Aerosol spread in a generic train entrance: Comparison between experiment and numerical simulation

Abstract

Computational fluid dynamics (CFD) rely on models and approximations that have to be validated. Thus, the present study focuses on the comparison of steady state CFD predictions with measurements of the distribution and spreading of aerosol particles, emitted by a passenger in a ventilated full-scale geometry representing the entrance area of a passenger train. The entrance area is represented by a simplified cubic geometry (see Fig. 1a), where two heated standing thermal manikins (TMs) are opposing each other. One of the manikins is exhaling aerosol at a constant rate and a size distribution typically produced during human breathing. Filtered and conditioned air is supplied through a rectangular panel in the ceiling, the air outlets are distributed symmetrically laterally near the floor. The simplicity of the utilized geometry allows to minimize differences between the experimental and the numerical setup, while also including all relevant mechanisms (e.g., ventilation and body heat) of a passenger cabin. Figure 1: a) Schematic of the simplified train entrance model used for the experimental and numerical studies. b) Position of the aerosol particle sensors on the three measurement planes (green, purple dashed lines in Fig. a)). Green squares mark the particle sensor-positions on the outward planes, purple squares additional sensor-positions on the middle plane. The air supply is positioned on the ceiling with the velocity being constant over the inlet area and over time. In the experiment, this is achieved through a membrane inlet, distributing the airflow equally over the whole area. The volumetric flow rate is 25 l/s at a temperature of 17°C. The walls are adiabatic. In the experiment, this is nearly achieved through thick layers of insulation. Both TMs simulate the geometrical dimensions and thermal influence of humans inside the model, with a constant heat flux of 100 W emitted from each of their bodies. Temperature probes are distributed in the experimental setup to measure the wall and air temperatures (not included in Figure 1b), for the determination of the experimental thermal boundary conditions. For the experimental determination of the aerosol distribution, we use 49 particulate matter sensors which are well-established and used in many studies regarding aerosol spread in airplane cabins [1, 2]. Aerosol particles with diameters from 0,3 – 10 µm are recorded at around 1 Hz frequency at the positions shown by the squares in Figure 1b in three parallel xz-planes. For the generation and distribution of the aerosol similar to human exhalation, an aerosol source is used [2]. It generates a total number concentration of 105 particles per cubic centimeter, using artificial saliva, at a constant aerosol exhalation of 12 liters per minute. As aerosol exhalation opening, a tube inlet with a diameter of 12 mm is used representing an opened generic mouth of an average opening-area of 1.13 cm². The resulting Reynolds number of the jet is in the range of 1400. To allow comparability between our measurements and the CFD simulation, we are evaluating the resulting aerosol concentration distribution, after a steady state temperature (and developed flow field) is reached in the experiment. For the numerical simulation, the open-source software package OpenFOAM is employed to approximate solutions of the Reynolds-averaged Navier-Stokes equations (RANS). From this package, the solver buoyantSimpleFoam is chosen, which is suitable for buoyancy driven convection flows caused by TMs. Turbulence is considered by employing a k-omega SST model. The mesh is locally refined around the TMs, around the expected breathing jet as well as the thermal plume. The ventilation inlet velocity and temperature are prescribed in accordance to the values realized in the experiment. The air outlets are defined with a passive outlet boundary condition on the positions shown in Fig. 1a. Heat flux and heat radiation boundary conditions are specified at the surfaces of the manikins by prescribing a power of 100 W and an emissivity of ε = 0.95. An additional air inlet is defined on one of the manikins faces, mimicking the mouth for the continuous exhalation. The particles are inserted directly into the breathing jet at the mouth opening. Fig. 2 shows blue-red streamlines through the x-orientated centerline of the air supply panel, where the color indicates the temperature in Kelvin. The streamlines reveal large, nearly symmetrical circulations that divide the room. The red colored streamlines above the heads reflect the buoyancy effects. In a steady state, the streamlines are equal to the path lines, illustrating the theoretical mean path of tracer particles that are solely affected by drag force, where gravity, inertia, and other forces are neglected. Therefore, the streamlines suggest that particles are recirculating symmetrically in the room before depositing or leaving the enclosure through the outlets near the floor. The additional gray streamlines represent the trajectories of aerosol particles emitted by the manikins’ mouth reflecting the case with exhalation (right TM) and also represent the trajectories of aerosol particles, which are crossing the breathing zone (in front of the mouth) of the left TM. For the comparison with the experiment, the influence of forces neglected by streamlines are considered by reactingParcelFoam for realistic particle tracing. The quantitative comparison presented at the Symposium will focus on temperature and particle concentration distribution measured in the experiment at selected positions. The objective is to validate CFD predictions with an experiment, addressing the aerosol spreading in a simple but relevant train geometry. The validation also serves as a basis for future prediction of the spreading of (potentially) pathogen-laden aerosols in indoor environments such as passenger compartments of trains and airplanes. [1] Netherland Aerospace Center (NLR) and National Institute for Public Health and the Environment (RIVM) (2021) “CORSICA final report” NLR-CR-2021-232. [2] Schmeling et al., Numerical and experimental study of aerosol dispersion in the Do728 aircraft cabin. (2022)