Effects of fuel and soot characteristics on the inception and development of contrails

Abstract

Fundamental questions related to the roles of fuel type, combustion parameters, and turbulence transport interactions in the inception and growth of contrails have remained intractable in remote sensing and in-flight measurements. Consequently, we developed a novel laboratory-scale facility for studying the inception, growth and persistence of contrails for aircraft-relevant conditions. The exhaust gas generated using an inverted co-flow soot generator at a set of global equivalence ratios for two fuels - ethylene and propane is supplied to the contrail tunnel, which then mixes with an ambient flow emulating long-haul aircraft cruise conditions (20.8 kPa and 190 K). Detailed soot characterization using a scanning mobility particle sizer and transmission electron microscopy is coupled with measurements of instantaneous and averaged scattering intensities from the generated contrails. The experimental results are complemented by numerical simulations of the contrail tunnel using solutions of the Favre-averaged Navier-Stokes (FANS) equation and a two-equation model for handling particulate matter, including soot and ice. Results show, for the first time, the cross-section of a contrail, and the interaction of turbulent mixing and microphysical growth scales involved in ice nucleation across the shear layers. The average scattering cross sections of contrails increase with equivalence ratio, due to higher soot number concentrations and water vapor content. Comparisons between ethylene and propane exhausts indicate that the scattering propensity of contrails is more sensitive to exhaust water vapor content than to soot concentrations. Finally, depolarization measurements are used to show asphericity in ice crystal habits. Thus, our study present a unique window into contrail formation, theoretical modeling and simulation.

Figures (8)

• Figure 1: Schematic of the contrail facility developed at the University of Toronto. Aircraft engine exit conditions at cruising altitude is simulated by the ambient and core jet flow in the test section where various measurements are combined to assess the formation of contrails. (Right) Snapshot of instantaneous scattering at $532$ nm due to ice formation for C2H4 flame at $\phi=0.161$, $T_j=560$ K, $T_\infty=190$ K, $p_{\infty}=20.8$ kPa and $S_\infty^v=0.35$.
• Figure 2: Properties of soot particles generated by the inverse diffusion flame for the two fuel types. (a,b) Observed fractal dimension $\mathcal{D}_F$ determined from TEM imaging of soot particles. The coefficient of fit for the scaling regimes in all cases is $R^2>0.87$, $L$ is characteristic length of soot aggregates and $N_{\textrm{act}}$ is the total number of primary particle in aggregates. The plots are shifted by a factor of 10 between cases for visual clarity. (c-d) Log-normal soot size distribution obtained through mobility measurements.
• Figure 3: (a) Temperature field which is maintained constant for all the experimental conditions. (b) Normalized soot concentration from exhausts of C2H4 flame at $\phi=0.161$ obtained from simulations by setting $\overline{\dot{\omega}}_\mathrm{ice}=0$ in Eq. \ref{['omegaIce01']} at low temperature and from scattering measurements at room temperature. (c) Normalized centerline temperature profile (left) and radial temperature profile (right) at $y=10d$.
• Figure 4: (a) Instantaneous scattering at target conditions revealing soot along the jet centerline and copious ice formation along the shear layers from $\phi=0.161$C2H4 flame. (b) Isolate scattering by ice particles after thresholding and removing soot scattering.
• Figure 5: Probability of ice formation obtained from instantaneous ice scattering for different experimental conditioned on $p_\mathrm{vap}$ and $T$. An increased probability of ice formation is observed with increasing $N_s$ across all values of $\phi$ and with increasing $x_\mathrm{vap}^{\ce{H2O}}$ emissions from C3H8.