Numerical Simulation of the Fan Cooling Effect on a 10000t/d Cement Rotary Kiln

DOU Haijian, CHEN Zuobing School of Mechanical and electronic Engineering

Wuhan University of Technology Wuhan, China

E-mail: dhj1230_171@126.com, zbchen01@126.com

XIAO Jiayun CBMEC Cement Industry Design Institute

China National Building Material Equipment Co., Ltd. Beijing, China

E-mail: tqh1208@126.com

Abstract—Industry production practice and CFD (Computational Fluid Dynamics) numerical simulation technology are integrated together in the study of the fan cooling effect on an overseas 10000t/d (ton/day) cement rotary kiln. The spot data of a 10000t/d cement production at home were collected and studied, which provided the vital data for numerical simulation of the fan cooling effect. Then aiming at the working ambient conditions of the overseas 10000t/d cement production line, the fan cooling effect of its rotary kiln was numerically simulated by considering natural convection and radiation heat-loss. Finally, based on the results of the simulation, a conclusion is drawn that the fan cooling effect is good enough to make the overseas rotary kiln safely run at its ambient conditions.

Keywords- rotary kiln; kiln shell; numerical simulation; fan cooling

I. INTRODUCTION With the rapid development of cement production,

especially the new type dry-process cement production technology of which the output is over 10000t/d (ton/day), many large cement enterprises and cement technology academe increasingly attach importance to the cooling method of rotary kiln shell, aiming at prolonging the life span of firebrick and improving the none-malfunction running ratio of main equipments of new type dry-process cement production process system.

As far as cooling method of rotary kiln is concerned, there are presently two kinds of cooling methods for rotary kiln shell [1]: one is water cooling, the other is fan cooling. Water cooling is not only simple and easily done, but also has obvious effect; however, during the process of long-term usage, it often results in the following problems: firstly, rotary kiln shell is easily rusted, leading to some concave points appearing in the surface of kiln shell due to rust falling off from kiln shell. Secondly, cooling effect is not uniform; as a result, some accidents easily take place. The temperature of kiln shell under wheel belt is a little higher for the reason that cooling water can not reach this area, then stress comes into being due to temperature difference between this area and other part that can be cooled by water, leading to the following problems such as the distortion and (or) desquamation of the wheel-belt underlay board, large longitudinal move of kiln body, frequent falling of fender wheel etc. what is more serious is that kiln shell may give birth to cracks through which cooling water enters into

firebrick layer and make it disintegrated, which causes grave craft accident and directly affects normal production.

The cooling effect of fan cooling is not obvious in comparison with water cooling, especially when the ambient temperature is high, the temperature of kiln shell often appears higher. On the other hand, the initial investment for fan cooling is large, in contrast with water cooling.

In this work, aiming at the overseas 10000t/d production line contracted by China National Building Material Equipment CO., Ltd., the rotary kiln cooling system of this production line is studied. On account of the 50℃ambient temperature of this production line, water cooling may be easier to cause the problems such as kiln shell rusting and eroding , large longitudinal move of kiln body, etc., consequently, the fan cooling was ascertained and adopted. Though the cooling effect of fan cooling is not so obvious than water cooling, a good cooling effect can also be obtained on condition that the number and position of cooling fan are reasonably ascertained and schemed.

II. EXPERIMENTAL PROCEDURE

A. Sketch map of kiln body structure and cooling fan position Kiln type is m982.6 ×φ , according to the design data

offered by China National Building Material Equipment CO., Ltd., the main structure and cooling fan position are described as Fig.1. The materials of kiln shell, firebrick and kiln inside-crust is specified in accordance to the design parameters of China National Building Material Equipment CO., Ltd.

B. Geometry model and calculation mesh Calculation region is made up of the following 4 parts

from outside to inside: the air layer outside kiln shell, kiln shell, firebrick layer, and kiln inside-crust layer. The radial thickness of the air layer outside kiln shell is 18 meter; fan outlet section size is 0.4x0.4m, and the normal distance from fan outlet to kiln shell outside wall is 0.2m. As is depicted in the Fig.1, 20 QZA-No7A mixing-current cooling fans are laid axially along kiln burning zone inside exterior air layer of kiln shell, furthermore according to the rotary direction of kiln, these cooling fans are placed in the side nearby the material surface inside kiln. Based on the upper, 3-dimension geometry model was constructed by adopting Solidworks software, then imported the 3-dimension geometry model to

International Conference on Computer Modeling and Simulation

978-0-7695-3562-3/09 $25.00 © 2009 IEEE

DOI 10.1109/ICCMS.2009.9

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Figure 1. Kiln body structure parameters and position parameters of cooling fan

Figure 2. Calculation meshes of a section perpendicular to kiln axis

the fluent6.2’s preprocess module Gambit2.2.30 and built calculation mesh. Due to the thinnest wall of kiln shell only has 34mm thickness, therefore, mesh locally refined technology was applied to the kiln shell. Moreover, the kiln shell meshes were looked upon as basic mesh, and then along radial direction of kiln, mesh transition technology was applied outward from kiln shell to exterior air layer (Fig.2). Mesh-built scheme was hexahedral scheme, and 826358 hexahedral meshes were built.

C. Physical and mathematical model According to the design data, the ambient temperature at

which the kiln runs is 50℃ , and fan cooling air flux is 22117m3/h, and then we can calculate the cooling air inlet velocity that is 38.4m/s, and Mach number 0.11. We can also obtain 50 ℃ air density 1.093 kg/m3 and dynamic viscosity1.96x10-5Pa.s from table look-at [2]; in addition, cooling air inlet hydraulic diameter 0.4m, consequently we can calculate cooling air inlet Reynolds number 856555. According to these data gained in the upper, the flow outside the kiln shell can be concluded as incompressible turbulent

flow [3], and we choose the ε−k turbulent model [4]; simultaneously, the air natural convection outside kiln shell, kiln body radiation as well as the heat-up effect of kiln body radiation on the air outside kiln shell are all included in the heat-loss calculation of kiln body.

1) Natural convection model equation as well as its realization

Cooling effect of natural convection is realized by adding buoyancy contribution source term bG the transport equations (1), (2) respectively for turbulent kinetic energy k and turbulent dissipation rateε .

ρεσμ

μρρ −++⎥⎦

⎤⎢⎣

⎡+

∂∂=

∂∂+

∂∂

bKk

t

ji

i

GGx

kux

kt

()()( (1)

kCGCG

kC

xxk

xt bk

j

t

ji

2

231 )()()()( ερεεσμ

μερρε εεεε

−++⎥⎥⎦

⎤

⎢⎢⎣

⎡

∂∂+

∂∂=

∂∂+

∂∂

(2)

11

Where: turbulent viscosityε

ρμ μ

2kCt = , kG is turbulent kinetic

energy source term produced by the gradient of mean

velocity, i

j

jik xu

uuG∂∂

′′−= ρ ; buoyancy contribution source

termit

tib x

TgG∂∂=

Prμβ , where, Hot coefficient of

expansionpT⎟⎠⎞

⎜⎝⎛

∂∂−= ρ

ρβ 1 , turbulent Prandtl number for

energy 85.0Pr =t ; ε3C is the degree to which turbulent dissipation rate is affected by buoyancy is determined according to [5].

2) Radiation heat-loss model and the heat-up effect of radiation on the air outside kiln shell

The heat-loss of kiln body radiation is realized by radiation transport equation (3), and the heat-up effect of radiation on the air outside kiln shell is done by adding radiation source term (4) into the energy equation of turbulent dynamics differential equations.

Ω′′•+=++ ∫ dsssrITansrIads

srdI ss )(),(

4),()(),( 4

0

42 φ

πσ

πσσ

π

(3) 44 TaGqS rr σ−=⋅−∇= (4)

Where: ),( srI denotes radiation intensity, which depends on position ( r ) and direction ( s ); radiation

flux GCa

qss

r ∇−+

−=σσ )(3

1 , where G is incident

radiation, and C is the linear-anisotropic phase function coefficient; a is air absorption coefficient; n is air refractive index; s ′ is scattering direction vector, and s is direction vector; Ω′ is solid angle; Stefan-Boltzmann constant

)/(10672.5 428 KmW ⋅×= −σ .

3) Model constants Main model constants used in these physical and

mathematical models are listed in TABLE І.

D. Boundary condition of solution 1) Temperature boundary conditions for inside wall of

firebrick layer and kiln inside-crust First of all, collected the spot temperature data of outside

wall of kiln shell from domestic 10000t/d production line, and then drew temperature curve of these data (see Fig.3) by using Microsoft Excel software.

As we can see from Fig.3, there are 3 temperature wave troughs in the temperature curve of kiln shell outside wall; the distance from outside wall of wheel belt to the inside wall of kiln liner (firebrick layer or kiln inside-crust) is larger than that from kiln shell outside wall to kiln liner, additionally, the heat-loss condition of wheel belt surpasses that of kiln shell outside wall, as a result, there must be larger temperature fluctuate in the zones of 3 wheel belts.

Secondly, because the rotary kiln of which the outside wall temperature has been collected is 90m long, and the kiln studied by us is 98m long, therefore, the temperature curve of outside wall of 90m-long kiln must be changed into that of outside wall of 98m-long kiln. Trend line can be appended into the temperature curve of kiln shell outside wall by using the 5-power polynomial method of Excel software, just as the dot line that is shown in Fig.3. Note that the trend line is appended for the highest temperature of outside wall of kiln shell. At practical working condition, the inside-wall temperature of firebrick layer and kiln inside-crust located in burning zone (30<x<60, x denotes the distance to kiln end) is the highest. In order to sufficiently consider this character, the temperature trend line of outside wall of kiln shell located in the burning zone is replaced with the three solid straight lines shown in the Fig.3, and then we can get the

TABLE I. MODEL CONSTANTS USED BY CALCULATION IN THIS PAPER

Constant ε1C ε2C μC kσ εσ a n sσ C

Value 1.44 1.92 0.09 1.0 1.3 2.57e-5 1 0 0

100

150

200

250

300

350

400

0

4.1

7.4

11.8

14.6

18.5

23.2

28

30.6

33.7

36.8

41

44.7

47.4

48.9

50.7

52.8

55.5

56.7

59.8

62.5

67.4

72.7

87.4

Distance to kiln end / m

Tem

pera

ture /℃

the lowesttemperatureof kiln shelloutside wallthe highesttemperatureof kiln shelloutside wall

Figure 3. Spot temperature data of rotary kiln shell outside wall of 10000t/d production line at home

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modified final temperature trend line (the three solid straight lines and the dotted temperature trend line located outside the burning zone) of kiln shell outside wall. According to the intersections of the modified final temperature trend line with the highest temperature curve of outside wall of kiln shell, we can get the coordinates (including temperature coordinates and position x coordinates) of these intersections.

In order to change the temperature tend line of outside wall of 90m-long kiln shell into that of outside wall of 98m-long kiln shell, we must get new position x coordinates of those intersections by using the formula 90/98 9098 −− ×= ii xx

(Where, 90−ix is 90m-long kiln position coordinate, 98−ix is 98m-long kiln position coordinate). New position coordinates of each intersection with the corresponding temperature coordinate constitute the point coordinates of outside wall temperature of 98m-long kiln. After we get these point coordinates of outside wall temperature of 98m-long kiln, we can draw the temperature points of outside wall of 98m-long kiln in a new coordinate system, and then we can get the temperature curve of outside wall of 98m-long kiln shell by sequentially connecting these temperature points with line.

Thirdly, from the temperature curve of outside wall of 98m-long kiln shell, we can get the highest temperature value of outside wall of 98m-long kiln, and then get a temperature difference value by subtracting the highest temperature of outside wall of 98m-long kiln shell form1500℃. In the end, we can get the temperature curve of inside wall of kiln liner (firebrick layer and kiln inside-crust) by parallelly moving the temperature curve of outside wall of 98m-long kiln shell a distance of the temperature difference towards the positive direction of temperature coordinate axis, which is shown as Fig.4.

Lastly, as is shown in Fig.4, the temperature boundary condition of inside wall of kiln liner is a non-uniform one, that is to say, the value for the temperature boundary condition is not a constant, on the contrary, varies in terms of certain functional regulation. For the sake of describing this kind of boundary condition, we use the “DEFINE_PROFILE [6]” macro of Fluent6.2 UDF (User-Defined Function) to realize the non-uniform temperature boundary condition of inside wall of kiln liner (firebrick layer and kiln inside-crust).

2) Boundary condition for air layer outside kiln The boundary conditions for air layer outside kiln are

listed in TableⅡ.

E. Numerical solution CFD commercial software Fluent6.2 is adopted to solve

the turbulent kinetic equations and those of heat transfer. With the second order upwind difference[7], finite-volume method is used to convert turbulent equations into difference equations ， and pressure-velocity coupling equations are solved by the classic SIMPLE[4] algorithm. The convergence standard for energy and radiation is that the consecutive two iterating error is respectively less than 10−6, 10−7, and those for the remained variables less than 10−3. Solution was converged after 17913 times iteration.

III. RESULTS AND DISCUSSION Fig.5 is the position sketch map of post-process faces,

and all these post-process faces are line faces that parallel to the axis of kiln and pass the positions of the dots in the sketch map, From Fig.6 and Fig.7, the largest temperature difference along the outside wall circumference direction of kiln shell located in fan cooling area is 212K (℃). The temperature of kiln shell outside wall directly facing to the

950

1050

1150

1250

1350

1450

1550

0 0.94 4.21 9.61 45.3 56.2 61.3 83.9 90.2 94.9 98

Distance to kiln end /m

Tem

pera

ture /℃

Figure 4. Position sketch map of post-process-faces for calculation result(line faces parallel to kiln axis)

TABLE II. MAIN BOUNDARY CONDITIONS AND PARAMETERS FOR AIR LAYER OUTSIDE KILN

Physical boundary Boundary condition type

Boundary value

Gas current temperature/K

Hydraulic diameter /m

Cooling air inlet velocity-inlet 38.4 /(m/s) 323.15 0.4 Outer circumference boundary of air layer pressure-outlet 0 /(Pa) 323.15 115.782

Two-end air boundary of kiln body pressure-outlet 0 /(Pa) 323.15 36

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Outsidewall of kiln shellor wheel

Insidewall of firebrick or kilninside crust

rotation direction

Kiln shell inside wall-1 of fan cooling axis section

Kiln shell outside wall-1of fan cooling axis section

Fan

Kiln shell inside wall-2 of fan cooling axis section

Kiln shell outside wall-2 of fan cooling axis section

Kiln shell

Firebrick layer kiln inside-crust

Fan cooling axis section

Figure 5. Position sketch map of post-process-faces for calculation result(line faces parallel to kiln axis)

Level K1 392.042 406.153 420.254 434.365 448.476 462.577 476.688 490.799 504.8910 519.0011 533.1112 547.2213 561.3214 575.43

1234567

89

10

11

12

13

14

1414

14

14

14

Level K 1 377.93 2 392.04 3 406.15 4 420.25 5 434.36 6 448.47 7 462.57 8 476.68 9 490.79 10 504.8911 519 001 2 3

456 7

8 9

10

11

12

13

Figure 6. Fan cooling area temperature contour of kiln shell outside wall

located in clinker burning zone Figure 7. Fan cooling area temperature contour of kiln shell outside wall

located in transition zone

outlets of cooling fans is only 378K (105 ℃ , NO.1 temperature contour in Fig.7), and the highest temperature of outside wall of kiln shell away from cooling area is 590 K (317℃). In addition, from the lowest temperature in cooling area to the highest temperature away from cooling area, the temperature drops gradually, and the gradually dropping magnitude is only about 15 K(℃), which, at root, avoids the damage effect of heat stress on the life-span of kiln shell as well as firebrick layer due to sharp change of temperature.

The temperature curves of inside and outside wall of kiln shell that faces to as well as opposites to the outlets of fans are respectively shown as Fig.8 and Fig.9(x=0m denotes kiln end, and x=98m denotes kiln head). On account of the inside wall of kiln shell coinciding with outside wall of firebrick layer, the temperature curve of inside wall of kiln shell is also that of outside wall of firebrick layer. From Fig.8, the temperature of inside wall of kiln shell in the cooling area is about 60 K(℃) higher than that of outside wall of kiln shell, and about 75 K (℃) higher than that of outside wall of kiln shell area without fan cooling. Fig.9 indicates that fan cooling has the least effect on the inside and outside wall of kiln shell area that opposites to the fan outlets, and there is about 50 K(℃) temperature difference between the inside and outside wall of kiln shell in the fan cooling axis section. We can also find in Fig.8 that the highest temperature 625K (352℃) of inside wall of kiln shell is located in the wheel

belt of kiln end near x=12m in which the heated states of kiln shell and interior firebrick layer can be noticeably improved on condition that a cooling fan is laid here.

Because the cooling fans are placed as is shown in Fig.5, higher as the temperature of inside wall of firebrick layer as well as kiln inside-crust layer in main burning zone (20<x<60) is (see Fig.4), the highest temperature of inside wall of kiln shell (outside wall of firebrick layer) in main burning zone located in fan cooling area(30<x<45) is only about 475K (202℃), which is certain to improve the heated states of interior firebrick layer, accordingly avoiding firebrick’s loosing or falling-off caused by larger temperature change( largest fluctuating temperature up to 400℃[1]) during kiln rotating a circle; however, the highest temperature of inside wall of kiln shell (outside wall of firebrick layer) in burning zone located in wheel belt area without fan (45<x<60) is about 575K(302℃). From Fig.8 and Fig.9, the highest temperature of outside wall for the 3 wheel belt is 540~570K(267~297℃), as a result, there is a suggestion that a cooling fan should be laid near every wheel belt for the sake of improving the heated states of wheel belts as well as upholding wheels.

In this work, the temperature of inside wall of firebrick layer and kiln inside-crust is uniform along circumference direction. However, in practical working condition of rotary

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Tem

prat

ure

/ KKiln shell inside wall-1 offan cooling axis section

Kiln shell outside wall-1 offan cooling axis section

Distance to kiln end / m

Kiln shell outside wall-2 offan cooling axis section

Kiln shell inside wall-2 offan cooling axis section

Distance to kiln end /m

Tem

pera

ture

/ K

Figure 8. Temperature curve of kiln shell inside and outside wall of fan

cooling axis section facing to fan outlet Figure 9. Temperature curve of kiln shell inside and outside wall of fan

cooling axis section opposite to fan outlet

kiln, the temperature of inside wall of firebrick layer and kiln inside-crust located in materials reacting zone is higher than other part along circumference direction. As a result, if the temperature boundary condition for the inside wall of firebrick layer and kiln inside-crust is set according to the practical working condition, a better cooling effect should be obtained from simulation results. On the other hand, the highest temperature of inside wall of firebrick layer and kiln inside-crust is set 1500℃ that is about 50℃higher than the common data1380~1450℃[8], the reason for this is also better to testify the cooling effect of the rotary kiln cooling system. From Fig.7, the highest temperature of outside wall of kiln shell is 590K (317℃) that is lower than the spot temperature 391℃ (see Fig.3) of outside wall of kiln shell collected in the 10000t/d production line at home, consequently, the cooling system of the rotary kiln has a better cooling effect. Considering all of the above factors, we can draw a conclusion that the rotary kiln cooling system in this work can ensure that the rotary kiln safely runs at the ambient temperature of 50℃.

IV. CONCLUSIONS

Based on the discussion of Ⅲ, some conclusions are drawn as the following:

⑴ The rotary kiln cooling system in this work can ensure that the10000t/d rotary kiln cooled by this cooling system safely runs at the ambient temperature of 50℃ ; However, there is a suggestion that a cooling fan should be laid near every wheel belt for the sake of improving the heated states of wheel belts as well as upholding wheels.

⑵ As far as fan cooling of rotary kiln is concerned, a good cooling effect can also be obtained on condition that the number and position of cooling fan are reasonably ascertained and schemed.

⑶ The combination of industry production practice with the CFD numerical simulation technology is an efficient research method for the study of rotary kiln cooling system, which can be popularized to carry out the study of relevant equipments and technologies of the new type dry-process cement production.

ACKNOWLEDGMENT The authors are very grateful to the supports provided by

China National Building Material Equipment Co., Ltd.

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[5] R.A.W.M. Henkes, F.F. van der Flugt, and C.J. Hoogendoorn, “Scaling of the Turbulent Natural Convection Flow in a Heated Square Cavity Calculated with Low-Reynolds-Number Turbulence Models”, Int. J. Heat Mass Transfer, vol 34, 1991, pp. 1543−1557.

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