dc.description.abstract | This thesis quantitatively measures the coupling effects of pressure and turbulence on minimum ignition energies (MIE) following by a physical model to explain these results. Lean methane-air mixtures at the equivalence ratio ? = 0.6 are used because much higher MIE is required to ignite such lean mixtures. Experiments are carried out in an already-established high-pressure, double-chamber explosion facility and a high-power pulse generator is used to control ignition energies of a pair of spark-electrodes with flat ends having a gap of 1 mm to increase required MIE for high pressure measurements positioned at the centre of a large inner cruciform burner. The inner burner lodged in a huge high-pressure absorbing outer chamber is equipped with a pair of counter-rotating fans and perforated plates capable of generating intense near-isotropic turbulence with negligible mean velocities and roughly equal magnitudes of turbulent fluctuation velocities in all three directions where the root-mean-square turbulent fluctuating velocities (u?) can be up to 8.42 m/s. Two statistical methods used to estimate MIE are reviewed and compared. Results show that values of MIEL and MIET noticeably as pressure (p) increases, where the subscripts L and T represent laminar and turbulent values. It is found that, similar to obtained at p = 0.1 MPa, the increasing slopes of MIET/MIEL = ? curves under elevated pressure conditions (p = 0.1 and 0.3 MPa) change drastically from linear to exponential when values of u?/SL are greater than some critical values depending on p showing ignition transition, where SL is the laminar burning velocity. Moreover, the Schlieren imaging technique is used to acquire flame kernel images at high pressure turbulent conditions in attempt to distinguish the structure difference of flame kernels before and after ignition transition. Finally, by introducing a pressure correction, we can modify the previous model at normal pressure condition, such that all data curves obtained at different pressure conditions with different critical values of u?/SL can be collapsed roughly into a single curve. Our previous model (Shy et al. 2010 [10]) based on a reaction zone (ignition kernel) Péclet number, Pe = u??K/?RZ, is used to explain ignition transition, where ?K is Kolmogorov length scale and ?RZ is the thermal diffusivity at the surface of the ignition kernel estimated at the mean temperature between flame adiabatic and reactant temperatures. In it when Pe < Pec for the pre-transition, ? = 1 + a1Pe, while ? = 1 + a2 (Pe4 –b2) for the post-transition when Pe > Pec, where a1, a2 and b2 are experimental constants. The present pressure modified correction is: Pe* = Pe(p/p0)-1/4, where p0 = 0.1 MPa. Using Pe* to take the pressure effect into consideration, all MIET/MIEL data at various values of u?/SL up to 50 and under different pressure conditions (p = 0.1, 0.3, 0.5 MPa) can be represented by a single curve having two drastically different increasing slopes with increasing Pe*: ? = 1 + a1Pe*,before transition and ? = 1 + a2 (Pe*4 –b3) after transition. These results are useful in many industrial devices such as spark-ignition-engines and internal combustion engines.
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