As a renewable fuel, hydrogen (H₂) may play an increasingly important role in the development and control of piston and gas turbine engines to achieve zero carbon emissions. Predictive modeling of H₂-fueled combustion processes requires a clear understanding of differential diffusion (DD) due to the high diffusivity of H₂. On the assumption that turbulent mixing is a far more dominant process than molecular mixing, DD effects are typically neglected in turbulent combustion simulations to reduce modeling complications. While this assumption is reasonable for hydrocarbon fuels, it is less valid for H₂ combustion, where DD is significant. In this work, two three-dimensional direct numerical simulations of temporally evolving turbulent H₂ jet flames with and without considering DD are performed and compared with laminar flamelet solutions to assess DD effects under turbulent conditions. The emphasis is placed on assessing the suitability of classical mixture fraction Z and Bilger mixture fraction Z_Bilger as conditioning variables for non-premixed turbulent combustion modeling through analyzing DD effects on flame structure, chemical reactions, and tangential diffusion (TD). Furthermore, the persistence of DD effects under turbulent conditions and the suitability of a conventional DD parameter are investigated by comparing the turbulent flames to laminar flamelet solutions. It is found that conditioning the thermochemical state on Z_Bilger helps to capture DD effects and mitigate the relative contribution of TD, which gives Z_Bilger advantages over Z when employing flamelet modeling. Due to close coupling between DD and local chemical reactions, DD can affect the turbulent/laminar flames in the form of thermal effects due to the change in flame temperature, chemical effects due to the change in chemical reactions, and transport effects due to multiple species with varying diffusivities that could result in the difference between Z and Z_Bilger. While the transport effects are suppressed, significant chemical and thermal effects of DD still persist under turbulent conditions, which indicates that the DD parameter is probably unsuitable for comprehensively characterizing and assessing DD effects on the structure of turbulent non-premixed flames.

作为可再生燃料，氢气（H ₂）在开发和控制活塞和燃气涡轮发动机以实现零碳排放方面可能发挥越来越重要的作用。由于H ₂的高扩散性，对H ₂燃烧过程的预测模型需要清楚地了解微分扩散（DD）。假设湍流混合比分子混合要占主导地位，在湍流燃烧模拟中通常忽略DD效应以减少建模复杂性。尽管此假设对于烃类燃料而言是合理的，但对于DD较大的H ₂燃烧而言，其有效性较差。在这项工作中，两个关于时变湍流H的三维直接数值模拟_进行2次有和没有考虑DD的喷射火焰，并与层流小火焰溶液进行比较，以评估湍流条件下DD的影响。通过分析DD对火焰结构，化学反应和切向扩散的影响，重点放在评估经典混合分数Z和Bilger混合分数Z _Bilger作为非预混湍流燃烧模型的条件变量的适用性。此外，通过将湍流火焰与层流小火焰溶液进行比较，研究了湍流条件下DD效应的持久性和常规DD参数的适用性。发现在Z _Bilger上调节热化学状态有助于捕捉DD及减轻TD的相对贡献，这给ž_比尔热了优势ž采用小火焰造型的时候。由于DD与局部化学反应之间的紧密耦合，DD会由于火焰温度的变化而以热效应的形式影响湍流/层流火焰，由于化学反应的变化而引起的化学效应以及由于多种物种引起的传输效应会影响湍流/层流火焰具有不同的扩散率，可能导致Z和Z _Bilger之间的差异。尽管抑制了输运效应，但在湍流条件下，DD仍具有显着的化学和热效应，这表明DD参数可能不适合全面表征和评估DD对湍流非预混火焰结构的影响。