Effect of the Chemical Nature of Fuels on the Combustion Rate of Condensed Mixtures
Abstract Generated abstract
This study examines how the chemical nature of solid fuels affects the combustion rate of condensed mixtures using ammonium perchlorate as a common oxidizer. Powdered mixtures of ammonium perchlorate with selected polymers and corresponding monomers were burned in a constant-pressure nitrogen bomb, and average combustion rates were measured over a range of pressures. The experiments showed that mixtures containing polymers burned faster than those containing the corresponding monomers, a difference not readily explained by calorific value or heat of gasification alone. The authors argue that the composition of fuel gasification products, including hydrogen-rich products, soot, hydrocarbons, nitrogen-containing fragments, and possible free radicals, is a principal factor governing heat release near the surface and hence combustion rate.
Full Text
Influence of the Chemical Nature of Fuels on the Combustion Rate of Condensed Mixtures
V. I. Bratinskii, N. N. Bakhman, G. V. Ivanov, N. Berlizov.
/ Presented by the scientific seminar of the Department of Radiation Chemistry /
The nature of the fuel and its physicochemical properties have a substantial influence on the combustion process of condensed mixtures; however, this question has been studied only poorly.
At present, it is important to establish the principal and secondary factors that determine the combustion rate when different fuels and a single oxidizer are used. So far there have been only isolated attempts to relate the value of the combustion rate of a number of mixtures to the calorific value of the fuels, to the strength of the bonds in their molecules \([1,2]\), and to the degree of crosslinking of the polymer \([3]\).
In order to accumulate factual material and determine directions for detailed investigations, we are studying the combustion of mixtures of ammonium perchlorate with polymers and their monomers; moreover, all the fuels selected are powders of solid substances of similar dispersity.
EXPERIMENTAL PART
In all experiments, perchlorate of dispersity \(\sim 5\,\mu\text{m}\) and fuel of dispersity \(\sim 80\,\mu\text{m}\) were used. The powdered components were mixed on tracing paper with a rubber stopper. The resulting mixture was pressed into brass cups (with an internal diameter of 10 mm) to a density close to the maximum. Expe-
Experiments were carried out in a constant-pressure bomb, in nitrogen at \(p \le 100\) atm. The average combustion rate was determined by means of a pressure transducer and a loop oscillograph. The names and chemical formulas of the fuels are given in Table 1.
Table 1
| Fuel | Structural formula | Note |
|---|---|---|
| \(p\)-phenylenediamine | \(H_2N{-}\mathrm{C_6H_4}{-}NH_2\) | |
| polymer of \(p\)-phenylenediamine | \(\left[{-}N{-}\mathrm{C_6H_4}{-}N{=}\right]_n\) | |
| benzidine | \(H_2N{-}\mathrm{C_6H_4}{-}\mathrm{C_6H_4}{-}NH_2\) | |
| polymer of benzidine | \(\left[{\equiv}N{-}\mathrm{C_6H_4}{-}\mathrm{C_6H_4}{-}N{=}\right]_n\) | |
| diaminodiphenylmethane | \(H_2N{-}\mathrm{C_6H_4}{-}CH_2{-}\mathrm{C_6H_4}{-}NH_2\) | |
| polymer of diaminodiphenylmethane | \(\left[{\equiv}N{-}\mathrm{C_6H_4}{-}CH_2{-}\mathrm{C_6H_4}{-}N{=}\right]_n\) | |
| diphenylamine | \(\mathrm{C_6H_5}{-}NH{-}\mathrm{C_6H_5}\) | |
| polyvinyldiphenylamine | \(\left[\begin{array}{c}\mathrm{C_6H_5}{-}N{-}\mathrm{C_6H_5}\\ \vert\\ HC{-}CH_2\end{array}\right]_n\) | \(n \approx 55\) |
| carbazole | \(\begin{array}{c}\text{carbazole ring}\\[-2pt] N\\[-2pt] H\end{array}\) | |
| polyvinylcarbazole | \(\left[\begin{array}{c}\text{carbazole ring}\\[-2pt] N\\[-2pt] \vert\\ HC{-}CH_2\end{array}\right]_n\) | \(n \approx 10\) |
| polyethylene | \(\left[CH_2{-}CH_2\right]_n\) | |
| polystyrene | \(\left[\begin{array}{cc} C_6H_5 & H\\ \vert & \vert\\ {-}C & {-}C{-}\\ \vert & \vert\\ H & H \end{array}\right]_n\) | |
| polymethyl methacrylate | \(\left[\begin{array}{c} C(=O){-}O{-}CH_3\\ \vert\\ CH_2{-}C{-}\\ \vert\\ CH_3 \end{array}\right]_n\) |
The experimental results are presented in Table 2.
Table 2.*
| Fuel | \(u\), mm/s (200 atm) | \(u\), mm/s (400 atm) | \(u\), mm/s (1000 atm) |
|---|---|---|---|
| n-Phenylenediamine | 9.8 | 18.7 | 20 |
| Polymer of n-phenylenediamine | 15.3 | 20.8 | 37.5 |
| Benzidine | 7.3 | 10.7 | 21.5 |
| Polymer of benzidine | 16.7 | 27.7 | 62.8 |
| Diaminodiphenylmethane | 9.4 | 14.5 | 24.2 |
| Polymer of diaminodiphenylmethane | 12.5 | 22.7 | 45 |
| Diphenylamine | 7.1 | 8.5 | 12.3 |
| Polydiphenylamine | 10.8 | 17.2 | 28.6 |
| Carbazole | 7.3 | 10.8 | 18.5 |
| Polyvinylcarbazole | 10.5 | 17.0 | 30.0 |
| Polyvinylacetylene | 8.2 | 11.1 | 13.1 |
| Polybutadiene | 6.0 | 8.1 | 15.2 |
| Polystyrene | 11.1 | 16.3 | 26.0 |
As is evident from the table, the combustion rate of all the mixtures studied with polymers is higher than with the corresponding monomers.
* All data are for a ratio between fuel and oxidizer of
\(\alpha = 0.8\).
DISCUSSION OF RESULTS
The observed sharp difference in the combustion rates of polymers and monomers cannot be related to their calorific value or heat of gasification. It can only be noted that all monomers are readily fusible \((T_{\mathrm{m}} \approx 100^\circ\mathrm{C})\) and volatile \((T_{\mathrm{bp}} \approx 200^\circ\mathrm{C})\), whereas polymers are highly heat-resistant substances that decompose at temperatures above \(350^\circ\mathrm{C}\). Although this fact is also of some significance, in our view the principal role is played by the composition of the gasification products of the fuels. Indeed, considering the combustion rates of mixtures of ammonium perchlorate with polystyrene, polyethylene, and polymethyl methacrylate (lower part of Table 2) and comparing them with the results of analysis of the gasification products of these polymers (Table 3), one can see the following:
Table 3.*
Composition of the gasification products of polymers
| Polymer | \(H_2\), % | \(CH_4\), % | \(C_nH_m\), % |
|---|---|---|---|
| Polymethyl methacrylate | 21.8 | 27.6 | 13 |
| Polystyrene | 71.4 | 21.7 | 1.4 |
| Polyethylene | 27.0 | 38.0 | 31.0 |
In the gasification of polystyrene, hydrogen is the main product. In the gasification products of polymethyl methacrylate it amounts to only 21%, whereas \(CO\) and \(CO_2\) amount to 31%.
Naturally, in the first case the gas mixture is not only more calorific, but should also have a much higher combustion rate [5]. In addition, during the gasification of polystyrene a large amount of soot is formed, particles of which may play the role of a catalyst [6].
* Note: data taken from monograph [4]
In the combustion of polymethyl methacrylate, soot is not formed at all. Thus, the reason for the high combustion rate of polystyrene is sufficiently obvious.
An analogous explanation may be proposed in the case of compositions with a polymer and a monomer.
Mass-spectrometric analysis showed that the polymers we studied gasify into a multitude of hydrocarbon and nitrogen-containing products, many of which are apparently free radicals. These fragments can immediately react with the oxidizer.
The reaction in the gas phase between the oxidizer and a large molecule of evaporating monomer will obviously proceed over a longer interval of time, which will lead to the displacement of the zone of maximum heat release away from the surface and, overall, to a low combustion rate of the mixture.
CONCLUSIONS:
-
The combustion rate of mixtures of ammonium perchlorate with various polymers and their monomers has been studied.
-
The observed difference in the combustion rates of the mixtures is associated with the composition of the gasification products of the fuels.
LITERATURE:
-
G. V. Ivanov, G. I. Shabaev et al. FGV, 1967, No. 2.
-
V. I. Bratishchev, N. I. Bakhman. Abstracts of papers of the Ninth Republican Conference on Problems of Evaporation, Combustion, and Gas Dynamics of Dispersed Systems. Odessa, 1969.
-
Rasota, Gutto. Rocket Fuels and Combustible Materials, 1969, 7, 3.
-
Gasification and Pyrolysis of Fuels. Collection of articles. Nauka Publishing House, 1964.
-
G. T. Gill and H. F. Calote. J. of Chem and Eng Data, 1963, Vol. 4, No. 3.
-
V. V. Rykalinov, N. I. Bakhman. FGV, 1969, 4.