Energy & Fuels 2005, 19, 1473-1483
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Flame Entrainments Induced by a Turbulent Reacting Jet Using High-Temperature and Oxygen-Deficient Oxidizers Weihong Yang* and Wlodzimierz Blasiak Royal Institute of Technology, Division of Energy and Furnace Technology, S-100 44 Stockholm, Sweden Received September 15, 2004. Revised Manuscript Received April 9, 2005
The entrainments of a turbulent jet in co-flow under high-temperature and oxygen-deficient oxidizers have been numerically and theoretical studied. By describing the chemical flame reaction, the entrainment induced by a turbulent reacting jet flame is calculated along the entire chemical flame length by mean of a flame entrainment ratio. The results in the near field of a reacting jet are verified by comparison with the results of published measurements. The effects of preheat temperature, the oxygen concentration of the oxidizer, the heat release, and buoyancy on the entrainment rate are investigated. The following results were obtained: (1) The uniformity of the heat release in reacting jets has a strong effect on the flame entrainment: the more uniform the heat release, the greater the entrainment. The effect of heat release reduces the entrainment in the near field of the reacting jets with the same factor of the characteristic ratio, which is r ) (Tf/To)0.5. (2) The entrainment increases as the oxygen concentration is decreased. Furthermore, the entrainment is independent of the fuel flow rate and the preheat temperature of the oxidizer for the investigated temperature range (1073-1573 K). (3) The effect of the oxygen concentration and preheat temperature of the oxidizer on buoyancy was examined. A correction Richardson coordinate, which includes the effect of the oxygen concentration (stoichiometric ratio), was derived to describe the local influence of buoyancy force along the chemical flame length under the hightemperature and oxygen-deficient oxidizer conditions. It can be concluded that the buoyancy force increases with the reduction of the oxygen concentration in the oxidizer. (4) The global behavior of the entrainment was revealed. The entrainment of jet flames can be identified as two regimes: (i) the near field, where the entrainment coefficient is positive, and (ii) the far field, where the entrainment coefficient is negative. Corrections for the entrainment rates were derived in terms of a Froude number (Fr) for the momentum-buoyancy transition jet flame under the high-temperature and low-oxygen-concentration oxidizer conditions. Furthermore, the maximum entrainments along the flame length are estimated.
1. Introduction Combustion technology that combines high-temperature preheated air with suitable hot-combustionproduct recirculation has become increasingly attractive in industrial furnaces and boilers recently.1-3 This technology, which is called high-temperature air combustion (HiTAC),1,2 flameless oxidation (FLOX)3, or mild combustion,4 offers significantly increased energy efficiency, very low CO, CO2, and NOx emissions, and high quality of the product at an increased production rate. * Author to whom correspondence should be addressed. Telephone: 0046-8-790 8402. Fax: 0046-8-207681. E-mail address: weihong@ mse.kth.se. (1) Hasegawa, T.; Mochida, S.; Gupta, A. K. Development of advanced industrial furnace using highly preheated air combustion. J. Propul. Power 2002, 18 (2), 233-239. (2) Tsuji, H.; Gupta, A. K.; Hasegawa, T.; Katsuki, M.; Kishimoto, K.; Morita, M. High-Temperature Air Combustion: From Energy Conservation to Pollution Reduction; CRC Press: Boca Raton, FL, 2003. (3) Wu¨nning, J. A.; Wu¨nning, J. G. Flameless oxidation to reduce thermal NO-formation. Prog. Energy Combust. Sci. 1997, 23 (12), 8194. (4) Cavaliere, A.; de Joannon, M. Mild Combustion. Prog. Energy Combust. Sci. 2004, 30 (4), 329-366.
This new combustion proceeds in an atmosphere of low oxygen concentration, as well as at high temperatures of the oxidizer, mostly above the auto-ignition temperature of the fuel. The technology has been studied in the past decade.1-10 A comprehensive view of these works is provided, in regard to the fundamental (5) Gupta, A. K.; Bolz, S.; Hasegawa, T. Effect of air preheat temperature and oxygen concentration on flame structure and emission. J. Energy Resour. Technol. 1999, 121, 209-216. (6) Lille, S.; Blasiak, W.; Jewartowski, M. Experimental Study of the Fuel Jet Combustion in High Temperature and Low Oxygen Content Exhaust Gases. Energy 2005, 30, 373-384. (7) Yang, W.; Blasiak, W. Numerical study of fuel temperature influence on single gas jet combustion in highly preheated and oxygen deficient air. Energy 2005, 30 (2-4), 385-398. (8) Yang, W.; Blasiak, W. Chemical flame length and volume in liquified propane gas combustion using high-temperature and lowoxygen-concentration oxidizer. Energy Fuels 2004, 18, 1329-1335. (9) Yuan, J.; Naruse, I. Effects of air dilution on highly preheated air combustion in a regenerative furnace. Energy Fuels 1999, 13, 99104. (10) Magnus, M.; Blasiak, W.; Gupta, A. K. Experimental Investigation of physical properties of a single fuel jet in cross-flow during highly preheated air combustion conditions. In Proceedings of High-Temperature Air CombustionsThe Quest for Zero Emissions in Industrial Furnaces, 23rd-24th October 2003, Stockholm, Sweden; pp 79-86. (ISBN 91-7283-662-8.)
10.1021/ef049763o CCC: $30.25 © 2005 American Chemical Society Published on Web 05/11/2005
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differences in the thermal, chemical, and fluid dynamics characteristics of the flame. It has been proven that this novel technology provides significantly higher flame stability at all fuel-air mixtures (including very lean mixtures), higher heat transfer, low heat loss from the stack (waste heat), and uniform temperature distributions. The entrainment of the jets is the key technique solution for the industrial applications of this novel combustion technology. Jet entrainment is the radial inward flux of ambient fluid drawn into a jet. The entrainment into a turbulent jet was investigated in several earlier studies.11-25 For the free nonreacting turbulent jets, Ricou and Spalding11 gave the general, well-known expression me/m0 ) Ce(F∞/F0)0.5x/d0, where Ce is the entrainment coefficient (Ce ) 0.32). In the reacting jets, the entrainment behavior is less straightforward, because of heat release and buoyancy. Important quantitative measurements and correlations of the entrainment rate based on this equation induced by turbulent gas diffusion flames have been further investigated by other researchers.12-24 For example, the constant Ce has been corrected for the cases of reacting jets that are described in refs 13 and 17. To obtain a general expression of the entrainment ratio for reacting jets, efforts have been made in refs 17, 19, and 20. The influences of the heat release and/or the buoyancy force on the jet entrainment rate were also investigated.12-24 However, little knowledge is available about the influences of preheat temperature and oxygen concentration on the jet-flame entrainment rate. When combustion occurs under the conditions of high temperature and an oxygen-deficient oxidizer, reaction zones are relatively widely distributed to yield a somewhat widespread and mild heat release, and, hence, a uniform temperature distribution. This also implies that the buoyancy force is decreased. The effects of the mild heat (11) Ricou, F. P.; Spalding, D. B. J. Fluid Mech. 1961, 11, 21-32. (12) Hill, B. J. J. Fluid Mech. 1972, 51, 773-779. (13) Becker, H. A.; Yamazaki, S. Entrainment, momentum flux and temperature in vertical free turbulent diffusion flames. Combust. Flame 1978, 33, 123-149. (14) Zukoski, E. E.; Kubota, T.; Cetegen, B. Entrainment in fire plumes. Fire Saf. J. 1980, 3, 107-121. (15) Cetegen, B. M.; Zukoski, E. E.; Kubota, T. Entrainment in the near field of fire plumes. Combust. Sci. Technol. 1985, 39, 305-331. (16) Delichatsios, M. A.; Orloff, L. Entrainment measurements in turbulent buoyant jet flames and implications for modelling. In Twentieth Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1984; pp 367-375. (17) Han, D.; Mungal, M. G. Direct measurement of entrainment in reacting/nonreacting turbulent jets. Combust. Flame 2001, 124, 370-386. (18) Delichatsios, M. A. Air entrainment into buoyant jet flames and pool fires. Combust. Flame 1987, 70, 33-46. (19) Blake, T. R.; Cote, J. B. Mass entrainment, momentum flux, and length of buoyant turbulent gas diffusion flame. Combust. Flame 1999, 117, 589-599. (20) Muniz, L.; Mungal, M. G. Effects of heat release and buoyancy on flow structure and entrainments in turbulent nonpremixed flames. Combust. Flame 2001, 126, 1402-1420. (21) Chigier, N. A.; Strokin, V. Mixing processes in a fire turbulent diffusion flame. Combust. Sci. Technol. 1974, 9, 111-118. (22) Takagi, T.; Shin, H. D.; Ishio, A. Properties of turbulence in turbulent diffusion flames. Combust. Flame 1981, 40, 121-140. (23) Westerweel, D.; Dabiri; Gharib, M. The effect of a discrete window offset on the accuracy of cross-correlation analysis of digital PIV recordings. Exp. Fluids 1997, 23, 20-28. (24) Clemens, N. T.; Paul, P. H. Effects of heat release on the near field flow structure of hydrogen jet diffusion flames. Combust. Flame 1995, 102, 271-284. (25) Han, D.; Mungal, M. G. Simultaneous measurements of velocity and CH distribution. Part 1: jet flames in co-flow. Combust. Flame 2003, 132, 565-590.
Yang and Blasiak
release and less buoyancy force on the entrainment could be of fundamental interest. The entrainment rate only affects the combustion when the ambient gases are entrained into the chemical reaction zone. Therefore, the study of entrainment into the chemical reaction zone is sufficient. However, the luminosity of this new combustion phenomenon differs from that observed in the traditional combustion.1-3,5,6,8,10 Studies1-3,5,6,8,10 of the global flame features of this new combustion phenomenon showed that the flame color changed from yellow to blue to bluish-green to green, and, in some cases, hybrid and purple-colored flames were also observed. Under certain conditions, flameless or colorless oxidization of the fuel has been demonstrated. Therefore, it is necessary to determine the flame (chemical reaction zone), for example, using numerical simulations. In the work of Yang and Blasiak,7,8 a “chemical” flame was used to describe this less-luminous (or invisible) chemical reaction zone, which is coincident with the internal zone in the outside border flame. The flame’s outside border is determined according to the flammability limits of the combustible gases in the combustion chamber, in terms of the oxidation mixture ratio. The length and volume of the “chemical” flame zone are used to describe the physical properties of the chemical reaction zone in the combustion chamber, which have been systematically studied in the previous work.8 In the context of this paper, the “flame” means a chemical flame zone. In this work, numerical studies were performed to understand the entrainment into a “chemical flame zone” induced by a turbulent jet flame using the hightemperature and oxygen-deficient oxidizer. The combustion with a high-temperature (above the fuel’s autoignition temperature) and oxygen-deficient atmosphere is called HiTOD combustion hereinafter, because those two parameters are unique characteristics that differ from any other combustion. A flame entrainment rate is proposed to describe the entrainment induced by a jet flame. The effect of the preheat temperature and the oxygen concentration in the oxidizer, heat release, and buoyancy on the entrainment rate is investigated. A correction Richardson coordinate (denoted as Ris), where the effect of the oxygen concentration (stoichiometric ratio) is included, was derived to describe the local influence of buoyancy force along the chemical flame length under the HiTOD conditions. The global behavior of the entrainment was revealed. Corrections of entrainment rates were derived in terms of a flame Froude number (Frf) for the momentum-buoyancy transition jet flame under the HiTOD conditions. 2. Experimental Method and Numerical Modeling The subject of the experimental and numerical modeling was a single jet flame of liquefied propane gas (LPG) in a coflow of high-temperature flue gas, as shown in Figure 1. The main LPG components are as follows (in terms of mass %): CH4, 0.02; C2H6, 0.95; C3H8, 98.35; and C4H10, 0.67. In this work, real flue gases were used as an oxidizer, instead of using a mixture of air and inert gas, as studied in other works.1,2,5,10 A flue gas generator fired with LPG based on a conventional burner (normal firing rate, 25 kW) was used to create an oxidizer with controlled temperatures and oxygen concentra-
Flame Entrainments Induced by a Reacting Jet
Energy & Fuels, Vol. 19, No. 4, 2005 1475 In the original model of Magnussen and Hjertager,28 the kinetic rates are assumed to be infinitely fast, so that turbulent mixing is guaranteed to be the controlling rate. Mathematically, these statements translate to the following equation:
REBU(kg m-3 s-1) ) -
[
]
mO m P F A min mF, ,B k sO sP
(2)
where sO ) nOMO/(nFMF) and sP ) nFMF. The fuel kinetically controlled reaction rate (RKIN) is defined as follows:
RKIN(kg m3 s-1) ) CMFTβ
∏ all j
Figure 1. Schematic of combustion chamber design and its computational domain. tions. The fuel nozzle was positioned coaxially to the main flow of the hot flue gas and also was fed with LPG. Only numerical studies are presented here. Other details of the experimental setup and results of experimental studies can be found in the literature.6,8 Variables chosen for numerical and theoretical studies were as follows: flow rate and temperature of fuel, oxygen concentration and temperature of the oxidizer, and diameter of the fuel nozzle (Table 1). The compositions of the oxidizer (O2, CO2, H2O, and N2) were obtained according to the level of oxygen based on the chemical balance of the conventional burner that was used to produce the oxidizer. Studied temperatures of the oxidizer are >1000 K, which is higher than the auto-ignition temperature of most of fossil fuels, and it is in the range of the HiTOD conditions. The co-flow speed is ∼4% of the jet speed used for most of studied cases. In addition, because, for all studied cases, this velocity was kept as constant, its influence was thus ignored. In addition, the axial distance, x, is counted from the virtual origin from the nozzle. There is no need for a pilot flame to stabilize the flame under HiTOD conditions. This is very important for the studies of the effect of the heat release on the entrainment. Although the mass flow rate of the pilot is very small, the heat release from the pilot occupies a large portion if, for example, the hydrogen is used as the pilot fuel. The heat-flux ratio between the jet and the pilot is only 22%, even the pilot-to-jet mass flux ratio is only 25 000 10 000 37 500
[O2] (mass %)
flame present?
5 10.2 13.4 16.9 23.2 23.2 23.2 23.2 23.2 23.2
yes yes yes yes yes no yes yes yes yes
uF (m/s)
u0 (m/s)
Lf/dF
Ce
25 25 25 25 25
0.98 0.98 0.98 0.98 0.98 0 0.45 2.0
1250 800 700 625 525
0.28 0.20 0.17 0.15 0.14 0.32 0.12a 0.076a 0.16b 0.13[1 - e-0.036(r-1)]
36 134
100 152
a The effect of instantaneous density and velocity fluctuations is neglected. C could be underestimated by ∼20%. b For the entrainment e coefficient at small ξ.
value of Ce ) 0.13 for a reacting free jet. A summary of the results from the present study and previous reported work are listed in Table 2. One major difference in the present work and other reported results is the conditions of the oxidizer. Here, a high-temperature and oxygen-deficient oxidizer was used. Another difference between the present work and other results is the definition of the entrainment: in this work, flame entrainment is used, whereas, in other works, jet entrainment is used. It can be observed that the entrainment rate for the studied cases are in the range of 0.14 e Ce e 0.28. They are smaller than those of nonreacting jets, but larger than those of reacting jets, under normal combustion conditions. It is been known from previous studies13,17,20 that heat release in reacting jets reduces the entrainment. This means that the entrainment coefficient for the reacting jets should be 82 for case 4. These values for cases 12 and
Flame Entrainments Induced by a Reacting Jet
Energy & Fuels, Vol. 19, No. 4, 2005 1481
Figure 7. Dimensionless entrainment rates versus the ratio of axial distance from the nozzle tip to flame height.
13 are 125 and 200, respectively. It is obvious that a larger initial momenta maintains a longer momentumcontrol regime and the Ris value decreases as a factor of u2o . Also, Figure 6 shows that a larger initial momentum causes a longer flame length exposed in the momentum-control regime. For case 13, ∼80% flame length is located in the monument-control regime. The values for cases 12 and 4 are 45% and 32%, respectively. 3.5. Global Field Behavior of the Entrainment. We have discussed, in detail, the entrainment behavior in the near field. In this section, the global field behavior of the entrainment is further discussed. The choice was made to present the data as a function of the axial distance (x) normalized by the flame height (Lf), because the general relationships of the flame length have been well-studied. Here, Lf is the chemical flame length, which can be obtained in Yang and Blasiak.8 Another dimensionless parameter (m*) is adopted; it is an expression of an entrainment rate normalized by the stoichiometric requirements:
me
m* )
(S + 1)mF
me (S + 1)mF
L* ) 11 L* )
m0 mF
()
x ) 1.77 Lf (S + 1)mF
(26)
Here,
(22)
stoic
(for x/Lf e 0.7)
(for Frf < 3)
(1 + 0.07Fr2f )0.2
(25)
(21)
Figure 7 shows the relationship of this dimensionless parameter (m*) and the parameter (x/Lflame). To observe the various trends clearly, the logarithmic scale of the distance axial is used. It can be observed that all the maximum normalized entrainment positions can be collapsed to a single point; this occurs at a location of x/Lf = 0.7. On the basis of this point, two regimes can be identified: a near field and a far field. Figure 7 shows that the mass entrainment of the near field can be expressed as
me
(for Frf g 3)
8.22Fr0.4 f
L* )
( )
)
Obviously, the entrainment coefficient is positive in the near field and is negative in the far field. It is possible to use the available flame length’s relationship to further simply the aforementioned relationships. In this work, the relationship of the flame length by means of the flame Froude number (Frf) under the HiTOD conditions obtained in ref 8 is used. Only the entrainment in the near field (x/Lf < 0.7) is discussed in this section. According to the results from Yang and Blasiak,8 the flame length under the HiTOD conditions can be decided by the Frf number, as follows:
where
S)
(
x x ) 3.67 - 3.31 ) 3.31 1.11 Lf Lf (for x/Lf > 0.7) (24)
(23)
and the mass entrainment of the far field can be stated as
Lffs dF(FF/Fo)
1/2
)
Lf d*(S + 1)
(27)
and
Frf )
uFf 3/2 s (FF/F∞)1/4[(∆Tf/T∞)gdF]1/2
fs )
1 (m0/mF)stoic + 1
(28) (29)
A simplification of the entrainment in the near field can be obtained by substituting eqs 25-29 into eq 23. They can be expressed as follows:
me x ) 0.16 mF d*
( )
(for Frf g 3)
me (1 + 0.07Fr2f )0.2 x ) mF d* 4.64Fr0.4 f
( )
(30)
(for Frf < 3) (31)
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Yang and Blasiak
Equation 30 shows that the entrainment coefficient is Ce ) 0.16 for a momentum-driven reacting jet (Frf g 3), which is comparable to the data reported by Becker and Yamazaki.13 These relationships can also be used to compare the entrainment coefficients that were obtained in the previous part in this paper. For example, cases 12 and 13 belong to the momentum-dominated regime (see Table 1), because of their Frf numbers are >3. The entrainment coefficient of these two cases then is Ce ) 0.16, according to eq 30. This value is only 6% larger than that obtained in previous data (Ce ) 0.15). A good agreement can be obtained. It is also very interesting to calculate the maximum mass entrainment, which can determine the optimal design of the furnace and the location of the burners, as discussed in ref 27. If we put a value of x/Lf ) 0.7 into eq 23, then the maximum entrainment can be estimated as
me ) 1.24(S + 1) mF
(32)
Comparison of the maximum entrainments calculated by eq 32 and the results presented in the previous part gives very encouraging results. 4. Conclusions The influence of combustion using a high-temperature and oxygen-deficient oxidizer on the flame entrainment induced by a turbulent reacting jet are investigated numerically and theoretically. A flame entrainment ratio is proposed. The results can be summarized as follows: (1) The uniformity of the heat release in the reacting jets has a strong effect on the flame entrainment. The greater the uniformity of the heat release, the larger the entrainment. The effect of heat release reduces the entrainment in the near field of the reacting jets with the same factor of the characteristic ratio (Tf/T0)0.5. (2) The entrainment increases as the oxygen concentration decreases. Furthermore, the entrainment is independent of the fuel flow rate and the preheat temperature of the oxidizer for the investigated temperature range (1073-1573 K). (3) The effect of the oxygen concentration and preheat temperature of the oxidizer on buoyancy was examined. A correction Richardson coordinate, ξHiTOD ≡ cHiTODRi1/3 o (x/d*), where the effect of the oxygen concentration (stoichiometric ratio) is included, was derived to describe the local influence of buoyancy along the axial distance from the nozzle under the HiTOD conditions. It can be concluded that the buoyancy force increases with the reduction of the oxygen concentration in the oxidizer. (4) The global behavior of the entrainment was revealed. Two regimes for the entrainment have been identified in jet flames: (i) the near field, where entrainment rates are positive, and (ii) the far field, where entrainment rates are negative. Corrections of the entrainment rates were derived in terms of a Froude flame number (Frf) for momentum-buoyancy transition
jet flame under the HiTOD conditions. Furthermore, the maximum entrainments along the flame length are estimated. Nomenclature u j ) average velocity (m/s) ) turbulent kinetic energy dissipation (m2/s3) [O2] ) oxygen concentration (%) Π(xj) ) denotes the product of all xj β ) temperature exponent of chemical reaction ∆Tf ) characteristic temperature increase resulting from combustion (K) A ) empirical coefficient; A ) 4 B ) empirical coefficient; B ) 0.5 C ) pre-exponential factor of chemical reaction Ce ) entrainment coefficient CHiTOD ) Richardson correction ratio for the HiTOD combustion Cp ) specific heat (kJ kg-1 K-1) D ) diameter (m) d*) equivalent diameter defined ds ) source diameter (m) Ea ) activation energy (kJ/mol) Fb ) the buoyancy force (N) Fm ) the initial inertia force of the fuel nozzle (N) Fr ) Froude number fs ) the stoichiometry g ) gravitational acceleration (m/s2) k ) turbulent kinetic energy (m2/s2) L ) flame length (m) L* ) dimensionless flame length M ) molecular weight (kg/mol) m ) mass fraction m* ) nondimension entrainment mass flux mo ) initial jet mass flux me ) the jet mass flux n ) stoichiometric coefficient (number of moles) R ) the universal gas constant (kJ kmol-1 K-1) r ) the density-weighted velocity ratio between the jet and co-flow; r ) [(FFu2F)/(Fou2o )]0.5 r ) radial distance to the flame centerline within the flame (m) Re ) Reynolds number REBU ) fuel consumption rate for eddy-break-up mode (kg m-3 s-1) Rent ) entrainment rate Rflame ) radial distance from the flame centerline to the flame board (m) RHiTOD ) rate of high-temperature and oxygen-deficient (HiTOD) combustion (kg m-3 s-1) Ri ) Richardson number RKIN ) Arrhenius reaction rates (kg m-3 s-1) Ro ) oxidation mixture ratio S ) stoichiometric air-to-fuel mass ratio T ) temperature (K) ui ) velocity in cell number i (m/s) vj ) species rate exponents x ) downstream distance from the virtual origin (m) δ ) jet width (m) ξ ) nondimensional streamwise coordinate ξ′ ) correction of the nondimensional streamwise coordinate ξHiTOD ) correction nondimensional streamwise coordinate for HiTOD combustion F ) density (kg/m3) Subscripts 0 ) initial value ∞ ) ambient a ) air ad ) adiabatic temperature
Flame Entrainments Induced by a Reacting Jet c ) species f ) flame F ) fuel Max ) maximum values O, o ) oxidizer
Energy & Fuels, Vol. 19, No. 4, 2005 1483 P ) product s ) source stoic ) stoichiometry EF049763O
