- Email: [email protected]
Flame velocity on the surface of volatile liquids
135
BRIEF COMMUNICATIONS
The burning-rate coefficient may be calculated by the equation [2]
8}; { 1+I [ Cp(Tr K=--In CpPL L
Q Yo ,a:]} h)+~
,
I
(I) where 5\and Cp are the mean thermal conductivity and specific heat respectively, PL is the density of liquid sulphur at the temperature of the droplet, L is the latent heat of vaporisation, Tf is the calculated flame temperature, TL the boiling point of sulphur, Q the heat of combustion, Yo,a: the mass fraction of oxygen in air (0.23) and i the stoichiometric coefficient. On this basis the calculated values of the burning-rate coefficients of droplets burning in air are 0.47 mrn? sec-I at 20°C and 0.52 at 300°C , and are in moderate agreemen t with the experimental values given in Fig. 2. This indicates that Eq . (1) could be used to calculate burning-rate coefficients in combustion chamber conditions. Thus unde r
typical plant conditions of 100% excess air and a reaction temperature of 1200° K the calculated burning rate coefficient is 0.47 mrrr' sec'": The cause of the curvature of the d 2 verses t plots in Fig. 1 probably arises from the relatively high boiling point of liquid sulphur and the consequent variation of PL and the surface and flame temperatures [3] with time . Measurements of the ratio of the radius of the outer edge of the flame to the droplet radius indicated that the value, which was 3.5 during the initial stages of combustion, did vary as combustion progressed indicating nonsteady-state burning. Further investigations are in progress to further elucidate the mechanism of combustion. References 1. Sulphur Manual, Texas Gulf Sulphur Company, New York (963) . 2. Williams, A. Oxidation and Combustion Reviews 3, 1 (1968). 3. Kotake, S. , and Okazaki, T., Int. J . Heat Mass. Transfer 12, 595 (1969).
(Received July, 19 72; revised version received August, 1972)
Flame Velocity on the Surface of Volatile Liquids ATSUSHI NAKAKUKI Fire Research Institute, Mitaka, Tokyo, Japan
Studies of the flame velocity on liquid surfaces have recently been reported by many workers. However, most of them treated the flame spread on the surface of a liquid at a temperature below its flash point, and few experiment al dat a have been reported on a liquid with its temperature higher than the flash point. The flame velocity generally increases with the liquid temperature and becomes a maximum at a specific liquid temperature 1~ which is a little higher than the flash point. At higher temperatures than Ts , conflicting results have been obtained with regard to the flame velocity: Kimbara [1] reported that it
decreases with rise in liquid temperature, Burgoyne and Roberts [2] that it is constant. There is a suggestion [3] that there will be essentially no change in the flame spreading velocity above Ts , because there is always some point above the liquid surface where a stoichiometric mixture exists . In this pape r. investigation of the cause of the difference of the flame spread tendency mentioned above are reported and a qualitative analysis of the flame spread is made. Two kinds of rectangular parallelepiped vessels of length SOD mm , width 40 mm, and depths 20 and 100 mm were used. Liquid fuels used were COMBUSTION AND FLAME 20, 135-137 (1973) Co py righ t © 1973 by The Combustion Instttute Publ ish ed by Ameri can Elsevier Publishing Company, Inc.
136
ATSUSHI NAKAKUKI
Fig. 1. The definition of h , hi, and hfz .
n-hexane, acetone, methanol , ethanol and n-propanol. The liquid in the vessel was ignited at one end by an electric spark and the flame spread was followed with a 16 mm motion picture camera. The film speeds were 64 and 200 frames per sec. Figure I shows how h, hs , and hf z are defined. The flame velocities of ethanol and zz-hexane at various liquid temperatures and distances h of the liqu id surface below the top of the vessel are shown in Fig. 2. The flame velocity of those liquids whose flash points are as high as o r higher than the room temperature, e.g. methanol, ethanol, and n-propanol, increases with the liquid temperature and approaches the maximum as in the case of ethanol in Fig. 2(a). It is seen from Fig. 2(a) that T, is approximately constant independent of the liquid depth. The higher the flash point, the larger was Ts . At large h, as the liquid temperature rises above T s , the flame velocity decreases with increasing liquid temperature and approaches a constant value, but at very small h, it is nearly constant above T s . The flame velocity of the liquid whose flash point is far lower than the room temperature, such as n-hexane and acetone, is approximately constant in the liquid temperature range near to room temperature, as shown for n-hexane in Fig. 2(b). However, at large h it tends to decrease a little with an increase of liquid temperature. In all liquids tested, the flame velocit y at large h was larger than that at small h as in Fig. 2 . Liebman, et al. [4], studied the flame spread at the boundary between an upper zone of flamm able gas and a lower zone of air. It is seen from Fig. 8 of Ref. 4 that in the rectangular parallelepiped with its base ope n the flame velocity at the interfacial layer between flammable gas and air increases with the flammable vapor zone thickness h f z and decreases with an increase of
distance hi of the composition corresponding to the lower flamm ability limit from the roof, and depends more upon hf z than upon h i- In the case of liquids , the flammable vapor is evaporated from the liquid and its concentration decreases with the distance upward from the liquid surface. In this case, therefore, hf z corresponds to the flammable vapor zone thickness above the liquid surface, and h 1 the distance of the composition corresponding to the lower flammability limit of the vapor from the liquid surface . Since h f z is inversely proportional to the concentration gradien t of flammable vapor and the concentration gradient decreases as h increases, h f z generally increases as h increases. At liquid temperatures lower than T s both h , and h f z increase as h increases, but the flame velocity increases not only because it depends more upon h f z than upon hi ' but also because the burning velocity of the mixture near the surface increases rapidly as the concentration rises from the lower flammability limit towards the stoichiometric value [1,2] . As the liqu id temperature rises above Ts , the concentration gradient of the vapor zone generally becomes steeper. The lower flammability limit of the vapor zone goes out of the vessel and h I becomes relatively constant so that the rate of increase of the height of the upper flammability limit becomes larger than that of hi, perhaps due to the rapid diffu sion of the flammable vapor in the open air and the dilution by the wind drawn into the flame . Then hf z and so the flame velocity decreases with an increase of liquid temperature. When the liquid temperature rises further , it is expe cted that a greater part of the flammable vapor zone goes out of the vessel, and hl z and hi will become approximately constant. When the liquid temperature is above T, at small h and very high at large h , the whole or a greater part of the
137
BRIEF COMMUNICATIONS
h(mm) • 3 095
o!---!=----:':--~-+----=--_:!=_-_+._
o
20
10
-00- -
-
~ -0•
•
•
0 •
10
--o;-n-"
-
••
60
0
-0
0 •
50
40
30
•
0
20
30 40 liquid temperature
50 'C
60
70
Fig. 2. Flame velocity on the liquid surface plotted against liquid temperature taking the distance h of the liquid surface below the top of the vessel as the parameter. Liquid fuel: (a) ethanol; (b) nhexane.• h = 3 mm; 0 h =95 mrn,
flammable vapor zone may go out of the vessel and therefore the flame velocity becomes constant and independent of the liquid temperature. In the case of liquids such as n-hexane, a greater part of the flammable vapor zone is outside of the vessel even at the room temperature, and the flame velocity becomes independent of liquid temperature in the range measured.
References 1. Kirnbara, T., Bull. Inst. Phys. Chern. Res. Japan 10, 37 (1931). 2. Burgoyne, J. H., and Roberts, A. F., Proc. Roy. Soc. (London) A308, S5 (1968). 3. Glassman, I., and Hansel, J. G., Fire Research Abstracts and Reviews 10,217 (1968). 4. Liebman, L., Corry, J., and Perlee, H. E., Comb. Sci. and Tech. 1,257 (1970).
BRIEF COMMUNICATIONS
The burning-rate coefficient may be calculated by the equation [2]
8}; { 1+I [ Cp(Tr K=--In CpPL L
Q Yo ,a:]} h)+~
,
I
(I) where 5\and Cp are the mean thermal conductivity and specific heat respectively, PL is the density of liquid sulphur at the temperature of the droplet, L is the latent heat of vaporisation, Tf is the calculated flame temperature, TL the boiling point of sulphur, Q the heat of combustion, Yo,a: the mass fraction of oxygen in air (0.23) and i the stoichiometric coefficient. On this basis the calculated values of the burning-rate coefficients of droplets burning in air are 0.47 mrn? sec-I at 20°C and 0.52 at 300°C , and are in moderate agreemen t with the experimental values given in Fig. 2. This indicates that Eq . (1) could be used to calculate burning-rate coefficients in combustion chamber conditions. Thus unde r
typical plant conditions of 100% excess air and a reaction temperature of 1200° K the calculated burning rate coefficient is 0.47 mrrr' sec'": The cause of the curvature of the d 2 verses t plots in Fig. 1 probably arises from the relatively high boiling point of liquid sulphur and the consequent variation of PL and the surface and flame temperatures [3] with time . Measurements of the ratio of the radius of the outer edge of the flame to the droplet radius indicated that the value, which was 3.5 during the initial stages of combustion, did vary as combustion progressed indicating nonsteady-state burning. Further investigations are in progress to further elucidate the mechanism of combustion. References 1. Sulphur Manual, Texas Gulf Sulphur Company, New York (963) . 2. Williams, A. Oxidation and Combustion Reviews 3, 1 (1968). 3. Kotake, S. , and Okazaki, T., Int. J . Heat Mass. Transfer 12, 595 (1969).
(Received July, 19 72; revised version received August, 1972)
Flame Velocity on the Surface of Volatile Liquids ATSUSHI NAKAKUKI Fire Research Institute, Mitaka, Tokyo, Japan
Studies of the flame velocity on liquid surfaces have recently been reported by many workers. However, most of them treated the flame spread on the surface of a liquid at a temperature below its flash point, and few experiment al dat a have been reported on a liquid with its temperature higher than the flash point. The flame velocity generally increases with the liquid temperature and becomes a maximum at a specific liquid temperature 1~ which is a little higher than the flash point. At higher temperatures than Ts , conflicting results have been obtained with regard to the flame velocity: Kimbara [1] reported that it
decreases with rise in liquid temperature, Burgoyne and Roberts [2] that it is constant. There is a suggestion [3] that there will be essentially no change in the flame spreading velocity above Ts , because there is always some point above the liquid surface where a stoichiometric mixture exists . In this pape r. investigation of the cause of the difference of the flame spread tendency mentioned above are reported and a qualitative analysis of the flame spread is made. Two kinds of rectangular parallelepiped vessels of length SOD mm , width 40 mm, and depths 20 and 100 mm were used. Liquid fuels used were COMBUSTION AND FLAME 20, 135-137 (1973) Co py righ t © 1973 by The Combustion Instttute Publ ish ed by Ameri can Elsevier Publishing Company, Inc.
136
ATSUSHI NAKAKUKI
Fig. 1. The definition of h , hi, and hfz .
n-hexane, acetone, methanol , ethanol and n-propanol. The liquid in the vessel was ignited at one end by an electric spark and the flame spread was followed with a 16 mm motion picture camera. The film speeds were 64 and 200 frames per sec. Figure I shows how h, hs , and hf z are defined. The flame velocities of ethanol and zz-hexane at various liquid temperatures and distances h of the liqu id surface below the top of the vessel are shown in Fig. 2. The flame velocity of those liquids whose flash points are as high as o r higher than the room temperature, e.g. methanol, ethanol, and n-propanol, increases with the liquid temperature and approaches the maximum as in the case of ethanol in Fig. 2(a). It is seen from Fig. 2(a) that T, is approximately constant independent of the liquid depth. The higher the flash point, the larger was Ts . At large h, as the liquid temperature rises above T s , the flame velocity decreases with increasing liquid temperature and approaches a constant value, but at very small h, it is nearly constant above T s . The flame velocity of the liquid whose flash point is far lower than the room temperature, such as n-hexane and acetone, is approximately constant in the liquid temperature range near to room temperature, as shown for n-hexane in Fig. 2(b). However, at large h it tends to decrease a little with an increase of liquid temperature. In all liquids tested, the flame velocit y at large h was larger than that at small h as in Fig. 2 . Liebman, et al. [4], studied the flame spread at the boundary between an upper zone of flamm able gas and a lower zone of air. It is seen from Fig. 8 of Ref. 4 that in the rectangular parallelepiped with its base ope n the flame velocity at the interfacial layer between flammable gas and air increases with the flammable vapor zone thickness h f z and decreases with an increase of
distance hi of the composition corresponding to the lower flamm ability limit from the roof, and depends more upon hf z than upon h i- In the case of liquids , the flammable vapor is evaporated from the liquid and its concentration decreases with the distance upward from the liquid surface. In this case, therefore, hf z corresponds to the flammable vapor zone thickness above the liquid surface, and h 1 the distance of the composition corresponding to the lower flammability limit of the vapor from the liquid surface . Since h f z is inversely proportional to the concentration gradien t of flammable vapor and the concentration gradient decreases as h increases, h f z generally increases as h increases. At liquid temperatures lower than T s both h , and h f z increase as h increases, but the flame velocity increases not only because it depends more upon h f z than upon hi ' but also because the burning velocity of the mixture near the surface increases rapidly as the concentration rises from the lower flammability limit towards the stoichiometric value [1,2] . As the liqu id temperature rises above Ts , the concentration gradient of the vapor zone generally becomes steeper. The lower flammability limit of the vapor zone goes out of the vessel and h I becomes relatively constant so that the rate of increase of the height of the upper flammability limit becomes larger than that of hi, perhaps due to the rapid diffu sion of the flammable vapor in the open air and the dilution by the wind drawn into the flame . Then hf z and so the flame velocity decreases with an increase of liquid temperature. When the liquid temperature rises further , it is expe cted that a greater part of the flammable vapor zone goes out of the vessel, and hl z and hi will become approximately constant. When the liquid temperature is above T, at small h and very high at large h , the whole or a greater part of the
137
BRIEF COMMUNICATIONS
h(mm) • 3 095
o!---!=----:':--~-+----=--_:!=_-_+._
o
20
10
-00- -
-
~ -0•
•
•
0 •
10
--o;-n-"
-
••
60
0
-0
0 •
50
40
30
•
0
20
30 40 liquid temperature
50 'C
60
70
Fig. 2. Flame velocity on the liquid surface plotted against liquid temperature taking the distance h of the liquid surface below the top of the vessel as the parameter. Liquid fuel: (a) ethanol; (b) nhexane.• h = 3 mm; 0 h =95 mrn,
flammable vapor zone may go out of the vessel and therefore the flame velocity becomes constant and independent of the liquid temperature. In the case of liquids such as n-hexane, a greater part of the flammable vapor zone is outside of the vessel even at the room temperature, and the flame velocity becomes independent of liquid temperature in the range measured.
References 1. Kirnbara, T., Bull. Inst. Phys. Chern. Res. Japan 10, 37 (1931). 2. Burgoyne, J. H., and Roberts, A. F., Proc. Roy. Soc. (London) A308, S5 (1968). 3. Glassman, I., and Hansel, J. G., Fire Research Abstracts and Reviews 10,217 (1968). 4. Liebman, L., Corry, J., and Perlee, H. E., Comb. Sci. and Tech. 1,257 (1970).