Transcript
8/6/2019 Displacement Ventiolation
1/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 1
Displacement Ventilation
by
Peter V. Nielsen,Aalborg University
8/6/2019 Displacement Ventiolation
2/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 2
Content of the lecture
Introduction
Vertical temperature gradient
Free convection flow
Contaminant control, zone model
Air diffuser
8/6/2019 Displacement Ventiolation
3/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 3
Mixing ventilation and displacement
ventilation
Mixing ventilation. Controlledby the momentum flow fromsupply openings
Displacement ventilation.Controlled by buoyancy(temperature differences)
8/6/2019 Displacement Ventiolation
4/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 4
Momentum flow
Momentum flux from diffuser n = 3 h-1
Io = 0.4 N
Io = 0.02 N
Io = 0.0002 N
(Io = 0.1 N at the air change rate of100 h-1)
8/6/2019 Displacement Ventiolation
5/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 5
Displacement ventilation
Charateristics:
Low location of supply
opening and highlocation of return opening
Free convection around heat
sources
Stratified flow in the room
Vertical temperature gradient (and vertical concentrationgradient)
Air movement is controlled by buoyancy
8/6/2019 Displacement Ventiolation
6/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 6
Air movement in room with displacement
ventilation, 1
Vertical air movement
Horisontal air movement
Short circuit of air
8/6/2019 Displacement Ventiolation
7/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 7
Best suited forDisplacement ventilation is usually preferable in the
following cases:
where the contaminants are warmer and/or lighterthan the surrounding air,
where the supply air is colder than the ambient air,
in tall rooms, e.g. where the room heights are more
than 3 metres,
8/6/2019 Displacement Ventiolation
8/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 8
Content of the lecture
Introduction
Vertical temperature gradient
Free convection flow
Contaminant control, zone model
Air diffuser
8/6/2019 Displacement Ventiolation
9/41
8/6/2019 Displacement Ventiolation
10/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 10
Temperature and concentration
distribution
Vertical temperature and concentration distribution in aroom with displacement ventilation
8/6/2019 Displacement Ventiolation
11/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 11
Temperature distribtion
Vertical temperaturedistribution atdifferent locations
in the room
Idea behinddimensionless
description
8/6/2019 Displacement Ventiolation
12/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 12
Simplified model for temperature
distributionThe 50%-rule for vertical temperature distribution
8/6/2019 Displacement Ventiolation
13/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 13
Content of the lecture
Introduction
Vertical temperature gradient
Free convection flow
Contaminant control, zone model
Air diffuser
8/6/2019 Displacement Ventiolation
14/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 14
Convective flows
Convective flows, the engine of displacement ventilation
8/6/2019 Displacement Ventiolation
15/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 15
Point source and line source
The value of the coefficient kis0,7-0,9 for pipes and ducts, 0,4-0,6 for smaller components and0,3-0,5 for larger machines and
components
8/6/2019 Displacement Ventiolation
16/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 16
Free convection around a person and a
flat plate
8/6/2019 Displacement Ventiolation
17/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 17
Source geometry
The thermal geometry
can be difficult toestimate
8/6/2019 Displacement Ventiolation
18/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 18
Weak thermal sources
Exhalation dT/dy = 0.5C/m
dT/dy = 0.1C/m
8/6/2019 Displacement Ventiolation
19/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 19
Cross Infection Problems
Source patient on theback, = 70
Source patient on theside, = 0.7
Qian Hua et al.
8/6/2019 Displacement Ventiolation
20/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 20
Content of the lecture
Introduction
Vertical temperature gradient
Free convection flow
Contaminant control, zone model
Air diffuser
8/6/2019 Displacement Ventiolation
21/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 21
The Idea of Contaninant ControlThe idea of contaminant control is to separate warm andcontaminated air below the ceiling from the fresh air
supplied at the floor
8/6/2019 Displacement Ventiolation
22/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 22
Contaminant distribution
8/6/2019 Displacement Ventiolation
23/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 23
Stratification
Gate open to theheight of 2.5 m
8/6/2019 Displacement Ventiolation
24/41
8/6/2019 Displacement Ventiolation
25/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 25
Ventilation Effectiveness
- Ventilation effectiveness of the occupied zonealso called the air quality index of the occ. zone
- Mean ventilation effectiveness. Also called thecontaminant removal effetiveness
- Local ventilation index. Also called the localair quality index at a given point
- Personal exposure index. Also called the airquality index of the inhaled air
c
coz
cP
cexp
8/6/2019 Displacement Ventiolation
26/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 26
Personal exposure index
The personal exposure index can be rather high, even if thestratification height is lower than the breathing zone
e = cR/ce
8/6/2019 Displacement Ventiolation
27/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 27
Content of the lecture
Introduction
Vertical temperature gradient
Free convection flow
Contaminant control, zone model
Air diffuser
8/6/2019 Displacement Ventiolation
28/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 28
Air Diffusers
Air supply without draught is as crucial in displacementventilation, as it is in mixing ventilation. Most draughtproblems reported from practical experience are due
to the incorrect choice of diffuser.
8/6/2019 Displacement Ventiolation
29/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 29
Air diffusers
8/6/2019 Displacement Ventiolation
30/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 30
Airflow from a low velocity diffuser
Isothermal airsupply
Supply ofwarm air
Cold supply air. Aradial flow will oftentake place
Th dj t
8/6/2019 Displacement Ventiolation
31/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 31
The adjacent zone
The length ln
, of the adjacent zone is defined as the distancefrom the diffuser to a point where the maximum velocity hasdecreased to a certain value, usually 0.2 m/s.
8/6/2019 Displacement Ventiolation
32/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 32
Radial Stratified Flow, 4
Virtual origin of the flow isclose to the diffuser
Influence of virtualorigin,xo = 0.5 m
8/6/2019 Displacement Ventiolation
33/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 33
Stratified Flow
Wall jet
flow
Stratifiedflow
8/6/2019 Displacement Ventiolation
34/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 34
Length Scale, Thickness or
Depth of the Flow
The profiles can be considered to be universal. The lengthscale, or depth, is the height to the velocity ux/2 = 0.5.
where
8/6/2019 Displacement Ventiolation
35/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 35
Flow Element for Radial Flow
+= oo yudxxq )(
mo
o
ebqq
=
o
Dr
o
x
xxK
q
u
+=
1
( )
+=o
xo
dfuxxq
)(
( ) df
ebK
o
o
mDr
=where
8/6/2019 Displacement Ventiolation
36/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 36
KDrValues for Diffusers, 1
Six differentwall-mounted
diffusers
8/6/2019 Displacement Ventiolation
37/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 37
KDrValues for Diffusers, 2
KDrversus productand Archimedes
Number
8/6/2019 Displacement Ventiolation
38/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 38
Plane Stratified Flow
The flow from a number of diffusers placed close to eachother will merge into a plane, stratified flow.
The flow will alsobe plane in a
narrow room.
Plane flow will alsobe generated when
the diffuser has alarge spreading ofthe supply air.
8/6/2019 Displacement Ventiolation
39/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 39
Flow Elements for Plane
Stratified Flow
where
8/6/2019 Displacement Ventiolation
40/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 40
Flow between Obstacles, 1
Flow through an opening in the occupied zone is similar
to the flow from a diffuser
8/6/2019 Displacement Ventiolation
41/41
Peter V. Nielsen, Aalborg University pvn@civil.auc.dk 41
Flow between Obstacles, 2
Kob versus flow rate and
temperature difference
Equation:
x
K
q
uob
ob
x 1=
top related