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Development of a Phase Stability-Based Fuel Condensation Model for Advanced Low Temperature Combustion Engines.

机译：为高级低温燃烧发动机开发基于相稳定性的燃料冷凝模型。

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摘要

Ever-more stringent legislative regulations on harmful emissions and fuel efficiency have driven researchers to develop cleaner and more efficient internal combustion engines. Research studies have shown that low temperature combustion can produce very low NOx and soot emissions while obtaining diesel-like high thermal efficiency. One strategy is reactivity controlled compression ignition (RCCI) combustion, which has been shown to be more practical and applicable than homogeneous charge compression ignition (HCCI) by providing extra controllability on the combustion processes, including for the combustion phasing and duration. However, recent experimental work has shown that more than 95% of the particulate matter from RCCI combustion consists of organic species, which is drastically different from conventional diesel combustion (CDC), which mainly produces carbonaceous soot. This distinctive character is believed to be related to condensation processes of large hydrocarbon species that cannot stably exist in the gas phase. Rather, under certain conditions the heavy gaseous species can condense and they become responsible for the organic fraction of the particulate matter.;To investigate this physical phenomenon, a thermodynamically consistent, robust and efficient phase equilibrium solver, which performs rigorous phase stability tests and phase splitting calculations with advanced numerical algorithms, was developed. This is a first step forward modeling condensation processes in engines. Potential phase separation and combination are considered using Gibbs free energy minimization and entropy maximization. The numerical solver was well validated on a number of mixtures in two- and three-phase equilibria with available data. It was also applied to study the complex phase behavior of mixtures, including multiphase dynamic flash calculations, supercritical fluid behavior, condensation and evaporation, PVT analysis and critical point behavior. In addition, the developed model was coupled with an open-source CFD code, KIVA, widely used for multi-dimensional engine spray and combustion simulations, thus enabling a consistent treatment of both the fluid dynamics and thermodynamics. The model was used to investigate a number of two-phase flow problems, including regular condensation in a nozzle, retrograde condensation in a shock tube, condensation processes during supercritical fuel injection, and condensation in an engine combustion chamber. The simulations were validated using available experiments for both pure species and mixtures, ranging from subcritical to supercritical flows.;The thermodynamic equilibrium analysis was also applied to study engine fuel condensation processes under non-reacting conditions. First, simulations were performed for Sandia optical combustion vessels and engines with direct injection of a diesel jet into a pure nitrogen environment. Consistent with experiments, the simulations show that condensation of previously evaporated fuel takes place during the expansion stroke. For high-pressure fuel injection of an n-alkane fuel, there are local sub-critical conditions under which phase separation can take place. This is because of the significant reduction of the mixture temperature caused by vaporization and cooling of the cold liquid fuel. Therefore, even though the ambient conditions during injection are supercritical relative to the fuel, the actual mixture temperature can be much lower so that the mixture enters into the two-phase region.;The phase equilibrium model was finally applied to study fuel condensation processes in a RCCI combustion engine. Condensation was predicted during the late stages of the expansion stroke, when the continuous expansion sends the local fluid into the two-phase region again. The condensed fuel is shown to affect emission predictions, including engine-out particulate matter and unburned hydrocarbons. Consistent with experiments, the organic fraction mass from the condensed fuel is predicted to be the majority (more than 99%) of the total particulate matter. Also, as the engine operation changes from low to high load, fuel condensation is significantly reduced due to the higher temperatures and pressures, and the engine-out PM is predicted to be mainly composed of solid carbonaceous soot particles.
机译：关于有害排放物和燃油效率的越来越严格的立法法规驱使研究人员开发更清洁，更高效的内燃机。研究表明，低温燃烧可产生非常低的NOx和烟尘排放，同时获得类似柴油的高热效率。一种策略是反应性控制压缩点火（RCCI）燃烧，通过提供燃烧过程的额外可控性（包括燃烧阶段和持续时间），它已显示出比均质充量压缩点火（HCCI）更实用和适用。但是，最近的实验工作表明，来自RCCI燃烧的95％以上的颗粒物是有机物，这与主要产生碳烟的传统柴油燃烧（CDC）完全不同。据信这种独特的特征与气相中不能稳定存在的大烃物种的缩合过程有关。相反，在某些条件下，重气态物质可以凝结，并成为颗粒物质中有机物的原因。为了研究这种物理现象，需要使用热力学上一致，可靠且有效的相平衡求解器，该求解器将执行严格的相稳定性测试和相变开发了具有高级数值算法的分割计算。这是对发动机中的冷凝过程进行建模的第一步。使用吉布斯自由能最小化和熵最大化来考虑潜在的相分离和组合。数值求解器已在两相和三相平衡的多种混合物上得到了很好的验证，并具有可用数据。它也用于研究混合物的复杂相行为，包括多相动态闪蒸计算，超临界流体行为，冷凝和蒸发，PVT分析和临界点行为。此外，开发的模型还与广泛用于多维发动机喷雾和燃烧模拟的开源CFD代码KIVA相结合，从而能够对流体动力学和热力学进行一致的处理。该模型用于研究许多两相流动问题，包括喷嘴中的定期凝结，冲击管中的逆行凝结，超临界燃料喷射过程中的凝结过程以及发动机燃烧室中的凝结。使用从亚临界流到超临界流的纯物种和混合物的可用实验，对模拟进行了验证。热力学平衡分析还用于研究非反应条件下的发动机燃料冷凝过程。首先，对Sandia光学燃烧容器和发动机进行了仿真，将柴油机喷嘴直接喷射到纯氮气环境中。与实验一致，该模拟表明在膨胀冲程期间发生了先前蒸发的燃料的冷凝。对于正构烷烃燃料的高压燃料喷射，存在局部亚临界条件，在该条件下可发生相分离。这是由于由于冷液体燃料的汽化和冷却引起的混合物温度的显着降低。因此，即使喷射过程中的环境条件相对于燃料而言是超临界的，实际的混合物温度也可以低得多，从而使混合物进入两相区域。 RCCI内燃机。在膨胀冲程的后期，当连续膨胀将局部流体再次送入两相区域时，预计会发生冷凝。凝结的燃料显示会影响排放预测，包括发动机排出的颗粒物和未燃烧的碳氢化合物。与实验一致，来自冷凝燃料的有机馏分质量预计将占总颗粒物的大部分（超过99％）。另外，随着发动机的运行从低负荷变为高负荷，由于较高的温度和压力，燃料凝结得到显着减少，并且发动机排出的PM预计主要由固体碳质烟灰颗粒组成。

著录项

  • 作者

    Qiu, Lu.;

  • 作者单位

    The University of Wisconsin - Madison.;

  • 授予单位 The University of Wisconsin - Madison.;
  • 学科 Mechanical engineering.
  • 学位 Ph.D.
  • 年度 2014
  • 页码 211 p.
  • 总页数 211
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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