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The decomposition flame of hydrazine in inert porous media
The Decomposition Flame of Hydrazine in Inert Porous Media B. YU. K O S H K I N , V. A. BUNEV, V. S. BABKIN* Institute of Chemical Kinetics and Combustion, Novosibirsk, 630090, Russia
YU. M. L A E V S K Y Computer Centre, Novosibirsk, 630090, Russia
The propagation of thermal waves of N 2 H 4 decomposition in capillary-porous media of two types has been studied with flltrational supply of the reactant to the decomposition zone. Steady-state decomposition regions have been examined. Parameters affecting the burning velocity and thermal wave propagation velocity have been determined. Thermal wave structure has been found to involve an anomalously wide preheat zone, due to the effect of capillarity. In this zone hydrazine appears in both liquid and gaseous states, the liquid and gaseous mass flows being, in fact, non-one-dimensional. The decomposition process causes internal capillary filtration. A physical model of decomposition flame hydrazone in porous media is discussed. The decomposition flame laws and peculiarities have been shown to depend on the properties of both the monofuel and the porous medium. Filtrational flows induced by external and internal (capillary) forces, as well as thermal interaction between porous and reaction media, have been shown to play an important role in the mechanism of wave propagation.
NOMENCLATURE D d
f 8
C
vo Pl, Ps Cs , Cg
Q S, U h
O(x) T(x)
o~,L /'0, r0
inner d i a m e t e r of the tube quartz rods d i a m e t e r grain size fraction porosity of m e d i u m (the v o l u m e ratio of voids to total volume of the p o r o u s medium) v o l u m e c o n s u m p t i o n of N z H 4 flow velocity of N z H a in p o r o u s medium densities of liquid and solid specific heat capacities o f the solid and gaseous phases heat release of the chemical reaction linear burning velocity p r o p a g a t i o n velocity of the decomposition flame of hydrazine relative to the porous medium wave zone size t e m p e r a t u r e profile in solid phase t e m p e r a t u r e profile o f reagent (and gas) equilibrium t e m p e r a t u r e initial pressure and t e m p e r a t u r e adiabatic t e m p e r a t u r e of the d e c o m p o sition flame o f hydrazine w i t h o u t porous medium
*Corresponding author. C O M B U S T I O N A N D F L A M E 1 0 3 : 1 4 3 - 1 5 0 (1995) Copyright © 1995 by The Combustion Institute Published by Elsevier Science Inc.
K
burning velocity sensitivity to changes in flow velocity critical value
INTRODUCTION Filtrational combustion usually implies exothermic conversion waves propagating t h r o u g h a p o r o u s m e d i u m u n d e r filtration of gases. As with combustion in h o m o g e n e o u s systems, the m e c h a n i s m o f wave p r o p a g a t i o n for filtration c o m b u s t i o n involves the heating of reactants and their chemical interaction, resulting in heat release within a relatively narrow space. A specific e l e m e n t of these processes is gas filtration, in which a vector determines the velocity and structure of the c o m b u s t i o n wave. Filtrational combustion takes place in blast-furnace processes, agglomeration of ores, selfpropagating h i g h - t e m p e r a t u r e synthesis [1] and in-situ thermal techniques for recovery of oil [2]. T h e r m a l waves propagating in a catalyst layer [3] and filtration combustion o f gases [4], are also c o m b u s t i o n processes of this kind. Filtrational combustion waves have a n u m b e r of interesting features which are absent f r o m the flames of h o m o g e n e o u s systems. F o r instance, these waves can change the direction o f their propagation, have wide chemical reaction zones, subadiabatic and superadiabatic equilibrium combustion temperatures, and essentially different phase t e m p e r a t u r e s [5, 6]. These spe0010-2180/95/$9.50 SSDI 0010-2180(95)00090-S
144 cific properties of filtrational combustion waves are related to the presence of two or more phases, the relative motion of reactants and intense heat and mass transfer in the wave zone. A classification of filtrational combustion processes is possible and the main characteristics of the steady-state and non-steady-state wave propagation have been established. The structure has been studied and a mathematical description proposed. The physical principles of the self-organization of steady-state thermal waves and the conditions of their existence have been formulated and thermal wave stability studied [5-7]. As already mentioned, filtrational combustion processes usually have been studied for gases. The object of the present study is the filtrational combustion of liquid. Liquids, compared to gases, have higher density, heat capacity per unit volume, chemical energy density and heat exchange coefficient. In addition, a liquid adds a number of new factors to the combustion process, such as phase transition and the action of surface tension and gravity forces. Thus, a study of the filtrational combustion of a liquid can reveal some new mechanisms and peculiarities of this process. Recently, the possibility has been demonstrated of waves of filtrational combustion of liquid with gas-phase chemical reaction that propagate at constant velocity in capillary-porous systems [8]. The present paper reports the results of further studies of the velocity and structure of stationary combustion waves, and formulates a physical model for further mathematical description of the process. Hydrazine (monofuel) whose characteristics have been studied in detail by many investigators, has been chosen as a model liquid. With catalytic surfaces absent, the flame of this fuel is maintained by a gas-phase exothermic decomposition reaction [9-11]. The objective laws obtained have been compared with those of the filtration combustion of gases [12-14], a wellstudied process with a similar chemical reaction. EXPERIMENTAL The apparatus is shown in Fig. 1. Liquid hydrazine was supplied from a tank with a pump,
B. YU. K O S H K I N E T AL.
DIFFUSION FLAME
I<::Li:l THE N2H/.' DECOMPOSITION WAVE
I
--I
\HYDRAZINE TANK
PUMP
lot <"
'
Fig. 1. Apparatus.
through a pipe, to a vertical tube of a transparent quartz filled with a porous medium. The decomposition flame of hydrazine was initiated at the upper end of the combustion tube, at which time the flow velocity, u 0, of N2H 4 in the porous medium was at, or close to, zero. After the flame had penetrated the tube the short distance of 20-30 mm, the pump was switched on, to provide the necessary liquid flow velocity in the tube. In the porous medium, after the start up period, a wave of N2H 4 decomposition was formed which propagated downwards with a constant spatial, or wave, velocity. Two types of porous media were used. The f i r s t comprised a 250-mm-long quartz tube filled with fractioned silicon carbide grains. The inner diameter of the tube, D, was 27.3 mm. Two fractions of grains were used: (1.0-1.2) mm and (2.0-2.5) mm. The porosity of this medium (the volume ratio of voids to total volume of the porous medium) was e = 0.50 _+ 0.02. The second comprised a bunch of parallel cylindrical quartz rods of equal diameters, inserted into the tube. The porosity of such a medium was e = 0.33 _ 0.04 and was practically independent of the rod diameter. These experiments employed 400 mm long tubes with internal diameters 10.5-35.6 mm, and quartz rods of diameter 1-5 mm. The following parameters were varied; the hydrazine flow (filtration) velocity and direction, V0, the tube inner diameter, D, the quartz rods diameter, d, the grain size fractions, f, and the medium porosity, e. The flow velocity was cal-
H Y D R A Z I N E F L A M E IN P O R O U S M E D I A
145
culated from V0 = C / [ e . (re. D2/4)], where C is the volume consumption of NzH4, measured from the liquid level change in the hydrazine tank. The propagation velocity of the N z H 4 decomposition wave relative to the porous medium (wave velocity), U, the linear burning velocity, S u, the wave zone size, and the temperature profile, O(x), in the solid phase were measured. A c h r o m e l - a l u m e l thermocouple measured the solid SiC t e m p e r a t u r e O(x). It was placed in a quartz glass capillary, with an outer diameter 1.1 or 2.2 mm, as shown in Fig. 1. The steady-state wave velocity U was measured from the change of the flame position in the tube. All experiments were at a pressure of P0 = 0.1 MPa and T 0 = 291-295 K.
on the flow velocity for two S1C grain fractions. The linear velocity, Su, of liquid burning out in the planar front of the wave is determined from the formula S, = V0 - U. It is seen that the dependence U(Vo) is U-shaped, as in the filtrational combustion of gases. The increase in flow velocity intensifies the decomposition of hydrazine and increases S~. The luminescence intensity of the wave front also increases. With a decreasing value of V0, the burning velocity decreases, and at some critical negative value of V0* the decomposition ceases (flammability limit). At this limit, the luminescence from the flame front is negligibly small. The presence of a wide gas-liquid zone is a peculiarity of the spatial wave structure. It was found that a zone with a partial filling of pores and complex spatial distribution of the gas and liquid phases occurs in front of the luminesing zone of chemical reaction. The length of this zone, h, increases with decreasing flow velocity V0, and reaches its maximum at the flammability limit, as shown in Fig. 3. The temperature of the solid in the gas-liquid zone, region AB in Fig. 4, rises continuously and attains values close to the liquid boiling point, of 113°C.
E X P E R I M E N T A L RESULTS
Porous Medium--Silicon Carbide Grains It was verified experimentally that the N 2 H 4 decomposition wave has a stable luminescing planar front and propagates at constant velocity. Figure 2 depicts the dependence of the wave velocity, U, and the burning velocity, S,,
03[
Porous Medium--Cylindrical Rods Figures 2, 3, and 5 show that both types of porous media exhibit the same general rules of a decomposition flame. An increase in flow
K 0
36
I °=27,3mm I D=2Z3 mm f =1,0-1,2 mm E:0,50-*0,02
t.
FLOW VELOCITY,Volmm/s) A
-0,1
-0,1
o
\
0,1
I---
> Ca : ~
< -0.2 3~ Fig. 2. Dependence of burning velocity and wave velocity of N2H 4 on flow velocity in SiC grains. A cross denotes limit flammability, l:f = 1.0-1.2 mm; 2:f = 2.0-2.5 mm.
i - 0,06
0
0,06 0,12 FLOW VELOCITY, Vo(rnm/$)
Fig. 3. Dependence of length of gas-liquid zone h on flow velocity V~ of Nell 4 flow. A cross denotes limit flammability.
146
B. YU. KOSHKIN ET AL.
>c }--
720
D= 27,3 mm f =1,0-1.2 mm E=0,50-*0,02
r~
o,41 E ~nz 0,37 H zoc rn -~ 0,33 E 45
Vo=0 360
uJ p-
Y
ic3
/
The normal boiling
i point of h
~
B
LU Z o
O0
•
i
L
10
20
3'0 DISTANCE, [mrn)
Fig. 4. Characteristic temperature profile of N2H 4 decomposition wave in SiC grains.
velocity increases the burning velocity and reduces the length of the gas-liquid zone. Similarly, with an increasing characteristic size of the porous medium, such as the quartz rod diameter of the SiC grain size, both the burning velocity and the length of the gas-liquid zone are observed to decrease in both cases (Figs. 2, 6). There are, however, some peculiarities. From Fig. 5 it follows that the thermal waves in the present experiments are observable only at positive values of flow velocity. Decomposition of hydrazine is impossible when the flow velocity is less than the critical value.
E
>"<
0
E
-0,o8
0.8U o, >~
• -
o
i
0.12
-7$c3 UJ Z
oN ,
"k,
cr
Ld D=26,Smm ] E =0,32.'-0,35 ] Vo: 0,33 mm/sJ
35 25 0
i
i
1
2
i
/i
i
3 5 ROD DIAMETER, d(mm)
Fig. 6. Dependence of wave velocity, U, burning velocity, S,, and gas-liquid zone length, h, on quartz rod diameter, d.
The geometry of the porous cavity when the medium consists of a bunch of quartz rods is more simple, capillary motion in this medium being regular. This allows five characteristic spatial zones in the wave structure to be distinguished (Fig. 7a). In the capillary zone (h l) the liquid is concentrated only at rod contacts (Fig. 7b). In the droplet zone (h2) the liquid is observed, not only near the contacts, but also throughout the surface of the rods as large droplets, in the vapor zone (h 3) no visible signs of the liquid phase were detected. In a number of experiments, a temperature plateau in this zone was observed, close to the boiling point of N2H 4. High temperatures are typical of the luminescent zone (h4). The maximum temperature and luminescence in this zone increase
C00LINGZONE
R00
LUNI NOUS ZONE VAPOUR
E=0,32+0,34 d=l,0mm
4o
N
S -o. o
t°4°" 0
80
-0,06
>
cr-
113
-0,02 E E
00
ZONE
OROPLET ZONE
S
~
~
CAPILLARY ZONE
~
A LIQUID"
1
•
0 0
i
i
i
0,2
0,4
0,6
i 0,8
1
1,0
LIQUID PHASE
(a)
~ ~----'-~---~-~I
(b)
FLOW VELOCITY, Vo(mm/s) Fig. 5. Dependence of wave velocity, U, burning velocity, S,, and gas-liquid zone length, h, on flow velocity in a
bunch of quartz rods. A cross denotes limit flammability.
Fig. 7. Scheme of structure of N2H 4 decomposition wave
in porous medium: (a) section through bunch of cylindrical rods; (b) position of liquid N2H 4 in capillary zone.
H Y D R A Z I N E F L A M E IN P O R O U S M E D I A
147
with flow velocity (see Fig. 8). In the cooling zone (h 5) the t e m p e r a t u r e and luminescence decrease continuously. Some data on the zone sizes in the tubes of various diameters are listed in Table 1. The preheat zone is 1 - 2 orders of magnitude thicker than that in the decomposition flame of hydrazine without porous media [15].
loss to the environment, and, as a result, a decrease in Su down to extinction. Table 1 shows that in porous media, for constant parameters V0, d, and e, the decomposition flame of hydrazine is impossible, with D < D * = 10.5 - 12.5 mm, in which D* is the quenching diameter. Consequently, the conception of a critical diameter also holds for a combustion regime in a porous medium. In this case, the critical conditions include such additional parameters as the porous medium characteristics and the flow velocity V0. A m o r e complex way of affecting S u via the chemical reaction rate is through the thermal interaction b e t w e e n the porous m e d i u m (carcass) and the reaction zone. The temperature distribution in the carcass O(x) and the reactants T(x) can be quite different in the wave zone, due to a finite value of the heat transfer velocity and the difference in the thermo-physical properties of the interacting phases. It is evident that the heat flows at the "carcass-reagent" interface determined by the t e m p e r a t u r e difference T ( x ) - O(x) depend on the flow velocity vector, which may result in two basically different situations. At relatively high flow velocities, ahead of the heat release zone, O ( x ) > T(x). In this case, heat transfers from the carcass to the liquid and its vapors. In contrast, in the zones of heat release and internal thermal relaxation (the zone in which O ( x ) and T(x) equalize once the chemical reaction has been completed) the heat passes to the carcass and is transmitted along the carcass by conduction to the preheat zone. The return of a portion of the heat from the product combustion zone accelerates liquid heating and evaporation; the enthalpy of reacting vapors increases and, correspondingly, so does the rate of chemical reaction. This heat recirculation is favored by a high (relative to the reactant) heat conductivity of the carcass material. At low flow velocities, particularly at negative values close to the propagation limits, the situation may change: ahead of the heat release zone O ( x ) < T(x), and the more heated liquid and its vapors transfer heat to the carcass in the preheat zone. This slows down the preliminary processes and, as a result, also the chemical reaction rate.
D I S C U S S I O N OF RESULTS The main conclusions are that the burning velocity is strongly dependent on p a r a m e t e r s of the porous medium, the motion of the liquid relative to the medium, and the inner diameter of the tube. For example, the burning velocity increases from 0.1 to 0.3 m m / s (Fig. 2) as the flow velocity increases from - 0 . 0 5 to +0.1 m m / s with SiC grains ( f = 1.0-1.2 mm), while in a similar tube, but without porous medium, the burning velocity is constant at 0.2 m m / s . What are the mechanisms that can influence these factors? Two main mechanisms have been assumed: (i) a chemical one that affects the chemical conversion rate, and (ii) a physical one that changes the area of the chemical reaction surface, or flame front. The chemical reaction velocity may be changed in different ways. The simplest one is the cooling of the combustion zone due to heat
ee?C g00
/
8OO
-
i
0.05
Calculated
-
•
Experimental
i
0
0,05
1
0,10 Vo. mm/s
Fig. 8. D e p e n d e n c e of m a x i m u m t e m p e r a t u r e of the solid p h a s e am(X)in N 2 H 4 d e c o m p o s i t i o n wave o n flow velocity VII. D = 27.3 mm, f = (1.0 + 1.2 mm).
148
B. YU. K O S H K I N E T AL. TABLE 1
Burning Velocity, S,, and the Lengths of Structural Zones, h, in Decomposition Wave in Tubes with Different Inner Diameters D at V0 = 0.55 + 0.01 r a m / s , d = 1.0 ram, e = 0.30 - 0.37 D
S.
(mm)
(mm/s)
Size of Zones (mm) hi
h2
10.5 0.04 0.09 0.12 0.125
37.5 35.0 25.0 17.5
In both cases the porous medium is important, on the one hand, as a heat transmitter (via conduction or, in principle, thermal radiation) and, on the other hand, as the accumulator of heat with a certain heat capacity and "charging" or "discharging" rates that are determined by heat exchange coefficients and differences in phase temperatures. In this case the filtration flow acts as a heat-carrier. Another important factor affecting the burning velocity is the equilibrium temperature of the decomposition flame of hydrazine, which, as in other filtrational combustion processes, depends on both the physico-chemical properties of the reactants, the thermophysical characteristics of the carcass and the flow velocity. Ignoring external heat losses in the heat and mass b a l a n c e s of the o n e - d i m e n s i o n a l "carcass-reactant" system, one obtains: Q Su -c g Su + o ' ( S u
Oe=T=To+ 1-~
8
h4
10.0 7.5 5.0 4.0
9.0 20.0 22.5 42.5
Flame Nonexistent
12.3 18.8 27.3 35.6
o-
h3
csp s
--,
Cg Pl
-
Vo)'
(1)
where Oe, Te, and T O are the equilibrium and initial temperatures, respectively; Q is the heat release of the chemical reaction; Pl and Ps the densities of liquid N e l l 4 and solid, respectively; c s and Cg the specific heat capacities of the solid and gaseous phases, respectively: and is the porosity. Figure 8 compares calculated (Eq. 1) and experimental values of temperature. The experimental data on V0 and S, (see Fig. 2) and the experimental value O//Cg = 1135 [16] are used to compute Te.
4.5 2.5 2.0 1.5
The above formulae show that if S u > V0, then Te < Tb, where T b = T o + Q / c g is the adiabatic temperature of hydrazine burning without the porous medium. When S u < V0, then T e > T b. In accordance with Eq. 1, temperature T e increases with increasing V0. This is confirmed by the measurements of temperature (Fig. 8) and, indirectly, by the increase in the intensity of luminescence from the chemical reaction zone. In the present experiments Su > V0, i.e., decomposition occurs at subadiabatic temperatures. However, the values of S u observed in a portion of the experiments (see Fig. 5) are much higher than the burning velocity of the decomposition hydrazine flame in a tube at P = 0.1 MPa and T = 20°C (S, = 0.3 - 0.4 m m / s [17]). As for the explanation of the effect of the experimental parameters on the burning velocity, it should be noted that according to the definition taken in this work, S u depends not only upon the burning rate (the rate of free burning of liquid hydrazine with a planar surface), but also upon the actual location of the chemical reaction, which, in turn, is dependent on the area of burning liquid. The actual burning liquid surface cannot be planar and uniform in a porous medium. Experiments in narrow tubes at V0 = 0 show the surface tension to cause considerable distortions of the liquid surface [9]. Under certain conditions the planar front may lose its stability for hydrodynamics reasons [18, 19]. Motion in narrow tubes gives additional reasons for the instability due to thermal interaction with the tube wall [20]. Experiments on N2H 4 decomposition in a tube of 5 mm i.d. indicate that when V0 < 0.3 m m / s
H Y D R A Z I N E F L A M E IN P O R O U S M E D I A
149
the normal decomposition flame of hydrazine produces no surface distortions. When V0 > 0.8 m m / s this flame is turbulent and involves intense boiling and dispersion of liquid, with intensified motion of the liquid and gaseous masses. The decomposition zone includes the extended regions, in which both boiling and decomposition take place. A transient decomposition zone lies within the above range of V0 values. In the porous medium, the hydrodynamic, thermal, and chemical wave structures are much more complex. Experiments on the porous medium of cylindrical rods revealed a few characteristic zones of abnormally wide gas-liquid heating (Fig. 7), the formation mechanism of which is thought to be related to the action of capillary forces. Porous media may be represented as a polycapillary body with a system of connected narrow and wide channels. The former act as a wick, transporting liquid to the evaporation and decomposition zones. The level of the liquid in wide channels falls, not due to liquid evaporation under heating, but due to the transport of liquid, first to narrow channels and then to the evaporation and decomposition zones. T h e r m o c o u p l e and visual m e a s u r e m e n t s show the length of the gas-liquid zone to correspond to that of the preheat one, i.e., to a smooth t e m p e r a t u r e increase prior to the drastic rise in the luminescence zone. Estimates show that such an extended thermal zone cannot be attributed to the thermal conductivity. A m o r e efficient mechanism of longitudinal heat transfer is required, such as the evaporation-condensation mechanism in heat pipes. Passing from a " v a p o r " zone to that with a lower temperature, the hydrazine vapors are condensed. The condensate returns to the evaporation zone in a liquid flow. The condensation-evaporation process results in the formation of a droplet zone. Two important conclusions follow from these observations. The flow in the preheat zone is not one-dimensional, which allows a hot-spot localization of the chemical reaction in the decomposition wave. Secondly, the decomposition process induces internal capillary filtration with a velocity that varies within the capillary
zone width, and may take place both with external filtrational flow and also without it, when V0 = 0. Figures 2 and 5 demonstrate that the flow velocity is an important system p a r a m e t e r determining the burning velocity. In order to estimate this influence let us introduce the concept of the burning velocity sensitivity to the changes in flow velocity, K = d S u / d V o. As can be seen from Fig. 9, the value of K is positive, variable and decreases with increase in V0. This change in K determines the shape of the U(V o) curve. The formula S, - V0 - U yields d U / d V o = 1 - d S , / d V o and it is seen that, at the minimum of the dependence U(V o) K = 1. When K > 1, the wave velocity increases (in its absolute value) with V0. Note that for filtrational combustion of gases K - 1 [12, 13]. U p o n decomposition of N z H 4 in narrow tubes K < 1 [20]. U n d e r the present experimental conditions of range of K variations is wider (0.5-1.9). Since the flow velocity influences the thermal interaction between the carcass and reagent, the high values of K also testify to the high activity of the porous medium in the combustion process. In this context the term "inert m e d i u m " loses its meaning. CONCLUSION The decomposition flame of hydrazine in inert porous media, involving gas-phase chemical reaction, has a n u m b e r of properties in c o m m o n with other types of filtrational combustion. These are the dependence of wave velocity on
0
-0,05
I
I
0
0,05
I
O.i Vo , mrn/s
Fig. 9. Dependence of K on the velocity V0. 1--D = 27.3 mm; d = 1.0 mm; e = 0.33, 2--D = 27.3 ram; f = 2.0 + 2.5 mm; e = 0.5.
150 the filtrational velocity, the dependence of equilibrium flame temperature on wave velocity, the probability of the wave reversing, and heat recirculation in the decomposition zone. However, the decomposition flame of hydrazine in inert porous media shows some qualitative and quantitative peculiarities, determined by the specific properties of the liquid. The are especially explicit in the decomposition wave structure with its abnormally wide preheat zone, involving complex spatial distribution of reaction medium phases and temperatures, as well as non-one-dimensionality of hydrodynamic flows that involve internal capillary filtration. Consequently, it is necessary to study further the fine structure of decomposition waves, including chemical reaction localization. A study of decomposition flame of hydrazine in inert porous media as a category of filtrational combustion is beneficial to both fields and reveals their common principles. Data on filtrational combustion may be used in studies of other processes of heterogeneous combustion waves, which involve the liquid phase of the product, the initial or intermediate components of the reactants.
The authors wish to thank Prof. D. Bradley for helpful discussion and the Russian Foundation for Fundamental Research for their financial support of this research (Grant No 93-03-18508).
B. YU. KOSHKIN ET AL. 4. 5.
6.
7.
8.
9.
10.
11. 12.
13. 14.
15.
16. 17.
REFERENCES 1. Merzhanov, A. G., in Combustion and Plasma Synthesis of High-Temperature Materials (Z. A. Munir, and J. B. Holt, Eds.), VCH, 1990, pp. 1 53. 2. Amr A., Combust. Flame 41:301 (1981). 3. Matros, Yu. Sh., Nestatzionarnye Protzessy ~ Katalititcheskikh Reaktorakh, Novosibirsk, 1982.
18. 19. 20.
Babkin V. S., Laevsky Yu. M., Combust. Explos. Shock Waves 23:531 (1987). Aldushin, A. P., and Merzhanov, A. G., Rasprostranenie Teplovykh Voln ~' Geterogennykh Sredakh (Yu. Sh. Matros, Ed.), s.9, Novosibirsk, 1988. Laevsky, Yu. M., and Babkin V. S., Rasprostranenie TeploLykh Voln L, Geterogennykh Sredakh (Yu. Sh. Matros, Ed.), s.93, Novosibirsk, 1988. Kisilev, O. V., Matros, Yu. Sh. and Chumakova, N. A., Rasprostranenie Teplotykh Voln c Geterogennykh Sredakh (Yu. Sh. Matros, Ed.), s.145, Novosibirsk, 1988. Koshkin, B. Yu., Bunev, V. A., Babkin, V. S., and Laevsky, Yu. M., DokladyAkad. Nauk SSSR 310:1395 (1990). Adams, G. K. and Stocks, G. W.: Fourth Symposium (International) on Combustion, The Combustion Institute, 1953, p. 239. Gray, P., Lee, J. C., Leach, H. A., and Taylor, D. C., Sixth Symposium (International) on Combustion, The Combustion Institute, 1957, p. 255. Antoine, A. C., Eighth Symposium (International) on Combustion, The Combustion Institute, 1961, p. 1057. Babkin, V. S., Drobyshevich, V. I., Laevsky, Yu. M., and Potytnyakov, S. I., Combust. Explos., Shock Waves 19:147 (1983). Potytnyakov, S. I., Laevsky, Yu. M., and Babkin, V. S., Combust. Explos., Shock Waves 20:15 (1984). Laevsky, Yu. M., Babkin, V. S., Drobyshevieh, V. I., and Potytnyakov, S. I., Combust. Explos., Shock Waves 20:591 (1984). Hildenbrand, D. L., and Whittaker, A. G., Fifth Symposium (International) on Combustion, The Combustion Institute, 1955, p. 212. Alecseev A. P., Manelis G. B., Combust., Explos., Shock Wal,es 16:405 (1980). Schmidt E. W., Hydrazine and Its Deriuatiues. Preparation, Properties, Applications. Wiley, New York, 1984. Landau, L. D.: Z. Eksp. Teoret. Fiz. 14:240 (1944). Levitch, V. C., Doklady Akad. Nauk SSSR 109:975 (1956). Babkin, V. S., Koshkin, B. Yu., and Laevsky, Yu. M,, Doklady Akad. Nauk SSSR 304:892 (1989).
Receit:ed 28 April 1994; ret,ised 26 March 1995
YU. M. L A E V S K Y Computer Centre, Novosibirsk, 630090, Russia
The propagation of thermal waves of N 2 H 4 decomposition in capillary-porous media of two types has been studied with flltrational supply of the reactant to the decomposition zone. Steady-state decomposition regions have been examined. Parameters affecting the burning velocity and thermal wave propagation velocity have been determined. Thermal wave structure has been found to involve an anomalously wide preheat zone, due to the effect of capillarity. In this zone hydrazine appears in both liquid and gaseous states, the liquid and gaseous mass flows being, in fact, non-one-dimensional. The decomposition process causes internal capillary filtration. A physical model of decomposition flame hydrazone in porous media is discussed. The decomposition flame laws and peculiarities have been shown to depend on the properties of both the monofuel and the porous medium. Filtrational flows induced by external and internal (capillary) forces, as well as thermal interaction between porous and reaction media, have been shown to play an important role in the mechanism of wave propagation.
NOMENCLATURE D d
f 8
C
vo Pl, Ps Cs , Cg
Q S, U h
O(x) T(x)
o~,L /'0, r0
inner d i a m e t e r of the tube quartz rods d i a m e t e r grain size fraction porosity of m e d i u m (the v o l u m e ratio of voids to total volume of the p o r o u s medium) v o l u m e c o n s u m p t i o n of N z H 4 flow velocity of N z H a in p o r o u s medium densities of liquid and solid specific heat capacities o f the solid and gaseous phases heat release of the chemical reaction linear burning velocity p r o p a g a t i o n velocity of the decomposition flame of hydrazine relative to the porous medium wave zone size t e m p e r a t u r e profile in solid phase t e m p e r a t u r e profile o f reagent (and gas) equilibrium t e m p e r a t u r e initial pressure and t e m p e r a t u r e adiabatic t e m p e r a t u r e of the d e c o m p o sition flame o f hydrazine w i t h o u t porous medium
*Corresponding author. C O M B U S T I O N A N D F L A M E 1 0 3 : 1 4 3 - 1 5 0 (1995) Copyright © 1995 by The Combustion Institute Published by Elsevier Science Inc.
K
burning velocity sensitivity to changes in flow velocity critical value
INTRODUCTION Filtrational combustion usually implies exothermic conversion waves propagating t h r o u g h a p o r o u s m e d i u m u n d e r filtration of gases. As with combustion in h o m o g e n e o u s systems, the m e c h a n i s m o f wave p r o p a g a t i o n for filtration c o m b u s t i o n involves the heating of reactants and their chemical interaction, resulting in heat release within a relatively narrow space. A specific e l e m e n t of these processes is gas filtration, in which a vector determines the velocity and structure of the c o m b u s t i o n wave. Filtrational combustion takes place in blast-furnace processes, agglomeration of ores, selfpropagating h i g h - t e m p e r a t u r e synthesis [1] and in-situ thermal techniques for recovery of oil [2]. T h e r m a l waves propagating in a catalyst layer [3] and filtration combustion o f gases [4], are also c o m b u s t i o n processes of this kind. Filtrational combustion waves have a n u m b e r of interesting features which are absent f r o m the flames of h o m o g e n e o u s systems. F o r instance, these waves can change the direction o f their propagation, have wide chemical reaction zones, subadiabatic and superadiabatic equilibrium combustion temperatures, and essentially different phase t e m p e r a t u r e s [5, 6]. These spe0010-2180/95/$9.50 SSDI 0010-2180(95)00090-S
144 cific properties of filtrational combustion waves are related to the presence of two or more phases, the relative motion of reactants and intense heat and mass transfer in the wave zone. A classification of filtrational combustion processes is possible and the main characteristics of the steady-state and non-steady-state wave propagation have been established. The structure has been studied and a mathematical description proposed. The physical principles of the self-organization of steady-state thermal waves and the conditions of their existence have been formulated and thermal wave stability studied [5-7]. As already mentioned, filtrational combustion processes usually have been studied for gases. The object of the present study is the filtrational combustion of liquid. Liquids, compared to gases, have higher density, heat capacity per unit volume, chemical energy density and heat exchange coefficient. In addition, a liquid adds a number of new factors to the combustion process, such as phase transition and the action of surface tension and gravity forces. Thus, a study of the filtrational combustion of a liquid can reveal some new mechanisms and peculiarities of this process. Recently, the possibility has been demonstrated of waves of filtrational combustion of liquid with gas-phase chemical reaction that propagate at constant velocity in capillary-porous systems [8]. The present paper reports the results of further studies of the velocity and structure of stationary combustion waves, and formulates a physical model for further mathematical description of the process. Hydrazine (monofuel) whose characteristics have been studied in detail by many investigators, has been chosen as a model liquid. With catalytic surfaces absent, the flame of this fuel is maintained by a gas-phase exothermic decomposition reaction [9-11]. The objective laws obtained have been compared with those of the filtration combustion of gases [12-14], a wellstudied process with a similar chemical reaction. EXPERIMENTAL The apparatus is shown in Fig. 1. Liquid hydrazine was supplied from a tank with a pump,
B. YU. K O S H K I N E T AL.
DIFFUSION FLAME
I<::Li:l THE N2H/.' DECOMPOSITION WAVE
I
--I
\HYDRAZINE TANK
PUMP
lot <"
'
Fig. 1. Apparatus.
through a pipe, to a vertical tube of a transparent quartz filled with a porous medium. The decomposition flame of hydrazine was initiated at the upper end of the combustion tube, at which time the flow velocity, u 0, of N2H 4 in the porous medium was at, or close to, zero. After the flame had penetrated the tube the short distance of 20-30 mm, the pump was switched on, to provide the necessary liquid flow velocity in the tube. In the porous medium, after the start up period, a wave of N2H 4 decomposition was formed which propagated downwards with a constant spatial, or wave, velocity. Two types of porous media were used. The f i r s t comprised a 250-mm-long quartz tube filled with fractioned silicon carbide grains. The inner diameter of the tube, D, was 27.3 mm. Two fractions of grains were used: (1.0-1.2) mm and (2.0-2.5) mm. The porosity of this medium (the volume ratio of voids to total volume of the porous medium) was e = 0.50 _+ 0.02. The second comprised a bunch of parallel cylindrical quartz rods of equal diameters, inserted into the tube. The porosity of such a medium was e = 0.33 _ 0.04 and was practically independent of the rod diameter. These experiments employed 400 mm long tubes with internal diameters 10.5-35.6 mm, and quartz rods of diameter 1-5 mm. The following parameters were varied; the hydrazine flow (filtration) velocity and direction, V0, the tube inner diameter, D, the quartz rods diameter, d, the grain size fractions, f, and the medium porosity, e. The flow velocity was cal-
H Y D R A Z I N E F L A M E IN P O R O U S M E D I A
145
culated from V0 = C / [ e . (re. D2/4)], where C is the volume consumption of NzH4, measured from the liquid level change in the hydrazine tank. The propagation velocity of the N z H 4 decomposition wave relative to the porous medium (wave velocity), U, the linear burning velocity, S u, the wave zone size, and the temperature profile, O(x), in the solid phase were measured. A c h r o m e l - a l u m e l thermocouple measured the solid SiC t e m p e r a t u r e O(x). It was placed in a quartz glass capillary, with an outer diameter 1.1 or 2.2 mm, as shown in Fig. 1. The steady-state wave velocity U was measured from the change of the flame position in the tube. All experiments were at a pressure of P0 = 0.1 MPa and T 0 = 291-295 K.
on the flow velocity for two S1C grain fractions. The linear velocity, Su, of liquid burning out in the planar front of the wave is determined from the formula S, = V0 - U. It is seen that the dependence U(Vo) is U-shaped, as in the filtrational combustion of gases. The increase in flow velocity intensifies the decomposition of hydrazine and increases S~. The luminescence intensity of the wave front also increases. With a decreasing value of V0, the burning velocity decreases, and at some critical negative value of V0* the decomposition ceases (flammability limit). At this limit, the luminescence from the flame front is negligibly small. The presence of a wide gas-liquid zone is a peculiarity of the spatial wave structure. It was found that a zone with a partial filling of pores and complex spatial distribution of the gas and liquid phases occurs in front of the luminesing zone of chemical reaction. The length of this zone, h, increases with decreasing flow velocity V0, and reaches its maximum at the flammability limit, as shown in Fig. 3. The temperature of the solid in the gas-liquid zone, region AB in Fig. 4, rises continuously and attains values close to the liquid boiling point, of 113°C.
E X P E R I M E N T A L RESULTS
Porous Medium--Silicon Carbide Grains It was verified experimentally that the N 2 H 4 decomposition wave has a stable luminescing planar front and propagates at constant velocity. Figure 2 depicts the dependence of the wave velocity, U, and the burning velocity, S,,
03[
Porous Medium--Cylindrical Rods Figures 2, 3, and 5 show that both types of porous media exhibit the same general rules of a decomposition flame. An increase in flow
K 0
36
I °=27,3mm I D=2Z3 mm f =1,0-1,2 mm E:0,50-*0,02
t.
FLOW VELOCITY,Volmm/s) A
-0,1
-0,1
o
\
0,1
I---
> Ca : ~
< -0.2 3~ Fig. 2. Dependence of burning velocity and wave velocity of N2H 4 on flow velocity in SiC grains. A cross denotes limit flammability, l:f = 1.0-1.2 mm; 2:f = 2.0-2.5 mm.
i - 0,06
0
0,06 0,12 FLOW VELOCITY, Vo(rnm/$)
Fig. 3. Dependence of length of gas-liquid zone h on flow velocity V~ of Nell 4 flow. A cross denotes limit flammability.
146
B. YU. KOSHKIN ET AL.
>c }--
720
D= 27,3 mm f =1,0-1.2 mm E=0,50-*0,02
r~
o,41 E ~nz 0,37 H zoc rn -~ 0,33 E 45
Vo=0 360
uJ p-
Y
ic3
/
The normal boiling
i point of h
~
B
LU Z o
O0
•
i
L
10
20
3'0 DISTANCE, [mrn)
Fig. 4. Characteristic temperature profile of N2H 4 decomposition wave in SiC grains.
velocity increases the burning velocity and reduces the length of the gas-liquid zone. Similarly, with an increasing characteristic size of the porous medium, such as the quartz rod diameter of the SiC grain size, both the burning velocity and the length of the gas-liquid zone are observed to decrease in both cases (Figs. 2, 6). There are, however, some peculiarities. From Fig. 5 it follows that the thermal waves in the present experiments are observable only at positive values of flow velocity. Decomposition of hydrazine is impossible when the flow velocity is less than the critical value.
E
>"<
0
E
-0,o8
0.8U o, >~
• -
o
i
0.12
-7$c3 UJ Z
oN ,
"k,
cr
Ld D=26,Smm ] E =0,32.'-0,35 ] Vo: 0,33 mm/sJ
35 25 0
i
i
1
2
i
/i
i
3 5 ROD DIAMETER, d(mm)
Fig. 6. Dependence of wave velocity, U, burning velocity, S,, and gas-liquid zone length, h, on quartz rod diameter, d.
The geometry of the porous cavity when the medium consists of a bunch of quartz rods is more simple, capillary motion in this medium being regular. This allows five characteristic spatial zones in the wave structure to be distinguished (Fig. 7a). In the capillary zone (h l) the liquid is concentrated only at rod contacts (Fig. 7b). In the droplet zone (h2) the liquid is observed, not only near the contacts, but also throughout the surface of the rods as large droplets, in the vapor zone (h 3) no visible signs of the liquid phase were detected. In a number of experiments, a temperature plateau in this zone was observed, close to the boiling point of N2H 4. High temperatures are typical of the luminescent zone (h4). The maximum temperature and luminescence in this zone increase
C00LINGZONE
R00
LUNI NOUS ZONE VAPOUR
E=0,32+0,34 d=l,0mm
4o
N
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t°4°" 0
80
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>
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113
-0,02 E E
00
ZONE
OROPLET ZONE
S
~
~
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~
A LIQUID"
1
•
0 0
i
i
i
0,2
0,4
0,6
i 0,8
1
1,0
LIQUID PHASE
(a)
~ ~----'-~---~-~I
(b)
FLOW VELOCITY, Vo(mm/s) Fig. 5. Dependence of wave velocity, U, burning velocity, S,, and gas-liquid zone length, h, on flow velocity in a
bunch of quartz rods. A cross denotes limit flammability.
Fig. 7. Scheme of structure of N2H 4 decomposition wave
in porous medium: (a) section through bunch of cylindrical rods; (b) position of liquid N2H 4 in capillary zone.
H Y D R A Z I N E F L A M E IN P O R O U S M E D I A
147
with flow velocity (see Fig. 8). In the cooling zone (h 5) the t e m p e r a t u r e and luminescence decrease continuously. Some data on the zone sizes in the tubes of various diameters are listed in Table 1. The preheat zone is 1 - 2 orders of magnitude thicker than that in the decomposition flame of hydrazine without porous media [15].
loss to the environment, and, as a result, a decrease in Su down to extinction. Table 1 shows that in porous media, for constant parameters V0, d, and e, the decomposition flame of hydrazine is impossible, with D < D * = 10.5 - 12.5 mm, in which D* is the quenching diameter. Consequently, the conception of a critical diameter also holds for a combustion regime in a porous medium. In this case, the critical conditions include such additional parameters as the porous medium characteristics and the flow velocity V0. A m o r e complex way of affecting S u via the chemical reaction rate is through the thermal interaction b e t w e e n the porous m e d i u m (carcass) and the reaction zone. The temperature distribution in the carcass O(x) and the reactants T(x) can be quite different in the wave zone, due to a finite value of the heat transfer velocity and the difference in the thermo-physical properties of the interacting phases. It is evident that the heat flows at the "carcass-reagent" interface determined by the t e m p e r a t u r e difference T ( x ) - O(x) depend on the flow velocity vector, which may result in two basically different situations. At relatively high flow velocities, ahead of the heat release zone, O ( x ) > T(x). In this case, heat transfers from the carcass to the liquid and its vapors. In contrast, in the zones of heat release and internal thermal relaxation (the zone in which O ( x ) and T(x) equalize once the chemical reaction has been completed) the heat passes to the carcass and is transmitted along the carcass by conduction to the preheat zone. The return of a portion of the heat from the product combustion zone accelerates liquid heating and evaporation; the enthalpy of reacting vapors increases and, correspondingly, so does the rate of chemical reaction. This heat recirculation is favored by a high (relative to the reactant) heat conductivity of the carcass material. At low flow velocities, particularly at negative values close to the propagation limits, the situation may change: ahead of the heat release zone O ( x ) < T(x), and the more heated liquid and its vapors transfer heat to the carcass in the preheat zone. This slows down the preliminary processes and, as a result, also the chemical reaction rate.
D I S C U S S I O N OF RESULTS The main conclusions are that the burning velocity is strongly dependent on p a r a m e t e r s of the porous medium, the motion of the liquid relative to the medium, and the inner diameter of the tube. For example, the burning velocity increases from 0.1 to 0.3 m m / s (Fig. 2) as the flow velocity increases from - 0 . 0 5 to +0.1 m m / s with SiC grains ( f = 1.0-1.2 mm), while in a similar tube, but without porous medium, the burning velocity is constant at 0.2 m m / s . What are the mechanisms that can influence these factors? Two main mechanisms have been assumed: (i) a chemical one that affects the chemical conversion rate, and (ii) a physical one that changes the area of the chemical reaction surface, or flame front. The chemical reaction velocity may be changed in different ways. The simplest one is the cooling of the combustion zone due to heat
ee?C g00
/
8OO
-
i
0.05
Calculated
-
•
Experimental
i
0
0,05
1
0,10 Vo. mm/s
Fig. 8. D e p e n d e n c e of m a x i m u m t e m p e r a t u r e of the solid p h a s e am(X)in N 2 H 4 d e c o m p o s i t i o n wave o n flow velocity VII. D = 27.3 mm, f = (1.0 + 1.2 mm).
148
B. YU. K O S H K I N E T AL. TABLE 1
Burning Velocity, S,, and the Lengths of Structural Zones, h, in Decomposition Wave in Tubes with Different Inner Diameters D at V0 = 0.55 + 0.01 r a m / s , d = 1.0 ram, e = 0.30 - 0.37 D
S.
(mm)
(mm/s)
Size of Zones (mm) hi
h2
10.5 0.04 0.09 0.12 0.125
37.5 35.0 25.0 17.5
In both cases the porous medium is important, on the one hand, as a heat transmitter (via conduction or, in principle, thermal radiation) and, on the other hand, as the accumulator of heat with a certain heat capacity and "charging" or "discharging" rates that are determined by heat exchange coefficients and differences in phase temperatures. In this case the filtration flow acts as a heat-carrier. Another important factor affecting the burning velocity is the equilibrium temperature of the decomposition flame of hydrazine, which, as in other filtrational combustion processes, depends on both the physico-chemical properties of the reactants, the thermophysical characteristics of the carcass and the flow velocity. Ignoring external heat losses in the heat and mass b a l a n c e s of the o n e - d i m e n s i o n a l "carcass-reactant" system, one obtains: Q Su -c g Su + o ' ( S u
Oe=T=To+ 1-~
8
h4
10.0 7.5 5.0 4.0
9.0 20.0 22.5 42.5
Flame Nonexistent
12.3 18.8 27.3 35.6
o-
h3
csp s
--,
Cg Pl
-
Vo)'
(1)
where Oe, Te, and T O are the equilibrium and initial temperatures, respectively; Q is the heat release of the chemical reaction; Pl and Ps the densities of liquid N e l l 4 and solid, respectively; c s and Cg the specific heat capacities of the solid and gaseous phases, respectively: and is the porosity. Figure 8 compares calculated (Eq. 1) and experimental values of temperature. The experimental data on V0 and S, (see Fig. 2) and the experimental value O//Cg = 1135 [16] are used to compute Te.
4.5 2.5 2.0 1.5
The above formulae show that if S u > V0, then Te < Tb, where T b = T o + Q / c g is the adiabatic temperature of hydrazine burning without the porous medium. When S u < V0, then T e > T b. In accordance with Eq. 1, temperature T e increases with increasing V0. This is confirmed by the measurements of temperature (Fig. 8) and, indirectly, by the increase in the intensity of luminescence from the chemical reaction zone. In the present experiments Su > V0, i.e., decomposition occurs at subadiabatic temperatures. However, the values of S u observed in a portion of the experiments (see Fig. 5) are much higher than the burning velocity of the decomposition hydrazine flame in a tube at P = 0.1 MPa and T = 20°C (S, = 0.3 - 0.4 m m / s [17]). As for the explanation of the effect of the experimental parameters on the burning velocity, it should be noted that according to the definition taken in this work, S u depends not only upon the burning rate (the rate of free burning of liquid hydrazine with a planar surface), but also upon the actual location of the chemical reaction, which, in turn, is dependent on the area of burning liquid. The actual burning liquid surface cannot be planar and uniform in a porous medium. Experiments in narrow tubes at V0 = 0 show the surface tension to cause considerable distortions of the liquid surface [9]. Under certain conditions the planar front may lose its stability for hydrodynamics reasons [18, 19]. Motion in narrow tubes gives additional reasons for the instability due to thermal interaction with the tube wall [20]. Experiments on N2H 4 decomposition in a tube of 5 mm i.d. indicate that when V0 < 0.3 m m / s
H Y D R A Z I N E F L A M E IN P O R O U S M E D I A
149
the normal decomposition flame of hydrazine produces no surface distortions. When V0 > 0.8 m m / s this flame is turbulent and involves intense boiling and dispersion of liquid, with intensified motion of the liquid and gaseous masses. The decomposition zone includes the extended regions, in which both boiling and decomposition take place. A transient decomposition zone lies within the above range of V0 values. In the porous medium, the hydrodynamic, thermal, and chemical wave structures are much more complex. Experiments on the porous medium of cylindrical rods revealed a few characteristic zones of abnormally wide gas-liquid heating (Fig. 7), the formation mechanism of which is thought to be related to the action of capillary forces. Porous media may be represented as a polycapillary body with a system of connected narrow and wide channels. The former act as a wick, transporting liquid to the evaporation and decomposition zones. The level of the liquid in wide channels falls, not due to liquid evaporation under heating, but due to the transport of liquid, first to narrow channels and then to the evaporation and decomposition zones. T h e r m o c o u p l e and visual m e a s u r e m e n t s show the length of the gas-liquid zone to correspond to that of the preheat one, i.e., to a smooth t e m p e r a t u r e increase prior to the drastic rise in the luminescence zone. Estimates show that such an extended thermal zone cannot be attributed to the thermal conductivity. A m o r e efficient mechanism of longitudinal heat transfer is required, such as the evaporation-condensation mechanism in heat pipes. Passing from a " v a p o r " zone to that with a lower temperature, the hydrazine vapors are condensed. The condensate returns to the evaporation zone in a liquid flow. The condensation-evaporation process results in the formation of a droplet zone. Two important conclusions follow from these observations. The flow in the preheat zone is not one-dimensional, which allows a hot-spot localization of the chemical reaction in the decomposition wave. Secondly, the decomposition process induces internal capillary filtration with a velocity that varies within the capillary
zone width, and may take place both with external filtrational flow and also without it, when V0 = 0. Figures 2 and 5 demonstrate that the flow velocity is an important system p a r a m e t e r determining the burning velocity. In order to estimate this influence let us introduce the concept of the burning velocity sensitivity to the changes in flow velocity, K = d S u / d V o. As can be seen from Fig. 9, the value of K is positive, variable and decreases with increase in V0. This change in K determines the shape of the U(V o) curve. The formula S, - V0 - U yields d U / d V o = 1 - d S , / d V o and it is seen that, at the minimum of the dependence U(V o) K = 1. When K > 1, the wave velocity increases (in its absolute value) with V0. Note that for filtrational combustion of gases K - 1 [12, 13]. U p o n decomposition of N z H 4 in narrow tubes K < 1 [20]. U n d e r the present experimental conditions of range of K variations is wider (0.5-1.9). Since the flow velocity influences the thermal interaction between the carcass and reagent, the high values of K also testify to the high activity of the porous medium in the combustion process. In this context the term "inert m e d i u m " loses its meaning. CONCLUSION The decomposition flame of hydrazine in inert porous media, involving gas-phase chemical reaction, has a n u m b e r of properties in c o m m o n with other types of filtrational combustion. These are the dependence of wave velocity on
0
-0,05
I
I
0
0,05
I
O.i Vo , mrn/s
Fig. 9. Dependence of K on the velocity V0. 1--D = 27.3 mm; d = 1.0 mm; e = 0.33, 2--D = 27.3 ram; f = 2.0 + 2.5 mm; e = 0.5.
150 the filtrational velocity, the dependence of equilibrium flame temperature on wave velocity, the probability of the wave reversing, and heat recirculation in the decomposition zone. However, the decomposition flame of hydrazine in inert porous media shows some qualitative and quantitative peculiarities, determined by the specific properties of the liquid. The are especially explicit in the decomposition wave structure with its abnormally wide preheat zone, involving complex spatial distribution of reaction medium phases and temperatures, as well as non-one-dimensionality of hydrodynamic flows that involve internal capillary filtration. Consequently, it is necessary to study further the fine structure of decomposition waves, including chemical reaction localization. A study of decomposition flame of hydrazine in inert porous media as a category of filtrational combustion is beneficial to both fields and reveals their common principles. Data on filtrational combustion may be used in studies of other processes of heterogeneous combustion waves, which involve the liquid phase of the product, the initial or intermediate components of the reactants.
The authors wish to thank Prof. D. Bradley for helpful discussion and the Russian Foundation for Fundamental Research for their financial support of this research (Grant No 93-03-18508).
B. YU. KOSHKIN ET AL. 4. 5.
6.
7.
8.
9.
10.
11. 12.
13. 14.
15.
16. 17.
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Babkin V. S., Laevsky Yu. M., Combust. Explos. Shock Waves 23:531 (1987). Aldushin, A. P., and Merzhanov, A. G., Rasprostranenie Teplovykh Voln ~' Geterogennykh Sredakh (Yu. Sh. Matros, Ed.), s.9, Novosibirsk, 1988. Laevsky, Yu. M., and Babkin V. S., Rasprostranenie TeploLykh Voln L, Geterogennykh Sredakh (Yu. Sh. Matros, Ed.), s.93, Novosibirsk, 1988. Kisilev, O. V., Matros, Yu. Sh. and Chumakova, N. A., Rasprostranenie Teplotykh Voln c Geterogennykh Sredakh (Yu. Sh. Matros, Ed.), s.145, Novosibirsk, 1988. Koshkin, B. Yu., Bunev, V. A., Babkin, V. S., and Laevsky, Yu. M., DokladyAkad. Nauk SSSR 310:1395 (1990). Adams, G. K. and Stocks, G. W.: Fourth Symposium (International) on Combustion, The Combustion Institute, 1953, p. 239. Gray, P., Lee, J. C., Leach, H. A., and Taylor, D. C., Sixth Symposium (International) on Combustion, The Combustion Institute, 1957, p. 255. Antoine, A. C., Eighth Symposium (International) on Combustion, The Combustion Institute, 1961, p. 1057. Babkin, V. S., Drobyshevich, V. I., Laevsky, Yu. M., and Potytnyakov, S. I., Combust. Explos., Shock Waves 19:147 (1983). Potytnyakov, S. I., Laevsky, Yu. M., and Babkin, V. S., Combust. Explos., Shock Waves 20:15 (1984). Laevsky, Yu. M., Babkin, V. S., Drobyshevieh, V. I., and Potytnyakov, S. I., Combust. Explos., Shock Waves 20:591 (1984). Hildenbrand, D. L., and Whittaker, A. G., Fifth Symposium (International) on Combustion, The Combustion Institute, 1955, p. 212. Alecseev A. P., Manelis G. B., Combust., Explos., Shock Wal,es 16:405 (1980). Schmidt E. W., Hydrazine and Its Deriuatiues. Preparation, Properties, Applications. Wiley, New York, 1984. Landau, L. D.: Z. Eksp. Teoret. Fiz. 14:240 (1944). Levitch, V. C., Doklady Akad. Nauk SSSR 109:975 (1956). Babkin, V. S., Koshkin, B. Yu., and Laevsky, Yu. M,, Doklady Akad. Nauk SSSR 304:892 (1989).
Receit:ed 28 April 1994; ret,ised 26 March 1995