A unified mechanism for unconfined deflagration-to-detonation transition in terrestrial chemical systems and type Ia supernovae
A unified mechanism for unconfined deflagration-to-detonation transition in terrestrial chemical systems and type Ia supernovae
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DOI：
10.1126/science.aau7365
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发表时间：
2019-10
期刊：
影响因子：
56.9
通讯作者：
A. Poludnenko;J. Chambers;K. Ahmed;V. Gamezo;B. Taylor
中科院分区：
文献类型：
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作者：
A. Poludnenko;J. Chambers;K. Ahmed;V. Gamezo;B. Taylor
实现无约束超音速爆炸在某些形式的超新星和化学爆炸中，以亚音速移动的火焰（爆燃）自发地演变成由超音速冲击（爆炸）驱动的火焰，从而大大增加了功率输出。人们对这种从爆燃到爆炸的转变（DDT）的机制知之甚少。波鲁德年科等人。开发了一个分析模型来描述 DDT，然后通过实验室实验和数值模拟对其进行测试。他们的模型成功地再现了实验中看到的 DDT，并预测了 Ia 型超新星中的 DDT，这与观测限制一致。同样的机制可能适用于任何无约束爆炸中的滴滴涕。科学，本期第 14 页。 eaau7365 通过实验室实验和数值模拟，开发了化学火焰和超新星的爆炸形成模型。简介 Ia 型超新星 (SNIa)——白矮星 (WD) 恒星的热核爆炸——的性质是天体物理学中的一个悬而未决的问题。人们普遍认为，SNIa 爆炸是由质量接近或低于 1.4 个太阳质量的钱德拉塞卡质量极限的 12C/16O WD 恒星的快速热核燃烧驱动的。然而，除了这个一般性陈述之外，SNIA 的确切机制仍不清楚，有许多可能的情况。事实上，所有现有的正常明亮 SNIa 理论模型（包括经典的单简并钱德拉塞卡质量和亚钱德拉塞卡质量场景，以及双简并合并模型）都需要形成超音速爆震波。当 WD 在爆炸过程中开始膨胀时，这种波会消耗掉所有的恒星物质。对于无约束系统（例如 WD 内部）中的爆炸起爆仍然知之甚少，并且在没有墙壁和障碍物的约束作用或预先存在或外部引入的强烈冲击的情况下特别难以实现。由于涉及的尺度范围极其广泛，SNIa 的数值模型无法根据第一原理捕获爆炸的形成。相反，他们被迫做出两个关键假设：（i）爆炸点火总是发生在爆炸期间；（ii）爆炸形成的时间和位置。因此，大多数现有 SNIa 模型中的爆炸起爆条件都是自由参数，这限制了它们的预测能力。基本原理 SNIa 中的热核燃烧波在性质上与地球上的化学燃烧波相似，因为它们受相同的物理机制控制。这种相似性使我们能够利用陆地化学系统获得的理论、数值和实验结果来深入了解控制 SNIa 爆炸的物理过程的基本方面。这包括也与地面应用相关的爆炸引发现象，范围从基于爆炸的推进和发电系统到与煤炭开采、燃料储存、化学加工和核发电相关的工业设施的爆炸安全。先前的直接数值模拟（DNS）表明，化学火焰与高强度湍流相互作用可以自发加速并产生强烈的冲击或爆炸。这种湍流驱动的爆燃到爆炸的转变（tDDT）可以在基本上不受限制的环境中发生。结果我们提出了无约束系统中 tDDT 的一般分析理论。该理论解释了快速湍流火焰变得不稳定、产生冲击并可能转变为爆炸的行为。当湍流燃烧速度超过 Chapman-Jouguet 爆燃速度时，就会发生这种情况，这是没有冲击的稳态反应波的最大可能速度。我们描述了在陆地氢气-空气火焰中这一过程的实验证实。接下来，我们对简并 12C 恒星等离子体中完全解析的湍流热核火焰进行了数值模拟，以表明在 SNIa 爆炸的典型条件下，这种机制也可以导致强激波的自发形成。我们证明，这些冲击可以通过与周围湍流火焰相互作用而迅速放大，并最终引发爆炸。最后，我们使用发展的理论来确定 SNIa 经典单简并钱德拉塞卡质量模型中的爆炸起爆标准。我们发现，在密度为 107 至 108 g cm−3 范围内，DDT 几乎是不可避免的，最大概率为 3 × 107 g cm−3。结论 我们开发了湍流引起的 DDT 理论，并通过化学火焰实验和热核爆燃数值模拟对其进行了验证。我们的结果描述了化学和热核反应流中无侧限滴滴涕的统一机制。该理论是无参数的，可用于自洽地预测 SNIa 爆炸中起爆的条件。了解 SNIa 中无侧限 DDT 的机制。 SNIa 中的 DDT 预计发生在约 103 至 106 厘米的尺度上，远低于 WD 恒星的特征尺度（109 厘米），并且大多低于三维模拟中可解析的最小尺度（约 105 厘米）。为了在实验和 DNS 中演示湍流驱动的无侧限 DDT，我们考虑了约 10−5 至 10 cm 的小尺度上的湍流-火焰相互作用。化学火焰和热核火焰的实验和 DNS 之间的协同作用导致了我们统一的 DDT 理论。 Ia 型超新星 (SNIa)（白矮星的热核爆炸）的性质是天体物理学中的一个悬而未决的问题。事实上，所有现有的正常、明亮 SNIa 的理论模型都需要爆炸产生爆炸，以消耗所有恒星物质，但爆燃到爆炸转变 (DDT) 的机制仍不清楚。我们提出了湍流引起的 DDT 的统一理论，描述了无约束化学爆炸和热核爆炸中引发爆炸的机制和条件。该模型通过化学火焰实验和热核火焰数值模拟进行了验证。我们使用开发的理论来确定单简并 Chandrasekhar 质量 SNIa 模型中的爆炸起爆标准，并表明 DDT 在密度为 107 至 108 克每立方厘米时几乎是不可避免的。
Achieving unconfined supersonic explosions In some forms of supernovae and chemical explosions, a flame moving at subsonic speeds (deflagration) spontaneously evolves into one driven by a supersonic shock (detonation), vastly increasing the power output. The mechanism of this deflagration-to-detonation transition (DDT) is poorly understood. Poludnenko et al. developed an analytical model to describe DDTs, then tested it with lab experiments and numerical simulations. Their model successfully reproduced the DDT seen in the experiments and predicted a DDT in type Ia supernovae, which is consistent with observational constraints. The same mechanism may apply to DDTs in any unconfined explosion. Science, this issue p. eaau7365 A detonation formation model is developed for chemical flames and supernovae by using lab experiments and numerical simulations. INTRODUCTION The nature of type Ia supernovae (SNIa)—thermonuclear explosions of white dwarf (WD) stars—is an open question in astrophysics. There is a general consensus that SNIa explosions are driven by fast thermonuclear burning in 12C/16O WD stars with a mass close to, or below, the Chandrasekhar-mass limit of 1.4 solar masses. Beyond this general statement, however, the exact mechanisms of SNIa remain unclear, with a number of possible scenarios. Virtually all existing theoretical models of normal, bright SNIa—including the classical, single-degenerate Chandrasekhar-mass and sub-Chandrasekhar-mass scenarios, along with the double-degenerate merger model—require formation of a supersonic detonation wave. This wave consumes all of the stellar material as a WD begins to expand during the explosion. Detonation initiation in unconfined systems, such as the interior of a WD, remains poorly understood and is particularly difficult to achieve in the absence of the confining effect of walls and obstacles or preexisting or externally introduced strong shocks. Numerical models of SNIa are unable to capture detonation formation from first principles because of the extreme range of scales involved. Instead, they are forced to make two crucial assumptions: (i) that detonation ignition always occurs during an explosion and (ii) the time and location of the detonation formation. As a result, detonation initiation conditions are free parameters present in most existing SNIa models, which limits their predictive power. RATIONALE Thermonuclear combustion waves in SNIa are qualitatively similar to chemical combustion waves on Earth because they are controlled by the same physical mechanisms. This similarity allows us to seek insights into the fundamental aspects of the physical processes that control SNIa explosions by using theoretical, numerical, and experimental results obtained for terrestrial chemical systems. This includes detonation initiation phenomena that are also relevant to terrestrial applications that range from detonation-based propulsion and power-generation systems to the explosion safety of industrial facilities related to coal mining, fuel storage, chemical processing, and nuclear power generation. Prior direct numerical simulations (DNS) have shown that chemical flames interacting with high-intensity turbulence can spontaneously accelerate and produce strong shocks or detonations. Such turbulence-driven deflagration-to-detonation transition (tDDT) can occur in essentially unconfined settings. RESULTS We present a general analytical theory of tDDT in unconfined systems. The theory explains the behavior of fast turbulent flames that become unstable, produce shocks, and can transition to detonations. This occurs when the turbulent burning speed exceeds the Chapman-Jouguet deflagration velocity, which is the maximum possible speed of a steady-state reaction wave without a shock. We describe an experimental confirmation of this process in terrestrial H2-air flames. Next, we used numerical simulations of a fully resolved turbulent thermonuclear flame in a degenerate 12C stellar plasma to show that under conditions representative of those in a SNIa explosion, this mechanism can also result in the spontaneous formation of strong shocks. We show that these shocks can rapidly amplify by interacting with surrounding turbulent flames and ultimately trigger a detonation. Last, we used the developed theory to determine the criteria for detonation initiation in the classical single-degenerate Chandrasekhar-mass model of SNIa. We found that DDT is almost inevitable at densities in the range of 107 to 108 g cm−3, with the maximum probability at 3 × 107 g cm−3. CONCLUSION We developed a theory of turbulence-induced DDT and validated it by using experiments on chemical flames and numerical simulations of thermonuclear deflagrations. Our results describe a unified mechanism of unconfined DDT both in chemical and thermonuclear reacting flows. This theory is parameter free and can be used to predict self-consistently the conditions for detonation initiation in SNIa explosions. Understanding the mechanism of the unconfined DDT in SNIa. The DDT in SNIa is predicted to occur on scales of ~103 to 106 cm, which is well below the characteristic scale of a WD star (109 cm) and mostly below the smallest scales resolvable in three-dimensional simulations, ~105 cm. To demonstrate the turbulence-driven, unconfined DDT in experiments and DNS, we considered turbulence-flame interaction on small scales of ~10−5 to 10 cm. The synergy between the experiments and DNS of chemical and thermonuclear flames led to our unified theory of DDT. The nature of type Ia supernovae (SNIa)—thermonuclear explosions of white dwarf stars—is an open question in astrophysics. Virtually all existing theoretical models of normal, bright SNIa require the explosion to produce a detonation in order to consume all of stellar material, but the mechanism for the deflagration-to-detonation transition (DDT) remains unclear. We present a unified theory of turbulence-induced DDT that describes the mechanism and conditions for initiating detonation both in unconfined chemical and thermonuclear explosions. The model is validated by using experiments with chemical flames and numerical simulations of thermonuclear flames. We use the developed theory to determine criteria for detonation initiation in the single-degenerate Chandrasekhar-mass SNIa model and show that DDT is almost inevitable at densities of 107 to 108 grams per cubic centimeter.