Australian Journal of Basic and Applied Sciences

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1 AENSI Journas Austraian Journa of Basic and Appied Sciences ISSN: Journa home page: Comparatie Studies of Heat and Mass Transfer by Conectie and Microwae- Conectie Drying for Nonhygroscopic Ceramic 1 Zawati Harun and 2 Tze Ching Ong 1 Associate Professor, Uniersity Tun Hussien Onn, Adanced Manufacturing and Materias Center (AMMC, Facuty of Mechanica and Manufacturing Engineering, Parit Raja, 86400, Batu Pahat, Johor, Maaysia 2 PhD candidate, Uniersity Tun Hussien Onn, Adanced Manufacturing and Materias Center (AMMC, Facuty of Mechanica and Manufacturing Engineering, Parit Raja, 86400, Batu Pahat, Johor, Maaysia A R T I C L E I N F O Artice history: Receied 15 September 2014 Accepted 5 October 2014 Aaiabe onine 25 October 2014 Keywords: Nonhygroscopic,ceramic, conectie, microwae, drying, mathematica mode, A B S T R A C T Drying is a compex process as it inoes a ot of mechanism and materia process particuar heat and mass transfer that eoes concurrenty during the process. In a simpe word drying can be defined as remoa of water or moisture from any porous substances. Drying may be accompished by conectie heat transfer, conduction from heated surfaces, by radiation or by dieectric heating. Different drying technique aso wi resut in the different structure and properties of the dried or sinter body since moisture remoa has strong correation with shrinkage, packing porous structure as we as pore formation. Thus, understanding drying mechanisms under different drying techniques can hep to contro faiure of the dried product. This aim of this paper is to propose a mathematica mode and compare the drying mechanism for both conectie and microwae-conectie drying for nonhygroscopic ceramic materias. This current work used a couped mathematica mode of mass, heat and gas transfer that embedded with finite eement method in two-dimensiona domain and numericay computed using Skyine soer to capture highy noninear transient process. The mode ariabes which proide anaysis of time eoution of saturation, temperature and gas pressure are used to obtain better understanding of the mechanism that occur during the process of drying at fundamenta ee. Vaidation of the proposed mode shows good agreement with the experimenta data and other mode resuts gained esewhere. The computed resuts showed effectieness of the drying process improes drasticay in conectiemicrowae drying when compared to conectie drying corresponding to mass and heat fuxes coincide in same direction, where from interna of matrix to the surface materia. Thus easier remoa of moisture is noted with higher temperature, moisture saturation and gas pressure accumuated at the bottom center of the materia. Howeer, the combination of those gradients may aso ead to increment in potentia interna defects on the dried materia. Neertheess, the increase understanding in fundamenta mechanism that occurs during both drying modes is acknowedged resuted from the proposed mode AENSI Pubisher A rights resered. Cite This Artice: Zawati Harun and Tze Ching Ong., Comparatie Studies of Heat and Mass Transfer by Conectie and Microwae- Conectie Drying for Nonhygroscopic Ceramic. Aust. J. Basic & App. Sci., 8(15: , 2014 INTRODUCTION Various materias undergo drying process in the industries. Drying process inoes moisture migration from the interna of the porous matrix to the externa surrounding enironment which occurs through one or more mechanism (Haghi, 2006; Harun and Gethin, 2008; Perré, P, et a., This process is further compex ia sophisticated as it is highy infuences by the materia properties and characteristic of the dried materia. Thus, drying rates are unique between each configurations of drying process in regards to the drying methods and dried materias itsef. Production efficiency which associate with time, cost, energy consume and quaity of fina product are cosey interreated with the drying rate of the seectie drying method (Mujumdar, Hence, in order to optimize a those aforementioned parameter, the combination of drying methods with different sources of energy suppy are more preferabe when compared to traditiona conectie method. The main aim of this paper is to present a proposed mathematica mode of heat and mass transfer for both conectie and conectie-microwae drying and its aidation with experimenta data and other mode resuts gained esewhere. The proposed heat and mass transfer in this mode may show combination of the different Corresponding Author: Tze Ching Ong, PhD candidate, Uniersity Tun Hussien Onn, Adanced Manufacturing and Materias Center (AMMC, Facuty of Mechanica and Manufacturing Engineering, Parit Raja, 86400, Batu Pahat, Johor, Maaysia. Te: ; E-mai: aex_ongtc@yahoo.com

2 219 Zawati Harun and Tze Ching Ong, 2014 methods of drying which expect to bring positie output in shortening the drying duration and better quaity of the dried products. Conectie drying is often use in many materia industries such as wood (Perré and Turner, 1999, fruit (Wang & Chen, 1999, fabric (A K Haghi, 2003 and others. The commony known adantage of the conectie drying when compared with others methods is its reatie ow cost as often associated to ambient or natura drying condition. Howeer, the significant drawbacks are onger drying duration and ow quaity of dried products. Thus, new drying methods which can offer better production time and quaity are becoming a necessity in many reated industry. Therefore, arge efforts to tacke such probems associated with conectie drying generate wordwide interest. Latey, studies that inoe combination of the drying methods hae gained increase attention due to many adantages. Recenty, the concept of microwae drying has been intensiey used for drying a wide ariety of materias and appications. Interna heating that offers by microwae drying resuts in ower consumption of energy, better structure of dried products of mechanica strength or bioogica aue is seems to the answer of improing the efficiency of the conectiona conectie drying (Turner et a., Conectie-microwae in ceramic drying process conducted by Kowaski et a., 2010 has shown a rapid improement in the effectieness of the drying process ia time and quaity of the dried materia. Howeer aso, experimenta works conducted by Kowaski et a., 2012 on microwae drying using aries microwae power shows materia may suffer destruction and shrink effect caused by the induced stress works especiay of high power configuration. This showed the microwae heating is intensiey reated to the microwae power suppied as the rapid heating cause sudden moisture gradient from interna towards the materia surface and further towards surrounding. This phenomena often causes interna destruction resuted from interna oerpressure and this may cause arious gradients effect from different eoe ariabes during the drying process (Kowaski et a., Therefore, the fundamenta anaysis of heat and mass transfer phenomena inoing the gradient that dominant during the different phase of the drying process is cruciay needed in conducting studies of drying works. Comprehensie modeing of conectie-microwae drying has been thoroughy treated in the past (Ni et a., 1999; Ratanadecho et a., 2001; Sanga et a., As noted in those works, a ery compex heat and mass transfer process arises during the drying process due to the nature of the process and compicated structure of the porous materia itsef. These compications resut a compex mathematica formuation to represent the detais of the physica phenomena associated with the process. Howeer, due to its adantages such as increase in drying rates and faster drying times that offered by the process has attract many detai inestigation in improing and extending this drying technique especiay when there is imited research in area of conectiemicrowae in ceramic membrane preparation. In this present works, the former conectie mode presented by Z Harun and Gethin, 2008; Z Harun and Ong, 2014; Zawati Harun et a., 2014 is modified and extended to suit with the proposed drying method. In this proposed mode, the work from preious study wi be reised to impement the couped mass, heat and gas transfer in two-dimensiona domain for nonhygroscopic porous materias using a more rigorous mathematicay microwae heating mode which is embodied after carefu studied. The main moisture migration transport mechanism are due to iquid fow by capiary action, apor by diffusion and gas by apor diffusion and buk air fow in conectie-microwae drying phenomena were taken into consideration in this mode. The next section of this paper wi discussed in detai the deeopment of the mathematica mode. To insti the confidence in the proposed mode, the computed resuts wi be aidated extensiey with experimenta data and others modes gained esewhere. (Turner et a., 1998; Stanish et a., 1986 Theoretica Formuation: Genera mass and energy conseration aw are used to define the mechanism of mass and heat transfers during the drying process in this mode. The hydrauic transports of the drying are represents iquid by capiarity action, diffusion by apor and air. The three measured system ariabes are water pressure P, temperature T and gas pressure P g. ( ρ S ( ρ S g.(ρv.(ρv.(ρvg t t (1 The water eocity, V and gas eocity, V g can be easiy deried from Darcy s aw as (A. K. Haghi, 2006: V K (P Z V K P g g g (3 where, K denotes the hydrauic conductiity of water, K g is the hydrauic conductiity of gas and Z is the ertica eeation from a datum (positie upward. Aso noted from eq. (3, effect of graity is significant in iquid fow but not for gas fow. Meanwhie apor eocity by diffusion is defined as (Kanno et a., 1996; V D atm a. (2 (4

3 220 Zawati Harun and Tze Ching Ong, 2014 where D atm denotes the moecuar diffusiity of water apour through dry air, α is the tortuosity factor, ν is a mass fow factor and θ a is the oumetric content of the air. In this work, the moecuar diffusiity of water apour through dry air is taken from the works of Stanish et a., 1986 as: D atm 2.20* Pa P T (5 The expression used in this study for the mass fow factor ν was introduced by Phiip and De Vries, 1957 and they suggested the use of an expression proposed as: P g P g P where P denotes the partia pressure of apour and can be cacuated from, P ρ R T (7 The tortuosity factor is set to a fix aue of 0.5 for both horizonta and ertica axis in ceramic structure. Rearranging the aboe equation according to the measured ariabes gies; D atm a h h h V o P ( o h T o Pg P T Pg (8 By appying a mass baance to the fow of dry air within the pores of the materia body dictates that the time deriatie of the dry air content is equa to the spatia deriatie of the dry air fux. ( ρa S g (ρavg ρv t (9 The ony effect of microwae towards the materia is that they generate heat inside the materia. Thus, the conduction, atent heat, conection and oumetric heat suppy are considered in the energy equation as gien as beow; L-( ((1- c ρ p s ia,, si ρici (λt (ρ V ρ V g (T-T ρ c V ia,, t (10 where denotes the oumetric heat suppy due to absorption of microwae energy depends on water oumetric therma conductiity and is expressed using a inear dependency of the absorbed microwae energy on moisture content by the foowing Kowaski et a., 2010: A BX s ( (11 where A and B express the amount of microwae energy absorbed by the porous matrix and moisture respectiey. For conectie drying, we use simiar equations as microwae-conectie drying with the difference that this time, 0. Furthermore, in order to get the set of equations for the drying mode, a ist of assumptions and restrictions is necessary. The foowing ist is the assumptions and restrictions that are impemented to the current proposed mode: i. The three phases, soid, iquid and gas phases within the porous structure are aways exist in oca thermodynamic equiibrium. ii. The fuid is consists of iquid and apour phases. iii. The temperatures of the iquid and apour phases in the body are equa at coincident arbitrary points due to moement of moisture in the porous skeeton is sow. i. Idea gas aw appied to the gas phase that fied the pores and is a binary mixture of air and apour.. A dimensiona changes that induced during drying are sma and negigibe. Hence, matrix is nondeformabe. i. Liquid is treated as non-bound water that is hed nonhygroscopicay within the soid phase. ii. The ony effect of microwae is they generate oumetric heat source inside the materia to be dried. iii. The matrix is rigid, homogenous and isotropic. ix. Porosity of matrix is uniform. x. Darcy s aw appies to gas and iquid phases. xi. Stresses induced aso insignificant and negigibe. xii. Finay, hysteresis phenomena are ignored. Thermodynamic Reationship: The existence of a oca equiibrium at any point within the porous is assumed. Kein s aw is appied to the equation beow. r i pi i (6

4 221 Zawati Harun and Tze Ching Ong, 2014 Pw Pg h exp RT (12 The apour partia pressure can be defined as a function of oca temperature and reatie humidity where the saturation apour pressure, ρ o is estimated from Mayhew and Rogers, 1976 with saturated apour density as a function of temperature as exp (T *10 3 (T ρo (13 The degree of saturation, S is an experimentay determined function of capiary pressure and temperature. S = S (p c, T (14 Saturation if expressed as combination of temperature effect [9] as in equation beow. m r 1 S n s r 1 ( ( T (15 where the parameters α, n and m are dependent of porous materia properties and infuence the shape of the water retention cure. The permeabiity of water and gas are based on Mueem s mode (Baroghe-Bouny et a., 1999as 1 m 2 S (1(1 S m k (S S Sirr 0 S Sirr (16a 1 m 2m 1 S (1 S m k (S S Scri(g g 0 S Scri(g (16b Materia Data: Figure 1 shows the meshing of the sampe in two-dimensiona domain using eight-nodes quadratic quadriatera eement. Fig. 1: Meshing of the rectanguar sampe. The sampe used is referred as a ery thin rectanguar sab of ength L= 3cm and height H= 3cm. The sab was meshed with 36 eements and 133 nodes and is assumed to be a porous medium that is homogeneous, isotropic and composed of soid phase, water and apor phase, gas phase and dry air phase. The initia saturation was set to be S 0 =0.7 and the initia temperature T 0 =30 C. The materia properties and transport parameter used as data for simuation are isted in Tabe 1. Tabe 1: Physica properties of ceramic body and transport parameter. Density of porous matrix ρ s kg/m Porosity Ø Intrinsic permeabiity K m Therma conductiity of the porous matrix λ W/m K 1.8 Specific heat capacity of porous matrix C p J/kg K 920 Critica saturation S cri Irreducibe saturation S irr Constant denotes the absorption of microwae energy by skeeton (Eq. 11 A J/m 3 s 126 Constant denotes the absorption of microwae energy by moisture (Eq. 11 B J/m 3 s 20 Boundary Condition: To refect the drying condition in conectie-microwae drying chamber, the boundary conditions for mass and heat transfer were chosen as foows: J m hm ( P Ps (17

5 222 Zawati Harun and Tze Ching Ong, 2014 J T (18 hc( T Tf The boundary conditions of both drying modes are appied to the top and side surfaces of the sampe whie the ower bottom surface is impermeabe and insuated for heat and moisture fow. Aso top and side surfaces are assumed to remain under atmospheric condition. The ambient temperature is set at 30 C with heat and mass coefficient of 20 W/m 2 K and ms -1. The maximum temperature in whoe oume of the sab is set equa to the boiing water temperature corresponding to water as the main absorption medium of microwae energy in microwae-conectie drying. Simiar configuration is used for conectie drying with the difference that the heat fux is pointed from outside to inside. The change of heat fux direction is impemented by mutipying both side of Eq. 18 by minus 1. Soution Of Goerning Equations And Numerica Method: The two dimensiona transient noninear couped heat and mass transfer equations described aboe are written in the form of a matrix as foows; [C(Φ {Φ} ([Kcx( Φ]ix [Kcy( Φ]iy {Φ} R( Z t (19 where {Φ} = {P w,t,p g } is the coumn of unknowns; [C],[K cx ]and [K cy ] are 3x3 matrices. Each eement of the matrix is a coefficient for the unknown{φ}; i x and i y are the unit direction ectors. In order to discretize this simpified second order non-inear couped partia differentia equation, finite eement method is used. Afterwards, Gaerkin method is used to minimize the residua error before the appication of Greens theorem, to the dispersie term inoing second order deriaties; this simpified combined equation set can be expressed in the foowing form. K( C( J( {0} (20 The transient matrix and noninear second order differentia equations aboe are then soed by using a fuy impicit backward time stepping scheme aong with a Picard iteratie method which is taken into account for non-inearity. RESULTS AND DISCUSSION The first aim of this section is to compare the drying resuts computed by the proposed mode with the experimenta data and other modes resuts. The aidation of current mode with the preious experimenta data and computed resuts by Turner et a., 1998 for microwae-conectie drying and Stanish et a., 1986 for conectie drying are present in Figure 2. Fig. 2: Comparison of drying cure: (a microwae-conectie; (b conectie. Figure 2 shows a comparison resuts between the predicted proposed mode, other s mode and experimenta data. As demonstrated in this figure that a the drying period are present for a times for both drying techniques respectiey. The good adherence of resuts generated by the proposed mode with experimenta data and other mode resuts simuates that the computed resuts presented in this paper are the proper ones. Fig. 3: Saturation (a and temperature (b cure with time for both drying modes. Figure 3 depicts the temperature distribution and saturation eoution with time for both drying modes. The drying saturation cure is characterized with the straight segments in the constant drying rate periods (denotes by A in Figure 3(a and noninear cure in the faing rate period (denotes by B in Figure 3(a. For microwaeconectie drying, temperature arises abrupty due to the heat generation from the microwae energy directy heated the interna moisture oumetric across the bodies. Then sampe temperature begin to rise drasticay at constant rate period corresponding to oumetric heat as energy is transferred directy to the whoe oumetric

6 223 Zawati Harun and Tze Ching Ong, 2014 iquid with homogenous distributed throughout the wet materia. This mechanism aso ery profitabe to moisture transport as depicted in Figure 3 aso saturation decreasing rate is constant. This is mainy generated by the interior heat fuxes proide higher fuxes of iquid to surface from the interior of the materia. Subsequenty, when saturation ee reaches the critica water content which is 0.3 for ceramic materias, the water rapidy changes into apour phase. Consequences from decining saturation ee, the temperature rising profie aso decrease which water is the main absorption of the heat energy by microwae when reaches the faing rate period. Eentuay, when saturation decrease graduay to the irreducibe aue where no water can be dried indicating the end of a drying condition. Temperature profies aso show no increment with decreasing of moisture content is decreasing as itte heat energy is produced. Meanwhie during conectie drying, saturation cure aso iustrated a those aforementioned drying period but with significant sower rates due to coiding heat and mass transfer. Aso noted, the surface materia temperature for microwae-conectie drying is ess than the center of the materia due to the oumetric heat generated is proportiona with the moisture content and this phenomena is conersed with the conectie drying method where minima temperature difference between surface and center of materia is noticed. The contour eoution of conectie-microwae drying on saturation and temperature distribution inside porous matrix wi be further discussed and anayzed in detai when compare to the conectie drying. Fig. 4: Contour of saturation and temperature: (a temperature at 1 hour for microwae-conectie drying; (b temperature at 6 hour for microwae-conectie drying; (c temperature at 1 hour for conectie drying; (d temperature at 23 hour for conectie drying; (e saturation at 1 hour for microwae-conectie drying; (f saturation at 6 hour for microwae-conectie drying. Heat suppied oumetricay eads to the different distribution of temperature obtained for conectiemicrowae drying as shown in Figure 4(a and 4(b. This temperature distribution is accordance with experimenta isuaized distribution due to infrared camera measurement as presented in Kowaski et a., This mechanism generated buk heating phenomena inside the porous matrix. As can be noticed, the highest temperature is generated around the bottom center of the rectanguar sampe when microwae enter the rectanguar sampe through the atera and upper surface. This corresponding to the accumuated moisture in the bottom center of the sab is due to the graity effect. The distribution of temperature gained in conectiemicrowae drying is ery profitabe for moisture remoa as the heat and mass fuxes coincide, which is contradict in conectie drying where the fuxes moes in the opposite direction. This aso eentuay eads to easier moisture remoa and is purey shown in the Figure 4(e and (f which iustrates the distribution of saturation content in the sampe at eary and end stage of drying. As it is isibe in this figure, the highest saturation is aso at the bottom center of the rectanguar sampe and the owest one is in the top side corner. This indicates the most intensie part of the free mass transfer takes pace at the top side corner of the rectanguar sampe. This is due to abiity of water to absorb microwae energy proides an adantage compare conectiona conectie drying mode. Conersey to conectiona conectie drying mode where heat is transfer from the ambient to the surface materia then to the interior of the materia reies mainy on the conduction and conection propagation (refer Figure 4(c and 4(d, in microwae drying the heat is generated within the accumuated iquid inside interior of the materia itsef. Thus, this eads to more uniform moisture distribution, reducing drying time as drying rate increase abrupty from this mechanism. Neertheess, saturation contours for conectie drying are simiar to microwae-conectie drying, thus not is not presented in here.

7 224 Zawati Harun and Tze Ching Ong, 2014 Fig. 5: Gas pressure eoution in time: (a microwae-conectie drying; (b conectie drying. Figure 5(a shows the gas pressure eoution in time at different positions inside the domain for conectiemicrowae and conectie drying modes. As iustrated, the gas pressure sowy buid up at the constant rate period of the drying process where moisture content is high inside pores at initia stage and capiary mechanism is the main suppy of moisture from interna to surface materia for eaporation to happen. When the drying stage reaches the faing rate period, pressure buid up more drasticay due to diffusion of apour cause by phase transition change of water to apor. Subsequenty, gas pressure recede to atmosphere condition drop as drying is amost accompished. As noted in figure 5, the surface of materia remains constant at atmosphere pressure and higher interna pressure buidup within interna of the materia. Figure 5 aso demonstrated higher gas pressure occurs during microwae-conectie drying when compared to conectie drying. Subsequenty the gas pressure gradient in the interna is high at the faing rate period which can cause interna damage in microwae-conectie drying as proen in experiment by Kowaski et a., 2012 that showed microwae tend to exhibit interna cracking compare to conectie drying. Thus, controing the drying rate for conectiemicrowae drying is ita to aoid faiure in dried materias. Concusion: The resuts computed by the proposed mode enabe the inestigation of fundamenta mechanism for both conectie-microwae and conectie drying. It is noticed that the drying rates can increase drasticay in conectie-microwae drying compared to conectie drying. To iustrate the aboe point, both heat and mass fuxes coincide in the same direction from internay to materia surface as moisture remoes continuousy. Thus, both maximum temperature and saturation can be seen accumuated at the center bottom of the sampe whie top side surface exhibit owest temperature and moisture content as the most profitabe moisture extraction happen at this position. Aso noted, the gas pressure is gathered at the same position and this coud ead to oerpressure internay. Lasty, this mode is considered to be a usefu too for simuating the process of conectie-microwae and conectie drying process due to its reasonabe accuracy at a times. Notations: T = Temperature, K P = pressure, Pa V = eocity, ms -1 ρ = density, kg/m 3 S = saturation Ø = porosity C p = specific heat capacity, J/(mo K θ = oumetric K = intrinsic permeabiity, m 2 k = reatie permeabiity λ = therma conductiity, W/mK t = time, s R = gas constant, J/(mo K µ = iscocity, N s m -2 h =reatie humidity, % L = Latent heat of aporization, J kg -1 J m = mass transfer fux J T = heat transfer fux h m = mass transfer coefficient h T = heat transfer coefficient Ω e = eement domain K ij = kinetic coefficients C ij = capacity coefficient N r = Shape function of residua error N s = Shape function of system ariabes Subscripts: a, c,, g,, b = air, capiary, apour, gas, iquid, bound water s = saturated irr = irreducibe cri = critica r = residua = cacuated f = fina ACKNOWLEDGEMENTS Authors thank to the financia support from Uniersity Tun Hussien Onn (UTHM and KPTM.

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