Experimental and Numerical Investigations of an Ice-slurry Generator Experimental and Numerical Investigations of an Ice-slurry Generator

Experimental and Numerical Investigations of an Ice-slurry Generator

  • 期刊名字:过程工程学报
  • 文件大小:127kb
  • 论文作者:洪若瑜,董梁,尚德义,徐建生,Kawaji M
  • 作者单位:Dept. Chem. & Chem. Eng.,Dept. Chem. Eng. & Appl. Chem.
  • 更新时间:2020-11-03
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第4卷第1期过程工程学报VoL.4 No.12004年2月The Chinese Journal of Process EngineeringFeb.2004Experimental and Numerical Investigationsof an Ice- slurry GeneratorHONG Ruo-yu(洪若瑜)',DONG Liang(董梁),SHANG De-yi(尚德义)尸XU Jian-sheng(徐建生)子,Kawaji M2(1. Dept. Chem. & Chem. Eng., Soochow University, Suzhou, Jiangsu 215006, China;2. Dept. Chem. Eng. & Appl. Chem, University of Toronto, Toronto ON M5S 3E5, Canada)Abstract: A new test facility equipped with refrigerant and brine circulation systems, and arotating-scraper ice slurry generator was constructed to analyze the ice-slurry flow and heat transferaccompanied by phase change in an industrial generator. The axial and transverse brine temperature andice fraction concentration profiles in the ice generator were measured. The heat transfer efficiency lowerthan the average was identified in the upper half of the ice generator and its cause was determined byconducting three-dimensional numerical simulation using a commercial CFD code, FLUENT.Approaches of improving the brine-side heat transfer rates were investigated by incorporating extramixing blades from numerical simulation.Key words: ice generator; ice-slurry; numerical simulation; heat transferCLC No: TQ022; TQ018Document Code: AArticle ID: 1009- -606X(2004)01-0001- -07INTRODUCTIONThe application of ice-slurry production in fishery, poultry and food processing industries has along history. A new emerging application is the ice generation, storage and processing for thermalstorage. All the above applications involve the generation of ice. Optimal design and operation ofice generators are important for enhancing energy efficiency, increasing ice-slurry productioncapacity, and reducing the cost of ice-slurry production.In some applications, space limitation and power requirements may make the use of ordinaryice generators unfeasible. The rotating-scraper ice-slurry generator is much smaller than theordinary ones. Analyzing the multiphase flow and heat transfer in rotating- scraper ice-slurrygenerators would reduce the investment and operating costs.Optimization of existing industrial apparatus requires knowledge of thermal-hydrauliccharacteristics inside ice-slurry generators, such as the rate of heat transfer with phase change andpressure drops with various ice fractions. Although some investigations have been carried out in thepast on pressure drop and ice- slurry flow characteristicsh-s, and the heat transfer characteristics bymelting ice'-s, the characteristics of ice-slurry flow and heat transfer in industrial ice-slurrygenerators are still poorly understood.Many companies and universities are doing related . 2中国煤化工”, but only a fewreports have been published'Gosman et al.4I propn (PISO-2P) and:YHCNMHG”Received date: 2003 -03- -25,Accepted date: 2003 -09 -29Foundation item: Supported by Key Lab. of Multiphase Reaction, IPE, CASBiography: HONG Ruo-yu( 1966 -), male, native of Suzhou city, Jiangsu Province, Ph. D.. Professor, chemical engineering.过程工程学报4卷developed an in-house CFD code. The turbulent flows in stirred vessels were simulated using thecode in that investigation. Fokema et al.' ! simulated fluid flow with strong rotation using acommercial code named FLOW3D. The computed results were compared with experimentalmeasurements. Xu et al.8 used the same CFD code to predict stirred tank flows, and the numericaldata were compared with LDA measurements. Similar numerical simulations were conducted byRanade et al.111Bakker et al.f12] using FLUENT code, and Djebbar et al.03] using FIDAP code.Recently, Sun et al.l4] conducted the 3-D numerical simulation of a stirred tank with an ASM(algebraic stress model) turbulence model. The model was embodied in their 3-D source code, andthe numerical results were verified by literature data.A new research facility was set up at Sunwell Technologies Inc., consisting of refrigerant(Freon R404A) and brine flow loops, and a rotating-scraper ice-slurry generator. The new facilitywas used to obtain heat transfer data in chilling and ice making modes. The brine- side flow and heattransfer characteristics were investigated both experimentally and numerically, and an inlet headerwas developed to distribute the refrigerant flow uniformly into the parallel cooling channels. Thispaper is concerned only with the brine-side behavior.2 EXPERIMENTAL2.1 Test Loop .The outlook of ice-slurry experimental facilityconstructed at Sunwell Technologies Inc. is shown inFig.1. The refrigeration capacity of the system is 19 kW,nominal production capacity (3.5% NaCl concentration, .and 0C makeup water temperature) is about 5 td. FreonFig. 1 Outlook of the ice-slurry generatorR404A is used as the refrigerant.2.2 Dimensions and Operating ConditionsThe dimensions of the ice-slury generator are listed in Table 1. The length of the brine flowchannel was 1.6 m, with anI.D. of 15 cm. The radius of the hollow rotating shaft was 1.9 cm andthe angel between wall and scraper was set at 60 degrees. The operating conditions of the ice-slurrygenerator are given in Table 2. At the inlet of brine, the temperature was 8°C (during the chillingmode) and the nominal velocity was 0.021 m/s (based on 15 cm I.D.). The shaft/scraper rotatingspeed was set at 0, 3, 6, or 10 r/s, respectively. The refrigerant temperature was assumed to beconstant everywhere at - _9°C and the heat transfer coefficient on the refrigerant side is about 7900W/(m2. K), which was calculated and provided by the Sunwell Technologies Inc.Table 1 Dimensions of the ice-slurry generatorLength (m)Length of outlet tube (em)0Inner diameter (cm)1Angle between wall and blade (°)50Shaft radius (cm)1.Wall thickness (between Freon and Brine)(mm) 3Outlet tube radius (cm)中国煤化工-Table 2 Typical operating conditionsMHCNM H G_Brine inlet temperature("C) 8(=-281.15 K)T Freon average temperature(U- y(=o4.15K)Brine inlet velocity (m/s) 0.332 (=6 g/min)| Rotating speed of scraper(r/s) 6 (=360 r/min)1期HONG Ruo-yu, et al : Experimental and Numerical Investigations of an Ice slurry Generator2.3 Experimental Procedure and ConditionsExperiments were performed during both thechilling and ice-making modes at different brinemass flow rates and cooling loads. Ice-slurygeneration under different conditions was conductedusing the test section. Typical flow conditions for theice -slurry experiments are shown in Table 2.2.4 Brine-side Temperature MeasurementsIn the present design, the ice scrapers inside thetubular ice generators are attached to a rotating shaft.The scrapers not only scratch ice crystals off therw ourinner wall of the generator, but also enhanceconvection heat transfer by mixing the brine. In orderFig.2 Sketch showing the structure of ice slurryto better understand the mixing effect, temperaturegeneratordistribution was measured inside the brine flow channel using thermistor arrays shown in Fig.2. Thescraper-rotating shaft assembly was equipped with 25 fast-response (~1 s in water) and high-accuracy (+0.1°C in flowing water) thermistors to measure the brine temperature at five axialpositions (A to E corresponding to 0.61, 0.91, 1.35, 1.83, and 2.23 m from the top) and four or fiveradial positions between the shaft and wall at each elevation. The transmission of 25 thermistorsignals from a rotating shaft to a data acquisition system required the use of an infrared transmitter.The brine temperatures obtained during thechilling mode of operation are shown in Fig.3. Ateach axial position, little variation is observed in theradial temperature distribution from the four radialpositions. Thus the fifth one was added, which isonly 1 mm away from the wall. When the refrigerantflow was stopped during the test between about 2102下Chiling modeST=10Kand 500 s, all the temperatures converged to the200400600800same value. An isothermal condition was reachedTime (s)inside the ice sury generator. With the refrigerant Fig.3 Brine temperature dstribution during a cllinlmode at a typical condition shown in Table 2flowing and cooling the brine, the axial temperatureprofiles obtained indicated lower heat transfer rates in the upper section compared to the lowersection of the ice generator. Such lower heat transfer phenomena are not clear and more measure-ments are needed to determine its cause. The overall heat transfer rate may be further improved bymodifying the scraper design and enhancing the radial velocity component and mixing of the brine.3 NUMERICAL SIMULATION中国煤化工MYHCNMHG3.1 Governing EquationsThe governing equations describing fluid flow in ice generator were listed in Table 3. The过程工程学报4卷mixture is assumed as incompressible Newtonian fluid since the ice concentration is relatively low.Thus the viscous stress tensor can be found easily in referencel4.14.Table 3 Governing equationsContinuity equation:div(p0)=0, .(1)U momentum equation:div(p0U)=-P+divr, +F,(2V momentum equation:div(p0V)- p"=_型+divr,-血+F,(3w momentum equation:div(pOW)+pW_1守+divrg +道+Fg,(4r a0k equation:div(pOk)=div| “gradk +(G, -pe), .(5(ar'E equation: .div(p0e)= div|其grade)+CG. C.pge).(6Note: The body forces, Fx, F, and Fo, include gravity, centrifugal and Coriolis terms, which arise only when a rotatingreference frame is used. Fx= Pg, Fr zp(o'2r+2wW), Fg= p( -2pN.3.2 Solution ProcedureIn order to further analyze the brine temperature distribution measured in the tubular ice-slurrygenerator as described above and to explore ways of improving the heat transfer rates, numericalmodeling and simulation of the brine flow in the tubular ice generator was conducted using acommercial CFD code, FLUENT version 5.3, which is based on control volume approach. Thesimulation involved modeling 3-D incompressible turbulent flow with strong rotation in complexgeometry. A steady flow was assumed in a rotating frame of reference, and a standard k- εturbulence model was used for turbulenceTOPmodeling. Other turbulence models werealso tried, it was found that the currentmodel was the most stable and gavesatisfactory results.3.3 Computational MeshScraperThe Gambit (version 1.3, Fluent Inc.)software was used to create unstructuredgrids of hexahedral mesh elements. Figure4(a) shows the surface meshes at the topof the ice generator, with two rotating-(a) Mesh showing top, inlet tube and (b) Mesh showing bottom and outletscraper blades attached to a rotating shaftscrapers of the ice-slurrytube of the ice- slury generatorat the center. Figure 4(b) shows theFig.4 Surface meshes .surface mesh at the bottom of the ice-slurry generator and the outlet tube.4 RESULTS AND DISCUSSION中国煤化工.MYHCNMHG4.1 Experimental Measurement vs. Numerical ResulsAll the cases modeled in this investigation were in the chilling mode. The physical properties1期HONG Ruo-yu, et al: Experimental and Numerical Investigations of an lce-slurry Generatorof water and ice at 0°C were used in theMeasured results from probes 1-4numerical computations.Figure 5 shows a comparison of numericalpredictions (solid line) with a measured axialtemperature profile (dashed line). The axialtemperature variation was predicted reasonablywell in the lower section, and some differenceMeasuredwas observed between the upper and lower0.0.5 1.0 1.5 2.0 2.5sections of the ice generator.Position from top (m)4.2 Influence of Rotation SpeedFig.5 Comparison of predicted and measured axial temp.profiles at a typical condition shown in Table 2The computed axial temperature profiles at 0and 3 r/s were shown in Fig.6(a) and Fig.6(b), respectively. The effect of rotating speed on the exitbrine temperature was significant at low rotation speeds (<3 r/s), but became reduced at higherrotation speeds. At a rotation speed of 6 r/s, the exit brine temperature was about 1.5 K lower thanthat at a rotation speed of 3 r/s.82 p280 foente280 10u0E278 7160200 278 hrere276 eoeaceines 1~4后2761a;274 E74GE信2742740:272 27a62270 feroau会268Line 5270 270or02660.51.C1.5264 L2.02.50.0Axial position (m)(a) Axial temperature profile along line 1 at 0 t/m(b) Axial temperature profiles along line 1~5at 3 t/mFig.6 Axial temperature profiles at 0 and 3 r/s [other conditions are shown in Table 2,lines 1~5 are vertically passing through points 1~5 in Fig.7(b)]中国煤化工(a) Overall velocity vectors in a cross-sectionFYHC N M H G across. sectionFig.7 Velocity vectors in a cross-section area (at a typical condition shown in Table 2)过程工程学报4卷At a rotating speed of 6 r/s, the computed velocity vectors of a cross section area in the middleof the ice- slurry generator are shown in Fig.7(a) and 7(b). The former shows the overall velocityvector in the cross -section, while the later shows the velocity vector near the scraper. It can be seenthat the flow pattern is very complicated in the rotating-scraper ice-slurry generator. The radialvelocity is relatively high, and it will be difficult to improve the heat transfer rate by increasing theradial flow.厄-1.0[H + Predicted信-1.8--■- PredictedMeasured苗-3各-2.0-2.20 110.000.04 0.080.12 0.16Inlet flow rate (g/min)Inlet ice fractionFig.8 Computed and measured exit temperatures at different Fig.9 Exit brine temperature vs. inlet ice concentrationflow rates (other conditions are shown in Table 2)(other conditions are shown in Table 2)4.3 Influence of Extra ScraperTo further improve heat transfer in the upper as well as the lower sections, extra mixingblades/scrapers near the shaft were added to the model to induce greater radial velocities in the brineflow field. A case with three scraper blades was also examined numerically for a rotation speed of180 r/min(=3 r/s). The results showed greater heat transfer rates, but further work is necessary to ;determine the optimum blade geometry and arrangement. The effect of rotating speed on the exitbrine temperature was significant at low rotation speeds (<3 r/s), but became reduced at higherrotation speeds. At a rotation speed of 6 r/s, the exit brine temperature was lower by 0.5 K if extrablades were used.4.4 Exit Brine Temperature vs. Inlet Flow RateFigure 8 shows the influence of inlet flow rate on exit brine temperature. The heat exchangerate increases with the increasing inlet flow rate. But the exit brine temperature increases as well.When the inlet flow rate is 6 g/min (=0.0227 m'/min), the exit brine temperature is about -2. 1°C,which was verified by experimental results. Further increase of the inlet flow rate will result in adecrease of ice productivity. Thus the inlet brine flow rate should be lower than 6 g/min.4.5 Exit Brine Temperature vs. Inlet Ice FractionThe viscosity of ice-slurry is influenced by ice fraction. Recently, the Danish TechnologicalInstitute that worked closely with the authors provided a better estimate to calculate the mixtureviscosity: μ=μ6(1+4.5cicc). Figure 9 shows the influ中国煤化工”on exit brinetemperature. When the ice is excessive in slurry, the mi:CYHcNMHchigh.whichwillresult in a low heat transfer between the ice-slury and tne cyinaer wall. 1 ne predicted result wasverified by experiments.1期HONG Ruo-yu, et al : Experimental and Numerical Investigations of an Ice slurry Generator75 CONCLUSIONSA new ice-slurry research facility with 150 cm I.D. has been developed that enables theinvestigation of ice-slurry flow and heat transfer characteristics in a rotating-scraper ice-slurrygenerator. The brine and refrigerant side flow loops have been completed and the ice-slurrygenerator test section was constructed with instrumentation for measuring brine temperature.Commissioning tests were conducted to collect temperature and heat transfer data. Axial and radialtemperature distributions were measured and numerically predicted. An apparently lower heattransfer rate from measurements was found in the upper section of the ice generator compared tothat in the lower section. The radial velocity around the scrapers was relatively high. An improveddesign with three scrapers was recommended to improve the heat transfer rate in the entire icegenerator.NOTATIONS:ice Ice fractionRadial direction (m)Gravity vector (m/s')Time ()Fluid average density (kg/m)Turbulent kinetic energy (m?/s)Velocity vector, =(U,V,W) (m/s)Angular velocity (r/s)Fluid pressure (Pa)xy2Spatial directions (m)Viscous stress tensor (N/m*)Gnd Peclet numberTime step (s)θ Azimuthal direction (m)Radius of the cylinder (m)Fluid average viscsity (Ns/m)REFERENCES:[1] Gupta R P Fraser C A. Effect of a New Friction Reducing Additive on Sunwell Ice Slurry Characteristics [R]. Report No.TR- LT- -023, NRC, No.32123. National Research Council of Canada, Institute of Mechanical Engineering, Low TemperatureLaboratory, 1990.[2] Bellas J, Chaer I, Tassou S A. Heat Transfer and Pressure Drop of Ice Slurries in Plate Heat Exchangers []. Appl. Therm. Eng.,2002, 22(7): 721-732.[3] Knodel B D, France D M, Choi U S, et al. Heat Transfer and Pressure Drop in Ice- water Slurries [J]. Appl. Therm. Eng.. 2000,20(7): 671- -685.[4] Gosman A D, Lekakou C, Politis S, et al. Multidimensional Modeling of Turbulent Two-phase Flows in Stirred Vessels [J]AIChE. J., 1992, 38(12): 1946 -1956.[5] Kitanovski A, Poredos A. Concentration Distribution and Viscosity of Ice slurry in Heterogencous Flow田Int. J. Refriger.,2002, 25: 827-835.[6] Ismail K A R, Radwan M M. Modeling of Ice Crystal Growth in Laminar Falling Films for the Production of Pumpable IceSlurries [J]. Energ. Convers. Manage, 2002, 44: 65 -84.[7] Fokema M D, Kresta S M, Wood P E. Importance of Using the Correct Impeller Boundary Conditions for CFD Simulations ofStirred Tanks [小. Can. J. Chem. Eng.. 1994, 72: 177-183.[8] Xu Y, McGrath G. CFD Predictions of Strred Tank Flows [J]. Trans. IChemE., 1996, 74(Part A): 471-475.[9] Ranade V V, Dommeti S M S. Computational Snapshot of Flow Generated by Axial Impellers in Baffled Strred Vessels [J].Trans. IChemE.. 1996, 74(Part A): 476- 484.[10] Tanguy P A, Bertrand F, Labrie R, et al. Numerical Modeling of the Mixing of Viscoplistic Slurries in a Twin-blade PlanetaryMixer [J]. Trans. IChemE., 1996, 74(Part A) 499- -503.[11] Morud K E, Hjertager B H. LDA Measurements and CFD Modeling of Gas- Liquid Flow in a Strred Vessel []. Chem. Eng,Sci, 1996, 51(2): 233-249.[12] Bakker A, Myers K J, Ward R W, et al. The Laminar and Turbulent Flow Patterm of a Pitched Blade Turbine [J]. Trans.IChemE., 1996, 74(Part A): 485- 491[13] Djebbar R, Roustan M, Line A. Numerical Computation of Turbuler中国煤化Ianical Agitated Vessels[]. Trans. IChemE, 1996, 74(Part A): 492- -498.:YCHCN MHG[14] SunH Y, Wang W J, Mao Z s. Numerical Simulation of the Whole Tre-nusonun riow m a sueu Tank with AnisotropicAlgebraic Stress Model [J] Chinese J. Chem. Eng., 2002, 10(1): 15-24.

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