Purpose

Double flow-focusing nozzles (DFFNs) form a coaxial flow of primary liquid with micro-crystalline samples, surrounded by secondary liquid and focusing gas. This paper aims to develop an experimentally validated numerical model and assess the performance of micro-jets from a DFFN as a function of various operating parameters for the water–ethanol–helium system, revealing the jet's stability, diameter, length and velocity.

Design/methodology/approach

The physical model is formulated in the mixture-continuum formulation, which includes coupled mass, momentum and species transport equations. The model is numerically formulated within the finite volume method–volume of fluid approach and implemented in OpenFOAM to allow for a non-linear variation of the fluid's material properties as a function of the mixture concentration. The numerical results are compared with the experimental data.

Findings

A sensitivity study of jets with Reynolds numbers between 12 and 60, Weber numbers between 4 and 120 and capillary numbers between 0.2 and 2.0 was performed. It was observed that jet diameters and lengths get larger with increased primary and secondary fluid flow rates. Increasing gas flow rates produces thinner, shorter and faster jets. Previously considered pre-mixed and linear mixing models substantially differ from the accurate representation of the water–ethanol mixing dynamics in DFFNs. The authors demonstrated that Jouyban–Acree mixing model fits the experimental data much better.

Originality/value

The mixing of primary and secondary liquids in the jet produced by DFFN is numerically modelled for the first time. This study provides novel insights into mixing dynamics in such micro-jets, which can be used to improve the design of DFFNs.

中文翻译：

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双流聚焦微射流的数值研究

 目的

双流聚焦喷嘴 （DFFN） 形成初级液体与微晶样品的同轴流，周围环绕着二次液体和聚焦气体。本文旨在开发一个经过实验验证的数值模型，并评估 DFFN 的微型射流的性能与水-乙醇-氦系统各种运行参数的函数关系，揭示射流的稳定性、直径、长度和速度。

设计/方法/方法

物理模型以混合物-连续体公式的形式表述，其中包括耦合的质量、动量和物质传递方程。该模型在有限体积法-流体体积法中以数值公式表述，并在 OpenFOAM 中实现，以允许流体材料属性随混合物浓度的非线性变化。将数值结果与实验数据进行了比较。

 发现

对雷诺数在 12 到 60 之间、韦伯数在 4 到 120 之间和毛细管数在 0.2 到 2.0 之间的射流进行了敏感性研究。据观察，射流直径和长度随着初级和次级流体流速的增加而变大。增加气体流速会产生更薄、更短和更快的射流。以前考虑的预混合和线性混合模型与 DFFN 中水-乙醇混合动力学的准确表示存在很大差异。作者证明 Jouyban-Acree 混合模型更符合实验数据。

 原创性/价值

DFFN 产生的射流中初级和次级液体的混合首次进行了数值建模。本研究为这种微射流中的混合动力学提供了新的见解，可用于改进 DFFN 的设计。