In this work we investigate moderately underexpanded n-hexane jets in a supercritical environment with respect to the injectant. At supercritical chamber pressure and constant nozzle pressure ratio of NPR ≈ 5.6, we systematically increase the reduced injection temperature from T_inj = 1.06−1.18. Our experiments comprise multiphase and single-phase injections. The latter expand close to the Widom line. Using a combined method of shadowgraphy and elastic light scattering (ELS), we study the structural evolution of the jets. Surprisingly, our measurements reveal ELS signals even for the supercritical single-phase jets. The measurements show periodic structures in signal intensity, which can be spatially associated with the train of shock cells in the moderately underex-panded jets that subsequently compress and expand the fluid. With chamber conditions close to the Widom line this implies that the fluid state is alternately brought across the Widom line and hence experiences repetitive changes in fluid properties. In this transition, the fluid exhibits changes in structural and thermophysical properties that can be associated with density fluctuations. The latter lead to an increase in scattering cross-section and, therefore, promote additional Rayleigh scattering contributions. The explanation of the scattering signal for our single-phase injections is therefore ascribed to the interaction of fluid dynamical processes within the jet and thermodynamic peculiarities across the Widom line. In each shock cell the fluid experiences a transition in fluid behavior attributed to density fluctuations. The resulting increase in scattering cross-section allows the detection of Rayleigh scattering, even with nonintensified optical CMOS sensors.

中文翻译：

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沿温差线超临界流体注射中增强的瑞雷散射

在这项工作中，我们调查适度欠膨胀ň相对于注射剂正己烷飞机在超临界环境。在超临界室压力和恒定的NPR≈5.6喷嘴压力比下，我们系统地提高了T _inj降低的注入温度= 1.06-1.18。我们的实验包括多相和单相注入。后者扩展到Widom线附近。使用阴影摄影和弹性光散射（ELS）的组合方法，我们研究了射流的结构演变。令人惊讶的是，即使对于超临界单相射流，我们的测量结果也显示了ELS信号。这些测量结果显示出信号强度的周期性结构，该结构可以在空间上与中度过度膨胀的射流中的一系列激波单元相关联，这些射流随后会压缩和膨胀流体。在腔室条件接近Widom线的情况下，这意味着流体状态交替穿过Widom线，因此流体特性会发生重复性变化。在这个过渡中 流体表现出结构和热物理性质的变化，这些变化可能与密度波动有关。后者导致散射截面的增加，因此，促进了额外的瑞利散射贡献。因此，对我们单相喷射的散射信号的解释是由于射流内的流体动力学过程与Widom线上的热力学特性之间的相互作用。在每个冲击单元中，由于密度波动，流体会经历流体行为的转变。所产生的散射横截面增大，即使使用非增强型光学CMOS传感器，也可以检测瑞利散射。促进其他瑞利散射贡献。因此，对我们单相喷射的散射信号的解释是由于射流内的流体动力学过程与Widom线上的热力学特性之间的相互作用。在每个冲击单元中，由于密度波动，流体会经历流体行为的转变。所产生的散射横截面增大，即使使用非增强型光学CMOS传感器，也可以检测瑞利散射。促进其他瑞利散射贡献。因此，对我们单相喷射的散射信号的解释是由于射流内的流体动力学过程与Widom线上的热力学特性之间的相互作用。在每个冲击单元中，由于密度波动，流体会经历流体行为的转变。所产生的散射横截面增大，即使使用非增强型光学CMOS传感器，也可以检测瑞利散射。