Experimental and Numerical Analysis of the Impeller Backside Cavity in a Centrifugal Compressor for CAES

Round 1

Reviewer 1 Report

The authros investigate the forces and the influence due to the back cavity in a centrifugal compressor on the global performance parameters. The study is gernerally interesting, but falls short in the expectations. After the authors discuss flow structures in the back cavity in the introduction, I expected to see measurements of flow simulations of these flow structures. However, none of that is presented.

Please justify the large deviations of the numerical and experimental results shown in figure 10. The differences are larger than 5%!

The authors discuss the flow structures found in rotating cavities. Centrifugal compressors are build in a wide range of dimensions. Please sumerize in a sentence, of which scale a centrifugal compressor must be that flow structures in the back cavities are important.

Please add a citation for publications that neglect the backside cavity. For example, Jyothishkumar et. al (2013) "Numerical Flow Analysis in a Centrifugal Compressor near Surge Condition". It would be also relevant, why it is often neglected. Cold flow assumptions, size considerations, ... .

The authors chose to model the back cavity as a small segment with no purge flow. Certainly, the flow structures, as described in the introduction, will not be contained in the flow simulations. Please mark that out in the manuscript.

Something happened in figure 11. Either the plots or the figure caption is wrong. I guess, it is the figure caption which states twice the same quantities for a and b.

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Minor:

The turbulence model is called "k-omega SST" closure.

Please provide some details about the mesh size and the y^+ valuse.

Please justify the chosen turbulence model and the modelling assumptions by references.

Please state the time step as a function of degree of revolution.

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Reviewer 2 Report

The article investigates the influence of the impeller backside cavity on centrifugal compressor performance. The novelty of this paper lies in the numerical simulations validated experimentally, on a high power facility. The article will be of great interest to those involved in numerical modelling. The paper is well organized in topics and sub-topics. However, there are some aspects of the article that require improvement:

1. In Table 2., the accuracy of the measurement equipment is stated. However, accuracy is a qualitative term that equipment manufacturers often wrongly use. Instead of accuracy, uncertainty values should be estimated. In lines 157-160 error ranges are given. Are these combined uncertainties? What coverage factor has been used to estimate them? While it may be not so crucial for pressure ratio, the discrepancies between the experiment and CFD simulation for isentropic efficiency in some places may well fall within the limits of uncertainty. But it should be stated more precisely what these error ranges mean, how were they estimated.

2. More information about the computational mesh should be given. What is the total number of elements, mesh quality, characteristic cell size? How was the boundary layer mesh generated? Was the boundary layer resolved with y+ < 1 or the wall function was used?

3. In lines 217-225, various possible reasons for discrepancies between CFD and experimental study are given. There is also a question of grid dependence of the solution. Was this studied as well?

4. What is the material description of the fluid used in the simulation? Which material properties are variable, and how are they described?

5. What inlet turbulence levels were used in the CFD study?

6. Description of series in Fig. 5 (the distinction between dashed and solid lines) is missing.

6. Shouldn't the total pressure ratio and isentropic efficiency deviations in Figs. 6a and 11a be negative? 

7. Some descriptions are missing from nomenclature; there are empty fields (e.g. AIGVs, some subscripts)

Author Response

Please see the attachment.

Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

The authors answered my questions. I don't have further comments.

This manuscript is a resubmission of an earlier submission. The following is a list of the peer review reports and author responses from that submission.