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Patient COVID-19 Breathing (Transient) in an Operating Room

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The problem simulates the flow of respiratory air from the mouth of a COVID-19 patient hospitalized in a room equipped with ventilation and air conditioning systems.

This ANSYS Fluent project includes CFD simulation files and a training movie.

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Description

Patient COVID-19 Breathing Project Description

The problem simulates the airflow from a patient (COVID-19) mouth in a hospital room. In fact, in the present case, a special hospital room has been designed that is equipped with ventilation and air conditioning systems. On the other hand, the patient receives oxygen and exhales carbon dioxide every time he inhales and exhales.

The main purpose of this simulation process is to allow fresh (oxygen-carrying) air to flow continuously into the interior of the room, rather than to expel polluted air from the patient’s mouth to the environment. Ventilation systems and air conditioners are designed on the ceiling and floor of the room are responsible for circulating fresh air inside the room and directing it from the side pores to the outside environment.

The flow of fresh air entering the room’s interior is composed of a combination of oxygen and nitrogen with a ratio of 3.76. Exhaust air from the patient’s mouth also contains carbon dioxide. Therefore, in the present simulation, the Species Transport model has been used.

Patient (COVID-19) Breathing Project Description

Thus, the airflow with an oxygen mass fraction of 0.23 and nitrogen with a mass fraction of 0.77 and without any percentage of carbon dioxide from the room’s air conditioners enters the room’s interior space. This fresh airflow has a speed of 1 m.s-1 and a temperature of 293.15 K.

Also, because the flow of air consisting of oxygen, nitrogen, and carbon dioxide in the form of exhaled air comes out of the patient’s mouth as a source, a Discrete Phase Model (DPM) has been used; Because in this model, the particles that make up the flow of the exhaled gaseous species are tracked, this type of view is called Lagrangian’s view in examining the fluid behavior of the particles.

In fact, the difference between Lagrangian’s view and Eulerian’s view is that fluid behavior in Lagrangian’s view is examined on the basis of a particle-by-particle of fluid flow; Whereas fluid behavior is considered in Euler’s view based on the assumption of a finite volume element in the fluid flow path.

Patient (COVID-19) Breathing Project Description

In this model, it is assumed that the exhaled air from the patient’s mouth has oxygen with a mass fraction of 0.16 and carbon dioxide with a mass fraction of 0.04, and the temperature of 310.15 K.

Also, the input flow rate from the oral area is defined as the UDF function. In reality, when breathing, the mouth constantly inhales and the nose exhales; But in the present model, only the mouth is assumed to be a constant source for both inhaling and exhaling. Therefore, both inhalation and exhalation should be defined for the oral area. So to define the velocity of the incoming airflow from the mouth to the interior of the room, a UDF function is used.

The magnitude of the airflow velocity is 0.25 m.s-1; So that in each 2.5 s interval, the value becomes negative or positive, depending on whether the action is inhaled or exhaled. So the input velocity in the mouth area has a positive value at the time of exhalation because it rejects the air and at the time of the inhalation it has a negative value because it receives air.

Due to the dependence of respiration over time, the present problem is unsteady in terms of time and has a time step of 0.01 s.

Geometry & Mesh

The present model is three-dimensional and is drawn using Design Modeler software. The model consists of a cubic room with dimensions of 2.9 m ⨯ 2.23 m ⨯ 3.7 m; So that a hospital bed and a patient are designed on it. Also, six circular holes are considered as fresh air inlet flow and five rectangular holes are considered as flow outlet sections in the sidewalls of the room.

Due to the fact that the main purpose of the problem is to focus on the exhaled airflow from the patient’s mouth, the patient’s oral surface is considered as the name-selection inlet boundary. The following figures show a view of the geometry.

patient

COVID-19

The meshing is done using ANSYS Meshing software. The mesh type is unstructured and the element number is 4354238. Meshing is smaller in the areas adjacent to the internal borders and has higher accuracy. The following figure shows the mesh.

Patient (COVID-19) Breathing

Patient (COVID-19) Breathing CFD Simulation

To simulate the present model, several assumptions are considered, which are:

  • A pressure-based solver has been performed.
  • The present simulation is transient; Because the purpose of the problem is to investigate the behavior of gaseous particles using the Lagrangian perspective over time.
  • The gravity effect is equivalent to -9.81 m.s-1.

Here is a summary of the steps for defining the problem and its solution in the following table:

Models (patient (COVID-19))
Viscous model k-epsilon
k-epsilon model RNG
near-wall treatment standard wall function
Species model species transport
species O2, N2, CO2
Discrete phase on
particle treatment unsteady particle tracking
physical model pressure gradient force
material in injection oxygen, nitrogen, carbon dioxide
particle type in injection inert
injection type surface
Energy on
Boundary conditions (patient (COVID-19))
Inlet Velocity inlet
velocity magnitude 1 m.s-1
temperature 293.15 K
O2 mass fraction 0.23
CO2 mass fraction 0
discrete phase BC type escape
Outlet (patient (COVID-19)) Pressure outlet
gauge pressure 0 Pascal
discrete phase BC type escape
Inlet – mouth Velocity inlet
velocity magnitude UDF
temperature 310.15 K
O2 mass fraction 0.16
CO2 mass fraction 0.04
discrete phase BC type escape
Walls Wall
wall motion stationary wall
heat flux 0 W.m-2
O2 mass fraction zero diffusive flux
CO2 mass fraction zero diffusive flux
discrete phase BC type reflect
Solution Methods (patient (COVID-19))
Pressure-velocity coupling   SIMPLE
Spatial discretization pressure second-order
momentum first-order upwind
turbulent kinetic energy first-order upwind
turbulent dissipation rate first-order upwind
energy second-order upwind
O2 second-order upwind
CO2 second-order upwind
(patient (COVID-19)) Initialization
Initialization method   Standard
gauge pressure 0 pascal
velocity (x,y,z) 0 m.s-1
O2 and CO2 0
temperature 300 K

Discrete Phase Model (DPM) for Patient (COVID-19) Breathing

We use the DPM when the aim is to investigate the behavior of the particles from a Lagrangian and discrete perspective. In the present model, the exhaled airflow, including oxygen, nitrogen, and carbon-oxide, is transmitted from the patient’s mouth as a particle to the inner space of the operating room. By selecting the unsteady particle tracking mode, the behavior of the discrete airborne particles is affected over time.

The discrete behavior of the exhaled particles is affected by the force caused by the pressure gradient force during exhalation.

Also, we define the injection process for the discrete phase. The material of the injected particles in the present model is the same as oxygen, nitrogen, and CO2, and it is injected from the patient’s mouth into the inner space of the hospital clean room. The type of injection process is in the form of a surface and the type of particles of gaseous species is inert.

Discrete Phase Model (DPM) for Patient (COVID-19) Breathing

The inert mode is an element of the discrete phase (particle, droplet, or bubble) that follows the balance of forces. Particle properties for each particle of the species present in the airflow in the present model include a diameter of 0.000001 m, a velocity of 0.25 ms-1, and a mass flow rate of 1 * 10-20 kg. s-1 and the temperature are 310.15 K. Also, the time required for this injection is 2.5 s, which is equivalent to the time required for an exhalation operation.

Also, to define the boundary conditions related to the discrete phase model, we use three types of discrete particle behavior relative to boundary areas; Thus, we use the Escape mode when the discrete phase crosses the desired boundary, and we apply the Trap mode when the discrete phase traps to the desired boundary. Also, we use the Reflect mode when the discrete phase reflects from the boundary after reaching the desired boundary.

In the present model, in the input section related to air conditioners and the input section related to the oral area, we use Escape mode. Also, we apply the Reflect mode in the inner walls of the room.

Results

At the end of the solution process, we obtain two-dimensional and three-dimensional contours of pressure, temperature, velocity, density, oxygen mass fraction, carbon dioxide mass fraction, and nitrogen mass fraction. We draw the two-dimensional contours on the XY section.

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

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