Bibliography - Y. Ju

1. Yuan, J., Y. Ju, and Chung K Law, 2007: On flamefront instability at elevated pressures. Proceedings of the Combustion Institute, 31(1), doi:10.1016/j.proci.2006.07.180 1267-1274
   [ Abstract ]

   Effects of pressure up to 3 atm on flame-front instability were numerically investigated for both linear and nonlinear growth stages at sub-unity and unity Lewis numbers. A sixth-order compact scheme and non-reflecting boundary conditions were used to capture the evolution of the flame front. Results show that in the linear instability growth stage, elevated pressure can extend the unstable range of flame-fronts and generate the fine flame cell structure. This effect can be qualitatively predicted by the theories when Le = 1.0; however the theories diverge at sub-unity Lewis numbers (e.g. Le = 0.7). In the nonlinear growth stage, the critical wave number (k_c) from the linear dispersion relation can be used as a reference length scale for the evolution of the flame cell structure. Since elevated pressure increases the critical wave number, small flame cells appear over large flame cells (deep folds) at high ambient pressures. Furthermore, flame-front hydrodynamic instability is excited when the lateral domain is enlarged.

2. Xue, Y., and Y. Ju, 2006: Studies on the Liftoff Properties of Dimethyl Ether Jet Diffusion Flames. Combustion Science and Technology, 178(12), doi:10.1080/00102200600626140 2219-2247
   [ Abstract ]

   The liftoff properties of the DME jet diffusion flame were investigated experimentally and analytically with Emphasis on the influences of flame stretch and fuel oxygen. The present experiments showed that the DME jet diffusion flame exhibited a distinct liftoff phenomenon that differed from other hydrocarbon fuels. This unique phenomenon was analyzed theoretically by taking into consideration the effects of flame stretch and the fuel oxygen. The results showed that the stretch effect had a significant impact on the critical liftoff Schmidt number and the flame liftoff height. Based on these observations, a new criterion for the lifted flame at the blowout limit was presented. The results also demonstrated that the appearance of fuel oxygen in DME increases the fuel mixture fraction at the stoichiometric condition and changes the flame liftoff phenomenon. The effect of fuel oxygen was further investigated by adding air into propane and n-butane diffusion flames. It was found that with the increase of oxygen addition, both propane and n-butane flames change from the direct liftoff regime to the direct blowout regime. The results well described the unique liftoff phenomenon of DME and also applicable to other oxygenated and air diluted hydrocarbon fuels.

3. Ju, Y., and Y. Xue, 2005: Extinction and flame bifurcations of stretched dimethyl-ether premixed flames. Proceedings of the Combustion Institute, 30(1), doi:10.1016/j.proci.2004.08.258 295-301
   [ Abstract ]

   Extinction limits and flame bifurcation of lean premixed dimethyl ether–air flames are numerically investigated using the counterflow flame with a reduced chemistry. Emphasis is paid to the combined effect of radiation and flame stretch on the extinction and flammability limits. A method based on the reaction front is presented to predict the Markstein length. The predicted positive Markstein length agrees well with the experimental data. The results show that flow stretch significantly reduces the flame speed and narrows the flammability limit of the stretched dimethyl ether–air flame. It is found that the combined effect of radiation and flow stretch results in a new flame bifurcation and multiple flame regimes. At an equivalence ratio slightly higher than the flammability limit of the planar flame, the distant flame regime appears at low stretch rates. With an increase in the equivalence ratio, in addition to the distant flame, a weak flame isola emerges at moderate stretch rates. With a further increase in the equivalence ratio, the distant flame and the weak flame branches merge together, resulting in the splitting of the weak flame branch into two weak flame branches, one at low stretch and the other at high stretch. Flame stability analysis demonstrates that the high stretch weak flame is also stable. Furthermore, a K-shaped flammability limit diagram showing various flame regimes and their extinction limits is obtained.

4. Qin, X., and Y. Ju, 2005: Measurements of burning velocities of dimethyl ether and air premixed flames at elevated pressures. Proceedings of the Combustion Institute, 30, doi:10.1016/j.proci.2004.08.251
   [ Abstract ]

   Laminar burning velocities of dimethyl ether (DME) and air premixed flames at elevated pressures up to 10 atm were measured by using a newly developed pressure-release type spherical bomb. The measurement system was validated using laminar burning velocities of methane–air flames. A comparison with the previous experimental data shows an excellent agreement and demonstrates the accuracy and reliability of the present experimental system. The measured flame speeds of DME–air flames were compared with the previous experimental data and the predictions using the full and reduced mechanisms. At atmospheric pressure, the measured laminar burning velocities of DME–air flames are in reasonable agreement with the previous data from spherical bomb method, but are much lower than both predictions and the experimental data of the PIV based counterflow flame measurements. The laminar burning velocities of DME–air flames at 2, 6, and 10 atm were also measured. It was found that flame speed decreases considerably with the increase of pressure. Moreover, the measured flame speeds are also lower than the predictions at high pressures. In addition, experiments showed that at high pressures the rich DME–air flames are strongly affected by the hydrodynamic and thermal-diffusive instabilities. Markstein lengths and the overall reaction order at different equivalence ratios were extracted from the flame speed data at elevated pressures. Sensitivity analysis showed that reactions involving methyl and formyl radicals play an important role in DME– air flame propagation and suggested that systematic modification of the reactions rates associated with methyl and formyl formations are necessary to reduce the discrepancies between predictions and measurements.

5. Xue, Y., X. Qin, and Y. Ju, January 2005: Study of Liftoff Mechanism of Nonpremixed Jet Flame near Unity Schmidt Number. AIAA-2005-546, 43rd AIAA Aerospace Sciences Meeting and Exhibit, Reno, Nevada, http://pdf.aiaa.org/preview/CDReadyMASM05_666/PV2005_546.pdf,
   [ Abstract ]

   Nonpremixed jet flames of dimethyl ether (DME) were studied both experimentally and theoretically to investigate flame liftoff near unity Schmidt number. It was found experimentally that although the DME nonpremixed flames have a Schmidt number larger than unity it cannot be lifted directly by increasing the flow rate. Lifted flames can only be established by igniting the mixture in a narrow region downstream of the jet at low flow rates. The results also show that the liftoff flow rate is less than that of the blowout limit of the attached flame. Theoretically, the self-similar Landau-Squire solution for a round jet is revisited and the combined effects of stretch and flame curvature on triple flame propagation speed were considered. It was found that the critical Schmidt number for liftoff shifts around unity. The critical Schmidt number is less than unity for fuel Le numbers larger than 0.5 and larger than unity for Le numbers less than 0.5.

6. Yuan, J., Y. Ju, and Chung K Law, 2005: Coupled hydrodynamic and diffusional-thermal instabilities in flame propagation at large Lewis numbers. Physics of Fluids, 17(O74106), doi:10.1063/1.1964845
   [ Abstract ]

   The dynamics of flame cell evolution due to the coupling between hydrodynamic and diffusional-thermal instabilities in subunity Lewis number flames was simulated using a sixth-order central difference scheme and newly developed nonreflective boundary conditions. Results show that the interaction between these two modes of instabilities yields distinct evolutions of cell splitting, merging, growth, local extinction, and lateral motion, leading to fluctuations of the flow and species concentrations as well as substantial increase in the flame speed. The study also demonstrates that small computational domains cannot correctly predict cell merging and transverse motion.

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