IGNITION OF PROPELLANTS. SHAPING AND DEVELOPMENT OF BURNING WAVE AND ITS CHARACTERISTICS
Victor S.Abrukov*, Alexander V.Khristoforov, Valery E.Nikonorov, Ilja V.Andreev
Chuvash State University, Moskovsky prosp., 15, Cheboksary 428015, Russia
ABSTRACT:
A results of experimental investigation for ignition process of transparent (glassy) solid propellant models are presented. The investigations have been made by holographic interferometric technique in terms of ignition process visualization in the condensed and the gaseous phases of burning sample as well as qualitative analysis. This technique has not been used earlier by other investigators in the solid ignition process research, however examples of partly analogously investigations are known.
A combustion wave of propellant consist of at least two parts: one bases in the gaseous phase (g-phase) - above the surface of propellant sample, and other part bases in the condensed phase (c-phase) - under the surface of propellant sample. Both parts equally important from standpoints of combustion mechanism studying. So studying their necessary to conduct simultaneously. Particularly this is important at the study of non-stationary modes of combustion.
It is known much methods of studying a structure of g-phase of combustion wave. Near 50 methods of determination of flame temperature are discuss by M.Elder 1. On the other hand, practically only one method - a method of thermocouples is broadly use for studying a structure of combustion wave in c-phase. But this method can be not use for studying of non-stationary modes of combustion, in particularly of ignition process. Amongst optical methods possible to note only one method - polarimetry, which was use for studying a structure of combustion wave in the c-phase of transparent thin propellant slices by H. Selzer (Germany, 1965) and thin powder slices by V. Kochakov (Russia, Chuvash St. Univ., 1980).
The holographic interferometric technique (HIT) permits to study the heat release and heat transfer in a transparent solid substance. However, in combustion diagnostics it is employed traditionally only in investigation of one part of the burning wave which is in the g-phase, except2-6. In these papers the applicability of the HIT was demonstrated for studying of polymer burning 2,3 and burning of liquid 4-6. The paper by A. Ito6 summarizes the results of HIT applied for the visualization and analysis of the process of liquid burning flame spread both in the c- and the g-phases simultaneously as well as pool fires leading to boilover.
ENTIRE STRUCTURE OF BURNING WAVE
The results demonstrating the potentialities of the HIT in the burning wave visualization in the c- and g-phases simultaneously during the laser radiation ignition (LRI) process of the solid propellant transparent (glassy) models are presented below. Under investigation was LRI of the polymethylmethacrylate (PM) and celluloid (weakly nitrated cellulose - NC) samples (laser radiation wavelength was 10.6 microns, laser spot size was 5 mm, radiation flux density is 80 W/cm2). The PM samples were parallelepiped (20*30*30 mm). The NC samples were 1-mm thick plates. The standard two-beam scheme of holographic interferometer has been used to produce the interference cine films by a real time method.
Figures 1 and 2 show the holographic interferometric film frames representing typical features of the burning wave shaping and development during LRI process (numbers under frames show moments of the time t in seconds with the origin at the
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Correspondence: Email: victor@chuvsu.ru; WWW: http://www.chuvsu.ru/~victor; Fax: 7-8352-42-80-90onset of the laser radiation). The time of laser radiation was 15 sec. (Figure 1) and 8 sec. (Figure 2). After the radiation was turned-off, the PM samples were extinguished; the NC samples were continued to burn.
The qualitative analysis of the films has revealed the number of LRI stages taking place both sequentially and concurrently, among which the following stages may be pointed out:
-formation in the c-phase of the "cone" layer heated by the laser radiation, the simultaneous generation of stresses within the sample (the elastic strain of the sample as a whole) and gasification (evaporation) of the sample substance (see t=0.1 ...3 sec. - Figure 1 and t=0.1 ...1 sec. - Figure 2);
-inflammation of the gasification products, the expansion of the inflammation front to the sample and stabilization of the flame near the sample surface (see t=4.5 sec. - Figure 1 and t=3 sec. - Figure 2);
-beginning of formation of the "hemisphere" layer heated due to the heat transfer from the flame (see t=4.5 sec. - Figure 1 and t=3 sec. - Figure 2);
0 0.1 3 4.5 6 7.5 9 10.5 12 13.5 15 15.1 18 21 22.5 24 25.5 34
Figure 1. The interference film frames showing typical features of the PM sample burning wave shaping and development.
-restructuring of the heated layer after stabilization of the flame near the sample surface (see t=4.5 ...6 sec. - Figure 1 and t=3 ...5 sec. - Figure 2);
-development of the stresses after the stabilization of the flame near the sample surface (see t=4.5 ...9 sec. - Figure 1 and t=3 ...5 sec. - Figure 2);
-burning-out of the sample substance, the development of a burnt-out "cone" (it appears as a result of self-focusing of the laser radiation) and restructuring of the general heated layer (see t=6 ...15 sec. - Figure 1 and t=5 ...8 sec. - Figure 2). The burnt-out "cone" can be definitely observed on the frame t=34 sec. (Figure 1);
-formation and development of the phase transition zone (see the dark zone: t=0.1 ...15 sec. - Figure 1);
-various transition processes in the g- and the c-phases after the interruption of the laser radiation followed by the flame extinction and dissipation of the heat accumulated in the sample (see t=15.1 ... 34 sec. - Figure 1).
0.1 1 3 5 8 12
Figure 2. The interference film frames showing typical features of the NC sample burning wave shaping and development.
As Figures 1 and 2 show, the burning wave structure formed during LRI, is strongly not one-dimensional due to the laser radiation self-focusing in the sample substance.
Based on the cine films data, the scheme of the c-phase zones (Figure 3) and time diagrams of the above mentioned stages (Figure 4) have been constructed.
Figure 3. Scheme of the c-phase zones: 1 - burnt-out substance zone, 2 - burnt-out "cone" which appears as a result of self-focusing of the laser radiation, 3 - heated layer zone, 4- sample elastic strain zone.
Figure 4. Time schemes (only qualitative character) of the some characteristics of PM sample ignition process: 1 - laser radiation flux, 2 - flame heat flux, 3 - degree of elastic strain, 4 - the deep of burnt-out "cone" which appears as a result of self-focusing of the laser radiation, 5 - thickness of the burnt-out substance zone, 6 - the whole thickness of the heated layer zone, tflame - the duration of the stabilization period of the flame near the sample surface.
When interference method are used for the strong optical ingomogeneity studying (type of burning solid or liquid substance) interference lines density in the field of part of image (in c-phase image) is much great and are to appear difficulties with at the decryption interference pictures. This seen on Fig. 1 and on drawings provides in paper by A.Ito 6. When the shadow method of slit and grating (SMSG) 9 are used it is possible to reduce sensitivity before the required value by means of selecting a grating constant. In this is concluding advantage of SMSG. Figure 5 show the shadow video film frames be received several days back before dispatch work on this conference and representing features of the burning wave shaping and development during LRI process (images orientation is opposite orientation on Figure 1 and 2) *.
Figure 5. The shadow video film frames showing typical features of the NC sample burning wave shaping and development (laser radiation flux density is about 10 W/cm2, the PM sample is parallelepiped (20*30*30 mm), numerals show the time t in hours : minutes : seconds).
We plan to get the cine films or moment frames simultaneously consist of shadow pictures of c-phase and interference pictures of g-phase. These cine films can be obtained by some ways: 1) by using one exposure holograms and SMSG, 2) by using a shift interferometry with small shift of interfering beams (wave fronts), 3) by using the SMSG in which the grating consist of very thin (about 0,04 mm) thread 10.
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This experiment be conduct by K.Muryschkin and S.Rybakov, students of Depart. of Thermophysics of Chuvash St. Univ.We plan also to obtain the holographic cine films simultaneously consist of photo- and interference pictures. These cine films can be obtained by small changing of real time holographic interferometry 11: interference picture is registered in the direction of the “reference” beam, not “working” beam as usually. Photo image of flame is directed on cine camera with small angle to the direction of registration of interference picture.
The detailed quantitative analysis of stages, observable in the condensed phase, is possible but calls for a further theoretical elaboration of the interferogram and shadow pictures calculation techniques described in 7,8 as well as the discussion of experimental problems and set up. In our opinion the development of these approaches will allow greatly increase the applications area of holographic interferometry and shadow technique and put new tasks of experimental investigation to ignition and combustion
. We invite specialist to participate in joint research.
Part of the research work described in this paper was supported by the Russian Foundation for Basic Research, grant rVolga No. 98-07-03375. In accordance with this grant Department of Thermophysics of the Chuvash State University carries out also the scientific project deals with the creation of Combustion Diagnostics Knowledge Base. We invite to participate in joint creation of this Base. Special forms are available by e-mail request to Prof. Victor S.Abrukov.
9. S.A.Abrukov, Shadow and Interference Methods, Kazan State Univ., Kazan, 1962, 88 pp. (in Russian)
10. S.A.Abrukov, “A Procedure for Determining the Temperature Field of CO-Air Flame”, Uchenye Zapiski Kazanskogo Universiteta (Kazan St. Univ.), Vol. 115, Book 12, 1955, pp. 3-23. (in Russian)
11. V.S.Abrukov, Interferometric Techniques in Combustion Research, Dr. Sci. dissertation, The Chuvash St. Univ., Cheboksary, 1995, 173 pp. (in Russian)