Two Phase Flow Analysis in Electro-Chemical Machining using CFD

Transcription

1 Two Phase Flow Analysis in Electro-Chemical Machining sing CFD 1 Usharani Rath, 2 Chandan Kmar Biswas 1,2 Department of Mechanical Engineering, National Institte of Technology, Rorkela, , India sha.rrath@gmail.com 1, ckbiswas@nitrkl.ac.in 2 Abstract - Electrochemical machining (ECM) is a nonconventional machining process based on the principle of reverse electroplating. Some genine characteristics like negligible tool wear, high precision machining in difficlt to ct materials, lower thermal and mechanical stress on work piece etc. makes ECM advantageos over other non-conventional machining processes. Still there are some challenges in ECM like generation of hydrogen bbbles and its effect on Material Removal Rate (MRR), complexity of tool geometry and its effect on varios process parameters, prediction of electrolyte flow pattern and its impact etc. which have been investigated by varios researchers. In the present stdy, three dimensional two phase flow pattern of ECM process has been simlated sing Comptational Flid Dynamics (CFD). The software sed for the analysis is ANSYS 13.0 CFX. Varios process parameters like volme fraction profile, velocity profile, trblent pattern of electrolyte flow in the Inter Electrode Gap (IEG) etc. have been evalated from the simlated environment. The reslts indicated hydrogen bbbles generation which in trn redces the volme fraction of brine. Redced brine volme fraction decreases MRR and may also affect the srface finish. Besides the temperatre at the bondaries are increased sbstantially de to redced convective heat transfer as hydrogen bbbles posses sbstantially low convective heat transfer coefficient. Keywords: comptational flid dynamics; convective heat transfer; electrochemical machining; electroplating; electrolyte; two phase flow etc. I. INTRODUCTION Analysis of single phase flow in Electro Chemical Machining process has been long evalated by varios researchers. However, it is an established fact that real ECM processes are mltiphasic. A two phase assmption in ECM comprising of brine as the primary phase along with formation of hydrogen bbbles as the secondary phase and the sbseqent analysis may provide a real insight into the complex ECM process. As discssed above, the secondary phase consists of hydrogen bbbles which are generated becase of the reaction taking place at cathode. More amont of hydrogen bbbles are generated if the electrolyte is sbjected to boiling becase of the high heat generated in the IEG as per the stdy carried ot by Ratkovich, Chan, Berbeand and Nopens [1]. This secondary phase affects varios process parameters like crrent density distribtion, volme fraction of brine, velocity pattern of brine etc. Therefore, it is highly desirable to stdy the second phase and its effect on overall machining process in order to accrately design the tool shape and predict the MRR. Good range of research has been carried ot so far on mathematical modelling and simlation of ECM processes. A brief review of existing literatre is carried ot. Evgeny I. Filatov [2] simlated the electrolyte in the IEG as a two phase continos mixtre of an incompressible flid and gas. He sed Finite Difference Method (FDM) to solve the system of nsteady conservation eqations for a two dimensional domain and predicted the inflence of nsteady technological factors on the accracy of machining process. Mont, Clifton, Howarth and Sherlock [3] also sed the FDM to simlate the ECM process thereby simlating the crrent transients and the configrations of tool and workpiece at any time for non-planar configrations. This approach has been shown to increase the accracy of both the material characterization and process simlation. Pattavanitch, Hindja and Atkinson [4] proposed the development and application of Bondary Element Method (BEM) to model the electrochemical machining of simple milling and trning featres. This simlation sed analytical integration rather than nmerical to calclate the element matrices. Bortels, Prcar, Bossche and Deconinck [5] sed a combination of 3D BEM for the potential distribtion in the electrolyte with a 2D Finite Element Method (FEM) for the ohmic potential drop in the electrode(s).this combined software was flly integrated with the CAD package solid works and enables to perform the ECM design by directly reading the existing CAD file. Prcar, Bortels, Bossche and Deconinck [6] proposed a nmerical method for the simlation of 3D electrode shape changes obtained in ECM process based on marker method. The reslts related to the 3D electrochemical machining of the letter E was presented. J.Kozak [7] developed the process modle in the Compter Aided Engineering system for ECM (CAE-ECM) on the basis of the physical and mathematical models of ECM. Kozak, Chchro, Rszaj and Karbowski [8] developed mathematical modelling of ECM with niversal spherical electrodes (ECM -CNC) and process analysis was done by sing artificial neral network. abib, Keasberry, Atkinson and Frost [9] proposed the development of a fzzy logic controller to add intelligence to the ECM process. Ryo Tsboi and Yamamoto [10] modelled the ECM process by copling between the gas and liqid phase in electrolyte and developed their own nmerical code. On the basis of the exhastive literatre review it was established that two phase analysis of ECM involving electrode shape change and varios technological factors inflencing the shape change may be comprehensively stdied with the help of mathematical or comptational models with a close proximity with real world processes. II. MATERIAS AND METHODOOGY A finite volme based commercial CFD software package, ANSYS 13.0 CFX, which operated nder the Windows VISTA 32-bit operating system was sed for the simlation Volme 3, Isse ICRASE13, May 2013, ISSN Online:

2 stdy. The physical model for the present stdy consists of a circlar work-piece and an -shaped tool with a central groove throgh which the electrolyte is pmped in. Key dimensional featres of the physical model are provided in Table I. The model was generated sing the design modeller modle of ANSYS 13.0 CFX as shown in Fig.1 and Fig.2. Generation of cell mesh is adeqate for the proposed two phase three dimensional simlation. For grid independence stdy three different types of mesh resoltions were investigated. The otcomes of the grid independence stdy are shown in Table II. The best reslts in terms of consistency and validation was obtained sing the fine resoltion mesh of cell. However, the reqired timing of simlation for the finer meshes was fond to be 35% higher than intermediate mesh and 190% higher than the coarse mesh. Materials sed are copper for tool, iron for work piece and brine (20% NaCl) as the electrolyte soltion. Varios physico-chemical and flid properties of these materials are provided in Table III [11,12]. III. GOVERNING EQUATIONS FOR TWO PHASE FOW The continity and Navier stokes eqation for gas phase (hydrogen bbbles) is given by the eqations (1) and (2) respectively. 2 ( αg ρg ) +.( αg ρgg ) = Г + CG αgρg (1) t 2 ( αgρgg ).( αg ρgg ) αg P αg DG G (2) t + = + The continity and Navier stokes eqation for the liqid phase (brine) is given by the eqations (3) and (4) respectively 2 ( α ρ ) +.( α ρ ) = Г CG αg ρ t G 2 ( α ρ ).( α ρ ) α P α µ t + = + Considering the gas phase velocity eqal to liqid phase (Homogeneos flow) characterised by = = ( 5) mix G Smming eqations (1) and (3) & eqations (2) and (5), eqations (6) and (7) are obtained [10]. ρmix +.( ρmixmix ) = 0 (6) t 2 ( ρmixmix ) + mix.( ρ mixmix ) = P + µ mix mix (7) t ρ = α ρ + α ρ (8) mix G G µ = α D + α µ mix G G IV. PROCEDURE The assmptions made in this analysis are 1. The IEG remains constant. 2. Material properties of the tool and work-piece never change throghot the analysis. 3. Tool and work-piece materials are homogenos. 4. Heat generation is only de to Jole s heating. 5. Material removal depends only on the crrent density. 6. Electrolyte is considered to be a two phase mixtre of an incompressible flid and gas. 7. Diameter of hydrogen bbbles prodced is taken as 30µm niversally. 8. Velocities of the two phases in the electrolyte are eqal (homogenos model). 9. For the simplicity of the process, properties of air are considered instead of hydrogen. (9) (3) ( 4) The bondary condition at the inlet velocity is given as 40 m/s whereas the temperatre remains constant at 298ᴼ K at inlet. The otlet bondary condition is the atmospheric pressre at Pa. The heat generated in the IEG de to Jole s effect is given by the eqation (10) Q = I 2 R e (10) The condctivity of the electrolyte in the IEG varies with the temperatre according to the eqation (11) K=K 0 (1+α(T-T a )) (11) There is convective heat transfer from the sides of the tool and work piece those are directly in the contact with atmosphere. Varios flid models considered for the simlation stdy are smmarized in Table IV. For the simlation, ato time scale is given with maximm of 30 iterations and for the convergence criteria, residal target of 1.0E-4 and conservation target of 0.01 is given while setting the volme fraction copling for the two phase flow. There are variations in the mass and momentm, heat transfer, trblence and volme fraction dring the iterations as given in the Fig.3. V. RESUTS Volme fraction pattern of brine in the IEG for a standard 40m/s inlet electrolyte velocity is shown in Fig.4. It depicts that the brine volme fraction is continosly redced from the otlet point of the groove towards the bondaries of the contact area. The redction in volme fraction of brine has the potential to redce MRR and may affect the heat transfer rates. The same is consistent with the temperatre contor as shown in Fig.5 which shows that, highest temperatre occrs near the otlet of the groove and it goes on decreasing otwards. This may be attribted to the increase in hydrogen volme fraction. As hydrogen has very low convective heat transfer coefficient than brine soltion, so transfer of heat is inhibited reslting in temperatre rise. The intensity of bbble formation and its effect on srface finish is another grey area in the domain of ECM which shold be investigated in frther research. The static pressre contor and velocity contors are shown in Fig.6 and Fig.7 respectively. It is crystal clear that velocity of two phase electrolyte is increased from the groove to the bondary partly de to redction in area of flow and partly de to formation of hydrogen bbbles reslting in more trblence. Ideally the enhanced velocity shold increase the pressre drop and a redced static pressre contor towards the bondary shold be obtained. However, the pressre contor shows nmeros ncles high pressre zones towards the bondary despite of enhanced velocity. This is de to the cavitation phenomena occrred there with increased bbbling of hydrogen gas. Finally Fig.8 shows the contor for interphase mass transfer. It may be seen that for the existing tool geometry and a fixed electrolyte flow rate, increased amont of blk interphase mass transfer rate reslting in hydrogen bbbles is obtained away from the groove towards the bondary in closely spaced ncles sites. VI. CONCUSION It is evident from the present investigation that ECM flow is trly polyphasic and the hydrogen phase has significant impact on all ECM process parameters. From this stdy, it is Volme 3, Isse ICRASE13, May 2013, ISSN Online:

3 depicted that formation of hydrogen bbbles is increased from the groove towards the oter bondary which is validated from the increased volme fraction contor of hydrogen, increased temperatre contor and increased static pressre contor. It is inferred that redced mass fraction of brine and redced heat transfer shold have significant impact on MRR and srface finish of ECM; however, the same is not evalated empirically and left for sbseqent research. The present investigation sggests an excellent comptational model for simlating the sites and the rate of hydrogen bbble formation, model validation along with its effect on varios ECM process parameters. α G = Gas volme fraction. NOMENCATURES = Velocity of liqid phase. ρ G = Density of gas phase. G = Velocity of gas phase. µ = Viscosity coefficient of liqid phase. ρ mix = Density of combined phase. CG = Diffsion coefficient (sed for eqation of mass conservation). DG = Diffsion coefficient (sed for eqation of momentm). α = iqid volme fraction. Γ= Sorce of mass per nit volme depending on the phase change on the cathode. TABES TABE I: EEMENT TYPE mix = Velocity of combined phase. ρ = Density of liqid phase. P = Pressre. Sl No. Components Material Physical State Shape Dimension (mm) 1 Tool Copper Solid Cylindrical 60 dia. X 20 ht. 2 Work piece Iron Solid Shaped Arm 30 X Width 15 X Height 50 X Central hole 3Ø 3 Medim Brine Flid - - TABE II: GRID INDEPENDENCE STUDY Sl No. Mesh Resoltion Fine Intermediate Coarse 1 Interval Size (mm) Nmber of cells 23,92,617 16,81,790 3,48,861 TABE III: MATERIA PROPERTIES Sl No Properties Brine Copper Iron Air 1 Molar mass (Kg/kmol) Density (Kg/m 3 ) Specific heat (J/Kg K) Dynamic viscosity (Pas) Thermal condctivity (W/mK) Electrical condctivity (S/m) E+07 1E+07-7 Convection coefficient (W/m 2 K) TABE IV: SUB-MODES FOR SIMUATION STUDY Sl No. Sb-model Type 1 Trblence model. RNG k-ε 2 Mltiphase flow Homogeneos 3 Interphase transfer Particle model 4 Mass transfer Cavitation model Volme 3, Isse ICRASE13, May 2013, ISSN Online:

4 FIGURES Figre 1. Geometric model. Figre 2. Meshed model. Figre 3. Properties dring iteration. Figre 4. Volme fraction contor of brine. Figre 5. Temperatre. Contor. Figre 6. Static pressre contor. Figre 7. Velocity contor. Figre 8. Interphase mass transfer. ACKNOWEDGMENTS The athor carried ot her comptational and prereqisite experimental work at the department of Mechanical Engineering, Machining and Comptational facilities ab, National Institte of Technology, Rorkela, Odisha, , India. REFERENCES [1] N. Ratkovich, C.C.V Chan., P.R. Berbe, I. Nopens Experimental stdy and CFD modelling of a two-phase slg flow for an airlift tblar membrane. Chemical Engineering Science 64(2009) [2] E.I Filatov. The nmerical simlation of the nsteady ECM process. Jornal of Materials Processing Technology. 109 (2001) [3] A.R Mont, D. Clifton, P. Howarth, A. Sherlock. An integrated strategy for materials characterisation and process simlation in electrochemical machining. Jornal of Materials Processing Technology 138 (2003) [4] J. Pattavanitch, S. Hindja, J. Atkinson. Modelling of the electrochemical machining process by the bondary element method. CIRP Annals - Manfactring Technology 59 (2010) Volme 3, Isse ICRASE13, May 2013, ISSN Online:

5 [5]. Bortels, M. Prcar, B.V.D Bossche, J. A Deconinck. Userfriendly simlation software tool for 3D ECM. Jornal of Materials Processing Technology 149 (2004) [6] M. Prcar,. Bortels, B.V.D Bossche, J. Deconinck. 3D Electrochemical machining compter simlations. Jornal of Materials Processing Technology 149 (2004) [7] J.Kozak. Mathematical models for compter simlation of electrochemical machining processes. Jornal of Materials Processing Technology. 76 (1998) [8] J. Kozak, M. Chchro, A. Rszaj, K. Karbowski. The compter aided simlation of electrochemical process with niversal spherical electrodes when machining sclptred srfaces. Jornal of Materials Processing Technology 107 (2000) [9] A.W abib, V.J Keasberry, J Atkinson, H.W. Frost. Towards next generation electrochemical machining controllers: A fzzy logic control approach to ECM. Expert Systems with 38 (2011) [10] R. Tsboi, M. Yamamoto. Modeling and applications of Electrochemical Machining process. Proceedings of the ASME 2009 International Mechanical Engineering Congress & Exposition IMECE 2009 November 13-19, ake Bena Vista, Florida, USA. Paper No- IMECE [11] R.B. Mcclesky, (2011). Electrical Condctivity of Electrolytes Fond In Natral Waters from (5 to 90) C. Jornal of Chemical and Engineering data, 56, [12] S. Sian. CFD analysis of flow pattern in electrochemical machining. B.Tech. Project Report (2011), National Institte of Technology Rorkela, Odisha, India. Athors Usharani Rath, M.Tech (Prodction Engineering), is a postgradate stdent in the Department of Mechanical Engineering, National Institte of Technology, Rorkela, Odisha, India. She completed her ndergradate Mechanical Engineering degree from Bijpattnaik University of Technology, Rorkela, Odisha in Presently she is contining her major thesis nder the spervision of Dr. Chandan Kmar Biswas, Associate Professor, NIT, Rorkela. Her research interests inclde modelling of non conventional machining processes sing comptational flid dynamics and experimental validation. Chandan Kmar Biswas, PhD, is crrently an Associate Professor in the department of Mechanical Engineering, National Institte of Technology, Rorkela, Odisha, India. His area of research expertise incldes non conventional marching processes, mathematical modelling and simlation. Volme 3, Isse ICRASE13, May 2013, ISSN Online: