Experimental investigation of annular air curtain domes

•

January 1985

-

'. • ~ ,.'..;...t','" ..r~ .• ~ . ,,'V".>.*1i.EXPEIÜMENTAL INVESTIGA'f.ION

OF ANNULAR AIR CURTAIN DOMES

by

B. Kamen and A. A. Haasz

TECHNISCHE HOGES HOOl OELFT

LUCHTVAART- EN RUI TEVAARTIECHN'EJ(

BmUOTHEEK

Kluyverweg 1 - DELFT

&.

3

JUN'

1~5

UTIAS Report No. 288 CN ISSN 0082-5255

•

EXPERIMENTAL INVESTIGATION OF ANNULAR AIR CURTAIN DOMES

by

B. Kamen and A. A. Haasz

Submitted August 1984

'<'

Acknowledgement

The authors wish to acknowledge the financial support provided by Air Roofs Canada for the construction of the 6m diameter facility, and the University of Toronto Institute for Aerospace Studies for providing the site for the facil ity. The research program was performed with the help of an NSERC grant.

-Abstract

The performance of annular air-curtains as protective barriers against precipitation was investigated experimentally using a 60 cm diameter laboratory model and a 6 m diameter outdoor test facility. Precipitation was sirnul ated by small si ze gl ass beads and water dropl ets. The scal i ng requirements for the particle-air jet interaction dynamics were reviewed and modified to explicitly include the effects of the simulation material • The effects of Reynolds number, however, are only implicitly included. Based on the results of this study a set of criteria for future tests that could lead to reliable projections for larger scale installations is presented in the form of dimensionless parameters.

Acknowl edgement Abstract

Table of Contents Li st of Symbo 1 s

1.0 INTRODUCTION

2.0 ANNULAR JET STRUCTURE • 3.0 SCALI NG LAWS • • • •

TABLE OF CONTENTS

4.0 FLOW CHARACTERISTICS OF THE 6m MODEL

4.1 Initial Cireumferential Veloeity Profile. 4.2 Model Modifieations • • • • • • • • • • 4.3 Format i on of the Ai r-curtai n Dome

4.4 Radial Veloeity Profiles • • • • • • • • • 4.5 Ve 1 ocity Measurement Teehni ques ••• 5.0 PRECIPITATION EXPERIMENTS WITH THE 6m MODEL

5.1 Sealing Requirements • • • • • • • • • • • • • 5.2 Production and Charaeteristies of Water Drops 5.3 Determination of Critical Jet Veloeity • • • •

.

.

.

6.0 PRECIPITATION EXPERIMENTS WITH THE 60 cm MODEL • • • • • •

6.1 Water Drop Experiments • • • • • • • • 6.2 Glass Bead Experiments • • • • • • • • • 7.0 COMPARISON OF PRESENT AND PREVIOUS RESULTS 8.0 CONCLUSIONS •

9.0 REFERENCES FIGURES

TABLES

APPENDIX A: ANALYSIS OF DROP SIZE RANGE PRODUCED BY WATER NOZZLES

i i i i i iv v 1 1 2 5 5 6 6 7 8 8 8 9 11 12 12 14 14 16 17

LIST UF SYMBOLS

d drop or particle diameter

9 gravitational acceleration

h height of annular nozzle

k₁ constant in the CD-Re function

m drop or particle mass

n power in the CD-Re function t thickness of the annular nozzle

-"

t dimensionless time

A ... ' \

u,v,w non-dimensional components of drop or particle velocity

û' ,v',w'

non-dimensional components of jet velocity

CD drag coefficient

D annulus diameter

Fr Froude number

Re Reynolds number

Re.

J Reynolds number based on jet velocity Vj and thickness t Rep Reynolds number based on actual particle velocity in jet V

particle di ameter d

Ret Reynolds number based on particle terminal velocity Vt and partiele diameter d

SG specific gravity

V_c critical jet exit velocity

Vj jet exit velocity

Vp drop or particle velocity in the jet V_t drop or particle terminal velocity

w wind velocity

~ fluid viscosity

V .; ( û -

a

i

F

+ (V - V i )2 + (w

-w

i )2

n₁,n₂,n₃ dimensionless parameters p density of air

P_p density of drop or partiele

Subscripts:

fs full scal e

m model

-1.0 INTRODUCTION

The basic air-curtain principle is the utilization of rapidly moving

air jets to separate two potentially different environments. Possible

applications of this concept include protection against rain and snow for

mall s, sports stadia and construction sites. Our research program at the

Uni versi ty of Toronto over the 1 ast decade 1-13 has concentrated on the

development of both theoretical and experimental models for the dynamics of the precipitating particles as they interact with the complex flows of the air-curtain jets. Extensive work on both of these fronts has been performed

for the horizontal air-curtain jet configuration.7- 9 Research on annular

jet air-curtains, on the other hand, has only involved small-scale experimentation. Theoretical treatment of particle dynamics in annular jets is comparatively more difficult due to the complexity of the annular jet flow field.

The major objective of our research with air-curtains is to establish the scaling laws for particle-jet interactions and to determine the feasibility of using air-jets for full-scale applications. The emphasis in this report is on the annular jet configuration. Previous studies with such a system were performed with models ranging in size from about 15 cm - 86 cm

diameter.10- 13 The simulation of rain was performed with either water

dropsl0,12 or glass beads. ll The present investigation with the use of a

6m diameter outdoor model provides a very important extension of physical size for studying the scaling laws.

Another important aspect of the present work is the rev i sion of the scaling laws, which was made necessary in order to achieve consistency among previous and present results. Proper scaling is extremely important for tlle projection of small-scale results to full-scale installations.

2.0 ANNULAR JET STRUCTURE

As previously mentioned, an air-curtain is just a jet of air formed and

oriented for a special purpose. In the case of the annular jet

configuration, air is discharged straight up through an annular nozzle to form a hemispherically shaped enclosure - hence the name "air-curtain domen. The protection against rain and snow within the enclosure is achieved via the interaction of the jet curtain and the precipitating particles. Falling rain and snow encountering the upward moving jet is slowed down, stopped and eventually accelerated up and away from the covered area.

The annular jet has been studied for its uses in air cushion

vehiclesl4 ,15 and in oil burners and combustion facilities. 16 ,17

Conse-quently, much is known about what happens and why when ai ris directed through an annular nozzle. There are four basic regions associated with an

annular air-curtain dome: (l) the exiting primary annular jet, (2) the

entrainment regions, (3) the recirculation zone within the dome, and (4) the

combined jet. The processes involved in creating these regions are

excellently described by Chigier and BeE!r 16 and are paraphrased in the

following paragraph for completeness.

"As the jet exits the annular nozzle, a potential core (first region) with essentially constant velocity is established in the center, with

mixing zones forming (second region) on the inner and outer edges due to the shearing action of the jet and the stagnant air. On the outside of the annular nozzle there is an endless supply of air for the entrainment but on the inner side of the jet there is only a limited, fixed amount of air to meet the entrainment requirements of the jet. This results in an internal vortex being set up (third region) which is toroidal in shape. The vortex creates a low pres su re region and the resulti ng pressure gradient across the annul ar jet causes the jet to bend towards the axis of the jet. As the converging streaml ines approach the axis, a stagnation region is set up and the resulting high pressure redirects the flow away from the axis until the combined jet expands in the same manner as a single jet (fourth region)." (See Fi gure 1.)

Chaplin,15 by making several assumptions about the flow pattern and the entrainment, as well as using the limiting case of infinitely thin nozzles, derives an equation for the height of the stagnation point above the floor. Adapted to the rnodels studied at UTIAS,lO the result yields a stagnation point occurring at a height equal to the radius of the annulus. However, this height (which also represents the length of the internal vortex) is a function of the nozzle thickness-to-annulus diameter ratio (t/D) as well as the exit speed of the jet. 16,17 For a constant t/D, as the exit speed of the jet increases, the internal circulation increases causing a greater pressure gradient across the jet which would tend to force it towards the axis sooner. However, the increased inertia of the faster jet resists the increased force and the height of the stagnation point remains essentially a constant.

Typical streamlines and pressure distributions for an annular jet are shown in Figures 2 and 3, respectively (taken from Chigier and Beér).16

The height of the stagnation point and the internal recirculation are of prime importance when applications of the annular jet are considered. In all previous studies the nozzles were positioned at what would be field level in a full-scale stadium case. However, in the case of a baseball or football stadium the nozzle would be mounted above the stands in some manner and the resulti ng vortex and rec i rcul ati on may be consi derab ly different. Future research must be directed to studying, and if necessary, reducing the effects of the internal recirculation on the activities and people shielded by the dome.

3.0 SCAlING lAWS

If the air dome is to be used to protect large areas - typically lOOm diameter - then the concept must be demonstrated with the use of small-scale 1 aboratory and outdoor model s. In order to confidently compare model s of various sizes and predict performance for full-scale facilities, the cases in question must all be dynamically similar. Formal dimensional analyses have been done10,11 which led to the dimensionless parameters that must be matched from case to case for dynamic similarity.

The first approach is based on a formal dimensional analysis involving the nine physical variables associated with particle-jet interactions: 10

These variables are th us reduced to the foll owi ng six non-dimensi onal parameters: p V· _J t d V ·tp _{J _J_.} V.2

- ,

_W'

'0' '0'

- - ,

Pp \..L Dg

The fact that it is impossible to simulate all of these parameters simultaneously in air is clearly demonstrated if we consider the following three: t/O, Vjtp/\..L and V_j 2/0g • As model size 0 increases, in order to keep t/O and V j 2/0g constant, it is obvi ous that both tand V j must increase; however, this conflicts with the requirement for Vjtp/\..L to be constant. This argument then leads to the conclusion that total dynamic simi 1 arity for the compl ete set of parameters consi dered is essenti a lly impossible to achieve.

An al ternati ve approach suggested by Lake and Etki n I 0 emphasi zes the particle dynamics as the particle encounters an arbitrary flow field. This approach yields the following non-dimensional equations of motion:

where and dû

=

'!t (û Û 1 )

v

dl

-

1

-dv

_{= -'!tl v-v}

("

"I)" _v df dQ

=

-'!tI(Q-Q')~-'!t2

dt

(la) (lb) (Ic) (2 ) (3 )

To reduce the complexity of the '!tI parameter, previous analyses involved the use of a simplifying assumption, namely that the particle was falling in still air at its terminal velocity. This resulted in '!tI being simpl ified to

'!tI

=

_{gO/V t 2} (4 )

So that the third equation of motion (Ic) became

d~

=

-'!tl

f(w-W1)V

+ ( Vt )2 ]

dt

~

Vj

(5)

Two seemingly important facts follow from such an analysis: (1) terminal velocity is scaled with jet exit velocity (i.e., Vt/Vj is a constant); and (2) '!tI has the form of a Froude number and hence is an important scaling parameter. It must be stressed that thi s i s not a true Froude number, but nevertheless in this report it shall be referred to as a Fraude number.

This Froude number allowed for the determination of the precipitation material and drop/paraticle size for any model based on a full-scale rainfall. lake and Etkin originallylO used water from a shower head to simulate rain. This was done mainly for convenience \'lÎth the implications of the decision being made af ter the tests. In their later tests,ll they used glass beads as the simul ation material in order to achieve proper dynami c scal i ng of the Froude number. Proper Froude number scal i ng was thought to be more important than other factors such as particle Reynolds number scaling and raindrop deformation and breakup (which cannot be obtained using solid particles). However, later work by Raimondo and Haasz7 on numerical simulations of particle dynamics in linear jet air-curtains showed a significant Reynolds number dependence. In particular, they found that under the influence of the jet a particle may experience a relative velocity much greater than its terminal velocity and therefore the drag must be eval uated for the Reynol ds number based on particl e velocity. Consequently, the drag and resulting trajectories are different from those expected fr om the simplified equations (4) and (5).

Hence, the method of scaling and the selection of the simulation materi al requi res further consi derati on, whi ch has been undertaken in the present study. Goi ng back to the compl ete non-dimensi ona 1 equati ons of motion, the two dimensionless parameters 'Ttl. and 'Tt₂ need to be conserved for dynamic similarity. For a spherical particIe, 'Ttl can be written as

=

3 C p D

'Tt 1 - D

-4 P_p d

Clearly the problem of maintaining 'Tt I constant arise through the drag coefficient. There is expressing CD that wil 1 simplify the analysis. important previous experiments as wel 1 as the Reynolds numbers, we shall consider CD = CD (Re)

(6)

for different scal es wi 11 no one analytic function However, since most of the present tests involve low in such a flow regime.

For Re < 1,000 the CD curve can be expressed as a series of piecewise functions of the form

CD

=

kl/Ren (7)

For example, for Stoke~ _{flow (Re<l), kl} = 24 and n = 1, so

CD

=

24/Re (8)

Of course the particle in the presence of a jet will experience a changing relative velocity, Vp' and hence a changing Reynolds number which wil 1 alter the drag coefficient over time. The Reynolds number for substitution into equation (7) is therefore

(9 )

Thus 'Ttl becomes

The Vt/V j term was i ntroduced here to al 1 ow for di rect compari sons wi th the simplif;ed analysis (equat;ons 4 and 5). Equation (10) can be rewritten as:

(11 )

or

nl

=

_{n 3 F(Re p ' Ret' Rej' 0, t, d}} (12 )

where = V j P

n3 - V t Pp

(13)

The new dimensionless parameter n₃ is important because it contains the

relationship generated from the simplified analysis with an additional term which can account for different material s used to s;mul ate ra;n. Al so important to note is that the funct;on F impl icitly contains the jet Reynolds number as well as the particle Reynolds numbers based on actual particle velocity and terminal velocity.

Thus the non-d;mens;onal parameter nl obtained fr om the equation of

mot;on has been expressed in terms of the new parameter n3 ~nd the function

F. The present study is aimed at investigating the benaviour of n3 for

different experimental scaling conditions. 4.0 FLOW CHARACTERISTICS OF THE 6m MODEL

4.1 Initial Circumferential Velocity Profile

The 6m model was des; gned so that the ai r for the annul ar jet could be provided by the exhaust ai r flow of the open ci rcuit UTIAS anechoic w; nd tunnel. The tunnel fan is driven by a 100 MW (150 HP) motor and has a throughput capacity of about 50 m3_{/s ( .... 100,000 cfm). Flow variation is}

achieved by adjusting the pitch angles on the variable pitch fan blades. The model consists of several stages (see Figs. 4 and 5). Attached to the exhaust port of the fan is a diffuser, followed by a circul ar-to-square cross section transiti on pi ece, an angul ar transiti on pi ece, a sl opi ng diffuser section, the plenum chamber and the annular nozzle. The latter two elements are the important ones in achieving a stable jet.

Since only one fan was available to drive the system, the plenum had to be constructed to efficiently manoeuvre the flow so that the air leaving the annul ar nozzl e was ci rcumferenti a lly uniform. Although an enclosed and protected area can be achieved without such uniformity, it is desirable in order to ensure symmetry and a final vertical single jet. The premise for this initial plenum design (see Fig. 6) was that the air leaving the fan would spil 1 into the lower level, then be forced out up through the center port and evenly spread rad;ally outwards toward the nozzle.

This design resulted in the circumferential velocity profiles that are shown in Figure 7. These profiles, taken with the fan blades in positions 1

and 2, show that the exiti ng ai r had the hi ghest velocity at the stati ons opposite to the entry point of the flow. Obviously, a significant amount of the fiow was simply travelling straight through from the lower level to the upper 1 evel •

4.2 Model Modifications

In order to overcome the non-uniformity problem, the plenum was modifi ed by i nserti ng baffl es in the lower 1 evel at the centermost structural supports, to redirect the flow. Figure 8 shows in detail an isometric view of the first baffle configuration while Figure 9

(configuration 1) shows the same when laid out flat. The resulting velocity distribution is shown in Figure IOa. As can be seen the profile is now the opposite of the original in that the faster flow was exiting at the stations directly above where the air entered the lower plenum. This must have been caused by the baffi es defl ecti ng the incomi ng ai r to the downstream end of the plenum and subsequently forcing it back and up resulting in a similar straight through condition but this time in the reverse direction.

In order to achi eve a reasonabl e ci rcumferenti al velocity profil e, vari ous baffl e confi gurati ons were tried (see Fi gure 9) with correspondi ng velocity profiles in Figures IOa-h.

The important modification in baffle configuration 9 is the addition of baffles in the upper level across stations 6-7 and 26-27 to eliminate velocity peaks. Although this resulted in velocity valleys, see Figure IOh, i t was fel t that the uniformity was now suffi cient for the purposes of the experiment. Thus, baffl e confi gurati on 9, shown in Fi gure 9, was used for all subsequent work.

The sixteen flow fins and sixteen structural supports, shown in Figure 5, allowed the annular nozzle to be constructed of thrity-two straight sections. Thus the nozzle is not circular, but a 32-gon, which is a very good approximation of a circle over the I8.85m circumference.

The original nozzle waS simply made from sheet metal strips that were 30cm wide and about 60cm long. The sheet metal was bent up from the hor.izontal by about 72° along a length of I2.5cm from the edge. These d imensi ons were chosen to yi el d a nozzl e thi ckness-to-nozzl e hei ght (t/h) rati 0 of about uni ty. A nozzl e thi ckness-to-annul us di ameter rati 0 of 3%

was selected in order to be similar to earlier work;12,13 this resulted in a nominal jet nozzle exit thickness of I8cm.

4.3 Formation of the Air-curtain Dome

The integrity of the annular jet was investigated in the following ways. Fi rst, sawdust was thrown into the jet at various 1 ocati ons and the particle trajectories were observed. In all cases the sawdust was entrained by the jet, carried upwards and curved towards the axi s, just as woul d be expected from the pattern of Fi gures 1 and 2. The second method invol ved flow visualization with the use of wool tufts attached to wires strung across the dome at various heights in a pl ane through the center and

perpendi cul ar to the fl oor. It was cl early evident that when the jet was turned on the tufts above the nozzle exit were pointing straight up, and started to bend inwards with increasing height, providing visual evidence of the formation and expected behaviour of the dome.

A further study was done by measuring the jet speed at several locations in the curtain jet using a pitot tube. The pitot tube was placed at the approximate center-l ine of the inwardly curving jet by noting the tuft deflections and then oriented to record the maximum pressure difference on the manometer so that the direction of the maximum speed of the jet could be determined. A plot consisting of four points is shown in Figure 11.

4.4 Radial Velocity Profiles

Idea11y the desired radial velocity profile across the nozzle is that of a plane two-dimensional jet, schematica11y shown in Figure 12a. The expected exit velocity of such a two-dimensional jet is uniform in speed and direction. Ra1i!1 velocity profiles measured in a parallel study by Diamant and MacKenzie on a 60cm mini-model of the 6m outdoor model were found to have the shape shown in Figure 12b, which is similar to the expected shape in Figure 12a. Radial profiles taken for the 6m model, however, were found to be completely different as shown by three representative profiles in Figure 13. These results give rise to two questions: what is the cause of such a velocity profile and what is the effect, if any, of such a profile on the overall performance of the air curtain jet?

The answer to the first question is simple. The flow to the nozzle is essenti a 11y hori zonta 1 and must qui ckl y turn through 90° to èxit. The hori zontal momentum carries the flow so that most of it fo" ows the outer edge of the nozzle (Figure 14). There is also flow separation at the base of the inner nozzle which creates losses.

The answer to the second question is more difficult. Baines and Wong12 had radial velocity profiles similar to that se en in Figure 13a, yet they were able to provide protection from precipitation. In terms of protecti on then, i t does not seem to matter very much. But thi s deviati on from the ideal profile does change the effective t/D ratio, which affects the energy requirements of the system.12 For a given model size and preci pitati on, the criti cal velocity necessary to stop that preci pita ti on depends on t/D, and thus, one can expect a negat i ve effect on the system

pe r fo rmance. .

To improve the radial velocity profiles, two modifications were conceived. The first involved extending the height of the nozzle to give the flow a chance to become more fully devel oped. It was arbitrari ly decided to make t/h = 1/3. The second modification involved placing 16-mesh screen i ng over the ci rcul ar annul us before the thi rty-two nozzl e secti ons were repositi oned. It was hoped that the screeni ng woul d reduce the separation at the inner edge. Unfortunately, all the screens did was load the fan to an over heat condition causing a circuit breaker to be tripped and hence stop running af ter about twenty minutes time. Thus the added nozzle height was to be the only usable modification. The resulting profiles for four stations are shown in Figure 15; no improvement was noted as aresul t of the modification.

A quick check showed that the circumferential velocity profile with this new nozzle configurtion was not significantly different from the

previous case. The system was now ready for the preci pi tati on/protecti on

s tudy.

4.5 Velocity Measurement Techniques

The velocity measurements of the jet on this scale were both difficult and tedious. The original circumferential velocity profiles were made with a hand held anemometer. Due to the highly turbulent nature of the jet there was no way to determine a reading error for this device as the reading was

constantly changing by as much as ±3m/s. The advantage was that readi ngs

could be taken quickly to give an indication of speeds and problem areas. Also, since the same instrument was used for the entire set of results, consistency was maintained and relative improvements could easily be observed, which was all that was needed at the initial stage of the investigation.

To record the radial velocity profiles across a nozzle section, a hot

wire was used at discrete interval s. The turbulence caused the voltage

readings to fl uctuate rapidly and the val ues were visually averaged over a

short period of time. The advantage of the hot wire was the confidence

level of the absolute velocities recorded as this was becoming more

important to the investigation. A disadvantage though was the tediousness

of the dismantling of the traverse machanism from station to station.

Finally, the last set of radial profiles was measured with a pitot

tube. Si nce the flow was coming strai ght up, the positi oni ng of the tube

was not difficult. The advantage over the hot wire was the slower response

time of the Betz manometer used to record the pressure difference. The

manometer did not record the fluctuations due to the turbulence, so it was easier to determine an average exit speed.

The velocity measurements in the curved portion of the jet above the

exit plane (Figure 11) were very difficult for two reasons. The

determination of the direction of the jet at heights well above the nozzle was difficult just by the awkwardness of manipulating the pitot tube through

manyangles. Also, any side wind interaction would cause the jet to bend

with a corresponding change in manometer reading. Nevertheless, with great care the results were found to be repeatable in still air conditions which

assures the accuracy and reliability of the results. In order to obtain

highly accurate and precise readings on large outdoor model s, it will be necessary to devise some new method of velocity sensing.

5.0 PRECIPITATION EXPERIMENTS WITH THE 6m MODEL 5.1 Sealing Requirements

In the first instance we must determine the type of rainfall (as rainfall rate control s the water drop size distribution) to be simulated

and then the model characteristics and requirements. It was shown in

section 3 that among various non-dimensional groups of parameters associated with air jet-water drop interactions, the most important ones appear to be:

t/D

1t₂

=

_{Dg/V j} 2

1tl

=

1tl(Rep,Ret, Rej' D, t, d)

where 1t3 = (Vj/Vt).( p/Pp'). Since one of the objectives of this

investigation is to assess the effect of the function F on the air dome, we shall, for the purposes of sel ecti ng the simul ati on materi al assume F to be

a constant. This resul ts in 1t3 replacing 1tl as one of the three scal ing

parameters.

Maintaining t/D constant in the model and full-scale cases results in

s imil ar jet flow confi gurati ons for the two cases. The constancy of 1t₂

establishes the relationship between linear and velocity sealing ratios. For examlple, if a full-scale installation is 16 times the size of the

model, as is the case in this experiment (i.e., Dfs

=

16Dm) , then the

jet velocity for the full-scale will be 4 times that for the model (i.e.,

Vj

=

4V j ). The parameter 1t3 in turn establishes the relationship between

th~Sjet ve~ocity

and the partiele terminal velocity. For model experiments using water as the simulation material the requirement for 1t3 to be constant

s imply reduces to V j/Vt bei ng a constant in both full-scal e and model

si tuati ons.

Simulation of a medium rainfall rate (~lnm/hour) with a characteristic

water drop diameter of about 2nm (corresponding to Vt

=

6.5m/s), and using

a linear sealing ratio of Dfs/Dm

=

16, requires a Vt of about 1.6m/s

for the simulating water drops. The corresponding characteristic water drop

diameter required for model testing is about 400~. The production of this

drop size was achieved with the use of a SPRACO full cone cluster nozzle

(15151014). Analysis of the drop size distribution yielded an average

diameter of ~380!lffi with a standard deviation of 140!lffi, see Appendix A. The

corresponding mean Vt is about 1.5m/s.

5.2 Production and Characteristics of Water Drops

The choice of water drops to simulate rainfall seemed appropriate for

two reasons: 1) it is the same substance and 2) there are a variety of

atomizing nozzles which are commercially available to produce almost any

des i red si ze range of drops. However, the choi ce of water drops does

introduce some difficulties as well. Full-scale raindrops of the order of

1-3nm diameter, deform and may experience breakup in the presence of an air

jet. Such events are not only difficul t to model experimentally but al so

present difficulties in the anlaysis of the full-scale case. The 380~

drops used in this investigation do deform, albeit less than full-size

drops, but they do not break up. The breakup of large raindrops into

smaller drops' with smaller terminal veloeities reduces the workload of the

lower portion of the jet, leading to the requirement of a reduced jet exit velocity. On the other hand, with no drop breakup in the case of the model, the jet must handle the simulated large drops requiring higher veloeities

than would be necessary in the presence of proper simulation of drop breakup.

For modelling purposes in order to produce a fine spray of the desired small size, a pressurized water line is needed as the degree of atomization is, in part, a function of the pressure. The finer the mist, the higher the pressure and hence a greater water flow for a given nozzle. This means that the simul ated rainfall rate from a parti cul ar nozzl e increases wi th decreasing size of drops produced, which is exactly the opposite of what happens in nature. The steady, all day, low rate type of rain is associated wi th smal 1 drops whi 1 e the heavy downpours wi th 1 arge rates are compri sed of large drops. This affects the drop concentration per unit volume of air which could result in different drop interaction effects. The quantitative effects associ ated with the full-scal e and the model concentrati ons are extremely complex and require statistical information which is not currently obtainable for the experimental models. Hence, the effects of different and changing concentrations are not explicitly known.

Now we consi der the source of the drops used in thi sexperiment. By observation it could be se en that the exit speeds of the drops at the water nozzle were greater than their terminal velocities due to the water line pressure. Thus the air jet velocity required to stop the penetration of these "faster" drops is bound to be "too high", leading to somewhat conservative results.

Another effect that could lead to conservative results is associated with the fact that the water drops in the model tests were introduced at a height of only 6m above the floor. Since the jet rises well above this point, a portion of the jet is not utilized for the slowing down process of the drops as would be the case in a full-scale facility.

In order to achieve a wide spray pattern for the tests, three nozzles were empl oyed. They were positi oned along a di ameter of the annul us such that one nozzl e was directlyon the axis of the annul ar jet and one on either side at a di stance of 1.8 meters from the axis. The height of the nozzles above the floor was about 6m.

Water was suppl ied to the nozzl es through ordinary hal f-inch diameter garden hoses which were hooked up to a standard water 1 ine with nominal pressure of about 550 kPa (80 psi). The corresponding flow through the nozzl es was about 3.4 1 i tres per minute.

The normal method of testing involved the determination of the quantity of water reaching the floor in a given time with the jet off and then with the jet on to see how much protection was achieved. With the outdoor facil ity this was not an easy task. First of all, side winds were a problem. Since the terminal velocity of the 380f..1m drops is of the order of 1.5

mis,

a side wind of that magnitude (3.5 mph

=

3 knots) is enough to cause the spray from the nozzl e to fall at a 45° angl e so that most of the spray does not even land within the area bounded by the annulus. The second problem was the efficient collection of the spray that did reach the floor.

At a water flow rate of 3.4 litres/minute, assuming a uniform coverage

of the area bounded by the nozzle (~25m2), the expected depth of water af ter

one minute is only about 150~. Consequently, for measurable quantities of

water on the floor long collection times are needed. In addition to the

difficulty of rnaintaining stable experimental conditions for long times, long collection times also pose an evaporation problem, especially for small drops. This problem is worse with the jet turned on since drops which reach the floor are subj ect to a greater rate of evaporati on caused by the recirculation flow.

Another problem, caused by the presence of side winds, is the wind interaction with the annular jet itself, causing the jet to bend in the

direction of the wind. As has been shown by Lake and Etkin,ll this bending

of the jet results in a significant 10ss in the ability to stop

precipitation, and if the wind is strong enough, the annular jet can

compl etely coll apse. Both of these cases were at one time or another

observed for our 6m model. As the maximum exit velocity of the jet was only 11 mis, the presence of steady side winds and the inevitable gusts made it nearly impossible to study fully quantitatively the performance of this air

roof. Fortunately there is a daily period when one can expect little or no

side winds. This occurs in the early morning, so all tests were therefore

carried out during the period from just before sunrise until about an hour and a half af ter.

5.3 Determination of Critical Jet Velocity

Even though the tests were performed in the near absence of winds when the water drops from the spray nozzle were falling nearly straight down,

quantitative measurements were still not possible. Instead a "qua litatively

quantitati veil estimate was made in the fo" owing manner. Wi th the jet off,

an observer coul d "determine" the quanti ty of water fall ing on an exposed

arm and eye gl asses over abri ef peri od of time. Thi s was then repeated

with the jet on so that comparisons could be made.

Photography was also used to document the protection. Many photographs from different vantage points were taken. Good photographs were obtained by standing on either side of the annular nozzle and 100king straight up; the

pictures show the . upward moving spray. The best photographs were the

sequence shots, taken with a 2.5 frames per second motor drive, showing the spray first coming down with the jet off, then being blown upward when the jet is turned on. Figure 16 shows such a typical sequence for the case when

the jet exit velocity was 7.5 mis (the time between photos is 5 s).

The exit speed of the jet was varied by changing the pitch angle of the

nine fan blades. Tests were performed with the blades in positions -1,0,1

and 3. The corresponding average jet exit velocities were 3.5, 5.5, 7.5 and 11 mIs, respectively.

At the 10west speed setting (blade position -1, Vj

=

3.5 mis) with

the presence of even the smallest of wind speeds, there was at least 50%

penetration. On one brief occasion when there was absolutely no side wind

present, almost 100% protection efficiency was achieved. This confirmed

Lake and Etkin's resultsl l _{regarding the possible catastrophic effects of a}

V· = 5.5 mis) the no-wind case yielded a protection efficiency of about 90-95%. With slight side winds, the upstream area was completely protected but the downstream end suffered 20-30% penetration.

For the two remaining cases, i .e., jet exit velocities of 7.5 and 11

mis, there was very 1 ittl e di fference. The only time that penetrati on was noticed coincided with a gust of side wind. Complete no-wind protection was achieved and only minimal penetration occurred (5-10%) in the presence of slight side winds. The only real difference of course was that for the higher speed setting the side winds had to be somewhat stronger to cause the same amount of performance deterioration as observed at the lower speed case.

Regarding the effect of side winds, it is important to no te that the wi nds were due to real full-scal e atmospheric air movements , whi 1 e the annul ar model jet velocities were scaled down by a factor of foue (the velocity scale factor). Thus, for full-scale installations, where the jet velocities will be considerably higher than in the model, the effect of side winds is expected to be less significant.

One of the goals of this experiment was to find the critical jet velocity needed to "stop" precipitation from penetrating the air-curtain dome. For the purpose of this study, we have defined the critical velocity,

V_{c '} to be the j et velocity that results in 95% protecti on effi ciency (i.e., 95% of the precipitation is prevented from penetrating the air dome). A discussion of the present results and comparisons with previous experiments will follow.

6.0 PRECIPITATION EXPERIMENTS WITH THE 60cm MODEL 6.1 Water Drop Experiments

The 60cm model studied was the one previously used by Baines and Wong12 and Diamant and MacKenzie13 (Fig.l7). This model was originally built to study the pressure losses in the system in order to determine how well the 6m outdoor model would perform. Once the 60cm model gave acceptable pressure loss resul ts, the des; gn of the 6m model was fi nal i zed. Thus, originally the 60cm and 6m models were identical except that the larger model contained more structural supports in the plenum.

Once testing began on the performance of the 60cm model to determine lts abil ities to provide protection as an air roof, a number of modifications sirnilar to those needed on the larger model (§4.2) were required. The evolution of the changes through two separate investigations are documented12 ,13 and the result was that the two models were no longer identical. Each had its own baffle configuration to provide the desired flow. Nevertheless the two models were similar enough so that the only essential difference was the factor of ten size difference.

The size of the water drops needed to simulate full-scale rain was determined by following the procedure discussed in section 5.1. That is, in order to keep 1t

3 , and in the case of water drops, Vj/Vt constant, the reduction of model size,

Dm,

by a factor of ten implies a corresponding

decrease in V_t by a factor of 110

=

3.16. Thus, the required water drop terminal veloclty for the 60cm model was about 0.5m/s, corresponding to a characteristic drop diameter of about 150~m.

In order to achieve this small size, a water nozzle such as employed on the 6m model could not be used. Instead a pneumatic nozzle using air to atomize the water was required. SPRACO nozzle number 3804456 was obtained to supply these smal 1 drops. Unfortunately, the average drop diameter produced was about 40~m (Vt

=

0.05 mis).

Nevertheless it was decided to test the 60cm model air curtain using these smal 1 water drops. The water nozzl e was mounted about 3.5m above the

fl oor of the model and poi nted strai ght down. A 95% protecti on effi ciency was achieved with an average circumferential exit velocity of 7.5 mis (i.e., Vc

=

7.5 mis). Thi s was totaly unexpected as it was inconsi stent with the 6m case which required the same jet exit velocity for similar protection.

There were two major causes for this anomaly. First of all, the air leaving the pneumatic nozzle was supersonic which resulted in water drop vel oci ti es bei ng greater than Vt. Consequently, the drops requi red some time and distance to decelerate to Vt before interacting with the jet. Without an initial condition for the water drop velocity, the required time and distance to reach Vt could not be calculated. Visual observations, however, indicated that a height of more than 3.5m was needed. Secondly, the spray was of a very high concentration in terms of droplets per unit volume which could have resulted in drop interaction due to wake effects, causing drops to fall in groups instead of individually. The terminal velocity of such groups is larger than that associated with single droplets, requiring higher Vc •

An attempt to simul taneously overcome both of these probl ems was made by pointing the nozzle upwards from its 3.5m height. By arcing the spray up and over, the droplets would disperse more and fall individually at their proper termi nal vel oc iti es. Unfortunately, the spray only di spersed and never made it back down to the model, making testing impossible.

A single fullcone water-only nozzle (SPRACO 11062004) was available that produced droplets of the order of 250~m (Vt = 1m/s). This was tried ih the "up and over" mode. At the top jet speed available, 7.5 mis, the average protection efficiency over the enti re floor was only 70%. The center of the floor had about 55% penetration while the edges had only about 10%, see discussion below. Once again, the critical velocity for the 60cm model turned out to be greater than that for the 1 arger 6m model. The anomaly in this case could again be attributed to particle interaction effects. With a high density of drops the possibility of coalescence exists when upwardly deflected drops collide with those still on their way down. The resulting larger drops wil 1 have a higher terminal velocity which would require a faster jet speed to prevent penetration through the dome.

Thus the conventional methods for producing the required small droplets could not achieve the necessary conditions for successful simulation.

Finallya few words are in order to exp1ain the varying penetration

rates over the f100r area. Hi gher penetrati on was observed in the center

with decreasing penetration occurring as one moved radia11y outward. There are two mai n reasons for thi s. Fi rst 1y, any drop penetrati ng the enc10sure wou1d encounter the recircu1ation region where it cou1d easi1y be entrained to be deposited in the center by the local downward velocity of the vortex. The second reason has to do with the jet velocity flow field encountered by the drop during its stay in the jet. The relative1y high jet veloeities in the vicinity of the nozzle will tend to move the drops upward a10ng the curved jet envelope either toward the outside or inside jet boundary. Thus, drops which are not ejected outward from the jet wil1 like1y enter the dome in the centre region, away from the nozzle. Similar behaviour was observed for glass beads, see disucssion bel ow.

6.2 Glass Bead Exeriments

In order to obtain useful small-sca1e results either of two things must be done: find a method of producing very fine water drops without the above mentioned problems or abandon water drop simu1ation in favour of glass beads

whi ch were extensi vely used in previous ai r-curtai n experiments. Although

glass beads were found to have 1imitations for particle trajectory

simulations associated with 1inear jets7 _{their app1icability for annu1ar jet}

simu1ations has not been ru1ed out so faro Therefore, glass bead tests were

undertaken with the following three objectives: (1) to test the

reproducibi1ity of earl ier experiments11 _{with glass beads, (2) to show}

whether or not partieles with a terminal velocity of about 0.5m/s (the

desired va1ue based on Vj/Vt being constant) cou1d be stopped by a 60cm

annular air-curtain whose highest jet exit speed was 7.5 mis, and (3) most

important1y, since the parameter 1t3 contains a term to account for the

density of the simu1ation material , this test cou1d provide usefu1

information regarding the val idity of 1t3 as a sca1 ing parameter. We have

proposed in section 3 that 1t3 should replace the Vj/Vt parameter widely

used in previous annu1ar air-curtain studies. The glass beads, from

Microbeads Division of Cataphote Corp., Toledo, Ohio, were in the size range

74-105~m and had a specific gravity of 2.39 (sample 1420, class 4B). A feed

system was improvised that neither ensured that the beads wou1d slow down to terminal velocity before encountering the jet nor that the beads were adequate1y separated to prevent wake interaction. Thus any resu1ts wou1d be

conservative. The experiments performed with these beads, at a jet exit

velocity of 7.5 mis, yielded an averape penetration of on1y about 2% which

was consi stent with previous results • 1 Thus, the fi rst two objectives of

the test were met with positi ve results • The fi na1 matter of checki ng the

va1idity of 1t3 as the relevant sealing parameter will be discussed below.

7.0 COMPARISON OF PRESENT AND PREVIOUS RESULTS

This chapter summarizes all the important resu1ts from the present and previous experiments in order that comparisons and conc1 usions may be made

based on the new general sealing 1aws. Tab1e 1 1ists the important

parameters and resu1ts from Lake and Etkin's water droplO and glass

beadll _{experiments, Baines and Wong's water drop tests}12 _{and the current}

work for both water and gl ass beads. For ease of compari son all resu1 ts

not directly stated anywhere in the respective reports, were either inferred from graphs or calculated from parameters that were explicitly stated.

The drop or particle Reynolds number in Table 1 is based on terminal velocity Vt • This Ret was calculated in order to determine Reynolds number for the drop as it fi rst encounters the jet. Thus, was computed as where and Re t p

=

1. 225 kg/m 3 ~

=

1.789 x 10-5 kg/(m s).

The quantity 1t3 was defi ned in secti on 3 as: 1t 3

=

(V C) (L) V_t P_p (17) the the Ret

Si nce pis essenti a lly a constant for simul ati on in ai rand Pp can be expressed as a particle specific gravity (SG) times the density of water, 1t3 can be rewritten as:

1t

=

(VC )

-.l

3 V

t SG

(18)

This 1t 3. is plotted as a function of the particle Reylnolds number in Figure

18. Neglecting the Lake and Etkin lO water tests because of the high Reynolds numbers, a horizontal straight line may be drawn through the data indicating that 1t3 is essentially independent of Reynolds number based on parti cl e termi nal velocity. Thi s has the very important impl i cati on that the function F(Re , Ret' Re·, 0, t, d) in equation (13) has remained essenti a lly constanE for the J various simul ati ons. Thi s confi rms the significance of 1t3 as the controlling scaling parameter, replacing 1tl.

Fi gure 19 shows 1t3 as a function of annul us diameter and again it appears that 1t3 i s i ndependent of model si ze, at 1 east for the si ze range tested so far (i.e. Dm<6m). This can be tied in with the Reynolds number independence. The model si zes used so far have necessarily demanded small water drops and hence low Reynolds numbers. This leads to a plot of Re as a function of annulus diameter shown in Fig. 20. Intuitively it is clear that a larger diameter model would require a larger jet velocity to maintain dynamic simil arity, and in fact from the equati ons of moti on arose the parameter 1t2 = Dg/V·2 which dictates how jet speed varies with model size. This greater jet ve10city would then be able to deflect larger drops which have greater Reynolds numbers. Hence, the Re-D curve should start near the origin and rise up and to the right, asymptotically approaching the real

rain's Reynolds numbers as model size increases. Figure 20 shows that this has not occurred, whi ch means that the individual tests simul ate di fferent rain drop sizes. In fact Lake and Etkin's water testslO used drops of the size of real rain but on a very small model.

Therefore, one must exercise caution in interpreting such a mixed variety of resul ts. These resul ts from all sources not only have not simul ated the same rainfall, but al so have not had dynamically simil ar annul ar jets due to various t/D ratios. However, the trends noted in Fi gures 18 and 19 indicate that the performance (namely, the 1t needed to achieve 95% protection efficiency) of the air-curtain dome is not seriously affected by the disparities among the various simulations.

8.0 CONCLUS IONS

The major objective of the present investigation was to verify the validity of the air-curtain sealing laws by extending previous small-scale laboratory simulations to the testing of a 6m diameter outdoor air-curtain dome facility. In addition to the successful testing of the 6m model with the use of water drops, tests were performed with a 60cm diameter small-scale version of the 6m model.

Analysis of the current experimental results as well as all previous air-curtain dome results led to the identification of a new sealing parameter, 1t3

=

Vc/(Vt SG), which plays an important role in determining

the critical jet velocity needed to provide a certain level of protection effi c iency. Thus 1t3 together wi tht t/D and 1t

2

=

Dg/V j 2 are the parameters

that need to be kept constant in model and full-scale sltuations. The ratio t/D assures kinematic similarity for the jets, 1t₂ establishes the

relationship between linear and velocity scaling, and 1t3 determines protection efficiency.

Di reet compari sons of our resul ts for the 6m model usi ng water drops and the 60cm model using glass beads yielded excellent agreement for 1t3.

Further compari sons wi th previously publ i shed resul ts al so yielded good agreement for 1t3 even though case-to-case variations of t/D, partiele Re, etc., were present. This implies that, at least for the particle Reynolds number and annulus diameter ranges considered, the effect of such variations .

does not significantly affect air-curtain performance.

With a major assumption that the validity of the three sealing parameters (t/D, 1tZ and 1t₃) could be extended to full-scale air curtain installations, we shall now attempt to make certain projections for a lOOm diameter air dome. Maintaining t/D

=

3%, as was the case in the present experiments, 1 eads to a jet thi ckness of 3m. For our 6m model experiments with water (SG = _{1) Vc ' Vt and} 1t3 were 7.5 mis, 1.5

mis

and 5.0,

respectively. Assuming a characteristic raindrop diameter of about 2mm with Vt

=

6.5 mis (this corresponds to a medium rainfall rate of about 1mm/hour), we obtain a critical jet velocity of about 33 mis. The corresponding air flow (Q) and power (P) requirements are 31,000 m3/s (66 Mcfm) and 10,000 kW (113,000 HP), respectively. Similar calculations for a heavy rainfall, with drops of about 3mm diameter, yield the following projections: Vt

=

8

mis,

_Vc = 40

mis,

Q

=

38,000 m3/s (80 Mcfm), and

9.0 ~EFERENCES

1. B. Etkin and P. L. E. Goering, Air-curtain walls and roofs - dynamic structures, Phil. Trans. Roy. Soc. London, A269 (1971) 527-543.

2. B. Etkin, Interaction of precipitation with complex flows, Proc. 3rd Int. Conf. on Wind Effects on Structures, Tokyo, Japan, Aug. 1971. 3. G. A. S. Allen, Experimental investigation of an air-curtain for

protection of an outdoor power installation from salt spray, UTIAS Technical Note No. 171, University of Toronto, Toronto, Canada, 1971. 4. G. A. S. Allen and R. T. Lake, Trajectories of raindrops in a jet

issuing into anormal crosswind, UTIAS Technical Note No. 165, University of Toronto, Toronto, Canada, 1971.

5. A. A. Haasz, B. Etkin, R. T. Lake and P. L. E. Goering, Laboratory simulation of an air-curtain roof for the Ontario Science Centre, UTIAS Technical Note No. 192, University of Toronto, Toronto, Canada, 1975. 6. A. A. Haasz and P. L. E. Goering, Intermittent enclosures - air-curtain

walls and roofs, Proc. IASS World Congress on Space Enclosures, July 1976, Montreal, Quebec, Canada, WCOSE-76, 1 (1976) 151-163.

7. S. Raimondo and A. A. Haasz, Single and dual air-curtain jets used as protection against precipitation, UTIAS Rep. No. 227, University of Toronto, Toronto, Canada, 1978.

8. A. A. Haasz and S. Raimondo, Effectiveness of an air-curtain canopy against precipitation, J. Wind Eng. and Ind. Aerodynamics, 6 (1980) 273-290.

9. A. A. Haasz and air-curtain roofs, 79-87.

S. Raimondo, Performance of adjacent dual-jet J. Wind Eng. and Ind. Aerodynamics, 10 (1982) 10. R. T. Lake and B. Etkin, The penetration of rain through an annular air-curtain dome, UTIAS Rep. No. 163, University of Toronto, Toronto, Canada, 1971.

11. R. T. Lake and B. Etkin, Experimental simulation of the interaction of wind-driven precipitation with an annular air-curtain dome, UTIAS Technical Note No. 182, University of Toronto, Toronto, Canada, 1973. 12. A. Parker and J. Wong, The penetration of rain through a one-meter

diameter annular air-curtain dome, AER304S Report, University of Toronto, Toronto, Canada, 1981. Also, P. Baines and J. Wong, Annular ai r curtai n study, AER408F Report, University of Toronto, Toronto, Canada, 1981.

13. J. Diamant and C. MacKenzie, Investigation of velocity profiles of an annul ar ai r-curta in, AER304S Report, Universi ty of Toronto, Toronto, Canada, 1982.

14. Gabriel D. Boehler, Aerodynamic theory of the annular jet, lnstitute of the Aeronautical Sciences, New Vork, N. V., lAS Report No. 59-77, 1959. 15. H. R. Chaplin, Effect of jet mixing on the annular jet, Navy Department

David W. Taylor Model Basin Jlerodynamics Labortory, Washington, O.C., Aero Report 953, 1959.

16. N. A. Chigier and J. M. Beér, The flow region near the nozzle in double concentric jets, Transactions of the ASME, Journalof Basic Engineering, Dec. 1964.

17. T. W. Davies and J. M. Beér, The turbulence chëlracteristics of annular wake flow, 1969.

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Fig. 4a Side view of 6m diameter annualar air-curtain test facility. (1) Circular-square transition piece, (2) Angular transition piece, (3) Diffuser section, (4) Plenum, (5) Annular nozzle.

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