Simulation of Evaporator for Two-phase Flow in the New Plate-fin Desalination Unit

Transcription

1 Research Journa of Appied Sciences, Engineering and echnoogy 5(13): , 2013 ISSN: ; e-issn: Maxwe Scientific Organization, 2013 Submitted: Juy 27, 2012 Accepted: September 17, 2012 Pubished: Apri 15, 2013 Simuation of Eaporator for wo-phase Fow in the New Pate-fin Desaination Unit 1, 2 Shu Xu and 1 Xiang ing 1 Nanjing Uniersity of echnoogy, Nanjing, , China 2 Department of Mechanica Engineering, Huaihai Institute of echnoogy, Lianyungang, China Abstract: In this study a new desaination unit is estabished. It has four ces such as cooing ce, heating ce, eaporation ce and condenion ce. Seawater is pumped into cooing ce to be preheated and then goes to eaporation ce. In the new desaination unit the eaporation and condenion ce is heated and cooed by the heating and cooing ces respectiey. he heating of the eaporation ce is ensured by hot water fowing upward aong heating ces. he cooing of the condenion ce is ensured by seawater in cooing ce. Fuent 6.3 is used to simuate gas-iquid two-phase fow of boiing eaporation numericay. A simuation cacuation to get fuid in a new desaination unit under the infuence of the fow, pressure distribution and heat transfer performance of the eaporator. Keywords: Desaination, eaporation, unit simuation INRODUCION he ack of potabe water poses a big probem in arid regions of the word where freshwater is becoming ery scarce and expensie. Shortage of fresh water is a major probem affecting many countries. Cean drinking water is one of the most important internationa heath issues today. Desaination is one of mankind s eariest forms of water treatment and it is sti a popuar treatment. Desaination uses a arge amount of energy to remoe a portion of pure water from a sat water source. Sat water is fed into the process and the resut is one output stream of pure water and another of wastewater with a high sat concentration. A number of seawater desaination technoogies hae been deeoped during the ast seera decades to augment the suppy of water in arid regions of the word. Due to the constraints of high desaination costs, many countries are unabe to afford these technoogies as a fresh water resource. Howeer, the steady increasing usage A ariety of desaination technoogies has been deeoped oer the years on the basis of therma distiation, membrane separation, freezing, eectro-diaysis, etc. (Akii et a., 2008). J.R. Lara (Lara et a., 2008) practice to operate mechanica aporcompression (MVC) seawater desaination systems at temperatures ower than 80. his study presents the detai engineering and economics of a MVC system. Luopeng Yang (Luopeng and Shengqiang, 2007) presents an economic anaysis of the energy cost of water production for dua-purpose pants by the Equiaent Enthapy Drop heory (EED) and the offdesign anaytica theory on steam turbines. hin iquid fims faing under the infuence of graity aong ertica or incined soid surface are encountered in a wide range of desaination industria process equipments which incude wetted-wa absorbers, faing fim reactors, condensers and ertica tube eaporators. Lee and Kim (Lee and Kim, 2008) deeoped a theoretica mode by improing Siddique s anaytica mode and inestigated steam condenion heat transfer in the presence of air or nitrogen gas in a ertica tube. heir resuts showed that the effects of the noncondensabe gas became weak as the condenser tube diameter decreased because of interfacia shear stress. Aso, Lee and Kim (Lee and Kim, 2010) deeoped four simpe modes using empirica correation. hey compared the theoretica mode and simpe modes with arious data obtained from the cooing jacket method and showed that the predictions of the theoretica mode and the simpe mode based on Lee and Kim s correation was in good agreement with the experimenta resuts. In these cacuations, the inner wa temperature profies were gien as boundary conditions. Zhang Ming (Zhang, 2009) designed noe grooed apor chamber.he grooed structure of the apor chamber can improe its axia and radia heat transfer and aso can form the capiary oop between condenion and eaporation surfaces. he effect of heat fux, fiing amount and graity to the performance of this apor chamber is studiedy experiment. Costa (Costa and Enrique, 2002), Prost, Aam and Gupta (Prost et a., 2006) and Miranda and Chen (Chen and Zhiming, 2004). hese modes are generay based on a set of inear or non-inear equations and can Corresponding Author: Shu Xu, Department of Mechanica Engineering, Huaihai Institute of echnoogy, Lianyungang, China. 3554

2 Res. J. App. Sci. Eng. echno., 5(13): , 2013 Fig. 1: Front side of integrated pate-fin desaination unit Fig. 2: Interna structure of eaporator and condense in pate-fin desaination unit accommodate effects of arying physica properties of fim eaporator and try to find an optimum eaporator apor/steam and iquor with change in temperature and performance from size point of iew, in other words, to concentration. Oh and Reankar deeoped a pure find an optimum oumetric cooing rate for a certain steam condenion mode and an empiricism-free or distance between the ertica pates. Moreoer this minimum empirica mechanistic mode for the study presents an attempt to improe upon the modes apor/noncondensabe mixture condenion. In the described earier and eiminate some of their mode for the mixture, they used genera momentum, imitations. heat and mass transport reations deried using an anaytic method and considered the surface suction DESCRIPION OF HE DESALINAION UNI effect. In a of the aboe mentioned references the apor he integrated pate-fin desaination unit is a pressure drop and apor eocity were not considered in patented distiation unit and it has two ayers (Fig. 1). the mathematica formuation. he objectie of this Fin, coer pate, capboard and sea are brazed into a study is to study the performance characteristics of a pate-fin. Its materia is stainess stee to preent 3555

3 Res. J. App. Sci. Eng. echno., 5(13): , 2013 heir major surface geometries in Fig. 4 are described by the fin height h f, fin space sf, fin thickness δf, fin ength L f. he fins are uniformy offset by haf the fin spacing and designed to be of equa size and shape to maintain the geometric periodicity. For the present study, the numerica serrated fin scheme is chosen in abe 1. PLAE-FIN DESALINAION UNI MODEL Fig. 3: Back Front side of integrated pate-fin desaination unit Boiing heat transfer: Boiing heat transfer of gas iquid two phase fow and phase change heat transfer is two kinds of compicated physica phenomena couped, in iew of the probems of heat transfer is affected by many factors and is compex, such as boiing heat transfer and heating surface, heat transfer path configuration, system pressure, temperature distribution, boiing medium physica properties; at the same time phase change probems inoing two-phase fow, in its fow patterns and phase interface are difficut to identify, so far, it has not yet been fuy understand the boiing heat transfer, aso did not estabish a mature mode to describe the boiing heat transfer parameters in the reationship between the. According to the experimenta boiing cure reationship, poo boiing heat transfer characteristics and can be expressed as the exponentia reationship between: Fig. 4: Serrated fins abe 1: Dimension of the serrated fins h f (mm) s f (mm) δ f (mm) L f (mm) corrosion and its size is 1m (Height) 0.5m (Width) 3cm (hickness). he Fig. 2 is the front side of integrated pate-fin desaination unit. he aboe two nozzes are seawater inet and non-condensabe gas extraction outet. he ower two nozzes are for brine outet and distied water outet. Remoing pate, we can see interna structure of eaporator and condenser (Fig. 3). In the eaporator, serrated fins are used and in horizonta mode.he seawater to be eaporated fows as a faing fim aong fins heated by hot water in the heating ce. Some soped hoes are machined in the strip sea to ensure steam go into condenser thus reducing therma osses and compex piping. Perforated fin are used and aso in horizonta mode. he cooing of the condenser is ensured by cod seawater in the cooing ce n w s = ψq (1) n in ype is 0.25 ~ 0.5. When the heat of formation of sma bubbes continues to be heated up to a certain size, the bubbe from the heating surface boundary ayer formed iquid phase perturbations and cod, heat medium of exchange to form a poo boiing. Bubbe departure diameter: D b σ = (2) ρ ρ g According to the recommendations of the Rohsenow, boiing heat transfer mathematica description is expressed as: Nu c p b = f ( ) w λ s ( Re ', Pr ) b q σ = C λµ g ρ 0.5 m ( Pr ) K (3) (4)

4 VOF mode: Euer Lagrange and Euer method are the current study of muti-phase fow method. In the Euerian method, different phase to each other throughout the continue treatment. Due to a phase took contro unit body space cannot be other possession, it is introduced the concept of oume fractions. he oume fraction of time and space of continuous functions, each phase oume fraction and. In the Fuent software proides three Euerian mutiphase mode, namey VOF (Voume of Fuid) mode, hybrid (Mixture) mode and Euer (EuerIan) mode. he VOF mode is obtained by soing the separate momentum equation and processing through the region of each fuid oume fraction to the simuation of mutiphase fuid. ypica appication: jet, perfusion, shaking, the gas-iquid interface of the steady and transient process, fuid bubbes moement. In the VOF mode, interface tracking is achieed by soing with the oume fraction of the continuity equation to compete and arbitrary contro in a phase oume fraction. he I phase continuity equation: Res. J. App. Sci. Eng. echno., 5(13): , 2013 n α S.. i α 1 i + i α i = + m ji mij (5) t ρ ρ i i i= 1 Momentum conseration equation: ( ) + ( ρ) = ρ + [ µ ( + )] + ρg + F ρ (6) t Energy conseration equation: t ( ρ ) + ( ρe + ρ) ( ) = ( keff ) + S h E (7) ype is j phase and I phase mass transfer, is the I phase to j phase mass transfer, property and (effectie heat transfer rate) is the sharing, source phase incudes the radiation, the oumetric heat source. In Fuent, the contro oume formua for soing is to draw phase interface of conectie and diffusie fux, but aso to ensure the contro oume of the interna source phase equiibrium. For the VOF mode, Fuent proides four soutions: geometry reconstruction, materia acceptance, Euer expicit and impicit. Geometry reconstruction: he geometric recons truction scheme, using the standard interpoation method to obtain interface fux and by piecewise inear representation between the phase interface. Fuent this program is the most accurate and suitabe for genera unstructured grid In this scheme, two phase fow soution are dependent on time. he first step is to cacuate the reatie to each section with unit inear interface ocation, based on the oume fraction and the unit eads out the information. he second step is to cacuate the fuid through each ee of fow, using the cacuated inear interface description and on the surface of norma and tangentia eocity distribution information. he third step is to use the preceding steps in computation of fux baance is cacuated for each unit of oume fraction. he materia receiing scheme: In the materia receiing scheme, Fuent uses standard interpoation method for soing interface fux. In this scheme, the interface is considered as a substance between the adjacent two units the donation and acceptance reation. Materia receiing scheme for soing time dependent and ony in the quadriatera and hexahedra meshes are used in. Euer expicit scheme: he Euer expicit scheme, Fuent uses standard finite difference method on time step unit oume fraction interpoation processing. he Euer expicit scheme ony depends on time for soing. Impicit scheme: In the impicit scheme, Fuent uses standard finite difference method for computing domains of a the eements of the surface fuxes, incuding near the interface unit. Impicit schemes can be used to soing the timedependent and steady state probem. UDF subroutine: he refrigerant is heated from the iquid phase to a gas phase process; two phase mass transfer phenomenon exists. In the Fuent software of muti phase fow mode assumes that the phase constant quaity principes, so essentia in two-phase fow properties increase describe eaporation during the phase transition process of mass transfer of the UDF subroutine. Li Wen he and mass transfer mode is adopted, which assumes that mass transfer is carried under the at constant pressure quasi-static heat baance condition of. Mathematica description is as foows:. m rα ρ =. rα ρ m = ( ) 0 ( ) 0,, <,, > (8) (9)

5 Res. J. App. Sci. Eng. echno., 5(13): , 2013 Fig. 5: Grid of the cross section BOILING SAE Fig. 6: Boiing simuation Meshing: Mesh is generated in software GAMBI 2.3. In this study the hybrid grid is used, as shown in Fig. 5.he simuations are carried out in a portion of pate-fin therma storage. A three-dimensiona computationa domain is empoyed and diided into 555, 520 cubic ces. he bue part is a iquid; the red part is apor in Fig. 6. Howeer the fow eocity is ow, with a arge bubbe aggregates such as square arrangement mode, before and after the tube between the areas. Figure 6 described the seawater in the desaination unit wa is heated to eaporate the bubbe formation process. At the same time away from the wa surface iquid ow temperature and wa temperature of iquid form temperature difference, the two-phase fuid to produce conection fow. Because of the buoyancy and fuid conection took effect, the bubbe departure wa into the fuid passageway; aong with the heating time increased, surface air bubbes that can grow quicky and by the high temperature iquid conection taking effect is more apparent, so that air bubbes aong the fuid toward the deeopment of eaporation. he eaporator side fow constant amounts of bubbe formation, growth and detachment, thereby reaizing the boiing heat transfer. CONCLUSION he thickness of the iquid fim in the fim is mainy infuenced by pressure of apor and iquid beside iquid-apor interface. At the apor-iquid interface, then magnitudes of ariations of the apor and iquid pressure and the pressure differences 3558

6 Res. J. App. Sci. Eng. echno., 5(13): , 2013 between apor and iquid increase with the heat fux. he iquid fim in heat source region is ery thin and this can enhance the performance of apor chamber. But if the starting point of the iquid fim out of the heat source region, the apor chamber may dry out. he heat fux spreads aong with the fow of apor. he apor condenes on whoe condenion surface, so that the condenion surface achiees a ery uniform temperature distribution. he optima fiing ratio shoud maintain a steady thin iquid fim in heat source region of the apor chamber. he agreement between the experimenta resuts and numerica simuation resuts erifies. ACKNOWLEDGMEN his study is supported by the study fund of HI at Weihai (No.IMVQ and IMJQ ). REFERENCES Akii, D.K., K.K. Ibrahim and W. Jong-Mihn, Adances in seawater desaination technoogies. Desaination, 221: Chen, F.C. and G. Zhiming, An anaysis of back iquor faing fim eaporation. Int. J. Heat Mass rans., 47: Costa, A.O.S. and E.L. Enrique, Modeing of an industria mutipe effect eaporator system. In Proceedings of the 35th Annua Congress and Exhibition deceuose and Paper (Purtuga), pp: Lara, J.R., G. Noyes and M.. Hotzappe, An inestigation of high operating temperatures in mechanica apor-compre. Desaination. 227: Luopeng, Y. and S. Shengqiang, Assessment of energy requirement for water production at duapurpose pants in China. Desaination, 205: Lee, K.Y. and M.H. Kim, Anaysis of steam condenion heat transfer with a non-condensabe as in a ertica condenser tube. Nuc. echno., 163: Lee, K.Y. and M. Kim, Experimenta and empirica study of steam condenion heat transfer with a noncondensabe gas in a sma-diameter ertica tube. Nuc. Eng. Desaination., 238: : Prost, J.S., M.. Gonzaez and M.J. Urbicain, Determination and correation of heat transfer coefficients in a faing fim eaporator. J. Food. Eng., 73: Zhang, M., he experimenta and numerica inestigation of a grooed apor chamber. App.herm. Eng., 29: