Initial, Secondary, and Final Target Interactions

In this method, the premixed reactants flow along a cylindrical tube in a uniform laminar flow to the flame, which is stabilized at the burner rim via heat loss, forming a cone-shaped flame. A mean flame speed across the flame area can be obtained by dividing the volumetric flow rate of the mixture by the luminous cone surface area [18], or by measuring the local velocity component of the flow normal to the flame surface, Uo.sinθ where Uo is the flow velocity and θ is the angle of the half cone angle. The disadvantages of the burner method include the non-adiabaticity of flame due to heat loss to burner rim which tends to decrease the flame speed. The tip of the flame is affected by stretch effects due to the high curvature number which may

Laminar flame speed measurement techniques area which could lead to inaccurate results. The local normal velocity measurement method gives results to within 20 % accuracy for example, but this will be for a locally strained flame, due the non-normality of the streamlines to the flame.

2.3.2 Unsteady flame in a tube method

This method involves igniting the combustible mixture filled in a long cylindrical tube with an open end. The rate of the propagation of the flame into the unburned reactant along the tube is determined as the flame speed. If the flame appears to be hemispherical in the tube, the flame speed can be determined through the relation S_LA_f=u_mπR², where S_L is the flame speed, A_f is the cross-sectional area of the tube, um is the local hemisphere velocity and R is the radius of the hemisphere. However, this method contains some inherent weaknesses: (a) one is the buoyancy effect which distorts the flame front, resulting in the non-uniformity of the flame that deviates from the geometric area of the tube, (b) wall quenching also has a significant impact on the propagating speed, (c) a pressure wave is formed when the flame is propagating, causing the mixture ahead of the flame to gain velocity due to the change of density.

These effects have to be accounted for in the determination of unstrained flame speed [19].

2.3.3 Soap bubble method

The gas mixture contained in a soap bubble is ignited at the centre by a spark to create a spherical flame that spreads radially through the mixture. The burned gases expand radially outwards, causing the soap film to expand. The flame velocity can be determined via the relation S_L = V_fr_i³/r_f³, where V_f is the average spatial velocity of the flame front, ri is the initial radius of the soap bubble, and rf is the final radius of the sphere of burned gas [20]. The growth of the flame front along the radius is photographed at high speed to determine the flame propagation rate. This method assumes that the spherical flame spreads uniformly in radial direction under constant pressure. Some of the difficulties of this method include the uncertainty in the

Laminar flame speed measurement techniques temperature ratio of the burned and unburned gases, heat loss to the electrodes and deformation of flame cellular structure for fast flames [19, 20].

2.3.4 Flat flame method

A flat flame is stabilised via heat loss on a porous metal plate or a series of small tubes. The rate of heat loss is controlled by the mixture flow rate. The gaseous mixture is ignited at high flow rate and adjusted until the flame is flat. The diameter of the flame is measured and divided by the volume flow rate of unburned gas to determine the flame speed. However, this method is only applicable to mixtures having low burning velocities, on the order of 15 cm/s or less [21]. Botha and Spalding [22]

extended the flat flame method to measure higher flame speed by using a water-cooled porous disk. The cooling effect induces heat loss from the flame and stabilises the flame closer to the disk. The tests are repeated at different cooling rates so that the values of flame speed S_L can be plotted against the cooling rates. To obtain the adiabatic flame speed S_L, extrapolation of the curve of S_L versus cooling rate back to zero cooling rate is performed. Some uncertainty is associated with this method including the unknown loss of radical species such as H to the porous plate. Van Maaren et. al. [23] utilized the flat flame method to measure the adiabatic flame speed of methane/air mixtures and good agreement was achieved when compared to the literature. The adiabatic flame speed is determined based on the measurement of the burner plate temperature profile. The uniform plate temperature profile indicates zero net heat loss of flame and hence the adiabatic flame speed is obtained. The adiabatic flat flame method was further extended to measure the laminar flame speed of ethane, propane, n-butane and isobutene by Bosschaart and De Goey [24].

2.3.5 Spherical bomb method

In this method, a quiescent combustible mixture situated in a constant volume environment is ignited by a spark, causing a variation of pressure due to adiabatic compression of the unburned gas as the flame propagates outwardly. By simultaneously

Laminar flame speed measurement techniques recording the pressure history and instantaneous flame radius, the flame speed can be determined through the following expression

γ

⎛ − ⎞

=⎜ − ⎟

⎝ ⎠

3 3

1 2 L 3

u

R r dp dr

S p r dr dt ^(2.1)

where S_L is the flame speed, R is the sphere radius, r is the instantaneous flame radius and γ_u is the specific heat ratio of the unburned gas [20]. For spherically expanding flames, the stretch created on the premixed flame-front is well defined. The outwardly propagating flame images can be used to determine the unstretched laminar flame speed by means of extrapolation to zero stretch. The associated Markstein length Lb

with the unstrained flame speeds is useful in expressing the onset of flame instabilities and the stretch influence on flame quenching. From the plot of unstretched flame speed Sn against flame stretch rate K, the Markstein lengthof burned gas can be derived from the linear relation of S_l -S_n = L_bK, where S_l is the unstretched flame speed. The unstretched flame speed S_n can be determined at the intercept value of K = 0 [25, 26].

This method is the one of the few that can reach higher pressure, and it has been applied extensively for such measurements [27]. Some of the limitations of this method are the effect of buoyancy on flame, heat loss to electrodes due to intrusive ignition, stretch effects, and the development of intrinsic pulsating and cellular instabilities [1].

These weaknesses are also present in other methods, but those usually cannot be used at high pressure conditions.

2.3.6 Counterflow flame configuration

The counterflow flame configuration consists of two opposing jets with same air-fuel ratio and exit velocity is used to create two flat flames stabilized on stagnation planes. Determination of the unstrained laminar flame speed is performed by extrapolating the reference flame speed back to zero strain rates. The reference flame speed is defined as the local flame speed at the position before the flow accelerates through the flame whereas the corresponding strain rate is derived from the upstream axial velocity gradient [28]. The counterflow flame technique presents the advantage of

Laminar flame speed measurement techniques difficulties can arise in determining the location of the stagnation point, as slight fluctuations or time variations in jet momentum cause the point to move in space during the experiment. Coupling of the acoustic properties of the two jets can also lead to oscillations and instabilities in the flame [29].

2.3.7 Jet-wall stagnation flame configuration

The jet-wall stagnation flame configuration consists of one burner and a stagnation plate downstream of the burner outlet. The impingement of premixed hydrocarbon/air on the wall creates a flat, one-dimensional flame that stabilizes through hydrodynamic strain when ignited. The velocity of the flow from the burner outlet decelerates upon approaching the flame front. The flow accelerates when passing through the flame due to the expanded gas volume before slowing down again when approaching the wall. The reference flame speed is identified as the location before the flow accelerates through the flame, while the velocity gradient upstream of the flame is referred as the strain rate. The unstrained laminar flame speed can be obtained by extrapolating the reference flame speed as a function of strain rate back to zero strain rates.

The difference between the counterflow flame configuration and the jet-wall setup lies primarily on the downstream adiabaticity of the established flame. The opposed flame method has the advantage of maintaining downstream flame adiabaticity, whilst the jet-wall configuration loses heat to the solid wall. The effect of heat loss to the wall on laminar flame speed has been addressed by Egolfopoulos et al. [30]: their experimental and numerical results suggest that the impinging wall has negligible effect on the laminar flame speed, even though the wall temperatures are set far below the flame adiabatic temperature. Mendes-Lopes [31] quantified the effect of heat loss to the water-cooled stagnation plate and reported minimum effect of the plate on laminar flame speed. The use of lower strain rates and larger nozzle separation distances can increase the accuracy of laminar flame speeds [28, 32]. However, although laminar flame speed is minimally affected, the presence of the wall alters the structures of the flame and significantly influences the flame extinction mechanism [30].

Previous results from the jet-wall stagnation technique