Review Reports

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Please find the attachment.

Comments for author File: Comments.pdf

Author Response

Please see the attached word file for the detailed list of responses.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The article "Two-Phase Flow Studies in Steam Separators Using Interface Capturing Simulations" presents single-phase and two-phase Direct Numerical Simulations of the first section of a steam separator, considering only the first pickoff ring.
The Level Set technique is adopted for two-phase flow. Due to the highly complex geometry, many severe simplifications and limitations needed to be adopted in order to carry out the simulations, to the point that the results shown risk being more towards ColorFul Dynamics instead of a real depiction of what happens in a real steam separator.

Authors are aware of the complexity and necessary simplifications that they introduce, but they somewhat downplay their importance.
In particular, when comparing to other approaches in the introduction, they claim (lines 87-111) the achievements of CFD simulations without any mention of the simplifications they will introduce later in the paper.

Another severe limitation is connected to the use of the Level Set method without any mechanism to preserve the phase masses, as stated by authors as well, who propose a conservative Level Set approach. I would argue that for regimes such as the ones shown in the paper, a hybrid LS/particle method may be more appropriate.

The authors cite several papers that propose experimental data for regimes different from the one required in a BWR separator. This data, even if not completely appropriate, could be used for validation of the proposed approach, which is completely lacking.

Additional detailed comments:

* lines 52-59: use consistent scale indications.

* lines 64-72. A picture would help to identify the regimes that are discussed here.

* Section 2. Have there been any previous papers describing the use of Level Set in PHASTA? Cite them here.

* Section 2. No description of the discretization technique adopted in PHASTA beyond the finite element method is provided.

* line 125. "dynamically adjust": does it refer to time (i.e., changed after any/some time step) or space (i.e., it changes locally in the computational grid)?

* line 144. Add a citation with the omitted details.

* Table 2. The increase in simulation time (core hours) scales strangely with the mesh size and simulated time. Please give some explanation on why the number of hours increases by a factor of about 20 between the coarse and intermediate mesh, with a mesh size increase of 8 and similar simulated time, while it grows only by 3 times from intermediate to fine with 1/4 simulated time and x8 mesh elements.

* Section 3.2.1. The swirler model simulates the steam phase alone. Is the geometrical domain reduced to take into account the liquid film at the wall? Do the boundary conditions at the interphase between steam and water consider the liquid velocity?

* Table 4. As stated by the authors, the resolution of this setup is not enough for DNS simulation. A subscale model is required to claim that the average velocity profile is correct.

* line 426-431. How do the authors select the amount of swirl with respect to the streamwise component?

* Figure 22. Since only two new numbers are shown in this picture, a table with previous results and new results would be clearer.

* Many references to sections in the paper are marked "0".

Author Response

Please see the attached document for the list of responses.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

No further revisions required.