mojet an improved jetfan

Road tunnels are often ventilatedby inductive systems withbooster fans, also known as jetfans, installed near the tunnel

ceiling. A brief summary of physical data isnecessary to better understand the scopeof this paper.

The theoretical impulse force for a jet fan,thrust, can be calculated as follows:

( 1 )

In practice a jet fan experiences lossesand so will not develop the full theoreticalthrust force.

The actual thrust can only be found bymeasurement. Action and reaction areequal and opposite so the thrust providedto the air can be evaluated by measuringthe reactive force on the fan, as specified inISO 13350. The difference betweentheoretical and real thrust ranges from 0.85to 1.05 (values greater than one happen insome cases where a high hub ratiogenerates a remarkable annulus velocity,

affecting outlet velocity). The authors warnthe user on calculating the thrust with theabove formula. It is recommended tomeasure the thrust and then calculate theVeff (effective velocity) from the appropriateformula [1].

Since the flow rate in the tunnel will notbe zero, formula (1) must be rewritten in thefollowing way :

( 2 )

Manipulating this formula

(multiplying by )

and solving the equation forwe have:

( 3 )

In order to free as much of the tunnel fortraffic as possible, booster fans are often

58

An improved

Tunnels & Tunnelling International JULY 2012

TECHNICAL | FAN DEVELOPMENT

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jet fanThis paper presented by Harald Rudelgass and Carlo Barbetta ofSystemair explores the results of converging nozzles on loss ofimpulse, a history of the research and the latest solutionive

Below: Testing a Mojet fan

058_062tun0712 Systemair:T&T 25/6/12 11:58 Page 58

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FAN DEVELOPMENT | TECHNICAL

JULY 2012 Tunnels & Tunnelling International

installed close to the ceiling. This results inthe exhaust jet stream not being able tofully develop. So a portion of jet energy islost to wall friction. Consequently, formula(3) has to be rewritten as follows:

( 4 )

The effectiveness number k, called theKempf factor, indicates what portion ofthe impulse force or momentum is actuallyprovided by the fan and not lost.Generally we should assume:

= 30m/s= 5m/s

And we immediately have a factor of

Studies, presented later in this paper,demonstrate that k can range between 0.65to 0.78, resulting in a correction factor onthe provided thrust of 0.54 to 0.64. Thismeans a loss in thrust of 36 to 46 per cent.This is a remarkable amount of energy loss,and needs an appropriate focus.

HistoryDuring the mid 1960s a Swiss engineer, IKempf, started to study the influence andeffect of walls on jet streams [2] in a cornerof rectangular tunnel section model. Thatwas the first approach trying to determinethe effects in an engineered way. Later in1979 E. Rohne [3] resumed the previouswork and began an analytical investigationof a simple jet on a plain plate. In 1982Rohne [4] also studied the simple jet in aniche. And again in 1985 [5] and 1988 [6]Rohne studied the effect of two and foursimple jets on a plain plate.All Rohne experiments were purely a

pipe or pipes positioned in the vicinity of aplain plate or plain niche, and not takinginto account the shape of the tunnel at all.This research showed the great influence oflosses generated through interactionbetween jet plumes and fixed surfaces.In 1991 [7] the prolific Rohne presented

research that in some ways reproduced,the circular shape of a tunnel: a halfcylinder in small scale (maximum radius of400mm and length 2,000mm), with onepipe positioned in the middle at variousdistances from vault.The research carried out to this point had

two technical limitations:Pipes do not correctly represent a jet fanoutlet, which has a swirl.Plain plates or semicircular sections donot create boundary surface existing in atunnel.

Also in 1991, N. Costeris [8] resumed theKempf research for a rectangular tunnelpresenting the dimensionless effectivenessof jets. Again the first technical limitation(above) was an issue.For the first time graphs showing the k

factors were of easy use and directlyapplicable to formula four (above, left).The author suggested how to improve

the results by using deflectors on airoutlets, correcting the final thrust by thecosine of the angle between the jet streamcentreline and the longitudinal tunnel axis.Costeriss comments on use of

deflectors are very clear, that the adoptionof jet stream deflectors is only consideredan improvement if the advantages outweighthe drawbacks caused by the additionallosses through the jet fan, andconsequently the increase of powerconsumption.During the 1990s there was a big jump

ahead in the research. The Department ofMechanical Engineering of the University ofPadua in Italy and the Centre for TunnelAerodynamics Research at the South BankUniversity in London, UK worked togetherin a joint research program and improvedthe analysis of k factor.The major improvements were:Creation of a scale model of a real tunnelcross section (ANAS 505, Italian RoadAuthority) at a scale of 1:10.Creation of a jet fan scale model 1:10including the typical swirl angles, withand without silencers.Both single and pair jet fan scale modelsat various distances between the vaultand fan centres. And also analysingeffect of distance between jets or pairs.Benefit analysis of inclining the jet.

The work done was presented in 1994[9], 1997 [10] [11], 2000 [12] and todayrepresents the more updated Kempf factordetermination.

Technical solutionFrom formula (4) it is clear that the Kempffactor is an extremely important influenceon the number of jet fans to be used andconsequently installation and running costs.The data in [9], [10], [11] and [12]

confirms that it could vary between 0.65 to0.78 so any improvement achieved will givebenefits as described above.

Jet fan inclinationFully reversible booster fans in abidirectional traffic tunnel, or in aunidirectional tunnel that, duringmaintenance allows two way traffic anddoes not allow the adoption of inclinationthat will only benefit one direction. In somecases the inclination is not achievable dueto tunnel clearance demands.From [10] [11] it was found that from 5

to 10 there is an optimisation of theinstallation efficiency. This method has noremarkable application as far as the authorsare aware.

DeflectorsAs far as the authors know Kempf [2]studied the effect of deflection vanes on apipe installed in a corner of a rectangularsection in 1965.The research was limited to establishing

the effectiveness of an installation that usesdeflectors. Effectiveness was surelyincreased due to the directing of the flowaway from the wall. However from a fanpoint of view, the installation of deflectorson the outlet side or, even worse (in thecase of a fully reversible jet fan), on both

= force, thrust ( N )

= fluid density (kg/m3)

= flow rate (m3/s)

= velocity (m/s)

= Kempf factor ( - )

= angle between jet stream and longitudinal tunnel axis ( )

= distance between fan axis and tunnel roof (m)

Subscript notation

= effective

= fan

= tunnel

Notation


60 www.tunnelsandtunnelling.com



the inlet and outlet, generates a negativeeffect on the fan performances through anincreased drop, and especially on the inletside, with a non uniform fluidodynamic loadon the axial impeller.

Lotsberg [13] investigated the effect ofdeflectors on large, 1,500mm jet fans in a6.6km long tunnel so the loss was limited.

The test was only done in one directionand they found a high energy loss insidethe fan.

On a fully reversible fan this approachdoes not give any real benefit. Should thedeflection vanes be attached at somedistance from the fan ends, the pressuredrop could be reasonably reduced but onlya proportion of the spreading plume wouldbe captured and turned, so again thebenefit is reduced.

The use of the deflectors has beenlimited and discontinued. No furtherresearch has been undertaken.

Slanted SilencersIt is clear that the use of silencers has abeneficial effect on the k factor [9]. A furtherimprovement is to slant the silencer downor away from the wall in the case of a nichepositioned jet fan. This technical solutionsurely increases the k factor.

In situ measurements on the Balmenrainand Uznaberg tunnels in Switzerland werecarried out by Pospisil et al [14] but flowmeasurement error was 19 per cent so

the conclusion was not a consistent resultdue to the large tolerance span.

In a second in situ measurement of theCollembey Tunnel in Switzerland, Mart et al[15] improved the measurement techniquereducing the uncertainty down to 12 percent. Here the jet fans were installed in thecorner of a rectangular tunnel section. Itwas concluded that the thrust of the slattedsilencer jet fan was between 11 per cent to21 per cent higher than the one ofconventional straight jet fans.

The above installation was possible sincethe slanted silencers didnt interfere with

tunnel clearance. If this is the case, then thetwo possible installation arrangements arefeasible (see Figures 1 and 2, above).

These two solutions have the same kfactor so there is no real need to install theslanted silencer jetfan type. Silencerslanting of 10 or more (normally it isbetween five and 25 per cent) means thatthe plume of the jet fan will be further fromthe vault of the tunnel, increasing the kfactor, but at the same time it will hit theground of the tunnel, generating a loss.

During a fire scenario, if the jet attachesitself to the tunnel floor and moves forward

Above, top: Figure 1, arrangement withslanted silencer jet fansAbove, bottom: Figure 2, arrangementwith standard jet fan

Pre

ssur

e

V1 V1 Volumetric owrateV2

Stall line

Fancharacteristic

Resistanceline without

nozzle

Constantpower line

Original operatingpoint

Resistanceline withnozzle

New operatingpoint (with nozzle)

Below: Figure 3, axial fan performance curves

Impeller Motor Mounting foot

Fan casing Silencer

Terminal box

Motor

Fan casing Tapered duct piece Tapered silencer

Impeller Terminal boxMounting foot


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as a wall jet, the air velocity above the jetmay be less than the critical velocity forsmoke control, possibly leading to localisedsmoke back-layering.This problem has been analysed by

Betta et al [16] and the conclusion was interms of energetic operating costsreduction:Optimal pitch angle 6 for a free tunnel.Optimal pitch angle between 2 to 4 fora tunnel with a traffic jam (e.g. congestedtraffic or fire scenario).

This issue may need to be addressedduring the design stage of the project,possibly resulting in a higher number of jetfans in a fire scenario.Due to the high silencer deflection angle

the thrust developed along the longitudinaltunnel axis has to be corrected by thecosine of the angle.

The latest technical solution

MojetSo far we have seen various technicalefforts in order to maximise the momentumexchange between booster fans and thefresh air flow induced into the tunnel.If we again consider formula one, we can

appreciate that:

= we have no control since it is aphysical property of the air inside the tunnel

= they are linked together in adirectly proportional way. But should qv bekept constant, and in some ways Vfincreased, the result is an increase in thefan-developed thrust.In more technical detail, Tarada [17] [18]

describes the concept of the MomemtumJet (Mojet) with converging silencersoutlets (nozzles) on one or both side of thefans depending on whether unidirectionalor reversible jet fans are needed. So asignificantly greater aerodynamic thrust canbe obtained, with this converging nozzlearrangement, by accelerating the dischargeair into a smaller outlet area.Naturally there are losses generated by

nozzles varying with a square law andpower demand with a cubic law.Axial fans have a steep performance

curve, so the additional pressure dropmoves up the working point of the fan whilethe volume flow reduction is not soremarkable, or it is possible to operate at

lower blade pitch angle and higher pressuredrops, while still delivering the requiredaerodynamic thrust.The latter seems to cancel all the

benefits but, it is not so, since movingupwards in the fan characteristic curve, thefan efficiency gets remarkably higher hencethere is a benefit.Another benefit comes from the

inclination of the nozzles, not by as muchas slanted silencers (around 5) but enoughto notably improve the k installation factor.It is important to note that the silencerdownward inclination can be asymmetric,keeping the lowest silencer side at thesame level, thus not increasing the fanenvelope and keeping the same clearanceas a standard jet fan.We can summarise the variables:Asymmetric converging silencers.Inclination (about 5).

Blade pitch angle.Area restriction ratio, of around 1:6.

Playing with all these possibilities wehave to optimise the combination, includingthe Kempf factor, to get the maximumperformances out of the installed jet fanconfiguration.A work example based on a 1.5km long

tunnel can give a clear picture of the finalbenefits (see Table 1).From the above Table 2 we can now

highlight the advantages.The above example has been developed

with a number of CFD calculations and ithas been optimised the many variables.In this particular case, due to a small fan

diameter, the thrust provided by the Mojetis lower than the one provided by astandard jet fan but the k factor and theabsorbed power are better.

Table 1: Mojet benefits based on 1.5km tunnel

Standard Jetfan Mojet

Design thrust needed in the 22,803Ntunnel including the effect ofair velocity inside the tunnelas per formula three

k factor Kempf 0.73 0.95

Static thrust needed from 31,237N 24,003Njetfans as per formula four

Table 2: Comparison of jet fan performance

Table 3: Financial implications of jet fan choice

Standard Jetfan Mojet Savings

Formula (4) 31,237N 24,003N -23%

Quantity of jetfans 38 32 -16%

Total power 1,246.4kW 915.2kW -27%

Total cost per year EUR 137,104 EUR 100,672 -27%

Standard Jetfan Mojet

Jetfan diameter 710mm

Jetfan thrust 822N 758N

Resulting Jetfan quantity 38 32

Jetfan absorbed power 328kW 28.6kW

Total power 1,246.4kW 915.2kW

Running hours per year 1000h

kWh per year 1,246,400 915,200

Electricity cost EUR 0.11/kWh

Total energy cost per year EUR 137,104 EUR 100,672

,

Left, top: Figure 4, tapered silencer jetfan arrangmentLeft, bottom: Figure 5, non-taperedsilencer jet fan arrangment


For larger jet fan diameters of 1,000 to1,250mm, the benefits can be achieved byincreasing the thrust and the restriction arearatio. So for every fan diameter we shouldevaluate all the possible variables to findthe best compromise:

Blade pitch angle and hence volumeflowArea ratio restriction and hence velocityhence thrustPowerk factor

Additional benefitsThis technical solution has other correlatedpositive effects. By reducing the number ofbooster fans we can reduce the installationcosts. The cost of original equipment isalso positively affected.The electrical cable cost represents

about 50 to 60 per cent of the total cost in alongitudinal ventilation system.Having fewer booster fans and less

power we can save in cable length anddiameter. In Italy, the Italian Road Authority(ANAS) prescribes that the jet fans andcorrelated equipments have to withstand400C for two hours, hence the saving inthis part of the installation is remarkable.The last but not the least important point

is that maintenance costs are not normallygiven much thought at the preliminarycontract stage of a jet fan order. A reductionin the number of booster fans means lesstime inside the tunnel for inspection andmaintenance purposes.The flexibility of inclined converging

nozzles can be done at any direction. On ahorizontal plane, on a vertical plane, or acombination of the two. This feasibilityallows the use of such technology in round,rectangular or any shaped tunnel section,even in niches.

Future stepsThe Mojet has been demonstrated to havea sound physical basis, and the CFDanalysis has supported this. However, trueconfirmation can only come with a realinstallation, or in a test carried out at a 1:1scale. It is even better if the test isperformed by a notified body that has nointerest at all to manipulate the relevantmeasurements.A 1:1 scale test was arranged for 2011 in

a 600m long tunnel in Spain. This waschanged to a test that will be carried out inItaly in 2012.

ConclusionsFrom the mid 1960s until the present day, alot of dedicated research to establish the kfactor more accurately has been carried

out. Concurrently, various fan engineeringtechnical solutions to achieve this have alsobeen developed.The latest, though surely not the final

one, shows the best technical achievement,

giving the possibility to increase thelongitudinal ventilation performances,especially on energy consumption. This is akey figure to be considered for present andfuture design.

62 www.tunnelsandtunnelling.com



[1] ISO 13350, Conversion Rules, Appendix C

[2] Einfluss der Wandeffekte aut die Treibstrahlwirkung eines Strahlgeblses I.Kempf -Schweizerische Bauzeitung, 1965

[3] The friction losses on wall caused by jet flows of booster fans E. Rohne BHRA, Sheffield,U.K., 1979

[4] Jet flow momentum losses of a booster fan when installed in a tunnel niche E. Rohne BHRA,New York, U.K., 1982

[5] The influence of axis distance of two parallel jet flows on the friction losses on walls E. Rohne BHRA, Lille, FR, 1985

[6] The friction losses on walls caused by a row of four parallel jet flows I,Rohne BHRA, Durham,U.K.,

[7] Friction losses of a single jet due to its contact with a vaulted ceiling I.Rohne BHRA, Brighton,U.K., 1991

[8] Impulse fans N. Costeris BHRA, Brighton, U.K., 1991

[9] An experimental study on the longitudinal ventilation system A.D. Martegani, G. Pavesi, C.Barbetta BHRA, Liverpool, U.K., 1994

[10] The influence of separation,inclination and swirl on single and coupled jet fans installationefficiency A.D. Martegani, G. Pavesi, C. Barbetta BHRGroup, Aosta Valley, IT, 1997

[11] The effect of jet plume configuration on the installation efficiency of jet fans R.D. Matthews, M.Tabarra, B. Kenrick BHRGroup, Aosta Valley, IT, 1997

[12] Experimental investigation of interaction of plain jet fans mounted in series A.D. Martegani, G.Pavesi, C. Barbetta BHRGroup, Boston, USA, 2000

[13] Investigation of wall-friction, pressure distribution and the effectiveness of big jet fans withdeflection blades in the Fodnes tunnel in Norway G. Lotsberg BHRGroup, Aosta Valley, IT, 1997

[14] Messungen an der Tunnellufftungsanlage der Tunnels Balmenrain und Uznaberg, HauptstrasseT8/A8 P. Pospisil, M. Mart, M. Brandt - HBI report 87 95 10 , CH, 2003

[15] Strmungsmessung, Tunnel de Collembey M. Mart, M. Brandt HBI report 03 100 02 ,CH, 2004

[16] Numerical study of the optimization of the pitch angle of an alternative jet fan in a longitudinaltunnel ventilation system V. Betta, F. Cascetta, M. Musto, G. Rotondo Tunnel and UndergroundTechnology, pages 164 172 , 2009

[17] Impulse ventilation for tunnels a state of the art review F. Tarada , R. Brandt BHRGroup,New Bruswick, USA , 2009

[18] Design, testing and application of an energy-efficient longitudinal ventilation system F. Tarada BHRGroup, Dundee, U.K., 2011

Bibliography

Jet fan blades
