Laminar flame speeds under engine-relevant conditions: Uncertainty quantification and minimization in spherically expanding flame experiments

Abstract

The spherically expanding flame method is the only approach for measuring laminar flame speeds at thermodynamic states that are relevant to engines. In the present study, a comprehensive evaluation of data obtained under constant pressure and constant volume conditions was carried out through experiments, development of a mathematically rigorous method for uncertainty quantification and propagation, and advancement of numerical models that describe the experiments accurately. The proposed uncertainty characterization approach accounts for parameters related to all measurements, data processing, and finally data interpretation. With the aid of direct numerical simulations, an alternative approach was proposed to derive laminar flame speeds in constant pressure experiments by eliminating the need for using extrapolation equations developed on simplifying assumptions, which are known to be susceptible to major errors under certain conditions. The propagation of spherical flames under constant volume conditions was investigated through experiments carried out in an entirely spherical chamber and the use of two numerical models. The first involves the solution of the fully compressible one-dimensional conservation equations of mass, species, and energy. The second model was developed based on thermodynamics similarly to existing literature, but radiation loss was introduced at the optically thin limit and approximations were made to allow for re-absorption with minimum computational cost. It was shown that neglecting radiation in constant volume experiments could introduce errors as high as 15%. Incorporating the aforementioned techniques, laminar flame speeds were measured and reported with properly quantified uncertainties for flames of synthesis gas for pressures ranging from 3 to 30 atm, and unburned mixture temperatures ranging from 298 to 550 K. Selected measurements were carried out as well for methane and propane flames for pressures ranging from 3 to 7 atm, and unburned mixture temperature of 298 K. The approaches introduced in this study allow for the determination of laminar flame speeds with notably reduced uncertainties under conditions of relevance to engines, which has major implications for the validation of kinetic models of surrogate and real fuels.

LES study on mixing and combustion in a Direct Injection Spark Ignition engine

“…The use of the Metghalchi and Keck correlation [26] allows to be consistent with the previous study of Robert et al [1,24,25]. Others most precise methods can be found in the literature to estimate the laminar flame speed [27,28].…”

Effects of stretch and thermal radiation on difluoromethane/air burning velocity measurements in constant volume spherically expanding flames

“…The range of smoothed ( t ) is narrowed: data at early times are eliminated for ( t ) < 1.25 P i , which is sufficient to remove ignition transients and electrical noise; data at longer times are eliminated beyond the maximum 2 nd derivative of the pressure trace, which has been suggested as a marker for when the flame begins to interact with the chamber walls [29]. Second, the relationship between the unburned/burned gas thermodynamic states and the flame radius ( R f ) was computed via the Hybrid ThermoDynamic-Radiation (HTDR) model [9] with species thermodynamic data taken from the NIST HFC mechanism [30]. Finally, the measured ( t ) and modeled ( P ) are combined to reconstruct S u and the flame stretch rate ( K ).…”

“…HTDR is a numerical, multi-zone, thermodynamic, spherically expanding flame model [9] which includes thermal radiation calculations based on RADCAL [31]. It assumes that the flame front is smooth, spherical, and infinitely thin; that no reactions occur in the unburned gas, which has uniform temperature and composition; that the pressure is spatially uniform but evolves in time; that the unburned gas compression occurs isentropically; and that the burned gas is in chemical equilibrium at any instant.…”

Measurements and modeling of spherical CH2F2-Air flames

“…In all of the above work, the simulations did not include the effects of stretch and radiation, which have typically been found to be important for slow burning flames [25]. Recently, Burrell et al [16] and Burgess et al [10] have presented new experimental data for CH2F2/O2/N2 mixtures, for a range of oxygen mole fraction and equivalence ratios ϕ, in constant volume experiments in which the effects of optically-thin thermal radiation have been included in the data reduction [26]. The 1-D, planar, adiabatic simulations in this work were performed using a newly developed detailed kinetic model for R-32 combustion based on the experimental burning velocity data.…”