STUDY OF HEAT TRANSFER FROM A SURFACE IMMERSED IN A VIBROFLUIDIZED BED

E. D. Zaitsev

A bed of granular material fluidized by the application of vibration is promising for drying \[1, 2, 3\], for carrying out chemical reactions accompanied by high heat release \[4\], and for the heat treatment of metals \[5\].

Questions of heat transfer in a vibrofluidized bed are treated in works \[6, 7, 8\], in which the dependence of heat transfer on many parameters was obtained: vibration, pressure, composition of the gaseous atmosphere, and the sizes of the particles and of the apparatus. There are no reliable generalized results in the literature for calculating heat transfer of surfaces with a vibrobed; therefore we carried out experiments on the experimental determination of the heat-transfer coefficient for corundum and quartz sand as a function of vibration parameters, moisture content, particle sizes, and the height of the bed of dispersed material.

The experimental setup consisted of a column 500 mm high and 100 mm in diameter, made of steel, and a BVS 70/200 vibration stand. In the experiments the oscillation amplitude was varied from 0.25 to 1 mm, and the frequency from 20 to 80 hertz.

In the experiments, white corundum with an average particle size of 10 microns was used, quartz sand with an average size of 1.2 mm, and quartz sand of polyfractional composition: (−0.05)—6%; (+0.05—0.125)—33%; (+0.125—0.2)—41.4%; (+0.2—0.4)—13.4%; (+0.4—0.63)—5.95%.

The experiments described were carried out without gas blowing into the bed and with free communication of the column volume with the atmosphere through fittings. The bed height was checked before the experiment with a measuring ruler. Heat transfer between the vibrofluidized bed and the heat-transfer surface was measured by a sensor-heater. The sensor was rigidly fastened to the upper flange of the column and was completely immersed in the bed to a depth of 10 mm from the bottom of the apparatus. The sensor had a cylindrical shape, 20 mm in diameter and 30 mm high, and was installed along the axis of the column. The electrical power dissipated by the sensor was measured by means of a bridge circuit balanced at 70°C \[9\]. Simultaneously, the temperature of the material was measured by two thermocouples located in the middle at a distance of 15 mm from the heater. From the measurement data, the heat-transfer coefficient between the surface and the vibrofluidized bed was calculated.

Figures 1 and 2 present the dependence of the heat-transfer coefficient on frequency for dry and moist materials at different amplitudes.

Fig. 1. Dependence of the heat-transfer coefficient \(\alpha\) on the vibration frequency of the column for corundum, \(d = 10\,\mu\).

1 — dry, \(H = 55\) mm, \(a = 0.86\) mm;
2 — moist, \(H = 55\) mm, \(a = 0.86\) mm;
3 — dry, \(H = 110\) mm, \(a = 0.5\) mm;
4 — moist, \(H = 110\) mm, \(a = 0.5\) mm;
5 — dry, \(H = 110\) mm, \(a = 0.86\) mm;
6 — moist, \(H = 110\) mm, \(a = 0.86\) mm.

Fig. 2. Dependence of the heat-transfer coefficient \(\alpha\) on the vibration frequency of the column for quartz sand.

1 — dry, \(H = 50\) mm, \(a = 0.82\) mm, \(d = 1.2\) mm;
2 — moist, \(H = 50\) mm, \(a = 0.82\) mm, \(d = 1.2\) mm;
3 — dry, \(H = 50\) mm, \(a = 0.82\) mm, polydisperse;
4 — moist, \(H = 50\) mm, \(d = 0.82\) mm, polydisperse;
5 — dry, \(H = 85\) mm, \(a = 0.82\) mm, polydisperse;
6 — moist, \(H = 85\) mm, \(a = 0.82\) mm, polydisperse.

and layer heights. Figure 3 shows the dependence of the heat-transfer coefficient on amplitude at various frequencies. A rapid, almost linear increase in heat transfer with amplitude is observed. The dependence of heat transfer on frequency has a more complex form. At frequencies of 20–40 hertz, a rapid increase in heat transfer is observed, followed by a slowing of the increase for small particles or the attainment of a maximum for large particles.

It has been established that, for moist corundum (moisture content 20%), the heat-transfer coefficient is almost twice as high and reaches large values at lower frequencies. This fact may be explained by the influence of mass transfer on heat transfer [10] and by aggregation of fine corundum particles toward optimum sizes, which, as established in [7], lie in the range 60–100 μm.

In the case of moist polydisperse sand (moisture content 10%), no increase in heat transfer was observed compared with dry sand. In all probability, the increase in heat transfer caused by mass transfer is offset by aggregation. All the more so, for moist quartz sand with an average size of 1.2 mm, an increase in heat transfer is observed compared with dry sand. In all cases, a decrease in the heat-transfer coefficient with increasing layer height was observed.

Fig. 3. Dependence of the heat-transfer coefficient $\alpha$ on amplitude for dry corundum, $d = 10$ microns.
1 — 70 hertz, $H = 55$ mm; 2 — 50 hertz, $H = 55$ mm; 3 — 30 hertz, $H = 110$ mm; 4 — 30 hertz, $H = 55$ mm.

This study made it possible to reveal the influence of the material moisture content and the layer height on heat transfer in a vibrofluidized bed and to confirm the influence of particle size and vibration parameters obtained in works [6, 7].

REFERENCES

1. V. A. Chlenov, N. V. Mikhailov. Drying of sand by the conductive method in a vibrofluidized bed. Stroitel’nye materialy, No. 11, 1964.

2. A. P. Rysin, S. A. Machikhin. Study of drying and cooling processes for kunzhut. Khlebopekarnaya i konditerskaya promyshlennost’, No. 3, 1963.

3. V. K. Siroedov. Study of the process of drying sugar sand. Candidate dissertation, MTIPP, 1966.

4. O. V. Isaev, L. A. Ruslanova, V. A. Chlenov, L. Ya. Margolis, N. V. Mikhailov. Catalytic oxidation of propylene to acrolein in a vibrofluidized bed. Khimicheskaya promyshlennost’, No. 6, 1965.

5. A. I. Tamarin, I. I. Kal’tman, L. A. Vasil’ev. Vibrofluidized bed as a hardening medium. Metallovedenie i termicheskaya obrabotka metallov, No. 3, 1968.

6. V. A. Chlenov, N. V. Mikhailov. Heat and mass transfer during drying of loose materials by the conductive method in a vibrofluidized bed. Teplo- i massoperenos, Kiev, “Naukova dumka,” vol. 6, part II, pp. 150–157, 1968.

7. I. L. Zamnnius, A. I. Tamarin, S. S. Zabrodsky. Study of heat transfer and hydrodynamics of a vibrofluidized bed. Teplo- i massoperenos, vol. 5, pp. 142–152, Minsk, 1968.

8. E. Yu. Laikovskaya, B. G. Sapozhnikov, N. I. Syromyatnikov. Heat-transfer processes and thermophysical properties of vibrofluidized disperse systems. Teplo- i massoperenos, vol. 5, pp. 153–163, Minsk, 1968.

9. N. V. Antonishin. Candidate dissertation, ITMO, Academy of Sciences of the BSSR, Minsk, 1963.

10. A. V. Lykov. Heat and mass transfer in drying processes. Gosenergoizdat, 1956.