INTERROGATING THE FLOW BEHAVIOUR IN A NOVEL MAGNETIC DESICCANT VENTILATION SYSTEM USING COMPUTATIONAL FLUID DYNAMICS (CFD)

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1 INTERROGATING THE FLOW BEHAVIOUR IN A NOVEL MAGNETIC DESICCANT VENTILATION SYSTEM USING COMPUTATIONAL FLUID DYNAMICS (CFD) Auwal Dodo*, Valente Hernandez-Perez, Je Zhu and Saffa Rffat Faculty of Engneerng, Unversty of Nottngham, Unversty Park, Nottngham, NG7 RD, UK *e-mal: enxam@nottngham.ac.uk ABSTRACT The ar flow behavour across a magnetc desccant ventlator model have been numercally analysed for the purpose of system desgn verfcaton. Smulatons were performed at dfferent nlet ar flow velocty and magnetc desccant wheel rotaton speed, and the way ths parameters can nfluence the ar flow around the ventlator geometry was studed. The smulatons were carred out usng the software package Fluent 6.3, whch s desgned for numercal smulaton of flud flow, heat and mass transfer. The model conssted of a magnetc desccant ventlator. A structured hexahedral mesh was employed n the computatonal doman. The condton of sngle phase flow was smulated, takng nto consderaton turbulence effects usng the k-ε model. The smulaton results showed that the ar flow pattern across the ventlator s not affected by an ncreasng nlet ar velocty or wheel rotaton speed. The results show that CFD can be a useful tool n the study of magnetc desccant ventlators. 1. INTRODUCTION The operatng condtons and geometry are among some of the maor factors that nfluence the performance of a magnetc desccant ventlator. Comprehensve studes of these parameters are usually lackng. Although the operatng prncples of desccant dehumdfcaton systems and magnetc refrgeraton systems may be relatvely smple, the flow behavour n both systems s consdered complex and not clearly understood. Computatonal flud dynamcs (CFD) numercal approach could provde reasonable solutons to the aforementoned problem. Computatonal Flud Dynamcs (CFD) s a numercal modellng technque that solves the Naver-Stokes equatons on a dscretsed doman of the geometry of nterest wth the approprate flow boundary condtons suppled. The Naver-Stokes equatons are a complex nonlnear set of partal dfferental equatons (PDEs) that descrbe the mass and momentum conservaton of a flud. Addtonal physcs can also be resolved by solvng addtonal conservaton equatons, e.g., heat transfer, multphase flow and combuston. The fundamental prncples behnd the process have been well establshed n the feld of flud dynamcs analyss and numercal methods for many years. CFD has the capacty to smulate flow and energy and delver smple to complex soluton. CFD offers the means of testng theoretcal advances for condtons unavalable expermentally as descrbed by Fletcher (1991). Accordng to Fletcher (1991), CFD has many maor advantages n comparson to expermental flud dynamcs. These advantages are: The lead tme n desgn and development s sgnfcantly reduced; CFD can smulate flow condtons not reproducble n expermental model tests; CFD provdes more detaled and comprehensve nformaton; CFD s ncreasngly more cost-effectve than wnd tunnel testng; CFD produces low energy consumpton. Although CFD modellng cannot be a complete substtute for real expermental nvestgatons, t however permts the smulaton of dfferent flow condtons and envronments wthout the rgours and expenses requred for real lfe experments, an opportunty whch would not have been possble wth physcal experments. The study presented heren wll attempt to nterrogate and determne the effect that dfferent operatng parameters of the ventlator, ncludng ar velocty and wheel rotaton speed wll have on the flud flow around the ventlator geometry usng computatonal flud dynamcs.. CFD MODEL The development and soluton of a representatve Computatonal Flud Dynamcs (CFD) model s a multstage process. In the present work, the CFD smulatons were carred out usng the commercal CFD package Fluent 6.3. Fluent software uses the Fnte Volume dscretzaton technque to 1

2 solve the governng flud flow equatons. The frst step n the Fnte Volume Method (FVM) nvolves the dvson of the flow doman nto dscrete control volumes. The equatons governng the flud flow are ntegrated over the control volume and the resultng ntegral equatons dscretzed to produce algebrac equatons at the nodal ponts. These algebrac equatons are then solved by an teratve method. The resultant steady state or tme dependent solutons may be graphcally vewed as a seres of two and three dmensonal, vector, streamlne or contour plots, etc. The process of model development and soluton descrbed above s requred of all CFD smulatons and can be dvded nto three man steps. These steps are: Pre-processng; Solver extracton; Post-processng..1 Computatonal doman It was mportant to ensure that the geometry of the CFD model represented as much as possble, the physcal magnetc desccant ventlator. Hence, much care was taken n defnng the geometry. The geometry for the case studes modelled s llustrated n Fgure 1. The geometry conssts of three dfferent sectons: process ar, regeneraton ar and the wheel. Two ar channels of dmensons 500 mm wdth, 40 mm depth and 17 mm heght each, were created to represent the process ar secton nlet and outlet ar streams. Also, another two separate ar channels were created to represent the nlet (500 mm wdth, 40 mm depth and 17 mm heght) and outlet (700 mm wdth, 40 mm depth and 17 mm heght) ar streams of the regeneraton secton. Both the process and regeneraton ar sectons have been created usng the brck tool under the geometry panel n Gambt. The wheel secton (magnetc desccant wheel) was created usng the cylnder tool under the geometry panel n Gambt. Dmensons 17 mm heght, 10 mm radus and z-axs orentaton were specfed for the wheel. The magnetc desccant wheel has been modelled as a porous medum. The porous zone s selected n ths work, for the fact that the real magnetc desccant wheel s made of a honeycomb structure wth parallel ar channels. The honeycomb matrx of the wheel s mpregnated wth slca gel desccant materal and gadolnum ngots but allowng suffcent ar to flow through the matrx of the wheel. The rotaton of the wheel has been specfed n the Moton Panel of the Magnetc desccant wheel. The moton type that was chosen for wheel rotaton s Movng Frame and rotatonal velocty s specfed as desre. Fg. 1: Computatonal doman.. Grd generaton The grd generaton process deals wth the dvson of the doman under consderaton nto small control volumes on whch the dscretsed governng equatons wll be solved. Ths process s also known as meshng. The meshng process s an ntegral part of the numercal soluton and must satsfy certan crtera to ensure a vald and accurate soluton, Lun et al. (1996). The geometry and grd generaton forms a large part n terms of person-hour tme of the CFD analyss. In meshng the flow doman, a structured mesh approach was adopted. Ths was done n order to acheve the desred grd densty at dfferent parts of the flow doman. The flow doman was therefore dvded nto three dfferent faces wth each face meshed separately to acheve the desred results. The geometres of the mesh employed s the structured hexahedral grd, whch has been successfully employed for smlar studes. There was need to cluster a large number of closely spaced grds n the regon of flow of man nterest. Fgure shows the generated grds for the ventlaton system.

3 Fg. : Computatonal mesh generated for the magnetc desccant ventlator used for CFD smulaton..3 Governng equatons To effectvely model the ventlator, the moton of an ncompressble sngle flow has been consdered as the flow scenaro. The mass and momentum conservaton equatons for the sngle flow through the doman are represented as: dvv t S m (1) Where ρ s the densty, t s the tme, V s the velocty vector of the flud and S m s a mass source. The momentum equaton s DV g p Dt x v x v x dvv () Where g s the gravtatonal vector, x s the spatal coordnate, v s the component of the velocty vector, s the ordnary coeffcent of vscosty, λ s the coeffcent of bulk vscosty, D s the Kronecker delta functon and Dt s the total or substantal dervatve. Solvng these sets of equatons has been done usng a software package Fluent Turbulence model The hgh operatng ar velocty across the ducts of desccant ventlators contrbutes to the occurrence of hghly turbulent flow, and as the ar flow through the matrx of the wheel, a developng vortex regon s created around the wheel. Therefore, turbulence must be consdered n the numercal smulaton. The accuracy of CFD smulatons for turbulent flows can be affected by turbulence modellng, especally because of the complex features of the flow. Accordng to Versteeg and Malalasekera (007), t has been recognsed by researchers n the feld of CFD that the choce of turbulence models used to represent the effect of turbulence n the tmeaveraged mean flow equatons represents one of the prncpal sources of uncertanty of CFD predctons. In order to smulate turbulence n the present work, the standard k-ε model, Launder and Spaldng (1974), whch requres that the flow s fully turbulent, was used for several reasons; the model s computatonally effcent, s mplemented n many commercal codes, the geometry s reasonably not complcated and t has demonstrated capablty to smulate properly ndustral processes, ncludng ventlated systems. The k- models are based on the Reynolds Averaged Naver Stokes (RANS) equatons and they focus attenton on the mean flow and the effect of turbulence on flow propertes. The standard k-ε model s today the most wdely used turbulence model n the engneerng ndustry, snce 1974, DeJesus (1997). The standard k-ε turbulence model s descrbed by the followng equatons, Hernandez-Perez (008): u u C k x k x x x t k u u t k x x x u x u t u C 1 t x k x x (3) u x In the above equatons, k s the turbulence knetc energy and s the turbulence dsspaton rate. k,, C 1 and C are constants whose values are 1.0, 1.3, 1.44 and 1.9, respectvely. Also, velocty u and u s the component of the flud x s the spatal coordnate. Furthermore, the flud vscosty must be corrected for turbulence n the Naver-Stokes equatons employng an (4) 3

4 effectve vscosty eff where s the dynamc vscosty and t s the turbulent vscosty..5 Boundary and ntal condtons The defnton of boundary condtons s performed n Fluent. However, the boundary types to be used n the smulaton have been prevously defned n Gambt, once the mesh was generated. The boundary type specfcatons defne the physcal and operatonal characterstcs of the model at those topologcal enttes that represent model boundares. The boundary types that were used n the CFD model are summarzed n Table 1 below: TABLE 1: SUMMARY OF BOUNDARY CONDITIONS Zone Name Process ar nlet Process ar outlet Regeneraton ar nlet Regeneraton ar outlet Magnetc desccant wheel Wall t Boundary Type Velocty nlet Pressure outlet Velocty nlet Pressure outlet Porous medum Wall Two nlets and two outlets are employed n ths model; the two nlets nclude the process ar nlet and the regeneraton ar nlet, whereas the two outlets nclude the process ar outlet and regeneraton ar outlet. In addton, a wall boundary type s appled to the rest of the boundares. At both process and regeneraton ar nlets, a velocty-nlet boundary type s used n whch the nlet ar velocty condtons are specfed. Pressure outlet boundary type was used for the process and regeneraton ar outlets of the model. At tme t 0 (s), all velocty and pressure components are set to 0 (m/s) and 0 (kpa), respectvely. These chosen ntal condtons ease the convergence process. In addton, an ntal guess for the turbulent knetc energy and dsspaton rate was appled n all the smulatons studed..6 Soluton algorthm In order to numercally solve the system of partal dfferental equatons n ths study, dscretzaton of the equatons has been carred out usng the Fnte Volume Method (FVM) wth an algebrac segregated solver and colocated grd arrangement, as mplemented n Fluent 6.3. Values of pressure and velocty are both stored at cell centres, n ths grd arrangement. Full detals of the (FVM) dscretzaton have been gven n the work carred out by Versteeg and Malalasekera (1995) and Zenkewcz and Taylor (000). The contnuty and momentum equatons are needed to be lnked consderng the fact that Fluent uses a segregated solver. Several of ths lnkage technques are avalable n Fluent and have been reported n the lterature. The Sem-Implct Method for Pressure Lnked Equatons (SIMPLE) algorthm, of Patankar and Spaldng (197), was employed due to ts good performance n fndng a fast and convergent soluton, Abdulkadr (011)..7 Mesh ndependence study CFD numercal smulatons are computatonally expensve. The sze of the computatonal grd specfed by the user s consdered among sgnfcant factors nfluencng the computaton tme. In order to dentfy the mnmum mesh densty to ensure that the converged soluton obtaned from CFD s ndependent of the mesh resoluton, a mesh senstvty analyss has been carred out, n the development and analyss of the CFD model. Four 3-Dmensonal meshes were nvestgated n the present study as llustrated n Fgure 3. The szes of the meshes nvestgated are; 14457, 34965, and cells. A CFD calculaton was performed to compare the performance of the above mentoned meshes. The results of the mesh ndependence study carred out for the four meshes are shown n Fgure 4. The velocty profle n a vertcal plane passng through the centre of the wheel has been used as the parameter for comparson. Due to the dffcultes nvolved wth comparng the velocty profles, the wheel rotatonal speed has been kept constant at 0 rpm, whle an nlet ar velocty of 1.63 m/s was appled. The results reveal that the velocty profle has the same shape for all the dfferent mesh denstes utlzed, but however, the magntude of the velocty s slghtly affected by the mesh. We can clearly see that as the number of cells n the mesh s ncreased, the maxmum velocty n the profle tends to be 1. m/s, as depcted n Fgure 4. It was observed that the changes from mesh c to mesh d are neglgble. Therefore, t can be concluded that mesh c wth cells s suffcent to carry out the smulaton, as t gves a good result and s less computatonally expensve than the mesh d wth cells. 4

5 (a) (b) (c) Fg. 3: Vew of dfferent szes of computatonal grd used for mesh ndependence study (a) cells (b) cells (c) cells (d) cells. (d) Fg. 4: Result obtaned from the CFD mesh ndependence studes. 3. RESULTS AND DISCUSSION The CFD smulatons of the flow n the vcnty of the desccant wheel model were bascally amed at analysng the flud flow n the wheel and determnng the drectons of flud flow n ths regon. Smulatons were carred out to determne the effect of varyng nlet ar velocty and magnetc desccant wheel rotaton speed on the flud flow across the ventlator. The results have gven an nsght on the flud behavour around the desccant wheel. The dfferent contour and vector dsplays avalable n the CFD software have been used as a post-processng tool to analyse the flud behavour around the wheel and then relate ths to flow verfcaton exercse. 5

6 3.1 Effect of varyng nlet ar velocty on flud flow Smulatons were carred out n order to observe how dfferent nlet ar velocty at the process and regeneraton ar sdes, affects the flud flow around the magnetc desccant ventlator. In these smulatons, a constant wheel rotaton speed of 1.3 rpm was used, the same as that used durng the expermental nvestgatons. Three dfferent nlet ar velocty condtons for process and regeneraton sdes were smulated, as shown n Table. TABLE : DIFFERENT INLET AIR VELOCITIES USED FOR SIMULATIONS Inlet Ar Velocty Settng Process Inlet Ar Velocty (m/s) Regeneraton Inlet Ar Velocty (m/s) Fgures 5(a) and 5(b) shows the ntal results of the smulatons. Intally, a cross sectonal vew at the horzontal central plane and the wheel was taken for all the three dfferent nlet ar velocty condtons. However, from the ntal results, the flow behavour for all condtons was observed to be very smlar. The velocty vectors showng flow behavour at the lowest and hghest nlet ar velocty condtons have been shown n Fgures 5(a) and 5(b), respectvely. In order to have a close and more detaled vew of the flow behavour n the system, a zoomed cross sectonal vew of the horzontal central plane and that of the wheel were taken separately for all the dfferent smulated nlet ar velocty condtons. These results have been presented n Fgures 6(a) and 6(b) to 8(a) and 8(b). From the obtaned results, t can be observed that, smlarly to the case of the mesh ndependence study, the flow behavour s very smlar for all smulaton condtons, wth the ar velocty at the central part of the wheel beng close to zero and the hghest ar velocty s obtaned far away from the centre of the wheel. The ar velocty wthn the wheel s relatvely less than that wthn the ar duct, whch can be explaned by the larger cross sectonal area avalable for the ar n the wheel. In addton, by comparng the results for all condtons, t can be observed that the as the nlet ar velocty s ncreased, the magntude of the maxmum velocty vectors ncreases accordngly. On a whole, the smulaton results show that there s proper and suffcent crculaton of ar n ducts of the ventlator, as ntended. It was also observed that the flow s expandng as t leaves the magnetc desccant and enterng nto the outlet ducts at both the process and regeneraton ar sdes of the ventlator. 5(a) 5(b) Process sde nlet ar flow velocty: 1.63 m/s Process sde nlet ar flow velocty: 7.75m/s Regeneraton sde nlet ar flow velocty: 1.64 m/s Regeneraton sde nlet ar flow velocty: 7.34m/s Fg. 5: Velocty vectors showng flow behavour at the (a) lowest nlet ar velocty (b) hghest nlet ar velocty. 6(a) 7(a) 8(a) 6(b) 7(b) 8(b) Fgs. 6, 7 and 8: Velocty vectors showng flow behavour for nlet ar velocty settng 1,, 3, respectvely. (a) Horzontal central plane (b) Wheel. 6

7 3. Effect of varyng wheel rotaton speed on flud flow Smulatons were also carred out n order to determne the effect of varyng the magnetc desccant wheel rotaton speed on the flud flow around the magnetc desccant ventlator. In these smulatons, ar velocty of 4.6m/s was utlsed, for the process and regeneraton ar nlets. A total of three dfferent wheel rotaton speed condtons were smulated, startng wth a statonary wheel. The smulated wheel rotaton speeds nclude; 0 rpm,.6 rpm and 6.4 rpm, respectvely. However, t s worth mentonng that the magnetc desccant wheel rotaton speeds used for ths smulaton study are the same as those used durng the expermental nvestgatons, n order to allow for a more accurate comparson of smulated and expermental results. The smulaton results for the effect of varyng wheel rotaton speed on flud flow have been presented n Fgures 9(a) and 9(b). Smlar to secton 3.1, a cross sectonal vew at the horzontal central plane and the wheel was taken for all the wheel rotaton speeds used n ths part of the smulaton. The obtaned results were found to be smlar for all the smulated wheel rotaton speed condtons, as observed for the smulatons carred out earler, for the effect of varyng the nlet ar velocty on flud flow. The velocty vectors showng flow behavour at 0rpm and at the hghest wheel rotaton speed of 6.4rpm have been shown n Fgures 9(a) and 9(b), respectvely. In order to have a more close and detaled vew of the flow behavour n the system, a zoomed cross sectonal vew of the horzontal central plane and that of the wheel were taken separately for all the dfferent smulated wheel rotaton speed condtons. These results have been presented n Fgures 10(a) and 10(b) to 1(a) and 1(b). From all the results obtaned, regardng the effect of varyng wheel rotatonal speed on the flud flow behavour, t can be concluded that, for the wheel rotaton speeds used n ths work (all of whch are relatvely slow), the effect has been found to be neglgble, as can be observed n the obtaned results. In order for the wheel rotatonal speed to have a sgnfcant effect on the flow behavour, the wheel rotatonal speed must be sgnfcantly hgher than those used for these smulatons. However, the smulaton results obtaned, shows that there s suffcent and proper crculaton of ar n the ducts and through the porous wheel of the ventlator, as ntended. 9(a) Fg. 9: Velocty vectors showng flow behavour (a) no wheel rotaton speed, 0rpm (b) hgh wheel rotaton speed, 6.4rpm. 9(b) 10(a) 11(a) 1(a) 10(b) 11(b) 1(b) Fgs. 10, 11 and 1: Velocty vectors showng flow behavour at wheel rotaton speeds of 0 rpm,.6 rpm and 6.4 rpm, respectvely. (a) Horzontal central plane (b) Wheel. 7

8 4. CONCLUSIONS CFD smulatons of the magnetc desccant ventlator were carred out for the purpose of system desgn verfcaton and also n order to vsualse the flow pattern and further nterrogate the ar flow behavour n the ventlaton system. A desgn of a magnetc desccant ventlaton system comprsng of a rotary magnetc desccant wheel as a porous medum and a seres of ar ducts at the both the process and regeneraton sectons of the ventlator has been developed. Detaled modellng studes of ar flow behavour across the ventlaton system have been conducted. Smulatons were ntally carred out under dfferent nlet ar velocty condtons at a constant wheel rotaton speed and later under dfferent wheel rotaton speeds, at constant nlet ar flow velocty. These smulatons were amed at flow vsualsatons under the above mentoned operatng condtons n order to fully nterrogate the ar flow behavour across the magnetc desccant ventlator. Smulaton results obtaned shows that the ar flow pattern across the ventlator s not affected by ncreasng nlet ar velocty or wheel rotaton speed. The results also revealed that there s suffcent ar flow through the porous magnetc desccant wheel, from the nlets to the outlets of both the process and regeneraton ar sdes of the ventlator. In order to acheve more realstc results n the numercal smulaton, further smulatons wll be requred. These wll need to ncorporate heat transfer and magnetc feld effects nto the CFD model. As a result, the complexty of the CFD model wll ncrease. 5. NOMENCLATURE Latn letters g Gravtatonal acceleraton [ m / s ] k Knetc energy of turbulence [ m / s ] p Pressure [ N / m ] t Tme [ s ] u Velocty [ m/ s ] D Dffuson coeffcent [ m / s Greek letters Volume fracton [-] Dynamc vscosty [ kg m. s ] / ] 3 Materal densty [ kg / m ] Surface tenson [ N / m ] Stress tensor [ N / m ] 3 Second (bulk) vscosty [ kg/ m s ] Eddy vscosty [ kg / m. s ] Rate of vscous dsspaton [ m / s ] Other symbols and operators Gradent operator t D DT Partal dervatve Total dervatve C, k,, C 1 and C Emprcal constants x dv Kronecker delta functon Spatal co-ordnate Dvergence of a vector feld 6. ACKNOWLEDGMENTS The authors wsh to thank Plkngton Energy Effcency Trust (PEET) for ther fnancal support through the postve envronmental ntatve of Plkngton Group Lmted. 7. REFERENCES (1) Abdulkadr, M., 011. Expermental and computatonal flud dynamc (CFD) studes of gas-lqud flow n bends. PhD thess, Unversty of Nottngham. UK () DeJesus, J. M., An expermental and numercal nvestgaton of two-phase slug flow n a vertcal tube. PhD thess, Unversty of Toronto, Canada (3) Fletcher, C.A.J., Computatonal technques for flud dynamcs. Volume 1, nd edton, Berln: Sprnger (4) Hernandez-Perez, V., 008. Gas-lqud two-phase flow n nclned ppes. PhD thess, Unversty of Nottngham. UK (5) Launder, B., and Spaldng, D., The numercal computaton of turbulent flows, Computer Methods n Appled Mechancs and Engneerng 3, (6) Lun I., Calay, R. K., and Holdo, A. E., Modellng two phase flows usng CFD. Appled Energy, Vol. 53, No.3, (7) Patankar, S. V., and Spaldng, D. B., 197. A calculaton procedure for heat, mass and momentum transfer n three dmensonal parabolc flows. Internatonal Journal of Heat and Mass Transfer 15, 1787 (8) Versteeg, H. K., and Malalasekera, W., An ntroducton to Computatonal Flud Dynamcs, The Fnte Volume Method. Prentce Hall (9) Versteeg, H. K., and Malalasekera, W., 007. An ntroducton to Computatonal Flud Dynamcs: the Fnte Volume Method. nd edton. Pearson Educatonal Ltd (10) Zenkewcz, O.C., and Taylor, R.L., 000. Fnte Element Method: Volume 3, Flud Dynamcs, 5 th edton, Butterworth - Henemann Subscrpts, Space drectons k Phase ndex 8