| 摘要(英) |
This thesis quantitatively measures laminar and turbulent burning velocities (SL and ST) of the premixed PRF95 (primary reference fuel with 95% volume of iso-octane and 5% volume of n-heptane) at high temperature and high pressure conditions. There are two objectives in this thesis: (1) To measure values of SL and ST of spherical premixed flames of PRF95 and investigate the pressure effect on SL and ST by keeping the r.m.s. turbulent fluctuation velocity (u′) constant. (2) To investigate the effect of pressure and turbulent flow Reynolds number (ReT,flow = u′LI/) on ST under constant ReT,flow condition, where LI and are the integral length scale of turbulence and the kinematic viscosity of reactants respectively. Experiments are conducted in a large dual-chamber, high-pressure, high-temperature, fan-stirred cruciform premixed turbulent explosion facility capable of generating isotropic turbulence for ST measurements of expanding spherical PRF95/air flames. While controlling the product of u′LI in proportion to the decreasing at elevated pressure to obtain the constant ReT,flow value, and we measure SL and ST of PRF95 at the equivalence ratio ( = 1.0, Le ≈ 1.44), T = 358K, and 373K, p = 1-5 atm, u′= 1.0-5.4 m/s, and at three constant ReT,flow = 6,700; 9,100; 11,600 in order to further investigate the ReT,flow effect on ST. Under high pressure condition, SL decreases with increasing pressure, where cellular structures all over the flame surface can be observed. For laminar flames, SL decreases with increasing pressure because SL is effected by a pressure chain reaction mechanism during flame propagating. Take H-O2 reaction for example, key reaction (H1): H + O2 → OH + O is a two-body branching reaction, while reaction (H9): H + O2 + M → HO2 + M is a three-body inhibiting reaction. While by increasing pressure, (H9) is enhanced relative to (H1) because three-body reactions are favored over two-body reactions as pressure increases. Therefore, in the overall reaction at high pressure conditions, the inhibition reaction is enhanced to cause a decrease in SL. For turbulence flames, when under constant u′ condition, ST increases with increasing pressure. In the past, this is because the flame thickness decreases with increasing pressure, making the flame surface interfered with Hydrodynamic Instability (i.e., Darrieus-Landau Instability). Flame surface results in the cellular structures which causes an increase in the area of the turbulent flame and increases the ST. However, previous studies have not considered the fact that pressure increases with the ReT,flow increasing.
Results show that when under the condition of constant u′, the values of ST increase with increasing p, which is mainly due to increase of ReT,flow with increasing p. On the contrary, when under the condition of constant ReT,flow, we find that the values of ST actually decrease with increasing p. Both ST and SL have an exponential decrease in pressure effect which indicates that the ReT,flow may be a key factor affecting the increase of ST.
This thesis also includes previous iso-octane ST at T=358K, 373K, and 423K obtained from data our laboratory using the same methodology, so that the normalization of these data by using five general correlations can be made for comparison as describe below (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), L is laminar flame thickness. (2) ST,c=0.5/SL = A(ReT,flame)0.5, where the turbulent flame Reynolds number ReT,flame= (u′/SL)(<R>/L). (3)ST,c=0.5/SL = A[(u′/SL)(p/p0)]B, where p0 is the atmospheric pressure. (4)ST/u′=AK-0.3, where Karlovitz number K = 0.157(u′/SL)2(ReT,flow)-0.5. (5)ST,c=0.5/SL= A(ReT,flame/Le2)B and A and B are experimental constants of different general correlations. As a result, it is found that the previously dispersed normalized ST/SL data can be converged into a single curve after being substituted into a function that considers the Le number as a correction. The five modified general correlations all show good fitting goodness. Among them, the general correlation (3) has better fitting goodness (R2 = 0.90). The results show that the flame propagation still has self-propagation similarity even though for different fuel and equivalence ratios. The results of this thesis are useful for high temperature and high pressure premixed turbulent combustion applications, such as combustion studies for vehicles engines. |
| 參考文獻 |
參考文獻
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