prof p mahanta

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Heat Transfer Characteristicsin Cyclone Separator of a

Circulating Fluidized BedUnit

DR. P. MAHANTA

IIT GUWAHATI



2

CFB- A novel and

more efficientcombustiontechnology for low

grade fuelsOffers wide fuel

flexibility, lowenvironmental

pollution, highavailability



3

Core-annulus structure Voidage variation



4

In CFB the bed materialsare entrained in the gasstream to form a refluxing

suspension

Intense gas-solids mixing

and good-solids contactcreate an isothermal system

DETAILS OF A TYPICAL

CFB BOILER



5

Staged combustion

Coal occupies 5 to 10%

of the bed volume

Bed material stores up

energy

STAGED COMBUSTION



6

Fuel flexibility

Bed materials acts as a large thermal flywheel

High heat release rate

Efficient sulphur removal

S + O2 So2+ 9260 kJ/kg.

Caco3

Cao + co2

- 1830 kJ/kg. Caco3

Cao + So2 + ½ O2 Caso4 + 15,141 kJ.

Low Nox emission



9

In the upper splash region of CFB dueto fully developed gas-solid flow bettersolid-gas contacting takes place

Low solid concentration on upper splashregion reduces the erosion problem on

heat exchanging surfaces



10

!"

Heat transfer to the walls of a CFB is due to the conduction fromclusters of particles falling along the wall, thermal radiation andconvection to uncovered surface area .

Fraction of the wall coverage by particles and the average

contact time of particles to the wall .

h = hpc + hgc + hrad

= (1- fo) hpc + fo hgc + hrad

Where fo : fraction of surface covered by gas bubbles.

dq = h (TB - TW) dA



11

Details of Set-up developed



12



13

# $

Details of Heat Transfer Probe



14

%



15

U-Tube Manometers

Dimensions:

Height= 120 cmWidth=92 cm

No. of Tubes=16

Pitch= 4 cm



16

!"



17

& !"



18

!" '



19

Temperature measurement on

upper splash region



20

Temperature Measurement on Upper Splash region



21

!" ! (



22

Experimental conditions

• Bed Material: Sand

• Mean particle size of sand: 271 µµµµm

• Fluidizing velocity : 2.9–4.6 m/s

• Solid circulation rate : 4-20 kg/m2s• Heat fluxes :

849.673 W/m2

1593.137 W/m2

Bed inventories:10 kg to 16 kg.



23

Working Formulae

• Voidage ( )

∆∆∆∆Pb = (1- εεεεmf). Lmf . (ρρρρs - ρρρρg ) . g

ε

gh

p W L

b .)

100

( ρ ∆

=∆

S

L

L

h

ρ .

.10 ∆ε =1-

: Difference of height in manometer

fluid, Cm of water. Lh∆

Experimental Set-Up and Procedures Contd..



24

&%

gssus ρ ε ε ρ ρ .)1(. +−=

Superficial Velocity

psm

p

∆=

∆

72.1 / 0104.0

0179.0

Uo = m/sSolid Circulation Rate

t

L mf as )1( ε ρ −

=sG

La : Accumulation height, m



25

Measurement of Mean Particle Size of Sand

1

1

__ 1

d

X d p

=

X1 : weight fraction of solids of diameter d1



26

))$

t

qh

∆

= "

)( BS ht T T A

VI h

−=

Aht : Area of heat exchanging surface.

TB:Bed temperature

Ts: surface temperature.



27

Stage – 1Stage – 2

Stage – 3

Stage – 4 Stage – 5

Stage – 6

Hydrodynamic Behavior



28

Axial voidage distribution

linear for low operatingconditions

decreases along the

cyclone heightincreases for higher

gas velocities

Voidage variations along the

cyclone height

Heat Transfer and Hydrodynamic Study in Cyclone

+

I = 16 kg

Distance from the inlet of cyclone , m

V o i d a g e f r a c t i o n

q" = 771.895 w/m2

Uo = 4.496 m/s

Uo = 3.881 m/s

Uo = 2.986 m/s

0.75

0.8

0.85

0.9

0.95

1

1.05

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8



29

I=10 kg

I=16 kg

Voidage variations along the cyclone height

Axial voidage distribution contd…

+



Uo = 4.429 m/s, Gs = 15.97 kg/m2s

I = 10 kg q" = 771.892 w/m2

Uo = 3.08 m/s, Gs=5.57 kg/m2s

Uo = 3.875 m/s, Gs =10.59 kg/m2s

0.84

0.86

0.88

0.9

0.92

0.94

0.96

0.98

1

1.02

1.04

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

+

I = 16 kg



q" = 771.895 w/m2

Uo = 4.496 m/s

Uo = 3.881 m/s

Uo = 2.986 m/s

0.75

0.8

0.85

0.9

0.95

1

1.05

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8



30

%

Uo = 3.875 m/s

Uo = 4.429 m/s

Uo = 3.084 m/s

I = 10 kg q" = 771.895 w/m2


S u s p e n s i o

n d e n s i t y ,

k g / m 3

−100

−50

0

50

100

150

200

250

300

350

400

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8

I=10 kg

I=16 kg

Suspension density variations along the cyclone height

S u s p e n s i o n d e n s i t y ,

k g / m 3


2I = 16 kg q" = 771.895 w/m

Uo = 2.986 m/s

Uo = 3.881 m/s

Uo = 4.496 m/s

−200

−100

0

100

200

300

400

500

600

0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8



31

* $

Variations of local heat transfer coefficient along the

cyclone separator height

I=10 kg

s

2

2

s

Distance from the inlet of cyclone, m

I = 10 kg

2 k

L o c a l h e a t t r a n s f e r c o e f f i c i e n t , w / m

q" = 771.895 w/m

Uo = 3.875 m/sUo = 4.429 m/s

U o = 3.084 m/s

250

255

260

265 270

275

280

285

290

295

300

305

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7



32

q" = 411.68 w/m2, Gs = 20.48 kg/m2s

q" = 771.985 w/m2, Gs = 20.53 kg/m2s+

L o c

a l h e a t t r a n s

f e r c o e f f i c i e n

t , w / m 2 k


I = 16 kg Uo = 4.496 m/s

120

140

160

180

200

220

240

260

280

300

320

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

Variations of local heat transfer coefficient along the cyclone heightfor different heat flux conditions



33

' $

Uo = 3.084 m/s

Uo = 3.875 m/s

Uo = 4.429 m/s

Non−dimensional distance across the cyclone width

k 2

L o c a

l h e a t t r a n s f e r c f f i c i e n t , w /

m

I=10 kg, q"= 771.895 w/m2

292

293

294

295

296

297

298

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

H=0.50 m

Radial distributions of heat transfer coefficient

I=10 kg



34

Uo = 3.084 m/s

Uo = 3.875 m/s

Uo = 4.429 m/s


k

2

L o c a l h e a t t r a

n s f e r c f f i c i e n t , w / m

I=10 kg, q"= 771.895 w/m2

292

293

294

295

296

297

298

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

I = 13 kg q" = 771.895 w/m2h = 0.50 m

Uo = 3.132 m/s, Gs = 8.23 kg/m2s

Uo = 3.913 m/s, Gs = 10.96 kg/m2s

Uo = 4.562 m/s, Gs = 18.03 kg/m2s

o c a

e a t t r a n s

e r c o e

c e n t , w m


293

294

295

296

297

298

299

300

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

I=10 kg

I=13 kg

H=0.50 m

' $ ++


H=0.50 m



35

h = 0.10

h = 0.30

L o c a l h e a t t r a n s f e r c o e f f i c i e n t , w / m 2 k


I = 16 kg Uo = 4.562 m/s, Gs = 18.98 kg/m2s q" = 411.68 w/m2

h = 0.50

120

130

140

150

160

170

180

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

' $ ++


H=0.10 m

H=0.30 m

H=0.50 m



36

m

I = 13 kg

I = 16 kg

I = 10 kg

L o c a l h e a t t r a n s f e r c o e f f i c i e n t , w / m 2 k


h = 0.10 Uo = 4.429 m/s − 4.562 m/s q" = 411.68 w/m2

122

123

124

125

126

127

128

129

130

131

0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9

' $ ++


I=10 kg

I=13 kg

I=16 kg



37

, %

I = 16 kgI = 13 kgI = 10 kg

2

k

2

Gas superficial velocity, m/s

q" = 411.68 w/m

A v e

r a g e h e a t t r a n

s f e r c o e f f i c i e n t , w / m

142

144

146

148

150

152

154

2.8 3 3.2 3.4 3.6 3.8 4 4.2 4.4 4.6

Variation of average heat transfer coefficient with gas superficial velocity

I=13 kg

I=16 kg

I=10 kg



38

Suspension density profile

Uo = 4.429 m/s, Gs = 14.42 kg/m2sUo = 3.875 m/s, Gs = 6.23 kg/m2s

Uo = 3.084 m/s, Gs = 4.05 kg/m2s

Height above the distributor plate, m

I = 10 kg

S u s p e n s

i o n d e n s i t y ,

k g / m

q" = 1593.137 w/m2 3

10

12

14

16

18

20

22

24

26

28

30

0.8 1 1.2 1.4 1.6 1.8 2 2.2

Suspension density variations along the cyclone height

C l i



39

Conclusion

The axial and radial distribution profiles of theheat transfer coefficient in the cyclone separatorand are consistent with the corresponding solids

concentration

The heat transfer coefficient in the cyclone isfound to be increasing with increase in the solid

loading as well as gas superficial velocity

At a certain distance from the entry of thecyclone downstream along the height, the local

heat transfer coefficient is found to be maximum



40

Conclusion contd…

On the upper splash zone of riser an increasing-

decreasing trend of local heat transfer coefficientHeat transfer coefficient increases with solidcirculation rate

The heat transfer coefficient generally increases

with the solids holdup, but their relationshipexperiences different trends under the differentoperating conditions and at different cyclone and

riser locations



41

prof p mahanta

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