| 摘要(英) |
This thesis measures quantitatively burning velocities of high-temperature, high-pressure premixed laminar and turbulent flames of iso-octane/air. We mainly explore the effects of thermodiffusive instability and pressure at constant turbulent Reynolds number (ReT,flow = u′LI/v) and constant r.m.s. turbulent fluctuation velocity (u′) on turbulent burning velocities ST, where LI and v are the integral length scale of turbulence and the kinematic viscosity of reactants respectively. Experiments were conducted in a high-pressure, high-pressure, double-chamber, cruciform fan-stirred premixed turbulent explosion facility, capable of generating a near-isotropic turbulence for conducting combustion experiments at fixed temperature (T), pressure (p), and u′ or ReT,flow conditions. We apply the liquid fuel (iso-octane) as a fuel which is first injected into a pre-vacuumed heating cylinder to make sure it is fully vaporized. Then we inject the pre-vaporized iso-octane into the cruciform fan-stirred burner by using the partial pressure method. The pre-vaporized iso-octane and are at T = 358K are well-mixed before a run. A run begins by centrally-igniting the combustible mixtures. The timing diagram of spherical expanding flames radii are recorded by the high-speed Schilieren imaging technique. Three equivalence ratios (phi = 0.9, 1.0, 1.25) with different Lewis numbers (Le ≈ 2.94, 1.43, and 0.93). Comparing the flames of phi = 0.9(Le > 1) and 1.25(Le < 1) are measured under the same ranges of experimental conditions i.e. T = 358K, p = 1 ~ 5atm, and u′ = 1.4 ~ 2.8m/s or ReT,flow = 6700, then the phi = 1.0 is measured more ranges of turbulent conditions i.e. u′ = 1.0, 1.4, 2.8m/s and ReT,flow = 6700, 9100, 11600 in order to further investigate the ReT,flow effect. Results show that when u′ is fixed, values of ST increase with increasing p, which is mainly due to increase of ReT,flow with increasing p. On the contrary, when ReT,flow is fixed, we find that values ST actually decrease with increasing p. Such decreasing trend of ST ~ p-0.36 is similar to laminar burning velocities (SL), showing a global response of burning velocities to the increase of pressure. When p is fixed, values of ST increase with increasing ReT,flow. In addition, it is found that the effect of Le plays an important role on ST. Le < 1 flame (phi = 1.25, Le ≈ 0.93) clearly have higher values of ST than that of Le > 1 flames (phi = 0.9, Le ≈ 2.94), both with the same SL ≈ 0.4m/s and u′ = 1.4 ~ 2.8m/s. This is because Le < 1 flames experience additional thermodiffusive instablility.
This thesis also apply that commonly used correlation of Damköhler number in the laboratory, ST,c = 0.5/u′ = A1(Da)B1, and two correlations proposed by Kobayashi et al. and Chaudhuri et al. respectively, ST,c=0.5/SL = A2(u′/SL)(p/p0)B2 and [(1/SLb)(d/dt)] = A3(ReT,flame)B3 to analyze present data with three different Le, where the subscript c is the mean progress variable, Damköhler number Da = (LI/u′)(SL/L), L is laminar flame thickness, p0 = 1atm, A1 ~ A3 and B1 ~ B3 are experimental coefficients, SLb is laminar burning velocity of product, and turbulent flame Reynolds number ReT,flame = (u′/SL)(/L). We discover that when using the Le-1 correction, all scattering ST data with different values of u′, p and phi can be collapsed onto a single curve, i.e. ST,c = 0.5/u′ = 0.087(DaLe-1)0.5, ST,c=0.5/SL = 3.5[(u′/SL)(p/p0)Le-1]0.24 and [(1/SLb)(d/dt)] = 0.35(ReT,flameLe-1)0.33. Unfortunately, the power constants of B2 = 0.24 and B3 = 0.33 are less than values (B2 = 0.38 and B3 = 0.5) of previous methane fuel and only B1 = 0.5 is close to the previous methane fuel (B1 = 0.53). We compare the only iso-octane ST data in previous literature (measured by the Leeds Prof. Lawes team) by the relationship of ST,c = 0.5/u′ vs. (DaLe-1)0.5 and get quite consistent results showing the relationship of ST,c = 0.5/u′ ~ (DaLe-1)0.5 is a better general correlation. These results are useful for the application of high-pressure, high-temperature premixed turbulent combustion such as in auto and aviation engines. |
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