validation of models implemented in Relap5/Mod3.2β thermal–hydraulic advanced code to simulate the hydrodynamic behaviour of helical pipe components in spite of the one-dimensional nature of the code. It is shown that much of the experimental data obtained at several national and international research laboratories are well predicted by this modified RELAP5 code for the examined helical pipe geometries.

Numerical investigation on the similarity of developing laminar flows in helical pipes

Abstract This paper presents numerical investigations into the similarity of developing laminar flows at the entrance region of helical pipes. It is shown that the new characteristic parameters can be used to represent the flow development in helical pipes. The development of friction factor and flow pattern along with the characteristic parameters is not significantly affected by the variations of Reynolds number, dimensionless pitch, or curvature ratio at a given condition of modified Dean number and inlet velocity profile. The development of the friction factor is investigated in a modified Dean number range of 20–400, and a new correlation for the fully developed angles of laminar flows in helical pipes is proposed.