To control the evolution of a pandemic such as COVID-19, knowing the conditions under which the pathogen is being transmitted represents a critical issue, especially when implementing protection strategies like social distancing and face masks wearing. For viruses and bacteria that spread via airborne and/or droplet pathways, this requires understanding how saliva droplets evolve over time after their expulsion by speaking or coughing. Within this context, the transition from saliva droplets to solid residues, due to water evaporation, is studied here both experimentally, considering the saliva from 5 men and 5 women, and via numerical modeling to accurately predict the dynamics of this process. The model assumes saliva to be a binary water/salt mixture and is validated against experimental results using saliva droplets that are suspended in an ultrasound levitator. We demonstrate that droplets with an initial diameter smaller than 21 μm will produce a solid residue that would be considered an aerosol of <5 μm diameter within less than 2 second (for any relative humidity less than 80% and/or any temperature greater than 20 °C). Finally, the model developed here accounts for the influence of the saliva composition, relative humidity and ambient temperature on droplet drying. Thus, the travel distance prior to becoming a solid residue can be deduced. We found that saliva droplets of initial size below 80 μm, which corresponds to the vast majority of speech and cough droplets, will become solid residues prior to touching the ground when expelled from a height of 160 cm.

In order to assess the remaining life of gas turbine critical components, it is vital to accurately define the aerothermodynamic working environments and service histories. As a part of a major multidisciplinary collaboration program, a benchmark modeling on a practical gas turbine combustor is successfully carried out, and the two-phase, steady, turbulent, compressible, reacting flow fields at both cruise and takeoff are obtained. The results show the complicated flow features inside the combustor. The airflow over each flow element of the combustor can or liner is not evenly distributed, and considerable variations, ±25%, around the average values, are observed. It is more important to note that the temperatures at the combustor can and cooling wiggle strips vary significantly, which can significantly affect fatigue life of engine critical components. The present study suggests that to develop an adequate aerothermodynamics tool, it is necessary to carry out a further systematic study, including validation of numerical results, simulations at typical engine operating conditions, and development of simple correlations between engine operating conditions and component working environments. As an ultimate goal, the cost and time of gas turbine engine fleet management must be significantly reduced.

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SARS‐CoV‐2 and its ever‐emerging variants are spread from host‐to‐host via expelled respiratory aerosols and saliva droplets. Knowing the number of virions which are exhaled by a person requires precise measurements of the size, count, velocity and trajectory of the virus‐laden particles that are ejected directly from the mouth. These measurements are achieved in 3D, at 15,000 images/s, and are applied when speaking, yelling and coughing. In this study, 33 events have been analysed by post‐processing ∼500,000 images. Using these data, the flow rates of SARS‐CoV‐2 virions have been evaluated. At high concentrations, 107 virions/mL, it is found that 136–231 virions are ejected during a single cough, where the virion flow rate peak is capable of reaching 32 virions within a millisecond. This peak can reach tens of virions/ms when yelling but reduced to only a few virions/ms when speaking. At medium concentrations, ∼105 virions/mL, those results are hundreds of times lower. The total number of virions that are ejected when yelling at 110 dB, instead of speaking at 85 dB, increases by two‐ to threefold. From the measured data analysed in this article, the flow rate of other diseases, such as influenza, tuberculosis or measles, can also be estimated. As these data are openly accessible, they can be used by modellers for the simulation of saliva droplet transport and evaporation, allowing to further advance our understanding of airborne pathogen transmission.Key points Advanced, optimized and combined laser‐based imaging techniques for temporally sizing and tracking respiratory droplets and aerosols. Understanding how pathogens are being ejected from the mouth when speaking, yelling and coughing. Quantifying and analysing the variation of SARS‐CoV‐2 flow rates emission during exhalation.

The National Jet Fuel Combustion Program (NJFCP) is an initiative being led by the Office of Environment & Energy at the FAA, to streamline the ASTM jet fuels certification process for alternative aviation fuels. To accomplish this, the program has identified specific applied research tasks in several areas. The National Research Council of Canada (NRC) is contributing to the NJFCP in the areas of sprays and atomization and high altitude engine performance. This paper describes work pertaining to atomization tests using a reference injection system. The work involves characterization of the injection nozzle, comparison of sprays and atomization quality of various conventional and alternative fuels, and uses the experimental data to validate spray correlations. The paper also briefly explores the application viability of a new diagnostic system that has the potential to reduce test time in characterizing sprays. Measurements were made from ambient up to 10 bar pressures in NRC's High Pressure Spray Facility using optical diagnostics including laser diffraction, phase Doppler anemometry (PDA), LIF/Mie imaging and laser sheet imaging to assess differences in the atomization characteristics of the test fuels. A total of nine test fluids including six NJFCP fuels and three calibration fluids were used. The experimental data were then used to validate semi-empirical models, developed through years of experience by engine original equipment manufacturers, and modified under the NJFCP, for predicting droplet size and distribution. The work offers effective tools for developing advanced fuel injectors, and generating data that can be used to significantly enhance multidimensional combustor simulation capabilities.