APPARATUS FOR INVESTIGATING THE CONCENTRATION LIMITS OF FLAME PROPAGATION IN GAS MIXTURES AT ELEVATED PRESSURE

A. V. BAIKOV

(Presented by the scientific seminar of the Department of Radiation Chemistry)

For the experimental study of the influence of various factors on the concentration limits (C.L.) of flame propagation in gases, all kinds of test devices are used, depending on the aims of the investigations and the conditions under which the gas mixture is found \[1, 3\].

One of the important factors affecting the C.L. is the total pressure of the gas mixture. The study of the influence of pressure on the C.L. is of considerable interest for safety engineering and for the theory of limiting phenomena.

Fig. 1. 1 — combustion chamber, 2 — mixer, 3 — mercury manometer, 4 — manovacuum gauge, 5 — vacuum pump, 6 — electromagnetic stirrer

To study the influence of pressure on the C.L. of flame propagation in gas mixtures, an apparatus was assembled and tested consisting of a mixer cylinder for preparing the gas mixture, a combustion chamber, an ignition source, a system for admitting the components of the mixture, and the connecting pipelines (Fig. 1).

The combustion chamber (Fig. 2) is a thick-walled metal cylinder 1 with an overall length of 1.5 m and an internal diameter of 7 mm, consisting of two sections, with flanges 6 welded to the ends. The lower end of the chamber is closed by a metal flange 2, to which an inlet fitting 3 is welded. The fitting serves for admitting and discharging the gas mixture from the chamber.

Fig. 2. 1 — combustion chamber, 2 — removable flange, 3 — fitting, 4 — Plexiglas, 5 — Teflon seals, 6 — flange, 7 — electrodes, 8 — getinax plate, 9 — thermocouples, 10 — removable flange

Fig. 3. 1 — mixer, 2 — removable flange, 3 — Teflon gasket; 4 — mixer blades, 5 — core, 6 — head, 7 — rod, 8 — solenoid

The upper end of the chamber is closed with Plexiglas 4, which is clamped through rubber gaskets by flanges 6 and 10. This makes it possible to observe visually the process of flame propagation. The removable flanges, as well as both sections of the chamber, are sealed by means of Teflon gaskets.

Along the chamber walls, diametrically opposite one another, thin getinax plates 8 are installed; along their length, at equal distances, a row of thermocouples 9 is arranged.

The signal from one row of thermocouples is fed to a loop oscillograph and makes it possible to determine the flame-propagation velocity of the given gas mixture. The signal from the other row of thermocouples is fed to an electrode potentiometer to determine the flame temperature. In addition, the thermocouples serve as recorders of the fact of propagation of the flame front along the length of the chamber. To ignite the gas mixture, electrodes 7 are located in the lower part of the chamber.

The mixer cylinder (Fig. 3) is a thick-walled steel vessel with a welded-on flange 1, which is covered by a removable flange 2 and sealed through Teflon gaskets 3. The mixer is equipped with an electromagnetic stirrer with blades made of aluminum

foil 4. The core 5 of the stirrer is enclosed inside a head 6 made of nonmagnetic steel. The solenoid, supplied with a current of 6 A, 220 V, freely moves the core with the stirrer along the cylinder. Solenoid 8 is switched on and off by a button once per second. All parts of the mixer are made of metal in order to prevent the accumulation of static-electric charges at the points of contact of the moving parts (the core and the head enclosing it, the rod with the stirrer). To avoid the formation of friction sparks during mixing, the rod is made of duralumin. A mixture of the required composition is prepared by sequential admission of the components, metered according to their partial pressures, into the previously evacuated mixer, after which the mixture is mixed for 10 min.

Experimental procedure

Before the components of the gas mixture are admitted into mixer 2 (Fig. 1), the entire system is evacuated by vacuum pump 5. For this purpose valve $K_8$ is opened, which makes it possible to evacuate the pipeline up to valves $K_6$ and $K_7$, and, with valve $K_9$ open, the mercury manometer. By opening valve $K_1$, the pipeline up to valves $K_2 + K_5$ is evacuated. Valves $K_2$ and $K_3$ are used to evacuate, respectively, the mixer and the chamber. When the mercury manometer reads $30$ mm Hg, valves $K_3$ and $K_8$ are closed. Then the components of the gas mixture are admitted into the mixer by means of valves $K_6$, $K_7$, $K_4$, in order of increasing partial pressure of each component, which are recorded by the mercury manometer. The pipelines are evacuated after admission of each component. The gas mixture prepared in this way is thoroughly mixed in the mixer by electromagnetic stirrer 6 for 10 min, and is then admitted, by means of valves $K_2$ and $K_3$, into the combustion chamber to the required pressure, which is recorded by manovacuum gauge 4, and ignited.

As the flame front passes along the length of the chamber, signals from the thermocouples are fed to a loop oscillograph and an electronic potentiometer. Upon completion of the experiment, the excess pressure is released from the chamber by valve $K_5$, the pipelines and the chamber are evacuated, and the next gas mixture is admitted into the chamber.

The described version is suitable for partial pressures below 2 atm, since this is limited by the mercury manometer. For higher partial pressures the mercury manometer is shut off by valve $K_9$, and the partial pressures are recorded directly by manovacuum gauge 4.

This apparatus makes it possible to investigate the normal propagation velocity of the flame of a gas mixture in the pressure interval $P = 2 \cdot 10^1$ mm Hg — $2 \cdot 10^4$ mm Hg.

LITERATURE

1. B. Lewis, G. Elbe. Combustion, Flames and Explosions of Gases. Moscow, “Mir,” 1968.
2. Journal “Physical Chemistry,” vol. 36, issue 12, 1962.
3. Proceedings of the IV International Symposium on Combustion. Moscow, Oborongiz, 1958.