This report presents a study of dissipation (irreversible production of entropy) in three-dimensional, temporal mixing layers laden with evaporating liquid drops. The purpose of the study is to examine the effects of evaporating drops on the development of turbulent features in flows. Direct numerical simulations were performed to analyze transitional states of three mixing layers: one without drops, and two that included drops at different initial mass loadings. Without drops, the dissipation is essentially due to viscous effects. It was found that in the presence of drops, the largest contribution to dissipation was made by heating and evaporation of the drops, and that at large length scales, this contribution is positive (signifying that the drops reduce turbulence), while at small scales, this contribution is negative (the drops increase turbulence). The second largest contribution to dissipation was found to be associated with the chemical potential, which leads to an increase in turbulence at large scales and a decrease in turbulence at small scales. The next smaller contribution was found to be that of viscosity. The fact that viscosity effects are only third in order of magnitude in the dissipation is in sharp contrast to the situation for the mixing layer without the drops. The next smaller contribution — that of the drag and momentum of the vapor from the drops — was found to be negative at lower mass loading but to become positive at higher mass loading.
This work was done by Josette Bellan and Nora Okong'o of Caltech for NASA's Jet Propulsion Laboratory. To obtain a copy of the report, "Irreversible entropy production in two-phase flows with evaporating drops," access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. NPO-30586
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Irreversible Entropy Production in Two-Phase Mixing Layer
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Overview
The document presents findings from a study conducted by Josette Bellan and Nora Okong’o at the Jet Propulsion Laboratory (JPL) under NASA's sponsorship, focusing on the effects of evaporating drops on turbulence in fluid flows. The research is detailed in the report titled "Irreversible Entropy Production in Two-Phase Mixing Layers," which examines how the presence of evaporating drops alters the dissipation mechanisms in turbulent flows.
The study employs direct numerical simulations to analyze three different mixing layers: one without drops and two with drops at varying initial mass loadings. The primary objective is to understand how these drops interact with the flow and influence turbulence characteristics. The findings reveal that, in the absence of drops, turbulence dissipation is predominantly due to viscous effects. However, when drops are introduced, the dynamics change significantly.
The research identifies that the largest contribution to dissipation in the presence of drops arises from the heating and evaporation processes. At larger length scales, this contribution is positive, indicating that the drops help reduce turbulence. Conversely, at smaller scales, the effect is negative, leading to an increase in turbulence. This dual behavior highlights the complex role that evaporating drops play in turbulent flows.
Additionally, the study finds that the second most significant contribution to dissipation is related to the chemical potential, which also exhibits a scale-dependent effect—enhancing turbulence at larger scales while diminishing it at smaller scales. Viscosity, traditionally considered a primary factor in turbulence dissipation, ranks third in importance when drops are present. The research also notes that the drag and momentum of vapor from the drops contribute to dissipation, with their effects varying based on the mass loading of the drops.
Overall, the work emphasizes the need for a deeper understanding of the interactions between evaporating drops and turbulent flows, as these insights can lead to more accurate modeling of two-phase flows with phase changes. The findings have implications for various applications, including environmental science, engineering, and aerospace, where understanding fluid dynamics is crucial.
For further details, the report can be accessed through the Technical Support Package (TSP) available online.
