One-Dimensional Annular Diffuser Model for Preliminary Turbomachinery Design
Round 1
Reviewer 1 Report
Overall comments
The paper deals with a numerical model dedicated to the preliminary design of diffusers and presents a sensibility analysis of various design parameters. This model works with an estimated friction coefficient (under the assumption of an unseparated flow). It is not clear how such a model can help in the preliminary design of diffuser if it is not associated to the definition of criteria ensuring an unseparated flow. It would be very interesting from an engineering point of view to provide additional information regarding design limitations (for instance, the highest divergence semi-angle value before flow separation).
One of the main novelty of the approach presented in the paper is the account of a heat transfer model. However, no sensibility analysis is proposed to quantify the heat transfer contribution compared for instance to the flow friction. I recommend to complete the sensibility analysis (section 4) by including this point.
Detailed comments
Page 3: please, be more specific about the approach proposed by Dubitsky and Japikse [8]: what kind of method is proposed and which “information required that might not be known in the early design phase”?
Page 5: A word is missing in the flowing sentence “The detailed derivation of these equations and a discussion of the physical meaning of the different terms is presented in the Appendix.”
Page 5: “This means that the diffuser can only be choked due to the meridional component of velocity and that the tangential component of velocity can be decelerated from supersonic to subsonic velocities without shock waves”. This is a very theoretical aspect, in practice the compatibility between a supersonic flow (tangential component) in the diffuser and the downstream static pressure (subsonic) will be ensured by a shock wave.
Page 11 – lines 159 to 161: The heat transfer is neglected for the test case validation. Since no sensibility analysis of heat transfer is presented in the paper, a justification of this assumption would be welcomed?
Page 14 – lines 180 to 182: it is mentioned that the “The divergence semi-angle was not included in the analysis because, in practice, increasing this parameter may lead to boundary layer separation close to the walls”. It is effectively well known that this parameter has a strong impact on the flow separation, but what about the other parameters studied in the paper? Regarding the flow separation, is the divergence semi-angle is an independent parameter or should it be correlated to other parameters to define the flow separation limit? Please, add comments regarding this point.
Author Response
See document attached.
Author Response File: 
Author Response.pdf
Reviewer 2 Report
The authors present a 1-D methodology for the design of diffusers coupled with the turbomachine. The methodology is extensively presented (also using some appendices), the code is verified and the calculation is validated against experimental data. As a plus, the code is freely redistributed.
I have no particular concern about the present paper, it is correctly written and presented, the topic is of interest (many industries are "going back" to 1-D models) especially in the "turbo expander" world. I support the publication as it is.
Author Response
See document attached.
