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Fluidization and surface-to-bed heat transfer in a aound assisted bubbling fluidized bed of fine powders.

Introduction

The heat transfer between a fluidized bed and immersed body has been part of many investigations in the past. Fluidization of fine particles can be improved by using vibration techniques, such as Pulsating magnetization, Mechanical vibrations or Acoustic vibrations etc. Morse (1955) found that at low frequencies, high intensity sound waves improved the fluidization on fine powders. Chirone, Massimilla and Russo (1993) proposed a cluster-subcluster model to describe influence of acoustic fields on fluidization. It was found that the quality of fluidization was appreciably improved by appropriate sound pressure level. Nowak et al. (1993) reported that high bed expansion and heat transfer coefficient could be obtained by adjusting the frequency of acoustic waves to resonance. Russo et al. (1995) proposed a modified cluster-subcluster model. They concluded that channel free and homogeneous fluidization could be obtained for the range of fine powders with an appropriate combination of frequency and SPL. Levy et al. (1997) concluded that quality of fluidization could be improved at low frequencies. They also reported that at natural frequency, high intensity helped in reduction of minimum fluidization and minimum bubbling velocity. Herrera (2000) concluded that minimum bubbling velocity was function of particle density, bed depth and SPL. Huang (2004) showed that bubble frequency increased with an increase in excess air velocity and SPL. The packet residence time and the fraction of packet contact time at the tube surface decreased with increase in excess air velocity and SPL. The convective heat transfer coefficient between the tube surface and the bed material was strongly affected by sound waves of appropriate frequency and SPL.

Fluidized bed heat transfer equipment

The experimental apparatus consists of a cylindrical fluidized bed, a sound wave generation system and horizontal copper tube for heat transfer measurement. The fluidized bed is a 115 mm ID transparent acrylic column with 610 mm height. The acoustic sound generation system included a sound amplifier, a digital signal generator and a loudspeaker. The loudspeaker was positioned on top of the bed column to generate the acoustic waves needed to achieve fluidization of the fine particles. The microphone was used for experimentation purposes. Digital storage oscilloscope was used to generate the waveform needed for estimation of SPL. The microphone was placed at a particular distance from the distributor for each set of experiments. The output of microphone was processed with digital storage oscilloscope and accordingly SPL was varied depending upon requirements. Sound pressure level measurements were performed with microphone and digital storage oscilloscope. The sound frequency was controlled with signal generator, and it was usually kept constant during the entire tests. The heat transfer tube was mounted horizontally in the fluidized bed. The properties of bed materials used in the present work are enlisted in table1.

[FIGURE 1 OMITTED]

Heat transfer in a bubbling fluidized bed

The local heat transfer coefficients were determined at four angular positions around the heated tube perimeter. Graph 1 show the effect of excess air velocity on local heat transfer coefficients at different angular positions for glass beads. In all cases, the highest heat transfer coefficients were obtained at [theta] = 90[degrees].The lowest value is attained at the topmost position of heated tube, at [theta] = 180[degrees]. The local heat transfer coefficient at all the angular positions with variation of excess gas velocity is shown in graph 2 for glass beads. Graph 3 shows variation of average heat transfer coefficients with and without acoustic field for large sized Glass beads with 462 [micro]m size with respect to excess air velocity. The value of average heat transfer coefficients increase marginally beyond SPL=120 db. The fluidizing behavior of large particles is marginally affected by acoustic field and hence there is relatively less effect on average heat transfer coefficients. The data show that beyond an excess air velocity of 17.52 cm/s, the average heat transfer coefficient was independent of air velocity.

Graph 4 show variation of local heat transfer coefficients with excess air velocity ([u.sub.g]-[u.sub.mf]) at different values of [theta] and graph 5 show variation of local heat transfer coefficients with angular position ([theta]) at different values of excess air velocity ([u.sub.g]-[u.sub.mf]) for microfumed silica (112 [micro]m) for SPL in range 112-144 dB. It can be concluded that at minimum fluidization and bubbling conditions, the position for maximum value of local heat transfer coefficients was located at the sides of the tube ([theta] = 90[degrees]).However, with further increase in gas velocity, the position of maximum value of local heat transfer coefficients moves towards the rear ([theta] = 180[degrees]).The high value of local heat transfer coefficients was obtained at the boundary between bubbling state and turbulent state. Graph 6 shows the effect of gas velocity for different acoustic conditions. Microfumed silica (112 [micro]m) exhibited poor fluidization behavior from 112-125 dB, and the average heat transfer coefficient were relatively low. Nevertheless, once the SPL was increased to 130-144 dB, the average heat transfer coefficient was improved appreciably.

For initial conditions, for [u.sub.g]/[u.sub.mf] upto 4,the location for maximum local heat transfer coefficients was at the sides of the tube ([theta] = 90[degrees]).With increase in gas velocity up to [u.sub.g]/[u.sb.mf] =4,the local heat transfer coefficient increases due to increasing replacement of packets of particles at the tube surface. The values of local heat transfer coefficients are higher for the bottom positions than the top positions on the tube. With further increase in gas velocity beyond [u.sub.g]/[u.sub.mf] = 4, it is found that the values of local heat transfer coefficients reduce for all conditions of SPL and sound frequency. Also, it is seen that the values of local heat transfer coefficients are improved in the SPL range 130-144 dB. It can be concluded that at minimum fluidization and bubbling conditions, the position for maximum value of local heat transfer coefficients was located at the sides of the tube ([theta] = 90[degrees]). However, with further increase in gas velocity, the position of maximum value of local heat transfer coefficients moves towards the rear ([theta] = 180[degrees]).The high value of local heat transfer coefficients was obtained at the boundary between bubbling state and turbulent state.

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[GRAPHIC 3 OMITTED]

[GRAPHIC 4 OMITTED]

[GRAPHIC 5 OMITTED]

[GRAPHIC 6 OMITTED]

Results and conclusion

Heat transfer with a heated tube in a sound assisted fluidized bed of fine powders is described. The fluidizing behavior of large particles (Glass beads with diameter 462 microns) is marginally affected by acoustic field beyond 120 db and hence there is relatively less effect on average heat transfer coefficients. In all cases, for microfumed silica (112 [micro]m), the average heat transfer coefficient improved with sound assistance. With sound assistance, the average heat transfer coefficient increased with an increase in excess air velocity up to a value 0f 2.9 cm/s([u.sub.g]/[u.sub.mf] =4).With further increase in excess air velocity up to [u.sub.g]/[u.sub.mf] = 6,the values of average heat transfer coefficient were reduced.

References

[1] R. D. Morse, "Sonic energy in granular solid fluidization", Industrial and Engineering Chemistry, Vol.47, No.6, pp.1170-1175, 1955.

[2] R. Chirone, L. Massimilla, S. Russo, " Bubble-free fluidization of a cohesive powder in an acoustic field", Chemical Engineering Science, Vol. 48, No. 1, pp. 41-51,1993.

[3] Wojciech Nowak, Masanobu Hasatani, Mieczyslaw Derczynski "Fluidization and heat transfer in an acoustic field", AIChE Symposium Series 89(296), pp. 137-149,1993.

[4] P. Russo, R. Chirone, L. Massimilla, S.Russo, "The influence of the frequency of acoustic waves on sound-assisted fluidization of beds of fine powders", Powder Technology, Vol. 82, pp 219-230, 1995.

[5] C. A. Herrera, "Bubbling characteristics of sound-assisted fluidized beds", Powder Technology, Vol. 119, pp. 229-240, 2001.

[6] DeShau Huang, "Heat transfer to fine powders in a bubbling fluidized bed with sound assistance", AIChE Jounal, Vol. 50, No.2, pp 302-310,2004.

[7] Liang-Shin Fan, Chao Zhu, "Principals of Gas-Solid Flows", Cambridge University Press, New York, USA, 1998.

[8] J.S.M. Botterill, "Fluid-Bed Heat Transfer", Academic Press Inc., New York, USA, 1975.

[9] O. Molerus, K.E.Wirth, "Heat transfer in fluidized beds", Chapman and Hall, London, 1997.

U. S. Wankhede Department of Mechanical Engineering, G. H. Raisoni College of Engineering, Digdoh Hills, Hingana Road, Nagpur(M.S.), 440016, India. Email: udaywankhede@yahoo.co.in
Table 1: Properties of Fluidized materials.

Bed material        dp           [C.sub.p]   [K.sub.p]   [[rho].sub.p]
                    ([micro]m)   (J/Kg-K)    (W/m-K)     (Kg/[m.sup.3])

Glass beads         462          753.6       0.89        2600
Microfumed silica   112          840         1.9         2500
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Author:Wankhede, U.S.
Publication:International Journal of Applied Engineering Research
Article Type:Report
Geographic Code:1USA
Date:Apr 1, 2009
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