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Heat transfer characteristics and particle behavior around a horizontal heating surface immersed in a sound assisted fluidized bed of fine powders.

Introduction

Fluidization quality is closely related to intrinsic properties of particles such as particle density, particle size and size distribution, and also their surface characteristics. Geldart group A particles fluidize nicely and show a region of expansion without much bubbling. Geldart group B and group D hardly show bed expansion when fluidized. Geldart group C particles are difficult to fluidize mainly because of cohesiveness. Acoustic energy of appropriate intensity decreases the minimum fluidization and minimum bubbling velocity. Sound waves with high amplitude and low frequency significantly improve the quality of fluidization. The heat transfer rates can be improved by increasing the SPL and acoustic frequency appropriately.

Morse(1955) found that at low frequencies, high intensity sound waves improved the fluidization of fine powders, preventing channeling and stagnation. Russo et al.(1994) concluded that channel free, homogeneous beds can be obtained with appropriate combinations of bed weight and acoustic field intensity and frequency. The experiments were carried out with SPL ranging between 110 to 140 dB and frequency varying between 30 to 1000 Hz for the range of fine powders from 0.5 to 45 [micro]m. Leu and Huang (1994) measured the interparticle forces of certain group C particles in fluidized beds by using sound waves to improve the quality of fluidization. Chirone and Russo(1995) developed an alternate elastic model. It was concluded that the contact points between particles were not rigid but elastic and the breakage of clusters occurred at certain frequency conditions. A. Schmidt and U. Renz(1999) formulated different physical models needed to give an Eulerian formulation of the particulate enthalpy equations. An empirical model of Gunn(1978) for interphase heat exchange, Kuipers et al.(1992) approach for thermal conductivity of the solid phase have been used for the simulation work. Herrera et al. (2000) investigated that high intensity sound disrupted the cohesive nature of powders, permitting both homogenous and bubbling fluidization.

Nowak and Hasatani(1993) ,in their experimentation for heat transfer, investigated the heat transfer between the acoustic energy and bed material at low frequency. However, they did not carry experimentation for heat transfer between fine powders and the heat transfer surface. Huang et al. (2004) concluded that the convective heat transfer coefficient between the heated tube surface and the bed material was strongly affected by the existence of the gas film between the heated surface and the emulsion phase.

Experimental apparatus and method

The fluidized bed used for experimentation purposes is a 115 mm ID cylindrical transparent acrylic column with 610 mm height. Figure 1 shows the arrangement for measuring heat transfer rates with acoustic sound generation system; a sound amplifier, a digital signal generator and a loudspeaker. SPL measurements were done with a microphone and digital storage oscilloscope. Loudspeaker was positioned at the top foe generating the acoustic waves. The signal generator was used to control the sound frequency. Foe most of the cases, the sound frequency was usually kept constant at the optimum level. The heating element was made up of copper and is placed horizontal in the fluidized bed.

[FIGURE 1 OMITTED]

Experimental results

The surface temperatures at four different locations are obtained for marble powder (90 [micro]m). The gas velocity was varied from 1.9 cm/s([u.sub.g]/[u.sub.mf]=1.2) to 4.7 cm/s

([u.sub.g]/[u.sub.mf] =2.9).Graph 1 show the plot of surface temperatures for marble powder(90 [micro]m) at different locations along the circumference of the heated tube at 144 dB. As shown in the graphs, variation of the surface temperature with respect to the angular position is minimal for the top thermocouple ([theta]=[180.sup.0]) under all conditions of SPL. This is due to low gas superficial velocity, where stagnant bed particles reside on the top of the tube. Under all conditions of SPL and sound frequency, the side thermocouple show higher thermal activity which is due to movement of the bubbles that replace the stagnant particles with fresh particles more frequently than for other thermocouple positions.

The local heat transfer coefficients are calculated from the surface temperatures for all angular positions. Graph 2 show variation of local heat transfer coefficients with excess air velocity ([u.sub.g]-[u.sub.mf]) at different values of 9 and graph 3 show variation of local heat transfer coefficients with angular position (9) at different values of excess air velocity ([u.sub.g]-[u.sub.mf]) for marble powder(90 [micro]m). At all the values of gas velocities ,the magnitude of local heat transfer coefficients is maximum at the sides of the tube ([theta]=[90.sup.]) and minimum at the top surface ([theta]=[180.sup.]).At lower gas velocities, the values of local heat transfer coefficients are smaller due to less solids motion along the circumference of the tube. Better contact with the emulsion phase is established at higher gas velocities due to improved solids motion. It has been noticed that the values of local heat transfer coefficients were more or less the same up to a SPL of 130 dB. However, the maximum values of local heat transfer coefficients increased appreciably beyond and reached a maximum value at 144 dB at 4.9 cm/s. This may be attributed to the fact that at SPL lower than 130 dB, the packets do not get sufficient momentum to carry the heat from the heating surface. Graph 4 show variations of local heat transfer coefficients with excess air velocity ([u.sub.g]-[u.sub.mf]) at different values of SPL.

[GRAPHIC 1 OMITTED]

[GRAPHIC 2 OMITTED]

[GRAPHIC 3 OMITTED]

[GRAPHIC 4 OMITTED]

Results and discussions

The poor fluidization of cohesive powders such as marble powder (90 ^m) in a conventional fluidized bed is improved by the acoustic waves. It has been found that the average values of heat transfer coefficients are seen to have maximum value at 144 dB and gas velocity 4.7 cm/s (i.e. excess air velocity of 3.1 cm/s). The average heat transfer coefficient increases for increasing gas flow rates and SPL due to increased replacement of particles on the tube. The values of average heat transfer coefficients were nearly equal for all conditions below 130 dB. However, the values increased gradually in the SPL range 130-144 dB. It can be concluded that, at higher values of SPL, the packets get enough energy to travel to the heat transfer surface and carry the heat from the heating surface to the bed material

References

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

[2] J.A.M. Kuipers, W. Prins, W.P.M. van Swaaij, "Numerical calculation of wall-to-bed heat-transfer coefficients in gas-fluidized beds", AIChE Journal, Vol. 30, No. 7, pp. 1079-1091, 1992.

[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] Lii-ping Leu, Chen-Tung Huang, "Fluidization of cohesive powders in a sound wave vibrated fluidized bed", AIChE Symposium series 90,301(1994).

[5] 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.

[6] A. Schmidt, U. Renz, "Eulerian computation of heat transfer in fluidized beds", Chemical Engineering Science, Vol. 54, pp. 5515-5522,1999.

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

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

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

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

[11] 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.

E-mail: udaywankhede@yahoo.co.in
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Author:Wankhede, U.S.
Publication:International Journal of Dynamics of Fluids
Article Type:Report
Geographic Code:9INDI
Date:Dec 1, 2009
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