Jordi Pallares

Slug frequency is a key parameter in the modeling and flow assurance management of gas–liquid slug flow encountered in the petroleum industry. While slug frequency has been extensively investigated in horizontal and vertical pipes, its behavior in inclined flows with high-viscosity liquids remains less documented. In this short communication, experimental datasets reported in the literature are re-analyzed to investigate the combined effects of pipe inclination and liquid viscosity on slug frequency. The results show that, for a given liquid viscosity, slug frequency increases with pipe inclination. In contrast, the influence of liquid viscosity depends largely on the regime, showing opposite trends in flows dominated by gravity and inertia. The transition between these regimes is observed at a mixture Froude number of approximately 1.2. Further experimental investigations of slug frequency in inclined gas–liquid flows with high-viscosity liquids are required to better explain this behavior and to develop more robust slug frequency models for gas-high-viscosity liquid two-phase flow in inclined pipes.

The COVID-19 pandemic has highlighted the critical role of respiratory aerosol dynamics in disease transmission, reinforcing the need for accurate biomedical models of exhalation and environmental dispersion. This study investigates the coupled heat and mass transfer phenomena governing bioaerosol cloud behavior during violent exhalations, such as coughing and sneezing, under varying ambient temperatures. A novel experimental setup was developed featuring a physiologically accurate, three-dimensional-printed replica of the human upper respiratory tract, including the nasal cavity, to closely replicate human exhalation mechanics and thermal conditions. Three exhalation flow rates, maintained at a body temperature of 37 °C, were tested in a climatic chamber set to ambient temperatures of 27, 17, and 7 °C. The experimental data enables an assessment of the influence of both ambient temperature and nasal airflow on aerosol dispersion. The results show that higher ambient temperatures enhance the vertical rise of the aerosol cloud and reduce ambient air entrainment, allowing the cloud to travel farther and maintain a higher particle concentration, even after the exhalation has ceased. In addition, nasal airflow alters the emission angle and modifies the subsequent trajectory of the cloud. These findings provide valuable guidance for optimizing indoor (heating, ventilation, and air conditioning) systems and serve as experimental validation for computational aerosol dispersion models.

This paper presents and discusses the results of the “2024 International Computational Fluid Dynamics Challenge on the long-range indoor dispersion of pathogen-laden aerosols” aimed at assessing the ability of different computational codes and turbulence models to reproduce the dispersion of particles produced by a turbulent natural convection flow enclosed in a room sized cubical cavity. A total of 12 research groups from ten different countries have conducted 15 simulations of the same flow configuration by solving the Reynolds averaged Navier–Stokes (RANS) equations, the unsteady Reynolds averaged Navier–Stokes (URANS) equations or using scale adaptive simulations (SAS), large-eddy simulations (LES), or hybrid (URANS-LES) techniques. Results for the velocity field and the particle dispersion provided by the different simulations are compared extensively, including the reference results provided by a direct numerical simulation (DNS). In general, LES and hybrid methods reproduce the time-averaged flow field correctly, the spatial distribution of the turbulence kinetic energy, and the particle dispersion. The performance of SAS is similar to that of LES and hybrid methods while the predictions of the RANS and URANS simulations exhibit larger deviations with respect to DNS. In general, the particle dispersion is better reproduced by simulations that capture correctly the spatial distribution of the turbulence kinetic energy.

Coughing and sneezing are critical mechanisms for the transmission of airborne respiratory diseases, dispersing pathogen-laden aerosols into the environment. Previous human volunteer studies provided valuable insight into aerosol dynamics but lacked reproducibility due to individual variations. This paper presents a novel, replicable experimental setup using three dimensional models of the upper respiratory tract and nasal cavity to simulate isothermal human-like coughs and sneezes. Results indicate that nasal cavity involvement decreases horizontal aerosol cloud spread while enhancing vertical dispersion. Incorporating this experimental data with theoretical models improves predictive accuracy of aerosol cloud evolution, particularly for indoor environments. Finally, a single novel analytical expression for the evolution of the particle cloud tip is derived that accurately predicts the cases studied.

The slug frequency (SF), which refers to the number of liquid slugs passing through a pipe during a specific time, is an important parameter for characterizing the multiphase intermittent flows and monitoring some process involving this kind of flow. The simplicity of the definition of SF contrasts with the difficulty of correctly measuring it. This manuscript aims to review and discuss the various techniques and methods developed to determine the slug frequency experimentally. This review significantly reveals the absence of a universal measurement method applicable to a wide range of operating conditions. Thus, the recourse to recording videos with high-speed cameras, which can be used only at a laboratory scale, remains often necessary. From the summarized state-of-the-art, it appears that correctly defining the threshold values for detecting the liquid slugs/elongated bubbles interface from physical parameters time series, increasing the applicability of instrumentations at industrial scales, and properly estimating the uncertainties are the challenges that have to be faced to advance in the measurement of SF.

We employ three-dimensional, fully resolved numerical simulations using the volume-of-fluid method to study the motion and interaction of two in-line bubbles ascending in both Newtonian and shear-thinning fluids. Additionally, we explore passive scalar transfer between the fluid phases across a variety of fluidic conditions, modeling shear-thinning behavior in non-Newtonian fluids through the Carreau model. The impact of the Galilei (Ga) and Bond (Bo) numbers, the bubble pair radius ratio, the inelastic time constant (λ), and the flow index (n) on the bubbles dynamics and the transient Sherwood number (Sht) and the surface-averaged Sherwood number (⟨Sh⟩) are reported. Using the well-known Ga–Bo regime phase diagram for a single rising bubble in a Newtonian ambient fluid, the present numerical experiments are used to study the departure from this reference case due to the presence and characteristics of a second bubble and the non-Newtonian nature of the ambient fluid. When categorized based on the single bubble phase diagram, we found that in regimes I (axisymmetric) and III (oscillatory), a pair of bubbles does not breakup or merge during our simulations. However, their behaviors vary due to the second bubble and change in non-Newtonian fluid parameters like the inelastic time constant and flow index. Likewise, we explored this parameter space for regime II (skirted), where the two bubbles eventually merge, and regimes IV (peripheral breakup) and V (central breakup), known for multiple bubble breakups. Additionally, we present results on differently sized bubbles, showing that their merging tendency depends on their arrangement as leading or trailing positions in the pair.