Experimental investigation of an air curtain for protection of an outdoor power installations from salt spray

Auguet 1971

'fUE!GTU1G1OU'W~UHOIL

.IUOTllEEK

EXPERIMENTAL INVESTIGATION OF AN AIR CURTAIN

1

S

JULI

1m

FOR PROTECTION OF AN OUTDOOR POWER

INSTALLATIONS FROM SALT SPRAY

by

Go Ao So Allen

EXPERIMENTAL INVESTIGATION OF AN AIR CURTAIN FOR PROTECTION OF,AN OUTDOOR POWER

INSTALLATION FROM SALT SPRAY

by

G. A. S. Allen

ACKNOWLEpGEMENT

The investigation reported herein was undertaken at the r~quest of.the Ontario Hydro E1ectric Power Commission on the initiative of the Salt Contamination Committee~ It was supported by a grant in aid of research fer the ,period August 15, 1970 to August 14,1971.

SUMMARY

An experimental investigation was carried out i~ the UTIAS subsonic wind .. tunnel of the .applicability of an air curta.in to protect outdoor electric power.installations from wind-borne salt spray. The salt .emanates·from an elevated roadway that is salted in the winter, being.thrown up.by passing traffic. The salt was simulated by.a tracer gas. (Helium), the concentrations.of which were measured in the region to be .. protected. The investigation, although not definitive, indicates ,

that.··reduetions of the order of 70% in contaIllination can be achieved. Ad~itional design studies and experimental .work would be needed to arrive at firm conclusions concerning cost and performance.

1. 2.

3.

4.

TABLE OF CONTENTS Acknowledgement Summary Notation INTRODUCTION EXPERIMENTAL TECHNIQUE 2.1 General Description 2.2 Tracer Gas Technique 2.3 Flow Field Simulation 2.4 Air Screen Simulation

2.5

Sampling Technique RESULTS OF THE INVESTIGATION 3.1 Concentration Profiles

Jet Curtain

for a Continuous 3.2 Flow Field Study of a Continuous Jet

Curtain

3.3 Concentration Profiles for a Segmented Jet Curt ain - Case 1

3.4 Concentrat ion Profiles for a Segmented Jet Curtain - Case 2

3.5

Smoke Flow Investigation

3.6

Concentration Profiles for a Segmented Jet - Effects of Wind Angle

3.7

Concentration Profiles for a Segmented Jet Screen - Case 3

CONCLUSIONS REFERENCES FIGURES 1 1 1 2 3

4

5

5

5

6 6 7 7 7 8 8 12

c*

w

w

x,y,z. p NOTATION

jet exit area

bloeked seetion of jet exit helium eonc~ntration

statie pressure drop through nozzle contraetion jet power

jet volume flow rate jet thiekness

jet exit veloeity

open section of jet exit wind tunnel speed

system ofaxes wind. angle air density

,1. INTRODUCTION

The concept of using air jets as aerodynamicstructures for controlling the atmospheric environment, recently expounded by B. Etkin and P. Goering (Ref. 1), has led the Hydro-Electric Power,Commission of Ontario to inquire into the applicability of air screens as a soluti9n

t~, the salt ,spray problem.

Salt spray emanating from salted roadways in thewinter ,is a cQstly nuisanc~ in the field of electric power transmission. Small particles of salt in the ,form of brine or fine crystals are deposited by the wind onto nearby insulators and bushings. If ,concentrations on

surfaces,are allowed to build up, a breakdown of functioning occurs. It ,was concei ved ,that an ail;' jet might be used as a curtain to protect the electrical installat~on fl;'om wind-borne salt.

Itwas propol?ed that an experimental investigation.be made;of,

the effectiveness and feasibil:i,ty of an upwardly-blown air scr~en to protect a spaçe downwind of an elevated contaminating source. To this end it was·decided that a series of tests in the UTIAS low-speed wind'

tunnel should be conducte~, simulating salt spray conditions a~ a typical tra~sformer station. Queen's Quay Transformer Station in

Toronto was chosen as the test site because of its ,small size and simplicity. A tracer gas was· to be used, to simulate salt spr?-y

contam-inant.

This paper is a description of the .technique devised for tracer gas investigation of mass transfer across an air screen and the results of an investigation using a scaled model of the Queen's Quay site and'

various air-scr~en configurations.

'2. EXPERIMENTAL TECHNIQUE 2.1 Gener~l ·Description

An

experimental investigation of the ,use of an upwardly-blown air-screen in a crosswind to protect an area frQm an upstream contaminant

requires (i) a system that will simulate the·contaminant and its

concen-trations,at various stations downstream, (ii) a flow field thatad~quately simulates the,one being studied, (iii) a jet source of variable parameters, and (iv) a device to.locate a probe thatwill take , samples for the

conçen-tration-measuring apparatus.

The choice of a tracer gas to simulate, salt-spray contaminant was ',due tG its measurability and, ease of handling. The concentrations were,measured using a thermal-conductivity gas analyzer, being the simplest and most vel;'satile. The flow field was generatedin the UTIAS low.speed

wind tunnel using blocks and spires upstream of tbe test seGtion to simul-ate ,atmospheric boundary layer condit ions. The j et screen apparatus produced,a 12 inches long low-turbulence jet of variable thickness blown at

90°

to the yind tunnel floor which could be rotatedto various angles to the flow and set .atvariou~ speeds. Because of the ,large number of

concentration measurements required, a motorized traver5ing rig was developed that could take,continuous vertical'profiles at any position behind the ,air screen. Two probes were used, one that drew in gas samples

f.or measuring concentrations, an~ one that gaue,tota+ pressure readings. ' In ,this way, the location of the .jet could,be found relative tG concen-·;6'l"ation levels.

. The model configuration is sho~ in;Fig. 1. It .is located on the roof .. ef the tunnel (i.e. upside~down) for acc~ssibility. 'Fhe axes are as shown,namely "x" perpendicular to the jet·screen, Hy" along its ' length, and "z" the height 'above grounq.. The origin was selected as

·the centre of the jet e~it. Model location and dimensions were determined, fr om the Queen's Quay site using a scaling f~ctor of 1:120 (i.e. 1 inch

=

10 feet). Since Reyn~lds' number effects a~ small f~r flow aro~nd sharp-edged objects, as is the jetcurtain and the elevatedroadway, thedom-inant non-:-dimensional parameter of the simulation is t.he jet-to-wind velocity ratio. These c;onsiderations' justify the simulati,on' technique ,used.

Each aspect of the ,experimental technique is discussed in detail in the following sections.

2.,2 Tracer Gas, Technique

Salt-spray contaminant was simulated with a gas. The spray was believed to consist of very fine particles of the order of 1 micron in diameter. As is the case with smoke,flow indicators, particles this small are,essentially trapped by the fluid since their momentum is negligible compared to their drag. A gas, then, behaves very much like

tiny particles,'

The concentration measure~ent system is shown in Fig. 2. The gas analyzer is a Gow-Mac the~al conductivity cell cons~sting of,four Rhenium-Tungsten hot-wire filaments in a bridge circuit powered by a Gow-Mac 40-001 Power Supply. The output of the system .is amplified 200 times, monitored on a digital voltmeter, and,recorded using a Honeywell visicorder. Gas samples are drawn by a vacuum pump past the cells and through two Brooks rotameters, one for a reference sample and one for the test sample. The reference and test gases each pass over.two cells ,on opposite arms of the·bridge. Current passing through the elements ca~ses

them to heat up until athermal balance is reached with the heat. dissip-ating into the gas streams. The bridge is balanceq. using a 'potentiometer that provides variable resistance to either side of the bridge (see Fig . . 2, zero adjust,). Since gases vary in conductivitr, the introduction of a different gas mixture into the,sample gas stream causes a change, in the rate of heat dissipation hence the thermistor ,temperature and resistance. The resultant bridge unbalance,is a measure of the concentration of a gas in,a carrier gas. Ashas been validated through calibrations, the output is linear with respect to concentration for small changes.

In this study, gas samples were taken in via a small probe connected to the thermistor cell by Tygon,tubing. The reference gas was taken fr om wind tunnel air upstream of the ,tracer gas source in order

r---

---~----that·.the baekground·.level accumulated in the closed-Gircuit tunnel wouJ,.d be"bal!aneed "eut ·1n·-the-bridge'. Concentration measurements are then only t-hose"due ,ilo direet propagation frGm the souree. The vacuum pump is suff-'..-a:e;ieltt1y.-etr,<!mg t,o' choke-the ,flow across tW0 neeq,le valves, one: forthe

;';eference flow and the other for the test flow. ' Any pressure fluctuations upstream of the valves do not affect flow rates;, hence, the flow rates are

.,he!hd, 'eonstant ~ . ,

Helium was selected as the tracer gas since its thermal

conduct-'ivity difference relative_to air is second only to hydrogen in magnitude. It is safe, inert andreadily available. Buoyancy effects in the flow .were consid~rednegligible compared with turbulent mixing.

The helium is introduce~ into the tunnel through a series of'

holes 1/8 in. in diameter along a 1/2' in. diameter pipe (see Fig. 3). The pipe is mounted on the top of a scaledmodel of the elevated roadway at the Queen's Quay Station. ' Fourteen holes were located 1 inch apart so as to produce an even distributi'on of helium across the whole jet , length. The hole d~ameters and helium flow rate were chosen,so as to give adequate concentrations down~tream without much flow disturbance. Concentrations at the jet exit were of the order of 1%; hence, for a 90% reduction by theair curtain, levels of, 110 maximum were to be read. Since the gas analyzer is sensitive to 1 in 10 5 parts helium in air, these levels provide adequately reliable readings. The helium is fed to the pipe from a pressurized gas cylinder through both ends and a baffling arrangement 'in the pipe insures equai delivery of heli~ to the holes. The flow is regualted by a constant-pressure supply valve, an on-off solenoid,valve for remote'operation, an adjustab1€ needIe valve, and a flowmeter. The helium, introduced in this way, pnovides a controllable SGurce of contaminant thatsimulates salt spray di~tribution fr9m an elevatedroad~ay.

Calibration curves have been made for the gas analyzer using an SKF Dilution Flask. Results indicate that concentration vs. voltage -is 1inear.to at least 1% a~d th~t very li~tle error exists. However, a few problems did arise. There was a zero drift for the first hour of running. Also, the signal from the bridge was, at times, noisy due to poor te~inal connections (later corrected). Nevertheless, the tracer gas system performed adequately weIl for the results required;

2;3 Flow Field Simulation

The salt spray problem OGcurs apparently in a low-wind-velocity, drift condition in which sal~, th~own up by traffic, is carried by the ' wind and deposits uniformly around,insulators. Since the turbulence and magnitude of the crosswind are'large factors in the interaction with a

jet ,screen, it was important to attempt some simulation of the actual

flow field expected.

It was deemed sufficient for this study to produce a boundary layer over the height of primary concern (i.e. 6 inches, equivalent tQ 60feet) with turbulence levels of comparable.magnitud~. To this end, a set ,ef 6 inch high spireiq were instalIed across the width of the

To increase the velocity gradient, a set of random b+ocks averaging 3 inches high were placed downstream of the spires .(Fig. 1). The velocity

and'turbulence~intensity profiles generated 30 inches dqwnstream are.

shown in Fig.

4.

This flow field ov~r the eleyate4-roadway simulation qualitatively represents ·the wind conditions to be expected at the Queeo I s Quay s~te. For the sake of simplicity and generality, the

whole,complex of ot her elevated roadways an4 obstructions were neglected. The wind tUlfnel speed, W, was set at ; 15 f. p. s. It 'did not

require varying since the independent parameter of the simulation is Vj/W which was more easily varied through Vj' The speed was ;.low enough

so that the corresponding helium flow rate, 1.08 ·c. f .m. ~ allowed . long running times before excessive accumulation in the tunnel or gas cylinder

depletion occurred. Lower velocities were more , difficult to set accurately. Wind angle variati.ons were achieved ,by ro~ating the jet .relative to the wind and realigning the elevated roadway.

The flow field generated, although not 'an accurate simulation

of.a particular case, was considered sui~able enough to give useful results.

2.4

Air Screen Simulation

The air screen configuration decided up on iS ,a straight

vertically-upward blo~ jet 'of air 12 inches long and a tbickness variable,between 0 and

.5

inches. This simulates a 120 foot jet, allowing a 10 foot extensiolf

béyond each end of the space. to be protected. The parameters to be invest-igated:for maximal design considerations were jet thickness; t

j , jet exit

velocity, V

j , and wind angle, a.

The final design of,the apparatus ,meeting these requirements.

is shown in Fig.

5.

In order to insure a constant-velocity, low-turbulence condition at the jet exit, a long diffuser section takes the flow from round

duçting to rectaogular without separation, honeycomb straightens the flow,

and a smoothly.rounded, sharp-edged e~it with a large area contraction .

produces a constant velocity across the jet width. Dynamic press~e

profiles we~e taken across the .width and along the length which varified

the desired results. The exit flaps are,hinged so that they can be secured.at the ends at variable separations. The air is supplied by a centrifugal blower through

4

inch diameter ducting. A slide valve in the ducting allows a variabIe f~ow rate. Jet velocity is monitored by

taking the. statie pressure drop through ·the nozzle contrE).ction. With the use of a hot-wire anemometer, the pressure drop, ~Pj, was calibrated·

with,vj for vari,ous tj' The resulting calibration is. expressable as

The whole diffuser .and nQzzle assembly is rotatabIe so that the jet screen angle, a, can be adjusted. With these features, the system provides

2.5 Samplins Technique ,

Rather than p0sitioning an inlet and drawing in a sample of gas f0r each point, it was decided to save time and effort by running a ve~tical traverse taking a continuous sample of contaminated air along the traverse. This method pr0duce~ a time lag between the time the probe was at a point and the actual time of recording cor:responding to,the time the sample takes to be drawn to the thermistor cello By taking traversès in opposite directions and noting the shift in the profiles, the time lag was deduced and accounted for. The method also leads to the possibi ity of.molecu ar diffusion thl'ough the tubing due to concentration graaients along the profile. Comparisons of profiles at various traversing speeds

indicated that in the cases being studied, this effect was negligible.

The signal produced from sampling at a point or continuously was,noisy as a re sult of concentration fluctuations in the turbulent flow. Although a mean value cannot be determined,by averaging at each point for continuous sampling, drawing smooth lines through the curves give an adequate,estimate

of the true mean values provided'that the traverses are slow compared with

c0ncentration fluctuati0ns. This sampling method, then, produces a great deal of information, rapidly, with little cost to accuracyo

In order,to make the traverses, a mechanical arrangement was

devised that made motor driven runs in one direct ion the z direction)

and allowed manual adjustments of'the (x,y) position of the probes. Figure

6' illust,rates the rig assembly. The vertical"7"dri ving motor, a Slo-Syn stepping'motor, is powered by a Slo-Syn translator triggered from a

square-wave function generator. The traversing speed is set by adjusting the generator frequency. Length of running is controlled by a time-delay

switch. - The 'x-position can be adjusted outside the tunnel by "turn~ng a

knob atthe end :of the traversing shaft., The probes are attached te the

end of a 'square sliding shaft. The sampling probe is 1/8" below the

total pressure probe with its opening to the side to minimize flow i

nter-ference.

The data is recorded on a Honeyw~ll visicorder that charts

inputs on a continuously moving photographic paper with a light beam,

l0cated by a mirror on a galvanometer. The inputs recorded simultaneously

were (1) a potentiometric voltage that gives the probe ocation, (il) a

filtered signal from a PACE transducer that'gives a voltage proportional

to the total head pressure, and (iil) the amplified signal from the gas

analyzer. The traverses were run at

.4

inches/seco, the same speed as

the chart ,recorder.

3. RESULTS OF THE INVESTIGATIGN

In the following sections, the results are explained and

discussed according to each aspect of the jet curtain investigated. 3.1 Concentration Profiles for a Continuous Jet,Curtain

The first configuration to be considered was the straight

Concen-tration and pressure measurements were made only behin~ the centre of,

the jet. The concentration levels are,non-dimensionalized with respect to the maximum concentration at 2.5 inches downstream of the jet exit centre when thE7 jet .,is off. Ih Fig. 7, the case V

J

/W=O (i. e. na jet)

shows the, typi~al distributiop of a,plume from the helium source on the elevated roadway. Tt was anticipate~ that the jet wouldforce the helium over the region to be protected ,with some ,helium gradient 'through

"thejet. However, only for low jet-to-'\find velo city ratios is this

,effect noticed (Fig.

7,

Vj/W=5.35; Fig.

8).

In addition, however, there

'-is a, coneiderable buildup beneath the jet centreline. Also, the gradient_ ',thr0ugh the curtain rapidly,diminishes.' rhe effect of increasing the

velocity ratio (Fig.

8)

is to il1crease levels behind the curtain and to spread.the helium higher. These findings suggested that helium was being drawn in from the sides and'the diffusion process thrqugh the

c~rtain was rapid. In order to understand,the process further and to gain someinsight as to 00W to correct t~e problem it was decided to

investigate the flow field around the curtain using tuft indicators.

··3.2 Flow Field Study of a COI;ltinuous Jet Curtain

Tufts of wool were attached to the tops of pins in ' suçh a way that they could rotate easily with flow directidn. A grid of them was laid out behind and te the side of the curtain

3/4

inches above the

ground. Figure 9(a) is a photograph of the ,tuft formation showing the flow pattern arQund a continuous jet. The pattern shows distinctly that '

a pair of vortices 'forms ,in the wake. Trese v9rtices cannot terminate

in,sp~ce; hence, they must continue ,up from the ground and along the inner edgesof the def+ected curtain. Figure 9(b) illustrates

qual-itatively the flow field around a jet screen in a crosswin~. The outer sides of the .screen are deflected more than the centre. The flowaround

the sides and,back towards the front partiallyexplains the large helium

concentrations observed,in the previous section. Bearing in mind that the flow around the ends is ,d~e to the ,low pressure c!,'eated in the wake

of the screen, one concludes tn.at relatively helium-free air is required

to flow into the wake preventing helium-c0nt~inated air from entering the wake. As.a preliminary effort in this direction , a tube was placed

across the jet ,exit, producing a "hole',' through the curtain which allows upstream air,to be drawn into the wake. Figure lO(a) shows thetuft pattern generated ',behind sueh a coqfiguration~ The flow around the sid~s

is greatly redueed. Since,helium concentrations are lowest near the ground, holes in the curtain should be at the bottom of the ,eurt~in. By taping over parts of the exit, this is achieved without having to change

the jet apparatu~. 'The resulting flow fi~ld is described graphically in

Fig. lO(b). The curtain effeetively closes within two widths of the',tape;

as has bee~ shown byKnystausus (Ref.

4).

3.3

Concentration Profiles for a Segmented Jet 'Curtain - Case 1

The jet ,exit was taped so that th~re was alternately an open section, w=.4 in. ~ ~nd a blocked ,section, b=.75 inches. ' The jet width

was

.4

inches. All eoncentrations, C*, were non-dimensionalized with respeçt 'to the maximum helium level yri-thout the jet, at x=O and W=15 f.p.s. ,The jet exitvelocity, V

j , was taken to be the average veloeity

The results shown in Figs. 11 and 12 indicate that levels can be reduced by increasing the jet-to-wind velocity ratio. This is a

significant' improvement over a contim,1ous jet since, when the .wind" velocity dr0ps, the protection improves rather tha~ worsens. Protection is still only around 60%reduction near,the outside of the curtaino For

reductions of 80%, very large ve10cities are.required. Tt was fe1t that

narrower holes and wider jet'exits were required to reduce penetration.

3.4 Concentration Profiles for a Segmented Jet Curtain - Case 2

i

The new choices.of jet parameters were b/tj=o8, w/tj=2, tj=.4

inches. Figures 13, 14,. and 15' show the concentration distrioutions

behind the curtain for various jet velocities. The results are similar

t~ tho'se for èase 1 except lower ve ocitie~ are required to achieve an 80% reduction. Figure 13 shows that for low velocity curtains, the helium is deflected by the jet put as the velocity increases, the helium is entrained 'by the jet aud spreads through it. The concentration

profile-at x=O indicates that the heliurri plume is drawn downward and into thecurtain. Behind the ,hole~ in tne curtain, as is seen in Fig.

14, helium levels are high near the ground and-aeCflrease further downstream

indicating thathel~um is dravn through the,hole~ andback into,the c~rtain. Figure 15 shows that near the curtain edges, the protection is still fairly

effectivewith'reductions of'70 to 80%. The helium levels thatdo arise

beh~nd the curtain appear 'to be a,result of three processes: jet entrain-ment with turbulent diffusion thro~gh the curtain, direct flow through the

curtain openings, and back flow intq the wakeregion. In order to

under-stand these processes better , a smoke visualization study was attemptedo

3.5 Smoke Flow Investigation

Smoke candles were used to produce a dense white smoke that was

fed into·the helium feed pipe. The photographs taken did not show the

smoke clearly'enough. ·Visual.observations indicated that for the

segmented'jetcase, the smokewas markedly drawn downwards into the jet andmixed·thoroughly. For the continuous jet, the effect was less

pronounced . . Tt may be concluded that jet.inta~es wil have to be placed some dis~ance from the elevated,roadway. Also, any holes to the wake

region should be kept as low as pos si bIe.

3.6 Conc entration Profiles for a Segmenteq. Jet .- Effect s of Wind Angle With the axes rema1n1ng fixed relative to the jet , profiles were taken for the Case 2 jet. The jet was at 45° to the wind. Figures 16,17,18 and 19 show the conce~tration distribution behind the curtain.

Thecurtain is less effectiye in the upwind region and becomes pr

og-ressively more effective towards the other side of the curtain. The jet centre~line is deflectedlessin the upwind region and m re in the downwind ,than the deflectionof.a similar jet at right ang es to the

wind. The 'large concentrations 'near the ground for Vj1W=500 and 700 in, Fig 0 17 ·suggest that ,the :'main SOUTce of helium in, the wake is

through the holes. Helium aroun~ the sides is drawn into the curtain. The overall effect of wind angle is to raise helium leve s in the

upwind:--s'~gment of the protected region and to lessen them in the downstream.

3

0

7

Concentration Profiles for a Segrnented Jet Screen - Case 3

-- -'I'lre~last-parameter to be , inves,tigated was the jet thicknel;ls.

The width-; -t"j:, -wa.s -"reduc ed 'to .2 inc he s but th~ open and, blocked lengths remairre-d-the'same a.s Case 2; hence, w/tj=4;b/tj=L6. The jet

velocri-ties examined were higher in order to obtain the same c1;lrtain

he:i:ght'S-~ - eomparing F~gures 20, 21, and 22 with 13, 14, and 15, the effect'S'of-jet 'thickness can.beobserved. In most cases, concentrationf?

arereduc-ed -by-about the' same amount. At 'y=3 inches, directly behind a hole in thecurtain, the helium tends to be drawn through more with

the thinner curtain. At 'y=5 inches, higher velocities appear to draw

helium around the sides in the thin jet.

4

0 CONCLUSIONS

4.1 --The nse of an' air -curtain as a screen for outdoor installations

against -up"Stream"cont-aminat.±on :does not rely on i ts ability to block and we-dge--contaminated flow as originally conceived. The essential

obj ective -is tQ 'maintain' the' concentration of contantinant at an acçept-ably lo~level'over'a certain volume. Sinçe the interaction of a jet

and a -crosswind -is adynamie process, there is a continued changing of the--air -in the -protected .volume. Air ' entrained into the curtain or

drawn -downstream-must 'be replaced ;with air of sufficiently low contaminant concentration;-in-most applications, however, it ,will be difficult to locate-and -introduce low-contamination air. In the case of the stratght.

upyrardly.;;.blown-jet-screen, the protected volUI!leis a wake region of the jet-; '- The-l-ow' pressure region created by jet entrainment and flow separation 'hends -the 'jet over -and induces flow around the sides of the

jet.-The-~mixing"pr0cess~across the-screen maintains a flux of c ontam-inatedair;---This-situation 'makes'it impossible to establish very low concentrations -with -this -configuration.' In summary; due to the dypamic nature of--air-screens 'of-the sort in this investigation, their usefulness

in protection against contaminated air relies on,eontinuous dilution of the protected'region.

4.2 The results of this-study along with those of a,study on annular jet diffusion indicate that-th~re is a large mass transfer aeross a 'jet screen whiehis essentially due,to turbulent diffusion. The concentration -profiles-relative to the jet centrelines. indieate

that ·,ratherthan 'defleeting -the cçmtaminant, it is drawn, into the curtain

and spreadsthroughout'th~-curtaininto-the wake regiono The tatal head profilesindieated hfgh-intensity turbulence in the wake and especially

in the jet. Research; 'by Teuni s sen and Apparao, at UT lAS , into annular

air currentproperties; 'showed-a 'negligible dependence of mass transfer

on jet 'veJ;ocity. The amount -of -mass -transfer was much greater than coul~ result 'from 'molecular -diffusion;- From these results, it ean be concluded that thereis a mass transfer across air screens resulting trom jet entrainment and large intensity turbul ent mixing.

4.3

The straight continuous jet cur.tain behaves much like asolid parabolie sheet. The flow separates from the edge of the sheet and a pair of,vortices shed off the sides. The vortex core extends from

the greund, upward along the inner edges of the sheet. The flow evolves

into a horseshoe shape similar to the development of the circular jet

in a crosswind (see Ref. 3). These observations suggest th~t experim-entation on tW0-dimensional jets in crosswinds must be performed with utmost consideration of the applicability to three-dimensional situations with their end effects. F0r the .purposes of our application, th~se

effects are adverse. The segmenteq. curtain creates "holes" at the base

that allow the passage of air to equalize the pressure behind the

curtain and reduces suction around the sides. If this air has low-level

c0ntamination, then the wake region will be diluted. Hence, the

contamination behind the segmented screen is due to turbulent diffusion through the .jet alone,and this is mixed with incoming air through,the

gaps. In our application, it is ~ssumed that the salt partieles emanate from the elevated roadway so that concentrations near the ground are low

and the wake is diluted. However, the flow creates a suction beneath the .roadway and'the contaminant plume is drawn downwards into the

curtain or through the h0les and back into the jet. The re sult of this

process is to disperse the contaminant over a greater height, thus

decreasing the maximum concentration. These findings are summarized as follows: a ;Eair ,of wake v~i~ are f?rmed behind the vert~lly­

u~ward plown jet·sheet in a crosswind which can be reduced or eliminate~

entir,elx. by ;mrovid,ing h0les ,in the curtain, for ventilation of the wake.'

4.4

The influence of jet velocity and thickness on the effectiveness of air curtain protection is predominantly to determine the height of influence rather than degree of proteGtion. Higher veloeities tend to

disperse c0ntaminant m0re but do not significantly prevent penetration

into the wake. ' An incr~asein jet thickness decreases diff'usion across

the curtain but has no overall effect for jets of equal momentum. In

selecting a jet thickness and v~locity, the factors tO,consider are the

power requirement and the volume flow rate for a jet of s'Ufficient momen-tum. The jet centreline location can be appr0ximately determined from the

semi-empirical expres sion for a two-dimensional jet in a crosswind

(~2

=

1.25

(".IJ2

~

(1) (Ref. 6)

Hence, for the same region of influence, the jet momentum or (

#-)2

t_j must remain constant. In our case, taking consideration of'

the difficulty of finding sufficient vo],umes of clean air, one should cp'oose a high velocipy, thin jet. Volume flow rate varies as tj~ whereas power varies as l/tj~ (see proof below).

butfrom (1)

The êhoice of et thickness and velocit de ends rimaril on the .re ·uired. jet momentum, the ~0wer of the jet, and.the volume f ow rate of the jet. In our case -(V'j/W) tj ;,. 1~0 feet ~houl~ give a 70% to 7,5% reduction in contaminant·, for the . station if the j et is 20 ft. in front of the station. Depend'ing on the accessibility of a clean air source, volume flow.rates should be çonsidered before power requirements.

4.5

Angling the wind relat~ve to the curtain reduces protection in the upstream·half of the proteçted;zone and increases .the effectiveness ~n the'downstream half.

4

.

6

The applicability of the investigation to·the full-sca~e outdoqr site depends mainly on access to relatively salt free air in.the vicinity, the origin of the salt spray, and the size of the salt-bearing particles. The .experimental curtain uses uncontaminated air. Also, the large volume flow rate required will cause a'local flow phenomeno~ that may distort the experimental results if the intake~ are. close to the jet. The exper-iment also .assumes that the only source .of salt spray is thèelevàted roadway. If additional roadways below are sal~ed, then contaminant levels behind t~e curtain may be sOmewhat higher. The use of hélium to simulate salt spray is a conseryative façtor of the testing. If the particles have masses significant.compared.to their d~ag, then,they will tend to follow . mean velocity paths, reducing turbulent diffusion, and causing them to be blown over top of the jet c~rtain. I~ is necessary, then, to point

out that ·knowledge of the actual salt-spray source distribution and particle size is necessary for an assessment of the validity of the experimental simulation. If salt spray ema~ates essentially from the elevated roadway alone and the particle size is in themicron range th en the results of the investigation will be applicable.

4.7

The suggested jet configuration for the ,Queen's Quay T.S. is shown in· Fig. 23. It is advised that if such.a jet screen is to be built, a model wind tunnel 'test . of the actual configuration wi th all st·ructures and jet intakes be performed. The jet specifications are:

t

j

=

2 ft. V

j

=

7 W which may.be varied with

W or set for a critical W

for W

=

15 fps Pj

=

1/2 p Vj3Aj

~

700 H.P.

•

This value is only the air power and does not take account of motor and blower efficiencies. Th~ proposed c~nfiguration d0es not solve the

problem of,d~cting air to the curtain or blower selection but is only,

1. 2.

3.

4,

5.

6.

Et~in, B. Goering, P.L.E. Campbell, G.S. Standen, N.M, ' Keffer, J.F. Baines, W,P. Knystautas, R. Vizel, Ya,M, Mostinskii , r,L, Strand, T, , Wei, M,H',Y. REFERENCES I

Air Curtain Walls and Roofs - Dynamie

Structure~. Phil. Trans. Roy, Soc. Lond.

A.269,527-543 (1971).

Progress Report 11 on Simulation of Earth's Surface Winds. National,Research Council, LTR-LA-37.

The Round Turbulent Jet in a Crosswind. Journalof Fluid Mechanics, Vol. 15, part 4, 481-496 (1962),

The Turbulent Jet ,from ,a Series of Holes in a Line. The Aeronautical Quarterly, Vol. XV,

Feq,

1964.

Deflection of a Jet Injected into a Stream, Inzhenerno-Fizichevskii Zhurnal, Vol. 8, No. 2, pp. 238-242 (1965).

Linearized Inviscid-Flow Theory of Two-Dimensional Thin Jet Penetrati~n into a Stream. ; Jour. of Aircraft~ vo1.5,N~. 6, pp. 551-554, 1968.

•

tracer gas feed

elevated road simulation

spires to produce turbulent boundary layer

random blocks

total pressure and sampling probes

traversing rig

b

VACUUM PUMP

c

/

,m--

i

d

POWER SUPPLY

THERMISTOR BRIDGE

AMPLIFIER

VOLTMETER

a

FLOW CONTROL

c

b

_e

ZERO ADJUST

a

f

reference

t

.

~

sample

VISICORDER

~

r

105

''

~

1/2" D plpe

3·3 "

FLOW BAFFLE

tracer gas enters

END VIEW

IA

r-

29"

"I

5"--t

_.p"r-

_/14

I/S"

0 holes

tunnel floor

Urms/Ü

(%)

ow-________

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8

Mean Velocity Profi Ie

-!

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7

Turbulence Intensity

~5

-N

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3

2

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FIG.

4:

MEAN VELOCITY AND TURBULENT INTENSITY PROFILES 30 INCHES DOWNSTREAM OF THE BLOCKS AND SPIRES'

•

mounting

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blower

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. . . - - - diffuser section

~----

honeycomb

. - - - - upstream pressure taps

r - - -

downstream tap

DETAILS OF

JET WIDTH ADJUSTMENT

spring meta I hinge

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. - J

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gears connecting

traversing gear.

motor and

potentiometer

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probe heads

posltion potentiometer

connections to

motor speed

and dtJration

control

/

5

tepping motor

block supporting

rock ond gear

to gas analyzer

to pressure transducer

FIG.

6:

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7:

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Jet Velocity

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