A Study On Spherical Expanding Flame Speeds Of Methane Ethane And Methane Ethane Mixtures At Elevated Pressures

High-pressure experiments and chemical kinetics modeling were performed for laminar spherically expanding flames for methane/air, ethane/air, methane/ethane/air and propane/air mixtures at pressures between 1 and 10 atm and equivalence ratios ranging from 0.7 to 1.3. All experiments were performed in a new flame speed facility capable of withstanding initial pressures up to 15 atm. The facility consists of a cylindrical pressure vessel rated up to 2200 psi. Vacuums down to 30 mTorr were produced before each experiment, and mixtures were created using the partial pressure method. Ignition was obtained by an automotive coil and a constant current power supply capable of reducing the spark energy close to the minimum ignition energy. Optical cine-photography was provided via a Z-type schlieren set up and a high-speed camera (2000 fps). A full description of the facility is given including a pressure rating and a computational conjugate heat transfer analysis predicting temperature rises at the walls. Additionally, a detailed uncertainty analysis revealed total uncertainty in measured flame speed of approximately +-0.7 cm/s. This study includes first-ever measurements of methane/ethane flame speeds at elevated pressures as well as unique high pressure ethane flame speed measurements. Three chemical kinetic models were used and compared against measured flame velocities. GRI 3.0 performed remarkably well even for high-pressure ethane flames. The C5 mechanism performed acceptably at low pressure conditions and under-predicted the experimental data at elevated pressures. Measured Markstein lengths of atmospheric methane/air flames were compared against values found in the literature. In this study, Markstein lengths increased for methane/air flames from fuel lean to fuel rich. A reverse trend was observed for ethane/air mixtures with the Markstein length decreasing from fuel lean to fuel rich conditions. Flame cellularity was observed for mixtures at elevated pressures. For both methane and ethane, hydrodynamic instabilities dominated at stoichiometric conditions. Flame acceleration was clearly visible and used to determine the onset of cellular instabilities. The onset of flame acceleration for each high-pressure experiment was recorded.

Flame speeds were determined for methane-air, propane-air, and ethylene-air mixtures at -73 C and for methane-air mixtures at -132 C. The data extend the curves of maximum flame speed against initial mixture temperature previously established for the range from room temperature to 344 C. Empirical equations for maximum flame speed u(cm/ sec) as a function of initial mixture temperature T(sub O) were determined to be as follows: for methane, for T(sub O) from 141 to 615 K, u = 8 + 0.000160 T(sub O)(exp 2.11); for propane, for T(sub O) from 200 to 616 K, u = 10 + 0.000342 T(sub O)(exp 2.00); for ethylene, for T(sub O) from 200 to 617 K, u = 10 + 0.00259 T(sub O)(exp 1.74). Relative flame speeds at low initial temperatures were predicted within approximately 20 percent by either the thermal theory as presented by Semenov or by the diffusion theory of Tanford and Pease. The same order was found previously for high initial temperatures. The low-temperature data were also found to extend the linear correlations between maximum flame speed and calculated equilibrium active-radical concentrations, which were established by the previously reported high-temperature data.

Flame speeds were determined for methane-air, propane-air, and ethylene-air mixtures at -73 degrees C and for methane-air mixtures at -132 degrees C. The data extend the curves of maximum flame speed against initial mixture temperature previously established for the range from room temperature to 344 degrees C. Both a thermal and diffusion theory predicted the effect of low initial temperatures within approximately 20 percent, as was found previously for high initial temperatures. The low-temperature data also extended the linear correlations between maximum flame speed and calculated equilibrium active-radical concentrations, which were established by the reported high-temperature data. Flame speed was determined from the total area of the outside edge of the shadow of a nozzle flame.

Flame speeds of methane-air mixtures and ethylene-air mixtures were determined as functions of mixture composition at initial mixture temperatures ranging from 34 to 344 degrees C by a Bunsen-burner method. The data were compared with reported data for propane and air; the maximum flame speeds increased with temperature at increasing rates and were affected on the percentage basis in the decreasing order, methane, propane, and ethylene. Both a thermal and diffusion theory predicted the effect of temperature on maximum flame speed within 20 percent. Straight-line correlations were found between maximum flame speed and calculated equilibrium hydrogen-atom concentration (at adiabatic flame temperature) for each fuel.