Drag Reduction In Pipe Flow Research Articles

Aqueous Solutions of Associating Poly(acrylamide-co-styrene): A Path to Improve Drag Reduction?

Hydrophobically modified associating polymers could be effective drag-reducing agents containing weak "links" which after degradation can reform, protecting the polymer backbone from fast scission. Previous studies using hydrophobically modified polymers in drag reduction applications used polymers with M w ≥ 1000 kg/mol. Homopolymers of this high M w already show significant drag reduction (DR), and the contribution of macromolecular associations on DR remained unclear. We synthesized associating poly(acrylamide-co-styrene) copolymers with M w ≤ 1000 kg/mol and various hydrophobic moiety content. Their DR effectiveness in turbulent flow was studied using a pilot-scale pipe flow facility and a rotating "disc" apparatus. We show that hydrophobically modified copolymers with M w ≈ 1000 kg/mol increase DR in pipe flow by a factor of ∼2 compared to the unmodified polyacrylamide of similar M w albeit at low DR level. Moreover, we discuss challenges encountered when using hydrophobically modified polymers synthesized via micellar polymerization.

A General Approach for Flow Optimization and Drag Reduction in Turbulent Pipe Flows

Flow optimization and drag reduction are of great importance in industrial applications. However, most of the structural optimization and drag reduction in pipe flows are based on industrial experience or a large number of experiments, and there is a lack of general theoretical guidance. In the present work, a general approach for flow optimization and drag reduction in turbulent pipe flows is developed based on the irreversibility of flow process and the principle of minimum mechanical energy dissipation. Considering that the effective viscosity coefficient is related to the space coordinates, the field synergy equation of turbulent flow is derived. The reliability and performance of the field synergy principle of turbulent flow as well as the general approach are then evaluated and validated in a turbulent parallel flow conduit, and finally applied to industrial pipe flows. It demonstrates that the present approach is able to optimize flow field for different purposes by adding speed splitter or deflector as an interface at proper locations to alter the interactions between fluid and wall. It is robust and easy to implement, which provides general theoretical guidance for flow optimization and drag reduction in turbulent pipe flows.

A Type B Drag Reduction in pipe flows using Complex Mixtures

Experiments were conducted to measure the drag reduction of the complex mixtures in the pipe flow. The friction factor was measured for bio-polymer solutions and aqueous suspensions of fine solid matter. The results indicated that the experimental data points of the test solutions diverged from the maximum drag reduction asymptote at and above Re√f ≅ 200 and aligned parallel to those of Newtonian fluids. Type B drag reduction was observed. The velocity profile in turbulent pipe flow was predicted using a semi-theoretical equation, in which the friction factors were determined using the difference between the experimental results of the tested solutions and Newtonian fluids.

Molecular Insight into the Possible Mechanism of Drag Reduction of Surfactant Aqueous Solution in Pipe Flow.

The phenomenon of drag reduction (known as the “Toms effect”) has many industrial and engineering applications, but a definitive molecular-level theory has not yet been constructed. This is due both to the multiscale nature of complex fluids and to the difficulty of directly observing self-assembled structures in nonequilibrium states. On the basis of a large-scale coarse-grained molecular simulation that we conducted, we propose a possible mechanism of turbulence suppression in surfactant aqueous solution. We demonstrate that maintaining sufficiently large micellar structures and a homogeneous radial distribution of surfactant molecules is necessary to obtain the drag-reduction effect. This is the first molecular-simulation evidence that a micellar structure is responsible for drag reduction in pipe flow, and should help in understanding the mechanisms underlying drag reduction by surfactant molecules under nonequilibrium conditions.

On the link between experimentally‐measured turbulence quantities and polymer‐induced drag reduction in pipe flows

AbstractIn this study, we investigate the hydrodynamics of polymer‐induced drag reduction (DR) in horizontal turbulent pipe flows. We provide spatiotemporally resolved information of velocity and its gradients obtained with particle image velocimetry measurements in solutions of water with dissolved polyethylene oxide of three different molecular weights (MWs), at various dilute concentrations and with flow Reynolds numbers from 35,000 to 210,000. We find that the local magnitudes of important turbulent flow variables correlate with the measured levels of DR irrespective of the flow Reynolds number, polymer weight, and concentration. Contour maps illustrate the spatial characteristics of this correlation. A relationship between the DR and the turbulent flow variables is found. The effects of the polymer MW, its concentration, and the Reynolds number on the flow are further examined through joint probability distributions of the fluctuations of the streamwise and spanwise velocity components.

Effects of Thermal Radiation on Mixed Convection Flow over a Permeable Vertical Shrinking Flat Plate in an Oldroyd-B Fluid

This study offers the numerical solutions for the problem of mixed convection stagnation-point flow along a permeable vertical flat plate in an Oldroyd-B fluid. The present investigation considers the effects of thermal radiation and heat generation/absorption in the fluid flow. The similarity transformation simplifies the complex model and the bvp4c function generates the numerical solutions according to the variations in the governing parameters. A higher degree of shrinking hastens flow separations. The dual solutions are visible in the range of buoyancy opposing flow. The results from this study may be useful for the scientist to understand the behaviour of the dilute polymer solutions in the industrial applications, for example, the drag reduction in pipe flows.

0306 ポリマー一様溶液の抵抗低減効果とポリマー会合体の分子量の影響(OS3-1 流れの抵抗低減,オーガナイズドセッション)

0306 ポリマー一様溶液の抵抗低減効果とポリマー会合体の分子量の影響(OS3-1 流れの抵抗低減,オーガナイズドセッション)

Drag reduction in pipe flow using microbubbles and acoustic energy

Methods and apparatus for reducing drag in fluid flow through pipes that utilizes microbubbles and acoustic energy are provided. A gas ejector for microbubble generation in a pipe is provided and includes a pipe comprising a wall having inner and outer diameters and at lease one orifice in the wall for ejecting gas microbubbles into the pipe. To cause the formation of the microbubbles, a flow restrictor is provided in the pipe, with the flow restrictor being spaced from the inner diameter of the pipe wall and positioned adjacent the at least one orifice. The flow restrictor causes and increase in the velocity of the fluid in the pipe past the at least one orifice, and produces a pressure drop which draws gas into the orifice. That gas is then ejected into the pipe in the form of microbubbles. Acoustic energy may be used to provide further reductions in drag.

Biaxial Extensional Characteristics of Drag-Reducing Surfactant Solution

The biaxial extensional characteristics of a cationic surfactant solution system showing an effective drag reduction in pipe flow have been studied by using two opposed nozzle devices. Aqueous solutions of cationic surfactant, oleylbishydroxyethylmethylammonium chloride, at the concentrations varied from 250 to 1,000 ppm with sodium salicylate, which ionized to counter-ion in an aqueous solution, of which molar ratio to surfactant was set at 1.5, were tested. Especially, time characteristics of extensional stress were focused in wide ranges of surfactant concentration and extensional rate. From the results, the apparent extensional viscosity is found to depend on the extensional rate and on the surfactant concentration. For the extensional rate dependency, the extensional viscosity behavior is classified into four regions. This characteristic is considered to be caused by the formation and the deformation of the surfactant rod-like micellar network structure. In each region, the build-up and relaxation times are not strongly affected by the concentration of the surfactant. On the other hand, the build-up time decreases with a gradient of -1 in a double logarithm plot to the extensional rate, while the relaxation time decreases weakly with the extensional rate. The former fact indicates the micellar network formation occurs due to the collision of surfactant micelles. It is also found that two relaxation processes exist for the extensional flow cases. The shorter or the longer relaxation time takes almost the same value as each value corresponding to that for the shear flow release condition. This indicates the network structure size formed in the biaxial extensional flow takes almost the same as that in the shear flow.

Surfactant Drag Reduction Caused by a Cationic Surfactant with Excess Addition of Counter-ions

Surfactant drag reduction is a promising technology to reduce the pumping power of district heating–cooling systems. Many investigations have been reported to give a clear prospect of the drag-reducing phenomenon. However, the mechanism of surfactant drag reduction and the microstructure of surfactant micelles are still left unknown. This study aims to provide an experimental evidence of the effect of counter-ions on the micelle structure and the surfactant drag reduction in a pipe flow. The present authors have been investigating the surfactant drag reduction with a cationic surfactant with suitable counter-ions (sodium salicylate). The effect of the molar ratio of the counter-ion to the cationic surfactant is discussed using the experimental results on the drag reduction in a turbulent pipe flow, on the micelle size and on the shear thinning–thickening viscosity. These experimental results suggest that a new drag-reducing state exists at very high molar ratios of the counter-ion to the surfactant. This new state gives a remarkable drag reduction, but shows very weak viscoelasticity. The optimum molar ratio to cause a strong viscoelasticity is (surfactant):(counter-ion) = 1:1.5, but at higher molar ratios, the elongational viscosity decreases, while high drag reduction effectiveness is kept. The micelle structure could be changed at higher counter-ion molar ratio, but this new structure may also be effective for drag reduction.

Drag Reduction in Pipe Flow with Riblet.

This paper presents drag reduction in a pipe flow with riblet. The experiment was carried out in the pipe flow apparatus with test section of inner diameter D=492 mm and length 4 000 mm. The parameters were selected 5 kinds, in order to investigate the details of drag reduction for the shape and size of the isosceles V groove riblet. The velocity profiles, static pressure and flow rate were measured for each experiment in the range of Re=3.0×105∼8.0×105. Consequently it was found that (i) the friction drag reduces in the range of non-dimensional riblet height h+l20 and the maximum drag reduction attains almost 8% at h+=11.4, and (ii) the optimum shape and size of riblet were shown for the maximum drag reduction in a pipe flow.

414 リブレットを用いた管内流の摩擦抵抗減少について

414 リブレットを用いた管内流の摩擦抵抗減少について

Surfactant Drag Reduction in Internally-Grooved Rough Tubes.

It is well known that the use of certain surfactants and counterions can cause drag reduction in pipe flow. This phenomenon can be applied to a district heating and cooling system. In such a system, however, heat transfer reduction correspondingly occurs as drag reduction does. This should be a major problem as far as heat transfer application is concerned. In our study in this regard, we focused on drag reduction in practical heat exchanger tubes and determined the effect of drag reduction caused by a surfactant solution of Ethoquad O/12. We used three rough tubes with different groove pitches and heights in our experiments to evaluate the effect of the roughness shape. From the results, we estimated the magnitude of the heat transfer reduction.

A qualitative assessment of the role of a viscosity depending on the third invariant of the rate-of-deformation tensor upon turbulent non-Newtonian flow

The numerical simulation of some non-Newtonian effects in wall and wall-free turbulent flows, such as drag reduction in pipe flows or the decrease in transverse normal Reynolds stresses, has been attempted in the past with a limited degree of success on the basis of modified wall functions applied to traditional turbulence models ( k– ε), rather than through more realistic rheological constitutive equations. In this work, it is qualitatively shown that if the viscosity function of a generalised Newtonian fluid is assumed to depend on the third invariant of the rate of deformation tensor, there is an increase of the viscous diffusion terms, but especially, of the dissipation of turbulence kinetic energy by a factor equal to the Trouton ratio of the fluid, divided by the Trouton ratio of the solvent, thus indicating a possible way to improve rheological–turbulence modelling.

Degradation effects of dilute polymer solutions on turbulent drag reduction in pipe flows

An important practical problem in the application and study of drag reduction by polymer additives is the degradation of the polymer, for instance due to intense shearing, especially in recirculatory flow systems. Such degradation leads to a marked loss of the drag-reducing capability of the polymer.

Application of V Shape Riblets to Pipe Flows

Pipes with V shape riblets were tested at Reynolds numbers between 5×103 and 4×104. All riblet pipes indicated some drag reduction. The model with h = 0.55 mm and h/S = 0.483 showed the maximum drag reduction of 8 percent and the widest range of Reynolds number over which the riblet reduces drag. The riblet shape desirable for drag reduction in pipe flows was almost the same as that in flat plate boundary layers, but the value of S+ which provided the maximum drag reduction was quite different; S+ = 23 for pipe flows and S+ = 12 for flat plate boundary layers.

An Experimental Study for The Flow of Dilute Polymer Solutions in Nozzles and Diffusers.(Dept.M)

This work presents an experimental study for the effect of drag reducing polymer additives upon a water flow in nozzles and diffusers. Five circular nozzles and diffusers of constant area ratio of 4 and half angle α = 4o ,6o , 10o, and 12o respectively are used. Two polymer types; Polyox coagulant and polyacrylamide are used at different concentrations. The results show that both polyacrylamide and polyox have no effect on the flow through nozzles at any flow rate and polyner concentrations. However, for diffuser flow, polymer additives results in a substantial increase in the pressure recovery of the diffuser. This effect increases with the flow rate and polymer concentration up to 10 wppm. For higher concentrations as 50 wppm, the effect is complicated and more interesting especially with polyacrylamide additives. The pressure recovery of the diffuser begins to decrease at some value of the flow rate. The decrease becomes more pronounced as the diffuser half angle increases. This is thought to be due to the increase of the viscoelastic properties with con centration and the elongation strains with the flow rate and diffuser half angle. The results also show that the pressure recovery increases with the diffuser half angle. They also show that polymer effect starts at a certain flow rate similar to the onset of drag reduction in pipe flow.

Turbulent diffusion and degradation of polymer molecules in a pipe and boundary layer

Results from a series of pipe-flow experiments using a range of water-soluble drag-reducing polymers are presented. Degradation has been investigated by means of multiple passes of the solutions through a pipe. A theory predicting drag reduction in pipe flow has been devised which agrees with the experimental results. Changes in polymer molecular weight due to degradation are taken into account. The analysis is then applied to a turbulent boundary layer with polymer injection.

Hydrodynamic Aspects of Drag Reduction with Additives

The hydrodynamic aspects of drag reduction with additives are presented. The fundamental properties of this remarkable phenomenon are described. A brief history is outlined from anomalous results in pipe flow, through strange effects in the Texas oilfields and to the current research efforts. Correlation by means of the velocity similarity laws of turbulent flow is explained for drag reduction in pipe flow and for the boundary layers on bodies. The limits of drag reduction are also explained on the basis of the interactive similarity law. The peculiar role of viscoelasticity is examined. Naval architectural applications are reviewed.

Effect of Polymer Additive on Grid Turbulence

ALTHOUGH many detailed investigations have been made of the drag reduction in pipe flow caused by polymer additives, very little quantitative information is available about the effect of polymers on grid turbulence. This is known to be small in the absence of the high shear stresses associated with boundary layer flow, but photographs show that there is a definite suppression of eddies in the high frequencies1–3. Hot film probes have been used in the past to study polymer flow in pipes4 but were not used in these investigations because their accuracy is known to be poor in non-Newtonian fluids. Instead, a laser velocimeter was constructed with a signal analysing system that gave a voltage output proportional to the instantaneous axial velocity, and could be used to determine both root mean square (r.m.s.) turbulent intensities and power spectral densities. Full details of the optical arrangement and electrical circuits are given in ref. 5.