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CFD Ventilation of a Hospital Room

Technical Challenge:
Efficient ventilation can contribute to reducing the energy consumption of buildings and minimize the risk of airborne infection in hospital rooms. In this problem we investigate ventilation in a hospital room containing a patient, a doctor, a bed, a wardrobe, a lamp, medical equipment, an inlet and exhaust. We account for both forced and natural ventilation as well as the flow of bacteria particles originating from a sick patient. Layout of the room is shown in Figure 1. Ventilation rate is 6 ACH (Air Change per Hour) for health care facilities, per ASHRAE Standard 170.

Figure 1. Room layout

Veryst Solution:
Veryst used CFD to simulate ventilation in the hospital rooms well as the dispersion of bacterial particles. Figures 2a and 2b show temperature distributions and velocity vectors adjacent to the doctor and patient. Average temperature of the room is measured 21°C. Upward air movement next to the doctor and patient is seen in Figures 2a and 2b, which is due to natural convection. There is also a flow of air from patient toward the doctor.

 

Figure 2a, 2b. Temperature distributions and velocity vectors from two perspectives within the room

We calculated Predicted Mean Vote (PMV)(1) and Predicted Percentage of Dissatisfied (PPD)(2) based on ASHRAE Standard 55-2013. The average air temperature and air speed experienced by the patient is 20.8°C and 0.06 meters/second, respectively. The metabolic rate of a patient who is sleeping is considered 1 Met(3). We assumed that the patient is wearing trousers and a long-sleeve shirt and therefore his clothing level is 0.61 clo(4). If humidity is 50%, then PMV is -1.59 and PPD is 56%. In that case, the probability of the patient being dissatisfied with the room temperature is 56%, with sensation of being cool.

We also simulated the release of bacteria due to patient coughing. Coughing characteristics are obtained from “Flow dynamics and characterization of a cough” by JJ. K. Gupta et al. (2009).  The animations below shows release of bacteria.

         

Animation 1a, 1b. Particle tracing showing the motion of bacteria particles resulting from patient coughing.
Particle color corresponds to velocity (m/s)

Figure 3 shows transmission probability of bacteria at the room ventilation exhaust. None of the bacteria leaves the room in 30 seconds after coughing. After three minutes, 8% of the bacteria still remain in the room.

Figure 3. Transmission probability of bacteria at the exhaust

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(1) The average thermal sensation response of a large number of subjects using ASHRAE thermal sensation scale.
(2) Quantitative measure of the thermal comfort of a group of people at a particular thermal environment
(3) Physiological measure expressing the energy cost of physical activities
(4) Unit to measure insulation of clothes

 

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