Experimental and Numerical Study of Double-Pipe Evaporators Designed for CO₂ Transcritical Systems

Round 1

Reviewer 1 Report

This manuscript reports an experimental and numerical study of a double-pipe evaporator for CO₂ trans-critical systems. The work is well structured, firstly reporting experimental results and then presenting a numerical model capable of describing the physics of the studied system with suitable accuracy. The numerical model is used to access the influence of the inner and outer tube diameters on the dry-out region, heat transfer coefficient and cooling capacity. Nonetheless, there are still some points that can be better explained and the following issues should be revised:

1 – On the 5^th line, in the introduction, the authors say “Initiatives like the Kigali Amendment to the Montreal Protocol on Substances that Deplete the Ozone Layer are anticipated to come into force on 1st January 2019.” – this should be updated since this date is in the past. So, are the policies already implemented or not?

2 – On section 3.2, line 6, the authors used the expression “vapor heart” which is probably a bad translation.

3 – Still on section 3.2, the authors suggest that due to high mass velocity the fluid cannot adequately perform heat transfer. Does the cooling capacity also decreases for the same velocities using higher tube lengths?

4 – In section 5.1, the authors reported that the heat transfer coefficient increases for lower outer tube diameters. It is not clear why the cooling capacity increases for bigger tube diameters, for which the heat transfer coefficient is lower.

5 – In section 5.2, it is not clear why the maximum cooling capacity is obtained with lower inner tube diameters, which correspond to lower heat transfer coefficients. In my opinion, this must be related to the constant mass flow that results in different fluid velocities for different tube diameters. Maybe the authors can perform further tests using constant fluid’s velocity instead of constant mass flow rate.

Author Response

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Author Response File: Author Response.pdf

Reviewer 2 Report

Please refer to the attached pdf.

Comments for author File: Comments.pdf

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

The authors addressed all the issues that were mentioned from the previous version and the manuscript was significantly improved. Therefore I recommend the acceptance of this manuscript.