Query on aspect ratios

[PDE-PT]

Hi, I would like to better understand what these aspect ratios means. Do we have to comply to all these ratios when modelling our cfast model.

Reason being, I have a room on fire, and I intend to have this smoke spill into a common corridor for extraction. The size of the corridor is approximately 15m (L) x 2.2m (W) x 3.6m (H). Under the corridor flow, I am only able to fulfil the criteria of L/W > 5. Where does this bring me next?

Thank you in advance!

[mcgratta]

Where does this table come from? Perhaps there is more explanation given in the reference. My guess is that these ratios represent the range of applicability of the various empirical models used for corridors or shafts.

In any event, these values are merely suggestions. You are the one who must determine if a particular assumption in the model is valid for your scenario. Nobody but you (or maybe the authority having jurisdiction) can make the decision.

[PDE-PT]

Hi. The table was referenced from the user guide for cfast.

[mcgratta]

What is the Table number? I cannot find it in the latest CFAST User's Guide.

[PDE-PT]

Hi. I believe it's in page 24 of version 6 user guide (see photo 1 below).

Reason for referring to V6 and not V7 is that because this table is not found in V7. However a similar query was brought up in Apr, and it appears that these values are still applicable for V7 (see photo 2 below).

[preneke]

Length and Width are essentially the same. The difference is about how they orient in space. However, for the table L is the longer dimension and W is the shorter. There are issues with flow down corridors, which can really change early answers. The other issue is as L/W grows the more heat is removed from the compartment via radiation and convection from the layers. The W/H is more about how much heat goes through the ceiling then the walls. Too much energy out the ceiling causes some issues with ceiling jets if I remember correctly.

[drjfloyd]

I thought W/H < 0.2 meant a shaft like geometry where the issue is the plume fills the plan area and can no longer be considered an unbounded, axisymmetric plume.

[PDE-PT]

Hi. Thanks for your clarification that length and width are the same, with length being the longer side. However, I would still like to understand, is it a necessity to adhere to all 3 ratios when modelling the corridor. Because it doesn't seem like it's possible.
Also, when I was referring to the validation model for ATF corridor, it doesn't seem like the aspect ratio were followed either.
Hence I am a little confused on what these ratios are meant for.

Once again, thank you in advance for your help.

[PDE-PT]

I thought W/H < 0.2 meant a shaft like geometry where the issue is the plume fills the plan area and can no longer be considered an unbounded, axisymmetric plume.

Hi Dr Flyod, does this mean that there is no issue for not complying with the aspect ratio. Since the corridor is 3.6m high, with a width of 2.2m, the W/H being 0.61 (> 0.2) does not fall into the corridor/shaft algorithim. However there shouldn't be any concern as I'm simulating for a corridor flow, and not a shaft flow.

And if you don't mind, what about the L/H ratio as well. As the calculated ratio is 4.17 (< 6.0).

Thank you.

[drjfloyd]

I cannot make the decision on the appropriateness of CFAST as a tool for your specific intended use. That is a decision you need to make as the modeler based on your understanding of how CFAST operates given the specific details of your use. L/H of 4 would, per the table you show, put you in the special consideration category ( 3 < L/H < 5).

CFAST is a zone model. It assumes that a room can be represented by two uniform layers that are coupled by an axisymmetric, unbounded plume.

If a room is very tall compared to its plan area (a shaft), then the plume can expand and fill the cross sectional area at which point you no longer have an unbounded plume.

If the aspect ratio of a room's plan area is very large (a corridor) then you will have the case where the time for the ceiling jet to reach the walls across the short distance is very different from the long distance. This impacts the assumption of a uniform upper layer and impacts calculations of ceiling jet times for things like detection and sprinkler operation.

[preneke]

My mistake. I believe you are correct. Sorry. From: Jason Floyd ***@***.***> Sent: Friday, August 15, 2025 9:41 AM To: firemodels/cfast ***@***.***> Cc: Reneke, Paul A. (Fed) ***@***.***>; Comment ***@***.***> Subject: [EXTERNAL] Re: [firemodels/cfast] Query on aspect ratios (Discussion #2207) I thought W/H < 0.2 meant a shaft like geometry where the issue is the plume fills the plan area and can no longer be considered an unbounded, axisymmetric plume. - Reply to this email directly, view it on GitHub<#2207 (reply in thread)>, or unsubscribe<https://github.com/notifications/unsubscribe-auth/ADDV4MDTVUYTATZZ64CGLZL3NXPNRAVCNFSM6AAAAACD7A53XKVHI2DSMVQWIX3LMV43URDJONRXK43TNFXW4Q3PNVWWK3TUHMYTIMJRG4YDIMI>. You are receiving this because you commented.Message ID: ***@***.******@***.***>>