Flame-holding in a recessed cavity is investigated experimentally in a Mach 2.5 preheated cross-flow for both stable and unstable combustion, with a relatively low preheating. Self-sustained combustion is investigated for stagnation pressures and temperatures reaching 1.4 MPa and 750 °K. In particular, cavity blowout is characterized with respect to cavity aspect ratio (L/D = 2.84 – 3.84), injection strategy ( floor - ramp), aft ramp angle (90 – 22:5°) and multi-fuel mixture (CH 4 – H2 or CH4 – C2H 4 blends). The results show that small hydrogen addition to methane leads to significant increase in flame stability, whereas ethylene addition has a more gradual effect. Since the multi-fuels used here are composed of a slow and a fast chemistry fuel, the resulting blowout region has a slow (methane dominant) and a fast (hydrogen or ethylene dominant) branch. Regardless of the fuel composition, the pressure at blowout is close to the non-reacting pressure imposed by the cross-flow, suggesting that combustion becomes potentially unsustainable in the cavity at the sub-atmospheric pressures encountered in these supersonic studies. The effect of preheating is also investigated and results show that the stability domain broadens with increasing stagnation temperature. However, smaller cavities appear less sensitive to the cross-flow preheating, and stable combustion is achieved over a smaller range of fuel flow rate, which may be the result of limited residence and mixing time. The blowout data point obtained at lower fuel flow rate fairly matches the empirical model developed by Rasmussen et al. [81] for oor injection Φ = 0:0028 Da–.8, where Φ is the equivalence ratio andDaa the Damkohler number. An alternate model is proposed here that takes into account the ignition to scale the blowout data. Since the mass of air entrained into the cavity cannot be accurately estimated and the cavity temperature is only approximated from the wall temperature, the proposed scaling has some uncertainty. Nevertheless the new Φ – Da scaling is shown to preserve the subtleties of the blowout trends as seen in the current experimental data.
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机译：在马赫数为2.5的预热横流中，以相对较低的预热对稳定和不稳定燃烧进行了实验，研究了凹腔中的火焰保持情况。研究了自持燃烧的停滞压力和温度达到1.4 MPa和750°K。特别是，型腔爆裂的特征在于型腔的长宽比（ L / D = 2.84 – 3.84），注射策略（底板-斜面），后斜角（90 – 22：5°）和多燃料混合物（CH 4 – H 2 或CH 4 – C 2 H 4 < / sub>混合）。结果表明，向甲烷中少量添加氢气会导致火焰稳定性显着提高，而乙烯的添加效果则更为缓和。由于此处使用的多种燃料由慢速化学燃料和快速化学燃料组成，因此，所产生的井喷区域具有慢速（甲烷为主）和快速（氢或乙烯为主）分支。无论燃料成分如何，井喷时的压力都接近于横流所施加的非反应压力，这表明在这些超音速研究中遇到的低于大气压的压力下，燃烧在腔体内变得可能无法维持。还研究了预热的影响，结果表明，随着停滞温度的升高，稳定域变宽。但是，较小的空腔似乎对横流预热不太敏感，并且在较小的燃料流速范围内实现了稳定的燃烧，这可能是滞留和混合时间有限的结果。以较低的燃油流量获得的井喷数据点与Rasmussen等人开发的经验模型完全匹配。 [81]对于口注Φ= 0：0028 D a –。8 ，其中Φ是等价的比率和 D a a Damkohler数。在此提出了一个替代模型，该模型考虑了点火以定标井喷数据。由于无法精确估计夹带在腔体内的空气质量，并且只能从壁温近似估计腔体温度，因此建议的缩放比例具有一定的不确定性。尽管如此，新的Φ– D a 缩放比例已显示出保留了井喷趋势的微妙之处，如当前的实验数据所示。
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