Using Macros

• If you did not select the Create internal part option in the Atmospheric boundary layerpanel, you must select and modify the opening object that makes up the boundarylayer.

25.2 Diffuser Macros

For simulations of room airflow, it is important to accurately model the behavior of airinlet diffusers and thus predict the penetration and spread of the air in the room. Thisperformance aspect of air inlet diffusers is referred to as the throw of the diffuser, which isdefined as the distance downstream of the diffuser where the jet centerline velocity equalsa specified terminal velocity. Due to the large scale differences between the dimensionsof the room and the important small geometrical features of diffusers, it is infeasiblecomputationally to explicitly model the detailed geometry of diffusers. Simply neglectingthe geometrical details of the diffuser usually leads to airflow simulations that deliver thecorrect amount of air into the room, but fail to achieve the proper jet penetration.However, simplified diffuser models that account for diffuser performance or the velocityfield in the vicinity of the diffuser exit can be used to predict the penetration and spreadof the air jet into the room. Airpak includes diffuser macros to help you accurately modelseveral different types of diffusers with simplified boundary conditions.

Information about modeling diffusers in Airpak is presented in the following sections:

• Section 25.2.1: Diffuser Modeling Methods

• Section 25.2.2: Diffuser Types

• Section 25.2.3: Steps for Adding a Diffuser to Your Airpak Model

• Section 25.2.5: Specifying Airflow Performance Data

• Section 25.2.4: Specifying Geometry, Position, and Size

• Section 25.2.6: Specifying the Extrusion and Flow Directions

• Section 25.2.7: Additional Inputs for Specific Types of Diffusers

• Section 25.2.8: Specifying Supply-Air and Room-Air Conditions

• Section 25.2.9: Specifying the Modeling Method

25.2.1 Diffuser Modeling Methods

The two primary methods for modeling diffusers with simplified boundary conditions [5]are the momentum method and the box method. These two methods are described below.

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25.2 Diffuser Macros

Momentum Method

The momentum method assumes that the airflow from a particular diffuser can be pre-dicted using the isothermal axisymmetric jet formula

umu0

= K1

√A0

x(25.2-1)

where um is the centerline or maximum jet velocity at a distance x from the diffuser, u0

is the initial jet velocity, A0 is the effective area, and K1 is the empirically-determinedcenterline velocity decay constant. K1 may be a function of many factors, includingdiffuser type, Reynolds number, jet confinement, or possible deflections [1]. In airflowsimulations, the volumetric flow rate, Q, is specified and diffuser performance data pro-vide information on the throw x for a specific terminal velocity um and a given flow rateQ. Equation 25.2-1 can then be rearranged to solve for the initial jet velocity:

u0 =u2mx

2

K21Q

(25.2-2)

where Q = u0A0.

The initial jet velocity, u0, may be larger than the velocity, u, that is calculated by simplyusing the volumetric flow rate and the geometric area that the diffuser occupies (Ageom):

u =Q

Ageom

(25.2-3)

To properly model the diffuser performance, the simulation needs to ensure that theairflow from the diffuser enters the room with momentum corresponding to velocity u0

instead of momentum corresponding to velocity u. Typically, the diffuser is modeledwith a volumetric flow rate Q and a cross-sectional area Ageom. A momentum sourcethat accounts for u0 being the initial jet velocity is added to the diffuser to reproducediffuser performance in the simulation.

The same approach is used for linear jets; however, the implementation in Airpak forceiling diffusers (circular, square, and vortex) maintains the radial or lateral jet behaviorof the diffuser by modeling the circumferential distance of the diffuser and extruding thedirection normal to the ceiling until the modeled flow area equals the computed effectivearea.

Box Method

Although the momentum method works well for most types of diffusers, entrainmenteffects of discrete air jets from nozzle, linear slot, or valve diffusers are more accuratelymodeled using the box method.

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Using Macros

The box method does not explicitly model the jet behavior in the immediate vicinityof the air inlet diffuser; instead, it specifies flow boundary conditions on the sides ofa rectangular region adjacent to the air inlet diffuser. For best accuracy with the boxmethod, velocity profiles from the inlet diffuser manufacturer or from measured datashould be used.

The box method in Airpak uses flat velocity profiles to account for entrainment effects,but you can improve the accuracy by supplying measured velocity profiles when you areediting the objects created by the diffuser macro

25.2.2 Diffuser Types

The following diffuser types are available in Airpak:

• Grille

• Ceiling

• Displacement

• Slot

• Nozzle

• Valve

• Vortex

Section 25.2.3 describes the steps for defining a diffuser in Airpak. See Figure 25.2.1 forgeneral information about inputs for each type of diffuser. Details about the inputs foreach type of diffuser are provided in Sections 25.2.5–25.2.9.

25.2.3 Steps for Adding a Diffuser to Your Airpak Model

To include a diffuser in your Airpak model, select Boundary conditions and then Diffuserin the Macros menu. This will open the Diffuser panel, shown for the different types ofdiffusers in Figures 25.2.2–25.2.14.

Macros−→Boundary conditions−→Diffuser

The procedure for adding a diffuser to your model is as follows:

1. Specify the Diffuser type by selecting Grille, Ceiling, Displacement, Slot, Nozzle, Valve,or Vortex. The panel will change (as shown in Figures 25.2.2–25.2.14) dependingon your selection of Diffuser type. Note that each Diffuser type panel has two tabs:Geometry and Properites.

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25.2 Diffuser Macros

Figure 25.2.1: Diffuser Macro Flow Chart

c© Fluent Inc. May 22, 2007 25-13

Using Macros

2. Specify the geometry, position, and size of the diffuser. See Section 25.2.4 fordetails.

3. Specify the airflow performance data. See Section 25.2.5 for details.

4. Specify the extrusion direction and, if required, the flow direction for the diffuser.See Section 25.2.6 for details.

5. Specify any additional inputs that may be required for the diffuser. See Sec-tion 25.2.7 for details.

6. Specify the supply-air and room-air conditions. See Section 25.2.8 for details.

7. Select the method of modeling the diffuser, and specify the related parameters. SeeSection 25.2.9 for details.

8. Specify whether you want the diffuser to be available as one part or as individualobjects:

• If you want the diffuser to be available as one part in your Airpak model,select Create internal part and specify a name in the Name text entry field.The default name is diffuser.n, where n is the next sequential number amongnumbered diffusers.

• If you want the diffuser to be available as individual block, source, fan, oropening objects, deselect Create internal part. The individual Airpak objectsthat comprise the diffuser model will have the default name of type.n, wheretype is the type of diffuser and n is the next sequential number among num-bered Airpak objects. For example, if you create a rectangular grille diffuserusing the momentum method, Airpak will create a fan object called grille.1 anda source object called grille.2.

9. If you want to save the current diffuser specifications to be reused in your cur-rent model or to be used in a different model, click the Save button to open theMacro information dialog box (which is the same as the File selection dialog boxdescribed in Section 2.1.8). Airpak will save the specifications in a subdirectorynamed diffuser info.

To load diffuser specifications that you have saved in this manner, click the Loadbutton and select the diffuser specifications to be loaded.

10. Click Accept to accept the parameters entered into the Diffuser panel and createthe diffuser.

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25.2 Diffuser Macros

i Objects created by the diffuser macro have hidden, performance-related informa-tion associated with the spatial location as defined in the Diffuser panel, and thusare not appropriate for copying or translating operations. To create several diffusersof similar geometry and performance, save the diffuser setup information and reuseit if you want to create a diffuser. The same restriction applies to running thediffuser at a different flow rate. Simply delete the previous diffuser and use thediffuser macro to create a new one.

i Objects created with the diffuser macro cannot be parameterized.

Modifying an Existing Diffuser

After the diffuser has been created and positioned in your Airpak model, it cannot bemodified using the Diffuser panel. There are two ways to modify an existing diffuser:

• If you selected the Create internal part option in the Diffuser panel, you can edit thediffuser in the same way that you edit a part (see Section 7.8.5).

• If you did not select the Create internal part option in the Diffuser panel, you mustselect and modify the object or objects that make up the diffuser.

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Using Macros

Figure 25.2.2: The Grille Diffuser Panel (Geometry Tab)

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25.2 Diffuser Macros

Figure 25.2.3: The Grille Diffuser Panel (Properties Tab)

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Using Macros

Figure 25.2.4: The Ceiling Diffuser Panel (Geometry Tab)

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25.2 Diffuser Macros

Figure 25.2.5: The Ceiling Diffuser Panel (Properties Tab)

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Using Macros

Figure 25.2.6: The Displacement Diffuser Panel (Geometry Tab)

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25.2 Diffuser Macros

Figure 25.2.7: The Displacement Diffuser Panel (Properties Tab)

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Using Macros

Figure 25.2.8: The Slot Diffuser Panel (Geometry Tab)

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25.2 Diffuser Macros

Figure 25.2.9: The Slot Diffuser Panel (Properties Tab)

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Using Macros

Figure 25.2.10: The Nozzle Diffuser Panel (Geometry Tab)

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25.2 Diffuser Macros

Figure 25.2.11: The Nozzle Diffuser Panel (Properties Tab)

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Using Macros

Figure 25.2.12: The Valve Diffuser Panel (Geometry Tab)

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25.2 Diffuser Macros

Figure 25.2.13: The Valve Diffuser Panel (Properties Tab)

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Using Macros

Figure 25.2.14: The Vortex Diffuser Panel (Geometry Tab)

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25.2 Diffuser Macros

Figure 25.2.15: The Vortex Diffuser Panel (Properties Tab)

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Using Macros

25.2.4 Geometry, Position, and Size

The geometry types that are available will depend on which type of diffuser you areusing. See Figures 25.2.2–25.2.14 and below for details. The inputs for the standardAirpak geometries are described in Section 7.4.4. See Section 7.3.6 for details on resizingan object and Section 7.3.7 for details on repositioning an object. one

i Note that or the diffuser macro, there are two additional geometry types that youcan specify for a specific type of diffuser: square geometry and perforated panelgeometry. The inputs for both square and perforated panel geometry are similarto those of standard rectangular geometry. See Section 25.2.4 for details.

Information about the availability of each geometry type and how Airpak uses the infor-mation you provide for it is provided below.

Rectangular Geometry

Rectangular geometry is available for grille, displacement, and nozzle diffusers. De-pending on the modeling method you select (momentum or box), specifying rectangulargeometry will instruct Airpak to create different objects to represent your diffuser in themodel.

Momentum Method

If you select the momentum method, Airpak will create a 3D source object (four-sidedtop and bottom) with a rectangular fan object on the supply side. The height of thesource object is determined by the effective area of the diffuser (A0 in Equation 25.2-1),which is defined as

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25.2 Diffuser Macros

A0 =(K1Q

umx

)2

(25.2-4)

where K1 is the velocity decay coefficient, Q is the volumetric flow rate, um is the terminalvelocity, and x is the throw.

Note that for rectangular displacement diffusers, the 3D source object will be created onlyif you specify an effective area ratio of less than 1. See Section 25.2.1 for more informationabout the momentum method, and Section 25.2.9 for information about inputs for it.

Box Method

If you select the box method (nozzle diffusers only), Airpak will create a hollow prismblock object with rectangular openings on five sides of the block. The opening on thesupply side will be the intake, and the four openings perpendicular to the supply sidewill be the exhaust. See Section 25.2.1 for more information about the box method, andSection 25.2.9 for information about inputs for it.

Circular Geometry

Circular geometry is available for grille, ceiling, displacement, nozzle, valve, and vortexdiffusers. Depending on the modeling method you select (momentum or box), specifyingcircular geometry will instruct Airpak to create different objects to represent your diffuserin the model.

Momentum Method

If you select the momentum method, Airpak will create different objects depending onthe type of diffuser you selected:

• Grille, displacement, or nozzle diffuser: Airpak will create a cylindrical source objectwith a circular fan object on the supply side. The height of the source object isdetermined by the effective area of the diffuser (A0 in Equation 25.2-1).

Note that for circular displacement diffusers, the cylindrical source object will becreated only if you specify an effective area ratio of less than 1.

• Ceiling or vortex diffuser: Airpak will create a cylindrical fan object. The height ofthe cylindrical fan object is determined by the effective area of the diffuser (A0 inEquation 25.2-1).

• Valve diffuser: Airpak will create a circular opening object. If you have specified aneffective area ratio of less than 1, Airpak will also create a circular partition object.The radius of the partition object is determined by the effective area of the diffuser(A0 in Equation 25.2-1).

c© Fluent Inc. May 22, 2007 25-31

Using Macros

See Section 25.2.1 for more information about the momentum method, and Section 25.2.9for information about inputs for it.

Box Method

If you select the box method (nozzle or valve diffusers only), Airpak will create a cylindricalblock object, a cylindrical fan object, and a circular opening object. The fan objectwill occupy the same space as the block object, with the block object acting as thesupport structure. The opening object will be on the supply side of the block object.See Section 25.2.1 for more information about the box method, and Section 25.2.9 forinformation about inputs for it.

Inclined Geometry

Inclined geometry is available for grille, displacement, and nozzle diffusers, for which youmust use the momentum method. Specifying inclined geometry will instruct Airpak tocreate a 3D source object (four-sided top and bottom) with an inclined rectangular fanobject on the supply side. The height of the source object is determined by the effectivearea of the diffuser (A0 in Equation 25.2-1).

Note that for inclined displacement diffusers, the 3D source object will be created only ifyou specify an effective area ratio of less than 1. See Section 25.2.1 for more informationabout the momentum method, and Section 25.2.9 for information about inputs for it.

Cylinder Geometry

Cylinder geometry is available for displacement diffusers, for which you must use themomentum method. Depending on the number of active corners you specify for thediffuser, specifying cylinder geometry will instruct Airpak to create different objects torepresent your diffuser in the model. See Section 25.2.7 for information about specifyingactive corners. See Section 25.2.1 for more information about the momentum method,and Section 25.2.9 for information about inputs for it.

Four Active Corners

If you specify the maximum of four active corners, Airpak will create a 3D fan object witha 16-sided top and bottom. If you have specified an effective area ratio of less than 1,Airpak will also create a 3D source object (four-sided top and bottom) on each of the16 equatorial sides of the fan. The height of the source objects is determined by theeffective area of the diffuser (A0 in Equation 25.2-1).

Less Than Four Active Corners

If you specify one, two, or three active corners, Airpak will create objects to representone, two, or three quarters of a cylinder. For each corner selected, Airpak will create a

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25.2 Diffuser Macros

3D “pie piece” block object with four inclined rectangular fan objects on the four outersides of the block. If you have specified an effective area ratio of less than 1, Airpak willalso create a 3D source object (four-sided top and bottom) on the four outer sides of theblock. The height of the source objects is determined by the effective area of the diffuser(A0 in Equation 25.2-1).

Prism Geometry

Prism geometry is available for displacement diffusers, for which you must use the mo-mentum method. Specifying prism geometry will instruct Airpak to create a prism blockobject with a rectangular fan object on the supply side. If you have specified an effec-tive area ratio of less than 1, Airpak will also create a 3D source object (four-sided topand bottom) on the supply side. See Section 25.2.7 for information about specifyingthe supply side for prism geometry. See Section 25.2.1 for more information about themomentum method, and Section 25.2.9 for information about inputs for it.

Polygon Geometry

Polygon geometry is available for displacement diffusers, for which you must use themomentum method. Specifying polygon geometry will instruct Airpak to create a 3D fanobject with a top and bottom that have the number of vertices specified by you. If youhave specified an effective area ratio of less than 1, Airpak will also create a 3D sourceobject (four-sided top and bottom) on each equatorial side of the fan. The height of thesource objects is determined by the effective area of the diffuser (A0 in Equation 25.2-1).See Section 25.2.1 for more information about the momentum method, and Section 25.2.9for information about inputs for it.

Perforated Panel Geometry

Perforated panel geometry is available for ceiling diffusers, for which you must use themomentum method. Specifying perforated panel geometry will instruct Airpak to createa 3D source object (four-sided top and bottom) with a rectangular fan object on thesupply side. The height of the prism source object is determined by the effective area ofthe diffuser (A0 in Equation 25.2-1). See Section 25.2.1 for more information about themomentum method, and Section 25.2.9 for information about inputs for it.

Square Geometry

Square geometry is available for ceiling diffusers, for which you must use the momentummethod. Depending on the number of active sides you specify for the diffuser, specifyingsquare geometry will instruct Airpak to create different objects to represent your diffuserin the model. See Section 25.2.7 for information about specifying active sides. SeeSection 25.2.1 for more information about the momentum method, and Section 25.2.9for information about inputs for it.

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Using Macros

Four Active Sides

If you specify the maximum of four active sides, Airpak will create a 3D polygon fanobject. The height of the polygon fan object is determined by the effective area of thediffuser (A0 in Equation 25.2-1).

Less Than Four Active Sides

If you specify one, two, or three active sides, Airpak will create a prism block object withrectangular intake fan objects on the specified active sides of the block object. The fanobjects will be perpendicular to the supply side of the block.

Linear Geometry

Linear geometry is available for slot diffusers. Depending on the modeling method youselect (momentum or box) and number of active sides you specify for the diffuser, spec-ifying linear geometry will instruct Airpak to create different objects to represent yourdiffuser in the model. See Section 25.2.7 for information about specifying active sides.See Section 25.2.1 for more information about the momentum method, and Section 25.2.9for information about inputs for it.

Momentum Method with Four Active Sides

If you select the momentum method and specify the maximum of four active sides, Airpakwill create a 3D polygon fan object. The height of the polygon fan object is determinedby the effective area of the diffuser (A0 in Equation 25.2-1).

Momentum Method with Less Than Four Active Sides

If you select the momentum method and specify one, two, or three active sides, Airpakwill create a prism block object with rectangular intake fan objects on the specified activesides of the block object. The fan objects will be perpendicular to the supply side of theblock.

Box Method

If you select the box method, Airpak will create a hollow prism block object with rect-angular openings on up to five sides of the block. The number of openings will dependon the number of active sides that you specify. The opening on the supply side will bethe exhaust, and the other openings perpendicular to the supply side will be the intake.See Section 25.2.1 for more information about the box method, and Section 25.2.9 forinformation about inputs for it.

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25.2 Diffuser Macros

Vertical Geometry

Vertical geometry is available for slot diffusers. Depending on the modeling method youselect (momentum or box), specifying vertical geometry will instruct Airpak to createdifferent objects to represent your diffuser in the model.

Momentum Method

If you select the momentum method, Airpak will create a 3D source object (four-sidedtop and bottom) with a rectangular fan object on the supply side. The height of thesource object is determined by the effective area of the diffuser, A0, which is defined inEquation 25.2-4. The air flow will be delivered normal or at a specified angle to thesurface on which the diffuser is located. See Section 25.2.1 for more information aboutthe momentum method, and Section 25.2.9 for information about inputs for it.

Box Method

If you select the box method, Airpak will create a hollow prism block object with rect-angular openings on five sides of the block. The opening on the supply side will bethe intake, and the four openings perpendicular to the supply side will be the exhaust.See Section 25.2.1 for more information about the box method, and Section 25.2.9 forinformation about inputs for it.

clearpage

25.2.5 Specifying Airflow Performance Data

There are two ways you can specify performance data for the diffuser:

• Specify data for a single point.

• Specify a curve or curves.

i For displacement and valve diffusers, only the first method is available.

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Using Macros

Specifying Performance Data for a Single Point

To specify performance data for a single point, select Performance data in the Diffuserpanel and specify the following values:

• Volumetric flow rate of air through the diffuser (Q in Equation 25.2-2)

• Terminal velocity (um in Equation 25.2-2) that corresponds to the air jet centerlinevelocity value for which the throw is specified

• Throw distance of the air jet from the diffuser (x in Equation 25.2-2) at which theair jet centerline velocity equals the specified terminal velocity

Note that, for displacement and valve diffusers, only the volumetric flow rate is specified.

Specifying Performance Curves

To specify performance data for a series of points, select Performance curves in the Dif-fuser panel. For grille, ceiling, slot, nozzle, and vortex diffusers, you can define up tothree curves using the Performance data panel (Figure 25.2.16). As noted above, thisspecification method is not available for displacement and valve diffusers.

The procedure for defining a performance curve is as follows:

1. Specify the number of the curve (1, 2, or 3) in the drop-down list above the Editbutton.

2. Click Edit to open the Performance data panel.

3. Specify a list of values in the Performance data panel. A curve specification consistsof a list of volumetric flow rate/throw sets, with one volumetric flow rate value andone throw value in each set. It is important to give the numbers in sets of two,but the spacing between numbers is not important. The sets must be entered inorder of increasing volumetric flow rate so that the data can be interpolated whenrequired.

4. Specify the Terminal velocity that corresponds to the air jet centerline velocity valuefor which the throw is desired. The throw distance of an air jet is a function of thevolumetric flow rate for a particular terminal velocity.

5. Select the appropriate Volumetric flow rate units for your listed values from the unitdefinition list.

6. Select the appropriate Throw units for your listed values from the unit definitionlist.

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25.2 Diffuser Macros

Figure 25.2.16: The Performance data Panel

7. Click Accept. This will store the values and close the Performance data panel. Airpakwill interpolate the data you provided for the volumetric flow rate and the throwto create a profile for the diffuser.

8. Click Edit in the Diffuser panel to re-open the Performance data panel.

9. In the Performance data panel, specify the Volumetric flow rate for the diffuser.

10. Click Compute throw to compute the throw distance value for the diffuser basedon the performance curve. In the Diffuser panel, a set of Performance data will beupdated (Volumetric flow rate, Terminal velocity, and Throw). The Message windowwill also print this information.

Loading a Previously Defined Curve

To load a previously defined profile, click on Load. (See Section 28.2.8 for details onsaving contour data and using them as a profile.) This will open the Load curve file

c© Fluent Inc. May 22, 2007 25-37

Using Macros

selection dialog box. Select the file containing the profile data and click Accept. SeeSection 2.1.8 for details on selecting a file. If you know the units used in the profile datayou are loading, you should select the appropriate units in the Performance data panelbefore you load the profile.

Saving a Curve

To save a profile, click on Save. This will open the Save curve dialog box, in which youcan specify the filename and directory to which the profile data are to be saved.

25.2.6 Specifying the Extrusion and Flow Directions

Specifying the Extrusion Direction

For all diffuser types, you will need to specify the direction of the Extrusion. For 3Ddiffusers, the height, or extrusion, is usually calculated using the effective area (A0 inEquation 25.2-1). The extrusion direction is the direction in which the Airpak objectsare extended. Depending on the location of the diffuser in the room, you need to makesure that the diffuser is extruded into the room volume and not out of the room volume.

There are two options for the Extrusion direction:

Max specifies that the extrusion points in the direction of increasing coordinate value.For example, if you have positioned a grille diffuser on a surface on the x-y planewith the positive z direction pointing into the room, you should select Max.

Min specifies that the extrusion points in the direction of decreasing coordinate value.For example, if you have positioned a grille diffuser on a surface on the x-z planewith the negative y direction pointing into the room, you should select Min.

Note that you will not specify the extrusion direction in this way for a cylindrical dis-placement diffuser. See Section 25.2.7 for details.

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25.2 Diffuser Macros

Specifying the Flow Direction

If you are modeling grille, slot, nozzle, or certain types of ceiling (perforated panel ge-ometry) or displacement (rectangular, circular, or inclined geometry) diffusers, you willneed to specify the Flow direction. There are two options:

• If the air flows into the room normal to the diffuser, select Normal.

• To specify the flow angle of the fluid entering the room through the diffuser, selectGiven. There are two specification methods:

Direction allows you to specify the direction vector (x, y, z) for the flow. Onlythe direction of the vector is used by Airpak; the magnitude is ignored.

Angles allows you to specify the direction of the flow entering the room in termsof angles relative to the inward-pointing normal direction. You can specifytwo angles, A (α) and T (θ), to account for incoming flow not aligned with thenormal direction. If the normal inward vector points in the positive direction,then α and θ must have values in the range −90◦ < angle < 90◦, with α=θ=0◦

representing flow in the positive normal direction. If the normal inward vectorpoints in the negative direction, then α and θ must have values in the range90◦ < angle < 270◦, with α=θ=180◦ representing flow in the negative normaldirection. Since the diffuser may be oriented in any one of three planes (x-y,y-z, or x-z), the roles of α and θ will vary depending on the orientation of theplane of the diffuser.

If you are modeling valve, vortex, or certain types of ceiling (square geometry) or displace-ment (cylindrical or prism geometry) diffusers, you will use other methods to specify theflow direction. See Section 25.2.7 for details. If you are modeling polygon displacementdiffusers or circular ceiling diffusers, the flow direction will be normal to the diffuser.

25.2.7 Additional Inputs for Specific Types of Diffusers

For specific types of diffusers, there are additional inputs that are required in the Diffuserpanel. These special inputs are described below.

Specifying Active Corners for a Cylindrical Displacement Diffuser

If you have selected cylindrical geometry for a displacement diffuser, you must specifythe active Corners of the diffuser. You must specify a minimum of one active corner andmay specify up to four. Depending on the plane you selected for the diffuser, the fourcorner options (+ +, − +, − −, + −) will represent the four quadrants of a completecylinder with its center at the location that you have specified.

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Using Macros

For example, if you have specified an x-z plane, the + + option represents the quadrant onthe positive x and positive z side of the center of the cylinder. When viewed from the Posi-tive Y orientation, the + + option represents the upper right quadrant of the diffuser. The− +, − −,and + − options represent the remaining three quadrants in a counter-clockwise direction(upper left, lower left, lower right), respectively.

Specifying the Supply Side for a Prism Displacement Diffuser

If you have selected prism geometry for a displacement diffuser, you must specify the Sidefrom which air will be supplied to the room (Xmin, Xmax, Ymin, Ymax, Zmin, or Zmax).Xmax, Ymax, and Zmax refer to the sides of the diffuser facing in the positive coordinatedirections. Xmin, Ymin, and Zmin refer to the sides of the diffuser facing in the negativecoordinate directions.

Specifying Active Sides for Ceiling and Slot Diffusers

If you have selected square geometry for a ceiling diffuser or linear geometry for a slotdiffuser, you must specify the Active sides of the diffuser. You must specify a minimumof one active side and may specify up to four. Depending on the plane you selected forthe diffuser, four different sides of the diffuser (Xmin, Xmax, Ymin, Ymax, Zmin, or Zmax)will be available for you to define as active sides. Xmax, Ymax, and Zmax refer to thesides of the diffuser facing in the positive coordinate directions. Xmin, Ymin, and Zminrefer to the sides of the diffuser facing in the negative coordinate directions.

For example, if you have specified an x-z plane, the Ymin and Ymax options will not berelevant, and you must choose from one to four sides from Xmin, Xmax, Zmin, and Zmax.

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25.2 Diffuser Macros

Special Inputs for Valve and Vortex Diffusers

If you have selected a valve diffuser, you will need to specify the Jet cone angle insteadof the flow direction.

If you have selected a vortex diffuser, you will need to specify the Swirl angle instead ofthe flow direction.

25.2.8 Specifying Supply-Air and Room-Air Conditions

Specifying Supply-Air and Room-Air Temperatures

For all diffuser types, Airpak calculates a local Archimedes number (Ar) to give an indica-tion of the relative importance of buoyant forces. For Airpak to calculate the local valueof Ar, you will need to estimate the temperatures of both the supply air and the roomair. Specify the temperature of the supply air in the Temperature field under Supply, andthe temperature of the room air in the Temperature field under Room.

Specifying Supply-Air Species Parameters

If you have enabled the Species option in the Problem setup panel, you can specify speciesparameters for the diffuser supply air using the Supply species concentration panel (Fig-ure 25.2.17). To open the Supply species concentration panel, select the Species option inthe Diffuser panel and click Edit.

Figure 25.2.17: The Supply species concentration Panel

See Chapter 21 for details on modeling species transport.

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Using Macros

Specifying Supply-Air Turbulence Parameters

If you selected the two-equation or RNG k-ε model in the Basic parameters panel, you canspecify turbulence parameters for the diffuser supply air using the Turbulence equationparameters panel (Figure 25.2.18). To open the Turbulence equation parameters panel,select Turbulence in the Diffuser panel and click Edit.

Figure 25.2.18: The Turbulence equation parameters Panel

Specify the Turbulent intensity as a percentage, and the Turbulent length scale in theappropriate units of length. See Section 6.4.3 for details on determining turbulenceparameters.

25.2.9 Specifying the Modeling Method

For all diffuser types, you will need to select the method of modeling the diffuser. SeeSection 25.2.1 for more information about diffuser modeling methods.

There are two methods of modeling diffusers in Airpak.

Momentum method specifies the momentum method for modeling the diffuser. Thisoption is available for all diffuser types, and is recommended for grille, ceiling,displacement, and vortex diffusers. See Section 25.2.4 for details about the types ofAirpak objects that are created when you choose the momentum method for specificdiffusers. See below for information about input options for the momentum method.

Box method specifies the box method for modeling the diffuser. This option is avail-able and recommended for slot, nozzle, and valve diffusers. See Section 25.2.4 fordetails about the types of Airpak objects that are created when you choose the boxmethod for specific diffusers. See below for information about input options for thebox method.

Inputs for the Momentum Method

The default approach in the momentum method is to use jet theory and published cor-relations to determine the effective area. You can modify this approach in several ways:

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25.2 Diffuser Macros

• Specify that corrections be applied

• Specify parameters that are used in the correlations for some diffusers

• Specify aspects of the diffuser performance that you want to control yourself

The options for controlling the momentum method are as follows:

Attached jet correction specifies a correction factor for the default value of K1. Thisoption is available for grille diffusers and accounts for jets from high side-walldiffusers attached to the ceiling.

Velocity decay constant directly specifies a value for K1 that overrides the default valueof the velocity decay constant. This option is available for grille, ceiling, slot, nozzle,and vortex diffusers.

Variable constant calculates a value for K1 based on the estimated discharge velocity.This option is available only for slot diffusers with linear geometry.

Effective area ratio directly specifies the ratio of the effective area (A0 in Equation 25.2-1)to the total area of the diffuser. This option is available for grille, displacement,slot, nozzle, and valve diffusers. Specifying the effective area ratio, which can besupplied by the diffuser manufacturer or obtained from your own diffuser perfor-mance measurements, allows you to bypass calculating the effective area based onjet theory and correlations.

Free area ratio directly specifies the free area ratio, which is a parameter in correlationsfor the velocity decay constant K1 for grille and perforated-panel ceiling diffusers.

The Compute button instructs Airpak to calculate values for various parameters anddisplay them in the Message window. These parameters include the discharge velocity,effective area, Archimedes number, and others that depend on the momentum methodoptions you have selected.

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Using Macros

Figure 25.2.19: The Box method parameters Panel

Inputs for the Box Method

You can specify parameters for the box method using the Box method parameters panel(Figure 25.2.19). To open the Box method parameters panel for a slot, nozzle, or valvediffuser, select the Box method option in the Diffuser panel and click Edit.

The procedure for specifying box method parameters for your diffuser is as follows:

1. For rectangular diffusers, enter values for the lengths of the sides (xL, yL, zL) of thebox under Box geometry type. For circular diffusers, enter values for the Height andRadius of the “box”.

2. Click the Check box button to check whether the box dimensions are large enoughto adequately model the diffuser.

i The Archimedes number (Ar) cannot be 0 if you are using the box method. Ifyou get an Error dialog box, click OK and adjust either the room temperature orthe supply air temperature so that they are different values. See Section 25.2.8for details.

The Message window will inform you if your box dimensions are too small or toolarge. Once you have specified adequate box dimensions, click Check box and theMessage window will display Check OK.

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25.3 Geometry Macros

3. Specify the Entrainment factor, which is the ratio of the volumetric flow rate of theair jet relative to the volumetric flow rate of air exiting the diffuser.

4. If you have enabled the Species option in the Problem setup panel, you can specifyspecies parameters for the diffuser exhaust air using the Exhaust species concentra-tion panel (Figure 25.2.20). To open the Exhaust species concentration panel, selectSpecies in the Box method parameters panel and click Edit.

Figure 25.2.20: The Exhaust species concentration Panel

See Chapter 21 for details on modeling species transport.

5. Click the Compute Tbox & Cbox button to compute the temperature and, if required,the mass fraction of species that exit the box. This accounts for mixing betweenthe supply air and the entrained room air. The Message window will display valuesfor Tbox (temperature) and Cbox (mass fraction).

6. Click Accept in the Box method parameters panel.

25.3 Geometry Macros

The following geometry-related macros are described in this section:

• Section 25.3.1: Polygonal Ducts

• Section 25.3.3: Closed Box

• Section 25.3.4: 1/4 Polygonal Cylinder

• Section 25.3.5: Cylinder Plates

• Section 25.3.6: Cylindrical Enclosure

• Section 25.3.7: Polygonal Circle

• Section 25.3.8: Polygonal Cylinder

• Section 25.3.9: Hemisphere

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