| Summary: | 碩士 === 國立中央大學 === 能源工程研究所 === 102 === This thesis measures quantitatively the turbulent flame speed of premixed flames over an initial pressure range of p = 1 ~ 5 atm. The main objective is to investigate the effect of the thermodiffusive instability on the self-similar propagation of expanding spherical premixed flames. Such a self-similar propagation phenomenon was first found by Chaudhri et al. (2012). In it they measured the turbulent flame speed (d<R>/dt) of unity Lewis number (Le) methane-air mixtures at the equivalence ratio = 0.9, such that all d<R>/dt data measured at various values of the root-mean-square turbulent fluctuation velocity (u') and pressures (p) can be represented by a normalized relationship: (d<R>/dt)/S_L^b ≈ 0.102ReT,flame0.54. <R> is the average flame radius, t is time, S_L^b is the laminar burning velocity before density correlation, and flame turbulent Reynolds number ReT,flame= u'<R>/DT where DT is the thermal diffusivity of unburned mixtures. All present experiments are carried out in a recently-built high-pressure, double-chamber, cruciform fan-stirred premixed turbulent explosion facility, capable of generating intense near-isotropic turbulence and making combustion experiments conducted at fixed p and u' conditions possible. Three different gas fuels/air mixtures with different values of Le are measured, respectively (i) syngas (35%H2/65%CO) at = 0.5 having Le ≈ 0.76 < 1, (ii) methane CH4 at = 0.9 with Le ≈ 1 (same as Chaudhri et al. for comparison), and (iii) propane C3H8 at = 0.7 having Le ≈ 1.62 > 1. Each case covers a wide range of u' = 1.4 ~ 6 m/s and p = 1 ~ 5 atm. Results show that the effect of Le has an important impact on the turbulent flame speed. The corresponding normalized relationships for the aforesaid three different mixtures were: (d<R>/dt)/S_L^b ≈ 0.190ReT,flame0.55 for Le < 1 syngas flames, d<R>/dt)/S_L^b ≈ 0.116ReT,flame0.54 for Le ≈ 1 methane flames, and (d<R>/dt)/S_L^b ≈ 0.102ReT,flame0.51 for Le > 1 propane flames. In comparison with methane flames, values of d<R>/dt)/S_L^b of syngas and propane flames are 1.64 times higher and 0.88 times lower, respectively. This is because Le < 1 turbulent flames are not only influence by the inherent hydrodynamics instability, but also strongly affected by the thermaldiffusive instability, while Le ≈ 1 and Le > 1 turbulent flames are only influenced by the hydrodynamics instability, resulting in lower values of (d<R>/dt)/S_L^b than that of Le < 1 turbulent flames at the same ReT,flame. Here we propose a correction function f(Le) = 2.15|Le - 1| based on the Lewis number for non-unity Lewis number turbulent flames and f(Le) = 1 if Le ≈ 1, such that the above-mentioned three different normalized relationship curves can be collapsed onto one single normalized relationship curve, f(Le)[(d<R>/dt)/S_L^b] ≈ 0.113ReT,flame0.54. These results should be useful to our understanding of high-pressure premixed turbulent combustion and applicable to automobile and aviation internal combustion engines.
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