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Numerical and experimental thermal analysis of an industrial kiln used for fritproduction

T.S. Possamai*, R. Oba, V.P. Nicolau

Department of Mechanical Engineering, Federal University of Santa Catarina, Campus Universitrio, Trindade, Florianpolis 88.040 900, SC, Brazil

h i g h l i g h t s

< Numerical simulation applied to a ceramic frits melting kiln.


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insulation of the walls and preheating of the combustion air and ofthe load can result in a fuel saving of up to 10%. Carvalho and

Nogueira[7]described the use of modeling tools together with on-line measurements for the optimization of ceramic and glass-melting furnaces, cement kilns and baking ovens providing

a contribution for the pollution abatement strategies in this type ofequipment. In the area of fusion kilns, Nieckele et al.[8]presenteda numerical analysis of an aluminum melting furnace solved incommercial CFD software (Fluent) with emphasis on the compar-

ison of the type of fuel used, natural gas or liquid. One of theirconclusions was that operation with natural gas leads to a moreeffective conguration, providing longer life for the refractory walls

as well as a lower amount of pollutants.Experimental data on kilns operating in the ceramic industry are

available mostly for tunnel and roller kilns. Kaya et al. [9] andNicolau and Dadam[6] have reported data collected on operating

tunnel kilns related to the wall and ow temperatures and energybalances indicating temperatures up to 1250 K in the ring zone.The latter study indicated a specic consumption (red material-based) of 3.47 MJ/kg. The specic consumption of ceramic kilns

varies with the required product but in general it lies within therange of 2.0e4.0 MJ/kg for most ceramic products produced on

industrial scale, such as bricks and roof and oor tiles[5]. In theceramic frits sector there is a lack of literature sources with regardto experimental data and the thermal analysis of the melting kilnused in a continuous process.

In this context, the aim of this study is to analyze the energy in

a ceramic frits kiln through experimental and numericalapproaches. The numerical model was solved in commercial CFDsoftware (Ansys CFX)modeling the phenomena associated with thecombustion of natural gas and oxygen, radiation with the

combustion gases as participating media and the internal ow. Thismodel is coupled to an independent code in FORTRAN whichenables the solid domain to be solved apart, composed of the wallsof the kiln and the load, with particular models. The resemblance of

this specic kiln to some types of metal melting furnaces enables

the extension of this study to other industrial sectors. The impor-tance of a reliable numerical model in the optimization of ceramickilns is enormous. The validation of the numerical model proposed

in this paper based on experimental data collected from realequipment is presented as part of the results of this study.

A model based on the eddy dissipation concept (EDC) of Mag-nussen[10]was used to solve the combustion of natural gas and

oxygen. The standard ke 3model was used for turbulence and theradiation was modeled with the rst-order Differential Approxi-

mation (P1). A method to calculate the absorption and diffusioncoefcients for the media composed of the combustion gasesproposed by Siegel and Howell [11]based on empirical data wasused for a composition ofxed components constant through the

uid domain. This study methodology is divided into four steps: (i)

physical problem and geometric model; (ii) acquisition of experi-mental data; (iii) numerical modeling; (iv) numerical model vali-dation from experimental data.

2. Physical problem and geometric model

The kiln is suspended at a height of 0.50 m above the ground bymetal beams. Two burners for natural gas and pure oxygen arepositioned at the front of the kiln. The primary one is placed in thefront wall producing a ame inside the kiln that is directed to the

raw material volume added to the inside of the structure throughthe back wall. The secondary burner acts right in front of the kilnexit, on the outside, in order to maintain the uidity of the ow ofproduct from the exit hole to the cooling tank. The fuel ratio

between the primary and secondary burner is high, concentrating

more than 90% of the fuel in the primary burner. The maincomponents of the kiln are presented in Fig.1, including an openingin the chimney bottom (1), the raw material inlet (2), the chimney(3), the frit outlet, the secondary burner and the water reservoir (4)

and the primary burner (5).The kiln does not possess any internal parts, consisting only of

an empty structure with a small slope in the base to assist the owof the frit mass in the uid phase to the front exit. The hot gas

products of the combustion enter the chimney through an exit inthe front portion of the left wall while the frit massows throughan exit in the front wall, under the primary and secondary burners,directly into a cooling tank lled with water at ambient tempera-

ture. The fragmented frits are accumulated at the base of thecooling tank and removed to storage bags.

On the chimney bottom there is an extra opening forambient airto mix with the hot combustion gases from the kiln in order to

reduce the ow temperature. The raw mass of frits when pushed tothe inside of the kiln piles up at the back forming a wallof solidmass with approximately 2 m length inside the kiln, as can be seeninFig. 2. The internal cavity of the kiln is rectangular with prime

dimensions of 1.80 m width, 1.20 m height and 5.65 m length ofwhich 2 m are occupied by the above-mentioned wall of solid

mass. Because of the high temperatures required in the process, thecombustion reaction is promoted by natural gas and pure oxygeninstead of atmospheric air.

The kiln model solved has the following dimensions: 2.60 mwidth, 2.00 m height and 6.45 m length. The gentle slope of the

bottom is neglected and a straight roof is considered. The wallshave a uniform thickness of 0.40 m. Two inlets along the verticalcenter line of the front wall represent the primary burner. Thesecondary burner was not modeled in this study as it affects the

product when it is on the outside of the kiln. The centers of the twoinlets are separated by a distance of 145 mm in the vertical direc-tion. The higher inlet is injected with a mix of fuel and a smallpercentage of oxygen while the lower one is injected only with

oxygen. The diameters are 40 and 80 mm for the inlets of the fuel

and oxygen mixture and the oxygen only, respectively. The centerof the lower inlet is positioned at a vertical distance of 690 mm

Fig. 1. Main components of the frit production kiln.

T.S. Possamai et al. / Applied Thermal Engineering 48 (2012) 414e425 415


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from the internal surface of the base wall. The lateral exit for the

ue gas is situated in the front portion of the left wall at the bottom.It is rectangular with dimensions of 100 150 mm (Fig. 3). The

wallof solid raw material mass is situated in the back portion ofthe kiln interior and is assumed to be L-shaped with dimensions of2 m length (starting at the internal surface of the back wall) with1.2 m height and a strip that runs along the whole of the internal

cavity up to the internal surface of the front wall with 150 mm ofheight. This strip represents the ow of frits in the liquid phase inthe bottom of the kiln.

Fig. 4 shows the internal cavity of the kiln, with only the space in

the internal cavity occupied by the ue gas. No inlets or outletswere considered for the frit mass as it was considered as packagesof mass per second.

3. Acquisition of experimental data

The experimental approach adopted was focused on obtainingthe variables required to determine the energy and mass balance in

the kiln. Table 1 presents the measured variables. A ow of ambient

air was admitted in the bottom of the chimney (0.40 kg/s, 303 K), inorder to reduce the ue gas temperature to 903 K. For the owmeasurement Pitots Tubes of 350, 1000 and 2000 mm length and4, 10 and 15 mm diameter, respectively, were used. The onlyexception was the measurement of the ow of surrounding air that

enters the chimney to cool down the ue gas, which was performedwith a vane anemometer (Testo, model 521-2). To measure theowtemperature, a type-K thermocouple probe (OMEGA KMQSS-020U-12) coupled to a digital thermometer (OMEGA HH-21) was used.

The temperature of the external surfaces and the liquid phase owof frit mass exiting the kiln were measured using an infraredcamera (FLIR ThermaCAM SC500).

4. Mathematical and numerical model

4.1. Physical phenomena in the internal cavity of the kiln

The conservation equations of mass, chemical species, linearmomentum and total energy were solved in order to obtain the

ow eld, temperature and species distributions for the combus-tion process inside the kiln. The following hypotheses were adop-

ted forthe uid and the ow. For the uid it isassumedthat:(a)it isa continuous medium; (b) its components adhere to the ideal gaslaw; (c) it is composed exclusively of methane gas (CH 4), oxygen(O2), carbon dioxide (CO2) and steam (H2O); (d) it has no soot

present; and (e) it obeys the concept of an ideal mixture e in orderto estimate the uid properties. The ow is considered to be: (f) in

a steady-regime; (g) compressible with regard to temperature; (h)turbulent; and to have (i) a Lewis number equal to one. In addition,

the kiln walls are assumed to be non-catalytic. The FavreeReynoldsaveraged conservation equations are given by

Fig. 2. Lateral sketch of the kiln.

Fig. 3. Geometric model including kiln walls.

Fig. 4. Cavity occupied by the ue gas inside the kiln.

Table 1

Measured variables.

Variable Mass ow [kg/s] Temperature [K]

Flue gas 0.57 5% 903 5

Fuel 0.020 1% 298 1

Oxygen 0.095 1% 298 1

Bottom of the chimney, ambient air 0.40 5% 303 2

Ceramic frit outlet 0.24 2% 1673 5Raw material inlet 0.27 2% 298 2

T.S. Possamai et al. / Applied Thermal Engineering 48 (2012) 414e425416


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vr

vt

v

r~Uj

vxj

0 (1)

v

r~Yi

vt

v

r~Uj

~Yi

vxj

v

vxj

Gref

v~Yivxj

! Smi; (2)

v

r~Um

vt

v

r~Um ~Uj

vxj

vp0

vxm

v

vxj

meff

v~Umvxj

v~Ujvxm

!! (3)

v

r~H

vt

vp

vt

v

r~Uj

~H

vxj

v

vxj

l

cp

vh

vxj

!

v

vxj

mtPrt

v~H

vxj

! Sreac

Srad

(4)

where ~Uis the velocity, ~Yiis the mass fraction,Gref Gi mt=Sctisthe effective diffusion coefcient, Giis the diffusion coefcient,mtisthe turbulent dynamic viscosity and Sct is the turbulent Schmidt

number. The subscript istands for the chemical component. Theturbulent dynamic viscosity is linked to the turbulent kineticenergyk and the turbulent energy dissipation 3through the rela-

tion mt Cmrk2= 3, where Cm 0.09 is the ke 3turbulence model

constant and r is the averaged mixture density dened as

1=r X

i

~Yi

1=~ri

(5)

In the species mass conservation equation (2) Smi is the masssource term which represents the production rate of component idened as the sum of the progress rate for every elementaryreaction,n, where that component participates, given by

Smi WiXnn 1

n00n;i n

0n;i

Rn (6)

whereWiis the molar mass,n00n;i is the stoichiometric coefcient onthe products side of the equation, n 0n;i is the stoichiometric coef-

cient on the reactants side of the equation and Rnis the elementaryreaction rate. The one-step reaction for the oxidation of methanegas was adopted as the combustion reaction. To solve theelementary progress rate for the chemical reaction the EDM model

was adopted based on the EDC concept proposed by Magnussen[10]. This model assumes that the elementary reaction rate is givenby the lowest value for equations (7) and (8)

Rn C1

3

kmincin0n;i!

(7)

Rn C1C23

k

0B@P

PciWiP

P

n00n;i

Wi

1CA (8)

where C1 4.0 and C2 1.0 arethe proportionality constants fromthe EDM model and ci is the molar concentration. In equation (7)

the component i refers exclusively to the reagents of theelementary chemical reaction, while in equation(8)it refers to theproducts.

In the linear momentum equation (3) the effective dynamicviscosity is dened as meff m mt and the modied pressure as

p

0

p 2=3r

k. In the total energy equation(4) l is the thermal

conductivity,cp the specic heat at constant pressure and Prt theturbulent Prandtl number. ~H 1=2~Ui ~Ui his the averaged total

enthalpy where h P

ihi~Yi is the specic enthalpy. The species

enthalpyhiis given by equation(9), withcp,ias the specic heat atconstant pressure, h0f;i as the enthalpy of formation at a tempera-ture of reference andTas the temperature.

hi ZT

Tref;i

cp;idT h0f;i (9)

The last two terms on the right side of equation (4) represent theenergy source term for chemical reactions from the combustion

Sreac and radiation Srad. The energy source due to chemical reactionsis given by

Sreac X

i

264h0f;i

Wi

ZTTref;i

cp;idT

375Smi (10)

and the energy source term due to radiation is solved by the rst-

order differential approximation radiative model (P1). The spectral

radiative transport equation is simplied by considering theintensity of radiation as the sum of a series of orthogonal harmonics

[11]. This allows the radiative transport equation to be solvedobtaining the divergence of the radiative ux for an isotropicemitting, absorbing and scattering gray medium as

Srad v

Qrad;j

vxj

KaG 4KasT4 (11)

where the second term from the left is the divergence of the totalradiative ux dened as the energy source due to radiation. Theradiative energy ux is Qrad, the absorption coefcient is Ka, the

incident radiation isGand the temperature isT. Assuming gray anddiffuse wall surfaces, equation (12)is used to compute the radiative

energy ux for the energy equation and for the incident radiationequation boundary condition at the walls. The subscript w standsfor wall. Further details on the P 1 radiation model canbe foundin Ref.[11].

Qrad;w 3mw

22 3mw

4sT4w Gw

(12)

In the calculation of the radiative properties of the ue gasa simplied model was used and the medium was considered asa homogeneous and isothermal mixture of gases composed

exclusively of 33% and 67% of CO2 and H2O, by volume, respectively,at 2000 K. In addition, the medium inside the kiln was assumed tobe gray. The mathematical model proposed by Siegel and Howell[11], based on empirical data, was used. Table 2presents the esti-

mated radiative properties used in the radiation model applied tothe ue gas inside the kiln. No scattering is considered.

Table 2

Properties.

Properties Value

Flue gas Absorption coef cient, Ka 1.0 m1

Mean optical thickness, s 1.12

Ceramic frits Fusion point 1680 K

Specic heat at constant pressure,cp 900 J/kg K

Thermal conductivity,l 3 W/m K

Latent heat, LH 400 kJ/kg

Refractory kiln walls Thermal conductivity, l At 1200 K, 5.8 W/m KAt 500 K, 7.8 W/m K

At 300 K, 20.8 W/m K



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Two additional transport equations are solved, for turbulentkinetic energy k and turbulent dissipation 3.

vrk

vt

v

r~Ujk

vxj

v

vxj

"m

mtsk

vk

vxj

# Pk r 3 (13)

vr 3vt

v

r~

Uj 3

vxj v

vxj

"m mt

s 3

v 3

vxj

# C

31Pk3

k C

32r 32

k (14)

wheresk, s 3, C31 and C 32 are the ke 3turbulence model constantsandPkis the turbulence production due to viscous forces, given by

Pk mt

v~Umvxj

v~Ujvxm

!v~Umvxj

! (15)

To solve the equations, wall functions[12]were adopted for thevariables near the walls. Table 3 shows the boundary conditionsapplied to the numerical problem on each surface. The surfaces areidentied inFig. 5. The heat ux at the boundary for surfaces front,

sides and top is dened as equation(16), whereqwis the heat rate

at the surface and Tw is the temperature at the surface. The externaltemperature, i.e., the ambient temperature,Tref, and the equivalentheat transfer coefcient, UAequi, are prescribed. Their values are

presented in Table 3. The equivalent heat transfer coefcient can bedetermined by the equivalent thermal resistance, which is equal tothe sum of the wall conduction resistance and external convectiveand radiative resistance.

qw UAequi

Tw Tref

(16)

4.2. Walls of the kiln, load and external surroundings

The temperatures at the refractory walls of the kiln are obtainedby solving the conduction heat transfer equation at the wall.

Convection and radiation heat transfer to the environment aretaken in account. The conduction equation was discretized with theFinite Volume method and implemented in a FORTRAN code, whichwas coupled with the CFX solver. A constant equivalent heat

transfer coefcient equal in value to that prescribed for the CFXnumerical model at the walls was used. Thetemperatureeld attheinternal surface of the walls obtained with the model in CFX was

prescribed as the boundary condition at the same surface in the

FORTRAN code. This coupling was based on a compromise betweencomputing time and accuracy since a conjugated heat transferproblem required a large number of mesh volumes in the solid

domain due to the thick kiln wall.The energy rate required for the frit mass to undergo the fusion

process is modeled using equation(17), where ql is the heat rate,

Tl,in is the initial temperature of the load, Tl,out is the fusion

temperature, _mlis the load mass ux,cp,lis the load specic heat atconstant pressure andLHis the load melting latent heat.

ql _ml

hl;out hl;in

_ml

cp;l

Tl;out Tl;in

LH

(17)

In this model the maximum temperature that the ceramic fritcan achieve is xed at a temperature dened as the fusion

temperature. A xed fusion point was adopted instead of a fusionzone, considering the ceramic frit as a pure component. Theproperties of the raw material and of the refractory walls of the kilnadopted for this study were estimated based on an average

composition of ceramic frits used in Brazil[13]. Thermal propertiesfor the raw material and refractory walls are presented in Table 2.

The kiln was considered enclosed in a large black body cavity at

a constant temperature. The corresponding shape factor for the kilnwalls is equal to one. The radiation loss from the external wallsurfaces is

qw;rad 3mwsAext

T4sur T4ext

(18)

whereqw,radis the radiative heat rate, 3mwis the emissivity,Aextisthe area and Tsur is the temperature considered at the externalsurface of the walls. The surrounding temperature is Text and

s 5.678 108 W/m2 K4 is the StefaneBoltzmann constant.

External convection was considered to be only due to naturalconvection originating from the heating of the air next to the walls.Equation (19) shows the convection energy losses through theexternal surface of the walls, whereqw,convis the convection heat

rate and htext is the heat transfer coefcient, both at the externalsurface of the walls. Correlations for the convection heat transfercoefcient were based on the work of Churchill and Chu[14]andGoldstein et al.[15]for isothermal walls.

qw;conv htextAextTsur Text (19)

4.3. Mesh

A non-uniform structured hexahedrical mesh composed of384,422 volume elements was used to solve the numerical probleminside the kiln. The fuel inlet, oxygen inlet and ue gas outlet aremore rened. The average volume element size is 29 21 32 mm,

varying from one-

fth of this value in the more re

ned regions to

Table 3

Boundary conditions.

Surface Flow condition Thermal condition

Fuel inlet Prescribed mass

ow, 0.04 kg/s(0.5 CH4 0.5 O2),5% prescribed

turbulent intensity

Prescribed

temperature, 298 K

O2inlet Prescribed mass

ow, 0.06 kg/s

of O2, 5% prescribed

turbulent intensity

Prescribedtemperature, 298 K

Outlet Prescribed mass

ow, 0.10 kg/s

Constant gradient

Bottom and

back mass

No-slip Isothermal wall,

1680 K, emissivity 0.4

Sides, front,

top

No-slip Constant heat transfer

coefcient, UAequi 9.16 W/m2 K, external

temperature, Text 303

K, Emissivity 0.4

Fig. 5. Identication of surface components.



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double this value in the less rened regions. The number ofelements in thex,y andzdirections are 61, 49 and 114, respectively.A second mesh of 196,000 hexahedrical volume elements was usedto solve the solid domain with the FORTRAN code with average size

of 20 20 40 mm. A grid test was performed with the totalnumber of volumes increased bya factor of 3. The average total wallheat ux varied less than 4% while the maximum kiln walltemperature changed less than 1%.

As a convergence criteria, an RMS residue lower than 2 104

was adopted for every transport equation in addition to a residue

lower than 1% in the conservation balance for the variables solved.The problem was solved using an Intel Duo Core, 3.24 GB RAM,2.80 GHz computer. The time required to obtain the convergencesolution for a typical case was approximately 112 h.

5. Results

5.1. Comparison between numerical and experimental results

The numerical and experimental results for the global energybalance for the kiln are shown in Table 4. The results related to eachenergy input and output of the kiln are expressed in terms ofenergy and the corresponding percentage based on the input

energy for each case. The difference between experimental and

numerical results is here dened as the difference between theenergy percentage of each energy output. The numerical modeloverestimates the energy losses through the refractory walls by

Table 4

Energy balancee experimental and numerical results.

Experimental energy Numerical energy Difference [%]

[kW] [%] [kW] [%]

Energy input 1050 100 1000 100 e

Flue gase chimney 346 32.9 251 25.1 7.8

Fusion product 428 40.8 388 38.7 2.1

Wall losses 314 29.9 363 36.2 6.3

Energy output 103.6 100Frit production [kg/s] 0.240 0.237 1.25

Fig. 6. Temperature distribution on the left wall (planex 0 m). (a) External surface e

experimental results; and (b) external and (c) internal surfaces e numerical results.

Fig. 7. Temperature distribution on the roof (plane y

2 m). (a) External surfacee

experimental results; (b) external and (c) internal surfacese

numerical results.



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6.3% compared with the experimental data. The energy leaving thekiln with the ue gas is underestimated by 7.8%. This difference is

attributed to the lower mass ow of ue gas prescribed in thenumerical model, that is, 0.10 kg/s compared with 0.17 kg/s as theexperimental result. The measurement uncertainty, air inltrations

in the kiln and oxygen excess from the burner are considered to bethe probable causes for the difference in the mass ow, since thenumerical model solves for a stoichiometric combustion in order tosimplify the combustion process. The energy consumption attrib-

uted to the production of frit mass was overestimated by 2%. Thiswas attributed to the differences in the values for the estimatedproperties of the ceramic frits and the real ones. Overall, the global

energy balance obtained with the numerical model is consistentwith the experimental results, indicating a similar behavior.

Fig. 6shows the temperature eld for the left wall, and similarresults can be observed inFig. 7for the roof. Experimental uncer-

tainty in the temperature measurements is included in a 5%deviation. The results are all presented based on the system ofcoordinates and origin point dened inFig. 3, located at the bottomof the kiln. The zero on the z-axis corresponds to the external

surface of the back wall, where the raw material is inserted into thekiln.It ends at 6.45 m at the external surface of the front wall where

the burner is xed. For they-axis, zero corresponds to the externalsurface of the bottom wall and 2 m to the external surface of theroof. For the x-axis zero corresponds to the external surface of theleft wall and 2.6 m to the external surface of the right wall.Figs. 6and7analyze planesx 0 m andy 2 m respectively.

The experimental data is limited to a portion of the wall surfacedue to space limitations around the operating kiln. The numericalresults arepresented at two separated surfaces, the external surfacewith 6.45 m length, Figs. 6(b) and 7(b), and the internal surface

with 3.65 m length (Figs. 6(c) and 7(c)).The highest temperatures appear in the central region on the

front portion of the kiln walls, near the burner, for both theexperimental data and numerical results. It is possible to identify

the position of the solid raw material wallinside the kiln at the

temperature experimental measurements on the external surfaceof the left wall, Fig. 6(a). It is evidenced by the low temperature

region located betweenz 0.6 m andz 2 m due the low thermalconductivity of the ceramic material (Table 2). The results inFig. 7show a similar behavior for the roof. On comparingFig. 6(b) and (c)

the effects of the refractory walls of the kiln can be observed, witha lowering of the internal to external wall temperature. Thisreduction is around 1000 K.

A detailed temperature distribution at the external wall is

compared with experimental measurements inFigs. 8e10for the

external surface of the left (plane x 0 m), right (x 2.6 m) androof (y 2 m) walls, respectively. In these gures the values for the

Fig. 8. Temperature proles for experimental and numerical results for three longi-

tudinal lines along the external surface of the left wall (x

0 m).

Fig. 9. Temperature proles showing experimental and numerical results for two

longitudinal lines along the external surface of the right wall (x 2.6 m).

Fig. 10. Temperature proles for experimental and numerical results for three longi-

tudinal lines along the external surface of the roof (y

2 m).



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Fig. 11. Convective (a) and radiative (b) heat uxes on the internal surface of the roof (y 1.6 m).

Fig. 12. Convective (a) and radiative (b) heat uxes on the internal surface of the left wall (x 0.4 m).



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temperature along horizontal lines on the z-axis, from the back tothe front wall of the kiln, are compared considering experimental

and numerical data obtained at different sections of the kiln. For theleft wall (Fig. 8) lines y 0.4 m, y 1.0 m and y 1.5 m wereanalyzed. On the right wall (Fig. 9) linesy 0.4 m andy 1.5 m are

considered. For the roof (Fig. 10) the analyzed lines were x 0.4 m,

x 1.2 m andx 2.0 m.The numerical solution presents the same behavior as the experi-

mental results along the entirelength of thecurves in Figs. 8 and 9, but

reaches higher temperatures near the burner for the external surfacesof the left and right kiln wall. The temperature difference between thenumericaland experimentalresultsstays below 100K. Probable causes

forthis discrepancy are thefact that therefractorywalls sufferabrasionfrom heat and friction during long periods of use and the numericalmodel uses thedesign dimensions for thethickness of thewalls. In thiskind of kiln the refractory walls are reconstructed every 4e8 months

due to this abrasion.In Fig. 10, similar behavior can be observed for the roof, but

a greater difference in the temperature values is found in the solidregion of the frit. This could be because in the numerical model the

wall of ceramic fritsis considered as a solid block ofxed lengthoccupying the entire space from the bottom to the roof of the kiln,

touching the internal surface of the roof and not leaving space forthe ue gas between the roof and the pile of mass. However, sucha space may be present in the real process. The creation of thisspace is unpredictable as it can occur at some point during theproduction and then be closed again by the action of the mass

movement through the kiln. Therefore, the numerical modelalways considers this space as being full of mass. The temperature

difference inside this zone reaches 200 K, while outside this zone itremains below 50 K.

5.2. Radiative and convective heatuxes on the refractory walls

Fig. 11 shows the convective and radiative heat uxes on theinternal roof surface (y 1.6 m). The minus sign indicates thedirection of the heat ux corresponding to heat transfer from the

ue gas to the refractory walls. The vertical axis starts at z 2.4 m,the interface between the raw material wallinside the kiln andthe ue gas. Due to the high temperatures inside the kiln, theradiative heat ux is around 10e30 times higher than the

convective heat ux. The same trend can be observed for theinternal surfaces of the left (x 0.4 m) and right (x 2.2 m)walls inFigs. 12and 13, respectively. The horizontal axis starts at

y 0.55m, the interface between the load material stripe inside the

kiln and the ue gas, while the vertical axis starts atz 2.4 m. Onthe left wall, where the ue gas exit is located, the convective heat

ux reaches values up to twice those for the right wall, indicatinga greater inuence from the ow itself on this wall. This indicates

that the ue gas exit position has strong inuence on the ue gasow. The radiation heat ux is stronger in the central region of the

wall, in the upper portion, since the ame is located in that area ofthe kiln due to the burner position.

5.3. Heatux for the load

Considering the load as the bottom and back interface between

the ue gas and solid domain, the heat uxes can be visualized in

Fig. 13. Convective (a) and radiative (b) heat

uxes on the internal surface of the right wall (x

2.2 m).



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Fig. 14(a) and (b), for convective and radiative heat transfer,respectively. Similarly to the other internal surfaces of the kiln, theradiative heatux dominates, reaching values 10e100 times higherthan those of the convective heat ux. On the back surface of the

load the participation of convection is greater, reaching 1/10 ofthe radiative heat ux due to the higher velocities and the impactof the jet ofue gas originating from the combustion ame. The

velocity of the ue gas in this region reaches 7 m/s, while on theother kiln surfaces it remains below 2 m/s. The radiative heat uxrepresents around 90% of the total heat ux for the load. Theregion of higher heat ux is situated on the bottom surface,

where the bath of frit mass is installed.

5.4. Application example

One of the rst approaches to saving energy in industrial kilns isto improve the insulation of the walls, decreasing the energy lossesby convection and radiation to the external ambient. For the type of

kiln analyzed in this study, the internal temperatures can reach

above 2500 K for 100% oxy-red combustion. The walls of the kilnneed to be reconstructed after a period of 4e8 months ofcontinuous operation due to the thermal abrasion. Thus,

improving the insulation of the walls has an effect in terms of

both energy savings and increasing the thermal abrasion of thewalls. Numerical results for the temperature on the internalsurface of the left wall (x 0 m) of the kiln for three cases of

wall insulation are presented in Fig. 15. Case (b) represents thestandard case with the insulation of the kiln analyzed in this paperand its thermal conductivity shown in Table 2. Case (a) correspondsto insulation ten times higher than the standard while case (c)

corresponds to a degree of insulation which is one-fth of thestandard. The global energy balance for each case is presented inTable 5.

Fig. 15shows that an improvement in the wall insulation causes

an increase in the temperature of the internal surfaces while lessinsulation reects in lower temperatures, as expected. However,with less insulation the higher temperature region is slowly shiftedto the left, toward the back bottom portion of the kiln, where the

Fig. 14. Convective (a) and radiative (b) heat uxes for the load.



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raw material is located, forming a zone of more homogeneoustemperatures near the load. As this occurs, in the front portion ofthe kiln, near the burners, more isotherms appear, showinga higher temperature gradient for less insulation. A more homo-

geneous temperature distribution near the load is favorable to theprocess, but low temperatures near the frit mass exit (near theburner) can cause the product to solidify beforeexiting the kiln. Theconsequence is a reduction in the product quality. With more

insulated walls, more energy is directed toward the load, increasingproductivity, while the energy carried with the ue gas to thechimney does not show a signicant change (Table 5). Additionally,the degradation of the walls due to a high internal temperature

tends to increase as the temperatures on the internal surface of thewalls were increased by around 50 K. Thus, this effect must also beconsidered in kiln operation analysis with the objective ofachieving an optimal agreement between energy savings and

thermal abrasion of the walls.

6. Conclusions

The investigation of a numerical model proposed to solve theturbulent ow inside a ceramic frit fusion kiln was describedherein. The model was applied to a real case and numerical and

experimental results were compared. Numerical results consistingof heat uxes on the internal surfaces of the kiln walls and load, theinternal and external surface temperatures and the global energybalances were presented and analyzed. Three different degrees of

wall insulation were compared.

It was observed that the numerical model presented a global

energy balance consistent with that obtained for the real kilnthrough experimental data. Furthermore, the external surfacetemperatures of the roof and the left and right walls presented the

same pattern in the numerical solution as in the experimental data,with slightly higher temperatures being obtained with thenumerical model.

It should be mentioned here that the actual physical process

involves more complex mechanisms than those idealized in thenumerical model, such as more chemical species resulting from thecombustion process and the dependence of the medium andsurface radiation on the wavelength. However, in spite of the

simplications hot spots could be identied in the refractory wallsand the estimate of the main energetic behavior was consistentwith the data for the real case. These results could provide valuableestimates for the global energy losses and energy optimization.

The major contribution to the total heat ux of the load and ofthe walls is radiation heat transfer, as expected. The highest heat

ux occurred around the ame, slightly shifted from the center ofthe kiln due totheue gas exit being positioned in the front portion

of the left wall causing a deviation in the ow. These higher uxesare due the higher temperatures concentrated in the ame region.

Improving the insulation of the kiln walls presented two con-icting outcomes; it increases the kiln productivity but resulted in

higher thermal abrasion of the walls due to higher temperatures.Also, using less insulation than normal contributes to a morehomogeneous temperature eld near the load, which can improvein the product quality, but leads to a lower estimated productivity.

The determination of the best furnace will be dependent onseveral factors, mainly the design of the geometry to give betterenergy optimization. However, from the experimental results it ispossible to identify energy saving procedures which can be applied,

such as through heat exchange with theue gas in the chimney anddetermining the optimum insulation conditions for the walls.

For better results in kiln optimization, the designer must

perform an extensive investigation of several different congura-

tions in order to achieve a conclusive case analysis. Even though

Fig.15. Temperature distribution on the internal surface of the left wall e numerical results obtained with three degrees of insulation: a-ten times the standard; b-standard; c-one-

fth the standard.

Table 5

Energy balance for kiln with different degrees of wall insulation.

Insulation Case (b) Case (a) Case (c)

Flue gas [%] 25.1 25.9 24.0

Frit outlet [%] 38.7 67.9 3.2Wall losses [%] 36.2 6.2 72.8

Frit production [kg/s] 0.237 0.416 0.0195



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absolute quantitative results are difcult to obtain, due to the highcomplexity of the problem, qualitative comparisons can provide

valuable information for the optimization process. Finally, it isimportant to note that experimental investigation, althoughexpensive and difcult to conduct in the case of industrial equip-

ment, is necessary to assess the validity of the proposed numericalmodel.

Acknowledgements

This work was funded by SCGAS e Gas Company of Santa Cat-arina. The author T. S. Possamai received a grant from the ANP

Human Resources Program for the Sector of Oil and Natural Gas ePRH09-ANP/MME/MCTand R. Oba from the CAPES program.

References

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[2] I.I. Akomolafe, L.E. Umaru, O.O. Ige, Inuence of glass frits on working prop-erties of kaolins, Applied Clay Ceramics 52 (2011) 428e431.

[3] E. Bou, A. Moreno, A. Escardino, A. Gozalbo, Microstructural study of opaqueglazes obtained from frits of the system: SiO2eAl2O3eB2O3e(P2O5)e

CaOe

K2Oe

TiO2, Journal of the European Ceramic Society 27 (2007)1791e1796.

[4] B. Baran, Y. Sarikaya, T. Alemdaroglu, M. Onal, The effect of boron containingfrits on the anorthite formation temperature in kaolinewollastonite mixtures,Journal of the European Ceramic Society 23 (2003) 2061e2066.

[5] G.B. Remmy Jr., Firing Ceramicse Advanced Series in Ceramics, vol. 2, WorldScientic Publishing, 1994.

[6] V.P. Nicolau, A.P. Dadam, Numerical and experimental thermal analysis ofa tunnel kiln used in ceramic production, Journal of the Brazilian Society ofMechanical Sciences & Engineering XXXI (4) (OctobereDecember 2009)297e304.

[7] M.G. Carvalho, M. Nogueira, Improvement of energy efciency in glass-melting furnaces, cement kilns and baking ovens, Applied Thermal Engi-neering 17 (8e10) (1997) 921e933.

[8] A.O. Nieckele, M.F. Naccache, M.S.P. Gomes, Combustion performance of analuminum melting furnace operating with natural gas and liquid fuel, AppliedThermal Engineering 31 (2011) 841e851.

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Nomenclature

A:surface area, m2

c:molar concentration, kmol/m3

cp:specic heat at constant pressure J/kg KC1:proportionality constant from EDM model, 4.0C2:proportionality constant from EDM model, 1.0Cm: ke 3turbulence model constant, 0.09C31: ke 3turbulence model constant, 1.44C32: ke 3turbulence model constant, 1.92Eb:black body radiation, W/m

2

G:incident radiation, W/m2

h:specic enthalpy, J/kgH:total enthalpy, kJ/kg

ht:heat transfer coefcient, W/m2 Kh0

f: enthalpy of formation, J/kg

k:turbulence kinetic energy, m2/s2

Ka:absorption coefcient, m1

LH:load melting latent heat, J/kg_m:mass ux, kg/s

p:pressure, PaPr:Prandtl numberPk:shear production of turbulence, kg/m s

3

q:heat rate, WQ:energy ux, W/m2

R:elementary reaction rate, kmol/m3 sS:energy source, W/m3

_S:specic energy source, J/kgSc:Schmidt numberSm:mass source, kg/m3 st:time, sT: temperature, KU:velocity, m/sUAequi:equivalent heat transfer coefcient, W/m2 KW:molar mass, kg/kmol

x:spatial coordinate, mY:mass fraction

Greek symbolsG:diffusion coefcient, m2/s3:rate of dissipation of turbulent kinetics energy, m 2/s3

3m: emissivityl:thermal conductivity, W/m Km:dynamic viscosity, Pa sn:spectral length, mmn0: stoichiometric coefcients on the reactants side of the equationn00: stoichiometric coefcients on the products side of the equationr:specic mass, kg/m3

s:StefaneBoltzmann constant, 5.678 108 W/m2 K4

s3: ke 3turbulence model constant, 1.3

sk:turbulence model constant for the k equation, 1.0

s:optical thickness

Subscriptconv:convectioneff: effectiveext: surroundingsi:chemical speciesin:inlet

j:indexl:ceramic frit loadm:indexout:outlet

p:productsrad: radiationreac:reactionref:referencesur:surfacet:turbulent termsw:wall

Superscriptsw:Favre averaged:Reynolds averaged


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