Abstract The effects of diluents including CO2, N2, and He on the lifted flame characteristics of laminar coflow propane jets are experimentally investigated by measuring their liftoff heights, HL, and visualizing flow fields with oil mist. For lifted flames at a specified diluent mole fraction, XD, HL increases in the order of He, N2, CO2 when the diluent is added to the coflow stream. However, the order of HL becomes opposite when the diluent is added to the fuel stream. In addition, HL increases with increasing XD for all cases. From the visualization of nonreacting flow fields, it is found that the negative buoyancy represented by the density difference between the fuel jet and the coflow induces a stagnation flow near the fuel nozzle by decelerating the fuel jet. As such, the lifted flame is found to be stabilized further upstream with increasing negative buoyancy. In addition to the buoyancy effect, the effects of diluents on HL via the edge flame speed of the lifted flames, Se, are estimated by evaluating the laminar burning velocity, S L 0 . Among the dilution, thermal, and chemical effects, the dilution effect is found to be dominant in reducing S L 0 for all cases, while the chemical effect is negligible. Finally, a correlation for HL is formulated using the ratios of the positive to negative buoyancy and the fuel jet velocity, U0, to S L 0 , which shows a satisfactory agreement with the experimental data.

中文翻译：

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稀释剂对层流非预混顺流丙烷射流离火特性的影响

摘要 通过测量升空高度、HL 和油雾可视化流场，实验研究了 CO2、N2 和 He 等稀释剂对层流共流丙烷射流的离焰特性的影响。对于指定稀释剂摩尔分数的离火火焰，XD、HL 在将稀释剂添加到协流流中时以 He、N2、CO2 的顺序增加。然而，当稀释剂添加到燃料流中时，HL 的顺序变得相反。此外，对于所有情况，HL 都随着 XD 的增加而增加。从非反应流场的可视化中发现，燃料射流和协流之间的密度差所代表的负浮力通过使燃料射流减速而在燃料喷嘴附近引起滞流。因此，发现随着负浮力的增加，升起的火焰在上游进一步稳定。除了浮力效应之外，通过评估层流燃烧速度 SL 0 来估计稀释剂对 HL 的影响，通过提升火焰的边缘火焰速度 Se。在稀释、热和化学效应中，发现稀释效应在所有情况下在减少 SL 0 方面占主导地位，而化学效应可以忽略不计。最后，使用正负浮力和燃料喷射速度 U0 与 SL 0 的比率制定了 HL 的相关性，这表明与实验数据具有令人满意的一致性。在稀释、热和化学效应中，发现稀释效应在所有情况下在减少 SL 0 方面占主导地位，而化学效应可以忽略不计。最后，使用正负浮力和燃料喷射速度 U0 与 SL 0 的比率制定了 HL 的相关性，这表明与实验数据具有令人满意的一致性。在稀释、热和化学效应中，发现稀释效应在所有情况下在减少 SL 0 方面占主导地位，而化学效应可以忽略不计。最后，使用正负浮力和燃料喷射速度 U0 与 SL 0 的比率制定了 HL 的相关性，这表明与实验数据具有令人满意的一致性。