EFFECT OF ELECTROSTATIC AND HIGH-FREQUENCY ELECTRIC FIELDS ON HEAT TRANSFER AND HYDRAULIC RESISTANCE IN A FLOW

I. P. Chashchin, V. P. Ignatov

(Presented by the scientific seminar of the Department of Processes, Apparatus, and Cybernetics of Chemical Production)

A large amount of heat-exchange equipment operates in various branches of industry, and, in this connection, intensification of heat transfer is an important task.

Some heat-exchange equipment operates under laminar flow regimes, which leads to an increase in heat-transfer surfaces, i.e., to bulky apparatus; therefore the problem of intensifying heat transfer is urgent.

At present, along with known methods of intensifying heat transfer (turbulizing inserts, finning, etc.), new methods of intensification are being proposed: high-frequency oscillations, and the use of magnetic and electric fields [1–4].

We carried out studies on the effect of a nonuniform electrostatic field and a high-frequency electric field on heat transfer and hydraulic resistance in an air flow.

The studies were carried out in an apparatus that was a vertical “tube-in-tube” heat exchanger, in the annular space of which electrodes were installed. Heat transfer occurred from saturated water vapor condensing in the inner tube to air flowing through the annular space of the apparatus. The air-flow regime was laminar.

The entire length of the heat-exchanger tube was conventionally divided into two sections in order to reduce the influence of the inlet conditions of the flow into the apparatus \((l/d_{\text{eq}} > 50)\). Temperature measurement was performed using chromel-copel thermocouples with an accuracy of \(\pm 0.5^\circ\text{C}\), connected to a PP-63 potentiometer. Air flow rate was measured with a throttling flowmeter with an accuracy of \(\pm 3\%\). Hydraulic resistance was measured with an inclined draft manometer with an accuracy of \(\pm 0.2\ \text{N}/\text{m}^2\).

The source of high electrostatic voltage was a VTM 1/10 rectifier, from which voltage was supplied to electrodes \(0.2\ \text{mm}\) in diameter. The experiments differed from one another in air flow rate, in the magnitude of the applied voltage \((E = 0 \div 8\ \text{kV})\), in the number \((n = 1, 2, 3)\), and in the length of the electrodes \(l_1 = 564\ \text{mm},\ l_2 = 282\ \text{mm}\).

As a result of the studies it was established that intensification of heat transfer is affected by the magnitude of the applied voltage causing corona discharge of the electrodes, by their number and length. Thus, when three electrodes, each \(564\ \text{mm}\) long, are installed in the stabilized section, the heat-transfer coefficient increases by a factor of \(1.58\) at \(E = 6.4\ \text{kV}\), and the hydraulic resistance increases by a factor of \(1.4\).

The effect of a high-frequency electrostatic field on heat transfer and hydraulic resistance was also investigated. The source of the high-frequency electric field was a ZG-10 audio generator, the signal of which was amplified by an amplifier to 3 kV, and the resulting high voltage, with a frequency from 8 to 20 kHz, was applied to three electrodes.

As a result of the studies conducted, no effect of the high-frequency electric field on heat transfer and hydraulic resistance in the flow was found.

As the experimental results showed, in order to intensify heat transfer it is necessary to use electric fields with higher intensity and over a broader frequency range.

REFERENCES

1. V. M. Buzyanik. Intensification of Heat Transfer in Shipboard Installations. Leningrad, “Sudostroenie,” 1969.

2. V. M. Borishanskii. Advances in the Field of Heat Transfer. Moscow, “Mir,” 1970.

3. N. F. Boboi, M. K. Bologa, K. N. Semenov. Electron-Beam Processing of Materials. Kishinev, 1965.

4. R. Ya. Gainutdinov, V. G. D’yakov, A. G. Usmanov. Collection of Postgraduate Papers of the Kazan Chemical-Technological Institute. Series of Mechanical Sciences, issue 1, 1970.