The flow of a drag-reducing surfactant solution in a bend with a curvature radius of 1.5 is considered, where 20 mm is the pipe inside diameter.BMWI, 03ET1197A, Energieoptimiertes Bauen (EnOB): Bionische Wärme- und Kältenetze (BioNet

Brandt, Daniel

Kriegel, Martin

Tawackolian, Karsten

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European Drag Reduction and Flow Control Meeting – EDRFCM 2017April 3–6, 2017, Rome, ItalyADDITIVES SHEAR-THINNING AND TURBULENCE DAMPING INFLUENCE ONTHE FITTING LOSS COEFFICIENT OF BENDSK. Tawackolian, D. Brandt, M. KriegelTU Berlin, Hermann-Rietschel-Institut,Chair of Heating, Ventilation and Air ConditioningINTRODUCTIONA large part of the auxiliary energy for cooling and heatingsystems in buildings is needed for the pumps that distributethe heat transfer medium in the thermal distribution network.Roughly half of the pressure losses in such systems are due tostraight pipes and roughly 10 % are due to bends and T-pieces.Drag reducing additives have not so far been widely used fordomestic thermal distribution networks. The BioNet-projectat the Hermann-Rietschel-Institute in Berlin aims at adoptingpromising solutions for this specific application and evaluatingpotential energy savings. The specific typical technical condi-tions for this application, such as Reynolds numbers and pipediameters are considered. In addition to measurements, nu-merical simulations (CFD) are used to adopt and evaluate theapproaches to heating and cooling nets as well as to explainthe physical principles of the developments.The flow of a drag-reducing surfactant solution in a bendwith a curvature radius Rk/D of 1.5 is considered, whereD=20 mm is the pipe inside diameter. It is known that thepressure drop of straight pipes can be reduced considerablywith additives such as surfactants due to the damping of tur-bulent structures. Measurements were performed on two testsetups to assess the influence of additives on the pressure dropin straight pipes as well as for a circuit containing ten bends.The experiments are further described in [1] and [5]. In themeasurements of the circuit, a drag reduction of 40 % wasfound as compared to 60 % for the straight pipes.The investigations could so far not clearly explain why thiswas the case. Is it that the pressure drop of fittings may actu-ally increase under specific circumstances if additives are used?The question already motivated other experimental studies,e.g. Gasljevic and Matthys[2]. This would mean that addi-tives are then disadvantageous in thermohydraulic nets thatcontain many fittings such as bends. The physical proper-ties of surfactant solutions depend on the actual chemical andphysical system conditions such as water quality and the mate-rials used for the pipes. In addition to the turbulence dampingby the micelle structures which leads to a reduction of pres-sure losses, a non-Newtonian viscosity with a shear thinningbehaviour is introduced that may actually increase the pres-sure losses. That is why a simple and straightforward modelwas needed to explain the basic influence of the shear-thinningbehaviour and the turbulence damping in bends.METHODIn this study two simple computational fluid dynamic mod-els are used to assess the effect of the turbulence damping andthe shear-thinning behaviour separately and combined. Themodels are available in the commercial computational fluiddynamics software package STAR-CCM+ version 11.06.010.The intermittency model is used for the turbulence damp-ing. The intermittency is a parameter that is used in thek-ω-SST turbulence model to reduce the turbulent productionand therefore obtain lower turbulence levels.The shear-thinning behaviour is considered by a generalisedNewtonian fluid model that allows for a dependence of thedynamic viscosity on the shear rate. In this case, the Carreau-Yasuda curve is used (see figure 1). The coefficients of theCarreau-Yassuda curve are fitted to an approximation givenin [4] and evaluated for a temperature of 22 ◦C, correspondingto the experimental conditions. Further conditions such asmedium degradation or chemical influences on the successfulmicelle formation were not considered. Measured viscositiesfrom two different sources are also shown as a reference. Themeasured shear viscosities are of a turbulent drag-reducingcationic surfactant solution CTAC with counter ion NaSal at aconcentration Cm of 200 ppm. Data is only available for shearrates below 150 s−1, but data for much higher shear rates wasneeded for this investigation. It was therefore assumed thatfor very high shear rates, an asymptotic value µ∞ = µWaterof the dynamic viscosity of pure water is approached. Anasymptotic value µ0 at low shear rates is not clearly visible inthe experiments, probably due to experimental uncertainties.Therefore, an approximation for µ0 given in [4] was used andevaluated for a temperature of 22 ◦C.The Reynolds number Re was varied between 2000 and140 000 (16 steps) and the intermittency I between 0.1 and 1.0(10 steps), resulting in 160 simulated cases for the straight pipeand for the bend, respectively. A two-dimensional mesh with aresolution of 200 points in the radial direction was used for thestraight pipe. Periodic boundary conditions were used to ob-tain a fully developed flow. For the bends, a three-dimensionalmesh of approximately 1× 106 cells was used. The dimension-less wall distance y+ of the computational meshes was alwaysbelow 0.7 in all investigated cases.RESULTSFigure 2 shows the predicted pressure loss coefficient for astraight pipe. The results at the lowest two Reynolds num-bers 2000 and 5000 should only be interpreted qualitativelybecause the used turbulence model is in principle not applica-ble for such low Reynolds numbers. The curve by Gersten[3]is the reference curve for a smooth pipe and water. The curveby Zakin[6] is the expected maximum drag reduction that can1be achieved by using surfactants as additives. The black linesshow the calculated pressure loss coefficients for different val-ues of the intermittency I between 0.1 and 1. The pressureloss coefficient decreases with decreasing intermittency. I=0.1is the smallest value used for the intermittency in this study;the calculated pressure drop is still above the curve by Zakinwhen this value is implemented. The turbulence in the simu-lation could, therefore, be even further reduced. The red (top)lines show the pressure losses for the shear-thinning fluid. Thecorresponding curves for the pressure loss coefficient of the sin-gle bend are depicted in figure 3. For low Reynolds numbersbelow 30 000, the pressure loss coefficients λ and ζ increasebecause of the additional viscosity at low shear rates.Compared to straight pipes, the influence of turbulencedamping on the drag reduction is lower for bends. It is knownthat the transition from laminar to turbulent flows occursmuch later for curved pipes than for straight pipes. This canbe seen in figure 4. Depicted is the effective viscosity thatresults from the water viscosity, the additional viscosity dueto the additive and the turbulent eddy viscosity. Especiallyfor low Reynolds numbers, the turbulent eddy viscosity is al-ready relatively low in the bend, therefore further dampingdoes not lead to the same larger reduction of the pressure losscoefficient that is observed for straight pipes. Furthermore,the additional viscosity at low shear rates becomes importantat low Reynolds numbers below 30 000.FIGURESFigure 1: Used model of shear-thinning behaviour and exper-imental data of Kawaguchi et al.[4] and Zhou et al.[7]Figure 2: Influence of intermittency and rheologic propertieson the pressure loss of a straight pipeFigure 3: Influence of intermittency and rheologic propertieson the pressure loss of a bendFigure 4: Influence of intermittency and rheologic propertieson the effective viscosity in a bend,Re=20 000REFERENCES[1] D. Brandt, K. Tawackolian, and M. Kriegel. Applicability ofdrag reducing agents in domestic heating systems. In submit-ted to European Drag Reduction and Flow Control Meeting– EDRFCM 2017 April 3–6, 2017, Rome, Italy.[2] K. Gasljevic and E.F. Matthys. Friction and heat transfer indrag-reducing surfactant solution flow through curved pipesand elbows . European Journal of Mechanics - B/Fluids,28(5):641–650, 2009.[3] K. Gersten. Fully Developed Turbulent Pipe Flow, in:FluidMechanics of Flow Metering. Springer, 2005.[4] Yasuo Kawaguchi, Bo Yu, Jinjia Wei, and Ziping Feng. Rhe-ological characterization of drag-reducing cationic surfactantsolution –shear and elongational viscosities of dilute solutions.In Proceedings of ASME FEDSM03 4th ASME JSME JointFluids Engineering Conference, Honolulu, USA, July 6–11,2003, 2003.[5] K. Tawackolian, H. Sagheby, D. Brandt, and M. Kriegel.Development and tests of bionic fittings for heating nets.In CLIMA 2016 - proceedings of the 12th REHVA WorldCongress: volume 9, Aalborg University, Department ofCivil Engineering, volume 9, 2016.[6] Jacques L. Zakin, Jiri Myska, and Zdenek Chara. New limitingdrag reduction and velocity profile asymptotes for nonpoly-meric additives systems. AIChE Journal, 42(12):3544–3546,1996.[7] Ying-Bo Zhou, Na Xu, Ning Ma, Feng-Chen Li, Jin-Jia Wei,and Bo Yu. On Relationships among the Aggregation Num-ber, Rheological Property,Effect of Surfactant Solutions. Ad-vances in Mechanical Engineering. 2011.2

Additives Shear-Thinning and Turbulence Damping Influence on the Fitting Loss Coefficient of Bends

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Additives Shear-Thinning and Turbulence Damping Influence on the Fitting Loss Coefficient of Bends

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