Turbulent drag reduction by polymer-fiber mixtures

The turbulent drag reduction studies by asbestos fiber mixtures with purified guargum, purified xanthangum, and their graft copolymers have been conducted at low concentration and Re 14000 using turbulent flow rheometer designed by Hoyt. A method for making stock suspension of asbestos fibers is also suggested which gives better drag reduction. It has been found from the present studies that purified guargum–asbestos fiber mixtures give positive synergistic effect even at low concentration and synergism in drag reduction may be caused by a mixture when its constituents are rigid. A simple mixture rule equation having interaction parameter may be used for the prediction of drag reduction by the mixture.     

Vortex flow of dilute polymer solutions

It has been observed that very d longchain polymers which are effective in turbulent drag reduction inhibit the formation of a vortex or air core as water drains from a tank. This paper considers the fluid mechanical velocity profile measurements have been performed. There appear to be at least two distinct mechanisms for the vortex inhibition—one involving the viscosity enhancement caused by polymer addition, and the other related to the viscoelastic properties of the polymer solutions. This second mechanism is shown to arise due to the generation of high normal stresses as the air core begins to form. The very close correlation between vortex inhibition and turbulent drag reduction suggests that normal stresses may also play an important role in this latter phenomenon.     

HYDRAULIC TRANSPORT OF COAL IN PIPES WITH DRAG REDUCING ADDITIVES†

The aim of the investigations was to determine the drag reduction produced by polymer additives in the hydraulic transport of coal. As no systematic results were available in the literature, experiments were undertaken using a wide range of flow velocities and polymer concentrations (up to 440 ppm) to obtain a clear picture about the processes involved. Special attention was paid to the effects of polymer degradation. The experiments were carried out on two different facilities for hydraulic transport with pipe diameters of 40 and 250 mm

Two criteria were used for selecting the polymer with the most favourable properties. It was to produce the largest possible amount of drag reduction and have the greatest stability against degradation. Six different types of polymers were investigated. Experiments were undertaken to establish the dependence of drag reduction on the polymer concentration for the polymer showing the best results according to the above-mentioned criteria. It was found that the drag reduction on coal-watei mixtures containing polymer was less than in “pure” polymer solutions (not containing any solid particles). Furthermore, for both coal-water mixtures and “pure” polymer solutions there was always found to be a polymer concentration at which drag reduction reached a maximum and this concentration was higher for the coal-water mixtures. The value of drag reduction increased as the flow velocity was increased. In the polymer degradation experiments the decrease in drag reduction as a consequence of polymer degradation was stronger when the polymer concentration was lower for both the coal-water mixtures and the “pure” polymer solutions. Experiments of longer duration showed that even after several hours of transport in the facility, there was still considerable drag reduction. Finally, it was found that for coal-water mixtures polymer degradation was greater at higher flow velocities.     

Turbulent drag effectiveness and shear stability of xanthan-gum-based graft copolymers

A number of graft copolymers of xanthan gum and polyacrylamide have been synthesized by grafting acrylamide onto xanthan gum using the ceric-ion-initiated solution polymerization technique. The effects of various synthesis parameters such as amount of catalyst, reaction time, and ratio of xanthan and acrylamide on drag reduction effectiveness of the graft copolymers have been studied. The scaling up of grafting reaction has been accomplished in 40-L reactor. The drag reduction effectiveness of the graft copolymers is investigated over a wide range of concentrations and Reynolds numbers. It is shown that the maximum drag reduction obtainable in xanthan gum solutions above 300 ppm can be obtained in solutions of graft copolymers at concentrations of 100–150 ppm. The grafting also improves the shear stability at higher Reynolds numbers. The shear stability of the graft copolymers at constant wall stress has been found to be superior to polyacrylamide and the mixtures of polyacrylamide and xanthan gum. In general, the shear stability of graft copolymers and polyacrylamide is shown to increase with concentration. The drag reduction characteristics and shear stability have been discussed in terms of structural features of the graft copolymers. The drag reduction characteristics of the graft copolymers are found to be similar to those of flexible polymers.     

SOME MOLECULAR EFFECTS IN DRAG REDUCTION: A SUMMARY†

Dilute solutions of high molecular weight polymers have drawn a great deal of interest in recent years because of their drag reducing characteristics. It is well-known now that a substantial reduction in turbulent frictional drag can be achieved with a very small amount of polymeric additives, usually only a few parts per million by weight (ppmw) in concentration. This unique phenomenon has offered a new dimension in the design development of new marine systems for higher speed, longer range, larger payload as well as possibly quieter machinery. Although the discovery of this turbulent drag reduction phenomenon may be traced back to Toms¹ and Mysels² in the 1940's, the U.S. Navy's exploration of the turbulent drag reduction effect did not begin until the pioneering effort of Hoyt and Fabula in the 1960's. ³ During a period of several years in the early 19707apos;s, an interdisciplinary group at the Naval Research Laboratory undertook an intensive basic research effort to study the effects of polymer molecular structure on turbulent drag reduction. Model compounds were synthesized in the laboratory, and their drag reducing properties characterized. Polymers including polyacrylamide and its derivatives, polyacrylic acid, poiyphosphate and association colloids have been investigated. In this report, an attempt is made to highlight some of the results from that program in a brief summary form.     

Drag reduction by polymer threads

A novel type of drag reduction is described which is caused by long polymer threads in turbulent flow. These threads are formed by axial injection of a concentrated, visco-elastic, polymer solution and persis over a distance of more than 200 tube diameters. The observed drag reduction is particularly effective at low Reynolds numbers. Depending on the properties of the injection system, the continuous supply of the polymer solution may change spontaneously into an oscillating supply resulting in periodic pressure drop variations.     

The drag reduction of dilute polymer solutions as a function of solvent power,viscosity, and temperature

The frictional drag reduction of high molecular weight poly(ethylene oxide) and polystyrene solutions under turbulent flow conditions has been studied as a function of temperature, solvent power, and solvent viscosity. A rotating-disc apparatus was used to make the drag reduction measurements. For aqueous poly(ethylene oxide) solutions, at concentrations well above that needed to produce maximum drag reduction, all drag reduction data reduced to a common curve when per cent drag reduction was plotted against the Reynolds number for the flow. However, for poly(ethylene oxide) solutions below this optimum concentration, the drag reduction-versus-Reynolds number curves showed decreasing drag reduction with increasing temperature. The data are explained primarily in terms of the inverse temperature solubility characteristics of poly(ethylene oxide) in water. The per cent drag reduction of polystyrene in nonaqueous liquids was found to be greater in good solvents than in poor ones. It was also found that increases in solvent viscosity and decreases in temperature increased the per cent drag reduction. The results are discussed in relation to the current drag reduction theories and are shown to be in opposition to Virk's theory. It is concluded from the data that drag reduction is very likely a function of a relaxation time phenomenon involving the polymer molecules and the flow system. The results also emphasize the importance of considering solvent power, viscosity, and temperature in the design of an efficient drag reduction system.     

SOME MOLECULAR EFFECTS IN DRAG REDUCTION: A SUMMARY†

Dilute solutions of high molecular weight polymers have drawn a great deal of interest in recent years because of their drag reducing characteristics. It is well-known now that a substantial reduction in turbulent frictional drag can be achieved with a very small amount of polymeric additives, usually only a few parts per million by weight (ppmw) in concentration. This unique phenomenon has offered a new dimension in the design development of new marine systems for higher speed, longer range, larger payload as well as possibly quieter machinery. Although the discovery of this turbulent drag reduction phenomenon may be traced back to Toms¹ and Mysels² in the 1940's, the U.S. Navy's exploration of the turbulent drag reduction effect did not begin until the pioneering effort of Hoyt and Fabula in the 1960's. ³ During a period of several years in the early 19707apos;s, an interdisciplinary group at the Naval Research Laboratory undertook an intensive basic research effort to study the effects of polymer molecular structure on turbulent drag reduction. Model compounds were synthesized in the laboratory, and their drag reducing properties characterized. Polymers including polyacrylamide and its derivatives, polyacrylic acid, poiyphosphate and association colloids have been investigated. In this report, an attempt is made to highlight some of the results from that program in a brief summary form.     

Adsorption measurements in dilute solutions of a drag-reducing polymer

Experimental evidence supporting an adsorptional mechanism for the reduction of friction drag in the turbulent pipe flow of dilute solutions of a polyoxyethylene in water has been obtained in the form of adsorption measurements, measurements indicating increased additive concentrations at the pipe wall, and additive axial dispersion measurements. An analytical method used to determine very low concentrations of polyoxyethylene in water (2–100 ppm) is reported.     

Turbulent flow behaviour of dilute polymer solutions in an annulus

An experimental study has been carried out to investigate the turbulent flow behaviour of dilute polymer solutions in an annulus. The polymers used are two grades of Separan, AP30 and MG500, both are known to exhibit drag reduction characteristics in turbulent pipe flow. Similar drag reduction phenomena have been observed in annulus flow. At a given Reynolds number, the friction factor decreases with increase in polymer concentration and appears to reach a minimum (or maximum drag reduction) at certain optimum concentration. An estimate of the critical wall shear stress, which marks the onset of drag reduction, is consistent with pipe flow results, suggesting that the critical value is independent of flow geometry and size. A lower drag reduction, achieved in an annulus in comparison with circular pipes, is attributed mainly to a diameter effect.     

表面活性剂添加对歧管式微通道阻力特性的影响

歧管式微通道（MMC）热沉具有热阻小、结构紧凑、冷却液流量小、流速低、沿着流动方向温度分布均匀等优点,但其小尺寸所产生的较大压降却增加了泵功的损耗.本文研究了表面活性剂添加对其阻力特性的影响,实验选用了纯度为95%的阴离子表面活性剂十二烷基硫酸钠（SDS）和纯度为98%的新型绿色非离子表面活性剂烷基多糖苷（APG）作为减阻添加剂,浓度分别为100和300 mg·kg^-1,结果表明阻力减小率与流速和温度有关.在层流区内减阻效果不是特别明显;但是当流体进入紊流区后阻力减小率开始明显增大,尤其是进入充分发展的紊流区后减阻效果大大加强.此外,温度的提升也可增加阻力减小率,但添加SDS后减阻效果的改善却不及APG.通过对两种不同类型表面活性剂的实验比较,发现温度较高时APG比SDS具有更佳的减阻效果.     

旋风除尘器减阻节能增产技术

通过安装在旋风除尘器内的减阻杆,可以在保证分离效率的前提下降低流动阻力。分别采用5孔球形探针、激光多普勒测速仪和粒子图像测速仪对旋风除尘器内安装减阻杆前后的时均流场与湍流场进行了测量,发现减阻杆降低了流场中与粉尘分离无益的内旋流切向速度,削弱了中心区域的湍流强度,使湍流耗散减弱,从而实现压降降低达到减阻目的。根据流动参数的变化,分析了减阻杆的减阻机理,提出了"加涡减阻"的假设。并介绍减阻杆在工业上的应用实例。     

An analysis of unsteady one-dimensional stretching of a viscoelastic fluid

The time-dependent response of a viscoelastic liquid to unsteady one-dimensional stretching deformations was examined. Oldroyd's three-constant model for a viscoelastic fluid was used. Two cases representing two different stretching histories were analyzed: a sine stretching pulse and a step stretching pulse. The results show that high elongational viscosity may be easily reached in both cases. As the relaxation time of the liquid becomes comparable to the pulse width, elongational viscosity increases with the increase in maximum stretching rates. Conditions to maintain high levels of elongational viscosity at a subsequently reduced stretching rate were given as functions of the relaxation time and initial stretching rates. In view of recent turbulent boundary layer data, the results were used to discuss possible explanations of turbulent drag reduction in polymer solutions. It was concluded that the basic mechanisms for drag reduction in polymer soluations. It was concluded that the basic mechanisms for drag reducation may be related to the effects of high elongational viscosity and local stabilization of small shear disturbances.     

Roles of drag reducing polymers in single- and multi-phase flows

It has become a well-known fact that finding sustainable solutions to the unavoidable high pressure losses accompanying pipeline flows to increase the pumping capacity without necessarily adding more pump stations is inevitable. Polymers, as one of the drag reducing agents which have been found to offer such an economic relieve, is the most widely investigated and most often employed in industries because they can produce drag reduction up to 80% when they are added in minute concentrations. In addition, polymer additives modify the flow configurations of multiphase flows to such an extent that stratification of individual phases is enhanced thereby making the separation of the phases at the fluid destination much easier. The achievements so far made and the challenges facing the use of polymers as drag reducers in turbulent single and multiphase flows are comprehensively reviewed. This review discusses the experimental studies of the effects of polymer additives in turbulent flows, the analytical studies, and the proposed models as well as the suggested mechanisms that explain the drag reduction. Likewise, specific areas of interest in the review include phenomena of drag reduction by polymers, factors influencing the effectiveness of the drag reducing polymers, methods of injecting the polymers into the base fluids, degradation of the polymers and industrial applications of polymers as drag reducing agents. The current and future research interests are also addressed. Although finding reveals that there are quite a lot of research in this area, most of the experimental and theoretical works are devoted to single phase flows while the remaining ones are mostly directed towards gas–liquid flows except in very recent time when investigation into the use of polymers in liquid–liquid flows is being focused. Despite this voluminous works on drag reducing polymers, there are no universally accepted models and hence the mechanisms of drag reductions by polymers.     

Flow visualization studies of a turbulent drag reducing solution

A nondisturbing photochromic dye trace technique has been employed to study the flow profiles of water-alcohol solutions containing the drag reducing polymer Polyox at concentrations of 0.55 to 5.5 ppm. Drag reductions of from 33 to 75% were observed in a smooth glass pipe. The dye traces show a thicker wall layer for the drag reducing solutions than that for the solvent alone. The core region is relatively unaffected by the polymer at 0.55 ppm and at 5.5 ppm the relative thickness of the turbulent core region has been greatly reduced. Earlier, unpublished work of Corinthios shows effectively no core at 33 ppm. Mean velocity and apparent axial turbulence intensity profiles obtained are in general agreement with previous data in the literature obtained with other methods and in the wall region the low frequency intensity is greater than that of the pure solvent. This is consistent with the results of Spangler but not those of Seyer. Higher low frequency intensities measured in the wall region suggest that higher frequency fluctuations are suppressed and/or that the time stability of eddies is increased as indicated by the reduced frequency of wall ejections in drag reducing solutions.     

Heat transfer to an immiscible liquid mixture and between liquids in direct contact

Heat transfer to a mixture of two immiscible liquids has been studied in connection with the development of a process for the desalination of sea water. The liquid system consisted of water and refined mineral oil, produced by BP under the trade name of Energol WM-2.Heat transfer to water drops descending through the mineral oil was also investigated. The drag coefficients of the drops in motion were expressed as a function of the Reynolds number. A good correlation was obtained for the heat transfer coefficient expressing the Nusselt number in terms of the Peclet number.Heat transfer to a mixture of two immiscible liquids in co-current turbulent flow without phase change was extensively studied. Friction factor and heat transfer for the oil-in-water type mixtures were theoretically expressed in terms of the volume fraction of oil. The experimental data checked the theoretical derivation quite satisfactorily. No correlations could be obtained for the water-in-oil systems.Similar studies were made for heat transfer in the laminar and turbulent flow with phase change, using a pilot-plant evaporator. Curves were obtained relating the convective heat transfer coefficient to fluid velocity for the liquid mixtures. It was established that, the heat transfer coefficient in evaporation decreased by velocity in the laminar region, but increased in the turbulent region.     

Experimental Study and Analysis on Drag Reduction Mechanisms of Reducing Pressure Drop Stick in a Cyclone Separator

The mechanisms of mean mechanical energy losses in a cyclone separator with a Repds (Reducing Pressure Drop Stick) are studied. The results show that the energy losses are mainly caused by direct viscous dissipation, turbulent diffusion, and turbulent energy production, which is the same as in a conventional cyclone separator. In particular, the direct viscous dissipation and the turbulent energy production are of importance. Laser Doppler Velocimetry (LDV) is used to measure the flow field in the cyclone body and the exit pipe of a cyclone separator with and without a Repds, respectively. The drag reduction mechanism of the Repds is analyzed by examining the effects of the Repds on flow parameters which are related to energy loss factors. The measured results and analysis reveal that the drag reduction results from decreases of the direct viscous dissipation, turbulent diffusion, and turbulent production as a whole when a Repds is inserted into a cyclone separator. The resultant drag reduction is a tradeoff between the increase and the decrease of energy losses caused by the Repds.     

Evidence for molecular interactions in drag reduction in turbulent pipe flows

Experiments which test the concentration and molecular weight dependence of turbulent pipe flow drag reduction for random coiling polymers in dilute solutions show correlations with concentration to the one-half power and molecular weight to the 0.8 power for good solvents. This result is not consistent with a model of extension of single1 molecules, but could be related to the increase in bulk viscosity of interacting molecules after some extension. In this work, measurements for very low amounts of drag reduction for rigid rod molecules arc reported, and the effect of tube diameter on the amount of drag reduction is examined for fiexible rod molecules. No diameter effect is observed for the rigid rods, but an increase in drag reduction with increase in pipe diameter is found for the flexible polyeleetrolytes. In all cases, the volume occupied by spheres which circumscribe the molecules is greater than the actual volume when drag reduction is found. The results indicate that combined effects of individual molecule stretching and molecular interactions are present in drag reduction for random coiling or flexible rod molecules.     

Drag reduction and structural turbulence in flowing polyox solutions

Structural turbulence has been detected in dilute aqueous solutions of Polyox Coagulant (also known to be a highly effective drag-reducing agent). The flow line which characterizes structural turbulence from its onset in the laminar region passes well into the fully turbulent region (Reynolds turbulence) with virtually no change in slope, implying that the same molecular oscillations or segmental motions responsible for structural turbulence are now operative in drag reduction. The persistence of structural turbulence at very low concentrations is rationalized on the basis of Busse's explanation of the role of polymer entanglements in viscosity and elastic turbulence.     

An improved correlation for turbulent drag reduction in dilute polymersolutions

The model for turbulent drag reduction in dilute polymer solutions developed by Darby and Chang (1984), and modified by Darby et al. (1989) has been further refined to significantly improve its ability to represent data for a variety of polymer solutions over a wide range of conditions. It is also shown how the rheological parameters in the model can be obtained from two or three data points for turbulent flow, in lieu of laminar viscometric measurements. This technique is applied to a wide variety of turbulent drag reduction data taken from the literature to demonstrate the general applicability of the model. The model is also shown to be consistent with Virk's maximum drag reduction asymptote (Virk, 1975) in the high Reynolds number limit.