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Turbulent Combustion Group

Intro block Prof. Zhuyin Ren's Group

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Introduction to the laboratory

        团队自2015年以来承担了重大研究计划项目,围绕着“受限空间内复杂湍流和燃烧的相互作用”核心科学问题,开展宽压力范围、强湍流下点火及火焰传播机理研究。在湍流化学反应耦合机制及其对点火和火焰传播的影响、湍流燃烧建模、高精度数值模拟及不确定分析等基础问题进行了深入研究。

        研究实现了适用于发动机工况的点火及火焰传播特性的实验研究,发现由火焰面不稳定性引起的火焰多级加速现象并揭示了其机制;研究了湍流对火核生成和传播的影响机制和湍流火焰传播速度标度率;发展了火焰传播速度的新的外推不确定性准则;揭示了发动机燃料着火负温度系数区转折点的控制机制,实现了详细化学反应动力学在数值模拟中的高效运用;揭示了湍流预混燃烧中湍流和火焰协同控制的反应标量混合机制,发展一个新的反应标量小尺度混合速率模型,提升了PDF模拟的适用性和准确性;发展了数值模拟的不确定性分析理论和方法,提出了运用低维子空间实现了动力学不确定性在模拟中的传递,用于揭示点火和火焰传播中的主导物理化学过程及模型参数;研究同时发展了基于湍流-化学反应耦合机制的低维发动机火核生成概率预测模型,为发动机可靠点火优化提供理论和预测工具的支撑。

混合气H2:O2:Ar=0.6:1:10(摩尔分数比)湍流点火过程

Prof. Ren gave an invited talk on KAUST Research Conference of Combustion in Extreme Conditions

Title: Transported PDF Simulations of Turbulent Flames with Uncertainty Quantification

Abstract:  Combustion modeling is now playing an important role in the design and optimization of advanced combustion devices. For high-fidelity combustion modeling, it is essential, though challenging, to resolve the highly nonlinear turbulence-chemistry interaction and to predict the near-limit combustion phenomena and pollutants. This requires the accurate description of turbulent mixing as well as the use of detailed chemistry. 

This talk will focus on presenting recent progress on transported PDF simulations of turbulent premixed flames, specifically the scalar mixing timescale modeling and adaptive chemistry acceleration. In addition, the propagation of kinetic uncertainty through active subspace will be demonstrated for transported PDF simulations of turbulent flames. It will conclude with a discussion on the subtle grid resolution issue in large eddy simulations of turbulent premixed flames.

Source: https://ccrc.kaust.edu.sa/cec/Pages/Event.aspx?i=17

Prof. Ren gave an invited talk on the opening of 2017 China National Sympisium on Combustion

报告题目:湍流燃烧概率密度函数模拟 (In Chinese)

摘要:高精度数值模拟是揭示湍流燃烧中复杂多物理耦合机制和燃烧组织优化的重要手段。基于概率密度函数方法(PDF)的模拟能精确求解有限反应速率,对研究强湍流-反应相互作用如局部熄火/再燃现象和污染物生成等有独到的优势。报告首先介绍PDF方法在湍流燃烧模拟中的应用现状。接着介绍提升PDF预测精度和效率开展的反应标量小尺度混合建模和详细反应动力学高效计算方法的工作。最后介绍研究湍流燃烧中控制物理过程和模拟不确定性的理论和方法。

Source: http://combust2017.csp.escience.cn/dct/page/70009

Congratulations to Hua Zhou for successfully defending his PhD thesis

Congratulations to Hua Zhou for successfully defending his PhD thesis. Wish him a bright future!

Uncertainty Quantificationh

Forward propagation of kinetic uncertainty in combustion simulations usually adopts response surface techniques to accelerate Monte Carlo sampling. Yet it is computationally challenging to build response surfaces for high-dimensional input parameters and expensive combustion models. This study uses the active subspace method to identify low-dimensional subspace of the input space, within which response surfaces can be built. Active subspace methods have previously been developed only for single (scalar) model outputs,

however. This paper introduces a new method that can simultaneously approximate the marginal probability density functions of multiple outputs using a single low-dimensional shared subspace. We identify the shared subspace by solving a least-squares system to compute an appropriate combination of single-output active subspaces. Because the identification of the active subspace for each individual output may require a significant number of samples, this process may be computationally intractable for expensive models such as turbulent combustion simulations. Instead, we propose a heuristic approach that learns the relevant subspaces from cheaper combustion models. The performance of the active subspace for a single output, and of the shared subspace for multiple outputs, is first demonstrated with the ignition delay times and laminar flame speeds of hydrogen/air, methane/air, and dimethyl ether (DME)/air mixtures. Then we demonstrate extrapolatory performance of the shared subspace: using a shared subspace trained on the ignition delays at constant volume, we perform forward propagation of kinetic uncertainties through zero-dimensional HCCI simulations – in particular, single-stage ignition of a natural gas/air mixture and two-stage ignition of a DME/air mixture. We show that the shared subspace can accurately reproduce the probability of ignition failure and the probability density of ignition crank angle conditioned on successful ignition, given uncertainty in the kinetics.

 

Mixing Models in PDF Method

Transported probability density function (TPDF) methods are attractive for modeling turbulent flames as the highly nonlinear chemical reactions appear in closed form.  The ability of the TPDF methods to capture complex phenomena such as extinction and re-ignition has been well demonstrated for turbulent non-premixed flames. However challenges remain when applying the TPDF methods to turbulent premixed flames, for which modeling molecular diffusion is difficult because the local species gradients may be strongly influenced by chemical reactions. The specification of a constant mechanical-to-scalar timescale ratio to relate the scalar mixing timescale to the turbulence timescale is questionable for premixed flames in the flamelet regime.

A new mixing timescale model is proposed to account for both flamelet-controlled and turbulence-controlled mixing to more accurately model scalar mixing rates [1,2]. In the limit of passive scalar mixing, the mixing timescale is proportional to the turbulence timescale according to the classical expression. Conversely, in the limit of laminar flamelets embedded in a turbulent flow field, the scalar mixing rate is expected to depend on the laminar flame structure, where the conditional scalar dissipation rate can be obtained from 1D laminar premixed reference flames.

A priori assessment using the DNS datasets of the H2-air premixed flames, as well as a posteriori tests in CH4-air Slot Bunsen flames, both demonstrate the model potential to better predict scalar dissipation and flame characteristics.

[1] Zhou, Z., Li, S., Wang, H., and Ren, Z. 2015. An investigation of a hybrid mixing model for PDF simulations of turbulent premixed flames. 68th Annual Meeting of the APS Division of Fluid Dynamics.

[2] Kuron, M., Ren, Z., Hawkes, E.R., Zhou, H., Kolla, H., Chen, J.H., and Lu, T. 2017. A mixing timescale model for TPDF simulations of turbulent premixed flames, Combust. Flame, 177, 171-183

Chemistry Acceleration

In numerical simulations of combustion processes, the use of dimension reduction to simplify the description of the chemical system has the advantage of reducing the computational cost, but it is important also to retain accuracy and adequate detail. In current research, dimension reduction of combustion chemistry is performed by using the ICE-PIC method. The ICE-PIC method is developed based on three major ingredients: constrained equilibrium; trajectory-generated manifolds; and, the pre-image curve method. The low-dimensional manifold identified, the ICE manifold, is invariant, continuous and piecewise smooth. The ICE-PIC method achieves local species reconstruction based on the constrained-equilibrium pre-image curve. In comparison to other existing methods such as QSSA, RCCE and ILDM, this method is the first approach that locally determines compositions on a low-dimensional invariant manifold.

The accuracy of the ICE-PIC method has been examined in autoignition and in one-dimensional laminar flames of hydrogen/air and methane/air mixtures. Studies demonstrate that the local errors incurred by ICE-PIC (e.g., the errors in the reconstructed composition) are well controlled. The capability of the ICE-PIC method for the reduced description of reactive flows is demonstrated through the calculation of the oxidation of CO/H2 in a CSTR. The reduced description provided by the ICE-PIC method is capable of quantitatively reproducing the complex dynamics.

The final goal is to develop an efficient and accurate algorithm - In Situ Adaptive Tabulation with Dimension Reduction (the ICE-PIC method) to incorporate detailed chemistry in turbulent combustion simulations.

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