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Experimental investigation of gas combustion regimes in narrow tubes
Brief Communication Experimental Investigation of Gas Combustion Regimes in Narrow Tubes V. V. ZAMASHCHIKOV Institute of Chemical Kinetics and Combustion, Not,osibirsk, 630090, Russia
Weinberg [1] has proposed excess enthalpy combustion as a means of burning mixtures of low heat content. In a series of studies, Weinberg et al. have shown that this concept successfully extends ranges of flammability [2, 3]. Takeno and Sato [4] have proposed a simple way of producing an excess enthalpy flame by inserting a high-conductivity porous solid into the one-dimensional flame zone to recirculate heat internally, through the solid, from the downstream high-temperature region to the upstream low-temperature region. Works by Takeno et al. [4, 5] have shown that flames can be stabilized inside porous solids. In porous solids of finite length, two combustion states exist with distinct solid temperatures. De Soete [6] and Babkin [7] have investigated gaseous combustion inside a porous solid when the boundaries of this solid do not influence the flame, and the mean size of the pores is smaller than the quenching diameter. They found that the combustion wave can be steadily propagated along the porous solid. Experimental work by Chen and Churchill [8] has shown that premixed propane-air flames can be stabilized inside a refractory tube without a flameholder. The stabilization is wholly due to thermal feedback (recirculated heat) by wall-to-wall radiation inside the tube and longitudinal thermal conduction along the tube wall. In Ref. [9], a model has been developed for the prediction of the behaviour of stationary flames of premixed propane and air inside a refractory tube of finite length. The model suggests the existence of multiple stationary states and Bernstein and Churchill [10] have investigated the existence of such multiplicity experimentally. A number of stable stationary states was found in a 3/8-in. tube with
the flame fronts grouped in two regions, just as predicted. The objective of the current experimental investigation was to study the combustion regime, the same as that obtained by De Soete and Babkin, but in a single tube. The apparatus consisted of fuel- and air-supply systems, a mixing chamber, a horizontal combustion tube, and a movable electric heater placed under the tube. The methane-air mixtures from the mixing chamber passed through a pressure reducer, adjustable valve, and rotameter into the combustion tube. All experiments were at room temperature and atmospheric pressure. After the flame cone attains a stable position at the rim of the tube, the flame can be shifted inside the tube by heating the tube wall with a supplementary burner. If this burner is removed, the combustion wave can exist independently, even though the inner diameter is smaller than that for quenching. In a steel tube, the position of the combustion wave can be determined by the appearance of a narrow red stripe at the tube wall, and in a ceramic tube by the appearance of a narrow light-green stripe. However, the flame exists only for a limited time. The reason for flame-quenching is the condensation of water from the reaction products on the tube walls. Hence, that part of the tube in which the water condensed was heated by the movable heater. The temperature of the heated part of the tube ( < 100°C) was substantially lower than that of the part in which the combustion wave was located ( > 300°C). Tubes made of stainless steel, ceramic, and quartz were used, with lengths greater than 0.6 m. After the supplementary burner was removed, the flame velocity attained a constant
COMBUSTION A N D F L A M E 108:357-359 (1907)
Copyright © 1997 by The Combustion Institute Published by Elsevier Science Inc.
0010-2180/97/$17.00 PII $0010-2180(96)00169- I
358
V.V. ZAMASHCHIKOV
value at a distance less than 0.1 m for all combustible gas flow rates. Two variants may be distinguished, in which the inner tube diameter is either smaller or greater than the quenching diameter.
6
5 ¢-
4
E 3 E 2
2
O
1
v
INNER TUBE DIAMETER SMALLER THAN THE QUENCHING DIAMETER 0
The dependencies of flame propagation velocity relative to the tube wall U on gas flow rate Q are shown in Figs. 1 and 2. The positive values of U correspond to flame movement against the flow. Experimental points in the figures cover the whole regime of the combustion wave's existence; there is no combustion wave at either higher or lower flow rates. There is no combustion wave in the ceramic tube with methane mole proportions of either more than 11.5%, or less than 8%. INNER TUBE DIAMETER LARGER THAN THE QUENCHING DIAMETER It is found that in this tube, two regimes exist: a low-velocity (LVR) and a high-velocity regime (HVR). In the usual H V R (squares, Fig. 3), U = A V + S, where V is a mean velocity of combustible mixture relative to the tube wall; A is a coefficient that depends on the gas velocity profile across the cross section of the tube; S is the combustion wave velocity with V = 0. In the HVR, the main practical difficulty is the formation of a combustion wave with U > 0. In this case, the H V R is realized by the transition from the LVR, induced by cooling of the outer surface of the tube at the flame position. A straight line 1 (U = A V + S)
2
-1
.4
1.7
I
I
~
I
I
I
2.0
2.3
2.6
2.9
3,2
3.5
Q (crn~/sec) Fig. 2. Combustion wave velocity as a function of combustible gas flow rate for different tubes. 9.5% CH 4. 1, 4, 5: quartz tubes: 5.1 mm o.d./3.1 mm i.d., 4.9/2.7, 5.0/2.6; 2: ceramic tube: 5.0/2.6; 3: steel tube: 3.0/2.5.
passes through the experimental points only when U < 0. It is seen that the points obtained for U > 0 fail to lie on this line. This is expected, as the flame propagates through that part of the tube heated by the electric heater. In the U < 0 region, H V R goes to LVR with increasing V in the vicinity of U = 0 (from curve 1 to curve 2). With U > 0, the flame propagates in the part of the tube previously containing combustion products. Since these combustion products have transferred a part of their heat to the wall, the flame propagates through the more heated gas. This is sure to lead to the fact that in HVR, as U decreases, one will first observe a deviation from the straight line U = A V + S and then, at fairly low values of U, will see a transition to LVR (dashed line). The author wishes to thank V. S. Babkin, V. A. Bunev, and A. A. Korzhavin for helpful discussions.
2 4
t-
E E v
o
14
16
18
20
22
Q (cm3/sec)
24
26
1.4 1.6 1.8 2.0 2.2 2.4 2.6
Q (cm'/sec)
Fig. 1. Combustion wave velocity as a function of combustible gas flow rate for different m e t h a n e - a i r flames. Ceramic tube: o.d.: 5.0 ram, i.d.: 2.6 mm. 1: 9.5% CH4; 2: 9.0% CH4; 3: 8.5% CH4, 4: 10.0% CH4; 5: 10.5% CH4; 6: 11.0% CH 4.
INVESTIGATION OF COMBUSTION REGIMES IN TUBES
359
50 40 30 20
//
E
2 a
A
A
p
20/1~ -10
40
60
80
I
100
V (era/see)
-20 Fig. 3. Combustion wave velocity as a function of the mean velocity of combustible gas. 9.5% CH 4. Quartz tube: o.d.: 7.3 mm, i.d.: 4.9 ram. 1: HVR; 2: LVR.
-30
-40 REFERENCES Wcinberg, F. J., Nature 233:239 (1971). Wcinberg, F. J., Fifteenth Symposium (International) , n Combustion, The Combustion Institute, Pittsburgh, 1975. 3. I.Ioyd, S. A., and Weinberg, F. J., Combust. Flame 27:391 (1976). 4. ['akeno, T., and Sato, K., Combust. Sci. Tech. 20:73 (1979). 5. Takcno, T., Sato, K., and Hase, K., Eighteenth Sympo.~ium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, pp. 465 472. I. 2.
6.
7. 8. 9. Ill.
De Soete, G., Elet,enth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1966, pp. 959-966. Babkin, V. S., PureAppl. Chem., 65:335-344 (1993). Chen, J. L.-P., and Churchill, S. W., Combust. [`'lame 18:37-42 (1972). Chcn, J. L.-P., and Churchill, S. W.. Combust. [''lame 18:27-36 (1972). Bernstein, M. H., and Churchill, S. W., Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1976, pp. 1737-1745.
Weinberg [1] has proposed excess enthalpy combustion as a means of burning mixtures of low heat content. In a series of studies, Weinberg et al. have shown that this concept successfully extends ranges of flammability [2, 3]. Takeno and Sato [4] have proposed a simple way of producing an excess enthalpy flame by inserting a high-conductivity porous solid into the one-dimensional flame zone to recirculate heat internally, through the solid, from the downstream high-temperature region to the upstream low-temperature region. Works by Takeno et al. [4, 5] have shown that flames can be stabilized inside porous solids. In porous solids of finite length, two combustion states exist with distinct solid temperatures. De Soete [6] and Babkin [7] have investigated gaseous combustion inside a porous solid when the boundaries of this solid do not influence the flame, and the mean size of the pores is smaller than the quenching diameter. They found that the combustion wave can be steadily propagated along the porous solid. Experimental work by Chen and Churchill [8] has shown that premixed propane-air flames can be stabilized inside a refractory tube without a flameholder. The stabilization is wholly due to thermal feedback (recirculated heat) by wall-to-wall radiation inside the tube and longitudinal thermal conduction along the tube wall. In Ref. [9], a model has been developed for the prediction of the behaviour of stationary flames of premixed propane and air inside a refractory tube of finite length. The model suggests the existence of multiple stationary states and Bernstein and Churchill [10] have investigated the existence of such multiplicity experimentally. A number of stable stationary states was found in a 3/8-in. tube with
the flame fronts grouped in two regions, just as predicted. The objective of the current experimental investigation was to study the combustion regime, the same as that obtained by De Soete and Babkin, but in a single tube. The apparatus consisted of fuel- and air-supply systems, a mixing chamber, a horizontal combustion tube, and a movable electric heater placed under the tube. The methane-air mixtures from the mixing chamber passed through a pressure reducer, adjustable valve, and rotameter into the combustion tube. All experiments were at room temperature and atmospheric pressure. After the flame cone attains a stable position at the rim of the tube, the flame can be shifted inside the tube by heating the tube wall with a supplementary burner. If this burner is removed, the combustion wave can exist independently, even though the inner diameter is smaller than that for quenching. In a steel tube, the position of the combustion wave can be determined by the appearance of a narrow red stripe at the tube wall, and in a ceramic tube by the appearance of a narrow light-green stripe. However, the flame exists only for a limited time. The reason for flame-quenching is the condensation of water from the reaction products on the tube walls. Hence, that part of the tube in which the water condensed was heated by the movable heater. The temperature of the heated part of the tube ( < 100°C) was substantially lower than that of the part in which the combustion wave was located ( > 300°C). Tubes made of stainless steel, ceramic, and quartz were used, with lengths greater than 0.6 m. After the supplementary burner was removed, the flame velocity attained a constant
COMBUSTION A N D F L A M E 108:357-359 (1907)
Copyright © 1997 by The Combustion Institute Published by Elsevier Science Inc.
0010-2180/97/$17.00 PII $0010-2180(96)00169- I
358
V.V. ZAMASHCHIKOV
value at a distance less than 0.1 m for all combustible gas flow rates. Two variants may be distinguished, in which the inner tube diameter is either smaller or greater than the quenching diameter.
6
5 ¢-
4
E 3 E 2
2
O
1
v
INNER TUBE DIAMETER SMALLER THAN THE QUENCHING DIAMETER 0
The dependencies of flame propagation velocity relative to the tube wall U on gas flow rate Q are shown in Figs. 1 and 2. The positive values of U correspond to flame movement against the flow. Experimental points in the figures cover the whole regime of the combustion wave's existence; there is no combustion wave at either higher or lower flow rates. There is no combustion wave in the ceramic tube with methane mole proportions of either more than 11.5%, or less than 8%. INNER TUBE DIAMETER LARGER THAN THE QUENCHING DIAMETER It is found that in this tube, two regimes exist: a low-velocity (LVR) and a high-velocity regime (HVR). In the usual H V R (squares, Fig. 3), U = A V + S, where V is a mean velocity of combustible mixture relative to the tube wall; A is a coefficient that depends on the gas velocity profile across the cross section of the tube; S is the combustion wave velocity with V = 0. In the HVR, the main practical difficulty is the formation of a combustion wave with U > 0. In this case, the H V R is realized by the transition from the LVR, induced by cooling of the outer surface of the tube at the flame position. A straight line 1 (U = A V + S)
2
-1
.4
1.7
I
I
~
I
I
I
2.0
2.3
2.6
2.9
3,2
3.5
Q (crn~/sec) Fig. 2. Combustion wave velocity as a function of combustible gas flow rate for different tubes. 9.5% CH 4. 1, 4, 5: quartz tubes: 5.1 mm o.d./3.1 mm i.d., 4.9/2.7, 5.0/2.6; 2: ceramic tube: 5.0/2.6; 3: steel tube: 3.0/2.5.
passes through the experimental points only when U < 0. It is seen that the points obtained for U > 0 fail to lie on this line. This is expected, as the flame propagates through that part of the tube heated by the electric heater. In the U < 0 region, H V R goes to LVR with increasing V in the vicinity of U = 0 (from curve 1 to curve 2). With U > 0, the flame propagates in the part of the tube previously containing combustion products. Since these combustion products have transferred a part of their heat to the wall, the flame propagates through the more heated gas. This is sure to lead to the fact that in HVR, as U decreases, one will first observe a deviation from the straight line U = A V + S and then, at fairly low values of U, will see a transition to LVR (dashed line). The author wishes to thank V. S. Babkin, V. A. Bunev, and A. A. Korzhavin for helpful discussions.
2 4
t-
E E v
o
14
16
18
20
22
Q (cm3/sec)
24
26
1.4 1.6 1.8 2.0 2.2 2.4 2.6
Q (cm'/sec)
Fig. 1. Combustion wave velocity as a function of combustible gas flow rate for different m e t h a n e - a i r flames. Ceramic tube: o.d.: 5.0 ram, i.d.: 2.6 mm. 1: 9.5% CH4; 2: 9.0% CH4; 3: 8.5% CH4, 4: 10.0% CH4; 5: 10.5% CH4; 6: 11.0% CH 4.
INVESTIGATION OF COMBUSTION REGIMES IN TUBES
359
50 40 30 20
//
E
2 a
A
A
p
20/1~ -10
40
60
80
I
100
V (era/see)
-20 Fig. 3. Combustion wave velocity as a function of the mean velocity of combustible gas. 9.5% CH 4. Quartz tube: o.d.: 7.3 mm, i.d.: 4.9 ram. 1: HVR; 2: LVR.
-30
-40 REFERENCES Wcinberg, F. J., Nature 233:239 (1971). Wcinberg, F. J., Fifteenth Symposium (International) , n Combustion, The Combustion Institute, Pittsburgh, 1975. 3. I.Ioyd, S. A., and Weinberg, F. J., Combust. Flame 27:391 (1976). 4. ['akeno, T., and Sato, K., Combust. Sci. Tech. 20:73 (1979). 5. Takcno, T., Sato, K., and Hase, K., Eighteenth Sympo.~ium (International) on Combustion, The Combustion Institute, Pittsburgh, 1981, pp. 465 472. I. 2.
6.
7. 8. 9. Ill.
De Soete, G., Elet,enth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1966, pp. 959-966. Babkin, V. S., PureAppl. Chem., 65:335-344 (1993). Chen, J. L.-P., and Churchill, S. W., Combust. [`'lame 18:37-42 (1972). Chcn, J. L.-P., and Churchill, S. W.. Combust. [''lame 18:27-36 (1972). Bernstein, M. H., and Churchill, S. W., Sixteenth Symposium (International) on Combustion, The Combustion Institute, Pittsburgh, 1976, pp. 1737-1745.