Strategic Control of Variable Density Jets in Crossflow

横流中变密度射流的策略控制

• 批准号: 0755104
• 负责人: Ann Karagozian
• 金额: $ 36万
• 依托单位: University of California-Los Angeles
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2008
• 资助国家: 美国
• 起止时间: 2008-09-01 至 2012-08-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0755104&HistoricalAwards=false
• 关键词: Strategic; Control; Variable; Density; Jets

CBET-0755104KaragozianThis study examines the fundamental nature and control approaches of near-field shear layer instabilities for the variable density crossflow or transverse jets. Transverse jets are widely applicable in energy systems, including dilution air jet injection for temperature pattern factor and NOx emissions control, film cooling jets for turbine blades, and hot reactive fuel jets in high speed combustors. Controlling the trajectory, spread, mixing, and temperature or density field associated with such jets could have important implications for improved energy efficiency. These studies build on the PI?s current NSF-funded research exploring shear layer instabilities associated with isothermal, isodensity transverse jets and their active forcing. These suggest that a range of potentially interesting phenomena may be present in the flow field with density and/or temperature variation. For the isothermal, isodensity jet at a fixed jet Reynolds number, the jet transitions from the ?free jet? to the ?transverse jet? as the crossflow velocity increases. Remarkable alterations in the spectral character of the upstream shear layer instability are then observed. The instability near the jet exit develops strong multiple modes that overtake one another in strength for jet-to-crossflow velocity ratios between 10 and 3.5. For lower ratios, the transverse jet shear layer instability develops a strong dominant mode in addition to higher harmonics, suggesting a nonlinear evolution. The alterations in these shear layer modes are seen in experiments, and their character is delineated in and supported by linear stability analyses and 3D numerical simulations. The stability characteristics suggest that at critical velocity ratios, the shear layer changes from being convectively unstable to possibly being absolutely unstable or self-excited. Response of the transverse jet shear layer to low level forcing is consistent with this interpretation. Such stability characteristics suggest reasons for the transverse jet?s ability to mix with surrounding gases in a superior manner to that of the free jet, but also suggest strategies for controlling jet behavior via jet forcing. Low to moderate level sinusoidal forcing can affect jet response for velocity ratios above approximately 3.5, so jet spread and penetration can be improved and controlled. However, for lower velocity ratios, very strong forcing with a prescribed temporal pulse width is required to significantly affect the jet. Based on these observations and the well known transition in instabilities associated with low density or heated free jets, the PIs plan to explore the shear layer instabilities associated with variable density and nonisothermal transverse jets, and to develop procedures for the strategic control of the jet based on specific applications relevant to energy generation systems. It is hypothesized that heated or low density jets in crossflow will experience an acceleration in the development of absolutely unstable flow, that is, the instability will occur at higher momentum flux ratios than in the isothermal case. The PIs postulate that the opposite effect would occur for the instabilities associated with the higher density or cooler jet in crossflow. Thus, for applications involving cooling dilution air jets, film cooling for turbine blades, and hot reactive jets in combustors, different control strategies based on the differing character of the instabilities will need to be developed. The PI?s current NSF project has had significant outreach to and laboratory experiences for local high school students and underrepresented students at other universities. In this project, undergraduates will continue to be trained and employed in the variable density transverse jet studies with hoped for NSF REU support. Outreach presentations and demonstrations will continue for local high school and potentially middle school and elementary school students to compensate for cutbacks in funding by the state of California to the public schools? MESA (Mathematics Engineering Science Achievement) programs.

CBET-0755104Karagozian 这项研究研究了变密度横流或横向射流的近场剪切层不稳定性的基本性质和控制方法。横向喷射广泛应用于能源系统，包括用于温度模式因子和氮氧化物排放控制的稀释空气喷射喷射、涡轮叶片的薄膜冷却喷射以及高速燃烧器中的热反应燃料喷射。控制与此类射流相关的轨迹、扩散、混合以及温度或密度场可能对提高能源效率具有重要意义。这些研究建立在 PI 目前由 NSF 资助的研究基础上，该研究探索与等温、等密度横向射流及其主动强迫相关的剪切层不稳定性。这些表明随着密度和/或温度变化，流场中可能存在一系列潜在有趣的现象。对于固定射流雷诺数的等温、等密度射流，射流从“自由射流”转变为“自由射流”。到？横向喷射？随着横流速度的增加。然后观察到上游剪切层不稳定性的光谱特征发生显着变化。射流出口附近的不稳定性产生了强烈的多种模式，当射流与横流的速度比在 10 到 3.5 之间时，这些模式的强度相互超越。对于较低的比率，横向射流剪切层不稳定性除了高次谐波之外还发展出很强的主模，这表明非线性演化。这些剪切层模式的变化可以在实验中看到，它们的特征在线性稳定性分析和 3D 数值模拟中得到描述和支持。稳定性特征表明，在临界速度比下，剪切层从对流不稳定变为可能绝对不稳定或自激。横向射流剪切层对低层强迫的响应与这种解释是一致的。这种稳定性特征表明横向射流能够以优于自由射流的方式与周围气体混合的原因，而且还提出了通过射流强迫控制射流行为的策略。低到中等水平的正弦强迫会影响速度比高于约 3.5 的射流响应，因此可以改善和控制射流扩散和穿透。然而，对于较低的速度比，需要具有规定的时间脉冲宽度的非常强的力才能显着影响射流。基于这些观察结果以及与低密度或加热自由射流相关的众所周知的不稳定性转变，PI 计划探索与变密度和非等温横向射流相关的剪切层不稳定性，并根据与发电系统相关的特定应用开发射流战略控制程序。假设横流中的加热或低密度射流将经历绝对不稳定流动发展的加速，即，与等温情况相比，在更高的动量通量比下会发生不稳定。 PI 假设，与横流中较高密度或较冷射流相关的不稳定性会产生相反的效果。因此，对于涉及冷却稀释空气射流、涡轮叶片的薄膜冷却和燃烧室中的热反应射流的应用，需要根据不稳定性的不同特征开发不同的控制策略。 PI 目前的 NSF 项目已经为当地高中生和其他大学的代表性不足的学生提供了重要的推广和实验室经验。在这个项目中，本科生将继续接受培训并从事变密度横向喷射研究，希望得到 NSF REU 的支持。将继续为当地高中以及可能的中学生和小学生进行外展演示和演示，以弥补加利福尼亚州对公立学校资金的削减？ MESA（数学工程科学成就）计划。