为明确MILD燃烧方式下炉内的传热特性和机制，建立了20 kW甲烷MILD燃烧炉的共轭传热（CHT）模型，开展了燃烧、流动及传热的耦合CFD数值模拟。通过对比模拟预测结果和试验测试数据，验证了模拟方法的可靠性。结果表明，Okafor反应机理虽能准确预测MILD燃烧炉内的温度及O2、CO分布，但会过高预测NO生成量。通过对Okafor反应机理部分含N基元反应进行修正能够提高NO预测精度。通过对比耦合CHT模型前后燃烧炉内各壁面上的温度与热流密度分布，发现未耦合CHT模型时在炉膛前墙壁面出现了逆换热现象，与实际情况不符，说明耦合CHT模型在描述炉内传热特性方面精度更高。在耦合CHT模型基础上比较了MILD燃烧与传统钝体燃烧2种方式下炉内传热机制的差异，发现相较MILD燃烧，传统燃烧在所有炉壁上的温度均高20～40 ℃，导致传统燃烧在炉膛前墙、侧墙及后墙上的换热量比MILD燃烧分别高0.018、0.622和0.028个百分点，排烟热损失减少0.67%。进一步对各炉壁上的对流和辐射换热进行区分，发现MILD燃烧在前墙和后墙上的辐射换热量比传统燃烧分别高2.21和24.62 W，但对流换热量分别减小3.93和27.27 W。在侧墙上，MILD燃烧相较于传统燃烧辐射换热量减少290.71 W，对流换热量增加231.63 W。总体上，辐射换热在MILD燃烧和传统燃烧方式下的占比分别为70.72%和81.92%，对流换热占比分别为29.28%和18.08%。侧墙上辐射换热量的减少是导致MILD燃烧总体换热效率下降的主要原因，而更深层次的原因为MILD燃烧方式下更低的燃烧温度。

To understand the heat transfer behavior and mechanisms inside furnaces under MILD combustion, the conjugate heat transfer (CHT) model was established for a 20 kW MILD combustion furnace firing methane, and CFD modeling by integrating combustion, fluid and heat transfer was conducted. By comparing the predicting results against experimental measurement in terms of temperature and gas species, the reliability of the numerical models was verified. The results show that the original Okafor chemical reaction mechanism can accurately predict the temperature, O2 and CO distributions inside the MILD combustion furnace, but overestimate the NO emission. However, the NO prediction can be improved by optimize the N-related elementary reactions in the original Okafor mechanism. By comparing the temperature and heat flux density distribution on different wall regions before and after coupling with CHT model, it is found that the flue gas absorbs heat from the furnace front wall, which disagrees with the actual condition, demonstrating the higher prediction accuracy of CHT model on describing the heat transfer behavior inside furnaces. Based on the coupled CHT model, the difference of heat transfer mechanisms between MILD combustion and traditional combustion was compared. It is found that the wall temperature is generally higher for traditional combustion than MILD combustion by 20-40 ℃ regardless of regions, resulting in a higher heat transfer amount of 0.018, 0.622 and 0.028 percentage points on front wall, side wall and back wall, respectively, in traditional combustion, together with 0.67% reduction of stack loss. By further examining the convection and radiation heat transfer on furnace walls, it is found that MILD combustion produces a higher radiation by 2.21 W and 24.62 W, but a reduction of convection by 3.93 W and 27.27 W on front wall and back wall, respectively. On furnace side wall, MILD combustion reduces the radiation heat transfer by 290.71 W and increases convection heat transfer by 231.63 W compared to conventional combustion. Overall, radiation accounts for 70.72% and 81.92% for MILD combustion and traditional combustion, respectively, while convection accounts for 29.28% and 18.08%, respectively. The reduction of radiation on side wall is the main reason for the decreased overall heat flux for MILD combustion, and the deeper reason comes from the lower combustion temperature under MILD combustion mode.

0 引言

1 甲烷MILD燃烧试验系统

2 数值模拟方法

3 计算结果及分析

   3.1 数值模拟方法验证

   3.2 耦合CHT模型前后MILD燃烧下传热特性对比

   3.3 耦合CHT模型下不同燃烧方式传热特性对比

4 结论