ASHRAE Journal - June 2019 - 39

TECHNICAL FEATURE

is desired. For ceiling mounted laminar diffusers the
unidirectional flow should ideally sustain downward
motion of the supply air without significant recirculation, mixing, and entrainment from the surrounding
air. This CFD study systematically analyzed the impact of
supply airflow rate (discharge velocity) on the dynamics
of unidirectional airflow under isothermal and nonisothermal conditions. It provides the following insights:
* Under isothermal conditions when the cooling capacity of the supply air is much higher than the sensible
cooling load in the space, such as in cleanrooms with high
air change rates, the supply airflow rate (discharge velocity)
has a little impact on the directionality of the laminar flow.
* When the flow rate or the cooling capacity of the
supply air is much lower than the sensible cooling load,
the discharge air cannot sustain the downward unidirectional motion due to upward resistance of the buoyant plume from the heat sources. However, such airflow
patterns can form a protective layer of hot air surrounding the heat sources and help create nearly isothermal
conditions in the space.
* When the cooling capacity of the supply air exceeds
about three times the cooling load, the air jet can sustain
the downward unidirectional motion. However, nonisothermal conditions surrounding the supply diffuser,
created by significant lighting load near the ceiling or
thermal stratification in the space, can cause entrainment and acceleration of the supply air jet. Reducing
such thermal gradients can help reduce entrainment of
the surrounding air into the supply air.
* Archimedes number, if calculated during the early
stages of design, can provide high level of guidance
about the directionality of the supply air jet. Archimedes
number much smaller than one indicates low resistance
from the upward buoyant forces which help sustain the
unidirectional downward flow.
This study indicates that in the case of ceiling mounted
laminar diffusers the local thermal gradients can significantly affect the directionality of laminar flow and
entrainment from the surrounding air. The layout of the
room including the locations of the supply and exhaust
grilles as well as location and strength of various heat
sources in the space can affect the airflow patterns and
temperature distribution in the space, which in turn
affects the performance of unidirectional flows. In
such cases an in-depth CFD analysis can predict airflow patterns, temperature distribution, and potential

TABLE 1 Archimedes number (Ar) for the non-isothermal cases analyzed.a

aQ

Supply Airflow Rate (cfm)

200

400

600

800

Discharge Velocity (fpm)

25

50

75

100

Archimedes Number (Ar)

14.9

1.9

0.6

0.2

= 3,412 Btu/h, Ts = 65°F, Diffuser Length (l) = 4 ft, and width (w) = 2 ft

entrainment from the surrounding air. Insights gained
through such analyses help in optimizing the HVAC
design to meet the desired goals.

References
1. 2017 ASHRAE Handbook-Fundamentals, Chapter 20.
2. 2016 ASHRAE Handbook-HVAC Systems and Equipment, Chapter 20.
3. Khankari, K. 2017. "Air change rate analysis for a
minienvironment cleanroom." ASHRAE Journal 59(8):38-46.
4. Cook, G., D. Int-Hout. 2009. "Air motion control in the
hospital operating room." ASHRAE Journal 51(3):30-36.
5. Khankari, K. 2018. "CFD analysis of hospital operating room
ventilation system part 1: analysis of air change rates." ASHRAE
Journal 60(5).
6. Khankari, K. 2018. "CFD analysis of hospital operating room
ventilation system part 2: analyses of hvac configurations." ASHRAE
Journal 60(6).

WELCOME TO

cooldrysaddles.com!
Designed for the Cold Piping Insulation Engineer.
Home / Products / Guide Specs / Technical Info / Projects / FAQ / Contact Us / Request Info or Samples

INTRODUCING

New CoolDryTM
Insulated Saddles
Today's Engineer's
Choice for Chilled
Water, Cold Water,
Refrigeration &
Ammonia Refrigeration
Piping Systems

Call us at 800.969.3113
or email us at info@cooldrysaddles.com
for your Free CoolDry™ Sample today!

INSULATION AND MOISTURE MANAGEMENT EXPERTS
FOR PIPE HANGERS SINCE 1990!

www.cooldrysaddles.com

J U N E 2 0 19

ashrae.org

ASHRAE JOURNAL

39


http://www.cooldrysaddles.com http://www.cooldrysaddles.com http://www.ashrae.org

ASHRAE Journal - June 2019

Table of Contents for the Digital Edition of ASHRAE Journal - June 2019

Contents
ASHRAE Journal - June 2019 - Intro
ASHRAE Journal - June 2019 - Cover1
ASHRAE Journal - June 2019 - Cover2
ASHRAE Journal - June 2019 - 1
ASHRAE Journal - June 2019 - Contents
ASHRAE Journal - June 2019 - 3
ASHRAE Journal - June 2019 - 4
ASHRAE Journal - June 2019 - 5
ASHRAE Journal - June 2019 - 6
ASHRAE Journal - June 2019 - 7
ASHRAE Journal - June 2019 - 8
ASHRAE Journal - June 2019 - 9
ASHRAE Journal - June 2019 - 10
ASHRAE Journal - June 2019 - 11
ASHRAE Journal - June 2019 - 12
ASHRAE Journal - June 2019 - 13
ASHRAE Journal - June 2019 - 14
ASHRAE Journal - June 2019 - 15
ASHRAE Journal - June 2019 - 16
ASHRAE Journal - June 2019 - 17
ASHRAE Journal - June 2019 - 18
ASHRAE Journal - June 2019 - 19
ASHRAE Journal - June 2019 - 20
ASHRAE Journal - June 2019 - 21
ASHRAE Journal - June 2019 - 22
ASHRAE Journal - June 2019 - 23
ASHRAE Journal - June 2019 - 24
ASHRAE Journal - June 2019 - 25
ASHRAE Journal - June 2019 - 26
ASHRAE Journal - June 2019 - 27
ASHRAE Journal - June 2019 - 28
ASHRAE Journal - June 2019 - 29
ASHRAE Journal - June 2019 - 30
ASHRAE Journal - June 2019 - 31
ASHRAE Journal - June 2019 - 32
ASHRAE Journal - June 2019 - 33
ASHRAE Journal - June 2019 - 34
ASHRAE Journal - June 2019 - 35
ASHRAE Journal - June 2019 - 36
ASHRAE Journal - June 2019 - 37
ASHRAE Journal - June 2019 - 38
ASHRAE Journal - June 2019 - 39
ASHRAE Journal - June 2019 - 40
ASHRAE Journal - June 2019 - 41
ASHRAE Journal - June 2019 - 42
ASHRAE Journal - June 2019 - 43
ASHRAE Journal - June 2019 - 44
ASHRAE Journal - June 2019 - 45
ASHRAE Journal - June 2019 - 46
ASHRAE Journal - June 2019 - 47
ASHRAE Journal - June 2019 - 48
ASHRAE Journal - June 2019 - 49
ASHRAE Journal - June 2019 - 50
ASHRAE Journal - June 2019 - 51
ASHRAE Journal - June 2019 - 52
ASHRAE Journal - June 2019 - 53
ASHRAE Journal - June 2019 - 54
ASHRAE Journal - June 2019 - 55
ASHRAE Journal - June 2019 - 56
ASHRAE Journal - June 2019 - 57
ASHRAE Journal - June 2019 - 58
ASHRAE Journal - June 2019 - 59
ASHRAE Journal - June 2019 - 60
ASHRAE Journal - June 2019 - 61
ASHRAE Journal - June 2019 - 62
ASHRAE Journal - June 2019 - 63
ASHRAE Journal - June 2019 - 64
ASHRAE Journal - June 2019 - 65
ASHRAE Journal - June 2019 - 66
ASHRAE Journal - June 2019 - 67
ASHRAE Journal - June 2019 - 68
ASHRAE Journal - June 2019 - 69
ASHRAE Journal - June 2019 - 70
ASHRAE Journal - June 2019 - 71
ASHRAE Journal - June 2019 - 72
ASHRAE Journal - June 2019 - 73
ASHRAE Journal - June 2019 - 74
ASHRAE Journal - June 2019 - 75
ASHRAE Journal - June 2019 - 76
ASHRAE Journal - June 2019 - 77
ASHRAE Journal - June 2019 - 78
ASHRAE Journal - June 2019 - 79
ASHRAE Journal - June 2019 - 80
ASHRAE Journal - June 2019 - Cover3
ASHRAE Journal - June 2019 - Cover4
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2021november
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2021november_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_XCODFR
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_QSLFGO
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_ILKVNM
https://www.nxtbook.com/nxtbooks/ashrae/ashraemexico_2021
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_OPDJKD
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_VJKSRY
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SDHUTC
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_JPPKRR
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SDLTTH
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_CKLLES
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_SLDOX
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_HJETUK
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_OLUHGE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2020october
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2020october_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_ZERDGH
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_QVMNEO
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_RTPOKE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_BBATRE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_STUBMW
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_TPEMPE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_JNMKDS
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_FBTTPA
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_WQMMNE
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_TVBRYN
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_showguide2020
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_KTUZMA
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_ABEDGD
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201910
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201909
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2019septmeber_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2019september
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201908
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201907
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201906
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201905
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201904
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2019april
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201903
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2019march
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201902
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201901
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_showguide2019
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2018december
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2018november
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2018fall_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2018fall
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_2018october
https://www.nxtbook.com/nxtbooks/ashrae/ashraemexico_2018
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201810
https://www.nxtbook.com/nxtbooks/ashrae/ashraeinsights_201806
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201805
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201804
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201803
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201712
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201711
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201710
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2017fall_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2017fall
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201709
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201705
https://www.nxtbook.com/nxtbooks/ashrae/ashrae_meetinginsert_201610
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2016fall_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2016fall
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_acrexindia
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2015summer_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_amca_2015summer
https://www.nxtbook.com/nxtbooks/amca/2014summer2
https://www.nxtbook.com/nxtbooks/amca/2014summer
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_acma_2014summer
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201311
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201309
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_acmasupp_2013fall
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201305
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201303
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2013winter
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201211
https://www.nxtbook.com/nxtbooks/ashrae/achr_expo_mexico2012
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201209
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201208_v3
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201208_v2
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2012summer
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201205
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201203
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2012winter
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201111_v2
https://www.nxtbook.com/nxtbooks/ashrae/ashraejournal_201109_v2
https://www.nxtbook.com/nxtbooks/ashrae/pubcatalog_2011summer
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201105
https://www.nxtbook.com/nxtbooks/ashrae/meetingplanner_201103
https://www.nxtbookmedia.com