| dc.description.abstract | This thesis measures self-acceleration characteristics of expanding cellular spherical flames and analyzes whether such self-acceleration propagation is either self-similar in time (t), if it holds for all t, or locally similar, if it holds only for a specific period of t. Propagation of laminar spherical flames is subjected to three possible flame front instabilities, namely: (1) thermal-diffusional instability (when the Lewis number, Le = DT/DM which is the heat and mass diffusion coefficient ratio, is less than 1, occurring at relatively small flame radius (R); (2) hydrodynamic instability (an inherent flame instability); (3) buoyancy instability (when R is approximately larger than 50mm). These flame front instabilities can result in cellular structures and/or wrinkles over the flame front surface upon flame propagation that continuously develop, so that the flame surface area is increased and thus the global flame propagation rate is also increased, leading to self-acceleration of expanding spherical flame. In previous studies, the power-law acceleration exponent (α) was commonly determined by a fitting formula of Rav = R0 + A(t-t0)α, where Rav is the flame average radius, R0 and t0 are the corresponding radius and time on the onset of flame wrinkling, and A is an experimental constant. However, values of R0 and t0 depend on the experimental conditions that can influence the correctness of α. Very recently, Law and his coworkers (Wu et al. 2013) found a better way to determine α without using R0 and t0 , that is to take the time derivation of Rav = Atα to obtain the flame propagation speed [SF = dRav/dt = αA1/αRav(α-1)/α] and thus α can be more accurately determined from the slope [d = (α - 1)/α] of the log-log plot of SF vs. Rav data. They found that α is a constant value at any given pressure (p) and equivalence ratio (φ) for H2/air mixtures, suggesting that self-accelerating cellular spherical flames are self-similar propagation, same as other previous studies. However, their α values ranging from 1.17~1.37 are generally smaller than 1.5, a value for self-turbulization.
The present study applies the same method as Law’s group to determine α. Focus is placed on high-pressure combustion experiments, including both lean hydrogen/air mixtures at φ = 0.6 and p = 0.5atm where Le ≈ 0.44 and near-stoichiometric methane/air mixtures at φ = 0.9 and p = 0.5atm where Le ≈ 1. Using high-speed high resolution Schlieren imaging technique to record and measure the time evolution of Rav, SF can be then determine from dRav/dt and α can be thus obtained. Results show that for lean H2/air case (Le << 1), cellular structures are observed at the very early stage of flame propagation due to thermal-diffusional and hydrodynamic instabilities. The cells continuously develop leading to flame self-acceleration. Most importantly, it is found that such acceleration is locally similar (not self-similar) where a transition on values of α is found. When 12mm ≤ Rav < 27mm, α = 1.30, while α increases to 1.42 when Rav ≥ 27mm. For the CH4/air case with Le ≈ 1, the spherical flame first expands at an almost constant propagation rate for a considerable long period of time where α = 1.02, then self-acceleration of cellular spherical flame starts to initiate around Rav ≈ 30mm where α increases to 1.15 for 30mm ≤ Rav < 43mm, and finally and surprisingly the value of α jumps to 1.47 for 43mm ≤ Rav < 70mm, a value closely matching with the theoretical vale (α = 1.5) for self-turbulization. These results are important to our further understanding of flames front instabilities and dynamics. | en_US |