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airpak常见问题问与答(一)建模基础


常见问题问与答( airpak 常见问题问与答(一)建模基础

1. My model is very complicated, what is the best way to build it? 2. How can I simplify my model? 3. How often shall I save? 4. I am making models w

ith only small changes between them, shall I save them under different names or get different solutions under the same name? 5. 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? 6. Should I include the effect of gravity in my model? 7. Should I include the effect of radiation in my model?
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8. The calculation for the view-factor matrix is taking too long, what is happening?
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9. How do I decide whether the flow is turbulent or laminar?
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10. Is is easier to import a model from a CAD package or construct the model inside Airpak? 11. What are the CAD package files I can import into Airpak?
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12. What units should I use?

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13. How do I take the effect of elevation into account?

14. My model is symmetric but my solution is not, what is the reason?
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15. What is a typical magnitude of flow velocity due to (pure) natural convection? 16. Can I predict condensation in a room?
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My model is very complicated, what is the best way to build it ? 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!

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
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How often shall I save? 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.
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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? 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.
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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.
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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.

Should I include the effect of radiation in my model? 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:
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natural convection cases sealed systems isolated hot components radiating to nearby objects or the ambient
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large areas radiating to the ambient solar loading
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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.
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The calculation for the view-factor matrix is taking too long, what is happening?
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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.
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How do I decide whether the flow is turbulent or laminar?

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

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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.
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What units should I use?

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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.
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How do I take the effect of elevation into account?

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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.

My model is symmetric but my solution is not, what is the reason?
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This is most likely due to one or both of the following reasons: The mesh is not symmetric.
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The solution hasn't converged..
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What is a typical magnitude of flow velocity due to (pure)

natural convection? It is of the order of 0.1 m/s.
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Can I predict condensation in a room? 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.


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