Effect of Superficial Velocity of Air and Riser Cross Sectional Area on Heat Transfer Characteristics of Circulating Fluidized Bed

Transcription

1 , July 2-4, 2014, London, U.K. Effect of Superficial Velocity of Air and Rier Cro Sectional Area on Heat Tranfer Characteritic of Circulatin Fluidized Bed R.S. Patil, P. Mahanta, and M. Pandey Abtract The preent paper decribe a numerical tudy on wall-to-bed heat tranfer characteritic of circulatin fluidized bed (CFB) rier of cro ection 0.15 (m) 0.15 (m), 0.30 (m) 0.30 (m), each of heiht 2.85 (m). 3-D CFD imulation for heat tranfer characteritic were carried out under ame operatin condition for heated portion (heater) of rier. For modelin and imulation, CFD code Any - Fluent verion 13 wa ued. Modelin and mehin were done uin ProE and Any ICEM CFD oftware, repectively. The wall of heater wa maintained at the contant heat flux q = 1000 (W/m 2 ). RNG k-ε model wa ued for turbulence modelin. Gidapow model for phae interaction wa ued for the imulation of two phae flow (air + and mixture flow). Effect of increae in uperficial velocity of air from 2.5 (m/) to 4 (m/) on heat tranfer characteritic wa tudied experimentally and numerically for the CFB rier of rier of cro ection 0.15 (m) 0.15 (m). Effect of chane in rier cro ectional area on heat tranfer characteritic wa alo tudied numerically when both the CFB rier were operated under ame operatin condition. Reult on heat tranfer characteritic were obtained in term of ditribution of bed (air + and mixture) temperature acro the heater and local heat tranfer coefficient alon the heiht of the heater of the CFB rier. Reult obtained throuh CFD imulation were compared with available experimental data which wa obtained uin available CFB etup of IIT Guwahati. Index Term Bed temperature, CFB rier, CFD imulation, Heat tranfer coefficient. I. INTRODUCTION Recently ue of circulatin fluidized bed (CFB) boiler in power eneration i ainin popularity becaue of it environmental compatibility and hih efficiency. CFB i widely ued for variou indutrial application which include power eneration, dryin, crackin, and combution. The increae in diverity in CFB application demand the need for the development of more efficient experimental technique, realitic imulation, and other reearch and dein tool. Veratile tool like CFD and related oftware may be therefore ued to accomplih the reearch with accuracy and alo to overcome the limitation of experimental apect. Manucript received March 04, 2014; revied April 06, 2014 R.S. Patil i with the Mechanical Enineerin Department, Birla Intitute of Technoloy and Science, Pilani K. K. Birla Goa Campu, Goa, India (correpondin author, phone: ; ranjitp@ oa.bit-pilani.ac.in, ranjitpatil48@mail.com). P. Mahanta i with the Mechanical Enineerin Department, Indian Intitute of Technoloy, Guwahati, India ( pinak@ iit.ernet.in). M. Pandey i with the Mechanical Enineerin Department, Indian Intitute of Technoloy, Guwahati, India ( manmohan@iit.ernet.in). Reference [1] [2] reported ome information on turbulence parameter which wa hard to obtain in laboratory condition, but can be eaily etimated uin CFD tool. Reference [3] reported dene flow hydrodynamic experiment meaure either only the particle velocitie or the particle concentration until Reference [4] reported tudie on both the particle velocitie and particle concentration, which were determined toether at the firt time for the rier flow. Since then, modeler were able to compare and evaluate their theoretical model with experimental tudie in detail [1]. Reference [5] reported on dipered a-olid flow with very hih velocity and tron interphae interaction in a CFB rier. CFD imulation i a veratile tool to imulate two phae problem to predict heat tranfer characteritic uch a temperature, heat tranfer coefficient, and hydrodynamic characteritic uch a preure, velocity, volume fraction.tc. Reference [6] reported detailed dicuion on the development of ranular flow model. Reference [7] [9] reported the Eulerian Eulerian (two-fluid) model with kinetic theory of ranular flow, which i currently the mot applicable approach to compute a olid flow in a CFB. Reference [10] [12] reported on different dra model, which were ued to predict the mot repreentative a olid interphae exchane coefficient. Reference [13] reported on development of a model uin a Particle Baed Approach (PBA) to predict accurately the axial preure profile in CFB. Thi imulation model alo account for the axial and radial ditribution of voidae and the volume fraction of oild. Reference [7] reported on a / particle flow behavior in the rier ection of a CFB, which wa imulated uin CFD packae Fluent to predict velocity, volume fraction, preure, and turbulence parameter for each phae. Reference [14] reported a multifluid Eulerian model interatin the kinetic theory for olid particle uin Fluent- CFD oftware, which wa capable of predictin the a olid behavior of a fluidized bed. Comparion of the prediction of the model like the Syamlal O Brien, Gidapow, Wen Yu dra function with experimental meaurement on the time-averae bed preure drop, bed expanion, and qualitative a olid flow pattern indicate reaonable areement for mot operatin condition. Intantaneou and time-averae local voidae profile howed imilaritie between the model prediction and experimental reult. Reference [15] reported on the a and olid velocity, and volume fraction predicted throuh 2-D imulation of a CFB rier.

2 , July 2-4, 2014, London, U.K. Reference [16] reported the imulation uin Any CFX oftware verion 10 and reported radial olid velocity profile, computed on even axial level in the circular rier of a hih-flux circulatin fluidized bed (HFCFB) uin a two phae 3-D computational fluid dynamic model. Reference [17] reported the flow multiplicity phenomenon in circulatin fluidized bed (CFB) rier, i.e. under the ame uperficial a velocity and olid circulation rate, the CFB rier may ometime exhibit multiple flow tructure which wa numerically and experimentally invetiated. Reference [18] reported that olid-phae wall boundary condition had little effect on axial voidae profile when the Gidapow dra model wa ued. The particle wall retitution coefficient e w play a minor role in the holitic flow characteritic. Literature review reveal that many reearcher have reported CFD imulation baed on Eulerian model to predict only hydrodynamic characteritic for two phae flow in the CFB rier. Reference [19] however reported that there will be eroion of the lower plah reion durin operation of CFB boiler becaue lower plah reion occupie dene hot tream of coal, limetone, and and etc. For deinin the entire CFB, the tudy on heat tranfer characteritic in lower plah reion i therefore equally important a like tudy on the hydrodynamic characteritic. Hence tudy in the preent i baed on the CFD imulation uin Any 13 followin Eulerian model to predict heat tranfer characteritic of air-and flow in the lower plah reion of CFB rier of cro ection 0.15 m 0.15 m. Reult obtained were compared with experimental data obtained uin developed CFB facility. In the preent paper, ame computational methodoloy ha been implemented for upper dilute reion of rier durin cale-up (increae in rier cro ectional area) tudy to predict the heat tranfer characteritic. Reference [20] reported that rier of quare and rectanular cro-ection are now widely employed in circulatin fluidized bed application in heat tranfer area (boiler, bioma dryer etc). Alo cale-up tudy i important in dein point of view. Hence, there i a hih demand to tudy the effect of cale-up on heat tranfer characteritic of quare cro-ection rier. It i alo reported that cale-up tudy ha been performed at IIT Guwahati to predict the effect of cale-up on heat tranfer characteritic uin three CFB unit a in Fi. 1, each of heiht 2.85 m with rier cro ection of 0.15 (m) 0.15 (m), 0.20 (m) 0.20 (m), and 0.25 (m) 0.25 (m), repectively. However, becaue of limitation like requirement of extra fund and extra pace in laboratory to launch new experimental CFB et up havin rier of cro ection 0.30 (m) 0.30 (m), preent paper approached to accomplih the further cale-up tudy throuh CFD imulation, which were performed under ame operatin condition on upper dilute reion of rier of different cro ectional area to predict heat tranfer characteritic. Hence preent paper decribe computational method to predict the effect of chane in operatin parameter like chane in uperficial velocity of air and chane in eometrical parameter like chane in cro ectional area of rier on heat tranfer characteritic at lower dener and upper dilute reion of CFB rier, repectively. and 0.25 (m) 0.25 (m) with each rier heiht of 2.85 (m) deined and fabricated at IIT Guwahati [21]. Rier of CFB etup wa made up of plexila to facilitate flow viualization. A poitive diplacement type blower powered by a 20 (HP) motor upplie air. Heater ection poition are U (upper dilute or plah reion of particle), M (middle dene reion of particle) and L (lower plah or dene reion). Experiment were conducted on the CFB unit with and a the bed material and air a the fluidizin medium. Heat tranfer characteritic alon the rier were tudied with incorporation of heater ection in the upper and lowe plah reion; havin the ame cro ectional area a that of the rier (0.15 m 0.15 m) and heiht of 0.6 (m). The heater ection wa fabricated with MS heet of 2 (mm) thickne with a heiht of 0.6 (m). Nichrome wire or heater coil of 2 (kw) capacitie wa wound over the mica heet of 1.5 (mm) thickne which cover the MS wall of the heater ection. Another mica heet, which act a an electric inulator, wa wrapped over the Nichrome wire. To avoid the heat loe by radiation, ceramic wool and abeto heet were wrapped over the aembly. Heat wa upplied to the heater ection with electrical upply throuh an auto tranformer. To meaure the temperature of the urface of the heater ection and the bed, calibrated T- type thermocouple were intalled at the ame heiht on the wall a well a inide the heater ection repectively a in Fi Motor 2. Blower 3. Bypa Valve 5. Orifice Plate 6. Rier Column 7. Cyclone Separator 9. Sand Meaurin Section 10. Butterfly Valve 11. Ditributor Plate Fi. 1 CFB Setup at IIT Guwahati II. EXPERIMENTAL SETUP Fiure 1 preent the three CFB etup havin rier of cro ection area 0.15 (m) 0.15 (m), 0.20 (m) 0.20 (m), Fi. 2 Poition of the Thermocouple

3 , July 2-4, 2014, London, U.K. Ten et of thermocouple with equal pacin of 5.5 (cm) alon the heiht of the heater ection were ued to meaure the bed temperature and urface temperature of the heater ection, a in Fi. 2. A ection AA wa taken in the lateral direction at 0.16 (m) above the inlet of the heater and another one ection wa taken in the lateral direction 0.44 (m) above the inlet of the heater. Five thermocouple were placed alon the horizontal direction in thee ection with equal pacin at the nondimenional ditance [X/b] of 0.1, 0.3, 0.5, 0.7 and 0.9, repectively a in Fi. 2. Here the nondimenional ditance [X/b] i the ditance X meaured from the left hand ide wall of the heater to the thermocouple end, normalized with repect to the width b of the heater. III. EXPERIMENTAL HEAT TRANSFER STUDY Steady tate experiment were conducted on the CFB etup under imilar operatin condition to examine the effect of chane in rier cro ection on heat tranfer characteritic. Experiment were carried out at uperficial velocitie 2.5 (m/) and 4 (m/) or at nondimenional velocity ratio U* (ratio of uperficial velocity and minimum fluidizin velocity) of 5 and 8, particle ize 460 (μm) and heat flux 1000 (W/m 2 ). Further, experiment were conducted with certain and inventory o that weiht per unit area of ditributor plate P of each CFB etup wa 3050 (N/m 2 ) maintained in each CFB unit. Experiment could not be conducted for the value of P more than 3050 (N/m 2 ) becaue of contraint of maximum capacity of experimental etup (blower) to puh the maximum weiht of inventory of and per unit area of ditributor plate into the fat fluidization. The local heat tranfer coefficient h i calculated by h Q / [A S. (T S - T B )] (W.m -2.K -1 ) (1) IV. CFD MODELING AND SIMULATION In Any Fluent [23] the overnin equation are dicretized uin the finite volume technique. The dicretized equation, alon with the boundary condition, are olved to obtain a numerical olution. Eulerian multiphae model wa ued for the imulation of air-and flow. 3-D CFD imulation were done on heater ection (portion L, U a in Fi. 1 each of heiht 0.6 m). While mehin the heater, cell type elected wa tetrahedral/hybrid with T rid mehin cheme. Selected boundary condition a in Fi. 3 were air velocity at inlet, bottom of rier = 2.50 (m/) or 4 (m/), and volume fraction of and = 0; volume fraction of and at inlet at riht hand ide wall of rier = 1 and and velocity = (m/) or (m/); outlet boundary condition wa preure outlet at top of the rier = 0 (Pa) aue preure of air-and mixture. While imulatin the rier in Any, 3D unteady tate olver wa ued. Other parameter elected were denity of and = 2600 (k/m 3 ), mean diameter of and = 460 (µm), denity of air = (k/m 3 ), turbulence model ued = RNG k-ε model, numerical method ued for preure velocity couplin = phae coupled SIMPLE, dicretization cheme = 1 t order upwind, under relaxation parameter for preure = 0.1, denity = 0.1, body force = 0.1, momentum = 0.1, volume fraction = 0.2, enery = 0.1; converence criteria = 0.001, olution initialization = from all zone, heiht of the and inventory above the ditributor plate in the different CFB rier wa maintained to obtain the tatic preure on ditributor plate a 3050 (N/m 2 ). In the Eulerian multiphae (a-olid, two fluid) model, conervation equation of ma and momentum for both phae are developed and olved imultaneouly. The link between the two phae i throuh the dra force in the momentum equation. where Q i rate of heat upplied to the heater, meaured uin a Wattmeter. A i the urface area of heater, q = Q/ A i the heat flux, T type calibrated thermocouple and data acquiition ytem with Day Lab oftware verion 8.0 wa ued to meaure the urface temperature T S and bulk mean bed temperature T B. The local heat tranfer coefficient i meaured at 10 location (y = 1 to 10 a in Fi. 2) alon the heiht of heater. Averae heat tranfer coefficient (h av ) alon the heater ection at it any particular location above the ditributor plate i calculated by H 1 h av h y. d y H (W.m -2.K -1 ) (2) 0 where H i the heiht of the heater (0.6 m), h y i the local heat tranfer coefficient. Local heat tranfer coefficient (h y ) i calculated at 10 different point (y = 1, a hown in Fi. 2) alon the heiht of heater ection. Uncertainty analyi wa carried out for the heat tranfer coefficient. Uncertainty i dependin upon connection of thermocouple, accuracy of T type thermocouple ± 0.5 ( C), wattmeter accuracy ± 5 (W), accuracy in lenth meaurement ± 1 (mm) etc. Uncertainty analyi, uin the method of Kline and McClintok [22], howed that the heat tranfer coefficient etimated in the preent tudy were within ± 4 %. Fi. 3 CFB Rier

4 , July 2-4, 2014, London, U.K. Continuity (k th phae) n ( ) ( u ) m t k k k k k (3) p1 pk where k f for fluid k for olid Momentum (fluid phae) ( f fu f ) ( f fu f u f ) t p K u u F f f f f f f f Momentum (olid phae) ( u) ( u u) t p p K u u F f f Total volume fraction conervation Equation (3) repreent the ma balance of each phae with temporal and patial radient on the left hand ide and ma creation ( m ) of the p th pecie (in thi cae, zero) by reaction or phae chane. Equation (4) and (5) are momentum conervation equation for the fluid (air in thi cae) and olid phae, repectively. The left ide repreent temporal and patial tranport term wherea the riht hand ide ha term for the variou interaction force involved. Note that the hydrodynamic preure i hared by both phae and hence, the radient of preure ( p ) i premultiplied by the repective volume fraction ( ) in both equation. ( ), (u ) and ( ) repreent to denity, velocity and acceleration due to ravity repectively. The tre term ( f ) repreent the hear tre in a phae in (4). Equation (5) repreent the olid phae equation, where ( ) repreent the hear tre term due to colliion amon particle. Term K f u f u and K f u u f repreent the momentum exchane or dra between the two phae in (4) and (5). Thee term are equal in manitude and oppoite in in and account for the friction at the interface between the phae. The term ( F ) in (4) and ( F ) in (5) f repreent all other force that may affect the flow, uch a electrical, manetic and other effect. The dra i an effective way of repreentin the urface interal of all the force that exit at the interface between the phae. Interphae momentum exchane factor of Gidapow dra cloure a in (7). (4) (5) 1 (6) f For 0.8, 3 K C u u C 2.65, f d, 4d d,.[ (.Re ) ].Re For 0.8,.. K u u 2, f d d (7) where Re and d are the Reynold number and diameter of olid particle repectively and other ymbol have their tandard meanin which already are defined. Entire CFB rier of 0.15 m 0.15 m wa modeled and mehed with tetrahedral cell, which were elected for it imulation after rid independence tet. Enery equation (8) wa applied durin heat tranfer 3-D imulation for heater ection a in Fi. 4.. ( E).( v( E p)).( keff T hj jj ( eff. v) Shj t (8) where effective conductivity ( k ) and i the diffuion flux eff ( j ) of pecie j. j The four term of the riht hand ide in (8) repreent enery tranfer due to conduction, pecie diffuion, vicou diipation and volumetric heat ource ( S hj ). Now, heat tranfer imulation (by enablin enery equation) for the heater ection were carried out uin both multiphae model to obtain the bulk mean bed temperature (T b ) and wall temperature (T ). Simulation were carried at contant heat flux q = 1000 (W/m 2 ) for different and inventory and particle ize for a uperficial velocity of 4 (m/). Local convective heat tranfer coefficient h i calculated by uin (1). Simulated reult were compared with experimental reult. Reult and dicuion i explained in the followin ection. V. RESULTS AND DISCUSSION A. Studie on Effect of Superficial Velocity of Air on Heat Tranfer Characteritic Effect of uperficial velocitie of air on heat tranfer characteritic have been preented in Fi Reult were compared with available experimental data obtained uin CFB etup of IIT Guwahati a in Fi. 1. Simulation were done in the lower plah reion for the two uperficial velocitie of air 2.5 (m/) and 4 (m/) at and inventory of 7 (k) i.e at tatic preure of 3050 (N/m 2 ) i due to weiht of and inventory per unit area of ditributor plate. j

5 , July 2-4, 2014, London, U.K. Fi. 4 Temperature (K) Contour for 0.15 m 0.15 m CFB Rier at U = 2.5 m/, P = 3050 N/m 2 Fi. 7 Temperature Counter (K) Upper Splah Reion of 0.15 m 0.15 m at U = 4 m/ and P = 3050 N/m 2 Fi. 8 Temperature Counter (K) Upper Splah Reion of 0.30 m 0.30m at U = 4 m/ and P = 3050 N/m 2 Bed Temperature, K Effect of bed Cro Section U* = 8, P = 3050 N/m 2 Expt._0.15 m x 0.15 m Fluent_0.15 m x 0.15 m Fluent_0.30 m x 0.30 m Fi. 5 Bed Temperature Ditribution Nondimenional Ditance acro Bed Width, X/ B Fi. 9 Bed Temperature Ditribution at a Section 2.24 m above the Ditributor Therefore bed temperature decreae with increae in uperficial velocity of air, which reult in increae in the drivin temperature difference (T S T B ) and decreae in heat tranfer coefficient, while the heat flux wa held contant. The percentae difference between predicted value and experimental value wa varyin from 2% to 18% for bed temperature ditribution and hence alo for axial ditribution of heat tranfer coefficient, becaue heat flux wa held ame (eq. 1). Fi. 6 Axial Ditribution of Heat Tranfer Coefficient Sand velocity at inlet at riht hand ide wall of rier wa (m/) and (m/) at U* = 5 and U* = 8 repectively for CFB unit 1. Remainin detail are mentioned in ection IV of preent paper. Temperature counter i preented in Fi. 4. It i oberved that bed temperature and heat tranfer coefficient decreae with increae in uperficial velocity of air. Thi i becaue and concentration decreae acro the ection of the rier column and alo near the wall of the heater with increae in uperficial velocity of air which reduce the heat tranfer from wall-to-bed due to conduction. B. Studie on Effect of Rier Cro Sectional Area on Heat Tranfer Characteritic Effect of rier cro ection on heat tranfer characteritic have been invetiated. Rier of bed cro ection 0.30 (m) 0.30 (m) ha been modeled and imulated uin Any. Reult obtained on bed temperature ditribution and heat tranfer coefficient have been compared with reult obtained 0.15 (m) 0.15 (m) CFB bed. Simulation have been completed for the upper plah reion for both the rier with uperficial velocity of air a 4 (m/), and maintainin the preure 3050 (N/m 2 ) due to weiht of and inventory per unit area of the ditributor plate. Eulerian multiphae model with Gidapow phae interaction cheme wa ued to imulate the two phae flow. Particle ize ued wa 460 μm and heat flux of 1000 W/m 2 wa applied at the wall of heater. Sand velocity at inlet at riht hand ide wall of rier wa

6 , July 2-4, 2014, London, U.K. Fi. 9 Effect of Bed Cro Section on Axial Ditribution of Local Heat Tranfer Coefficient at U* = 8, P = 3050 N/m (m/) at U* = 8 for CFB unit 1 (0.15 m 0.15 m) and CFB unit of cro ection 0.30 m 0.30 m. Remainin detail are mentioned in ection IV of preent paper. Temperature counter are preented in Fi Fiure 9-10 repreent the bed temperature ditribution acro a bed cro ection at 2.24 m above the ditributor plate and axial variation of local heat tranfer coefficient, repectively. Averae bed temperature at lateral ection at 2.24 m above the ditributor plate wa more in maller cro ection heater than larer cro ection heater. Thi i expected becaue and inventory in larer cro ection CFB etup wa kept proportionately more than the maller ize CFB etup o a to maintain the ame weiht of and per unit area of the ditributor plate. Therefore, weiht of and particle per unit urface area of the larer cro ection heater wa more than the maller heater. Therefore at ame heat flux applied at heater wall of each CFB etup, ditribution of amount heat extracted due to conduction from wall of the heater took place into lare number particle, which were comparatively more in larer cro ection CFB etup, hence averae bed temperature wa le for laer ize heater than maller heater. It i oberved that in the wall-to-bed heat tranfer tudy, heat tranfer coefficient increae with increae in bed ize. Thi i becaue the drivin temperature difference (T S T B ) in the larer ize bed wa leer than the maller ize bed, while the heat flux wa held fixed. Thi i becaue of the hiher concentration of olid particle near the wall of the larer bed, and conequently, lower thermal reitance from the bed-to-wall cauin better heat conduction. VI. CONCLUSIONS Heat tranfer experiment were conducted on in-houe fabricated CFB etup at the bottom reion (dener reion of and particle) via heater, and 3-D numerical imulation uin Any. Numerical and experimental reult were in ood areement for the rier of cro ectional area 0.15 m 0.15 m. To accomplih the cale-up tudy, further CFD imulation were done on rier havin cro ectional area 0.30 m 0.30 m. Eulerian-Eulerian multiphae model with Gidapow phae interaction cheme i found to be accurate model to imulate the two phae flow. Bed temperature decreae with increae uperficial air velocity and rier cro ectional area. Heat tranfer coefficient increae with decreae uperficial velocity of air and increae in rier cro ectional area. REFERENCES [1] A. Almuttahar and F. Tahipour, Computational fluid dynamic of hih denity circulatin fluidized bed rier: Study of modelin parameter, Powder Technol., vol. 185, 2008, pp [2] V. Ranade, Computational flow modelin for chemical reactor enineerin, Acad. Pre, pp [3] A. Miller, D. Gidapow, Dene, vertical a olid flowin a pipe, AIChE J., vol. 38, 1992, pp [4] R. Bader, J. Findlay, T. 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