Secondary flame

Alumina trihydtate is also used as a secondary flame retardant and smoke suppressant for flexible poly(vinyl chloride) and polyolefin formulations in which antimony and a halogen ate used. The addition of minor amounts of either zinc borate or phosphoms results in the formation of glasses which insulate the unbumed polymer from the flame (21). 

Alumina Trihydrate. Alumina trihydrate is usually used as a secondary flame retardant in flexible PVC because of the high concentration needed to be effective. As a general rule the oxygen index of flexible poly(vinyl chloride) increases 1% for every 10% of alumina trihydrate added. The effect of alumina trihydrate on a flexible poly(vinyl chloride) formulation containing antimony oxide is shown in Figure 5. 

Fig.l2.n Definition of the primary flame and the secondary flame of a rocket plume. 

Fig. 12.11 shows the structure of a rocket plume generated downstream of a rocket nozzle. The plume consists of a primary flame and a secondary flame.Fil The primary flame is generated by the exhaust combustion gas from the rocket motor without any effect of the ambient atmosphere. The primary flame is composed of oblique shock waves and expansion waves as a result of interaction with the ambient pressure. The structure is dependent on the expansion ratio of the nozzle, as described in Appendix C. Therefore, no diffusional mixing with ambient air occurs in the primary flame. The secondary flame is generated by mixing of the exhaust gas from the nozzle with the ambient air. The dimensions of the secondary flame are dependent not only on the combustion gas expelled from the exhaust nozzle, but also on the expansion ratio of the nozzle. A nitropolymer propellant composed of nc(0-466), ng(0-369), dep(0104), ec(0 029), and pbst(0.032) is used as a reference propellant to determine the effect of plume suppression. The burning rate characteristics of the propellants are shown in Fig. 6-31. Since the nitropolymer propellant is fuel-rich, the exhaust gas forms a combustible gaseous mixture with the ambient air. This gaseous mixture is ignited and afterburning occurs somewhat downstream of the nozzle exit. The major combustion products in the combustion chamber are CO, Hj, CO2, N2, and HjO. The fuel components are CO and H2, the mole fractions of which at the nozzle throat are co(0.47) and iH2(0.24).

Fig. 12.12 Flame photographs of rocket plumes, showing that the dimensions of the secondary flame decrease with increasing concentrations of KNO,.

Fig. 12.12 shows a typical set of flame photographs of a nitropolymer propellant treated with potassium nitrate. From top to bottom, the photographs represent KNO3 contents of 0.68%, 0.85%, 1.03%, and 1.14%. Each of these experiments was performed under the test conditions of 8.0 MPa chamber pressure and an expansion ratio of 1. Though there is little effect on the primary flame, the secondary flame is clearly reduced by the addition of the suppressant The secondary flame is completely suppressed by the addition of 1.14% KNO3. The nozzle used here is a convergent one, i. e., the nozzle exit is at the throat... 

Fig. 12.13 Shrinkage of secondary flame zone by suppression with KNO, at e = 6.3 and 11.7.

K2SO4. This temperature decrease is sufficient to inhibit ignition of the inter-diffused mixture of exhaust gas and ambient air, thereby preventing the generation of a secondary flame. 

Fig. 12.13 shows the extent of the secondary flame zone as a function of the concentration of KNO3 at a chamber pressure of 4 MPa and with Dt = 5.0 mm with nozzle area expansion ratios of e = 6.3 and 11.7. No clear difference is seen for the different values of 8. It is evident that the zone shrinks with increasing concentration of KNO3 and thus also with increasing mass fraction of potassium atoms contained within the propellant Fig. 12.14 shows the extent of the secondary flame zone as a function of the concentration of K2SO4 at a chamber pressure of 4 MPa with D, = 5.0 mm and e = 1. Like KNO3, K2SO4 is seen to be effective as a plume sup-... 

Explosives with a secondary flame which have not enough oxygen for complete combustion. On the detonation of these materials combustible products are formed such as CO, H2, CH4 etc. These products become mixed with air, producing... 

Fig. 127. Flame from picric acid. The primary flame (relatively small) is visible with the secondary flame above it. The duration of both flames is shown on the scale the primary flame of short duration, the secondary of long duration. The material was fired in a mortar (according to Will [11]).

As a result of these investigations, group (1) explosives were withdrawn from use in coal-mines. The duration of the secondary flame is rather long so that the gaseous mixture ignites easily. 

Fig. 190. Muzzle flame I—primary flame, 2—secondary flame, 5—initiation of the...

The burning of the mixture may end with an explosion (detonation) if its composition is suitable. The explosion of the gas mixture is accompanied by a bright flame. This secondary flame is elliptical and visible for a great distance. The dimensions of the flame and the intensity of the flash depend to a great extent on the calibre of the gun e.g. a shot from a 30 cm naval gun gives a secondary flame up to 50 m long, visible for a distance of 50 km. 

The following circumstances favour the formation of a secondary flame ... 

On the other hand the presence among the products of explosion of substances that prevent the explosion of the gas mixtures reduces the possibility of the secondary flame. 

Methane has the narrowest limits so that a shot in which a large amount of this gas is generated is unlikely to produce a secondary flame. A high pressure in the bore favours the formation of a large proportion of methane (p. 533), hence a high charge density reduces the probability of the secondary flame. 

Another factor in reducing the likelihood of a secondary flame is an increase in the concentration of non-flammable gases (C02 and N2) in the products formed... 

The presence of water vapour in a methane-air mixture may exert the same effect as the presence of C02, but small amounts of water vapour increase the possibility that the gas mixture will explode. This accounts for the observation that the secondary flame develops much more easily when the firing takes place in a moist atmosphere. 

The temperature of the propellent gases depends on the heat of explosion and on the gas composition. The greater the heat of explosion, and hence the temperature of gases, the more readily the secondary flame arises. Powders of a low calorific value may therefore give, no secondary flame if the temperature of the gas mixture is lower than the temperature of ignition. 

Substances inhibiting the development of the secondary flame are those which inhibit burning reactions. The strongest of the substances known to possess this property is the potassium ion. Its ability to prevent the development of a secondary flame was demonstrated by Dautriche [39] as early as 1908. Since then potassium nitrate has been employed in the manufacture of flash-reducing charges added to the charges of ordinary propellant. 

Investigations showed that the salts of other alkali metals are not so efficient in suppressing secondary flame as potassium salts. Fairly numerous experiments were carried out to clear up whether or not known antiknock substances, such as tetraethyl lead or nickel carbonyl prevent the development of a secondary flame. They proved to have no effect on its development. In practice, two methods for removing gun-flash may be employed, i.e. either a special flashless powder is produced, containing nitroguanidine or DNT and a small admixture of potassium sulphate,... 

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