Modeling Hospital Operating Room Air Quality in Autodesk CFD

by Matt Bemis23 January 2018

Surgical infections due to airborne bacteria are an area of concern for doctors, hospitals, and patients. According to a 2002 paper published in the Journal of Surgical Research, 2.6% of all operations were complicated due to Surgical Site Infections. Traditionally, industrial high efficiency particulate air (HEPA) filtration systems have been deployed to supply operating rooms with clean air that adheres to strict CDC standards. While these initiatives have proven to decrease the chance of SSI’s, many unknowns still exist.

·  How long after a patient enters a room and HEPA filters are turned on can surgery start?

·  Are there areas in the room prone to air stagnation? If so, is the location in a critical area?

·  What effect does adjacent room leakage/air quality have on operating room air quality?

Irregular room dimensions, retrofitting existing operating rooms, and tightly packed surgery schedules have led to a need for a better understanding of operating room air quality.  In recent years CFD has been leveraged to better understand, predict, and improve airflow in operating rooms.

This transient scalar mixing analysis simulates the cleaning, or flushing out, of a dirty room from time = 0. This is similar to some surgical patient or hospital personnel entering a vacant operating room, then turning on the HEPA filtration system and waiting for the room air quality to reach an acceptable level to begin surgery. This analysis will be done using a scalar mixing analysis.

CAD Model:

This CAD model consists of an operating room table, 5 air supply diffusers, 4 outlets, and 5 mannequins.

Operating Room in CAD:

Materials:

The internal volume is assigned as air. The walls, diffusers, and operating room table is assigned as steel. The solid materials will be suppressed from meshing.

Materials in Autodesk CFD:

Initial Conditions:

Location

Initial Condition

All air volumes

Scalar = 0.2

Boundary Conditions:

Location

Boundary Condition

Supply Diffuser 1, 2, 3, 4

100 l/s, Scalar = 0.01

Supply Diffuser 5

400 l/s, Scalar = 0.01

Outlets

0 Pressure Gauge

The scalar values are arbitrary and scaled to industry standards. The initial condition of scalar = 0.2 represents dirty air. The boundary conditions of supply air with a scalar of 0.01 represent clean air. In this particular situation, air which has a scalar quantity of 0-0.05 is considered clean.

Meshing:

All walls and diffusers were suppressed, along with the operating table and mannequins. An automatic mesh with Surface and Gap refinement was used for this model. After an initial Autosize, the sizing was refined by selecting the air domain and changing the Size Adjustment bar to 0.7. A refinement region was applied below the diffusers.

Mesh in Autodesk CFD:

Solve:

The Solution Mode was set to Transient via the Solve dialogue box. A 0.25 second time step was used along with 3 Inner iterations. The Stop Time was set to 240 seconds. The scalar module was enabled via “Advanced” on the Physics Tab. A save interval was set to record intermediate results every 10 seconds.

Results:

An isovolume can be used to visualize where air in the room meets surgical-quality. The save intervals were set to 10 seconds. An animation can be made showing how clean air enters the room and circulates over the 240 second time period. Each frame represents 10 seconds of time.

Velocity Profile at 240 Seconds: