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Numerical modeling of two-phase flow in the sodium chloride-water system with applications to seafloor hydrothermal systems.

机译：氯化钠-水系统中两相流的数值模拟及其在海底热液系统中的应用。

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摘要

In order to explain the observed time-dependent salinity variations in seafloor hydrothermal vent fluids, quasi-numerical and fully numerical fluid flow models of the NaCl-H2O system are constructed. For the quasi-numerical model, a simplified treatment of phase separation of seawater near an igneous dike is employed to obtain rough estimates of the thickness and duration of the two-phase zone, the amount of brine formed, and its distribution in the subsurface. Under the assumption that heat transfer occurs mainly by thermal conduction it is shown that, for a two-meter wide dike, the maximum width of the two phase zone is approximately 20 cm and that a zone of halite is deposited near the dike wall. The two-phase zone is mainly filled with vapor. After 13 days, the two-phase zone begins to disappear at the base of the system, and disappears completely by 16 days. The results of this simplified model agree reasonably well with transient numerical solutions for the analogous two-phase flow in a pure water system. The seafloor values of vapor salinity given by the model are compared with vapor salinity data from the "A" vent at 9-10°N on the East Pacific Rise and it is argued that either non-equilibrium thermodynamic behavior or near-surface mixing of brine with vapor in the two-phase region may explain the discrepancies between model predictions and data. For the fully numerical model, the equations governing fluid flow, the thermodynamic relations between various quantities employed, and the coupling of these elements together in a time marching scheme is discussed. The thermodynamic relations are expressed in terms of equations of state, and the latter are shown to vary both smoothly and physically in P-T-X space. In particular, vapor salinity values near the vapor-liquid-halite coexistence surface are shown to be in strong agreement with recently measured values. The fully numerical model is benchmarked against previously published heat pipe and Elder problem simulation results, and is shown to be largely in agreement with those results. Additionally, code output from an approximately one-dimensional scenario is compared to the analytic solution of the classical one-dimensional thermal advection-diffusion equation, and it is found that the numerical output and analytic solution are in strong agreement. A number of simulation results are presented in the context of two-phase flow and phase separation within the framework of the single pass model, a model that has been shown to be useful in the study of seafloor hydrothermal systems. It is found that a quasi-stable two-phase (liquid + vapor) zone at depth below the hydrothermal discharge outlet gives rise to vent fluid with lower than normal seawater salinity. Additionally, it is shown that increasing the spatial extent of the two-phase zone can lower vent fluid salinity, even with the average temperature of the two-phase zone held constant. As the two-phase zone evolves, brine of high salinity and density collects at the bottom of the system and is held there primarily via the effect of vapor on the liquid phase's relative permeability; however, it is found that lowering the temperature of the heat source until the two-phase zone vanishes and allowing the system to evolve for some time results in the flushing of this brine from the system. The resulting pattern of vent fluid salinities resembles that described in a widely held conceptual model of vent fluid salinity variation in seafloor hydrothermal systems, where low salinity fluids emerge from venting systems during early stages, and high salinity fluids emerge at later stages as brine is flushed from the system. The effect of varying the permeability is investigated, and it is found that peaks in vent fluid salinities occur later in time for lower permeabilities than one might expect for a simple linear relationship. Finally, it is argued that the numerical approach used in this thesis may be able to explain the vent fluid salinities and temperatures found at the Main Endeavour Vent Field on the Juan de Fuca Ridge, as this approach is able to produce simulated vent fluid salinities that match observed values from the Endeavour Field vents Dante and Hulk.
机译：为了解释在海底热液排放流体中盐度随时间的变化，构建了NaCl-H2O系统的准数值和全数值流体模型。对于准数值模型，采用火成岩堤坝附近海水相分离的简化处理方法，可以粗略估算出两相区的厚度和持续时间，形成的盐水量及其在地下的分布。在热传递主要通过热传导发生的假设下，表明对于两米宽的堤坝，两相区域的最大宽度约为20 cm，并且在堤壁附近沉积了盐岩区域。两相区主要充满蒸气。 13天后，两阶段区域在系统底部开始消失，并在16天后完全消失。该简化模型的结果与纯水系统中类似两相流的瞬态数值解非常吻合。将模型给出的海盐盐度海底值与东太平洋上升沿9-10°N处“ A”排放口的水盐度数据进行了比较，并认为这是由于非平衡热力学行为或近地表混合引起的。两相区域中含有蒸汽的盐水可能解释了模型预测与数据之间的差异。对于全数值模型，讨论了控制流体流动，采用的各种量之间的热力学关系以及这些元素在时间行进方案中的耦合的方程式。热力学关系用状态方程表示，后者在P-T-X空间中既平滑又物理变化。特别地，显示出气-液-卤化物共存表面附近的蒸气盐度值与最近测量的值非常一致。完全数值模型以先前发布的热管和Elder问题模拟结果为基准，并显示出与这些结果基本一致。另外，将近似一维情况下的代码输出与经典一维热对流扩散方程的解析解进行了比较，发现数值输出与解析解具有很强的一致性。在单程模型的框架内，在两相流和相分离的背景下，提出了许多模拟结果，该模型已被证明可用于研究海底热液系统。发现在热液排放出口下方深度处的准稳定的两相（液体+蒸气）区域会产生排放液，其海水盐度低于正常水平。另外，显示出即使两相区的平均温度保持恒定，增加两相区的空间范围也可以降低排出流体的盐度。随着两相区的发展，高盐度和高密度的盐水聚集在系统的底部，并主要通过蒸汽对液相的相对渗透率的影响而保持在系统底部。然而，发现降低热源的温度直到两相区消失并且允许系统发展一段时间会导致从系统中冲洗该盐水。最终的排放液盐度模式类似于在海底热液系统中广泛存在的排放液盐度变化概念模型中描述的模式，该模型在早期阶段从排放系统中排出低盐度流体，而在冲洗盐水后在后期阶段出现高盐度流体。从系统。研究了改变渗透率的影响，发现在渗透液盐度中出现峰值的时间较晚，其渗透率低于对简单线性关系的预期。最后，有人认为，本文中使用的数值方法可能能够解释在胡安·德·富卡山脊上的主要努力通气区域发现的放空流体盐度和温度，因为这种方法能够产生模拟的放空流体盐度，从而匹配“奋进号”野外喷口但丁和绿巨人的观测值。

著录项

  • 作者

    Lewis, Kayla C.;

  • 作者单位

    Georgia Institute of Technology.;

  • 授予单位 Georgia Institute of Technology.;
  • 学科 Geophysics.
  • 学位 Ph.D.
  • 年度 2007
  • 页码 157 p.
  • 总页数 157
  • 原文格式 PDF
  • 正文语种 eng
  • 中图分类
  • 关键词

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