Purpose

The mixing of fuel and air plays a pivotal role in enhancing combustion in supersonic regime. Proper mixing stabilizes the flame and prevents blow-off. Blow-off is due to the shorter residence time of fuel and air in the combustor, as the flow is in supersonic regime. The flame is initiated in the local subsonic region created using a flameholder within the supersonic combustor. This study aims to design an effective flameholder which increases the residence time of fuel in the combustor allowing proper combustion preventing blow-off and other instabilities.

Design/methodology/approach

The geometry of the strut-based flameholder is altered in the present study to induce a streamwise motion of the fluid downstream of the strut. The streamwise motion of the fluid is initiated by the ramps and grooves of the strut geometry. The numerical simulations were carried out using ANSYS Fluent and are validated against the available experimental and numerical results of cold flow with hydrogen injection using plain strut as the flameholder. In the present study, numerical investigations are performed to analyse the effect on hydrogen injection in strut-based flameholders with ramps and converging grooves using Reynolds-averaged Navier–Stokes equation coupled with Menter’s shear stress transport k-ω turbulence model. The analysis is done to determine the effect of geometrical parameters and flow parameter on the flow structures near the base of the strut where thorough mixing takes place. The geometrical parameters under consideration include the ramp length, groove convergence angle, depth of the groove, groove compression angle and the Mach number. Two different strut configurations, namely, symmetric and asymmetric struts were also studied.

Findings

Higher turbulence and complex flow structures are visible in asymmetric strut configuration which develops better mixing of hydrogen and air compared to symmetric strut configuration. The variation in the geometric parameters develop changes in the fluid motion downstream of the strut. The fluid passing through the converging grooves gets decelerated thereby reducing the Mach number by 20% near the base of the strut compared to the straight grooved strut. The shorter ramps are found to be more effective, as the pressure variation in lateral direction is carried along the strut walls downstream of the strut increasing the streamwise motion of the fluid. The decrease in the depth of the groove increases the recirculation zone downstream of the strut. Moreover, the increase in the groove compression angle also increases the turbulence near the base of the strut where the fuel is injected. Variation in the injection port location increases the mixing performance of the combustor by 25%. The turbulence of the fuel jet stream is considerably changed by the increase in the injection velocity. However, the change in the flow field properties within the flow domain is marginal. The increase in fuel mass flow rate brings about considerable change in the flow field inducing stronger shock structures.

Originality/value

The present study identifies the optimum geometry of the strut-based flameholder with ramps and converging grooves. The reaction flow modelling may be performed on the strut geometry incorporating the design features obtained in the present study.

中文翻译：

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支柱几何形状对模型燃烧室流动特性影响的数值研究

目的

燃料和空气的混合对于增强超音速状态下的燃烧起着关键作用。适当的混合可以稳定火焰并防止吹熄。吹扫是由于燃料和空气在燃烧室中的停留时间较短，因为流动处于超音速状态。火焰在超音速燃烧室内使用火焰稳定器产生的局部亚音速区域中引发。本研究旨在设计一种有效的火焰稳定器，它可以增加燃料在燃烧器中的停留时间，从而实现适当的燃烧，防止吹熄和其他不稳定性。

设计/方法论/途径

在本研究中，基于支柱的火焰稳定器的几何形状被改变，以引起支柱下游流体的流向运动。流体的流向运动是由支柱几何形状的斜坡和凹槽引发的。数值模拟是使用 ANSYS Fluent 进行的，并根据使用普通支柱作为火焰稳定器的氢喷射冷流实验和数值结果进行了验证。在本研究中，使用雷诺平均纳维-斯托克斯方程与 Menter 的剪切应力传递 k- ω湍流模型相结合，进行数值研究，分析对具有斜坡和会聚凹槽的基于支柱的火焰稳定器中氢气喷射的影响。进行分析是为了确定几何参数和流动参数对发生彻底混合的支柱底部附近流动结构的影响。考虑的几何参数包括坡道长度、凹槽会聚角、凹槽深度、凹槽压缩角和马赫数。还研究了两种不同的支柱配置，即对称支柱和非对称支柱。

发现

在不对称支柱配置中可以看到更高的湍流和复杂的流动结构，与对称支柱配置相比，它可以更好地混合氢气和空气。几何参数的变化引起支柱下游流体运动的变化。与直槽支柱相比，通过会聚凹槽的流体会减速，从而在支柱底部附近将马赫数减少 20%。发现较短的斜坡更有效，因为横向方向上的压力变化沿着支柱下游的支柱壁传递，增加了流体的流向运动。凹槽深度的减小增加了支柱下游的再循环区域。此外，凹槽压缩角的增加还增加了喷射燃料的支柱底部附近的湍流。喷射口位置的变化使燃烧器的混合性能提高了 25%。喷射速度的增加显着改变了燃料喷射流的湍流。然而，流域内流场特性的变化是微乎其微的。燃料质量流量的增加引起流场的显着变化，从而引起更强的激波结构。

原创性/价值

本研究确定了带有斜坡和会聚凹槽的基于支柱的火焰稳定器的最佳几何形状。反应流建模可以在结合本研究中获得的设计特征的支柱几何形状上进行。