displacement ventiolation

8/6/2019 Displacement Ventiolation

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Peter V. Nielsen, Aalborg University [email protected] 1

Displacement Ventilation

by

Peter V. Nielsen,Aalborg University


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Content of the lecture

Introduction

Vertical temperature gradient

Free convection flow

Contaminant control, zone model

Air diffuser


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Mixing ventilation and displacement

ventilation

Mixing ventilation. Controlledby the momentum flow fromsupply openings

Displacement ventilation.Controlled by buoyancy(temperature differences)


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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)


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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


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Air movement in room with displacement

ventilation, 1

Vertical air movement

Horisontal air movement

Short circuit of air


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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,


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Introduction




Air diffuser


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Temperature and concentration

distribution

Vertical temperature and concentration distribution in aroom with displacement ventilation


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Temperature distribtion

Vertical temperaturedistribution atdifferent locations

in the room

Idea behinddimensionless

description


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Simplified model for temperature

distributionThe 50%-rule for vertical temperature distribution


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Introduction




Air diffuser


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Convective flows

Convective flows, the engine of displacement ventilation


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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


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Free convection around a person and a

flat plate


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Source geometry

The thermal geometry

can be difficult toestimate


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Weak thermal sources

Exhalation dT/dy = 0.5C/m

dT/dy = 0.1C/m


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Cross Infection Problems

Source patient on theback, = 70

Source patient on theside, = 0.7

Qian Hua et al.


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Introduction




Air diffuser


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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


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Contaminant distribution


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Stratification

Gate open to theheight of 2.5 m


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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


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Personal exposure index

The personal exposure index can be rather high, even if thestratification height is lower than the breathing zone

e = cR/ce


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Introduction




Air diffuser


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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.


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Air diffusers


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Airflow from a low velocity diffuser

Isothermal airsupply

Supply ofwarm air

Cold supply air. Aradial flow will oftentake place

Th dj t


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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.


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Radial Stratified Flow, 4

Virtual origin of the flow isclose to the diffuser

Influence of virtualorigin,xo = 0.5 m


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Stratified Flow

Wall jet

flow

Stratifiedflow


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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


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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


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KDrValues for Diffusers, 1

Six differentwall-mounted

diffusers


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KDrValues for Diffusers, 2

KDrversus productand Archimedes

Number


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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.


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Flow Elements for Plane

Stratified Flow

where


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Flow between Obstacles, 1

Flow through an opening in the occupied zone is similar

to the flow from a diffuser


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Flow between Obstacles, 2

Kob versus flow rate and

temperature difference

Equation:

x

K

q

uob

ob

x 1=

displacement ventiolation

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