UDC 537.52

EFFICIENCY OF ELECTROPLASMA AIR HEATING IN A HIGH-FREQUENCY TORCH DISCHARGE

Vl. L. TEPL0UKHOV, G. N. KOLPAKOV, Vik. L. TEPLOUKHOV,
V. P. DRYAMOV, V. A. VERNYAEV

(Presented by the scientific seminar of the Physico-Technical Faculty)

The efficiency of air heating in high-frequency torch-type plasmatrons is considered. For this purpose, energy losses are estimated at various sections in the power-source–plasmatron circuit. Along with the calorimetric method, a procedure is proposed for measuring the discharge power from the electrical parameters characterizing the operation of the high-frequency generator. It is shown that the overall efficiency of a high-frequency torch plasmatron can reach 0.7–0.8.
Illustrations 3, tables 2, bibliography 5.

Important questions in improving the technology of inorganic compounds are intensification, reduction in the number of process stages, more complete use of raw materials and expensive reagents, and automation of processes. In this respect, plasmochemical processes are of great interest, since they make it possible to combine intensification of processes and their continuity with the possibility of constructing simpler technological schemes.

The studies we have carried out on various plasmochemical processes using high-frequency induction and torch discharges have made it possible to accumulate a certain amount of experience in their operation. High-frequency induction-type plasmatrons have been better mastered and are more often used, but since quartz discharge tubes have low mechanical and chemical resistance, they cannot be used for operation with toxic media. In this connection, we have developed plasmatrons using a high-frequency torch discharge [1, 2], on the basis of which plasmochemical reactors have been created for both homogeneous and heterogeneous processes.

Operation of high-frequency torch-type plasmatrons has shown that, during their operation, high purity of the gas-discharge plasma and a practically unlimited service life are ensured. The considerably larger spatial volume of the high-frequency torch discharge, as compared with an electric arc, makes it possible to ensure good heat and mass transfer between the gas discharge and the plasma-forming gas, and also ensures efficient heating of solid particles.

The plasma jet in an arc plasmatron passes through the nozzle orifice at sonic or supersonic velocity. In this case a high efficiency, 0.7–0.8, of arc plasmatrons is achieved. Plasmatrons with a laminar plasma jet, however, are of no interest because of their low efficiency. When solid particles are fed into the plasma jet of arc plasmatrons, the contact time is \(10^{-2} \div 10^{-3}\) sec [3]; as a result,

therefore, the efficiency of heating solid particles is low. According to the data of the authors of [3], the efficiency of heating solid particles is 1–3%.

In high-frequency electroplasma heating, the plasma jet often moves at subsonic velocities, which can be varied over wide limits while maintaining the overall high efficiency of the high-frequency plasmatron (induction or torch). The contact time of solid particles with the plasma jet can be increased by one or two orders of magnitude, i.e., to 1.0–0.1 sec; correspondingly, the efficiency of heating solid particles increases to 20–30% [3]. However, the effectiveness of using high-frequency torch plasmatrons is often called into question. Therefore, we carried out studies with the aim of determining the efficiency of conversion of electrical energy into heat.

The studies were carried out on an LD4-10 type generator, specially re-equipped for such a load. The generator operates in the frequency range 37–40 MHz. Air was used as the plasma-forming gas.

The design of the high-frequency torch-type plasmatron (Fig. 1) consisted of a water-cooled electrode 1 placed in a water-cooled sectional copper calorimeter 2. This made it possible to measure the value of the thermal power in different sections of the high-frequency torch discharge. Naturally, such a design could not ensure optimal introduction of high-frequency power into the discharge; therefore, we determined the efficiency of operation of the tube generator by the method proposed in [4]. This method was first checked calorimetrically on the above-described design. The heating efficiency, however, was evaluated for a free (not disturbed by walls) high-frequency torch discharge.

Fig. 1. Diagram of calorimetry of a high-frequency torch discharge: 1—electrode; 2, 3—copper calorimeter; 4—thermometers.

The high-frequency power introduced into the discharge from the tube generator was determined by the procedure proposed in [4]. This procedure had first been checked by the calorimetric method on the above-described design. The heating efficiency was evaluated for a free (not disturbed by walls) high-frequency torch discharge.

The method for measuring calorimetric power was as follows. The power consumed from the mains was measured with a three-phase kilowattmeter (class 1.5). The power from the rectifier \(P_0\) was measured with an M-1108 type ammeter (class 0.2) and an S-96 kilovoltmeter (class 1.5). The power dissipated at the anode was determined by calorimetry; the power of the plasma jet and plasma radiation \(P_a\) was also determined by calorimetry.

On the other hand, calorimetric measurements of the power dissipated at the anode were duplicated by measurements of the efficiency of the tube generator from its electrical characteristics. For this purpose, the values of the dc components of the anode \(I_0\) and grid \(I_{0c}\) currents, the supply voltage of the generator tube \(E_a\), and the oscillatory voltage \(U_k\) were measured. The principle for calculating the efficiency \((\eta)\) of the tube generator from its operating parameters is based on the classical quasilinear theory of the tube generator and is set forth in [4, 5]. According to this procedure, \(\eta\)—the efficiency of the tube generator, characterizing the power losses at the anode—is calculated from the well-known formula

\[ \eta = 0.5 \cdot \gamma \cdot \xi, \tag{1} \]

Fig. 2. Determination of the cutoff angle of the autogenerator anode current.

Fig. 3. Dependence of the form factor of the anode-current curve on the cutoff angle.

where \(\gamma\) is the form factor of the anode-current curve; \(\xi\) is the utilization factor of the anode voltage, equal to the ratio

\[ \frac{U_k}{E_a}; \]

The form factor of the anode-current curve is determined from the dependence shown in Fig. 2, where \(\Theta\) is the cutoff angle, determined by formula 2 and the graph in Fig. 3.

\[ \tg \Theta - \Theta = -\frac{\pi}{S} \left( \frac{I_0}{I_{0c}R_c - E_{0c} + D E_a} \right). \tag{2} \]

Here \(S\) is the static slope of the characteristic of the generator tube; \(D\) is the grid permeability; \(R_c\) is the resistance in the grid circuit; \(I_0\) is the dc anode current; \(I_{0c}\) is the dc grid current.

In the course of the experiments, the high-frequency generator LD4-10 was operated with 50% anode voltage (3 kV). The power of the high-frequency discharge varied insignificantly; calorimetry was carried out both for a free high-frequency torch discharge and with axial air blowing.

Table 1 presents the results of measurements of the parameters of the high-frequency generator and of the thermal power of the discharge in the calorimeter.

It follows from the data obtained that the results of the calorimetric measurements are in good agreement with the results of the electrical measurements. In addition, the analysis shows that the efficiency \((\eta_{\text{r–v}})\) of the rectifier–plasmatron can be increased by increasing the utilization factor of the anode voltage \(\xi\), which in our measurements corresponded to values of \(0.135 \div 0.258\). Ways of increasing the values of \(\xi\) are possible through the development of special designs of high-frequency ...

torch plasmatrons, gas blowing, and the use of special circuits of high-frequency generators.

Table 1

To confirm this, we give the values of \(\eta_{a-b}^{e}\) when measuring a free high-frequency torch discharge (Table 2).

Table 2

These results indicate a sufficiently high efficiency of conversion of the power supplied to the rectifier into the power released in the high-frequency torch discharge.

The substantial increase in efficiency in this case, compared with the results of Table 1, occurs due to an increase in

\[ \xi = \frac{U_a}{E_a}, \]

which was raised to values of 0,51–0,55.

There is no need to carry out a complete analysis of the losses of the power supplied to the high-frequency generator, since this has been done in the authors’ work [3]. According to their data, the overall efficiency for a high-frequency induction plasmatron, whose power source is the same type of high-frequency generators, can be represented as

\[ \eta_0 = \eta_n \cdot \eta_t \cdot \eta_v \cdot \eta_a \cdot \eta_k; \]

where

\[ \eta_n = \frac{P - P_n}{P} \]

is the efficiency accounting for the power losses \(P_n\) in the filament;

\[ \eta_{\mathrm{t}} = P_{\mathrm{v}}/P_{\mathrm{t}} \]
— efficiency of the anode transformer;

\[ \eta_{\mathrm{v}} = P_{\mathrm{o}}/P_{\mathrm{v}} \]
— efficiency of the high-voltage rectifier;

\[ \eta_{\mathrm{a}} = P_{\mathrm{a}}/P_{\mathrm{o}} \]
— efficiency in the anode circuit;

\[ \eta_{\mathrm{k}} = P_{\mathrm{p}}/P_{\mathrm{a}} \]
— oscillatory system–plasma.

According to the work of the authors [3], \(\eta_{\mathrm{n}} = 0.910 \div 0.965\), \(\eta_{\mathrm{t}} = 0.915 \div 0.975\), \(\eta_{\mathrm{v}} = 0.995\); according to the data we obtained for an H.F. torch discharge,

\[ \eta_{\mathrm{a-v}} = \eta_{\mathrm{a}} \cdot \eta_{\mathrm{k}} = 0.50 \div 0.61. \]

Then the overall efficiency of an H.F. torch-type plasmatron that can be realized in our experiments is

\[ \eta_{\mathrm{o}} = 0.42 \div 0.58. \]

Apparently, the maximum possible efficiencies of an H.F. torch plasmatron will be comparable in magnitude with the maximum possible efficiencies of induction plasmatrons, i.e., will reach values of \(0.7 \div 0.8\) [3]. This increase in efficiency can be achieved by operating H.F. generators in the mode with \(\xi = 0.7\); \(\Theta = 50 \div 60^\circ\).

It should also be noted that the losses of thermal power in the water-cooled electrode amounted in our experiments to \(6 \div 10\%\) of the total thermal power of the plasmatron.

Thus, high-frequency torch-type plasmatrons are effective air heaters. In addition, they can be used to heat other process gases and their mixtures. The mass-average temperature of the plasma air jets of such plasmatrons, according to our estimates, can reach \(3500 \div 4000^\circ\mathrm{K}\).

REFERENCES

1. I. A. Tikhomirov, V. L. Teploukhov, G. N. Kolpakov. Proceedings of the III All-Union Conference on Low-Temperature Plasma Generators. Minsk, 1967.

2. A. A. Vorob’ev, I. A. Tikhomirov, A. I. Karelin, V. L. Teploukhov, G. N. Kolpakov, G. P. Khandorin. Author’s certificate No. 252300 with priority of 10/VI-1968.

3. L. M. Sorokin, I. D. Kulagin, N. N. Rykalin. Proceedings of the III All-Union Conference on Low-Temperature Plasma Generators. Minsk, 1967.

4. I. V. Egorov. Elektrotermiya. 1970, 92, p. 10.

5. S. V. Person, A. I. Lebedev-Karmanov, V. Ya. Khatskelevich. Theory and Calculation of Amplitude-Modulated Tube Generators. Sov. Radio Publishing House, Moscow, 1955.