Numerical analysis on combustion characteristics of n-heptane co-firing with methane
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摘要:
针对燃气轮机的双燃料混合燃烧问题，采用直接关系图法对NUI（National University of Ireland）机理进行简化（含204组分，902步反应），在此基础上数值研究了甲烷质量分数和体积分数对正庚烷/甲烷混合燃烧特性的影响。结果表明：提高甲烷体积分数会使混合燃料的着火延迟时间非线性增加，层流火焰速度、绝热火焰温度、一氧化碳排放下降；当甲烷体积分数超过70%时，混合燃料的层流火焰速度和着火延迟时间对甲烷体积分数较为敏感。此外，对于采用正庚烷/甲烷的环形燃烧室，在保持混合燃料热值不变的条件下，两种燃料的火焰锋面位置基本相同，且随着甲烷质量分数的升高，火焰长度和托举高度逐渐增加，燃烧效率、总压损失、一氧化碳和氮氧化物排放逐渐下降；当甲烷质量分数低于30%时，火焰呈“V”型，高于30%时火焰呈“M”型。
Abstract:In view of the problem of dual-fuel mixed combustion of gas turbine, based on the NUI (National University of Ireland) mechanism, a reduced mechanism of n-heptane/methane combustion with 204 species and 902 reactions was obtained by applying the directed relation graph method. Effects of methane mass fraction and volume fraction in the n-heptane/methane mixture on combustion characteristics of n-heptane/methane were numerically studied based on the reduced mechanism. Results showed that increasing methane volume fraction in the fuel could increase ignition delays nonlinearly, and decrease laminar flame speeds adiabatic flame temperature and Carbon monoxide emission nonlinearly. When methane volume fraction in the fuel was above 70%, ignition delays and, laminar flame speeds were sensitive to fuel’s methane volume fraction. In the combustor, the positions of the flame fronts of n-heptane and methane were the same when co-firing n-heptane/methane. With the increase of methane mass fraction in the fuel, the flame length and flame lifted distance increased, while the combustion efficiency, pressure loss, Carbon monoxide emission and nitrogen oxide emissions decreased. When methane mass fraction in the fuel was below 30%, the flame was “V” shaped, and above this value the flame was “M” shaped.
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Key words:
- gas turbine /
- n-heptane /
- methane /
- mechanism reduction /
- combustor
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图 1 正庚烷详细化学反应机理对比结果
Figure 1. Comparsion between the detailed chemical reaction mechanism of n-heptane
图 2 层流火焰速度验证结果（p=1.013×105 Pa, T =393 K）
Figure 2. Validation results of laminar flame speeds （p=1.013×105 Pa, T =393 K）
图 3 着火延迟时间验证结果 （p=1.013×106 Pa，
$ \phi $ =1.0）Figure 3. Validation results of ignition delays （p=1.013×106 Pa，
$ \phi $ =1.0）
图 4 关键组分验证结果 （正庚烷体积分数为50%，甲烷体积分数为50%，p=1.013×106 Pa，T=1000 K）
Figure 4. Validation results of some important intermediate species （n-heptane volume fraction of 50%, methane volume fraction of 50%，p=1.013×106 Pa，T=1000 K）
图 5 当量比、点火温度、甲烷体积分数对正庚烷/甲烷/空气着火延迟时间的影响（p=1.013×106 Pa）
Figure 5. Influences of equivalence ratio, ignition temperature, methane volume fraction in n-heptane/methane mixtures on ignition delays of n-heptane/methane/air （p=1.013×106 Pa）
图 6 甲烷体积分数对正庚烷/甲烷/空气层流火焰速度的影响
Figure 6. Influence of methane volume fraction in the fuel on the laminar flame speeds of n-heptane/methane/air mixture
图 7 甲烷体积分数对正庚烷/甲烷绝热火焰温度及CO排放的影响
Figure 7. Influence of methane volume fraction in the fuel on ambient flame temperature and CO emission
图 12 甲烷质量分数对释热率分布的影响
Figure 12. Influence of methane mass fraction on heat release rate
表 1 燃料组成及燃料质量流量
Table 1. Composition and mass flow rate of different fuels
组合编号 质量分数 质量流量/（kg/s） 正庚烷 甲烷 1 1 0 0.0301 2 0.7 0.3 0.0291 3 0.5 0.5 0.0285 4 0.3 0.7 0.0279 5 0 1 0.0270 
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表 2 甲烷质量分数对燃烧室性能和排放的影响
Table 2. Influence of methane mass fraction on combustor performances and emission
甲烷质量分数 效率/% 总压损失/% 出口温度分布系数 CO排放/（mg/m3） NOx排放/（mg/m3） 0 99.91 7.75 0.23 47.28 302.30 0.3 99.70 7.74 0.25 28.28 227.86 0.5 99.65 7.72 0.23 28.27 189.32 0.7 99.18 7.71 0.24 23.15 151.53 1 98.89 7.70 0.24 18.12 105.09
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