Covid 19 Airborne Risk Measuring in a Classroom, CFD Study

Description

Project Description

Corona (COVID 19) is currently recognized as the most significant human challenge globally; Because this disease, in addition to being dangerous to human health, has a high transmission power between a sick person and healthy people. Breathing of a person without mask with corona virus in a closed public environment, transmits the disease to their neighbors. One of the doctors’ critical recommendations regarding preventing disease transmission between people is to maintain a proper social distancing between people in such places. For example, a short distance between a university student seat or a schoolboy in a classroom can increase the risk of transmitting COVID-19 disease from a carrier to other people nearby.

In this project, based on the CFD method and using ANSYS Fluent software, an attempt has been made to simulate the respiration of viral air from the mouth of sick corona virus carrier students in the classroom. This model includes a computational domain in the form of a classroom and chairs inside which a student is modeled on each of the chairs. For each of these students, a surface is defined as the mouth as the source of the virus’s respiration and transmission. This work aims to investigate the effectiveness of the ventilation system installed in the classroom to eliminate polluted air and clean the air.

Project Description

In this project, a ventilation system has been used, which has several vents for the entry of fresh air flow from the ceiling of the classroom and several panels for the exit of the old airflow at the bottom of the classroom’s sidewalls. For the present simulation, the discrete phase model (DPM) is used; Because this model allows us to study a mass of particles discretely in a continuous fluid (air). The discrete phase corresponding to the virus particles is defined in the steady-state solver. After activating the discrete phase model, the injection process must be described, determining the type and quality of discrete particles injected into the classroom.

In this model, the virus particles are defined as inert particles, and the injection type is surface and through the inner surface of the mouth of each student (mouth). According to the injection definition, virus particles have a diameter of 0.000001 m, a temperature of 308 K, a velocity of 0.05 m.s-1, and a flow rate of 0.00002 kg.s-1. The discrete phase model’s boundary conditions are defined as particles at the boundaries of the patient’s mouth and the inlet and outlet sections of the ventilated airflow in the classroom with Escape mode, meaning that the particles pass through these boundaries.

Project Description

All the people and the body of the chairs and the classroom’s side walls have a Trap mode, which means that particles are trapped and accumulate in these borders. Also, the ventilation and air circulation system inside this classroom is such that fresh air flow enters the classroom environment as a continuous fluid through several vents installed on the roof of the classroom with a speed of 0.4 ms-1 and a temperature of 293 K. While the old airflow inside the classroom comes out of several outlet panels at the bottom of the sidewalls of the classroom to the outside environment with a pressure equivalent to atmospheric pressure.

geometry & Mesh

The present model is designed in three dimensions using Design Modeler software. The model’s geometry includes the interior of a classroom with a length and width of 9.5 m and 6.75 m and a height of 3 m. Inside the classroom, a teacher is designed with 25 students sitting on chairs. The ventilation system of this classroom is designed so that it has eight panels for the entry of fresh air on the ceiling of the room and four side panels at the bottom of each side wall of this class. Also, for each student sitting on a chair, a border is defined as the mouth, which is considered a place where people.

The meshing of the model has been done using ANSYS Meshing software, and the mesh type is unstructured. The element number is 2745511. The following figure shows the mesh.

CFD Simulation

Several assumptions are considered to simulate the present model:

• We perform a pressure-based solver.
• The simulation is steady.
• The gravity effect on the fluid is equal to -9.81 m.s-2 along the Y-axis.

The following table represents a summary of the defining steps of the problem and its solution:

Models (Covid 19)
Viscous		k-epsilon
	k-epsilon model	realizable
	near-wall treatment	standard wall function
Discrete phase model		On
	particle treatment	steady particle tracking
Injection		active
	injection type	inert
	release from surfaces	mouth
	diameter	0.000001 m
	temperature	308 K
	velocity magnitude	0.05 m.s-1
	total flow rate	0.00002 kg.s-1
Energy		On
Boundary conditions (Covid 19)
Floor, Roof, Side Walls & Chairs and Bodies		Wall
	wall motion	stationary wall
	heat flux	0 W.m-2
	discrete phase conditions	trap
Inlet-Air		Velocity Inlet
	velocity magnitude	0.4 m.s-1
	temperature	293 K
	discrete phase conditions	escape
Inlet-Mouth		Velocity Inlet
	velocity magnitude	0.05 m.s-1
	temperature	308 K
	discrete phase conditions	escape
Outlet-Air		Pressure Outlet
	gauge pressure	0 Pascal
	discrete phase conditions	escape
Methods (Covid 19)
Pressure-velocity coupling		SIMPLE
	pressure	second order
	momentum	second order upwind
	turbulent kinetic energy	first order upwind
	turbulent dissipation rate	first order upwind
	energy	second order upwind
Initialization (Covid 19)
Initialization methods		Hybrid

Results

At the end of the solution process, a particle tracking of the virus particles is obtained based on the residence time of 60 s. Also, two-dimensional and three-dimensional contours related to temperature and airspeed inside the classroom interior have been obtained. The two-dimensional contours are drawn in a section that includes a row of students. The flow path lines are also obtained in 3D. It is obvious that this ventilation system is not appropriate for the classroom and increase the risk of the virus dispersion.

All files, including Geometry, Mesh, Case & Data, are available in Simulation File. By the way, the Training File presents how to solve the problem and extract all desired results.