Lu_Heat Transfer Characteristics in a Horizontal Swirling Fluidized Bed

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Heat transfer characteristics in a horizontal swirling uidized bed

Ping Lu a ,⇑ , Yan Cao b , Wei-Ping Pan b , Chengguo Ma c

a School of Energy and Mechanical Engineering, Nanjing Normal University, Nanjing 210042, Chinab ICSET, Western Kentucky University, Bowling Green, KY 42101, USAc Black Dragon Double Boiler Co. Ltd., Shuangyashan, Heilongjiang 155110, China

a r t i c l e i n f o

Article history:Received 16 May 2010Received in revised form 30 November 2010Accepted 22 March 2011Available online 29 March 2011

Keywords:Swirling uidized bedHeat transferSecondary air injection

a b s t r a c t

An innovative horizontal swirling uidized bed (HSFB) with a rectangular bafe in the center of an airdistributor and three layers of horizontal secondary air nozzles located at each corner of uidized bedwas developed. Experiments on heat transfer characteristics were conducted in a cold HSFB test model.Heat transfer coefcients between immersed tubes and bed materials in the HSBF were measured withthe help of a fast response heat transfer probe. The inuences of uidization velocity, particle size of bedmaterials, measurement height, probe orientation, and secondary air injection, etc. on heat transfer coef-cients were intensively investigated. Test results indicated that heat transfer coefcients increase withuidization velocity, and reach their maximum values at 1.5–3 times of the minimum uidization veloc-ity. Heat transfer coefcients are variated along the circumference of the probe, and heat transfer coef-cients on the leeward side of the probe are larger than that on the windward side of the probe. Heattransfer coefcients decrease with increasing of measurement height; heat transfer coefcients of thelongitudinal probe are larger than that of the transverse probe. The proper secondary air injection andparticle size of bed materials can generate a preferred hydrodynamics in the dense zone and enhanceheat transfer in a HSFB.

Ó 2011 Elsevier Inc. All rights reserved.

1. Introduction

Circulating uidized beds (CFBs) have been successfully used infossil fuel combustion, coal and biomass gasication, and uid cat-alytic cracking. In combustion and gasication, CFB technology of-fers signicant advantages such as fuel exibility, in-bed sulfurcapture, and relatively low NO x emissions with high efciencies[1,2] . The advanced swirling uidized bed combustion (SFBC) isone of the alternative ways to improve FBC performance by intro-ducing the secondary air through the tangential direction into thecombustor, in which the residence time distributions of feed mate-rials as well as gas products in the reactor can be controlled. Theswirling uidized bed combustion technology can be applied inwaste management to reduce an amount of wastes in communi-ties, cities, or countries all over the world. The SFBC is expectedto possess the following advantages: (1) high combustion ef-ciency, (2) wide fuel exibility, (3) low emissions, (4) enhancedmass/heat transfer, and (5) large ring intensity [3,4] .

In many uidized bed applications, it is necessary to add or ex-tract heat in order to maintain the operating temperature at a de-sired value. The design and scale-up of heat transfer surfaces

require the knowledge of the bed hydrodynamics and heat transfercoefcient at the wall surfaces in contact with the uidized mass[5,6] . However, the thermo-physical behaviors of gas–solid in a u-idized-bed reactor are highly complicated and sometimes randomowing to the irregular contacting and ow behaviors of gas and so-lid. Numerous experimental investigations have been carried outto measure heat transfer in CFBs at room and high temperatures.Grace [7] , Leckner [8] , and Basu and Nag [9] have presented com-prehensive reviews of CFB heat transfer. Tian et al. [10,11] studiedheat transfer of an immersed tube in an internal circulating uid-ized bed, and indicated heat transfer characteristics were found tobe signicantly different from that in a bubbling bed. Up to now, afew published works are available to explore hydrodynamics andheat transfer in a horizontal swirling uidized bed, especially heattransfer among gas–solid and immersed surfaces.

In this paper, a HSFB with a rectangular bafe in the center of anair distributor and three horizontal secondary air nozzles located ateach corner of uidized bed was developed. Heat transfer charac-teristics in a horizontal swirling uidized bed were studied underdifferent operating conditions. Heat transfer coefcients betweenthe immersed tubes and bed materials were measured with thehelp of a fast response heat transfer probe. The effects of uidiza-tion velocity, particle size of bed materials, measurement height,probe orientation, and secondary air injection rate, etc. on heattransfer were intensively analyzed.

0894-1777/$ - see front matter Ó 2011 Elsevier Inc. All rights reserved.doi: 10.1016/j.expthermusci.2011.03.007

⇑ Corresponding author. Tel.: +86 25 8548 1123; fax: +86 25 8540 0095.E-mail address: [email protected] (P. Lu).

Experimental Thermal and Fluid Science 35 (2011) 1127–1134

Contents lists available at ScienceDirect

Experimental Thermal and Fluid Science

j ou rna l homepage : www.e l sev i e r. com/ loca t e / e t f s

http://dx.doi.org/10.1016/j.expthermflusci.2011.03.007

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2. Experimental

The experimental apparatus consists of a HSFB cold model, airsupplying and dust collecting system, and heat transfer coef-cient measurement system. Fig. 1 shows the schematic of exper-imental horizontal swirling uidized bed. A rectangular chamberis 457 mm (width) Â 584 mm (length) and 1192 mm in height.At the bottom of the chamber there is an air distributor and itsair caps connected to an air box. A bafe with rectangle cross-area of 51 Â 318 mm and 305 mm in height is located at the cen-ter of the air distributor. The uidization air (primary air, PA) sup-plying by a compressor was introduced through the airdistributor. Total twelve secondary air (SA) injection nozzles

(9.5 mm ID) at the height of 152 mm, 228 mm, and 305 mm away

from the air distributor are placed at four corners of the HSFB,respectively. The nozzles at each corner are inserted 38 mm intothe chamber and connected with each outside corner main pipe.Each main pipe has a control valve to adjust airow rate to t theexperimental needs. The secondary air (horizontal swirling air) isintroduced into the chamber through these nozzles, which makeinside bed materials swirling. Three layers of test ports at theheight ( Z h) of 76 mm, 178 mm, and 305 mm above the air distrib-utor are located at the each side walls. The exhausted gas waspuried in a lter bag before exiting to air. The air ow rateand pressure drop are measured by rotameter and digital pres-sure meter, respectively.

Nomenclature

C pc specic heat capacity, W kg À 1 KÀ 1

d p particle size, mm f b contact time ratio between bubbles and the probehb average of local heat transfer coefcient at the different

probe orientation angle, W m À 2 KÀ 1

h average heat transfer coefcient, W m À 2 KÀ 1

hl longitudinal heat transfer coefcient, W m À 2 KÀ 1

hlocal local heat transfer coefcient, W m À 2 KÀ 1

hmax calculated maximum heat transfer coefcient,W m À 2 KÀ 1

ht transverse heat transfer coefcient, W m À 2 KÀ 1

ke thermal conductivity of emulsion, W m À 1 KÀ 1

k g gas thermal conductivity, W m À 1 KÀ 1

n uidization numbernw bubble frequency, s À 1

q heat ux, W/m 2

T bed bed temperature, KT out outside surface temperature of a probe, Ku f uidization velocity, m/sumf minimum uidization velocity, m/sus secondary air velocity, m/s Z h height of test ports away from the distributor, mm

Greek lettersb probe orientation angle, °

q e density of emulsion phase, kg m À 3

q p particle density, kg m À 3

q sb bulk density of bed, kg m À 3

Fig. 1. Schematic of a horizontal swirling uidized bed.

1128 P. Lu et al. / Experimental Thermal and Fluid Science 35 (2011) 1127–1134



The schematic of heat transfer coefcient measurement systemis shown in Fig. 2 . It includes a Teon tube, a mini-heater, a heatux sensor, a thermocouple, a controller, and a computerized dataacquisition system (DAS). Two miniature cylindrical Teon probeswith 25.4 mm OD and 9.5 mm ID, and 324 mm/597 mm in lengthare developed for measuring the transverse and longitudinal heattransfer coefcients, respectively. A HFS-3 heat ux sensor (manu-factured by Omega company) with the dimensions of 0.18 mm(thickness) Â 35.0 mm (length) Â 28.6 mm (width) is stuck to theoutside surface of each probe, whose working temperature isÀ 200 to 205 ° C. A mini-heater (manufactured by Omega company)with maximum power of 75 W is placed inside each probe.

The heat transfer probes are installed in three different loca-tions along the measurement height above the air distributor.The schematic of location and orientation of the transverse andlongitudinal probes is shown in Fig. 3 . During the experiments,two probes are placed at the same level, heat transfer coefcientsof different probes and its orientation angles b (0 ° –180 ° ) can be ob-tained by changing test ports and rotating the probes.

Heat generated by a heater inside the probe is transferred frominside to outside of the probe, then transported into the densephase of the uidized bed. Heat ux is measured by the thin lmheat ux sensor stuck on the surface of the probe. All signals of bed temperature, heat ux, and temperature of the probe surfaceare transferred to computer through a high-speed computerized

data acquisition system. The uniform bed temperature will be ob-

tained due to high gas and solids ow, vigorous mixing and a smallamount of heat generated by the heating probes. With theassump-tion and the negligible heat losses through the end sides of a probe,the time-average local heat transfer coefcient, h local was deter-mined as follows [6,12] :

hlocal ¼q

T out À T bedð Þð1 Þ

in which, T bed is the bed temperature measuredby a K-type thermo-couple which was installed approximately 50 mm away from theprobe,K; T out is theoutside surfacetemperature of a probe measuredbya K-typethermocouplewhichwasmountedon theHFS-3 heat uxsensor, K; and q is heat ux, W/m 2 . Precision of temperature mea-surementwas±0.5 ° C.Thesampling interval is0.187–0.201 s. It takesmore than 5 min for each sampling condition. An actual samplingtime-average local heat transfer coefcient is shown in Fig. 4 . As itis canbe seen, themean and standard deviationof local heat transfercoefcient of 2298 sampling time intervalsare194.7 W m À 2 KÀ 1 and21.8 W m À 2 KÀ 1 , respectively. The average heat transfer coefcient,h, which is the arithmetical average of transverse and longitudinalheat transfer coefcients ( h t , h l), can be obtained by

1

2

4

3

5

2

2

2

1

2

4

3

5

2

2

2

Downwards

Heat flux sensor

Probe oritation angle

Flow direction

Probe

(a) Probe position in HSFB (b) Probe orientation

1-HSBF cross-area; 2-secondary air nozzle;3-tansverse probe; 4-longitudinal probe; 5-baffle

β

Fig. 3. Schematic of probe position and probe orientation.

0 100 200 300 400 500 6000

50

100

150

200

250

300

Time (s)

Z h = β = o; d p = 0.718 mm; u f =0.516 m/s

Transverse probe

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 - 1 )

178 mm; 180

Fig. 4. Local heat transfer coefcient.

Fig. 2. Schematic of a heat transfer probe measure system.

P. Lu et al. / Experimental Thermal and Fluid Science 35 (2011) 1127–1134 1129



h t ðor hlÞ ¼h0 þ 2 h45 þ 2 h90 þ 2 h135 þ h180

8ð2 Þ

h ¼hl þ ht

2 ð3 Þ

in which, hb (including h0 , h45 , h90 , h135 , and h180 ) is the average of local heat transfer coefcient at the different probe orientation an-gle. Considering on the uncertainty both highly stochastic operationof uidized bed and temperature measurement, experimental er-rors on computed values of average heat transfer coefcient ( h)were less than 12%.

Three particle sizes of quartz sands with the particle density q p

of 2650 kg/m 3 were used as bed materials. During the experimentsthe static height is 300 mm. All the experiments were carried outat room temperature and atmospheric pressure. The experimentalparameters are summarized in Table 1 .

3. Results and discussion

3.1. Effect of uidized velocity on heat transfer

The effect of uidization number on average heat transfer coef-cient is shown Fig. 5 , in which uidization number, n , is denedas the ratio of uidization velocity ( u f ) to minimum uidizationvelocity ( umf ) of different particles, i.e. n = u/umf . It can be seen thatthe average heat transfer coefcient is very small at a minimum

uidization velocity ( n = 1), and increases quickly with uidizationnumber. The uidization numbers corresponding to the maximum

heat transfer coefcient of coarse and middle particles are veryclose, i.e. 1.75 and 1.5 respectively. However, the uidization num-ber of ne particles with maximum average heat transfer coef-cient is about 3.0. The relevant maximum heat transfercoefcients are 290.2W m À 2 KÀ 1 , 373.2 W m À 2 KÀ 1 , and599.4 W m À 2 KÀ 1 of coarse, middle, and ne particles, respectively.

Based on Zabrodsky [13]

hmax ¼ 35 :8 q0 :2 p k0 :6

g dÀ0 :36

p ð4 Þ

in which k g is the gas thermal conductivity, W m À 1 KÀ 1 . The calcu-

lated maximum heat transfer coefcients are 266.8 W mÀ 2

KÀ 1

,342.5 W m À 2 KÀ 1 , and 439.1 W m À 2 KÀ 1 of coarse, middle, and neparticles, respectively. The relative errors are less than 10% forcoarse and middle particles, but there is a big relative error of 30% for ne particles. This big relative error of maximumheat trans-fer coefcients of ne particles should be the accumulative effectsfrom measurement errors and chose correlations. Most of the rela-tive errors of ne particles are less than 12% based onthe other cor-relations (such as Baerg et al. correlation in Fig. 5 ) [14] . Althoughthe uidization number of ner particles at maximumheat transfercoefcient is very bigger than others, the minimum uidizationvelocity is very small. So the uidization velocity is not very highat a maximumheat transfer coefcient. The corresponding uidiza-tion velocities are 0.27 m/s, 0.31 m/s, and 0.62 m/s of ne, middle,

and coarse particles at maximum heat transfer coefcient,respectively.

Table 1

Summary of the experimental conditions.

Average particle size d p, mm 0.180 0.359 0.718Particle size range, mm 0.148–0.210 0.297–0.420 0.595–0.841Direction of probe Transverse, longitudinalLocation of probe Lower, middle and upper; Z h = 76, 178, and 305 mmOrientation angle of probe, b , ° 0 ° , 45 ° , 90 ° , 135 ° , and 180 °

Minimum uidization velocity, m/s 0.089 0.177 0.414Fluidization velocity, m/s 0.089–0.331 0.161–0.509 0.318–0.882Secondary air velocity, m/s – 0–0.561 0–0.726

0 1 2 3 40

50

100

150

200

250300

350

400

450

500

550

600

650

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 - 1 )

Fluidization number (-)

d p =0.718 mm (Experimental)



d p =0.718 mm (Equ.(4))

d p =0.359 mm (Equ.(4))

d p =0.180 mm (Equ.(4))

d p =0.180 mm (Equ. in Fig. 5)

6max 120.8ln(7.05 10 )sb

p

hd

ρ −= ×

Fig. 5. Effect of uidization velocity on heat transfer.




3.2. Effect of probe orientation on heat transfer

The effects of probe orientation on transverse and longitudinalheat transfercoefcient areshownin Figs. 6 and 7.As itcanbe seen,heat transfercoefcient of angle 0 ° is smaller than most of other an-gles. Heat transfer coefcient backward the ow is bigger than thatupward the ow at the most of test conditions. That is because theconvection heat transfer is determined by contact ratio betweentheprobesand thebubblephaseor theemulsionphase.Due tosmal-ler thermal capacity of gas phase, so heat transfer based on bubble

phaseat upward orientation is weaker than that based on emulsionphase at backward orientation. Ref. [15] elucidates the effects of increasing of the bubble phase on heat transfer based on

h ¼ 1 À f bð Þ keq eC pc nw

1 À f b 1 =2

ð5 Þ

inwhich, f b is contacttimeratiobetweenbubblesand theprobe; nw isbubble frequency, s À 1 ; ke is thermal conductivity, W m À 1 KÀ 1 ; q e isdensity of emulsion phase, kg/m 3 ; C pc is specic heat capacity,W kg À 1 KÀ 1 . The rapid increase of local surface heat transfer coef-cient is due to the increasing of bubble frequency at that velocitywhen uidization velocity is just larger than critical uidizationvelocity. The bubble frequency is a limitedvalue,which is controlledby formation and growth of bubbles [16] . Generally the bubble fre-

quency is less than 14 bubble/s, and is variational with the distanceaway fromthe wall. The closer to the wall, the lower the bubble fre-

quency. Based on Eq. (5) wecan see that the local heat transfercoef-cient is directproportional to squareroot of bubblefrequency. Thatis to say, there will bea better effect onheat transferat the beginning

of bubble formation. With increasing of bubbles, the increasing of bubble frequency will be slower. So the effects of bubble frequency

020406080

100120

140160180200220240260

Transverse Probe

Fluidization Velocity (m/s)

0o

45o

90o

135 o

180o

d p =0.718 mmZ h =178 mm

0.3 0.4 0.5 0.6 0.7 0.8 0.9 0.0 0.1 0.2 0.3 0.4 0.5 0.60

50

100

150

200

250

300

350

Transverse probe


0o

45 o

90o

135o

180o

d p =0.359 mmZ h =178 mm


- 2 K - 1 )

(a) (b)


- 2 K - 1 )

Fig. 6. Effect of transverse probe on heat transfer.

0

100

200

300

400

500

Longitudinal probe

0o

45 o

90 o

135 o

180 o

d p =0.718 mm

Z h =178 mm

(a)

0.05 0.10 0.15 0.20 0.25 0.30 0.35 0.40 0.45 0.50 0.550

100

200

300

400

500

600

Longitudinal probe

0o

45 o

90 o

135 o

180 o

d p =0.359 mm

Z h =178 mm

(b)


0.3 0.4 0.5 0.6 0.7 0.8 0.9



C o e f f i c i e n t ( W m -

2 K - 1 )


C o e f f i c i e n t ( W m - 2

K - 1 )

Fig. 7. Effect of longitudinal probe on heat transfer.

0.2 0.3 0.4 0.5 0.6 0.7 0.8100

150

200

250

300

350

400

450


Z h = 76 mm

Z h = 178 mm

d p =0.718 mm

Longitudinal probe 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 - 1 )

Fig. 8. Effect of measurement height on heat transfer.




on heat transfer will decrease also. Otherwise, heat transfer will de-crease a little due to the increasing of contact time ratio of bubbleswithprobesundertheconditionof sufcientuidization.Atthesametime, the bubble frequency and contact time ratio are almost thesame in all of the probe orientation, so the change of heat transfercoefcient in different directions is very small.

3.3. Effect of probe position and particle size on heat transfer

It also can be seen from Figs. 6 and 7, there are notable differ-

ences between transverse and longitudinal heat transfer coef-cient. As Fig. 3 shows, the transverse probe is much closer thanthe longitudinal probe to the wall, this makes longitudinal probeget bigger bubble frequency than transverse one, and get higherlongitudinal heat transfer coefcient at the same uidizationvelocity. As Fig. 5 shows heat transfer coefcient of small particlesis bigger than that of bigger ones. The cause is there are more con-tact points of small particles with probes than that of big ones. Thismakes small particles get more contact area at the probe surface,and get more heat transfer area at the same refresh ratio of theemulsion phase, so heat transfer is enhanced.

3.4. Effect of measurement height on heat transfer

The longitudinal heat transfer coefcients at the middle andlower position are shown in Fig. 8 , in which particle size d p is

0.718 mm. As it can be seen, heat transfer coefcient decreaseswith the measurement height. The results agree with Ref.

[17,18] . Jin et al. [19] measured the local heat transfer coefcientat different measurement height and radial position in uidized

100

120

140

160

180200

220

240

260

280

300

Transverse probe

d p =0.718 mm

Zh =76 mm

u f =0.726 m/s

(a)100

120

140

160

180

200

220

240

260

Transverse probe

u s = 0 m/s

u s = 0.357 m/s

u s = 0.516 m/s

u s = 0.726 m/s

d p =0.718 mm

Zh =305 mm

u f =0.726m/s

(b)

0 45 90 135 180

β (o)0 45 90 135 180

β (o)



- 2 K - 1 )



- 2 K - 1 )u s = 0 m/s

u s = 0.357 m/s

u s = 0.516 m/s

u s = 0.726 m/s

Fig. 9. Effect of secondary air injection on heat transfer.

0 45 90 135 180200

250

300

350

400

450

500

550

600

Longitudinal probe

β (o)

u s = 0 m/s

u s = 0.357 m/su s = 0.516 m/su s = 0.726 m/s

d p =0.718 mm

Zh=76 mmu

f =0.726 m/s

(a)

0 45 90 135 180200

250

300

350

400

450

500

550

600

Longitudinal probe

d p =0.718 mm

Zh=305 mmu

f =0.726 m/s

u s = 0 m/s

u s = 0.357 m/s

u s = 0.516 m/s

u s = 0.726 m/s

(b)



- 2 K - 1 )



- 2 K

- 1 )

β (o)


0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8225

250

275

300

325d p =0.718 mm

u f = 0.726 m/s

Transverse and longitudinal probes

u s (m/s)

Z h=76 mm

Z h=178 mm

Z h=305 mm



- 2 K - 1 )

Fig. 11. Effect of secondary air injection rate on heat transfer.




bed of U 280 mm with small ball of U 19.4 mm. Test results shownheat transfer was variated with the measurement height and radial

position. These all agree with the discussion about bubble fre-quency and bubble sizes in Section 3.3 . Generally the bubble fre-quency is higher and bubble size is smaller at the lower positionclose to the air distributor. So the contact time ratio between bub-bles and the probe is smaller. Based on Eq. (5) these factors willmake heat transfer coefcient decrease with measurement height.

3.5. Effect of secondary air injection on heat transfer

Secondary air injection is a main difference between a HSFB anda bubbling uidized bed. So it is important to obtain the effect of secondary air injection on heat transfer. The effects of secondaryair injection on heat transfer of the transverse and the longitudinalprobes are shown in Figs. 9 and 10 , respectively, in which the aver-age particle size of bed material d p = 0.718 mm, the uidizationvelocity ( u f ) is 0.726 m/s, the secondary air (swirling air) velocity(us), which is the cross-sectional velocity of HSFB, is 0–0.726 m/s(the injection velocity of secondary air is 0–120 m/s). As the guresshow, thesecondary air injection has a bad inuence on heat trans-fer at most of operating conditions. The causes are that: (1) in abubbling uidized bed the effects of mixing and disturbance arevery intense at high uidization velocity, so the mixing and distur-bance effects of secondary air injection are weakened; (2) the sec-ondary air will increase the contact time ratio between probes andbubbles (or gas phases), and decease heat transfer coefcient; and(3) coarse particles and high uidization velocity are very powerfulto weaken the rigidity of secondary air injection.

Fig. 11 shows the effect of secondary air injection rate on aver-

age heat transfer coefcient at different tested locations. As the g-ure shows, with increasing of secondary air injection rate, heattransfer coefcient of the probes at low location decreases, how-ever, heat transfer coefcient of the probe at middle and upperlocations almost keeps constant. This indicates the SA injectionshould be helpful to get a stable heat transfer coefcient in a HSBF.Based on the design of secondary air injection of this model, moresecondary air will come out from the upper injection nozzles dueto higher uid resistance at the low injection location, and inhibitsmixing and disturbance at the low location. So heat transfer coef-cient at low location decreases with the increasingof SA injection.This indicated some modication of secondary air injection needbe done to get the same effects at different injection location.

Fig. 12 shows the effect of secondary air injection on heat trans-

fer of the transverse and longitudinal probes, respectively, in whichd p = 0.359 mm, u f = 0.363 m/s. Comparing with a HSBF with coarse

particles, the secondary air injection of a HSBF with moderate par-ticles has a signicant effect on increasing of heat transfer coef-

cient at the most of operating conditions. That is due to astronger penetrability and disturbance ability of SA injection forsmall particles, and all these makes heat transfer enhanced.

The results give us a good suggestion about design and opera-tion for a HSFB. The effects of bed particle size, secondary air injec-tion rate and its velocity, injection position will have importantinuences on heat transfer in a HSFB. As well known, more second-ary air injection will increase the contact time ratio between bub-blephases and probes, and make heat transfercoefcient decrease;on the other hand, less secondary air injection cannot make bedmaterials swirling in the dense zone, and cannot enhance heattransfer in a HSBF. So in order to obtain an optimum secondaryair injection rate and its velocity and form a stable horizontalswirling at the condition of certain particle size, uidization veloc-ity, and bed location in a HSFB, some change of design and furtherstudy should be carried out.

4. Conclusion

Heat transfer characteristics of an innovative horizontal swirl-ing uidized bed (HSFB) were measured with the help of a fast re-sponse heat transfer probe. Based on the experimental ndings of this work it could be concluded as following:

(1) Heat transfer coefcients increase quickly at minimum u-idization velocity, and reach their maximum values at 1.5–3 times of the minimum uidization velocity.

(2) Heat transfer coefcient in the backward orientation ishigher than that in the upward orientation. Heat transfer

coefcient of the longitudinal probe is larger than that of the transverse one.

(3) Heat transfer coefcients decrease with increasing of mea-surement height. The ner bed material size has a goodeffect on heat transfer in a horizontal swirling uidized bed.

(4) The secondary air injectiongenerates the preferred hydrody-namics in the dense zone and enhanced heat transfer forner bed materials. Further study of formation of horizontalswirling and heat transfer characteristics in a HSFB shouldbe carried out.

Acknowledgments

Financial supports from National Natural Science Foundation of China (51076067), Natural Science Foundation of Jiangsu Province

400

450

500

550

600

650

700

(a)

u s = 0 m/su s = 0.318 m/su s = 0.561 m/s

d p =0.359 mm

Z h =176 mmu f =0.363m/s

Longidutinal Probe

0 45 90 135 180 0 20 40 60 80 100 120 140 160 180100

150

200

250

300

350

(b)

d p =0.359 mm

Z h =76 mmu f = 0.363 m/s

Transverse probe

u s = 0 m/s

u s = 0.318 m/s

u s = 0.561 m/s

β (o) β (o)



- 2 K - 1 )



- 2 K - 1 )





of China (BK2010081) and Scientic Research Foundation for theReturned Overseas Chinese Scholars, MOP are gratefully acknowl-edged. The authors also expressed sincere gratitude to ProfessorR. Troutman for constructive advices and valuable assistance inpreparing this paper.

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Lu_Heat Transfer Characteristics in a Horizontal Swirling Fluidized Bed