airflow patterns and flow path of airborne contaminants

Kishor Khankari, Ph.D.Fellow ASHRAE

ASHRAE Distinguished Lecturer

www.ansight.com

Airflow Patterns and Flow Path of Airborne Contaminants

© 2020 ASHRAE (www.ashrae.org). For personal use only. Additional reproduction, distribution, or transmission in either print or digital form is not permitted without ASHRAE's prior written permission. 1

CopyrightCopyright 2020 by ASHRAE. All rights reserved. No part of this presentation maybe reproduced without written permission from ASHRAE, nor may any part of thispresentation be reproduced, stored in a retrieval system, or transmitted in anyform or by any means (electronic, photocopying, recording, or other) withoutwritten permission from ASHRAE.

ASHRAE has compiled this presentation with care, but ASHRAE has notinvestigated and ASHRAE expressly disclaims any duty to investigate any product,service, process, procedure, design or the like, that may be described herein. Theappearance of any technical data or editorial material in this presentation doesnot constitute endorsement, warranty or guaranty by ASHRAE of any product,service, process, procedure, design, or the like. ASHRAE does not warrant that theinformation in this publication is free of errors. The user assumes the entire risk ofthe use of the use of any information in this presentation.


AIA/CES Registered Provider

ASHRAE is a Registered Provider with The American Institute of ArchitectsContinuing Education Systems. Credit earned on completion of this program will bereported to CES Records for AIA members. Certificates of Attendance for non‐AIAmembers are available on request.

This program is registered with the AIA/CES for continuing professional education.As such, it does not include content that may be deemed or construed to be anapproval or endorsement by the AIA of any material of construction or any methodor manner of handling, using, distributing, or dealing in any material or product.Questions related to specific materials, methods, and services will be addressed atthe conclusion of this presentation.


Learning Objectives

• Review airflow patterns in an indoor environment.• Evaluate the impact of airflow patterns on thermal comfort and probable flow path of airborne contaminants.

• Evaluate the impact HVAC design and locations of supply and return on probable flow path of airborne contaminants.

• Understand how Computational Fluid Dynamics (CFD) be employed for the design and optimization of a resilient indoor environment.


Purpose of Building Ventilation• Create comfortable, healthy, and productive environment for people.• Minimize infection and hazard due to airborne pathogens and toxins.


ASHRAE Mission and Value Statement


Talk of the Town

“Good Ventilation”


COVID‐19: Airborne Transmission

Two different interpretations

July 09, 20202

“ beyond any reasonable doubt that viruses are released during exhalation, talking, and coughing in microdroplets small enough to remain aloft in air and pose a risk of exposure at distances beyond 1 to 2 m from an infected individual”


Ventilation Strategies

• Containment ‐ Pressurization

• Dilution ‐ Supply of clean air

• Removal ‐ HVAC design


Containment‐ Room Pressurization

+Supply > Exhaust

‐Supply < Exhaust Open Door


Containment‐ Room Pressurization – Zonal Transport

• Pressurization is to control zonal transport.• Does not address transport within the space.• Mostly employed for “contaminant sensitive” spacesHealthcare, laboratories, cleanrooms, etc.

• “Directional Transport” – a misleading phrase.• It is a “containment strategy” – not “directional”


Dilution Philosophy – Steady State

• Dilution Airflow Rate Calculation

Q: dilution airflow rateG: contaminant generation rateC: target concentrationK: safety factor

• Assumes well‐mixed conditions – a myth.


Dilution Philosophy ‐ Transient

0

5

10

15

20

25

30

0 20 40 60 80 100 120 140 160

Concen

tration (ppm

)

Time (min)

Steady StateBuild‐up Decay

𝐶 1 𝑒 𝐶 𝑒𝐺𝑞

How fast? How fast?How much?

Assumes well‐mixed conditions – a myth.

n: air change rate


Dilution – Air Change Rates

• Higher ACH – Higher Dilution• Supply flow rate ‐> Dilution• With diminishing returns!• ACH ‐> Rate of purge

0

5

10

15

20

25

30

0 20 40 60 80 100 120 140 160

Concen

tration (ppm

)

Time (min)

4 6 8 10ACH

Assumes“well‐mixed” conditions – a myth!


Dilution and Infection

• Dilution of gases is different than dilution of particulates.• No good value for “threshold” concentration for viruses!• What is the “dose”?• No strong evidence yet to prove impact of “dilution”.• “Infection” is not just about “fluid dynamics”!


Removal of Dirty Air

Clean Air

SourceDirty Air


Removal of Dirty Air – Airflow Patterns!

• Location, number, and type of air supply diffusers• Supply airflow rate – discharge velocity • Location and number of exhausts • Location and extent of heat sources• Locations of contaminant generation• Layout of the facility – obstructions to airflow


Why Airflow Patterns Matter!

Airprimary carrier of

Heat, Moisture, Airborne Contaminants

Airflow Paththe key

for Efficient Operation


Why Airflow Patterns Matter!

Air movement determines:• Air speeds – drafty and stuffy zones• Distribution of heat – hot and cold spots• Contaminant flow path – concentration levels• Thermal comfort of occupants• Effectiveness of ventilation


Airflow Patterns – Can you see?

Yet, we cannot see the air movement!


Arrows on the Drawings – Wishful Thinking!

• Arrows on your drawings !• Your “intent” of air behavior• A wishful thinking!• Air has its own “mind”!


Common Sense Laws of Air Motion

• Air takes “short cuts”!• Follows path of least resistance

• Hot air goes up !

• Cold air stays near floor – “cold feet” !


Computational Fluid Dynamics (CFD)

• Computational Fluid Dynamics (CFD)• Simulates the processes that involve:• Fluid flow – flow of air, water, and other fluids• Heat transfer – heating, cooling, effect of body heat sources, solar gain, etc.

• Mass transfer – movement/transport of water vapor, gaseous contaminants, airborne particles, etc.

• Chemical reactions – fire simulation


How CFD Can Help?

• Visualization of airflow patterns• Visualization of flow path of airborne contaminants• Low and high zones of contaminant concentration• Drafty and stuffy zone, hot and cold spots!• Risk identification• Physics based mitigation strategies• Takes guess work out• Avoids expensive trial‐and‐error efforts


Particle Dynamics• Large particles/droplets fall near the source.

• Small particles/aerosols remain airborne.

• Aerosols follow the flow path of air.• Density, size, and shape of particulates can affect the dynamics.

• Droplets can evaporate and become aerosol.

• External forces can affect the trajectory.


Bathroom Exhaust

Room Return

Supply Diffusers Supply Diffuser

Door leakage

Glazing (South facing)

CFD Model of a Patient Room


Airflow Patterns


Temperature Distribution


Flow Path of Aerosols


High Induction – Mixing Diffusers

Low Pressure ZoneHigh Velocity Air Jet

Induced Air


Modified Return

Mitigation Strategy

Supply

Return


Airflow Patterns – Return Over Patient


Flow Path of Aerosols – Return Over Patient


Displacement Ventilation

BathroomExhaust

Room Return

Supply DiffuserDoor leakage


Displacement Ventilation


Displacement Ventilation – Return location


Displacement Ventilation – Considerations

• Sensitive to strength and location of heat sources

• Careful design to locate stratified layer• System response to varying heat loads• Heating and cooling applications• Detail particle dynamics consideration• Evaluate Archimedes Number

2vgLAr

6 ach (1/h)

4 ach (1/h)


Laboratory Ventilation

Person 3

Room Supply4‐way diffusers

Source

Exhaust grille 1

Exhaust grille 2

Person 2

Person 1


Airflow Patterns


Spread of Contaminant Fog


Contaminant Concentration at 5 feet


Spread of Contaminant ‐ Cloud of 25 PPM


Concentration at Breathing Level

1

2 3

S


Occupant Exposure (Dose) 1

2 3S


Impact of Air Change Rates or Ventilation Rates

4 ACH 6 ACH

8 ACH 10 ACH


Two versus Three Exhaust GrillesTwo Exhaust grills Three Exhaust grills


Contaminant Concentration at 5 feet – 720s

Two exhaust grilles Three exhaust grilles


Contaminant Concentration at 5 feet



Contaminant Cloud of 25 PPM – 720s


26.6% 16.4%


Spread of Contaminant ‐ Cloud of 25 PPM



Occupant Exposure (Dose) – Person 1 1

2 3

SThree grilles

Two grilles


Occupant Exposure (Dose) – Person 2 1

2 3

SThree grilles

Two grilles


Occupant Exposure (Dose) – Person 3

1

2 3

S

Three grilles

Two grilles


Airflow Assessment of a Space

• Design and layout of HVAC configuration• Effectiveness of airflow management devices• Airflow patterns and flow path of airborne pathogens• Mitigation strategies• Implementation


Efficacy of Air Moving and Cleaning Devices

• Capacity• Locations• Capture efficiency• Impact of local airflow patterns


Assessment of Airflow Patterns

• Initial assessment of the layout• CFD analysis to predict:

• Airflow patterns• Flow path of airborne pathogens• Assessment of mitigation strategies• Assessment of local air moving/cleaning devices• Optimization for social distancing


FINANCIAL CASE FOR HIGH PERFORMANCE BUILDINGSSTOK.COM

How much it costs to perform CFD analysis?

Cost of CFD ≈ $0.1


Control the Source!


Research and Development Needs• Quantification of contaminant generation rates.• Role of dilution for particulate contaminants.• Impact of airflow patterns on particulate transmission.• Importance of HVAC design – diffusers and returns.• Development of new air distribution systems to balance thermal comfort and indoor air quality.

• Revision of Standards and guidelines.


Insights

Airflow patterns determine the flow path of airborne contaminants


Insights

Locations and number of supply diffusers and exhaust grilles matter!


Insights

Just increasing the ventilation rate is not enough. Understand the flow path of contaminants


Insights

Just increasing the filtration and adding air cleaning devices are not enough. Understand the flow path of contaminants


Insights

Computational Fluid Dynamics (CFD) is an effective tool to optimize flow path of contaminants.


ASHRAE Publications

• CFD Analysis of Hospital Operating Room Ventilation System Part 1: Analysis of Air Change Rates. ASHRAE Journal May 2018

• CFD Analysis of Hospital Operating Room Ventilation System Part 2: Analyses of HVAC Configurations. ASHRAE Journal June 2018

• Airflow Path Matters: Patient Room HVAC. ASHRAE Journal June 2016

• Dynamics of unidirectional flow. ASHRAE Journal, Vol. 61, no. 6, June 2019.

• Analysis of Spread Index: A Measure of Laboratory Ventilation Effectiveness. ASHRAE Conference Paper for ASHRAE Annual Conference, Houston, TX.


Acknowledgements

• AnSight LLC providing funding for the research

• Ansys for providing software support


Thank You!

Questions?

Kishor Khankari, Ph.D.

[email protected] (734) 327 4079


ASHRAE values your comments about this course. You will receive your Certificate of Attendance when you finish the online course evaluation form at this URL: https://www.ashrae.org/2020july21Be sure to add your appropriate license numbers.

If you have any questions about ASHRAE Certificates, please contact Kelly Arnold, Professional Development Coordinator, at [email protected].

If you have any questions about ASHRAE courses, please contact Tiffany Cox, Course Administrator, at [email protected].

Evaluation and Certificate


ASHRAE Professional CertificationVisit www.ashrae.org/certification to learn more:More than 3,000 certifications earned to-dateElevate your reputation among peers, in the workplace and

among clientsNEW! Digital Badging:

Embedded metadata uniquely linked to you Shareable in electronic media, including LinkedIn and email Instant recognition, with real-time, third-part verification


airflow patterns and flow path of airborne contaminants

Documents