摘要(英) |
This study measures high-pressure, high-temperature turbulent burning velocities (ST) of iso-octane/air mixtures (iC8H18 is a major component of gasoline surrogate) relevant to gas turbine conditions. Two equivalence ratios (= 1.0, 1.2) with different Lewis numbers (Le ≈ 1.43, 0.93) are used, and the ranges of experimental conditions are: The initial temperature T = 373K, the initial pressure p = 0.5~3 atm, and the r.m.s. turbulent fluctuation velocity u′ = 0~4.2 m/s. Experiments are conducted in a high-pressure, high-temperature, double-chamber, cruciform fan-stirred premixed turbulent explosion facility, capable of generating a near-isotropic turbulence for conducting combustion experiments at constant T, p, and u′ conditions. Before a run, the iso-octane is injected into a pre-vacuumed heated vessel to make sure that it is fully vaporized. Then we inject the pre-vaporized iso-octane into the cruciform fan-stirred burner by using the partial pressure method, and the pre-vaporized iso-octane and air are well-mixed. A run begins by centrally-igniting the combustible mixture. The radii of spherical expanding flames as a function of time are recorded by high-speed Schilieren imaging to estimate the corresponding burning velocities. The experimental results are mainly consisted of four parts: (1) Together with our previous data, the effect of T (358K, 373K, 423K) on SL and ST is discussed, which show that when =1.0 at 1 atm, values of SL and ST increase with increasing T, i.e. SL ~ (T/T0)1.3, ST¬ ~ (T/T0)0.07 at u′ = 1.4 m/s, ST ¬~ (T/T0)0.64 at u′ = 2.8 m/s, ST¬ ~ (T/T0)1.07 at u′ = 4.2 m/s, where T0 = 298K. At modest u′ = 1.4 m/s, the incremental effect of T is rather weak, but such effect becomes stronger and stronger as u′ increases. (2) Results show that when =1.0 at 1atm, SL ~ (p/p0)-0.26but ST¬ ~ (p/p0)0.15 when u′ = 1.4 m/s, ST¬~ (p/p0)0.22 when u′ = 2.8 m/s, ST¬ ~ (p/p0)0.17 when u′ = 4.2 m/s, where p0=1atm. The latter is because when p increases, the kinematic viscosity (v) decreases resulting in a decrease of turbulent flow Reynolds number ReT,flow = u′LI /v, where LI is the turbulent integral length scale. This ReT,flow decrease is the major reason causing the decrease of ST, not the commonly-held view that ST increases with increasing p due to the enhancement of wrinkling flame instability by the decrease of the flame thickness. When ReT,flow is kept constant, ST decreases with increasing p in the same manner as SL, showing the global response of laminar and turbulent flame speeds to pressure. (3) It is found that the effect of Le plays an important role on ST, because Le < 1 flame ( = 1.2, Le ≈ 0.93) have higher values of ST than that of Le > 1 flames ( = 1, Le ≈ 1.43), both at the same SL ≈ 0.4 m/s and u′ = 1.4 m/s. This is because Le < 1 flames experience additional thermodiffusive instability. (4) Finally, this paper applies four correlations to analyze the present ST data: (1) ST,c=0.5/u′ = A(Da)B, where the subscript c is the mean progress variable, Damköhler number Da = (LI/u′)(SL/L), where L is laminar flame thickness, and A and B are experimental coefficients; (2)A correlation of [(1/SLb)(d/dt)] = A(ReT,flame)B proposed by Chaudhuri et al., where SLb is the burned laminar burning velocity, and the turbulent flame Reynolds number ReT,flame = (u′/SL)(/L); (3)A correlation of ST,c=0.5/SL = A[(u′/SL)(p/p0)]B proposed by Kobayashi et al.; (4) A correlation of ST/u′=AKB proposed by Bradley et al., where the Karlovitz number K=0.157(u′/SL)2(ReT,flow)-0.5. In the above four correlations, Le < 1 flame have higher values of ST than Le > 1 flames, suggesting a need for Le modification. The Le modifications we propose are as follows: (1) ST,c=0.5/u′ = 0.084(DaLe-1)0.5; (2) ST,c=0.5/SL = 0.24(ReT,flameLe-1)0.5; (3) ST,c=0.5/SL = 2.97[(u′/SL)(p/p0)Le-1]0.32; (4) ST,c=0.5/u′ = 0.32(KLe)-0.45; all scattering ST data are collapsed onto single curves showing self-similar propagation turbulent of spherical flames regardless of different Le, T, p, u′. These four general correlations with the consideration of Le are important for high-pressure, high-temperature turbulent premixed combustion relevant to auto and aviation engines. |
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