losses across multiple fittings in ventilation

7/26/2019 Losses Across Multiple Fittings in Ventilation

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Hindawi Publishing CorporationTe Scientic World JournalVolume , Article ID ,pageshttp://dx.doi.org/.//

Research ArticlePressure Losses across Multiple Fittings in Ventilation Ducts

Z. T. Ai and C. M. Mak

Department of Building Services Engineering, Te Hong Kong Polytechnic University, Hong Kong

Correspondence should be addressed to C. M. Mak; [email protected]

Received September ; Accepted October

Academic Editors: J. Niu and G. M. ashtoush

Copyright Z. . Ai and C. M. Mak. Tis is an open access article distributed under the Creative Commons AttributionLicense, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properlycited.

Te accurate prediction o pressure losses across in-duct ttings is o signicance in relation to the accurate sizing and good energyefficiency o air-delivery systems. Current design guides provide design methods and data or the prediction o pressure losses onlyor a single and isolated tting. Tis study presents an investigation o pressure losses across multiple interactive in-duct ttingsin a ventilation duct. A laboratory measurement o pressure losses across one tting and multiple ttings in a ventilation duct iscarried out. Te pressure loss across multiple interactive ttings is lower than that across multiple similar individual ttings, whilethe percentage decrease is dependenton the conguration andcombination o the ttings. Tisimplies that the pressureloss acrossmultiple closely mounted ttings calculated by summing the pressure losses across individual ttings, as provided in the ASHRAEhandbook and the CIBSE guide, is overpredicted. Te numerical prediction o the pressure losses across multiple ttings using thelarge-eddy simulation (LES) model shows good agreement with the measured data, suggesting that this model is a useul tool in

ductwork design and can help to save experimental resources and improve experimental accuracy and reliability.

1. Introduction

In air-delivery ductworks o HVAC systems, pressure lossesacross duct ttings such as dampers, sensors, bends, transi-tion pieces, duct corners, branches, and even splitter attenu-ators are important in counteracting the pressure differencecreated by ans. Accurately predicting pressure losses acrossduct ttings at the design stage is thus crucially importantto proper duct sizing and an selection, which could nallyresult in great economic benets in terms o both the initial

investment cost and the operational cost o duct systems.Te commonly adopted data o pressure losses across

HVAC duct ttings are those provided in the well-knowndesign guides, such as the ASHRAE handbook [], the CIBSEguide [], and the handbook by Idelchik []. Tese datawere summarized rom many experimental works, most owhich were conducted based on ASHRAE Standard P[]. However, in terms o scope, the data are limited to thetypes o duct ttings, the range o duct sizes, and the rangeo mean duct velocities. In addition, the accuracy o theexperimentally obtained data available in these handbooksand guide has been questioned by a number o investigators[]. One possible reason or their inaccuracy is that the

measurements were conducted on single, isolated duct t-tings without consideration o the inuence o the interactiono other ttings []. In practice, there are commonly multiplettings in a HVAC ductwork, and very requently, someo these are relatively close to each other. Rahmeyer [ ]experimentally studied the effect o the interaction betweenbends and ound that the pressure loss across two closelycoupled bends is related to their distance. Tis ndingimplies that the traditional calculation method, which sumspressure losses across each individual duct bend, could

sometimes be inaccurate. Later, Atkin and Shao [] appliedcomputational uid dynamics (CFD) modeling to analyzethe effects o the separation and orientation o two closelyconnected bends on total pressure loss. Tey ound that ata separation distance o to hydraulic diameters, thepressure drop across the two bends is highly dependenton their relative orientation. Unortunately, it is not knownwhether these ndings rom bends can be applied to otherttings, especially in-duct ttings. Tus, more studies arerequired.

Another concern is that obvious differences are oundin part o the pressure loss data between the ASHRAEhandbook and the CIBSE guide []. One actor that may


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Te Scientic World Journal

possibly contribute to these discrepancies [] is that due tothe lack o understanding on airow patterns at duct ttings,pressure sensors are occasionally located at inappropriatesections such as disturbance sections,which could cause largemeasurement errors. However, since the in-duct airow pat-tern is strongly associated with the duct conguration, mean

duct velocity, and local aerodynamic conguration o ttings,it is difficult and time consuming to nd the appropriatelocations or placing pressure sensors beore every test. Insuch a condition, a numerical method should be helpulin terms o saving experimental investment and time. Evenin an experimental way, it is more reliable and economicalto know the ow patterns beore a real test is set up andconducted. Asa numerical method, CFDhas been sufficiently

veried and validated as a method o predicting uid ow.

Shao and Riffat [, ] studied the possibility and accuracyo using the CFD method to predict the pressure loss actorandin turn determine pressure losses acrossduct bends. Teyevaluated the effect o a set o computational parameters on

the accuracy o numerical results. Teir numerical resultsare supported by the experiments o Gan and Riffat [].In addition, the CFD method has been used to predict thepressure loss coefficient o many other duct ttings, suchas damper, orice, transition [, ], and junction []ttings, in HVAC ductworks as well as to conduct other duct-related studies (e.g., air leakage) []. Without exception,

these previous numerical simulations used the ReynoldsAveraged Navier-Stokes (RANS) method [], specically

the steady standard- turbulence model. However, thismodel may be not accurate and reliable enough to predictthe ow eld inside a duct with multiple ttings, where

the airow is more strained and swirling as well as highlyuctuating. As an alternative CFDmodel,the advanced large-eddy simulation (LES) model is well known or its accuracyin predicting airow in the building-related eld [, ].Te LES model, which resolves large turbulent eddies andmodels small eddies, has the capacity to reproduce tran-sient turbulent uctuations and handle ow intermittency,although it consumes more numerical cost. In this study,the accuracy and reliability o the LES turbulence model inpredicting pressure losses across multiple in-duct ttings areevaluated.

Te specic problems that motivated this study arethe inaccuracy o the available data in the current guides

and the lack o a predictive method or pressure lossesacross multiple in-duct ttings. Te purposes o this study,thereore, are to examine the pressure losses across multiplein-duct ttings and to evaluate the accuracy and reliabilityo a predictive method. Te effect o the interaction ottings on the total pressure loss across multiple ttingsis analyzed by experimental tests. Te predictive method,namely, LES modeling, is then evaluated by comparing itwith tested data. It is expected that this study will reveal thepressure losses across multiple in-duct ttings and providedesigners with a predictive method that can be used eitherindependently as a design tool or to assist experimentaltesting.

2. Conceptual Models

Tere are two types o pressure loss in duct systems, namely,riction loss and dynamic loss. Tese losses are derivedrom different mechanisms and are thereore calculated bydifferent methods [].

Friction loss is due to uid viscosity and results romthe momentum exchange between molecules or betweenadjacent uid layers moving at different velocities. It occursalong the entire length o a duct. Friction losses in uidductworks can be calculated by the Darcy equation:

= 22, ()

where is a riction loss, a dimensionless rictionactor, duct length, hydraulic diameter, area-averagedstreamwise velocity, anduid density. Friction actorisdetermined by the Colebrook equation:

1= 2 log 3.7 + 2.51Re , ()whereis the absolute roughness actor o a material and Reis the Reynolds number computed rom

Re=]

, ()where ] is kinematic viscosity. Te hydraulic diameteris dened as4/, where is the duct area and is theperimeter o the cross-section.

Dynamic losses in ttings result rom ow disturbancescaused by duct ttings that change the airow direction or

ow path area and can be calculated by

= 2

2 , ()where is the dimensionless local loss coefficient (alsocalled actor), which is determined by the local dynamiccharacteristics.

3. Experimental Method and Simplification

Tis experiment was a part o our previous test on ow noisecaused by in-duct elements []. Te experimental system isshown inFigure . Air ow was provided by a centriugal an

driven by a variable speed motor. Te an was enclosed ina . . . m3 enclosure. Te ..m2 test ductwas made o steel. Te total length o the duct was .m, inwhich, counted rom the air ow inlet, the rst tting (p) waslocated . m rom the duct inlet section and the third tting(p) was located m rom the duct outlet. Tese upstream anddownstream lengths [] were generally sufficient to ensurethat the test o the rst and third ttings were not affectedby the inlet and outlet o the duct, respectively. Te inlet andoutlet o the experimental system were placed on the outsideso as to eliminate the effect o relative pressure difference.

As shown inFigure , at plates were used to generallyrepresent the in-duct ttings in HVAC ductworks. Here,


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: Fifeen tting congurations tested in this study.

Case Conguration o tting(s) Mean ow velocities

(m/s)

rr = 0.025 m at p1

, , , ,

r

r = 0.05 m a t p1

, , , ,

r

r = 0.075 m a t p1

, , , ,

r = 0.025 m a t p1

r/2r/2 , , , ,

r = 0.05 m a t p1r/2r/2

, , , ,

r = 0.075 m a t p1r/2r/2

, , , ,

r = 0.025 m a t p1

rr = 0.025 m a t p2

r , , , ,

r = 0.05 m a t p2r = 0.025 m a t p1

r r , , , ,

r = 0.075 m a t p2r = 0.025 m a t p1r r

, , , ,

r r

r = 0.05 m a t p2r = 0.05 m a t p1

, , , ,

r = 0.075 m a t p2r r

r = 0.05 m a t p1

, , , ,

r = 0.075 m a t p1 r = 0.075m a t p2r r

, , , ,


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: Continued.

Case Conguration o tting(s) Mean ow velocities

(m/s)

r = 0.05 m a t p1 r = 0.05 m a t p2

r/2r/2 r/2r/2 , , , ,

r = 0.05 m a t p1

r/2r/2

r = 0.05 m a t p2

r/2r/2

r = 0.05 m a t p3r/2r/2 , , , ,

r = 0.05 m a t p1r

r = 0.05 m a t p2r

r = 0.05 m a t p3r

, , , ,

the control variable was the obstructed ratio o the tting

area to the cross-sectional area o the duct. Te plates weremade rom mm thick steel plate, and they were xed tothe adjoining anges o the test duct. Te gap was sealedwith compressed oam rubber. As shown in able , congurations were tested. In Cases , only a single ttingwas inserted into position p. In Cases , two ttings wereinserted at two different positions (p and p), and in Cases-, three ttings were inserted at three different positions(p, p, and p).

Te velocity prole in the empty test duct was measuredto make sure that the ow could be symmetrically developedinside the duct. A pitot tube was used to sample the dynamicpressure at specied points in the duct cross-section. Based

on the measurements obtained using this empty duct, arelationship between the mean duct velocity () and thevelocity measured at the center o the duct () was developed( = 0.9639 0.3289(2 = 0.9997)), and this was usedto calibrate the mean duct velocity in later tests using themeasured center velocity. Te mean airow velocities testedor each case are listed inable .

Te static pressure losses across the ttings were mea-sured using two piezometric rings placed at positions p, p,and p (Figure ). Each ring consisted o our static pressuretappings, one in each duct ace. Te downstream ring wassufficiently araway (ve times the duct dimensions) rom thetest tting to ensure that ull static pressure recovery could

take place in the wake o the ow obstructions under test.

4. Numerical Modeling

Tis section briey discusses the numerical method usedby LES modeling and introduces the test cases selected toevaluate it.

.. Governing Equations of LES. Te governing equationsused or large eddies can be obtained by ltering the time-dependent Navier-Stokes equations. Eddies whose scales aresmaller than the ltering width or gridspacing adopted in thecomputations are effectively removed by the ltering process.

Inlet

Outlet

Pitot

1.25 m 1 m 2 m

5.75m

PS3PS2PS1

0.5 m

0.2 m0.5 m0.2 m 0.2 m

0.5 m0.5 m

p1 p2 p3

F : A schematic diagram o the experimental system (p, p,and p are the positions where the rst, second, and third ttingsare inserted, respectively;1,2, and3 represent the staticpressure loss across the rst, second, and third ttings, resp.).

Ten, the resulting equations only govern the dynamics olarge eddies.

In this study, the ltering operation provided by thenite volume discretization method, as described in [], is

employed:

()=1

0 , , ()

where () represents a ltered variable and the volume oa computational control cell. Te lter unction,(,), is

, =1, or , = 0, .()


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0.1 m

0.1 m

r

(a)

0.1 m

0.1 m

r/2 r/2

(b)

F : Cross-section o the duct with two types o at-plate ttings (shaded area). (a) Centrally placed ttings;

= 0.025m, .m,

. m. (b) Te geometries consisted o plates protruding symmetrically rom both sides o the duct, leaving a central vertical strip o theduct open, later called a centrally opened tting; = 0.025 m, . m, . m.

In this study, the governing equations o LES or incom-pressible ows were obtained by ltering the Navier-Stokesequations:

= 0,

+

=

,()

where is the stress tensor due to molecular viscosity,dened by

+

23

()andis the subgrid-scale (SS) stress dened by

. ()Since the subgrid-scale stress term in the LES model

is unknown, it requires modeling to close the governing

equations. Currently, the most adopted subgrid-scale turbu-lence model, which employs the Boussinesq hypothesis [],computes subgrid-scale turbulent stresses by

13= 2, ()where is subgrid-scale turbulent viscosity. Te isotropicpart,, which is not modeled, is added to the lteredstatic pressure term.is the rate-o-strain tensor under theresolved scale, dened by

=1

2

+

. ()

In this study, the subgrid-scale turbulent viscositywas modeled by the Smagorinsky-Lilly model, which wasinitially proposed by Smagorinsky []. In the Smagorinsky-Lilly model, the turbulent viscosity coefficient is calculated by

= 2 , ()where|| 2, is the subgrid mixing length,calculated by

=min ,1/3 , ()where is the von Kaman constant, the distance to theclosest wall, and the Smagorinsky constant, empiricallygiven as ..

.. Mesh Work, Boundary Conditions, and Numerical

Scheme. In this study, a ull straight square duct (. .m2in section and . m in length) was simulated (seeFigure ).Te uid air was assumed to be incompressible, and thegravitational acceleration was not considered. Te Reynoldsnumber based on the mean duct velocity and square duct

dimensions was .. 5. Mean duct velocity wasimposed on the inlet boundary and the ow turbulence ischaracterized by turbulence intensity (%) and hydraulicdiameter (. m). On the outlet boundary, it is assumed thatthe ow is ully developed, with zero normal gradients andzero background pressure. Tere was no slip o uid at thesuraces o the duct and tting(s). Structured grids were usedto discretize the computational domain in which the rstgrids are2.5 105 m away rom the tting(s). Ten, the+(+ = /) value or the rst grid points was around. depending on the mean duct velocity, which indicatesthat the rst grids are within the laminar sublayer. Te meshesbecome coarser in the region ar away rom the tting(s) but


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Inlet Outlet

0.2 m

0.2 m

0.2 m0.5 m

0.5 m

0.5 m

5.75 m

1.75m

p1 p2 p3

2 m1 m

PS1 PS2PS2

F : A schematic diagram o the duct system in the numerical simulations.

remain high density near the duct walls (+ < 5). When themesh is ne enough to resolve the laminar sublayer, the LESmodel applies the laminar stress-strain relationship to obtainthe wall shear stress:

= . ()

Te sensitivity o mesh number was systematically tested. Foreach case, three different mesh systems (a coarser, a medium,and a ner) were constructed and the nal numerical solu-tions based on these three meshes were compared. Finally,in compromise between the numerical accuracy and cost,

meshes with around .6, .6, and .6 gridswere selected or the cases with one tting, two ttings, andthree ttings, respectively. Te time step size used in the LESsimulations was . s, which ensures that the convergencecan be achieved within iterative steps or each time

step.Based on the nite volume method (FVM), the governing

equations are discretized to algebraic equations on the gridsystem. Te convection term was discretized by the boundedcentral differencing scheme, while the pressure staggeringoption scheme (PRESO) was selected or pressure interpo-lation. Finally, the SIMPLEC algorithm was used to couplethe pressure and velocity equations.

.. Cases Simulated. In order to evaluate the accuracy andreliability o the LES model in predicting the pressure lossesacross multiple in-duct ttings, two tested cases are selectedto be numerically reproduced: Case at a mean ow velocity

o m/sand Case at m/s. Te predicted pressure lossesare compared with those measured in the experiments.

5. Results and Discussions

As shown in able , ve mean ow velocities were testedor each case. However, due to the difficulty in accuratelycontrolling the mean velocity during the tests, the tested

velocities were not necessarily the same or all cases. Tis doesnot inuence the later analysis. In this section, the measuredor simulated pressure losses (Pa) across in-duct ttings aredirectly presented and analyzed. I one is interested in theactors, these can be obtained using () inSection .

.. Effect of Reynolds Number (). In practice, there arevarious types o HVAC ducts in terms o cross-sectionalshape and dimension and mean ow velocity. Despite thiscomplexity, the dimensionless Re can be used to representthese duct characteristics given that the same Re indicatesaerodynamic similarity. In this section, the effect o Re on thepressure losses across ttings is examined when the ttingconguration remains unchanged. It is ound that the pres-sure loss across a tting almost has a linear relationship withthe duct Re (the example o Case is shown inFigure ).Tis implies that any actors increasing the duct Re, such asan increase in velocity and cross-sectional dimensions, canresult in an increase in pressure loss across an in-duct tting.In other words, pressure losses across the in-duct tting(s) oa larger duct with a higher velocity remain high.

.. Effect of Fitting Conguration. In order to study the effecto tting conguration on the pressure losses, this sectiondiscusses the cases with the same Re to exclude the inuenceo Re on the comparison o different tting congurations.

Te effect o tting type on pressure losses across ttingsis studied when the obstruction ratio is kept constant. Teobstruction ratio is dened as the area ratio o the tting tothe duct cross-section, namely, the ratio o the shaded areato the whole duct section (seeFigure ).able summarizesthe comparison o pressure losses across the two types ottings, namely, the centrally placed tting and the centrallyopened tting (in Figure ). From able , it can be seenthat the pressure loss across a centrally placed tting isremarkably larger than that across a centrally opened tting.

Tis can be explained by the act that the velocity prole inthe cross-section o a duct ollows a parabolic distribution,namely, the largest in the center and the smallest on the ductsuraces. Tus, centrally placed ttings obstruct the astestcentral airow and lead to the largest pressure losses, whereascentrally opened ttings allow this strongest airow to passthrough and offer much less resistance to the airow. It canalso be observed that the deviation ratio o pressure lossbetween these two types o ttings is not the same and isdependent on the obstruction ratio.

Te effect o the obstruction ratio on pressure lossesacross ttings is studied at a Re o .4, and the resultsare shown in Figure . For all cases, an increase in the


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: Effect o tting type on pressure losses (Pa) across tting(s).

Re(104) Case versus (only p)

Case versus (only p)

Case versus (only p)

Case versus (p and p)

Case versus (p, p and p)

. versus versus versus at p

versus at p

versus at p versus at p versus at p

. versus

: Effect o downstream tting(s) on pressure loss (Pa) across an upstream tting.

(a) Base case: Case

Re =6.67 104 Case Case Case Case 1 1 2 1 2 1 2Pressure loss

Percentage decrease .% .% .%

(b) Base case: Case

Re =

6.67 104 Case Case Case Case

1 1 2 1 2 1 2 3Pressure loss Percentage decrease .% .% .% .%

(c) Base case: Case

Re =6.67 104 Case Case 1 1 2Pressure loss

Percentage decrease .%(d) Base case: Case

Re =6.67 104 Case Case Case

1

1

2

1

2

3

Pressure loss Percentage decrease .% .% .%

ttings obstruction ratio signicantly increases the pressureloss across it. However, the percentage increase in pressureloss is dependent on the obstruction ratio, tting type, andthe interaction o neighboring ttings. For a centrally placedsingle tting (Cases ), whenthe obstruction ratio increasesrom . to . (rom Case to ), the percentage increaseis approximately %, which is almost one time higher thanthe increase (%) when the obstruction ratio increasesrom. to . (rom Case to ). It is also observed that this

percentage increase is relatively lower in the case with thecentrally opened tting (Cases -) and is also inuenced bythe presence o neighboring ttings (Cases and Cases ). However, regardless o the change in the obstruction ratioo a neighboring tting, the pressure loss across a tting isonly changed slightly.

.. Effect of Interaction of Multiple Fittings. able sum-marizes the pressure loss across an upstream tting and itspercentage decrease as a result o its downstream ttings.From able (a), it can be seen that the presence o adownstream centrally placed tting can reduce the pressureloss across its upstream tting, and the percentage decrease

increases remarkably with the increase in the obstructionratio o the downstream tting. However, a comparison oable (a)(c) shows that with the increase in the obstructionratio o the upstream tting, the decrease in pressure lossacross it is gradually decreased, becoming.% when theobstruction ratio reaches .. As tabulated inable (d), orthe centrally opened tting, the presence o a downstreamtting can reduce the pressure loss across it, whereas twodownstream ttings complicate this situation.

Te effect o upstream ttings on the pressure loss acrossa downstream tting is also evaluated, and the results arepresented in able . For the centrally placed tting, the pres-ence o upstream ttings signicantly reduces the pressurelosses across its downstream tting(s) (see able (a)(c)).Contrarily, or the centrally opened tting, this pressure lossis negligibly affected by the presence o an upstream tting,whereas it is signicantly decreased by the presence o twoupstream ttings (seeable (d)).

Te above results suggest that as a result o the effect odownstream and upstream ttings, the pressureloss across anin-duct tting is changed substantially. Tis can be explainedby the act that the presence o a tting changes the airow


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: Effect o upstream tting(s) on pressure loss (Pa) across a downstream tting.

(a) Base case: Case

Re =6.67 104 Case Case 1 1 2Pressure loss

Percentage decrease .%

(b) Base case: Case

Re =6.67 104 Case Case Case Case 1 1 2 1 2 1 2 3Pressure loss

Percentage decrease .% .% .% .%

(c) Base case: Case

Re =6.67 104 Case Case Case Case 1 1 2 1 2 1 2Pressure loss

Percentage decrease .% .% .%

(d) Base case: Case

Re =6.67 104 Case Case Case 1 1 2 1 2 3Pressure loss

Percentage decrease .% .% .% : Comparison o pressure losses (Pa) across multiple interactive and individual ttings.

Re =6.67 104 Case Case Case Case Case Case Case Case Case Across interactivettings

Across individualttings

Percentage decrease .% .% .% .% .% .% .% .% .%

direction and turbulence around its neighboring tting(s)and consequently modies theactor (see () inSection ).Te results also demonstrate that the effect o a neighboringtting is complex; that is, different tting type, location(upstream or downstream), obstruction ratio, and duct Recan result in very distinctive pressure losses. Tis implies thattheactors or individual ttings provided in the ASHRAEhandbook and the CIBSE guide are inaccurate in a condi-

tion when the interaction o neighboring ttings cannot beignored.

Te pressure losses across multiple interactive and indi-vidual in-duct ttings are compared (seeable ). Inable ,the pressure losses across interactive ttings or Cases are directly measured in the tests. In order to evaluate theeffect o the ttings interaction on the total pressure loss,or each case, the pressure losses across every individualtting are summed or comparison. aking Case as anexample, the pressure losses across its individual ttings aresummed rom Case and Case . Based on thesummations oindividual pressure loss, the percentage decreases in pressurelosses across multiple ttings are calculated. It can be seen

that the pressure losses across multiple interactive ttingsare lower than those across multiple individual ttings, andthe percentage decrease is dependent on the congurationand combination o the ttings. Tis nding is supportedby the previous study on two bends by Rahmeyer [].Again, this demonstrates that the calculation o pressurelosses across multiple closely mounted ttings via summingthose across individual ttings is inaccurate. Tis method

overpredicts the total pressure loss, which may consequentlyresult in energy waste owing to the selection o larger ans. Insuch a condition, exploring an accurate, reliable, and high-efficiency predictive method, such as a validated CFD model,is crucially important.

.. Validation of LES Modeling. In order to validate the LESmodel in predicting pressure losses across multiple in-ductttings, the predicted values o the pressure losses acrossttings inCase at m/s and Case at m/sarecomparedwith corresponding data measured in the tests. Te results arepresented inable . It can be seen that the predicted results


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0

200

400

600

800

1000

60000 90000 120000 150000

Pressure

loss

(Pa

)

ReCase 10-p1

Case 10-p2

F : Effect o Reynolds number on pressure loss acrosstting(s).

Pressure

loss

(Pa

)

0

200

400

600

800

1000

1200

0 1 2 3 4 5 6 7 8 9 10 11 12

Case number

At p1

At p2

F : Effect o obstruction ratio on pressure losses acrosstting(s).

agree well with the measured data, with a relative deviationless than %. Tis indicates that the LES model can accuratelyresolve the ow eld in a HVAC duct with multiple in-ductttings.

Compared to the experimental measurement, numerical

modeling has an incomparable advantage in obtaining in-duct ow details such as velocity and pressure distributions.Figure presents the pressure distribution along the ductcenterline in Case at a mean ow velocity o m/s. Figures and show thepressure andair speed contourson the centerplane o the duct, respectively. Tese kinds o pressure andair speed distribution are useul because it can not only beused independently or ductwork design (i the numericalmodel is validated beore) but also to indicate the locationswhere pressure sensors should be placed in the tests; or thelatter, the involvement o numerical modeling can save manyexperimental resources and help to produce more reliableexperimental data. Tus, the successul use o numerical

0

200

400

600

800

1000

0 1 2 3 4 5 6

Pressure

(Pa

)

Distance along streamwise direction (m)

800

600

400

200

p1 p2

F : Pressure distribution along centerline o the duct (Case at m/s).

modeling is o great signicance in optimizing ductworkdesign and improving the database or pressure losses acrossttings.

6. Conclusions

Tis study examines the pressure losses across multiple in-duct ttings o a ventilation duct using experimental tests.wo tested cases are reproduced by LES modeling to evaluatethe accuracy and reliability o this numerical method inpredicting the pressure eld inside a duct with multiplettings. Te ollowing conclusions can be drawn.

Te ow resistance o a centrally placed tting is remark-ably larger than that o a centrally opened tting basicallydue to the act that the velocity prole o a cross-sectiono a duct ollows a parabolic distribution. For all cases, anincrease in the obstruction ratio o a tting signicantlyincreases the pressure loss across it. However, this pressureloss does not linearly increase with the obstruction ratio;there is a substantial increase in pressure loss when theobstruction ratio increases rom . to .. Again, this isbecause thevelocity prole on a cross-section is nota uniormdistribution.

Since the presence o a tting changes the airow direc-tion and turbulence around its close neighboring tting(s)and consequently modies the

actor, the pressure losses

across the neighboring tting(s) are changed substantially.However, the magnitude o this change is affected by manyactors, such as tting type, location (upstream or down-stream), obstruction ratio, and Re. In addition, the pressurelosses across multiple interactive ttings are lower thanthose across multiple similar individual ttings, althoughthe percentage decrease is dependent on the congurationand combination o the ttings. Tese ndings imply thatthe calculation o pressure losses across multiple closelymounted ttings via summing those across individual ttingsis inaccurate. Tis method overpredicts the total pressure lossand could result in energy waste via the selection o largerans. Tus, a more accurate, reliable, and high-efficiency


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0

0.05

0.1

Z

(m)

1.8 2 2.2 2.4 2.6 2.8 3

X (m)

Pressure: 300 200 100 0 100 200 300 400 500 600

F : Pressure (Pa) contour around the in-duct ttings on the center plane o the duct (Case at m/s).

1.8 2 2.2 2.4 2.6 2.8 3

Velocity-magnitude: 5 10 15 20 25 30

0

0.05

0.1

Z

(m)

X (m)

F : Air speed (m/s) contour around the in-duct ttings on the center plane o the duct (Case at m/s).

: Comparison o predicted and measured pressure losses(Pa).

Case (m/s) Case (m/s)

1 2 1 2 3Measurement

LES simulation

predictive method, such as a validated CFD model, shouldbe explored.

Te predicted results by LES modeling agree well withthe measured data in the tests, which demonstrates thatthe LES model can accurately resolve the ow eld in aHVAC duct with multiple in-duct ttings. Compared tothe experimental measurement, the numerical modeling canprovide the details o pressure distribution. Tis predictedpressure distribution can be used not only independently in aductwork design (i the numerical model has been validatedbeore) but also to assist in tests to nd correct locations to

place pressure sensors. In the latter case, the use o numericalmodeling can potentially save many experimental resourcesand help to produce more reliable experimental data.

References

[] ASHRAE HandbookFundamentals, SI Edition, chapter -, American Society o Heating, Rerigerating and Air-Conditioning Engineers, Atlanta, Ga, USA, .

[] CIBSE,Te Chartered Institution of Building Services Engineers,Heating, Ventilating, Air Conditioning and Refrigeration : CIBSEGuide B, Chartered Institution o Building Services Engineers,London, UK, rd edition, .

[] I. E. Idelchik, Handbook of Hydraulic Resistance, rd edition,.

[] ASHRAE Standard P, Methods o testing to determine owresistance o HVAC air ducts and ttings, American Society oHeating Rerigerating and Air-conditioning Engineers, Atlanta,Ga, USA, .

[] B. Abushakra, I. S. Walker, and M. H. Sherman, A studyo pressure losses in residential air distribution systems, in

Proceedings of the American Council for an Energy EfficientEconomy (ACEEE ), Washington, DC, USA, , LBNLReport . Lawrence Berkeley National Laboratory, Cali,USA, .

[] L. Shao and S. B. Riffat, Accuracy o CFD or predictingpressure losses in HVAC duct ttings,Applied Energy, vol. ,no. , pp. , .

[] S. M. Atkin and L. Shao, Effect on pressure loss o separationand orientation o closely coupled HVAC duct ttings, BuildingServices Engineering Research and echnology, vol. , no. , pp., .

[] L. Shao and S. B. Riffat, CFD or prediction o k-actors o ductttings,International Journal of Energy Research, vol. , no. ,pp. , .

[] R. R. Rend, E. M. Sparrow, D. W. Bettenhausen, and J. P.Abraham, Parasitic pressure losses in diffusers and in theirdownstream piping systems or uid ow and heat transer,International Journal of Heat and Mass ransfer, vol. , pp. , .

[] J. P. Abraham, E. M. Sparrow, J. C. K. ong, and D. W.Bettenhausen, Internal ows which transist rom turbulentthrough intermittent to laminar,International Journal of Ter-mal Sciences, vol. , no. , pp. , .

[] E. M. Sparrow, J. P. Abraham, and W. J. Minkowycz, Flowseparation in a divergingconicalduct: effect o reynolds numberand divergence angle,International Journal of Heat and Massransfer, vol. , no. , pp. , .


11/12


[] W. J. Rahmeyer, Pressure loss coefficients or close-coupledpipe ells,ASHRAE ransactions, vol. , pp. , .

[] G. Gan and S. B. Riffat, k-actors or HVAC ducts: numericaland experimental determination, Building Services EngineeringResearch and echnology, vol. , no. , pp. , .

[] S. A. Mumma, . A. Mahank, and Y.-P. Ke, Analytical determi-

nation o duct tting loss-coefficients,Applied Energy, vol. ,no. , pp. , .

[] G. Gan and S. B. Riffat, Numerical determination o energylosses at duct junctions,Applied Energy, vol. , no. , pp. , .

[] S. Moujaes and R. Gundavelli, CFD simulation o leak inresidential HVAC ducts, Energy and Buildings, vol. , pp. , .

[] J. P. Abraham, J. C. K. ong, and E. M. Sparrow, Breakdowno laminar pipe ow into transitional intermittency and subse-quent attainment o ully developed intermittent or turbulentow,Numerical Heat ransfer, Part B, vol. , no. , pp. , .

[] W. J. Minkowycz, J. P. Abraham,and E. M. Sparrow, Numerical

simulation o laminar breakdown and subsequent intermittentand turbulent ow in parallel-plate channels: effects o inletvelocity prole and turbulence intensity,International Journalof Heat and Mass ransfer, vol. , no. , pp. ,.

[] J. P. Abraham, E. M. Sparrow, and J. C. K. ong, Heat transerin all pipe ow regimes: laminar, transitional/intermittent, andturbulent,International Journal of Heat and Mass ransfer, vol., no. , pp. , .

[] R. D. Lovik, J. P. Abraham,W. J.Minkowycz, and E. M. Sparrow,Laminarization and turbulentization in a pulsatile pipe ow,Numerical Heat ransfer: Part A, vol. , no. , pp. ,.

[] J. P. Abraham, E. M. Sparrow, and W. J. Minkowycz, Internal-

ow nusselt numbers or the low-reynolds-number end o thelaminar-to-turbulent transition regime, International Journalof Heat and Mass ransfer, vol. , no. , pp. , .

[] D. A. Kose and E. Dick, Prediction o the pressure distributionon a cubical building with implicit LES, Journal of WindEngineering and Industrial Aerodynamics, vol. , no. , pp., .

[] C.-H. Hu, M. Ohba, andR. Yoshie, CFD modellingo unsteadycross ventilation ows using LES,Journal of Wind Engineeringand Industrial Aerodynamics, vol. , no. , pp. ,.

[] C. M. Mak, J. Wu, C. Ye, and J. Yang, Flow noise rom spoilersin ducts,Journal of the Acoustical Society of America, vol. ,no. , pp. , .

[] Fluent, Ansys Fluent . Teory Guide: urbulence, ANSYS,Canonsburg, Pa, USA, .

[] J. O. Hinze, urbulence, McGraw-Hill, New York, NY, USA,.

[] J. Smagorinsky, General circulation experiments with theprimitive equations: the basic experiment, Monthly WeatherReview, vol. , pp. , .


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