Drug Reduction - WSEAS：药物还原模式.doc

Drag Reduction in District Heating Networks with Surfactant Additives DARKO GORICANEC, JURIJ KROPE, ZELJKO KNEZ Faculty of Chemistry and Chemical Engineering, University of Maribor Smetanova ul. 17, 2000 Maribor, SLOVENIA Abstract: In the paper drag reduction with surfactant additives and their application in district heating systems is presented. Dilute solutions of certain surfactants affect the flow behavior of fluids in turbulent flow and show a much lower coefficient of friction than does the solvent alone. Surfactants are low molecular chemical substances with a great interfacial activity. At higher concentrations they form aggregates of molecules known as micelles. The superstructures made of associations between rod-like micelles are responsible for drag reduction. Their main effect is to cause a thickening of the buffer layer on account of damping of the turbulent layer. The mathematical model is composed of non-linear equations for determination of incompressible fluid flow characteristics and extra determined empirical equation of friction coefficient in dependence of Reynolds number for solution of surfactant in hot water. Thus, a new district heating network can be designed with smaller pipe diameters, pumps with reduced pump power and decreased pumping energy. Economic benefits of this reductions are considerable. Key Words: Additives, Surfactants, Drag reduction, Pressure drop, District heating 1 Introduction Growth of environmental pollution and decrease of primary energy supplies demand consistent rational use of primary energy, therefore is this activity an important topic of numerous national and international research projects particularly in recent years. If we limit ourselves to energetic system of district heating, we found out that it ensures savings at consumption of primary energy and ecologically acceptable heat energy supply. One of the possibilities for the rational use of primary energy is to improve effectiveness and economy of district heating systems with application of drag reducing surfactants. High molecular polymer drag reducing additives were used in hot water systems in the past but they did not prove well due to irreversible degradation in regions of high shear stress. Nowadays, low molecular weight surfactants which demonstrate drag reducing behaviour at very low concentrations and have reversible structure are applied in district heating system. Already very small quantities of this additives could cause large reduction of pressure drop. Because of this many costs savings could be achieved in the existing district heating systems especially at costs of electrical energy used for electrical drive of pumps and gas or electrical energy used for heating of smaller water volume. Likewise is possible to enlarge transport capacities considerably or reduce investment in new district heating system because of smaller pipe diameters and pumps with reduced pump power. Numerous laboratory researches have been carried out so far 1, 2, 3. On their basis the criterion for general use of additives in district heating supply systems could be set and computer applications which simulate influence of additives on flow - pressure conditions in pipe networks could be developed. The aims of our research are following: to find out physical and chemical properties of surfactants and their operating principle, to research the influence of combined surfactant Dobon G / NaSal on the district heating system and flow - pressure conditions in the system. 2 Drag Reduction Principle The phenomenon of drag reduction with surfactants in aqueous solutions is based on the decrease of turbulence intensity and can be explained with the formation and the shape of micelles. The detailed measurements of the velocity profile in the near-wall region has revealed that the main effect of the additives is to cause a thickening of the buffer layer, which is intermediate between the laminar and turbulent layer, and damping of the turbulent layer 4. Surfactants are low molecular chemical substances with a great interfacial activity 2. Their molecules are formed from a water soluble and a water insoluble part, referred as hydrophilic and hydrophobic respectively. The hydrophilic part is polar or ionizable, whereas the hydrophobic (head group) part of the molecule is a long alkylchain. Depending on the electric charge of the hydrophobic part, the surfactants are classified as cationic, anionic, and non-ionic. In aqueous solutions, above critical concentration known as CMC1 the surfactant monomers associate to form aggregates called micelles (Figure 1). If the concentration is further increased, the number of surfactant molecules per micelle will increase and the transition from spherical to rod-like micelles occurs 2. The concentration at which this transformation happens is called CMC2. Temperature Monomeres CMC1 CMC2 Spherical micelles Rod-like micelles Concentration Figure 1: Critical micelle concentrations CMC1 andCMC2 2 It is believed that not rod-like micelles alone, but larger superstructures made of associations between rod-like micelles are responsible for the drag reducing effect 3. These superstructures are formed under the action of shear stress, and thus the condition at which this happens is called “Shear Induced State (SIS)” (Figure 2). After the critical value of shear stress is exceeded, the rod-like micelles align completely in the flow direction and form a permanently oriented viscoelastic network (SIS) which expands the buffer layer and reduces the layer of turbulent main stream flow. If the concentration is only just above CMC2, only few, relatively large micelles will be formed. These are not well capable of forming superstructures, which is why their drag reduction effect is only small. Therefore, for the sufficient reduction of drag higher concentration has to be exceeded. 2.1 Maximum Drag Reduction Asymptote Typically, the relationship between the pressure drop and the flow rate, is represented by plotting the friction coefficient as a function of the Reynolds number. The most well known correlation in drag reduction is the maximum drag reduction asymptote (MDRA) proposed by 5: (1) 58, 0 Re58 . 0 Recently 6 proposed a new limiting MDRA for surfactants which is even higher than the 5 MDRA: (2) 55, 0 Re315 . 0 The relationship vs. Re is shown in Figure 3, where four regions are distinguished, namely: onset, intermediate, asymptotic and supercritical 2: Onset: In laminar region of flow with no or small shear stress the rod-like micelles form a spatial. network by the electrostatic repulsion because of their surface charge, in which they occupy energetically beneficial positions. In this state surfactant solution shows Newtonian behavior. Intermediate: A rise of shear stress and turbulent flow lead to orientation of the rod-like micelles and formation of the SIS what causes laminar flow. In this range surfactant solution shows pseudoplastic flow behavior. Asymptotic: A further rise of shear stress leads to an increase of drag reduction. In this range the maximum drag reduction appears. Micelles organized in SIS are able to incorporate more energy, because deforming and stretching causes reset forces, which act against the turbulent fluctuation movement and therefore reduce the energy dissipation. In this range pseudoplastic behavior exist as well and this flow condition is known as pseudolaminar flow. Supercritical: Very high shear rates finally effect the destruction of the viscoelastic micelle superstructure. With that drag reducing effect ceases and the characteristic surfactant solution curve reaches the curve of water. In this case a full developed turbulent flow appears, which again shows Newtonian behavior. Figure 2: Shear induced state (SIS) 3 log Re Onset Hagen- Poiseuille Intermediate Virks MDRA Prandtl Asymptotic Supercritical log Water Surfactant solution Figure 3: Friction coefficient as a function of Reynolds number 2 2.2 Cationic Surfactant System Quaternary ammonium surfactants with counterions have been shown to be particularly useful for district heating systems. The effect of the counterions is to suppress the electrostatic forces of the head groups and to help the molecules to pack more densely and form rod-like micelles (i.e. reduce the CMC2). As leading candidates for use as a drag reducers in district heating systems cationic surfactant substances Habon G and Dobon G with added sodium salicylate (NaSal) as counterion have been reported 3. Their molecules form large rod-like micelles, which are very effective in causing drag reduction 4. They have different temperature ranges where they exhibit drag reducing effect and so different areas of application at flow velocities up to about 4 m/s. Their chemical structure and temperature ranges are shown in Figure 4. CH3 rod-like micelles shear stress CnH2n+1NCH3 (C2H4O)1-2H n-Alcyldimethylpolyoxethylammonium Cation COO OH COO OH Salicylat3-Hydroxy-2-naphothoat n = 16 (Habon G / NaSal):from 25 to 105C n = 18 (Obon G / NaSal):from 35 to 120C n = 22 (Dobon G / NaSal):from 45 to 140C Figure 4: Chemical structure and temperature range of cationic surfactants 3 3 Mathematical Model The mathematical model is composed of a system of non-linear equations which determinate the hydraulic characteristic of pipe network with N nodes and m pipe sections 7. The model could be solved iteratively with linearisation of empirical Hazen - Williamson equation (3) or non-linear Darcy - Weisbach equation (4) 7, 8: (3) 852, 1 87, 4 78,10 C q d L pp j iV ji . (4) 2 4 2 81, 0 j iVij j iV ji qK d L d q pp The linearisation of the equations could be done either by flow or by pressure. The Linear Theory Method - LTM is most suitable method for the flow - pressure calculation of hot water systems 9, 10. This method linearises the non-linear Darcy - Weisbach equation (4) by the pressure, equation (5): (5) k jij i k ji iV ppK ppq 1 1 m , 3, 2, 1, kN, , 3, 2, 1, j N, , 3, 2, 1, i In planning the flow - pressure model we must consider: the continuity of nodes flow (the sum of flows which flow into a node must be equal to the sum of flows which flow out of a node), the preservation of energy around closed loop (the pressure drops sum in any closed loop of the pipe network must be equal to zero), the continuity of the pipe network flow (the sum of flows into the pipe network must be equal to the sum of flows out of the pipe network). The calculation of flow-pressure conditions in existing and redesigned pipe network was done with the software application for hydraulic analysis HAPN/HW 11. The change of flow conditions, due to application of surfactant, was considered with the empirical equation for friction coefficient () determined on the basis of full-scale experimental data for the surfactant solution Dobon G/NaSal in hot water: (6) 001266, 01989, 0 Re00603, 0Re17442, 0 4 Simulation of Pipe Network Conditions The part of the hot water pipe network of the district heating system of city Maribor (TOM) is shown in Figure 5. It is composed of 18 nodes and 17 pipe sections. Calculation data are following: water density and kinematic viscosity at the operating temperature of 130C (because of low surfactant concentration the density of surfactant solution is presumed to be equal as the density of pure water), required relative precision of calculation, pressure of the flow in the system input, outflow in individual nodes, diameters and length of pipes, pipe roughness and coefficients of local losses. The concentration of surfactant is 1500 wppm. Pipe network data and results are shown in Table 1. 4.1 Results of Flow - Pressure Analysis Flow - pressure analysis of conditions in district heating system of city Maribor has been done for following two alternatives: existing system operating without surfactant and existing system operating with surfactant Dobon G / NaSal. The results of hydraulic analysis, pressures in nodes, are shown in Figure 6. Figure 5: Scheme of the analysed district heating system Figure 6: Pressure of hot water in nodes for water and water with surfactant Table 1. Power, flow and pressure in nodes, diameters, flow and length of pipe sections Input dataResultsInput dataResults NodePower (kW) Input / Output (m3/h) Water Pressure (Pa) Surfactant Pressure (Pa) FromTod (mm)L (m)Flow (m3/h) TOM24797.+374.01550000550000TOMJ201495.4110.0374.01 J2010.0.00549400549832J201J202343.0198.4374.01 J2020.0.00541497546880J20260282.595.03.26 602-216.-3.26540923546589J202601125.08.116.61 601-1100.-16.61541321546782J202J203343.01.6343.64 J2030.0.00539129545144J203J207100.842.511.34 J2070.0.00537828544435J20760382.57.66.42 603-425.-6.42537441544144J20760451.2112.64.92 604-326.-4.92520214538242J203J208309.7295.7343.29 J2080.0.00519415537706J208J209309.7134.8224.53 J2090.0.00514262534834J208605150.035.0118.76 605-7864.-118.76500287526499J209J210309.7214.8197.35 J2100.0.00509841533177J209606125.0197.027.18 606-1800.-27.18505401531350J210J211309.7131.3197.35 J2110.0.00506390531413J211J217309.7181.0197.35 J2170.0.00501962529314J217608309.740.019.07 608-11771.5-19.07500562528326J21760782.5100.0177.30 607-1295-177.30482002522711Roughness of pipe 0.4 mm TOM J202 J203 J207 603 602 601 605606 J208J209J210J211J217 608 607 604 470000 480000 490000 500000 510000 520000 530000 540000 550000 560000 TOM J201 J202 602 601 J203 J207 603 604 J208 J209 605 J210 606 J211 J217 608 607 Nodes Pressure (Pa) Water Surfactant 5 Conclusions Addition of small quantity of certain surfactant additive (measured in ppm) to a fluid can produce large reduction of drag in turbulent pipe flow. This is very interesting because many practical applications could eventually profit from the energy savings that such reduction could offer. Despite more than five decades of research, a full understanding of the fundamentals of this phenomenon is still not complete. The amount of friction in the transport system dictates the pipe diameters and the pump power to be installed for hot water circulation. With the application of drag reducing surfactants in district heating water the pressure drop in pipelines can be reduced significantly. Application of surfactants is recommended in all district heating systems with the operating temperature ranged from 40 to 140C, especially in the following systems: in existing district heating systems where the mass flow is constantly reduced, in district heating systems where the problem of insufficient mass flows is present which causes disturbed heat supply of consumers, in systems in which the incorporation of new heat stations is envisaged, in new district heating systems, where their application assures short- and long-term the best economic operation of this systems. Nomenclature: Ccoefficient of Hazen-Williamson equation dpipe diameter (m) Kcoefficient of Darcy-Weisbach equation kpipe roughness (mm) knumber of iteration Lpipe length (m) Nnumber of nodes ppressure (Pa) qvflow volume (m3/s) ReReynolds number Greek letters friction coefficient density (kg/m3) coefficient of local losses References: 1G. Aguilar, An Experimental Study of Drag and Heat Transfer Reductions in Turbulent Pipe Flows for Polymer and Surfactant Solutions, PhD Dissertation, University of California, Santa Barbara, CA, 1999. 2M.C. de Groot, & E.A. Kievit, Experiments on the Effects of Friction Reducing Ad