Abstract
In order to experimentally study whether or not the density ratio σ substantially affects flame displacement speed at low and moderate turbulent intensities, two stoichiometric methane/oxygen/nitrogen mixtures characterized by the same laminar flame speed SL = 0.36 m/s, but substantially different σ were designed using (i) preheating from Tu = 298 to 423 K in order to increase SL, but to decrease σ, and (ii) dilution with nitrogen in order to further decrease σ and to reduce SL back to the initial value. As a result, the density ratio was reduced from 7.52 to 4.95. In both reference and preheated/diluted cases, direct images of statistically spherical laminar and turbulent flames that expanded after spark ignition in the center of a large 3D cruciform burner were recorded and processed in order to evaluate the mean flame radius bar {R}_{f}left (t right ) and flame displacement speed S_{t}=sigma ^{-1}{dbar {R}_{f}} left / right . {dt} with respect to unburned gas. The use of two counter-rotating fans and perforated plates for near-isotropic turbulence generation allowed us to vary the rms turbulent velocity u^{prime } by changing the fan frequency. In this study, u^{prime } was varied from 0.14 to 1.39 m/s. For each set of initial conditions (two different mixture compositions, two different temperatures Tu, and six different u^{prime }), five (respectively, three) statistically equivalent runs were performed in turbulent (respectively, laminar) environment. The obtained experimental data do not show any significant effect of the density ratio on St. Moreover, the flame displacement speeds measured at u′/SL = 0.4 are close to the laminar flame speeds in all investigated cases. These results imply, in particular, a minor effect of the density ratio on flame displacement speed in spark ignition engines and support simulations of the engine combustion using models that (i) do not allow for effects of the density ratio on St and (ii) have been validated against experimental data obtained under the room conditions, i.e. at higher σ.
Highlights
As reviewed elsewhere [1,2,3,4], turbulent flame speed St was in the focus of experimental research into premixed turbulent combustion for decades, with a number of new experimental data bases being built over the past twelve years, e.g. see [5,6,7,8,9,10,11,12,13,14,15,16,17,18]
After an initial phase associated with the spark discharge influence on the flame kernel, the data obtained from different runs collapse to a mean curve, which indicates an increase in the flame displacement speed with the flame kernel radius
At a higher u, both the scatter of the data around mean curves and the increase in St with Rf are more pronounced. Because these trends are beyond the scope of the present study, we refer the interested reader to [4, 64] where the growth of the flame displacement speed was well predicted in RANS simulations or to [65, 66] where two simplified models of the growth of St ( b/ u)dRf /dt (m/s)
Summary
As reviewed elsewhere [1,2,3,4], turbulent flame speed St was in the focus of experimental research into premixed turbulent combustion for decades, with a number of new experimental data bases being built over the past twelve years, e.g. see [5,6,7,8,9,10,11,12,13,14,15,16,17,18]. We restrict ourselves to pointing out that a substantial effect of the density ratio on turbulent flame speed is widely expected in theoretical and numerical combustion community, but such an effect is not indicated by experimental parameterizations of St
Sign-in/Register to view highlights & summary Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a callDisclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.
