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    Characterization of laminar premixed methanolair flames

    S.Y. Liaoa,b,*, D.M. Jianga, Z.H. Huanga, K. Zenga

    a State Key Laboratory for Multiphase Flow in Power Engineering, Xian Jiaotong University, Xian 710049, Peoples Republic of Chinab Department of Power Engineering, College of Chongqing Communication, Chongqing 400035, Peoples Republic of China

    Received 30 September 2005; received in revised form 15 December 2005; accepted 16 December 2005

    Available online 7 February 2006

    Abstract

    This study focuses on the effects of initial temperature and pressure on the propagation characteristics of laminar premixed flame of methanol

    air mixtures. Spherically expanding laminar premixed flames, freely propagating from spark ignition sources in initially quiescent methanolairmixtures, are continuously recorded by a high-speed CCD at various equivalence ratios and temperatures. The flames are then analyzed to deduce

    the flame speed. The stretch imposed on the spherical flame front is explored experimentally; as a consequence, the unstretched laminar burning

    velocities of methanolair flames have been derived. The present measurements are compared with the experimental data reported previously, and

    good agreements are obtained. Combined previous results, a correlation in the form ofulZuloTu=Tu0aTPu=Pu0

    bp has been developed to describe

    the dependences of initial temperature and pressure on the burning velocities of methanolair flames. The global activation temperatures are

    determined in terms of the burning mass flux. And then, the Zeldovich numbers for methanolair flames are estimated as a function of equivalence

    ratio. On the basis of the mass burning flux, an alternative correlation of laminar burning velocities has been proposed, and agreements can still be

    found in the comparison between this alternative forms and the power law correlation above.

    q 2006 Elsevier Ltd. All rights reserved.

    Keywords: Methanol; Laminar burning velocity; Premixed flame; Mass burning flux

    1. Introduction

    Methanol has been demonstrated to be one of the

    promising alternative fuels for spark ignition engines. The

    most important characteristic of methanol is that it is

    undoubtedly the cheapest liquid alternative fuel per calorific

    unit, which can be produced from the widely available

    fossil raw materials including coal, natural gas and bio-

    substances, that it can be produced from biological sources

    and therefore represents a renewable energy source [1,2].

    This essentially means that many countries can solve their

    energy imbalance problems due to petroleum shortage by

    using methanol as a source of energy.

    It is known that, laminar burning velocities are fundamen-

    tally important in regard to developing and justifying the

    chemical kinetics mechanism of fuel, as well as in regard to

    predicting the performance and emissions of the internal and

    external combustion system [3]. Generally, chemical kinetics

    of flames can be studied by performing numerical calculation

    by solving balance equations of one-dimensional laminar

    flames, but the chemical kinetic data in such models are not

    always sufficiently well known to be used in confidence. In

    practice, the measured laminar burning velocities are

    commonly used to validate these chemical kinetic schemes.

    There are many techniques for experimentally measuring the

    laminar burning velocity of combustible mixture, such as

    counterflow double flames [4], flat flame burner [5], heat flux

    method [6], and closed bomb technique [714]. For spherically

    expanding flames in closed combustion bomb, the stretch

    imposed on the premixed flame front is well defined.

    Furthermore, the asymptotic theories and experimental

    measurements have suggested a linear relationship between

    flame speeds and flame stretches [15]. As we know, the term of

    laminar burning velocity is generally defined for one-

    dimensional planar flames in theory, where flames are

    unstretched. Therefore, measured flames can then be used

    systematically to determine the fundamental burning velo-

    citythe unstretched laminar burning velocity by mean of an

    extrapolation at zero stretch [3]. Meanwhile, Markstein lengths

    can, in principle, also be deduced. Here, Markstein length

    characterize the variation in the local flame speed due to the

    influence of external stretching, which is important in

    Fuel 85 (2006) 13461353

    www.fuelfirst.com

    0016-2361/$ - see front matter q 2006 Elsevier Ltd. All rights reserved.

    doi:10.1016/j.fuel.2005.12.015

    * Corresponding author.

    E-mail address: [email protected] (S.Y. Liao).

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    expressing the onset of flame instabilities and the stretch

    influence on flame quenching as well.

    The measurements of burning velocities have been

    extensively studied for a wide variety of hydrocarbon fuels,

    including methane, ethane, propane, butane, and octane etc.

    However, as to methanol, it has not been studied so far in the

    literatures. Metghalchi and Keck, Gulder, and Saeed and Stonemeasured the burning velocities of methanolair mixture in

    closed combustion bombs. However, in those previous studies,

    a continuous observation of flame growth were not always

    implemented and the laminar burning velocities were

    generally, determined by resolving various combustion models

    based on combustion pressure traces, but those procedures

    commonly ignored the influence of stretch imposed on the

    flame, hence the Markstein lengths have not been obtained as

    well. Meanwhile, discrepancies still exist, to some extent,

    within these experimental results. In addition, Gibbs and

    Calcote [16], Muller et al. [17], and Westbrook and Dryer [18]

    conducted flame chemical kinetics computations to determinethe laminar flame speed, however, the results obtained still

    appear to show somewhat obvious scatterings.

    In view of considerations above, the motivation of this work

    is mainly due to that the burning velocity data of methanol is

    still scare and less accurate. This work presents experimental

    measurements of the laminar burning velocities of methanol

    air premixed flames. Spherically expanding laminar premixed

    flames, freely propagating from spark ignition sources in

    initially quiescent methanolair mixtures, are continuously

    recorded by a high-speed CCD at various equivalence ratios

    and temperature conditions. The flames are analyzed to

    estimate flame size, consequently, the flame speeds are derived

    from the variations of the flame size against time elapsed.

    Following the linear relation between flame speed and flame

    stretch, the unstretched laminar burning velocities and

    corresponding Markstein lengths of flames have been deduced.

    The effects of the fuel/air equivalence ratio, initial temperature

    and pressure on the laminar flame propagation have been

    studied in detail. As a result, empirical formulas for the laminar

    burning velocities are suggested, and comparison with the

    previous results is made as well.

    2. Experiments and procedures

    Shown in Fig. 1 are the schematic diagrams of the

    combustion bomb and the optical system used for recording

    the flame growth. The combustion bomb has an inside size of

    108!108!135 mm, as shown in Fig. 1A. Two sides of this

    bomb are transparent to make the inside observable; these sides

    are to provide the optical access, and the other four sides are

    enclosed with resistance coils to heat the bomb to the desired

    preheat temperature. The inlet/outlet valve is used to let fresh

    air or combustion product in or out. The liquid fuel needed is

    pre-calculated corresponding to the given equivalence ratioand then injected into the combustion chamber using a small

    capability injector. The normal function of the motion of

    perforated plate is to provide a turbulent combustion

    environment if needed, where it is only used to enhance the

    vaporization of methanol and make the reactants mixed well.

    Two extended stainless steel electrodes are used to form the

    spark gap at the center of this bomb. A conventional battery-

    coil ignition system is used for producing the spark. The history

    of the shape and size of the developing flame kernel is recorded

    by a REDLAKE HG-100K high-speed CCD camera, operating

    at 5000 pictures per second with a schlieren optical system, as

    given in Fig. 1B. Dynamic pressure used to determine burned

    gas and unburned gas properties herein, is measured since

    spark ignition with a piezoelectric absolute pressure transdu-

    cer, model Kistler 4075A, with a calibrating element Kistler

    4618A.

    Fig. 1. Schematic diagram of experimental system.

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    The laminar burning velocity can be deduced from the well-

    established expanding flames method as described in [7]. In

    this approach, the stretched flame velocity, Sn, is derived from

    the flame radius versus time data t as,

    SnZdrudt

    (1)

    where flame size ru is defined as the schlieren flame size. The

    definition of the flame stretch, a, of a flame front in a quiescent

    mixture is given by,

    aZ1

    A

    dA

    dt(2)

    where A is the flame front area. For a spherically outward

    expanding flame, the flame stretch is well defined as,

    aZ1

    A

    dA

    dtZ

    2

    ru

    drudtZ

    2

    ruSn (3)

    Following an early idea of Markstein, it is suggested a linear

    relationship between flame speeds and flame stretches, given as

    Eq. (4),

    SlKS

    nZL

    ba (4)

    Thereby, the unstretched flame speed, Sl, can be obtained as

    the intercept value at aZ0, in the plot of Sn against a. The

    value of burned gas Markstein length, Lb, can also be obtained

    from the plot of experimental values of Sn against a, as

    mentioned in Eq. (4), using linear regression shown in

    Appendix A.

    For the observation limited to the early stage of the flame

    expansion, where the pressure does not vary significantly yet,

    there is a simple relationship linking the spatial flame velocity,

    Sl, to the fundamental one, i.e. the unstretched laminar burning

    velocity, ul, as

    ulZrbSl=ru (5)

    Here, ru is the density of the unburned gas and rb that of the

    burned gas, which can be determined from a quasi-dimensional

    two-zone combustion model [19].

    3. Results and discussions

    After the spark occurs, ignition takes place in the center of

    the chamber, and the flame propagates spherically to the whole

    mixture. Shown in Fig. 2 is a typical case of flame propagation,

    it can be seen that the schlieren flame images show a disk

    whose diameter increases with time. As far as the visualization

    of flame propagation was concerned, the key aim is to obtain

    the flame propagating speed. The flame image analysis can

    then be performed to deduce the evolution of flame radius

    versus time; hence the spatial flame speed is evaluated as the

    rate of variation in flame size (Eq. (1)). In order to characterize

    the flame propagation, one can plot the evolution of the flame

    speed in function of the flame radius, as shown in Fig. 3. As we

    know, the role of spark is to initialize a flame, which is to

    overcome the tendency for the flame to quench because of the

    high curvature stretch rate, during the early stage of flamepropagation. It is obvious that the flame speeds illustrate a

    decrease initially, and then increase gradually as the radius

    increases, because of the relatively higher flame stretch and

    heat loss during the early stage of the flame propagation [7].

    The effects of the fuel/air equivalence ratio and preheated

    temperature on the flame speed are studied in this figure. It is

    clearly shown that the increase in flame speed with radius is

    Fig. 2. Typical growing schlieren flame kernels for methanolair mixture with fZ1.2. The time interval is 4 ms, TuZ358 K, PuZ0.1 Mpa.

    Fig. 3. Variations of Sn with flame radius, ru for methanolair mixtures

    (0.1 MPa), and (A) equivalence ratio, f, (B) initial temperature, Tu.

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    much greater for the stoichiometric flames and high tempera-

    ture can accelerate the flame propagation.

    The local stretch imposed on the spherical flame can beevaluated via Eq. (3). Thereby, the spatial flame velocity can

    be plotted as a function of the stretch rate, to characterize the

    stretch effect on the flame propagation. Shown in Fig. 4 is a

    selection of experimental data showing the variations of flame

    speed, Sn with total stretch rate, a, at different equivalence

    ratios and initial temperatures. According to Eq. (4), the

    unstretched flame speed can be obtained by a zero stretch

    extrapolation, also shown in Fig. 4 with solid lines, and the

    slope of straight line indicates the burned gas Markstein length

    Lb. Then, the fundamental burning velocity, unstretched

    laminar burning velocity can be derived from the value of

    the unstretched spatial flame speed via Eq. (5). Fig. 4 shows

    that the flame accelerates with the decrease of stretch rate as the

    flame propagates, which denotes positive burned gas Markstein

    lengths for all tested flames. This figure also presents that the

    flame speed of mixture with fZ0.8 is more sensitive to the

    flame stretch than that of richer flames, relatively. However, it

    also appear that there is no obvious difference between mixture

    offZ1.0 and 1.2. This can be an indication that the curves of

    mixture with fZ1.0 and 1.2 are relatively flatter, compared to

    that of flames of fZ0.8 (Fig. 4A). The effect of initial

    temperature on the flame propagation is investigated as well.

    The increasing temperature results in a corresponding increase

    in the flame speed. Shown in Fig. 4B is the variation of flame

    speed with stretch for stoichiometric flames at different initial

    temperature conditions. However, apart from the overall

    magnitude of the flame speed, there appears little influence

    of temperature on flame behaviour because it is indicated thatthe slopes of flame speed curves do not change obviously.

    Fig. 5 exhibits the influences of fuel/air equivalence ratio

    and initial temperature on the flame/stretch interaction,

    namely, quantified by the burned gas Markstein lengths Lb.

    With regard to the values of Lb of methanolair flames, few

    experimental results have been reported previously. Our

    measurements report that the value of Lb decreases slightly

    with the increasing equivalence ratio, however, when fO1.0,

    the value ofLb almost appears no change with the variation of

    the equivalence ratio. The effect of the preheat temperature on

    Lb can be neglected, as it is shown that Lb varies little for

    different temperature conditions. Generally, all of the

    Markstein lengths are positive. This can be demonstrated bythe fact that the measured flames are smooth, in which no

    instability occurs during the atmospheric pressure experiments,

    for the present range of fuel/air equivalence ratio.

    Shown in Fig. 6 are the measured laminar burning velocities

    for methanolair flame at 358 K and the atmospheric pressure,

    over a wide equivalence ratio range. In this figure are also

    plotted the results obtained previously for comparison.

    Obviously, a good agreement of our measurements with

    f=0.8

    f=1.0

    f=1.2

    a/s1

    Fig. 4. Variations of flame speeds with different stretch rates, and (A)

    equivalence ratio, f, (B) initial temperature, Tu.

    Fig. 5. Measured Markstein lengths of methanolair flames as a function of

    fuelair equivalence ratio. The curves are second order fitting to measured

    points.

    Fig. 6. Experimental laminar burning velocities for methanolair flames at

    358 K and 0.1 MPa.

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    literature values of Metghalchi and Keck can be found, while

    we slightly underestimate the laminar flame velocity for rich

    mixtures, compared to those obtained by Saeed and Stone.

    According to these results, at normal pressure of 0.1 MPa and

    temperature of 358 K, the maximum burning velocity for

    methanolair flame appears to be approximately between 54

    and 61 cm/s for Metghalchi et al. and Saeed et al., respectively,whereas the present measured value is about 56.2 cm/s. Over

    the measured range of the equivalence ratio, the present results

    can be fit by a second order polynomial of the equivalence ratio

    as Eq. (6), also shown in Fig. 6 with a solid curve.

    ul0ZK195:6f2C419:92fK169:43 (6)

    where the subscript o represents the reference conditions, i.e.

    358 K and 0.1 MPa in this work. The maximum burning

    velocity in Eq. (6) is about 55.9 cm/s at an equivalence ratio of

    1.07, corresponding to 56.2 cm/s of experiment. Generally

    speaking, one can see that, the fitting burning velocities showgood agreements with the results of experiment over a wide

    range of equivalence ratio. In particular, the study of the effect

    of temperature on the burning velocities has been emphasized

    in this work. Fig. 7 illustrates temperature dependence of the

    laminar burning velocities of methanol at atmospherical

    pressure. More often, this dependence generally is expressed

    as a simple power law relation of nondimensional temperature

    (Tu/Tu0) and nondimensional pressure (Pu/Pu0), at the datum

    conditions, as

    ulZuloTu=Tu0aTPu=Puo

    bP (7)

    where aT and bP represent the parameters of temperature and

    pressure dependences. The power law relation is also presented

    in Fig. 7 as solid curves, where initial pressure of premixed

    mixture has no change. It can be seen that the plotted curves of

    the laminar burning velocities exhibit good agreements with

    experimental data over the equivalence ratio range. The

    parameter aT is clearly influenced by the fuel/air equivalence

    ratio. Table 1 lists the present results for aT. Moreover, in this

    table are also presented the results reported previously for

    comparison. Metghalchi and Keck reported that aT is function

    of the equivalence ratio, which was formulated as aTZ2.18K

    0.8 (fK1) at 300 K and 0.1 MPa, while Gulder was given a

    constant of 1.75. Although slight differences can be found in

    our results against previously studies, partially because of thedifference of the datum temperature, a similar variation of

    parameter aT against equivalence ratio can also be obtained, as

    given in Eq. (8),

    aTZ1:85K0:6fK1 (8)

    Fig. 7 also shows the comparisons of the burning

    velocities for methanolair flames at various temperatures

    derived from Eq. (7) with those derived from the published

    empirical formulas. On the whole, it is found that our

    predictions are more consistent with those of Metghalchi and

    Keck, and especially, better agreements can been obtained

    for the mixture at relatively low temperature. However, theresults of Saeed and Stone gradually underestimated the

    laminar burning velocities with the increase of initial

    temperature.

    The pressure effect on the laminar burning velocities has not

    been investigated experimentally in this work. Metghalchi and

    Fig. 7. Laminar burning velocity of methanolair flames on temperatures at

    different equivalence ratios.

    Table 1

    Evolution ofaT with equivalence ratio

    aT(Metghalchi and Keck)

    aT(Gulder)

    aT(present study)

    fZ0.8 2.47 1.75 1.98

    fZ1.0 2.11 1.83

    fZ1.2 1.98 1.75

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    Keck, Saeed and Stone had reported that the pressure

    dependence, bP is weakly influenced by the fuel/air equival-

    ence ratio. On the basis of the available data reportedpreviously (Fig. 8), we can obtain an empirical relation to

    describe the dependence bP on the fuel/air equivalence ratio, as

    given in Eq. (9).

    bTZK0:1651C0:2fK1 (9)

    Thereby, combining Eqs. (6), (8) and (9), Eq. (7)

    provides an approach to empirically estimate the laminar

    burning velocities of methanolair mixtures by functions of

    temperature, pressure and equivalence ratio. In order to

    validate these empirical formulas, it is informative to

    compare its behaviour over more wide temperature and

    pressure ranges, with those reported previously, as shown inFigs. 9 and 10. It is known that, high temperature can

    accelerate the rate of combustion chemistry, thus, the

    laminar burning velocities increase consistently with the

    increase of initial temperature of combustible mixtures. In

    Fig. 9, we can find that, the predictions of Metghalchi and

    Keck are more sensitive to temperature changes. When

    temperature varies from 358 to 480 K, the derivations

    among these empirical relations relatively become signifi-

    cant, showing that the predicted results of Metghalchi and

    Keck are slightly overestimates the laminar burning

    velocities than the others at 480 K. The effect of initial

    pressure on the laminar burning velocities is presented in

    Fig. 10. The general trend is that, the increase of pressure

    results in the decrease of the laminar burning velocities. No

    direct experimental comparison has been made herein.Similarly, the available predicted results of Metghalchi

    and Keck, Saeed and Stone are presented for comparison.

    The overall agreement among these results is very good

    over the whole range of equivalence ratios.

    It is suggested that the expression of the laminar burning

    velocity in terms of the square root of an Arrhenius expression

    in adiabatic flame temperature Tb with overall activation

    energy E, has more significant theoretical basis than does the

    power law correlations as given above [2022]. Peters and

    Williams [21] has derived an asymptotic structure of the flame

    that introduces the inner layer temperature T0 in the fuel

    consumption, and express the activation temperature E/R in

    terms of the mass flux (ruul) and adiabatic flame temperature Tbas following,

    E

    RZ2T2b

    dlnruul

    dTb(10)

    where R denotes gas constant and an assumption that T0

    remains constant was adopted. In practice, Eq. (10) can also be

    rewritten as a alternative form as

    E

    RZK

    d2lnruul

    d1=Tb(11)

    This indicates that the activation temperature E/R can be

    derived directly from the linear plot of 2ln(ruul) against 1/Tb.

    Meanwhile, Eq. (11) also suggests that the evaluation of E/R

    requires the determination ofruul in advance. It is known that,

    for a given mixture with Tu, the adiabatic flame Tb and the mass

    burning flux ruul can be determined from chemical equilibrium

    calculation and Eqs. (6)(9). That is to say, the power law

    relations of Eqs. (6)(9) just provide an approach to determine

    activation temperature of flames.

    Plots of 2ln(ruul) against 1/Tb are shown in Fig. 11. The

    activation temperature at each pressure is given by the slope of

    appropriate straight fitting line. It is found that E/R of

    methanolair flames is a weak function of temperature, while

    strongly depends on initial pressure of mixtures, which appears

    Fig. 8. Parameter bP for methanolair mixtures, the straight line shows the least

    squares fit for Eq. (9) to points.

    Fig. 9. Effect of temperature on burning velocities for methanolair flames.

    Fig. 10. Effect of pressure on burning velocities for methanolair flames.

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    to have a similar behaviour for methaneair [7] and iso-octane

    air flames [1]. Over the studied range, E/R can be

    approximately expressed by,

    E

    RZ2566PuC16085 (12)

    With the pressure Pu in MPa, and the intercept value of

    fitting lines at 1/TbZ0, C is found to be,

    CZ10:88P0:253u (13)

    Integration of Eq. (11) gives:

    lnruulZKE

    2R

    1

    TbC0:5C (14)

    Or rearrangement gives:

    ulZ exp KE

    2R

    1

    Tb

    exp0:5C

    ru(15)

    When E/R and Cpresumed from Eqs. (12) and (13), Eq. (15)

    gives an alternative correlation of laminar burning velocities to

    that presented by Eq. (7). The calculated laminar burning

    velocities using Eq. (15) together with Eqs. (12) and (13) are

    also plotted in Figs. 9 and 10. We can see that, the predicted

    burning velocities of Eq. (15) exhibit good agreements withthose of Eq. (7), and still has no significant deviation with the

    predicted results by Metghalchi and Keck, and Saeed and

    Stone. Certainly, Eq. (15) is directly derived from the empirical

    correlation of Eq. (7), rather than experimental measurements,

    that is to say, the former can only be thought as an alternative

    form of the later, the good agreement for Eq. (15) just

    demonstrates that the derivation in deduction of Eq. (15) from

    Eq. (7) is acceptable. However, as mentioned above, the former

    has more physical relevance, which just is the significance of

    this procedure.

    From the values of E/R, the Zeldovich number, b can be

    determined [7] as,

    bZE

    2RT2bTbKTu (16)

    Here b represents the sensitivity of chemical reactions to the

    variation of the maximum flame temperature and the inverse of

    it denotes an effective dimensionless width of the reaction zone

    [20]. Shown in Fig. 12 are the variations ofb against pressure

    for methanolair mixtures. The values ofb is clearly influenced

    by equivalence ratio, temperature and pressure. The increase of

    pressure results in increasing b, because of the increase of inner

    flame temperature T0 [17]. Also, it is out of question that, b

    attains a minimum for increased Tu and near-stoichiometric

    mixture, which has a maximum Tb. More often, another form of

    Zeldovich number, Ze, defined as

    ZeZbTbKTu

    T0KTu(17)

    The predicted results of Ze for methanolair are plotted in

    Fig. 13, and the values of methaneair and iso-octaneair

    mixtures are presented for comparison. The present results and

    those of methaneair generally show weak dependence on

    pressure than those computed values for iso-octaneair

    mixtures. And methanolair gives a similar tendency against

    the variation of equivalence ratio with methaneair flames,

    which is obviously contrast to iso-octane mixture in air.

    Fig. 12. Variations of b with the pressure for methanolair mixture with

    different equivalence ratio.

    Fig. 11. Plots of 2ln(ruul) against 1/Tb for methanolair mixtures. Where full

    lines are linear fits through the points obtained from Eqs. (6)(9), and dashed

    lines denote burning mass flux correlation of Eq. (14).Fig. 13. Variations of Ze with the pressure for methanolair mixture with

    different equivalence ratio, and the results of methaneair [7] and iso-octane

    air [17] are presented for comparison, where &, 0.1 MPa; B, 0.5MPa; 6,

    1.0 MPa.

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    4. Conclusions

    In this paper, centrally ignited, spherically expanding

    flames have been measured using schlieren photography to

    determine the influence of fuel/air equivalence ratio and

    initial temperature on the laminar burning velocities of

    methanolair mixtures. The stretch imposed at the flamefront has been explored experimentally and the Markstein

    lengths are estimated to characterize its effect. Combined

    the measurements and previous results, an empirical relation

    by the function form ulZuloTu=Tu0aTPu=Pu0

    bT , has been

    obtained, to formulate the laminar burning velocity

    dependencies on the equivalence ratio, initial temperature

    and pressure. On the basis of the asymptotic theory of flame

    structure, an alternative burning velocities correlation has

    been derived from above power law formula, and the

    expressions for mass burning flux, Zeldovich numbers have

    been obtained. The comparisons of our empirical corre-

    lations for burning velocities with the results reported

    previously show a good agreement, which validates the

    present study.

    Acknowledgements

    This work is supported by the state key project of

    fundamental research plan of Peoples Republic of China

    (No. 2001CB209206), and the key project of NSFC (No.

    50156040).

    Appendix A. Linear regression

    Consider Npairs of computed or measured values of (a, Sn)i.

    They satisfy Eq. (4):

    SlKSnZLba (A1)

    The value ofLb is calculated from

    LbZK

    PNiZ1

    aiK aSniKSn

    PNiZ1

    aiK a2

    (A2)

    in which

    Sn Z1

    N

    XNiZ1

    Sni; aZ1

    N

    XNiZ1

    ai; SlZ SnCLb a (A3)

    The standard derivation is given by

    eZ

    ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1

    NK2

    XNiZ1

    SnKSn 2CLb

    XNiZ1

    aiK aSniKSn

    " #vuut(A4)

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