Combustion Regimes of a Jet Diffusion Flame in Hot Co-flow

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Combustion Regimes of a Jet Diffusion Flame in Hot Coflow Feifei Wang, Jianchun Mi, and Pengfei Li Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef400500w • Publication Date (Web): 01 May 2013 Downloaded from http://pubs.acs.org on May 8, 2013

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Combustion Regimes of a Jet Diffusion Flame in Hot Coflow F. Wang, J. Mi*, and P. Li State Key Laboratory of Turbulence and Complex Systems, College of Engineering, Peking University, Beijing 100871, China *Tel: +86 10 6276 7074; Email: [email protected]. ABSTRACT

In this paper a classification of combustion regimes is investigated for the diffusion flame of a hydrocarbon fuel jet in hot flue-gas coflow (JHC) with varying oxygen fraction up to 100%. Numerical simulations by computational fluid dynamics (CFD) are performed to obtain both forced-ignition and auto-ignition temperatures for the present investigation. All calculations use the Eddy Dissipation Concept (EDC) model with the well-known detailed chemistry-reaction mechanism of methane combustion (GRI-Mech 3.0). To validate the modeling, the predicted JHC flame characteristics are compared with those measured by Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154]. It is found that the predictions agree well with the meassurements.

Use of the predicted ignition temperatures can qualitatively classify the JHC combustion, based on the suggestion of Cavaliere and de Joannon [Prog. Energy Combust. Sci. 30 (2004) 329-366] for the combustion in a well stirred reactor (WSR), into three distinct regimes: traditional combustion (TC), high temperature combustion (HTC), and flameless combustion (FLC). The FLC regime can be further divided into three distinct zones: MILD (moderate or intense low-oxygen dilution), MILD-like and quasi-MILD. The MILD and MILD-like combustion regimes share the same neccessary conditions proposed by Cavaliere and de Joannon while the quasi-MILD combustion does not. It is found that theoretically the MILD-like combustion should occur at any oxygen fraction as long as the preheating temperature of coflow prior to their reactions is sufficiently high. By comparison, all previous diffusion MILD combustions were established only for highly diluted reactants with the oxygen fraction less than 10%. In this paper, a fundamental analysis of the classification of combustion regimes is provided.

Keywords: Diffusion flame; Auto-ignition; Forced-ignition; MILD combustion; High temperature air combustion (HiTAC); Flameless oxidation (FLOX).

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Nomenclature Symbols Tai

auto-ignition temperature (K)

Tfi

forced-ignition temperature (K)

YO*2

inlet O2 mass fraction of the coflow

X O* 2

inlet O2 volume fraction of the coflow

* Tcof

inlet temperature of the coflow (K)

Vcof*

inlet coflow-injection velocity (m/s)

Tjet*

inlet temperature of the fuel jet (K)

Vjet*

inlet fuel-injection velocity (m/s)

ReD

fuel-jet Reynolds number

ν

kinematic viscosity (m2/s)

Tmax

downstream maximal temperature (K)

YCO

mass fraction of CO after reaction

YOH

mass fraction of OH after reaction

YO2

mass fraction of O2 after reaction

ξ

mixture fraction

∆T

maximum temperature rise (K)

Cp

coflow specific heat (at constant pressure (kJ/(m3·K)))

Λ

reaction zone volume (m3)

L

reaction zone length (m)

W

reaction zone width (m)

2


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Acronym MILD

moderate or intense low-oxygen dilution

FLOX

flameless oxidation

HiTAC

high-temperature air combustion

WSR

well stirred reactor

FC

feedback combustion

HTC

high temperature combustion

HFDF

hot fuel and diluted fuel

HODF

hot oxidant and diluted fuel

HODO

hot oxidant and diluted oxidant

JHC

jet in hot coflow

RANS

reynolds-averaged navier-stokes equations

EDC

eddy dissipation concept

WSGGM

weighted sum of gray gas model

DO

discrete ordinate

AC

auto-ignition combustion

FLC

flameless combustion

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1. INTRODUCTION In last two decades or so, several clean and efficient combustion technologies such as flameless oxidation (FLOX), high-temperature air combustion (HiTAC), excess enthalpy combustion and moderate or intense low-oxygen dilution (MILD) combustion1-4 have been successfully developed. These four technologies are not fully identical, but generally similar in the sense that they all recover exhaust heat (using, e.g., regenerators or recuperators) and rely on high reactant dilutions or high recirculation rates to ensure appropriate mixing of one or two reactant streams with flue gases prior to their confluence and main combustion reactions. These technologies demonstrated significant benefits for industrial applications producing, e.g., stabilized flames due to high oxidant temperature and uniform temperature distributions, low NOx emissions and high thermal efficiency as a result of the exhaust heat recovery. Hence, considerable research has been carried out on the relevant combustion in past twenty years or so5-34.

Based on the well stirred reactor (WSR) combustion, Cavaliere and de Joannon4 introduced the term “MILD combustion”, which may be similar to, but more rigorous than, FLOX defined by Wünning & Wünning1. These authors defined the MILD combustion as a combustion process that satisfies: (i) the inlet temperature (Tin) of the reactant mixture is higher than the mixture auto-ignition temperature (Tai), or the inlet temperature is high enough to allow a distributed auto-ignition in the combustion chamber, and (ii) the allowable maximum temperature rise (∆T) above the inlet temperature during combustion is lower than the auto-ignition temperature (Tai); i.e., simply, the MILD combustion is the one which meets the following condition: Tin > Tai and ∆T < Tai

(1)

Using condition (1), they drawn a schematic Tin-∆T diagram that unambiguously shows different WSR combustion regimes of burning CH4/O2/N2 with a residence time of one second and at atmospheric pressure; they termed these regimes respectively as the feedback combustion (FC), high 4


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temperature combustion (HTC) and moderate or intense low-oxygen dilution (MILD) combustion, see their Fig. 3.

Following the work [4], a number of numerical investigations on the MILD non-premixed combustion in the counter-flow configuration were late performed for different combinations of heating and dilution of the reactants [5-8]. MILD combustion was successfully established under various initial conditions: e.g., hot fuel and diluted fuel (HFDF) [5], hot oxidant and diluted fuel (HODF) [6, 8] and also hot oxidant and diluted oxidant (HODO) [7]. In particular, the original three combustion regimes (FC, MILD and HTC) have been extended to four regimes for some HFDF and HODF cases [5, 8]. Using the Oppdif module of the ChemKin package, de Joannon et al. [5] investigated the structure of the reaction zones developing in a steady one-dimensional diffusive layer in the configuration of opposing air and methane jets, where the preheated fuel was diluted by hot nitrogen. Using the similar method of Ref. [4], these authors built up the maps of combustion regimes based on the allowable maximum temperature rise (∆T) against the inlet diluted-fuel temperature (Tfuel), i.e. schematic Tfuel − ∆T diagrams, for two environmental pressures (1 bar, 10 bar). Interestingly, they showed four characteristic regions of counter-flow diffusion combustion for the low pressure case (against three regions for the other case, similar to the WSR), i.e. feedback combustion (FC), high temperature combustion (HTC), MILD and new flameless combustion. Note that the ‘flameless’ region with Tfuel < Tai (and either ∆T < Tai or ∆T > Tai), appearing as a transitional zone between FC and non-reaction zone, is new and different from the MILD [4]. Such a transitional zone was termed as ‘flameless’, not ‘MILD’, by de Joannon et al. [5], who claimed that, in this zone, Tfuel is below Tai but flame thickens and the maximum heat release and pyrolysis depression are not correlated with the stoichiometric condition.

However, both the premixed WSR combustion and non-premixed counter-flow flame processes are much simpler than, and so significantly different from, those occurring practically in furnaces or 5


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combustors, and accordingly the quantitative definitions for the former in Refs. 4 to 8 are difficult to be implemented to the latter. For example, it is not feasible to determine that ∆T = Tmax − Tin since the maximum furnace temperature Tmax is hardly measurable. Previous studies of the furnace MILD combustion focused mainly on effects of the inlet conditions or burner configurations, e.g., Refs. 9 to 16. No research has been performed for the dependence of a flame on its surrounding flue-gas conditions inside a furnace, due to difficulties in defining such conditions and also in performing in-furnace measurements. Therefore, it is not surprising that, unlike the WSR and counter-flow diffusion combustions, the furnace MILD combustion has yet to be well defined in a quantitative manner, despite the available diagram from Ref. 1 of combustion stability limits of firing CH4/air in a practical furnace, based on the furnace temperature (which was not precisely defined in Ref. 1) and dimensionless recirculation rate, which are both internal parameters. It would be ideal if we could classify furnace combustion regimes using inlet or operational parameters.

By comparison, through a device generating an open jet flame in hot coflow (JHC), Dally et al.17 and others18-22 could well define and control the coflow temperature and O2 mass fraction, particularly generating a diluted low-oxygen coflow to emulate the MILD combustion observed in closed furnaces. Advanced laser diagnostics techniques were used to evaluate the interaction of turbulence and chemistry17 and the effects of fuel type on flame stability and lift-off height18-22, providing highly valuable data for numerical modelers of those flames. Importantly, Medwell et al.18-20 investigated the evolution of the OH radical and the formaldehyde intermediate to better understand the auto-ignition phenomenon prevalent in the MILD flames, while Oldenhof et al.21,22 further investigated the role of entrainment in the stabilisation of the JHC flames as well as their ignition kernel formation and lift-off behavior. However, to our best knowledge, there are not any studies available that classify combustion regimes of the JHC flame under different coflow conditions.

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Besides, nearly all previous studies on the MILD combustion1-34 have concerned the diluted cases with the volume concentration of oxygen XO2 well below 21%, which is consistent with the true meaning of ‘MILD’ being ‘moderate or intense low-oxygen dilution’. Perhaps it is only an exception that Rao and Levy23 attempted to differentiate operating regimes of flames, in terms of reactant temperature and oxygen concentration up to 30%. However, they simply summarized the combustion modes and named ‘oxy-rich flames’ for XO2 = 21% − 30%. So far, no attention has been paid to combustion in less-diluted or even pure oxygen cases. On the other hand, it is interesting to note that only the quantitative condition (1) is set for the MILD definition in Ref. 4, which does not require any information on XO2 or whether any reactant is diluted. Cavaliere and de Joannon4 found that the allowable temperature rise ∆T for the mixture of CH4/O2/N2 is related to the mixture inlet temperature Tin and also that an increase in Tin results in a decrease in ∆T. Nevertheless, they did not analyze any case for XO2 > 21% and Tin > Tai.

Accordingly, an attempt is presently made to lay a theoretical foundation mapping of the combustion regimes of the fuel jet flame in preheated oxidant coflow, similar to Ref. 4. To this end, a great number of numerical simulations for various conditions have been performed through RANS (Reynolds-averaged Navier–Stokes equations) modeling by using the Eddy Dissipation Concept (EDC) model with the detailed chemistry-reaction mechanism (GRI-Mech 3.0)35. The forced-ignition and auto-ignition temperatures of a hydrocarbon mixture in a coflow at different oxygen concentrations, preheating temperatures and velocities are, for the first time, systematically estimated and investigated. The main objectives of the study are: (1) To map different combustion regimes of the JHC flame by using coflow oxygen concentration and preheating temperature; and (2) To check whether the MILD-like combustion can take place at any coflow oxygen fraction.

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2. COMPUTATIONAL DETAILS The present simulations deal firstly with the burner system of Dally et al.17, which produces a fuel jet in hot coflow (JHC). The details of the system are described in Ref. 17 and here only a brief description is given. The JHC device consists of an insulated and cooled central jet nozzle of inner diameter (4.25 mm) and an annulus nozzle of inner diameter of 82 mm with a secondary burner mounted upstream of the perforated plate. The secondary burner provides hot combustion products which are mixed with air and nitrogen via two inlets aside to control the temperature and O2 levels in the mixture. The fuel of Dally et al.17 is a mixture of CH4 and H2 equally in volume. In their experiments, the JHC burner was mounted in a wind tunnel providing a surrounding air stream at the same velocity as the hot coflow but with room temperature.

In the present work, to validate the RANS modeling, the flames from the above JHC device are at first simulated under the identical conditions of Dally et al.17. Then, to realize the designated objective, a different configuration of the JHC system is utilized to produce a central jet flame surrounded only with hot flue gases24, i.e., by removing the outer cold air stream from the JHC * system of Dally et al.17. This removal ensures that YO* 2 and Tcof remain constant across the whole

computational domain, thus properly mimicking effects of the local O2 concentration, temperature and velocity of the hot flue gas on the global flame characteristics in furnaces. The fuel-jet Reynolds number is kept at ReD ≈ 10,000 for all the simulations, where ReD ≡ Vf D/ν with Vf being the fuel-injection velocity and ν the kinematic viscosity. Note that this value of ReD is identical to that of Dally et al.17.

Due to the symmetry of the system, a geometrically simplified axisymmetric model of computation is constructed in Fig. 1 together with the coordinate system. The computational domain starts at the exit plane of the burner, and extended 500 mm downstream in the axial direction and 800 mm in the 8


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radial direction. A primary orthogonal structured mesh with about 30,000 cells is used to simulate the flames. The grid-independency of the results is verified using a finer grid with a number of 80,000. Results are kept in high consistency for the different grids. Therefore, the former mesh is finally employed in each case to save the computational time.

(a) (length unit is mm)

(b)

Fig. 1. Boundary conditions (a) and structrured grid (b) of the computational domain. Note: present calculations have used two JHC configurations, i.e., (i) original JHC system coflowing with hot flue-gas mixture and outer cold air stream, and (ii) modified JHC system coflowing only with hot flue-gas mixture.

The present study implements a modified k–ε model with standard wall function to model turbulent flows, adjusting the constant C1ε between 1.44 and 1.625, 26 to achieve the best agreement with the measurements17. Pressure boundaries of 1.0 atm are set as the boundary conditions for the outlet and sides of the cylindrical computational domain, see Fig. 1. The calculations are found to be sensitive to the turbulent kinetic energy (k) at the fuel and hot coflow inlets but to depend weakly on the turbulence intensity at the shroud air inlet. Thus, k is set to be 60 m2/s2 and 1.8 m2/s2, respectively, at the fuel and coflow inlets (so that the fuel turbulence intensity ≈ 7.5%, which was also used in the previous works25, 26 to obtain the best agreements between the calculated and measured results. 9


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The turbulence/chemistry interactions are often modeled with two different approaches: the Eddy Dissipation/Finite Rate (ED/FR) model and the Eddy Dissipation Concept (EDC) model36. The EDC hypothesis is expected to provide satisfactory results for MILD combustion25-28. Thus, the EDC model with the detailed chemical kinetic mechanism of GRI-Mech 3.035 is used for the modeling of reactions. The present calculations implement the mechanism of GRI-Mech 3.0 without NOx reactions, so that 219 reversible reactions occur, involving 36 chemical species. Note also that, although GRI-Mech 3.0 was optimized for methane combustion, it still performs better than the global mechanism for other hydrocarbon fuels such as LPG24. Besides, large quantities of previous studies25-28 have used GRI-Mech 3.0 to model the MILD combustion of hydrogen and hydrocarbon blends. Therefore, the present study also uses it for the combustion simulations of C3H8 and CH4/H2.

A total of seven transport equations (continuity, momentum, turbulence kinetic energy k and its dissipation rate ε, energy, and radiative intensity) are solved using the commercial CFD package FLUENT6.3. The discrete ordinate (DO) radiation model with weighted sum of gray gas model (WSGGM) to model the gas emissivity is applied for radiation. A structured non-uniform grid system is used to mesh the whole domain, see Fig. 1. The SIMPLE algorithm method is utilized to solve pressure velocity coupling. The second-order upwind scheme is employed for discretizing the equations in order to improve the accuracy of the simulations. Solution convergence is determined by two criteria. The first one is to ensure the numerical residuals to be less than 10-6 for the energy and 10-5 for all the other variables. The second criterion is to make sure that the variations between consecutive iterations of temperature and velocity are kept within 1.0 K and 0.1 m/s, respectively, at the downstream outlet of the computational domain.

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3. FORCED- AND AUTO-IGNITIONS AND THEIR TEMPERATURES (Tfi AND Tai) Ignition is a complex time-dependent process of starting with reactants and evolving in time towards a steadily burning flame. Experience shows that there are two general modes of ignition: i.e., forced-ignition and auto-ignition.

Forced-ignition: it occurs as a result of local energy addition from an external source such as an electrically heated wire, an electric spark, an incandescent particle, a pilot flame, etc. A flame is initiated locally near the ignition source and, if sustainable, propagates into the rest of the mixture. The forced-ignition temperature (Tfi) is the lowest temperature of a flammable gas mixture at which the external heat source can ignite the mixture into a flame.

Auto-ignition: Auto-ignition occurs as a result of raising the temperature of a considerable volume of a reactant mixture by containing it in hot boundaries or by subjecting it to adiabatic compression. Because the heat generation rate is a strong exponential function of temperature whereas the heat loss rate is a simple linear function, even a slight increase in the temperature of the reacting mixture would greatly increase the rate of its temperature rise. As a consequence, once the generation rate exceeds the loss rate, ignition occurs in the whole volume almost instantaneously. The reaction then proceeds by itself without any other external heating. The auto-ignition temperature (Tai) is the lowest temperature of the reactant mixture at which the auto-ignition occurs. It is worth noting that, different from the forced case, the auto-ignition temperature does not necessarily refer to a flammable mixture since the MILD combustion mainly occurs in the non-flammable mixture where the oxygen fraction is often smaller than 5%-10%.

How to obtain Tfi and Tai via CFD: Recently, a procedure of obtaining Tai and Tfi37 has been 11


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developed for the present work based on the time-dependent nature of iterations in the RANS modeling and validated by the measurements of Katsuki and Hasegawa2. The validation confirms that Tfi and particularly Tai of a LPG jet impinging perpendicularly on air or diluted-air stream, a flame quite different from the present JHC combustion, can be effectively found through several steps of modeling calculations using this procedure, whose brief description is given below.

Fig. 2. Flow chart for the present method to identify the forced-ignition and auto-ignition temperature (Tfi or Tai).

As shown in the flow chart of Fig. 2, the present calculations determine the ignition temperatures Tfi and Tai in a sequent order as described below:

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*

(1) Setting T1 as the coflow temperature ( Tcof ), then switch off the CFD module of chemical reactions and initialize the fuel-jet flow-field (so that the convergence of subsequent computations is more quickly achieved); (2) When the convergence of flow calculation has been achieved, a) for Tfi, set the entire computational domain, which is easy to implement, or a line ‘source’ at the nozzle exit, with a sufficiently high temperature (> T1) such as 1800 K (the magnitude of this value is found not critical) as the first iteration value of temperature (acting like an ‘external source of heat’) and then activate the module of chemical reactions for subsequent iterations; or b) for Tai, just activate the module of chemical reactions (so that T1 is the first iteration temperature, i.e., without any ‘external source of heat’) for iterative computations of reactive flows (if T1≥ Tai chemical reactions must take place); (3) Once all the calculations have converged, check the resulting temperature (T) in the mixing-layer: if T > T1, the combustion must occur and the fuel jet is considered to ignite in the coflow with T1; (4) Repeat the above process when taking lower or higher values (e.g., by 50K for the present calculations) of T1 until Step (3) finds T < T1 (no reaction) and hence the minimum of T1 which enables combustion. The resulting value of T1 used in the above procedure is the forced-ignition or auto-ignition temperature (Tfi or Tai). Furthermore, although the present RANS modeling is steady, the time-dependent nature of ignition can be realized through the above process of modeling which uses the time-dependent nature of iterations for achieving convergence of the calculations.

*

Fig. 3 illustrates how Tfi and Tai are obtained for the jet flame of CH4/H2 in hot coflow when Tcof is

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* varied from 400 K to 1800 K. As demonstrated in Fig. 3(a), for Tcof ≤ 400 K, combustion cannot

sustain even initially ‘forcibly igniting’ the CH4/H2 jet and thus eventually no reaction takes place. *

*

When Tcof = 450 K, one can find that, in the mixing layer, the temperature is higher than Tcof , which reflects the occurrence of combustion reaction, i.e., a flame being ‘forcibly’ ignited. When * Tcof ≥ 1050K, a stable combustion is established no matter what initial value of temperature is set

over the entire domain; namely, a flame is naturally or ‘automatically’ ignited. Hence, the two temperatures, i.e., 450K and 1050K, are found approximately as the forced-ignition and auto-ignition *

temperatures, respectively. Moreover, Fig. 3(b) indicates that, as Tcof is increased from 450 K to *

1800 K, the difference between Tmax and Tcof decreases from 1750 K to 900 K, i.e., the reaction zone becomes more uniform, thus approaching the MILD regime defined in Ref. 4, see Section 4.2 for more discussions.

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(b) 3000 Auto-ignition

no reaction

Forced-ignition

2000

*

Tmax, Tcof [K]

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Tmax

1000 *

Tcof 0 300

800

1300 * Tcof

1800

[K] *

Fig. 3. Indication of how to obtain Tfi and Tai of the JHC flame of CH4/H2 when varying Tcof from *

400 K to 1800 K. (a) Temperature contours, and (b) the maximum temperature (Tmax) versus Tcof . Coflow: 23% O2, 6.5% H2O, 5.5% CO2 and 65% N2 (by mass), velocity = 3.2 m/s. Fuel jet: 12% H2 and 88% CH4 (by mass), velocity = 70 m/s and temperature = 305 K.

4. RESULTS AND DISCUSSION 4.1 Validation of the Present CFD Modeling Present calculations, using the EDC model with full detailed chemistry of GRI-Mech 3.0, are validated by predicting the flame characteristics under the same conditions as for the experiments of Dally et al.17. The corresponding flame is in a hot flue-gas coflow surrounded by a cold air stream at the same velocity. The coflow inlet compositions (O2, H2O, CO2 and N2 mass fraction) and temperature are assigned the experimental values of Dally et al.17 measured approximately at 4 mm downstream from the jet exit.

Figure 4 compares the predictions and the measurements of the radial distributions of the mean temperature (T) and mass fractions YCO, YOH, and YO2 obtained at x = 30 mm for YO*2 = 3%, 6% and 9%. Although not perfect, all the predictions of T, YCO, YOH, and YO2 for the test cases are quite 15


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satisfactory. In particular, T and YO2 are predicted very well across the entire field. In contrast, the obvious difference in YCO between the predicted and measured results is seen between r = 20 mm and r = 40 mm. This can be explained. Dally et al.17 used a secondary burner to provide combustion products which were mixed with air and nitrogen via two side inlets at the bottom of the annulus to control the O2 levels. These authors claimed that the carbon monoxide product initially existed in the coflow in the experiments due to the cooling and extinction effects of the secondary flame outside the primary burner and near the burner outer wall. However, there is no carbon monoxide present in any calculated coflow of the present investigation.

0

4

r/D 8

12

0

4

r/D 8

12

0

4

r/D 8

12

1800 T [K]

1300 800 300

YCO

0.02 0.01

YOH

0 0.002

0.001

0

2

0.2 YO

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0.1 0 0

20

40

60 0

20

40

60 0

20

40

r [mm]

r [mm]

r [mm]

(a) Y *O = 3%

(b)Y *O = 6%

(c) Y *O = 9%

2

2

60

2

Fig. 4. Comparison between present predictions (curves) and previous measurements17 (symbols) of the mean temperature (T) and mass fractions of CO (YCO), OH (YOH) and O2 (YO2) for x = 30 mm made at the same conditions.

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Moreover, the present predictions perform quite well for the mixture fraction (ξ), obtained from Bilger’s formula36. This is demonstrated in Fig. 5, which shows the mean mixture fraction for YO*2 = 3% along the axis and its radial distributions at different x locations. These good agreements provide a sufficient confidence on the spreading and mixing characteristics of the simulated jets. That is, the CFD model used presently can correctly capture the features of the JHC combustion processes of Dally et al.17. Hence, the use of the same model is appropriate for investigating the combustion characteristics of the JHC diffusion flames surrounded by an entirely hot flue-gas coflow (using uniform gas compostions), different from the above JHC cases, which is reported below in Section 4.2 and Section 5. 4

8

12

0

10

x/D 20 30

40

1.0

1.0

(b)

(a) x = 30 mm

0.8 0.6

0.8 0.6

60 mm along axis

0.4

0.4

0.2

0.2

120 mm

0

Mixture fraction (ξ)

r/D 0

Mixture fraction (ξ)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

0 0

20

40

60

0

50

r [mm]

100 x [mm]

150

200

Fig. 5. Present predictions (curves) and previous measurements17 (symbols) of mixture fraction at (a) *

axial and (b) radial directions for Tcof = 1300K and YO*2 = 3%.

4.2 Combustion Regimes of the JHC Flames The CH4/H2 JHC flames with a wide range of the coflow oxygen concentration X O* 2 (volume * concentration) and temperature Tcof are modeled to find the ignition temperatures Tfi and Tai as well

17


Energy & Fuels

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Page 18 of 37

* as the maximal temperature increment during combustion, i.e., ∆T = Tmax– Tcof , where Tmax is the

maximum temperature obtained in the whole computational region. With the resulting data of ∆T, Tfi * and Tai, we produce a map in Fig. 6 of ∆T/Tai against Tcof /Tai for X O* 2 = 2% to 100% at the coflow

*

* / Tai = 1 velocity of Vcof = 3.2 m/s. Following Ref. 4 for the WSR combustion, a vertical line at Tcof

and a horizontal line at ∆T / Tai = 1 are drawn to delimit the combustion regimes: (i) traditional combustion (TC); (ii) high temperature combustion (HTC); and (iii) MILD combustion. It is interesting to note that an additional combustion zone emerges on the map as a small quasi-triangular region which is formed by the blow-off curve (connecting the blow-off points obtained at X O* 2 ≥ 8%) * and the lines of Tcof / Tai = 1 and ∆T / Tai = 1. This combustion regime satisfies the condition that

* Tcof < Tai and ∆T < Tai and so should be flameless, according to Refs. 5 and 7. However, it is not a

* combustion which is auto-ignited since Tcof < Tai (not > Tai) and is thus rational to name it

“Quasi-MILD”. Within this region, although the preheated coflow temperature is not high enough to ignite the reactants or cause fast combustion reactions, the accumulation of the input heat energy carried by the coflow and the heat release by exothermic reactions can sustain relatively slow combustion reactions (if forced-ignited) in this region.

Fig. 6 demonstrates that the maximum temperature increment ∆T increases monotonously as the coflow oxygen fraction X O* 2 is increased. This is because the combustion reaction at a higher value of X O* 2 must occur at a higher rate, thus releasing more heat to increase ∆T. More importantly, it is * observed that, generally for any X O* 2 , ∆T reduces as Tcof is increased; also, the ∆T

reduction

* accelerate by rising X O* 2 . This dependence of ∆T on Tcof and X O* 2 should result from three

factors listed below:

18


Page 19 of 37

*

HTC

TC

3

∆T/Tai = (Tmax - Tcof ) /Tai

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Energy & Fuels

*

XO = 100% 2

2

80% HTC

50% 21%

Blow-off

1

15% 13% Quasi-MILD

MILD + MILD-like

8% 5% 2.5%

No reaction

2%

0 0

1

2

3

* Tcof /Tai

*

Fig. 6. Normalized maximum temperature rise ∆T / Tai against Tcof / Tai for the flames of CH4/H2 in *

the coflow at X O* 2 = 2% to 100% and Vcof = 3.2 m/s. On plot, TC = traditional combustion; HTC = high temperature combustion; MILD = moderate or intense low-oxygen dilution.

* (1) Chemical pathway: When Tcof > 1100K, as deduced from Ref. 29, the variation of controlling

kinetic pathway changes the temperature of the mixture (reactants + flue-gases) because numerous endothermic reactions take place to recombine some intermediates. And, very * importantly, increasing Tcof will accelerate those endothermic reactions and thus reduce ∆T. In

other words, the higher is the reactant temperature, the more does the fuel evolve following the recombination path (which thus ‘sucks’ more heat) and the lower is the ∆T during combustion. * (2) Mixing effect: As Tcof is increased, the coflow density decreases. Wang et al.28 showed that the

entrainment ratio, i.e., the entrained gas mass flow rate to the initial jet mass flow rate, is reduced 19


Energy & Fuels

* when only increasing Tcof and keeping other conditions unchanged. This means that less oxidant

* is entrained from the coflow and then mixed with fuel. It follows that the increase of Tcof slows

* down the fuel oxidation and thus decreases the temperature rise from Tcof .

* (3) Coflow heat capacity: As Tcof grows, the coflow specific heat (Cp) increases and consequently

* the same heat release from combustion will cause the flame temperature to rise less above Tcof .

In other words, the coflow temperature growth lessens ∆T due to increased specific heat. 3300 T (Cp fixed @ 1200K)

∆T (2400K)

2800

T (Cp varying)

*

Tcof = 2400K ∆T (1200K)

2300 T [K]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 37

1800

T (Cp varying) *

Tcof = 1200K

1300

800

300 0

1

2

3

4

5

r/D

Fig. 7. Radial distributions of the mean temperature (T) at x = 30 mm in the JHC flames of X O* 2 *

= 100% for Tcof = 1200K and 2400K. Dependences of ∆T on endothermic reactions and specific heat (Cp) reveal that the ∆T reduction due to the former is significantly greater than that due to the latter.

It can be proven below that, among the three factors, the thermo-chemistry one plays the very dominant role. To check the relative importance of specific heat Cp in the ∆T reduction, we * calculated the JHC flames of X O* 2 = 100% for Tcof = 1200 K and 2400 K with varying Cp and the

20


Page 21 of 37

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Energy & Fuels

* fixed value at Tcof = 1200 K. Fig. 7 shows radial distributions of the mean temperature (T), hence ∆T,

* obtained at x = 30 mm. It is seen that, for Tcof = 1200 K and 2400 K, ∆T ≈ 1854 K and 772 K when

Cp is temperature-dependent and ∆T ≈ 1854K and 834 K when Cp = Cp (1200 K) for both cases. * Accordingly, for X O* 2 = 100%, as Tcof is increased from 1200 K to 2400 K, the total reduction in ∆T

is approximately 1100 K over which only 62 K results from the factor of Cp and the rest from other two factors. It follows that the specific-heat factor contributes less than 6% to the ∆T reduction.

* Fig. 7 also reveals that, if Tcof is sufficiently high, the MILD-like combustion can occur at any

value of X O* 2 . This is obviously at odds with the true meaning of MILD which is the acronym of “moderate or intense low-oxygen dilution”4. To avoid such a perplexity, the term “MILD-like” is * used in this paper to identify the combustion satisfying the MILD condition4 of Tcof ≥ Tai and ∆T
Tai)”. In addition, the maximum temperature Tmax is obtained from the present simulation; * namely, the curve of Tcof = Tmax−Tai cannot be estimated just based on the inlet coflow and fuel

parameters. Nevertheless, the limit borders of the combustion regimes are not sensitive to the change of other gasous fuel, as shown late, so that the map of Fig. 8 may still be valid qualitatively for all the JHC flames of the gasous hydrocarbon fuel. Moreover, considering in such an unconfined JHC burner, the map might also apply when firing other liquid and solid hydrocarbons. At least, the method to classify the combustion regimes is always available no matter whatever fuel is used.

*

Fig. 8. Classification of the JHC flames of CH4/H2 in the coflow at X O* 2 = 0.5% to 100% and Vcof = 3.2 m/s. Symbols: , corresponding to the three cases of Dally et al. 17; I, error bar (±ΔT/2, ±25K for the present study). On the plot, TC = Traditional Combustion; HTC = High Temperature Combustion; FLC = Flameless Combustion; MILD = Moderate or Intense Low-oxygen Dilution; AC = auto-ignited combustion. 22


Page 23 of 37

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Energy & Fuels

Table 1. Criteria for the combustion classification of a diffusion JHC flame. Combustion regime Traditional combustion (TC)

Auto-ignited combustion (AC)

Flameless Quasi–MILD combustion MILD (FLC) MILD-like High temperature combustion (HTC)

Coflow inlet condition * cof

T

< Tai

Working condition ∆T > Tai

* < Tai Tcof

∆T < Tai

* > Tai and X O* 2 ≤ X O(1)2 Tcof

∆T < Tai

* > Tai and X O* 2 > X O(1)2 Tcof

∆T < Tai

* > Tai Tcof

∆T > Tai

The small zone of “Quasi-MILD combustion” requires more comments. We classify this zone as a subset of the FLC, i.e. treating it as the flameless combustion, for the following reason. The previous investigations4-8, have demonstrated that, when the feeding conditions (dilution and pre-heating) are obtained separately for two reactants, the resulting combustion locally releases heat in a spatial structure considerably different from the traditional diffusion flame. Relatively, the former combustion is characterized by flame thickening and pyrolysis depression (and, for some cases, non-correlation between the stoichiometric and maximum heat release conditions5). The pyrolysis depression is a necessary condition to yield colorless products2 whereas the flame thickening is required for distributed non-front-like flames. This has justified the MILD (also with a great quantity of experimental evidence) and MILD-like combustion as being ‘flameless’. And so is the “Quasi-MILD combustion”, which is to some extent similar to the transitional ‘flameless’ zone of de Joannon et al.5. In Ref. 5, the map of counter-flow combustion regimes for p = 1 bar clearly identifies the transitional zone between the feed-back combustion and the non-combustion region with the inlet temperature lower than the ignition one (this identification however does not apply for the case of p = 10 bar). The flame thickening and the absence of pyrolytic region found in Ref. 5 justify the identification of such a transitional zone as ‘flameless’. This justification should apply for the present * “Quasi-MILD combustion” with Tcof < Tai, which is not fitting the numerical MILD conditions by

23


Energy & Fuels

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Page 24 of 37

definition.

* Importantly, the Tcof - X O* 2 map enables the combustion regime to be approximately identified a

* priori directly from the given operative conditions and is hence advantageous over the ∆T- Tcof map

of Fig. 6, from which the similar identification is impossible. For example, the three JHC flames of * Dally et al.17 for YO*2 = 3%, 6% and 9% at Tcof = 1300K could be identified all in the MILD regime

(see Fig. 9), if the fuel jet were surrounded only by coflowing hot exhaust gas without cold air stream outside. This is well supported by the measurements of the maximum temperature rises at x = 30 mm, i.e. ∆T ≈ 388K, 210K and 90K, which are all well below the auto-ignition Tai ≈ 1100K-1300K, thus proving that ∆T < Tai. Not unexpectedly, the flames for YO*2 = 9% and 6% were actually visible from locations at x < 30 mm, given that those flames were surrounded by cold air at room temperature. We believe that all flames should be invisible if Dally et al.17 could perform their experiments in a furnace or under the present condition without cold air involved.

Moreover, Fig. 8 demonstrates that, as X O* 2 is increased from zero to 5%, the auto-ignition temperature Tai drops rapidly from beyond 2200K to around 1100K, and then Tai decreases only very slightly to about 950K until X O* 2 = 100%. For the forced-ignition temperature Tfi, it is nearly identical to Tai at X O* 2 = X（O12） (≈ 8% for the present study) and falls off as the coflow oxygen levels * * up. The curve Tcof = Tmax−Tai onsets from its intersection with Tcof = Tfi at X O* 2 = X（O22） (≈ 13% for the

* present study) and climbs up until X O* 2 = 100%; it also has an intersection with Tcof = Tai at X O* 2 =

X（O32） (≈ 15% for the present study). Note that X（O12） , X（O22） and X（O32） correspond to the * / Tai = 1 and the horizontal line intersections of the blow-off curve with the vertical line Tcof

∆T / Tai = 1 and that of the latter two on the map of Fig. 6. This suggests that, for the diffusion JHC 24


Page 25 of 37

flame, a critical value of X O* 2 appears to occur at X O* 2 = X（O12） below which combustion can take *

place only in the MILD regime at Tcof > Tai. For the present flame of CH4/H2, X（O12） is approximately 8%.

*

For X O* 2 ≤ X（O12） , if Tcof is less than the forced-ignition Tfi of the reactant mixture, the heat released in the combustion process will be less than the heat transferred to the coflow, so that no stable oxidation can take place, then the diffusion flame will be blown off and hence “no reaction” *

develops. On the other hand, if Tcof > Tfi, the released heat during the oxidation process is sufficient to balance the heat transfer to the coflow, stable flameless oxidation then can be established.

*

*

XO = 100%, Tcof = 200 K 2

∆T / 200

10

100%, 950 K

*

∆T / 950

T/Tcof

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1 ∆T / 2400

100%, 2400 K 2.5%, 2400 K 2.5%, 1250 K 0.1 0

1

2 r/d

3

4

* Fig. 9. Radial distributions of the normalized flame temperature (T/ Tcof ) obtained at x = 30 mm for

X O* 2 = 2.5% and 100%. Note that the vertical axis of plot is in the logarithmic scale.

25


Energy & Fuels

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Page 26 of 37

When the coflow oxygen is increased to X（O12） < X O* 2 < X（O22） , stable flameless combustion (FLC) may *

* occur in the MILD-like zone at Tcof > Tai or in the quasi-MILD zone at Tcof < Tai. Further for X O* 2 >

X（O32） , stable combustion may occur in all the three regimes depending upon the coflow temperature. * * * < Tai causes the evolution from When Tcof < Tfi, no reaction occurs. Increasing Tcof for T fi < Tcof

“no reaction” region to traditional combustion (TC). Inside the TC region, the predicted ∆T is much higher than Tai, which characterizes the TC structure as similar to classical diffusion flame, i.e., * lift-off flame or attached flame. Further increasing Tcof to beyond Tai helps the JHC flame develop

into the high temperature combustion (HTC) regime. Once more increasing the coflow temperature to Tcof > Tmax − Tai enables the JHC flame to reach the FLC state (MILD-like). *

Of note, all previous MILD combustions found by experiments or CFD appear to have been established at X O* 2 ≤ X（O12） , which is truly of “moderate or intense low-oxygen dilution”. However, it is found that the MILD-like combustion whose quantitative definition is identical to that by Cavaliere and de Joannon4 can occur at X O* 2 > X（O12） . For X O* 2 ≥ 21%, the MILD-like combustion takes place in the hot coflow which is not low-oxygen diluted but rich with oxygen. In other words, Fig. 8 suggests that the JHC flame can develop into the FLC regime at any value of X O* 2 providing * > Tai and ∆T < Tai is met. To understand this better, Fig. 9 shows radial that the condition of Tcof

* distributions of the normalized flame temperature (T/ Tcof ) obtained at x = 30 mm for two extreme

cases with X O* 2 = 2.5% and 100%. Evidently for X O* 2 = 2.5%, the normalized maximum temperature *

* * rise (∆T/ Tcof ) is incredibly small at Tcof = 1250 K and also at Tcof = 2400 K. That is, the combustion

for this case occurs always in the flameless MILD regime. In contrast, for the pure oxygen coflow, *

*

*

when Tcof = 200K and 950 K (nearly auto-ignition), ∆T ≈ 14 Tcof (= 2800K) and ∆T ≈ 2.3 Tcof (= 26


Page 27 of 37

2185K); namely, both the flame temperature rises are greater than Tai ≈ 950K and so the related JHC flames are not in the FLC regime but in the TC and HTC regimes. By comparison, for the case of *

* X O* 2 = 100% and Tcof = 2400 K, the temperature rise is ∆T ≈ 0.3Tcof = 720K < Tai ≈ 950K, so that

the flameless MILD-like combustion is established. In other words, when the coflow is highly preheated to the temperature of 2400K, the flame temperature rise will constantly fall below the corresponding auto-ignition temperature, regardless of X O* 2 = 2.5% or X O* 2 = 100%. This not only reflects that both combustion regimes occur well in the FLC regime but also demonstrates that a sufficiently high coflow or oxidizer temperature is always required for the flameless MILD or MILD-like combustion. On the other hand, Fig. 9 certainly reveals that the low-oxygen (and also low-fuel) dilution1-32, which is perhaps essential in practice for the MILD combustion, mathematically does not act as a necessary condition for the flameless combustion. So, the phrase ‘MILD’ for the flameless combustion should be used together with “MILD-like” and “Quasi-MILD”. (b) CH4

(a) CH4/H2

(c) C3H8

2200

FLC

FLC

FLC 1800

HTC

HTC

1400

HTC

*

Tcof [K]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Energy & Fuels

1000

FLC

FLC 600

FLC TC

TC NR

TC

NR

NR

200 0

20

40 * 60 XO [%] 2

80

100 0

20

40 60 * XO [%]

80

100 0

20

40 * 60 XO [%]

80

100

2

2

Fig. 10. Combustion regimes of the JHC flames of (a) CH4/H2, (b) CH4 and (c) C3H8 at X O* 2 = 0.5% *

to 100% and Vcof = 3.2 m/s. On plots, NR = No Reaction, TC = Traditional Combustion, HTC = High Temperature Combustion, FLC = Flameless Combustion. On part (c), the symbol () stands for the experimental data of Ref. 2. 27


Energy & Fuels

Further, the characteristic combustion regimes observed from the JHC flame of firing CH4/H2 *

mixture are expected to be valid for firing other gaseous fuels. Figure 11 compares the Tcof - X O* 2 *

maps for CH4/H2, CH4 and C3H8 at X O* 2 = 0.5% to 100% and Vcof = 3.2 m/s. Overall, they are similar. More specifically, the auto-ignition temperature is nearly identical for CH4 and C3H8 but higher for the CH4/H2 mixture. The difference is expected to result from the fact that the thermal diffusivity of H2 is many times greater than any hydrocarbon fuels. Worthy to note, too, the results of Fig. 10 are consistent with the recent observation of Medwell and Dally 20 that the fuel type does not have a significant effect on the reaction zone structure under MILD combustion conditions. Moreover, from Fig. 10(c), one can observe that the predicted auto-ignition temperatures agree quite well with experimental data of Katsuki & Hasegawa2, although the combustion systems of the two studies are totally different. It follows that the influence of the combustion system on the auto-ignition temperature is minor. *

*

2200

*

(b) Vcof = 3.2 m/s

(a) Vcof = 0.1 m/s

(d) Vcof = 32 m/s *

FLC

FLC

FLC

*

(c) Vcof = 9.6 m/s

2200

Vcof ↑

1800

1800 1400

*

HTC

HTC

HTC

1400

Vcof ↑

*

Tcof [K]

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 37

1000

1000 600 200

*

25

50 *

XO [%] 2

75

0

600

NR

NR 100

Vcof ↑

TC

TC

TC

NR 0

FLC

FLC

FLC

25

50 *

X O [%] 2

75

100

0

200 25

50

75

*

XO [%] 2

100

0

25

50

75

100

*

XO [%] 2

Fig. 11. Classification of the JHC flames of firing CH4/H2 at three different coflow velocities. (a) * Vcof = 0.1 m/s; (b) 3.2 m/s; (c) 9.6 m/s; and (d) 32 m/s, dashed curves from (a)-(c). On plots, NR =

No Reaction, TC = Traditional Combustion, HTC = High Temperature Combustion, FLC = Flameless Combustion. 28


Page 29 of 37

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Energy & Fuels

The above findings should apply generally for any normal diffusion flame in air. For instance, if a *

CH4/H2 jet discharges at Vf ≈ 70 m/s into an air coflow with Vcof = 3.2 m/s, a diffusion flame can develop into all the three regimes of combustion by preheating air to different temperatures. More *

specifically, in the case of analysis, the traditional flame may be forcibly ignited in air at Tcof ≥ 480K whereas high temperature combustion (HTC) and flameless combustion (FLC) are established *

*

spontaneously at Tcof ≥ 1025 K and Tcof ≥ 1540 K, respectively. Note that, when using higher values *

of Vcof , both forced-ignition and auto-ignition are found to occur at higher temperatures. Examples *

are shown in Fig. 11 which compares the results for Vcof = 0.1 m/s, 3.2 m/s, 9.6 m/s and 32 m/s *

when Vf ≈ 70 m/s. It is obvious from Fig. 11(d) that the increase of Vcof shifts up the three regimes of combustion as a whole to higher temperature regions. More specifically, both the ignition temperatures enlarge appreciably as the coflow speeds up. For instance, with the air coflow, when * Vcof grows from 0.1 mm/s to 3.2 m/s, the auto-ignition/forced-ignition temperature increases

* approximately by 136/92 K from 884/480 K to 1020/572 K, and the curve of Tcof = Tmax−Tai shifts

* * up by 70 K. The observed effect of Vcof may be interpreted as follows. Firstly, as Vcof is increased,

the relative velocity between the fuel jet and the coflow reduces and so does the mean shear in the mixing layer, consequently the fluid mixing weakens between fuel and oxidant, which should lessen their probability to chemically react with each other and hence results in the increase of the two * ignition temperatures. Second, increasing Vcof shortens the residence time of the oxidant in the

reactive region (or computational domain) and thus heightens the auto-ignition temperature, see Ref. * 32. Apparently, the alteration of Vcof can change the combustion regime: i.e., it is seen from Fig. 11

* that the flame occurring in the HTC or FLC regime with Vcof = 0.1 m/s may switch to the TC or HTC

29


Energy & Fuels

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Page 30 of 37

* regime as Vcof grows. In other words, we may affect the general classification proposed by

changing the coflow speed.

Of great importance, a careful inspection of Figs. 10 and 11 reveals that the three special values of oxygen volume concentration, i.e., X（O12） , X（O22） and X（O32） which define the quasi-MILD zone of FLC, vary hardly (< 0.5%) with different fuels and raise slightly (< 2.5%) with increasing the coflow velocity. In other words, the three oxygen volume concentrations are apparently not sensitive to the initial conditions, i.e., fuel type and reactants velocities. The reason is that the predicted Tfi and Tai do not change significantly when firing other fuel and using different coflow velocity. Thus, it can be inferred that the classification of Fig. 8 can be qualitatively applied to other JHC flames. Especially, for the cases at X O* 2 ≤ X（O12） , only flameless MILD combustion occurs. X（O12） is approximately 8% for the present JHC configuration that is consistent with that previously required for the achievement of MILD combustion. This might be why nearly all previous studies were focused on low-oxygen MILD combustion (< 10%).

5 FURTHER DISCUSSION Further discussion is worthwhile on the flameless combustion defined in the present work. The classification of combustion regimes proposed originally by Cavaliere and de Joannon4 is based on the quantitative condition (1) and hence can be claimed rigorous. However, the identification of the combustion regimes is practically expected to reflect physical differences in the flame behavior, e.g., visually observable. In particular, as demonstrated in Figs. 8, 10 and 11, the mathematical definition of MILD combustion implies that the flameless combustion can be achieved for any fraction of oxygen in the oxidant, provided that the coflow temperature is high enough. By comparison, all previous MILD combustions have been experimentally established for diluted reactants with the 30


Page 31 of 37

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Energy & Fuels

oxygen fraction generally less than 10%. In those previous cases, both the typical “traditional” features of visible emissions and the “front-like” structure of flame disappear. Since the properties of flame thickening and pyrolysis depression are coincident with flameless properties5, the domain over which the MILD combustion takes place has been classified commonly as the flameless regime. However, it may be fundamentally significant to address the following question: whether does it make sense or not to refer to the flameless regime in the case of the MILD-like combustion with high oxygen fraction and high coflow temperature? Or to what extent does the mathematical definition reflect the consequence of a flame behavior which can be observed experimentally? To explore such an interesting issue, we quantify the flame thickening by estimating the flame volume for the cases of

X O* 2 = 7%, 13%, 30% and 100% at selected coflow temperatures, which refer respectively to all the combustion regimes of Fig. 8. Following our previous work24, the contour of RCO= 0.01 is regarded as the approximate border of the two-dimensional ellipsoid-like reaction zone, see Fig. 12, where RCO is the ratio of the local carbon monoxide (CO) molar fraction to its maximum over the whole

computation domain (1600 mm × 4500 mm). The volume (Λ) of the three-dimensional reaction zone, a quasi-oblate spheroid, is approximately estimated by Λ =

π 6

LW 2 , where L and W are the maximal

length and width of the reaction zone24. Fig. 13 displays the results, together with the marked regime of each case. Evidently, the reaction volume for each case grows, and thus the overall reaction rate * drops, nearly exponentially as Tcof increases. In other words, rising the coflow temperature results

in a substantial flame-thickening, consequently generating a necessary condition for the establishment of flameless combustion5. It is no doubt that all flames for the cases of X O* 2 = 7% are truly in the invisible state, whose reactive volume varies from the mimimum of 7.2×10-2 m3 to 0.56 * m3 at Tcof = 2400 K. In contrast, the reaction volume of the traditonal combustion (TC) is much

* * smaller with 7.2×10-3 m3 ( Tcof = 800 K) and 1.3×10-3 m3 ( Tcof = 400 K), respectively, for X O* 2 = 30%

31


Energy & Fuels

and 100%, thereby producing highly visible flames. 4500 mm Coflow

Length Width

Fuel Coflow

RCO=1

1600 mm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 32 of 37

RCO=0.01

Fig. 12. Definitions of the length and width of the reaction zone.

* . The vertical axis of plot is Fig. 13. Volume of the reaction zone versus the coflow temperature Tcof

in the logarithmic scale. The shadow area corresponds to the practically flameless state.

Consider the mimimum volume Λmin = 7.0×10-2 m3 as the threshold to simply identify the practically flameless state for all the present MILD-like combustions. Then, the calculations suggest that the 32


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* invisible combustion may be established when taking high coflow temperatures at Tcof ≥ 1450 K,

1950 K and 3000 K for X O* 2 = 13%, 30% and 100%, respectively. By comparison, the quantitative * ≥ 1040 K, 1545 K and 2145 K, MILD condition is much easier to be met that requires that Tcof

drastically smaller values than the above, for the same cases. As a result, there are a number of the MILD-like points out of the shadow area on the plot (see Fig. 13). It is thus suggested that the realization of practically flameless combustion may require higher coflow temperatures than does the the mathematical condition to be satisfied. In other words, the mathematical definition of Cavaliere and de Joannon4 for the MILD regime might not totally reflect the consequence of the flameless behavior which can be observed experimentally. Nevertheless, Fig. 13 also suggests that the practically flameless state of MILD-like combustion theoretically can be realized, despite requiring higher oxidizer temperatures than those by the mathematical definition.

6 CONCLUSIONS The present numerical study has classified the combustion regimes of a hydrocarbon jet flame in a hot and diluted or non-diluted oxidizer stream; the coflowing stream contain combustion products with varying oxygen concentration up to 100%. The modeling effectiveness has been proven by the predicted (mean) JHC flame characteristics which compare well with the previous meassurements17 under the same conditions. The classification is based on the suggestions of Cavaliere and de Joannon4 and others5-8 using the forced-ignition and auto-ignition temperatures (Tfi, Tai). The main conclusions drawn from the present work are summarized below: (1) From the predicted Tfi and Tai, the JHC or diffusion (in a more common term) flame can be classified into three distinct regimes of combustion: traditional combustion (TC) at the coflow temperature between Tfi and Tai, high temperature combustion (HTC) at both the coflow 33


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* temperature Tcof > Tai and the flame temperature increment ∆T > Tai, and flameless combustion

* (FLC) at ∆T < Tai. Within the FLC regime, the MILD or MILD-like combustion occurs at Tcof >

* Tai but ∆T < Tai whereas the quasi-MILD combustion takes place at ∆T < Tai and Tfi < Tcof < Tai.

Noteworthy is that the new findings of the MILD-like and quasi-MILD combustion are the contribution of the present work to the above classification. (2) In all the JHC flames predicted, a critical coflow oxygen concentration appears to occur at X O* 2 = X（O12） below which only the flameless MILD combustion can take place. The magnitude of

X（O12） appears to depend weakly on the fuel type and perhaps other initial conditions. The present value of X（O12） is approximately 8%, which agrees well with the inlet or local oxygen fraction leading to the MILD combustion obtained by most of previous studies1-32. In other words, the previous diffusion MILD combustions were generally established at X O* 2 ≤ X（O12） < 10%. (3) Significantly, the flameless combustion of any gas fuel appears to occur in a hot coflow with any oxygen concentration as long as the reactants temperature before chemical reactions is sufficiently high. (4) More generally, to establish the flameless combustion in any combustor system, either the low-oxygen or low-fuel dilution, claimed constantly in the past, does not appear to be necessary. Instead, sufficiently high temperatures of reactants prior to their main reactions do appear to be required. It is important to note, nontheless, that this requirement can be easily met without external air preheating, see Refs. 13 to 15.

ACKNOWLEDGEMENT The support of both Specific Research Fund for Doctoral Program of Higher Education of China (No. 34


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20110001130014) and Nature Science Foundation of China (No. 51276002) is gratefully acknowledged.

Reference [1] Wünning, J. A.; Wünning, J.G. Prog. Energy Combust. Sci. 1997, 23, 81-94. [2] Katsuki, M.; Hasegawa, T. Proc. Combust. Inst. 1998, 27 3135-3146. [3] Tsuji, H.; Gupta, A.K.; Hasegawa, T.; Katsuki, K.; Kishimoto, K.; Morita., M. High temperature air combustion: from energy conservation to pollution reduction; CRC press; boca Raton; FL;

2003. [4] Cavaliere, A.; De Joannon, M. Prog. Energy Combust. Sci. 2004, 30, 329-366. [5] De Joannon, M.; Sorrentino, G.; Cavaliere, A. Combust. Flame 2012,159, 1832-1839. [6] De Joannon, M.; Sabia, P.; Sorrentino, G.; Cavaliere, A. Proc. Combust. Inst. 2009, 32, 3147-3154. [7] De Joannon, M.; Sabia, P.; Cozzolino, G.; Sorrentino, G.; Cavaliere, A. Combust. Sci. Tec. 2012, 184(7-8), 1207-1218. [8] Chen, S.; Mi, J.; Liu, Z.; Zheng, C. Int. J. Hydrogen Energy 2012, 37, 5234-5245. [9] Li, P.; Dally, B. B.; Mi, J.; Wang, F. Combust. Flame 2013,160, 933–946. [10] Kumar, S.; Paul P. J.; Mukunda, H. S. Proc. Combust. Inst. 2002, 29, 1131-1137. [11] Dally, B. B.; Riesmeier, E.; Peters, N. Combust. Flame 2004,137, 418-431. [12] Weber, R.; Smart, J.P.; Kamp, W. Proc. Combust. Inst. 2005, 30, 2623-2623. [13] Li, P.; Mi, J.; Dally, B. B.; Craig, R.A.; Wang, F. Energ. Fuel 2011, 25, 2782-2793. [14] Mi, J.; Li P.; Dally, B.B.; Craig, R.A. Energ. Fuel 2009, 23, 5349-5356. [15] Mi, J.; Wang, F.; Li, P.; Dally, B. B. Energ. Fuel 2012, 26, 265-277. [16] Mi, J.; Li, P.; Zheng, C. Energy 2011, 36, 6583-6595. [17] Dally, B. B.; Karpetis, A.N.; Barlow, R. S. Proc. Combust. Inst. 2002, 29, 1147–1154. 35


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[18] Medwell, P. R.; Kalt, P.; Dally, B. B. Combust. Flame 2007,148, 48-61. [19] Medwell, P. R.; Kalt P.; Dally, B. B. Combust. Flame 2008, 152, 100-113. [20] Medwell, P. R.; Dally, B. B. Combust. Flame 2012, 159, 3138-3145. [21] Oldenhof, E.; Tummers, M.J.; Veen, E. H.; Roekaerts, D. Combust. Flame 2010, 157, 1167– 1178. [22] Oldenhof, E.; Tummers, M.J.; Veen, E. H.; Roekaerts, D. Combust. Flame 2011, 158, 1553– 1563. [23] Rao, G.. Levy, Y. A new combustion methodology for low emission gas turbine engines, The 8th HiTAC conference 2010, Poznan, July 5-8. [24] Mei, Z.; Mi, J.; Wang, F. Energ. Fuel 2012, 26, 3257-3266. [25] Christo, F. C.; Dally, B. B. Combust. Flame 2005, 142, 117-129. [26] Frassoldati, A.; Sharma, P.; Cuoci, A.; Faravelli, T.; Ranzi, E. Appl. Therm. Eng. 2010, 30, 376 -383. [27] Galletti, C.; Parente, A.; Derudi, M.; Rota, R.; Tognotti, L. Int. J. Hydrogen Energy 2009, 34 8339-8351. [28] Wang, F.; Mi, J.; Li, P.; Zheng, C. Int. J. Hydrogen Energy 2011, 36, 9267-9277. [29] De Joannon M.; Cavaliere, A.; Faravelli, T.; Ranzi, E.; Sabia, P.; Tregrossi, A. Proc. Combust. Inst. 2005, 30, 2605–2612.

[30] Plessing, T.; Peters, N.; Wünning, J. G. Proc. Combust. Inst. 1998, 27, 3197–3204. [31] Mancini, M.; Schwöppe, P.; Weber, R.; Orsino, S. Combust. Flame 2007, 150, 54-59. [32] Schaffel-Mancini, N.; Mancini, M.; Szlek, A.; Weber, R. Energy 2010, 35, 2752-2760. [33] Mancini, M.; Weber, R.; Bollettini, U. Proc. Combust. Inst. 2002, 29, 1155-1163. [34] De Joannon, M.; Saponaro, A.; Cavaliere, A.; Proc. Combust. Inst. 2000, 28, 1639–1646. [35] Smith, G. P.; D. Golden, M.; Frenklach, M.; Moriarty, N. W.; Eiteneer, B.; Goldenberg, M.; Bownan, C. T.; Hanson, R. K.; Song, S.; Gardiner, W.C; Lissianski, Jr. V.V.; Qin, Z.; GRI-Mech 36


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[38] Bilger, R. W.; Starner, S. H.; Kee, R. J.; Combust. Flame 1990, 80, 135-149. [39] Cavaliere A.; Joannon M de; Mild Combustion，Proc. of the European Combustion Meeting, 2005，http://www.gfcombustion.asso.fr/ecm/2005/261_Cavaliere.pdf.

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Combustion Regimes of a Jet Diffusion Flame in Hot Co-flow

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