airpak常见问题问与答-整理版

airpak常见问题问与答（一）建模基础

1) 2)

My model is very complicated, what is the best way to build it ? .......................................................... 6 Start simple. First include the most important items such as the cabinet, big power devices,

fans, vents and openings, if any. For first level analysis, ignore small details. For example, you may smear power sources over blocks or plates, instead of modeling them in detail using small blocks. Make sure the simple model runs well and the solution makes sense. You can then gradually include more details into your model. It is absolutely essential that you build your model step by step! ......... 6 3) 5)

How can I simplify my model? ............................................................................................................................. 6 As with all software products, it is advisable to save as often as possible. This will ensure that 4) How often shall I save? ............................................................................................................................................... 7 your data is not lost in the event of machine or system failure. In particular, it is important to save the model before starting the solution process. So, if you need to terminate your solution process for any reason using the control C option, the latest model will be saved. ........................................................... 7 6) I am making models with only small changes between them, shall I save them under different names or get different solutions under the same name? ...................................................................................... 7 7)

If the changes are geometric (so that the mesh between the two is different) you should save them under different names. For example, if you have a model named job1, you may copy it to job2. Then make the changes you want in job2 and run it. However, if you are just changing the ventilation flowrate or inlet temperature (so that the mesh is the same), you may either save it to a different name, or, if you wish to use the Restart option, run a second solution under the same name. .............. 7 8) 9) 10) 11)

My area of interest is not surrounded by walls on all sides, is it OK to use the room walls or Should I include the effect of gravity in my model? .................................................................................... 8 Should I include the effect of radiation in my model? ................................................................................ 8 In most forced convection cases (where fluid is pushed through by fans, pumps, etc.), the should I put openings on these walls? .......................................................................................................................... 8

effect of radiation is under 2-5% and may be ignored. In some cases, the effect may be of the order of 10% or even greater and must be included. Examples are: ............................................................................. 8 12) 13) 14)

The calculation for the view-factor matrix is taking too long, what is happening? ......................... 9 How do I decide whether the flow is turbulent or laminar? ................................................................... 10 Airpak estimates the non-dimensional numbers which will help you determine whether the

flow is turbulent or laminar. For forced convection problems, the number to watch is the Reynolds number, for natural convection problems, look at the Rayleigh number. You can look at the estimates of these numbers by clicking the \"Reset\" button in Solution/Setup. The numbers as well as the recommended flow setting will be displayed in the messages window. In general, the flow is considered tubulent if the Reynolds number is greater than 5000 or the Rayleigh number if greater than 1e+8. The user is advised to use his/her own judgement as to this setting, particulary when the

flow regime is in the transition region (between laminar and turbulent). .................................................... 10 15) 16) 17) 18) 19)

Is is easier to import a model from a CAD package or construct the model inside Airpak? ....... 10 What are the CAD package files I can import into Airpak? ..................................................................... 11 What units should I use? ...................................................................................................................................... 11 How do I take the effect of elevation into account? .................................................................................. 11 The effect of elevation can be taken into account by simply altering the density of air in the

materials database. The default value of air density can be seen in \"Model/Materials\". Go to \"Fluid/Gases/Air\" and enter the new air density for the elevation under consideration. Make sure to click \"Update\" after editing the material properties. ............................................................................................ 11 20) 21) 22) 23)

My model is symmetric but my solution is not, what is the reason? ................................................... 12 What is a typical magnitude of flow velocity due to (pure) natural convection? ........................... 12 Can I predict condensation in a room?............................................................................................................ 12 In Airpak, you can track water vapor as a species and look at the relative humidity. Although

there is no condensation mass transfer model in Airpak, seeing results in which the relative humidity approaches 100% in some areas is a good indication that condensation is likely. .................................... 12 24) 25) 26) 27) 28)

My Airpak jobs are taking up too much disk space, what should I do?.............................................. 12 Use \"Cleanup\" under \"File\" menu and delete mesh, scratch, and solution files ............................. 13 How can I free-up disk space without deleting model, mesh and postprocessing files?............. 13 What are the three most essential files of an Airpak model? ................................................................. 13 They are \"job\and \"problem\". These files contain all the information specified to

build the model, such as the size and location of objects, properties of objects, problem setup data, mesh specifications, and solver settings. Any Airpak model must have atleast these three files. ....... 13 29) 30) 31) 32)

What does \"pack\" and \"unpack\" mean? ......................................................................................................... 13 How do I print out an Airpak model? .............................................................................................................. 14 Where do I define my units? ............................................................................................................................... 14 You can define units in \"File/Configure\". Click \"Edit unit definitions\" and define the units for

any quantity (length, conductivity, power, etc.) and click \"Set as default\". The units so defined will be applied to any new object introduced into the cabinet. You can also alter the units of any variable of an object by editing the object and changing the unit definitions selected. To do this, just click the unit indicator beneath the quantity under consideration and change it to any other unit in the pick-list. The user also has the ability to simply set the type of units used for the entire problem to SI units or Imperial (English) units. ................................................................................................................................... 14 33) 34)

How do I run a time-dependent or unsteady simulation? ....................................................................... 15 First select \"Transient\" by going to \"File/Problem\" and then specify \"Start\

step increment\specifiy the \"Iteration/timestep\". ................................................................................................................................. 15 35) 36)

What are initial conditions? ................................................................................................................................. 15 \"Radiation temperature\" is the partial enclosure temperature applied when the default

surface-to-surface radiation model is used. When a given surface radiates to some surfaces, normally all of the radiation does not reach these surfaces. The portion of radiation that doesn't reach these surfaces is assumed to go to the ambient maintained at a reference temperature called the \"Radiation temperature\". ......................................................................................................................................... 15 37) 38)

What is the \"Minimum object separation\"? .................................................................................................. 16 This is a value you specify in \"File/Configure\". It sets a tolerance limit where if any two adjacent

faces are separated by a distance less than this tolerance limit, Airpak warns you of the situation. You may choose to cancel the meshing and examine the model for any errors or ignore the warning and go ahead with the meshing. It is not advisable to let Airpak make the changes automatically for it may make many unintended changes. ........................................................................................................................ 16 39) 40) 41)

What is the meaning of \"Radiation temperature\" on the \"Problem/Setup\" menu? ...................... 16 My Airpak model is so cluttered, what should I do to see the model more clearly? ..................... 16 You can suppress the views of some objects. See the next FAQ on suppressing views for how to

do it. You can also use the \"Gouraud shading\" on a given class of objects (or just one object) to see these objects clearly. .......................................................................................................................................................... 17 42) 43) 44)

How can I suppress the view of objects in Airpak? .................................................................................... 17 Sometimes the dimensions of objects contain too many decimal places: Is there a way to avoid Before beginning to build your model, set the \"x-grid\

this situation? ....................................................................................................................................................................... 17 decimal place you want to have (for example 0.0001). This will force objects to move or locate with fixed increments and you will not get the unwanted decimals unless you scale the model. ................. 17 45) 46) 47) 48) 49) 50) 51) 52) 53)

How do I move, copy or scale a group of objects at the same time? .................................................. 18 Use the \"Groups\" option in the \"Model\" menu to group the objects. You can then move, copy, How do I move a group of objects using the mouse? ............................................................................... 18 Use the \"Groups\" option in the \"Model\" menu and group the objects. Then toggel on the How can I change the specifications (material type, power, etc.) of a group of objects in my What is a part? .......................................................................................................................................................... 19 How can I scale an object with different scale factors in each of the coordinate directions? .... 19 How do I reverse the order of object creation? ........................................................................................... 19 You can change the priority values in \"Model/Priorities\". Objects that were created first have a

or scale as you wish. Please refer to the training notes or the Airpak manual for more details. .......... 18

\"Move group\" button and move the group using the mouse (shift + middle mouse button). ............ 18 model? ..................................................................................................................................................................................... 18

lower priority number than objects that were created later. By changing these numbers, you can change the order of creation. This in turn affects, among other things, the meshing hierarchy. ........ 19 ) 55) 56)

How do I change the default values of material properties? .................................................................. 20 How do I define new materials in my database? ......................................................................................... 20 How do I view my model in shaded form? .................................................................................................... 21

57) 58) 59)

What is the difference between \"Device\" and \"Approach\" methods in the calculation for What is a macro? ..................................................................................................................................................... 22 A macro is a compact and easy representation of a complex system. Examples are heat sink

pressure losses in vents and resistances?................................................................................................................... 21

macros and macros for packages (BGA). Such models may be used when detailed modeling is time consuming and/or impractical. ...................................................................................................................................... 22 60) 61) 62) 63) )

I have created a heat sink macro and want to send it to my colleague. What is the quickest Save the heat sink macro settings (in the heat sink creation form) and send that file to your How do I apply a transient thermal loading (power) to my block? ...................................................... 23 Can I prescribe several transient power loading segments to my model? ........................................ 23 Yes. For example, you can ramp up the power for 5 minutes, maintain the maximum power for option? .................................................................................................................................................................................... 23 colleague. The settings can be saved in a file insied the directory \"heatsink_info\". ................................. 23

10 minutes, and ramp down the power for 15 minutes. To do this, use the \"Text editor\" (under \"Piecewise linear\") and specify the time-dependent power. .............................................................................. 23 65) 66) 67) 68) 69) 70) 71) 72) 73) 74) 75) 76)

I am only interested in getting the flow distribution and not the temperature. How do I solve Go to \"File/Problem\" and click off \"Temperature\" in the \"Variables solved\" option. ................... 24 I have obtained the flow distribution from a previous run and want to solve my model for I am modeling a pole-mounted enclosure and want to include the effect of solar loading, what I am modeling external airflow. How large should my room be? ......................................................... 24 Can I build my model in inches and use the SI system of units to specify the other parameters Yes. You can build your model in inch and still use other system of units for the other The origin of my room is a non-zero number, can I convert it to (0,0,0)? ......................................... 25 Yes. Use the \"Move\" command in \"Model/Room\" and translate the room in the X,Y,Z directions I am modeling a telecom rack and do not know whether I should use a plate or PCB for my How can I model small air gaps or interface material between surfaces? ......................................... 26 This can be done using contact resistances or conducting thin plates. Both are planar objects for the flow distribution only? ....................................................................................................................................... 24

another power loading. Can I only solve for temperature? ................................................................................. 24 is the best approach? ......................................................................................................................................................... 24

such as conductivity, power, etc.? ................................................................................................................................. 25 specifications. ....................................................................................................................................................................... 25

by giving the proper offset. ............................................................................................................................................ 25 motherboard? ....................................................................................................................................................................... 25

that provide thermal resistance normal to the plane. The difference between the two is that conducting thin plates have both in-plane and normal condction, whereas contact resistance plates have normal conduction only. In both cases, you must specify an effective thickness. For contact resistances, you can also specify thermal resistance explicitly. ......................................................................... 26

77) 78) 79)

What is the difference between conducting thick partitions and contact resistances? ................ 26 My region of interest is not a rectangular prism, how can I construct the room in Airpak? ...... 27 You can use polygonal hollow blocks to block off a part of the room. Hollow blocks not only

allow you to shape your room but also reduce the mesh size of your model, since no mesh is constructed inside them. .................................................................................................................................................. 27 80) 81) 82) 83) 84)

When should I use hollow blocks? .................................................................................................................... 27 I have a table of volume flow vs pressure data for my fan, can I load this data directly, instead Yes, you can load it directly using the \"Load\" option in the fan curve \"Graph editor\". How do I take the effect of elevation into account? .................................................................................. 28 The effect of elevation can be taken into account by changing the default density of air for

of entering it manually? .................................................................................................................................................... 27 Alternatively, you can also copy the text using the mouse and transfer it to the \"Text editor\". .......... 27

constant density problems. The default value of the density of air can be seen in \"Model/Materials/Fluid/Gases\". Calculate the air density at the elevation under consideration and enter that value by clicking \"Edit\". Click \"Update\" to save changes. For variable density problems, change the operating pressure to account for elevation. .................................................................................... 28 85) 86) 87) 88) ) 90) 91) 92)

How do I determine the operating point of a fan? ..................................................................................... 28 What is the difference between blocks and partitions? ........................................................................... 28 What is the difference between internal and external fans? .................................................................. 29 Internal fans are meant to be placed inside the room. External fans must be on room walls or Is there flow through the hub of my fan? ...................................................................................................... 29 There is no flow through the hub of a fan. The flow comes from the no-hub area only. ............ 29 How do I incorporate a swirl effect into my fan flow? .............................................................................. 29 You can provide a swirl factor in the fan specification form. Check the fan manufacturer's data

on the interior faces of hollow blocks; they must on boundary surfaces. ..................................................... 29

or the appropriate handbook for this factor. You can also specify the fan RPM and the swirl factor will be calculated for you in Airpak. In this case, the the slip is assumed to be 95%. ....................................... 29 93) 94) 95) 96)

Can I use partitions on the sides of the room? ............................................................................................ 30 What is the difference between partitions and walls? .............................................................................. 30 How do I specify a non-uniform conductivity to my blocks or partitions? ....................................... 30 You may do this by modifying the conductivity values in \"Model/Materials\". Solids materials

can have Isotropic, Orthotropic or Anisotropic conductivities. Isotropic solids have uniform conductivities in all directions. Orthotropic materials can have different conductivities in X, Y, and Z directions. The conductivity of Anisotropic materials is defined by a nine component tensor. So, depending on the complexity of your materials, you can choose either Orthotropic or Anisotropic options for your materials. .............................................................................................................................................. 30 97) 98)

I am specifying power on a block. Can I specify the power to the volume as well as to the faces?31

When I write out an IGES file from IDEAS, Airpak can not read it. How can I fix this? ................. 31

99) 100) 101) 102) 103)

Is it possible to copy one object type to another? ...................................................................................... 32 What is the difference between \"Total heat\" and \"Fixed value\" in \"Model/Source\"? ............... 32 \"Total heat\" is the total power given to the source. Whereas, \"Fixed value\" is a constant How do I calculate the thermal resistance given conductivty data? ................................................ 33 The fixed temparture option in sources is disabled. Please use blocks to fix the temperature

temperature given to the source. .................................................................................................................................. 33

on one or more faces. ........................................................................................................................................................ 33

1) My model is very complicated, what is the best way to build it ? 2) Start simple. First include the most important items such as the cabinet, big power devices, fans, vents and openings, if any. For first level analysis, ignore small details. For example, you may smear power sources over blocks or plates, instead of modeling them in detail using small blocks. Make sure the simple model runs well and the solution makes sense. You can then gradually include more details into your model. It is absolutely essential that you build your model step by step!

3) How can I simplify my model?

To simplify your model, you may consider:

reducing your model by using symmetry conditions

using planar objects instead of thin objects

breaking the model into two or more smaller models

neglect small curves, small protrusions, etc.

aligning nearly-aligned faces of objects

4) How often shall I save?

5) As with all software products, it is advisable to save as often as possible. This will ensure that your data is not lost in the event of machine or system failure. In particular, it is important to save the model before starting the solution process. So, if you need to terminate your solution process for any reason using the control C option, the latest model will be saved.

6) I am making models with only small changes between them, shall I save them under different names or get different solutions under the same name?

7) If the changes are geometric (so that the mesh between the two is different) you should save them under different names. For example, if you have a model named job1, you may copy it to job2. Then make the changes you want in job2 and run it. However, if you are just changing the ventilation flowrate or inlet temperature (so that the

mesh is the same), you may either save it to a different name, or, if you wish to use the Restart option, run a second solution under the same name.

8) My area of interest is not surrounded by walls on all sides, is it OK to use the room walls or should I put openings on these walls?

In general, it is not necessary to put openings on all room walls. It is sufficient to put openings on the walls which are perpedicular to the dominant flow direction(s). However, make sure that the walls that don't have openings are far enough so that they don't influence the flow distribution near the area of interest.

9) Should I include the effect of gravity in my model?

Generally, you should include gravity because room air flows are relatively low velocity flows and the buoyant plumes from local heat sources or non-isothermal behavior of air jets can be important to model. However, if you have an isothermal application, gravity will have no effect.

10) Should I include the effect of radiation in my model?

11) In most forced convection cases (where fluid is pushed through by fans, pumps, etc.), the effect of radiation is under 2-5% and may be ignored. In some cases, the effect may be of the order of 10% or even

greater and must be included. Examples are:

natural convection cases

sealed systems

isolated hot components radiating to nearby objects or the ambient

large areas radiating to the ambient

solar loading

It is often a good practice to ignore the effect of radiation in the first level of analysis and include it later, in the final stages of analysis.

12) The calculation for the view-factor matrix is taking too long, what is happening?

Your model probably has a large number of objects. Even if the number of objects participating in radiation is small, Airpak still involves the other objects in the calculation. One way of speeding the calculation is to construct the radiation.ff file from a simpler Airpak model and use this file in the actual model. To construct the simpler model, make a copy of the actual model and delete some components that are either not participating in radiation or have minimal effect on radiation. Then compute the view factor matrix and use the resulting radiation file

(radiation.ff) in the actual model. Another option is to activate the discrete ordinates radiation model which alleviates the need for a time-consuming view-factor calculation although the ensuing flow solution will require more computational time.

13) How do I decide whether the flow is turbulent or laminar? 14) Airpak estimates the non-dimensional numbers which will help you determine whether the flow is turbulent or laminar. For forced convection problems, the number to watch is the Reynolds number, for natural convection problems, look at the Rayleigh number. You can look at the estimates of these numbers by clicking the \"Reset\" button in Solution/Setup. The numbers as well as the recommended flow setting will be displayed in the messages window. In general, the flow is considered tubulent if the Reynolds number is greater than 5000 or the Rayleigh number if greater than 1e+8. The user is advised to use his/her own judgement as to this setting, particulary when the flow regime is in the transition region (between laminar and turbulent). 15) Is is easier to import a model from a CAD package or construct the model inside Airpak?

It varies from case to case. In many cases, you can easily and rapidly construct your model in Airpak. In some cases, however, it may be advantageous to import

from CAD packages directly. This is true in situations where transfering the dimensions (sizes and locations) is time consuming.

16) What are the CAD package files I can import into Airpak?

You can import an Airpak model from Pro/E directly. You can also import IGES files from any CAD package and DXF files from AutoCad.

17) What units should I use?

You can use any system of units or a combination of them (e.g., SI, English system, etc.). You can define your units in the \"Edit unit definitions\" under \"File/Configure\". You can also define units locally for any particular object (dimensions, properties, etc.). However, in general, it is advisable to use a consistent set of units so that the possibility of error is minimized.

18) How do I take the effect of elevation into account?

19) The effect of elevation can be taken into account by simply altering the density of air in the materials database. The default value of air density can be seen in \"Model/Materials\". Go to

\"Fluid/Gases/Air\" and enter the new air density for the elevation under consideration. Make sure to click \"Update\" after editing the material properties.

20) My model is symmetric but my solution is not, what is the reason?

This is most likely due to one or both of the following reasons:

The mesh is not symmetric.

The solution hasn't converged..

21) What is a typical magnitude of flow velocity due to (pure) natural convection?

It is of the order of 0.1 m/s.

22) Can I predict condensation in a room?

23) In Airpak, you can track water vapor as a species and look at the relative humidity. Although there is no condensation mass transfer model in Airpak, seeing results in which the relative humidity approaches 100% in some areas is a good indication that condensation is likely.

24) My Airpak jobs are taking up too much disk space, what should I do?

25) Use \"Cleanup\" under \"File\" menu and delete mesh, scratch, and solution files

26) How can I free-up disk space without deleting model, mesh and postprocessing files?

You can compress the files using the \"Compress\" option under \"File/Cleanup\or, you can delete the scratch files in \"File/Cleanup\" and save as much as 20% of the disk space.

27) What are the three most essential files of an Airpak model? 28) They are \"job\information specified to build the model, such as the size and location of objects, properties of objects, problem setup data, mesh specifications, and solver settings. Any Airpak model must have atleast these three files.

29) What does \"pack\" and \"unpack\" mean?

The \"Pack\" and \"Unpack\" facility in Airpak allows you to communicate with Airpak support or with another Airpak user rather easily.

\"Pack\" means packaging the component files of an Airpak model (job, model

and problem) into one file by tarring and zipping these files. You can send that file to Airpak support or another Airpak user for examination. The packaged file is named \"jobname.tzr\".

\"Unpack\" means unpacking a packaged job automatically. The unpack routine unzips and untars the zipped and tarred file and saves the component files \"job\\"model\" and \"problem\" under the the directory \"jobname\name of the model specified in the packaged job.

30) How do I print out an Airpak model?

First display the model in the Airpak inteface the way you want the model to appear in the print (including the size, view, shading, etc.). Then, in the options menu at the far right of the Airpak main inteface, click \"Print\" and save the image under one of the file formats allowed, e.g. gif, tiff, etc. You can then send the saved file to your printer.

31) Where do I define my units?

32) You can define units in \"File/Configure\". Click \"Edit unit

definitions\" and define the units for any quantity (length, conductivity, power, etc.) and click \"Set as default\". The units so defined will be applied to any new object introduced into the cabinet. You can also alter the units of any variable of an object by editing the object and

changing the unit definitions selected. To do this, just click the unit indicator beneath the quantity under consideration and change it to any other unit in the pick-list. The user also has the ability to simply set the type of units used for the entire problem to SI units or Imperial (English) units.

33) How do I run a time-dependent or unsteady simulation? 34) First select \"Transient\" by going to \"File/Problem\" and then specify \"Start\interval\" of your simulation. Then go to \"Solution/Setup\" and specifiy the \"Iteration/timestep\". 35) What are initial conditions?

36) \"Radiation temperature\" is the partial enclosure temperature applied when the default surface-to-surface radiation model is used. When a given surface radiates to some surfaces, normally all of the radiation does not reach these surfaces. The portion of radiation that doesn't reach these surfaces is assumed to go to the ambient maintained at a reference temperature called the \"Radiation temperature\".

How do I include the effect of natural convection in my model?

In \"File/Problem\appropriate direction.

37) What is the \"Minimum object separation\"?

38) This is a value you specify in \"File/Configure\". It sets a tolerance limit where if any two adjacent faces are separated by a distance less than this tolerance limit, Airpak warns you of the situation. You may choose to cancel the meshing and examine the model for any errors or ignore the warning and go ahead with the meshing. It is not advisable to let Airpak make the changes automatically for it may make many unintended changes.

39) What is the meaning of \"Radiation temperature\" on the \"Problem/Setup\" menu?

These are initial values of velocity and temperature that will be assigned to each cell in the mesh to serve as the first guess values for the solver. These are just initial guess values and are replaced by new values after the first iteration is completed. The default initial conditions are sufficient for most cases.

40) My Airpak model is so cluttered, what should I do to see the

model more clearly?

41) You can suppress the views of some objects. See the next FAQ on suppressing views for how to do it. You can also use the \"Gouraud shading\" on a given class of objects (or just one object) to see these objects clearly.

42) How can I suppress the view of objects in Airpak?

At the top right hand corner of the Airpak interface, there are two columns of boxes. Click on \"Visible\" and toggle off the object type (e.g. blocks) the views which you want to supress. Note that the object class that is suppressed is still part of the model, it is just not visible. For that reason, it is a good practice to toggle on these objects (after you are finished viewing) so that you don't forget about them. You can also suppress the view of a particular object (rather than a class of objects) by editing that object and selecting \"Invisible\" under \"Shading\" in the object edit form.

43) Sometimes the dimensions of objects contain too many decimal places: Is there a way to avoid this situation?

44) Before beginning to build your model, set the \"x-grid\and \"z-grid\" to the minimum decimal place you want to have (for example 0.0001). This will force objects to move or locate with fixed

increments and you will not get the unwanted decimals unless you scale the model.

45) How do I move, copy or scale a group of objects at the same time? 46) Use the \"Groups\" option in the \"Model\" menu to group the objects. You can then move, copy, or scale as you wish. Please refer to the training notes or the Airpak manual for more details. 47) How do I move a group of objects using the mouse?

48) Use the \"Groups\" option in the \"Model\" menu and group the objects. Then toggel on the \"Move group\" button and move the group using the mouse (shift + middle mouse button).

49) How can I change the specifications (material type, power, etc.) of a group of objects in my model?

First, group all the objects. Then change the specifications of one of the objects. Then use the \"Copy parameters\" option in the \"Groups control\" form to copy the specifications from that object (which is highlighted in the Airpak edit form) to all other object in the group. You will see the following messages in the messages window: Copying parameters from ... to ...

50) What is a part?

A part is an assembly of objects which you create and store that can then be used in different models. To create and store a part, go to \"Model/Groups\" and make a group and then click \"Create part\". This will create an internal part, i.e., a part specific to the current model. To create an external part, go to \"Parts\\"Model\part\" in the Parts form. This will open a \"Save job\" form for you. Save the part (with a given name) inside a directory of your choice.

51) How can I scale an object with different scale factors in each of the coordinate directions?

Use the \"Move\" option in the \"Model\" menu. Toggle \"Scale\" and enter scaling factors for each of the three directions. For example entering 0.5, 1.0, 1.0 reduces the X dimension of the object by 50%.

52) How do I reverse the order of object creation?

53) You can change the priority values in \"Model/Priorities\". Objects that were created first have a lower priority number than objects that were created later. By changing these numbers, you can change the order of creation. This in turn affects, among other things, the meshing hierarchy.

) How do I change the default values of material properties?

Go to \"Model/Materials\" and then select \"Solid\\"Fluid\" or \"Surface\" properties. For example, under \"Solid\default propreties of materials such as Al-Extruded, Cu-Brass, etc. Just click \"Edit\" and alter one or more of the properties. If necessary, you can also define a linear or curve variation of a property value with temperature.

55) How do I define new materials in my database?

If the new materials to be defined are specific to the current model only, go to \"Model/Materials\" and add new materials in one of the three catergories available (Solid, Fluid and Surfaces). To do this, select the proper subclass (e.g. \"Metals/Alloys\" under \"Solid\") and click \"New\". Then, click \"Edit\" and edit the name of the new material and specify its properties.

If the new materials to be defined are common to all other Airpak models, you need to create them differently. In this case, create a new Airpak model called \"materials\" (just the default room is sufficient). Then create new materials following the steps described above and save the Airpak model (called materials). The newly defined materials will then be available to any Airpak model, provided the correct path is specified in \"Edit library path\" under \"File/Configure\". To specify the path, type the complete path (to the directory containing the model called materials) in the \"Edit entry\" and click \"Add\". For more details, call your Airpak support.

56) How do I view my model in shaded form?

Go to \"view\" and select \"Gouraud shading\" instead of the default \"Wireframe\" display. In this case, all the objects in the cabinet will be shaded. If you need to shade one class of objects only, go to \"File/Configure\" and click \"Edit graphical styles\" and select \"Gouraud\plates, etc.). It is also possible to have Gouraud shading for one object only. To do this, edit the particular object under consideration and, under \"Shading\select \"Solid\".

In my fan curve, the volume flow/pressure numbers are not what I specified in the \"Edit\" form, what is happening?

The numbers are scaled by a factor shown along the axis. For example, 1.5 Pa with a scale factor of 1e2 is equal to 150 Pa.

57) What is the difference between \"Device\" and \"Approach\" methods in the calculation for pressure losses in vents and resistances?

The two differ in the way the pressure drop is calculated. In the \"Approach velocity method\provided and the flow velocity approaching the device (vent or resistance); the free area ratio data is not used. In the \"Device velocity method\however, the flow velocity through the device is used, as well as the loss coefficient and the free area

ratio data provided. The following relationships may be noted:

V-dev = V-app/A

L-app = L-dev/A**n

where,

V-app = Approach velocity

V-dev = Device velocity

L-app = loss coeff. based on approach velocity

L-dev = loss coeff. based on device velocity

A = free area ratio

n = 1 for linear relationship and 2 for quadratic relationship

58) What is a macro?

59) A macro is a compact and easy representation of a complex system. Examples are heat sink macros and macros for packages (BGA). Such models may be used when detailed modeling is time consuming

and/or impractical.

60) I have created a heat sink macro and want to send it to my colleague. What is the quickest option?

61) Save the heat sink macro settings (in the heat sink creation form) and send that file to your colleague. The settings can be saved in a file insied the directory \"heatsink_info\".

62) How do I apply a transient thermal loading (power) to my block?

Edit the block and give the Total power. Then select \"Transient\instead of \"Constant value\". Now, click \"Edit\" and you will need to specify tS and tE (the starting and ending times within which the specified power is on), and the time variation of power with time (Linear, Power law, etc.). Note that you can specify trainsient loading for solid or fluid blocks only. Sources (planar or volumetric) can also be used to specify transient power loading.

63) Can I prescribe several transient power loading segments to my model?

) Yes. For example, you can ramp up the power for 5 minutes, maintain the maximum power for 10 minutes, and ramp down the power for 15 minutes. To do this, use the \"Text editor\" (under

\"Piecewise linear\") and specify the time-dependent power.

65) I am only interested in getting the flow distribution and not the temperature. How do I solve for the flow distribution only?

66) Go to \"File/Problem\" and click off \"Temperature\" in the \"Variables solved\" option.

67) I have obtained the flow distribution from a previous run and want to solve my model for another power loading. Can I only solve for temperature?

Yes. Go to \"File/Problem\" and click off \"Flow (velocity/pressure)\" in the \"Variables solved\" option. In order to use the previous flow solution, you need to use the \"Restart\" option in \"Solution/Solve\".

68) I am modeling a pole-mounted enclosure and want to include the effect of solar loading, what is the best approach?

First, you need to calculate the incident flux (from text books) to each of the faces exposed to solar loading. Then, put planar sources on these faces with the appropriate heat flux (power) conditions.

69) I am modeling external airflow. How large should my room be?

In general, if the building has height H and width W, you would want your room to be at least 5H high, 10W wide, with at least 2H upstream of the building and 10 H downstream of the building. You would want to verify that there are no significant pressure gradients normal to any of the boundaries of the computational domain. If there are, then it would be wise to enlarge the size of your room.

70) Can I build my model in inches and use the SI system of units to specify the other parameters such as conductivity, power, etc.? 71) Yes. You can build your model in inch and still use other system of units for the other specifications.

72) The origin of my room is a non-zero number, can I convert it to (0,0,0)?

73) Yes. Use the \"Move\" command in \"Model/Room\" and translate the room in the X,Y,Z directions by giving the proper offset.

74) I am modeling a telecom rack and do not know whether I should use a plate or PCB for my motherboard?

The PCB is a very simple representation of a printed circuit board. For first level calculations and approximate answers, the use of the PCB is acceptable. If more

detailed results are desired, plates or blocks should be considered, perhaps with discrete chips (blocks or sources) attached to the surfaces. The advantage here is that the conductivity of the board can be modified in each direction to take into account the multi-layered nature of the board. PCBs do not have inherent conducivity. In addition, plates and blocks allow the specification of other effects such as radiation.

75) How can I model small air gaps or interface material between surfaces?

76) This can be done using contact resistances or conducting thin plates. Both are planar objects that provide thermal resistance normal to the plane. The difference between the two is that conducting thin plates have both in-plane and normal condction, whereas contact resistance plates have normal conduction only. In both cases, you must specify an effective thickness. For contact resistances, you can also specify thermal resistance explicitly.

77) What is the difference between conducting thick partitions and contact resistances?

Conducting thick partitions have both in-plane and normal (to the plane) conduction. Contact resistance partitions have normal conduction only, and are meant for partitions of very small thickness.

78) My region of interest is not a rectangular prism, how can I construct the room in Airpak?

79) You can use polygonal hollow blocks to block off a part of the room. Hollow blocks not only allow you to shape your room but also reduce the mesh size of your model, since no mesh is constructed inside them.

80) When should I use hollow blocks?

Use hollow blocks in any of the following situations:

to make a cut-out section in your room

the represent a flow obstruction of no thermal interest inside your room

to represent a region that has a constant temperature or heat flux

to use recirculating openings inside your room and model such things as air conditioners, etc.

81) I have a table of volume flow vs pressure data for my fan, can I load this data directly, instead of entering it manually?

82) Yes, you can load it directly using the \"Load\" option in the fan

curve \"Graph editor\". Alternatively, you can also copy the text using the mouse and transfer it to the \"Text editor\". 83) How do I take the effect of elevation into account?

84) The effect of elevation can be taken into account by changing the default density of air for constant density problems. The default value of the density of air can be seen in \"Model/Materials/Fluid/Gases\". Calculate the air density at the elevation under consideration and enter that value by clicking \"Edit\". Click \"Update\" to save changes. For variable density problems, change the operating pressure to account for elevation.

85) How do I determine the operating point of a fan?

The easiest way is to use \"Report\" and calculate the mass flow rate through the fan. Then, read the correspoding pressure point from the fan curve.

86) What is the difference between blocks and partitions?

The main differences are:

Partitions are meant to be used in place of thin objects/blocks.

Blocks allow six-sided control for meshing and thermal specifications, whereas partitions allow for only two-sided control.

Partitions can be conducting thin or contact resistance partitions. In addition, both blocks and partitions can be solid, fluid or hollow objects.

87) What is the difference between internal and external fans? 88) Internal fans are meant to be placed inside the room. External fans must be on room walls or on the interior faces of hollow blocks; they must on boundary surfaces.

My model has 100,000 elements, how much RAM and disk space do I need to solve the model?

You need roughly 100 Mb of RAM and 600 Mb of disk space.

) Is there flow through the hub of my fan?

90) There is no flow through the hub of a fan. The flow comes from the no-hub area only.

91) How do I incorporate a swirl effect into my fan flow?

92) You can provide a swirl factor in the fan specification form. Check

the fan manufacturer's data or the appropriate handbook for this factor. You can also specify the fan RPM and the swirl factor will be calculated for you in Airpak. In this case, the the slip is assumed to be 95%.

93) Can I use partitions on the sides of the room?

In general, you should use walls to give meaningful properties to room boundaries. Partitions are meant for interior walls.

94) What is the difference between partitions and walls?

The main differences are:

Walls are mainly used on room boundaries, whereas as partitions are used inside the room.

Walls can be moving whereas partitions are stationary.

Walls can have fixed temperature, but partitions cannot.

95) How do I specify a non-uniform conductivity to my blocks or partitions?

96) You may do this by modifying the conductivity values in

\"Model/Materials\". Solids materials can have Isotropic, Orthotropic or Anisotropic conductivities. Isotropic solids have uniform conductivities in all directions. Orthotropic materials can have different conductivities in X, Y, and Z directions. The conductivity of Anisotropic materials is defined by a nine component tensor. So, depending on the complexity of your materials, you can choose either Orthotropic or Anisotropic options for your materials.

97) I am specifying power on a block. Can I specify the power to the volume as well as to the faces?

Yes, you can specify power at one or more places. However, they are additive, so, the total power specified to the block is the sum of all powers specified to the volume and faces of the block.

98) When I write out an IGES file from IDEAS, Airpak can not read it. How can I fix this?

By default IDEAS uses the NURBS (b-spline) approximation for lines and circles. Airpak does not recognize this format. To correct this, do the following when exporting an IGES file from your IDEAS model:

In the \"IGES export\" window click on \"IGES Options...\"

Under \"Surface type to export\" select \"None\" and hit \"OK\"

Next from the \"IGES export\" window click on \"Flavors...\"

Select \"Curve Representation...\"

Next to \"Line\" click on the toggle window that says \"NURB\" and select \"Line\"

Next to \"Circular Arc\" click on the toggle window that says \"NURB\" and select \"Circular_Arc\"

Click \"OK\" in the open windows then \"Export\"

99) Is it possible to copy one object type to another?

Yes. For example, you can copy the geometry (shape and position) of a partition from a wall. This is accomplished by the \"Copy from\" option. The \"Copy from\" button is located at the bottom of the \"edit form\" of any object. Objects in the following two sets of objects can be copied from one to the other (in the same set). Note that the 'Copy from' feature only copies the shape. Other object specifications such as properties, heat flux, velocity etc. should be specified after executing 'Copy from'.

100) What is the difference between \"Total heat\" and \"Fixed value\" in \"Model/Source\"?

101) \"Total heat\" is the total power given to the source. Whereas, \"Fixed value\" is a constant temperature given to the source.

I am using a source to fix the temperature on a block, but I am not getting the right result. What is happening?

The relationship between thermal resistance and material conductivity is as follows: Thermal resistance = Thickness / (Area * Conductivity)

102) How do I calculate the thermal resistance given conductivty data? 103) The fixed temparture option in sources is disabled. Please use blocks to fix the temperature on one or more faces.