Single-pin assempblies in DASSH

[fpepe-polito]

Hi @MilosAtz,
I'm working with dassh and I have some troubles understanding the following behavior.
In the theory manual, section 3.9, Figure 9, a corner-corner adjacency for subchannels along the duct wall is shown as possible, and it is also listed among the possible options in the same paragraph. This configuration would imply the possibility of having single-pin assemblies, which is instead not possible with the actual code. In fact, if 1 ring is designated in the input file, the following error comes out : 'DASSH... single pin assemblies not supported'.
This seems generated by the instructions at lines 244-245 of file region_rodded.py. This if statement works appropriately when using the CTD correlation for mixing, but also in accordance with the theory manual (pp. 74), it could be avoided when using the MIT correlation, which is said to be 'indipendent on the bundle size'. Is this true? Or are there any restrictions in the usage of MIT correlation also for one pin assemblies? Thanks

[MilosAtz]

Hi there! Thanks for the question. The corner-to-corner subchannel connection is indicated as possible in the manual, but if I remember correctly, it was never added to DASSH because it's a bit of a corner case (no pun intended) and at the time, we didn't need that functionality.

Although it's not unreasonable to have a single-pin assembly and is theoretically possible to mesh one, all of the correlations in DASSH require at least seven pins in the bundle (two rings); you can see that in Table 1 on page 62. The MIT mixing correlation is "independent of bundle size" in that it does not depend on the number of rings, but it does depend on the pin pitch; for a bundle to have pitch, there must be more than one ring.

I do believe there was work done previously on SE2 to accommodate single pin bundles, but I don't recall the details. I was not able to develop that capability while I was at Argonne. One possible workaround could be to use a 7-pin (2-ring) assembly with a small pin pitch relative to the duct flat-to-flat distance.

I'll leave this issue open for now, as adding single-pin assembly capabilities to DASSH could be a good opportunity for future work. I hope this is helpful; feel free to reach out with any more questions.