2013-D. Bradley-PCI
2013-D. Bradley-PCI
2013-D. Bradley-PCI
com
Proceedings
of the
Combustion
Institute
Proceedings of the Combustion Institute 34 (2013) 1519–1526
www.elsevier.com/locate/proci
Abstract
The implosion technique has been used to extend measurements of turbulent burning velocities over
greater ranges of fuels and pressures. Measurements have been made up to 3.5 MPa and at strain rate
Markstein numbers as low as 23. The implosion technique, with spark ignition at two opposite wall posi-
tions within a fan-stirred spherical bomb is capable of measuring turbulent burning velocities, at higher
pressures than is possible with central ignition. Pressure records and schlieren high speed photography
define the rate of burning and the smoothed area of the flame front. The first aim of the study was to extend
the previous measurements with ethanol and propane–air, with further measurements over wider ranges of
fuels and equivalence ratios with mixtures of hydrogen, methane, 10% hydrogen–90% methane, toluene,
and i-octane, with air. The second aim was to study further the low turbulence regime in which turbulent
burning co-exists with laminar flame instabilities.
Correlations are presented of turbulent burning velocity normalised by the effective rms turbulent veloc-
ity acting on the flame front, ut =u0k , with the Karlovitz stretch factor, K, for different strain rate Markstein
numbers, a decrease in which increases ut =u0k . Experimental correlations are presented for the present mea-
surements, combined with previous ones. Different burning regimes are also identified, extending from that
of mixed turbulence/laminar instability at low values of K to that at high values of K, in which ut =u0k is
gradually reduced due to increasing localised flame extinctions.
Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Keywords: Premixed turbulent flames; Turbulent burning velocity; High pressure; Turbulent/laminar flame instabilities;
Explosion measurements
1540-7489/$ - see front matter Ó 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.proci.2012.06.060
1520 D. Bradley et al. / Proceedings of the Combustion Institute 34 (2013) 1519–1526
flame front and the mass rate of burning during to study the regime of K < 0.1, in which there is
the implosions, to yield associated values of tur- strong evidence of coupling between turbulence
bulent burning velocity, ut. and laminar flame instabilities, giving increased
Over a wide range of conditions the ratio of ut, values of U [5,6]. The fuels in [2,3] were ethanol
to the effective rms turbulent velocity, ut =u0k , that and propane and in the present study, hydrogen,
allows for the development of the flame kernel, methane, toluene and i-octane, up to a maximum
can be expressed in terms of the Karlovitz stretch pressure of 3.5 MPa and 480 K.
factor, K, by [2,3]:
ut =u0k ¼ U ¼ aK b for K > 0:05: ð1Þ 2. Experimental method
a and b are constants expressed by first order Key dimensions are represented for the propa-
expressions in terms of the strain rate Markstein gating flame kernels on Fig. 1. Turbulent flame
number, Masr. K is given by [3]: surfaces are represented by smoothed spherical
K ¼ 0:25ðu0 =u‘ Þ2 R0:5
l ; ð2Þ surfaces, with the mass of unburned gas within
the surface equal to the mass of burned gas out-
0
where u is the measured rms turbulent velocity side it. The surface is defined in relation to the
and Rl the turbulent Reynolds number, u0 l/m, with schlieren front. The centre of the curvature of
l the integral turbulence length scale and m the the smoothed flame front, of flame radius r, is usu-
kinematic viscosity. ally outside the inner wall of the spherical bomb.
The study has two principal aims. The first is The complete analysis leading to the derivation
to measure ut with five additional and contrasting of ut, assumed to be the same for both kernels,
fuels at different pressures and equivalence ratios, is given in [2]. The mass burned, mu, is deduced
/, to ascertain whether Eq. (1) is generally appli- from the flame front geometry. This must be com-
cable. The range of Masr values is extended from patible with that deduced from the measured pres-
11 to 3 in [2] to 23 to 5. This parameter is sure, p. Values of k, which controls the position of
important, in that it expresses fuel effects arising the centre of the flame cusp radius, see Fig. 1, for
from flame stretch sensitivities. The maximum each kernel were fine-tuned to achieve this. The
laminar flame speed has been similarly used as a area of each single flame surface was found and
correlating parameter in [4]. The second aim is this together with dmu/dt, yielded [2]:
Table 1
Ranges of experimental parameters in present study, pressure in MPa and u0 in m/s.
H2 CH4 C7H8 C8H18 10% H2–90% CH4
/ 0.3,0.4,0.5,0.6,0.8 0.9 1.0,1.2 1.0,1.4 0.8, 1.0,1.2
po 0.5,0.7,1.0 0.1,0.5,0.75,1.0,1.25 0.5,0.75,1.0 0.5,1.0 0.5,0.75,1.0
u0 1,2,3,4 0.2,0.25,0.5,0.75,1,1.5,2 1,2,3,4 1,2,3 1,2
D. Bradley et al. / Proceedings of the Combustion Institute 34 (2013) 1519–1526 1521
Fig. 3. Variations of ut with u0k for CH4–air, / = 0.9, po = 0.5 and 1.25 MPa, for different u0 . Solid and broken lines are
best fit curves. Values of velocity are for u0 . Each large symbol represents a particular pressure in MPa, indicated by an
adjacent number.
Fig. 4. Variations of ut with u0k for different lean H2–air mixtures, / = 0.4, 0.6 and 0.8, po = 1.0 MPa and different u0 .
Solid and broken lines are best fit curves. Values of velocity are for u0 . Each large symbol represents a particular pressure
in MPa, indicated by an adjacent number.
1522 D. Bradley et al. / Proceedings of the Combustion Institute 34 (2013) 1519–1526
3. Experimental results
Fig. 6. Correlations of present measurements for negative Masr, K > 0.05. Full line curves are best fit curves. Broken
curves express Eqs. (1), (4), and (5), for K P 0.1. Cross and multiplication symbols are from data in [2] and [3].
Variations of ut with u0k during implosions are Values of ut increase linearly with u0k in Figs. 3
given in Figs. 3 and 4. Figure 3 is for CH4–air, with and 4, up to 3.5 MPa. For the lean H2 mixtures in
initial pressures of po = 0.5 and 1.25 MPa for / Fig. 4, values of ut at a given u0k tend to increase
= 0.9, and at different u0 . Figure 4 is for hydro- with /, as Masr decreases. Space limitations
gen–air mixtures, po = 1.0 MPa, with / = 0.4, 0.6 preclude the presentation of all the experimental
and 0.8, also at different u0 . Different symbols show data, but these are available from the correspond-
the scatter in ut, while the lines represent the best fit. ing author.
1524 D. Bradley et al. / Proceedings of the Combustion Institute 34 (2013) 1519–1526
Table 3
Initiation and termination of instability.
Masr Ks [16] Initiation Maximum value Termination
Ki ls/lG Km ls/lG Kt ls/lG
3 2270 0.004 4.6 0.0073 15.5 0.009 23.5
19 50 0.011 0.77 0.048 14.7 0.07 31.4
23 50 0.012 0.64 0.045 13.0 0.064 26.2