SCRAPED SURFACE HEAT EXCHANGER (SSHE)

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1 Intenational Jounal of Engineeing Reseach and Geneal Science Volume 4, Issue 6,Novembe-Decembe, 2016 SCRAPED SURFACE HEAT EXCHANGER (SSHE) Rajesh S C, D. B. Sadashive Gowda, D. G.S.V.L. Naasimham VVPIET Solapu, ajesh.sc044@gmail.com Abstact: This pape deals with Computational Fluid Dynamic (CFD) analysis of 3D model of an SSHE is developed in ode to study the fluid flow and heat tansfe with a steady state lamina, non-isothemal flow of pue glyceine, which can be teated as a newtonian fluid. The vaiation of the local heat tansfe coefficient based on the inne wall tempeatue and bulk fluid tempeatue as a function of the main pocess paametes, namely, otational Reynolds numbe (Re ot ), axial Reynolds numbe (Re axi ) and dimensionless cleaance, is obtained. The esults have shown that viscous dissipation has a significant effect on the cooling of glyceine. The local heat tansfe coefficient inceases slightly when the tip cleaance inceases. Lage inceases of the same occued with highe otational Reynolds numbe and axial Reynolds numbe. A coelation of the aveage Nusselt numbe is obtained in tems of the otational and axial Reynolds numbes. This is expected to be useful in the design of SSHEs handling highly viscous fluids. Keywods: Scaped suface heat exchange, Newtonian fluid, Inne wall tempeatue, Bulk fluid tempeatue, Local heat tansfe coefficient, Nusselt numbe, dimensionless tip cleaance 1. INTRODUCTION SCRAPED SURFACE HEAT EXCHANGER (SSHE) is a device which consists of an annula gap with a cylindical oto and stato as shown in Fig 1. Scaping means emoval of fluid fom the bounday. This beaks the themal and hydodynamic bounday layes at the suface and thee by enhances the heat tansfe. Specifically an SSHEs is used fo the themal teatment of high-viscous fluids. SSHE is widely used in food industy fo steilizing o cooling highly viscous fluids such as mayonnaise, ceam cheese, peanut butte and ice ceam. As the teated fluids in SSHE ae highly viscous, fouling poblems may occu and educe significantly the themal efficiency of SSHE. The pesence of otating blades in the annulus makes it possible to avoid the possible fouling poblem on the heat exchange suface, and impove the heat tansfe teatment eceived by the poduct. A shot discussion of the pevious studies of authos who attempted to focus on the heat tansfe and basic flow pattens in SSHE is pesented hee. Tommelan et al. [1] was the fist peson to descibe the fluid flow and heat tansfe mechanisms fo the design and opeating conditions of the SSHE. The pesence of blades in the annulus makes the fluid flow moe complex compaed to the Coutte-Taylo flow configuations with a blade cleaance of 1µm. Penny and Bell [2] suggested that the cleaance between the edge of the scape blade and the stato wall is not constant, but it is dependent on the opeating conditions of the SSHE. De Gode et al. [3] studied the feezing of wate-ethanol sluies using an ice geneato with a blade cleaance of 3 mm and 1 mm, to examine the heat tansfe coefficient on the exchange suface. Toh and Muikami [4] studied the shape of two scape blades, one cuved and anothe, pependicula to the wall. Bott and Rameo [5] numeically studied an SSHE to compae the oveall heat tansfe coefficient with diffeent numbes of scape blades like 2, 4 and 6. The study shows that inceasing the numbe of scape blades inceases the powe consumption and that thee ae no advantages in inceasing the numbe of scape blades beyond fou. Yataghene et al. [6] conducted a numeical study of the fluid flow and heat tansfe within a SSHE, using 130 µm cleaance of the blade gap with pue glycein fluid and examined the mixing time fo the exit tempeatue of the SSHE. Haod [7] studied the SSHE using a newtonian fluid (wate). One of the key puposes of the pape was to model the heat tansfe fo both lamina and vetical otational flow, to find the tansition between lamina and vetical flow. D Addio [8] analyzed the themal behavio of a new kind of SSHE with an altenate scape blades aangement, namely, A-SSHE, duing the heating and cooling of hazelnut paste, which is a high viscous fluid. He detemined the heat tansfe coefficient by vaying otational Reynolds numbe (5 < Re < 250), axial Reynolds numbe (0.06, 4) and Pandtl numbe (6800, 60000). Stanzinge et al. [9] studied the flow patten in SSHE, using expeimental flow visualization (PIV technique) in a simplified geomety (blades fixed on the stato) and numeical simulation based on finite volume method (FVM). Hatel [10] gave a eview of the SSHE in which paticula attention was given to the cystallization pocess of ice ceam. He concludes that the mechanical enegy dissipated can each 50% of the caloific enegy evacuated by the cooling fluid. Moe ecently viscous heating within SSHE was investigated by Fayolle et al. [11] both expeimentally and numeically. They found that fo Newtonian fluid with high viscosity, the effect of viscous heating was vey impotant

2 Intenational Jounal of Engineeing Reseach and Geneal Science Volume 4, Issue 6,Novembe-Decembe, 2016 All though wok is epoted in the liteatue on the topic of heat tansfe in a SSHE, the conjugate heat tansfe due to finite wall thickness of the stato has not eceived much attention. Moeove issues like inne wall tempeatue, bulk fluid tempeatue and local heat tansfe coefficient fo the themally fully developed flow and the effect of the cleaance between the tip of the blade and the stato wall have not been clealy addessed. The objective of the pesent wok is to ceate a 3D model of an SSHE and study the fluid flow and heat tansfe in steady state lamina non-isothemal flow of pue glyceine (newtonian fluid) with tempeatue dependent viscosity. The cleaance between tip of the blade and the stato is taken as δ gap = 2mm, and 65µm. The elevant dimensionless numbes fo the pesent study ae the Rotational Reynolds numbes (Re ot ), axial Reynolds numbe (Re axi ) and the dimensionless cleaance. The fou otational Reynolds numbes (Re ot ) investigated ae 26.39, 79.17, , and the axial Reynolds numbes (Re axi ) ae 0.6, 1.21, 1.82, and Paametic study is done by keeping the otational Reynolds numbe constant and vaying the axial Reynolds numbe (Re axi ), and epeating the same pocedue as above fo diffeent (Re ot ). Thus fo each cleaance, 16 simulations ae to be pefomed and fo thee diffeent cleaances, 48 simulations ae to be caied out. The local heat tansfe coefficient fo the heat exchange suface is based on the inne wall tempeatue and bulk fluid tempeatue fo each coss section of the SSHE. The paametes fo which themally developed flow occus ae examined. Scape blade Stato Annula gap Fig 1: Tansvesal coss-section of SSHE [6] 298

3 Intenational Jounal of Engineeing Reseach and Geneal Science Volume 4, Issue 6,Novembe-Decembe, MATHEMATICAL FORMULATION 2.1 The physical model and coodinate system A 3D model of SSHE is ceated in Gambit (Ve ). In view of the geomety, the Catesian co-odinate system is chosen to descibe the geomety, whee X, Y and Z axis ae taken in the hoizontal, vetical and axial diections of the SSHE espectively. The model is shown in Fig. 2. Outlet Z Oute wall Stato Roto Inlet t c/δ gap Fig 2: 3D model of SSHE with 2mm cleaance The SSHE consists of a oto with two fixed scape blades. The stato suface epesents the wall heat exchange, whee a constant wall tempeatue is applied to the oute wall. In this study the thee diffeent cleaances between the tip of the blade and stato wall ae 2 mm and 65 µm. Othe dimensions of the SSHE device ae: stato diamete (Ds) =0.065m, oto diamete (D) =0.040m, stato length (Ls) =0.6m, numbe of blades (n) =2, aea of coss section =2.945e-3 m², and numbe of blades (n) = GRID INDEPENDENCE STUDY A commecially available meshing tool is used to geneate good quality hexagonal gids. A stuctued mesh ovelaying the hexahedal elements is geneated with the Gambit (Ve ) softwae meshing tool. The mesh density is inceased nea the wall and especially in the cleaance egion to ensue accuacy thee, as geate tempeatue changes in the fluid ae expected to occu in these egions. The mesh is highly concentated at the tip of the blades and ensued thee wee at least 4 mesh points acoss the tip of the cleaance. This was done by pefoming a numbe of simulations with diffeent mesh sizes, stating fom a coase mesh and efining it until physical esults wee no moe dependent on the mesh size. The hexahedal cells wee etained to have good discetization accuacy. Fou mesh efinements of , , and cells wee tested and compaed with a physical paamete of bulk tempeatue of the fluid fo each coss section of SSHE as shown in Fig 4. Fo good accuacy of the esults, the case with cells can be employed as this gives no significant diffeence compaed to that with cells. A fine mesh of cells is chosen fo futhe analysis as shown in Fig

4 Bulk tempeatue (K) Intenational Jounal of Engineeing Reseach and Geneal Science Volume 4, Issue 6,Novembe-Decembe, µm cleaance cells cells cells cells a) b) c) Figue 3: a) Computational of physical domain of a SSHE and used gid, all physical domains was meshed with hexahedal cells of ( ) b) SSHE Tansvese coss section of the mesh topology of physical c) Vaiation of bulk fluid tempeatue fo diffeent mesh cells fo 65µm cleaance. 4. GOVERNING EQUATIONS In this section the govening equations ae stated fo the heat tansfe and fluid flow including viscous dissipation in the SSHE. Steady state, non-isothemal, incompessible flows have been consideed. The flow is assumed to be lamina in the entie computing fluid domain with otation Reynolds numbe lowe than 250 and axial Reynolds numbe lowe than 4. The consevation equations fo continuity, momentum and enegy fo incompessible flows have been solved in dimensional fom. It is petinent to use the otating efeence fame fomulation to solve the continuity, momentum and enegy equations. The pincipal eason fo employing moving efeence fame is to ende a poblem which is unsteady in the stationay (inetial) fame steady with espect to the moving fame. In the otating efeence fame, the continuity equation in steady state fo the elative velocity is witten as: 0 v (1) The momentum equation is witten as: v v 2 v p (2) Momentum equation contains two additional acceleation tems: the coiolis acceleation 2 v and the centipetal acceleation Enegy equation fo the fluid in steady state fo otating fame is expessed as: v. H k T v (3) Foced heat convection Heat conduction Viscous heating 300

5 Intenational Jounal of Engineeing Reseach and Geneal Science Volume 4, Issue 6,Novembe-Decembe, 2016 In the pesent wok we will neglect the effects of gavity; howeve, it is staightfowad to include it in the axial pessue gadient if the SSHE is mounted with the axis vetical. The stato wall (heat exchange suface) was taken into account with 5 mm thickness. The enegy equation has been solved in the fluid and the conduction equation in steady state applicable to solid egions is: Enegy equation fo solid: C T k T. (4) steel psteel steel The system of non-dimensionalisation is: Re axi v in d equ Re ot.d equ c p P k 5. BOUNDARY CONDITIONS We have examined in this wok the fluid cooling pocess occuing in an SSHE. Fo the fluid flow, the momentum equation (Eq. 3) bounday conditions ae specified as follows: On the stato: v. At the oto and scape blade: v 0. Flow at inlet : v in m/s. Outflow: zeo velocity gadient v/ n=0. The fluid (glyceine) is intoduced at the SSHE inlet with the tempeatue of T inlet =288K, and was cooled with the constant oute wall tempeatue T w =278K. The tempeatue diffeence between the wall heat exchange and fluid inlet was T=10K. Adiabatic conditions wee assumed fo the oto and scape blade. Zeo tempeatue gadient is assumed at the outlet. ANSYS Fluent code uses the finite volume method fo discetization. The govening steady-state equations fo mass and momentum consevation ae solved with a segegated appoach. In this appoach, the equations ae sequentially solved with implicit lineaization. Volume-faces advective fluxes wee appoximated using a second- ode upwind intepolation scheme. Because of the effect of the viscosity vaiation due to heat tansfe, a coupling between velocity and tempeatue fields must be consideed. The pessue-velocity coupling is implemented using iteative coection pocedue (SIMPLEC algoithm). Fo the enegy equation too, a second-ode upwind intepolation scheme is used. 6. CODE VALIDATION The poblem is solved using Fluent CFD code. In ode to check fo the accuacy and coectness of the code, the esults of Yataghene [9] ae epoduced. The poblem deals with the CFD analysis fo vaious otational velocities with a constant volumetic flow ate. The dimensionless exit tempeatue is calculated using Eq. (7) fo pue glycein by vaying otational velocities in the ange 3-10 ev/s. Fig. 4 shows the compaison of the esults of Yataghene[9] and the pesent Fluent esults. Fom this we can comment that the vaiations ae almost identical except that the pesent esults show slightly highe values. This level of ageement is consideed satisfactoy. It may be noted that when inceasing the otational velocities, the tempeatue is inceases due to viscous dissipation of the fluid. T T out wall (5) in T T wall 301

6 Dimensionless exit tempeatue Intenational Jounal of Engineeing Reseach and Geneal Science Volume 4, Issue 6,Novembe-Decembe, 2016 N ev/sec Yataghene Pesent Fig 4: Compaison of tempeatue contous 7. RESULTS and DISCUSSION In ode to obtain the pefomance chaacteistics of 3D model of SSHE fo the fluid flow and heat tansfe, a paametic study is caied out by vaying the cleaance between the tip of the scape blade and the stato wall, the angula velocity of the oto and the axial velocity of the fluid enteing the SSHE. The dimensionless paametes coesponding to the above dimensional paametes ae the dimensionless tip cleaance, the otational Reynolds numbe (Re ot ) and the axial Reynolds numbe (Re axi). Contou plots ae constucted to depict the flow and tempeatue distibutions. Gaphs ae plotted to show the vaiation of inne wall tempeatue, bulk tempeatue and the local heat tansfe coefficient. The local heat tansfe coefficient is calculated with Eq. (8) with espect to the axial distance. Coelations ae obtained fo the aveage Nusselt numbe based on the aveage heat tansfe coefficient in tems of the pocess paametes. h T in, w qw T bulk (6) 7.1 Flow and tempeatue distibutions Figs. 5 (a, b) show the tempeatue contous fo the axial position coesponds to the nea-exit section (Z=0.55 m) of the SSHE, fo vaious dimensional cleaances. The colo code gives infomation about the tempeatue limits, the blue colo coesponding to 278 K and the ed colo coesponding to 288 K. In Fig (5a), coesponding to 2 mm cleaance, the hot fluid is moe located in the coe and the extent of cooled fluid is vey less. Hence highe blade cleaance does not poduce effective cooling of the fluid. When we obseve Figs. 5 (b) fo 65 µm cleaance, bette cooling pocess of the fluid takes place with a pope distibution of the cooling tempeatue is achieved with deceasing the tip of the cleaance and thee is good heat emoval at the bounday suface. a) b) Fig. 5 Tempeatue contous of SSHE coss section in case of Z=0.55 m a) 2 mm cleaance b) 65 µm cleaance Re ot = and Re axi =

7 Local heat tansfe coefficient based on inne wall tempeatue and Local heat tansfe coefficient based on inne wall Inne wall tempeatue and Bulk fluid Inne wall tempeatue and Bulk fluid Intenational Jounal of Engineeing Reseach and Geneal Science Volume 4, Issue 6,Novembe-Decembe, Axial vaiations of tempeatue diffeence and local heat tansfe coefficient The inne wall tempeatue and bulk fluid tempeatue distibutions along the SSHE axial distance ae shown in Figs. 6 (a, b) at diffeent values of axial Reynolds numbe, otational Reynolds numbe and also fo diffeent dimensionless tip cleaances. Fo 2 mm cleaance with a otational Reynolds numbe (Re ot ) of , the inne wall tempeatue is vey high at the inlet section and the tempeatue of the inne wall gadually deceases along the axis of the SSHE. When compaing the two cuves of the inne wall tempeatue and the bulk fluid tempeatue at diffeent axial Reynolds numbe (Re axi ), it can be seen that these two lines ae almost paallel to each othe and we can say that the tempeatue diffeence is unifomly distibuted to the fluid. At the tip cleaances of 65 µm, the scape blades continuously emove the bounday laye suface and bette cooling occus with the inne wall and bulk tempeatues deceasing with axial distance along the SSHE. When inceasing the otational Reynolds numbe, the inne wall tempeatue and bulk fluid tempeatue incease due to viscous dissipation, which in tun deceases the viscosity of the fluid. Reot = (Reaxi=0.6) Reot= (Reaxi=0.6) (Reaxi=1.21) (Reaxi=1.21) (Reaxi=1.82) (Reaxi=1.82) Fig. 6(a) Inne wall tempeatue and bulk fluid tempeatue fo 2mm cleaance with diffeent axial Reynolds numbe Fig. 6(b) Inne wall tempeatue and bulk fluid tempeatue fo 65 µm cleaance with diffeent axial Reynolds numbe The local heat tansfe coefficient based on inne wall tempeatue and bulk fluid tempeatue calculated using Eq. (8) along the axial distance of SSHE is shown in Figs. 7 (a, b) fo diffeent Reynolds numbes and also fo diffeent tip cleaances. The geneal vaiation of local heat tansfe coefficient eveals that it is of high value nea the inlet section of the exchange because of the thickness of the bounday laye is vey small. It deceases continuously due to the inceasing themal bounday laye thickness. When the dimensionless tip cleaance deceases (i.e., 65 µm as against 2 mm cleaance), bette local heat tansfe coefficient is achieved and impovement in the pefomance of the SSHE is obtained. It can be obseved in Fig. 8(a) (2 mm cleaance) that the local heat tansfe coefficient is less compaed to the othe cleaance. This is because the scape blades only bush against but not scape the exchange suface. When tip cleaance deceases, the scape blades emove the bounday laye and esult in highe values of local heat tansfe coefficient. A themally fully developed flow of the fluid is also achieved. Reot = Reaxi = 0.6 Reot= Reaxi=0.6 Reaxi = 1.21 Reaxi= 1.21 Reaxi = 1.82 Reaxi= 1.82 Reaxi = 2.43 Reaxi= 2.43 Fig. 7(a) Local heat tansfe coefficient based on inne wall Fig 7(b) Local heat tansfe coefficient based on inne wall tempeatue and bulk fluid tempeatue fo 2 mm cleaance tempeatue and bulk fluid tempeatue fo 65 µm cleaance with diffeent axial Reynolds numbe with diffeent axial Reynolds numbe 303

8 log10 (Nu log10(nu Intenational Jounal of Engineeing Reseach and Geneal Science Volume 4, Issue 6,Novembe-Decembe, Coelations fo aveage Nusselt numbe The following coelation has been established fo aveage Nusselt numbe by using the Eq. (9) unde seveal opeating conditions of SSHE. Nu avg hdequ (7) k In obtaining the heat tansfe coelation, the otational and axial Reynolds numbes ae taken as paametes. The Pandtl numbe vaiation does not occu because esults ae obtained fo only a single fluid, namely, glyceine. The coelation obtained using multiple egession analysis is: Nu a Re Re (8) b ot c axi whee a, b and c ae the constants of the coelation. In ode to detemine the constants a, b and c of the model seveal numeical simulations wee caied out. Figs. 8 (a, b) show the paity plots between computed and coelated Nusselt numbe in which 16 data points petaining to one dimensionless cleaance ae shown fo each plot. Although not pesented, simila plot is also obtained fo the emaining cleaance. The coelation constants ae given in Table 1: Table 1: Coelation constants Coelation constants 2 mm Cleaance 65 µm Cleaance log10 (Nu log10(nu- a b a) b) c R Fig. 8 Paity plots between computed and coelated values of Nusselt numbe fo a) 2 mm cleaance b) 65 µm cleaance Accoding to obtained esults it appeas that the incease of the otating speed impoves the efficiency of the SSHE fo all consideed dimensionless cleaances. 8. CONCLUSIONS A 3D CFD model is employed to examine the themal pefomance of an industial scaped suface heat exchange device. A paametic study is done and the following conclusions ae eached: 1) The diffeence between inne wall tempeatue and bulk fluid tempeatue inceases due to viscous dissipation of the fluid

9 Intenational Jounal of Engineeing Reseach and Geneal Science Volume 4, Issue 6,Novembe-Decembe, ) The local heat tansfe coefficient based on inne wall tempeatue and bulk fluid tempeatue inceases along the length of the SSHE in the axial diection with deceasing tip cleaance. 3) In case of 2 mm cleaance, the local heat tansfe coefficient is vey less compaed to othe cleaance because the scape blade only bushes but does not to scape the suface. Fo 65µm cleaance, the scape blades scape the bounday laye at the suface and this inceases the local heat tansfe coefficient. 4) The local heat tansfe coefficient inceases with inceasing otational Reynolds numbe; this impoves the pefomance of the SSHE. 5) A heat tansfe coelation is obtained fo the aveage Nusselt numbe in tems of the otational and axial Reynolds numbes, which ae the pocess paametes. REFERENCES: [1] Tommelen A. M, Beek W. J, Flow Phenomena in a Scaped Suface Heat Exchange ( votato type), Chem. Engg Sci, Vol. 26, No. 11, pp [2] Penney W. R, and Bell K. J, Ind. Engng Chem (ap) 40. [3] De Goede R, De Jong E. J, Heat Tansfe Popeties of Scaped Suface Heat Exchange in the tubulent Flow Regime, Chem. Eng. Sci. Vol. 48, No. 1993, pp [4] Toh and Muikami. Y, (1982), Powe Consumption of a Fluid Loaded Floating Scape Blade, J Chem Engg JPN. Vol. 15, No. 6, pp 493 [5] Bott T. R, Romeo. JJB, (1966), The Chaacteistic Dimension in Scaped Suface Heat Exchange. J Chem Engg, 44, 226. [6] Mouad Yataghene. Jack Legand, A 3D-CFD model themal analysis within a scaped suface heat exchange, Chem Engg. Vol 71, No.2013, pp [7] Häöd. M, (1987), Residence time distibution, heat tansfe and powe equiements, Jounal of Food Pocess Engg 9: 1-62 [8] D Addio. L, Dejong E. j, 1993, Heat Tansfe Popeties of a Scaped Suface Heat Exchange in the Tubulent Flow Regime, Chem, Engg, Sci, Vol No, 48, pp [9] Stanzinge M, Feigl k, Windhab E, Newtonian Flow Behavio in Naow Annula gap Reactos, Chem Eng Sci 2001, Vol. 56, No. 11, pp [10] Hatel R. W, Ice Cystallization duing Manufactue of Ice Ceam, Tends Food Sci Technol 1996, Vol. 7, No. 10, pp [11] Fayolle F, Legaand J, Expeimental and numeical analysis of heat tansfe including viscous dissipation in a scaped suface heat exchange Chem Eng pocess, Vol. 2009, No. 48, pp