Influence of Thermal Expansion on Fluid Dynamics of Turbulent Premixed Combustion and Its Modelling Implications
Influence of Thermal Expansion on Fluid Dynamics of Turbulent Premixed Combustion and Its Modelling Implications
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DOI：
10.1007/s10494-020-00237-8
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发表时间：
2021-03
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影响因子：
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通讯作者：
N. Chakraborty
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文献类型：
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作者：
N. Chakraborty
本文的目的是证明热膨胀的影响，作为放热化学反应所产生的热量释放的结果，在湍流预混燃烧的情况下，对底层湍流流体动力学及其建模。由于热释放的热膨胀引起的膨胀率在湍流预混火焰，这已被证明有显着的影响，流动拓扑分布，湍流动能和拟能演化的主要正值。已经证明，主要是正膨胀率的大小提供了热膨胀强度的度量。随着Karlovitz数和特征刘易斯数的减小以及未燃气体与已燃气体密度比的增大，热膨胀对湍流度的影响增强。这反映在流动拓扑结构的贡献，这是只获得了积极的值的膨胀率，随着Karlovitz数的减弱。预混湍流火焰中的热膨胀不仅引起正膨胀率，而且由于火焰法向加速引起火焰诱导压力梯度。压力和膨胀波动之间的相关性以及密度和压力梯度之间的矢量积分别通过压力膨胀项和斜压扭矩项显著影响湍流预混火焰中湍流动能和涡度拟能的演化。与湍流动能和涡度拟能输运方程中的其他项的大小相比，压胀和斜压力矩的相对贡献分别随着Karlovitz数和特征刘易斯数的减小而加强。这将导致显着的增大湍流动能和拟能内的火焰刷的小值的Karlovitz和特征刘易斯数，但湍流动能和拟能衰减从未燃到燃烧气体侧的火焰刷的Karlovitz和特征刘易斯数的大值。预混火焰内的热释放也引起显着的各向异性的子网格应力，并影响他们的路线与解决应变率。这种各向异性在亚网格应力的建模中起着关键作用，并且已经证明了亚网格应力的各向同性部分的显式闭合可以提高亚网格应力和湍流动能闭合的性能。此外，通常的动态建模技术，这是用于非反应湍流，已被证明是不适合湍流预混火焰。此外，由于火焰法向加速度的火焰速度增加，可能会导致湍流动能，反应标量，标量梯度和标量方差在预混湍流火焰在某些条件下的反梯度传输。反梯度输运的倾向随着均方根湍流速度和特征刘易斯数的减小而增大。已经发现，涡主要对准中间主应变率本征方向，但最广泛的和最压缩的主应变率本征方向的涡的相对程度的变化响应于热膨胀的强度。结果表明，对于小的次单位刘易斯数和大的Damköhler数与小的Karlovitz数的组合，膨胀率几乎等于最广泛的应变率。
The purpose of this paper is to demonstrate the effects of thermal expansion, as a result of heat release arising from exothermic chemical reactions, on the underlying turbulent fluid dynamics and its modelling in the case of turbulent premixed combustion. The thermal expansion due to heat release gives rise to predominantly positive values of dilatation rate within turbulent premixed flames, which has been shown to have significant implications on the flow topology distributions, and turbulent kinetic energy and enstrophy evolutions. It has been demonstrated that the magnitude of predominantly positive dilatation rate provides the measure of the strength of thermal expansion. The influence of thermal expansion on fluid turbulence has been shown to strengthen with decreasing values of Karlovitz number and characteristic Lewis number, and with increasing density ratio between unburned and burned gases. This is reflected in the weakening of the contributions of flow topologies, which are obtained only for positive values of dilatation rate, with increasing Karlovitz number. The thermal expansion within premixed turbulent flames not only induces mostly positive dilatation rate but also induces a flame-induced pressure gradient due to flame normal acceleration. The correlation between the pressure and dilatation fluctuations, and the vector product between density and pressure gradients significantly affect the evolutions of turbulent kinetic energy and enstrophy within turbulent premixed flames through pressure-dilatation and baroclinic torque terms, respectively. The relative contributions of pressure-dilatation and baroclinic torque in comparison to the magnitudes of the other terms in the turbulent kinetic energy and enstrophy transport equations, respectively strengthen with decreasing values of Karlovitz and characteristic Lewis numbers. This leads to significant augmentations of turbulent kinetic energy and enstrophy within the flame brush for small values of Karlovitz and characteristic Lewis numbers, but both turbulent kinetic energy and enstrophy decay from the unburned to the burned gas side of the flame brush for large values of Karlovitz and characteristic Lewis numbers. The heat release within premixed flames also induces significant anisotropy of sub-grid stresses and affects their alignments with resolved strain rates. This anisotropy plays a key role in the modelling of sub-grid stresses and the explicit closure of the isotropic part of the sub-grid stress has been demonstrated to improve the performance of sub-grid stress and turbulent kinetic energy closures. Moreover, the usual dynamic modelling techniques, which are used for non-reacting turbulent flows, have been shown to not be suitable for turbulent premixed flames. Furthermore, the velocity increase across the flame due to flame normal acceleration may induce counter-gradient transport for turbulent kinetic energy, reactive scalars, scalar gradients and scalar variances in premixed turbulent flames under some conditions. The propensity of counter-gradient transport increases with decreasing values of root-mean-square turbulent velocity and characteristic Lewis number. It has been found that vorticity aligns predominantly with the intermediate principal strain rate eigendirection but the relative extents of alignment of vorticity with the most extensive and the most compressive principal strain rate eigendirections change in response to the strength of thermal expansion. It has been found that dilatation rate almost equates to the most extensive strain rate for small sub-unity Lewis numbers and for the combination of large Damköhler and small Karlovitz numbers, and under these …