Research on High-Pressure Hydrogen Pre-Cooling Based on CFD Technology in Fast Filling Process

Round 1

Reviewer 1 Report

Manuscript ID: processes-1464182

Title: Research on high-pressure hydrogen pre-cooling based on CFD technology in fast filling process

Review

The paper refers to the hydrogen pre-cooling process in hydrogen filling systems for vehicle tanks. The authors modelled the transient heat transfer process between gaseous hydrogen and boiling R23, implemented in a shell-and-tube heat exchanger. The presented topic is interesting and of significant utilitarian interest, but the paper requires the following comments to be addressed.

Critical remarks

1. How do the authors define the term "flow rate (m/s)"
2. Line 72-74, sentence "At the same time, in the heat exchange..." needs clarification. The critical parameters of hydrogen are T_kr=-240.18^oC, p_kr=1.29 MPa. Under charging conditions (-40^oC, 77 MPa), the hydrogen is far from the critical point. What phase changes can it undergo?
3. It is advisable to supplement Section 2 with a simple diagram showing the tank filling method and the location of the shell-and-tube heat exchanger in it.
4. The authors mention that the shell-and-tube heat exchanger was designed based on the Péclet equation (logarithmic mean temperature difference). For what parameters of hydrogen and R23 was this exchanger designed?
5. The method for adopting the initial conditions for transient heat transfer in the exchanger is not clear. How the calculations were implemented over time (0-30 s).
6. There is a contradiction in lines 204-206 and line 280: In line 280 we read: "refrigerant R23 has a certain heat exchange with the outside world" , while lines 204-206 state: "Therefore, the outer wall surface of the outer wall surface other than the entrance and exit of the heat exchanger is set as an adiabatic boundary.”
7. Section 4.1. Please consider the following interpretation: The decrease in the dryness of R23 vapour in the final cooling phase may be due to the decrease in the temperature difference between hydrogen and R23 and the resulting decrease in flux of transferred heat and the heat transfer coefficient on the R23 side.
8. Line 300 states: "Second, according to Table 2, the heat transfer coefficient" where it should be: "...thermal conductivity"
9. Lines 295-307, the interpretation provided is not clear. Note that the hydrogen flow in the exchanger is not isobaric. In the range of pressures 42 MPa-77 MPa and temperatures (-40...25^oC), the pressure drop during hydrogen flow is accompanied by a temperature increase, which increases together with hydrogen pressure (Joule-Thomson effect for hydrogen).
10. Lines 331-334. Interpretation is not adequate for the results. With determined parameters on the R23 side and a fixed surface area of the exchanger, higher hydrogen temperature renders R23 unable to cool the hydrogen to lower temperatures.
11. Lines 37, 43, 354-355 contain text that should be removed
12. The list of abbreviations does not include an explanation of the designations used in Equations 1–4. Subscript "1" in Equations 1-4 should be replaced with the subscript "l" (for liquid).
13. Line 290 – mistake in figure numbers, Fig. 7 should be Fig. 8
14. Line 359 – mistake in figure numbers, Fig. 9 should be Fig. 10

The paper may be accepted for publication in the journal Processes after proofreading and addressing the above comments.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

Reviewer 2 Report

The heat transfer and phase transformation of shell and tube heat exchangers are simulated at two-dimensional level. The influence of hydrogen inlet pressure, temperature and refrigerant flow rate on refrigerant phase transformation was studied, which has certain reference significance for hydrogen pre-cooling in hydrogenation stations. However, there are many things to be revised before publishing.

1. FIG. 1 is too crude to describe the heat exchange mechanism.
2. It is recommended to add the heat transfer flow chart of high temperature heat exchanger and low temperature heat exchanger, so that the text can be more intuitive to understand.
3. FIG. 2 is suggested to be modified as the comparison of the “commonly used baffle” and the “flower baffle”. Neither text nor pictures can tell the difference between the two baffles and why the “commonly used baffle” is not suitable.
4. The structure parameters of heat exchanger in the table are too abstract. It is suggested to supplement the structure diagram of heat exchanger. Whether these data are parameters of the simplified model?they should be marked on the simplified model in FIG. 3 accordingly.
5. The change of temperature has a great influence on the viscosity of fluid, but the author ignores the viscosity. How does the author consider it?
6. For numerical simulation, the number of grids will affect the simulation results, but the author did not do grid independence verification. The reliability of the simulation data obtained is questionable.
7. The existence of baffle will make the shell side fluid deviate from the axial flow of the heat exchange tube, that is to say, the shell side fluid does not always flow along the axial horizontal flow of the heat exchange tube actually. However, the two-dimensional model simplified by the author just avoids this point. Is the applicability of this model reliable?
8. What does the vapor volume fraction in Figs. 8, 9 and 10 represent and how is itobtained need to be supplemented.
9. Why can't we see the obvious temperature change at the shell side from FIG. 6?

Author Response

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Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

The author's reply has clear logic, sufficient evidence and reasonable revision, but there are still some problems. It is recommended to publish after modification.

1. Comparing Table 1 and Figure 4, it seems that the number of baffles is inconsistent. Also, the spacing of baffle is not given.
2. In section 2.2, it is mentioned“Except for density, other physical properties are constant”. However, temperature has a great influence on viscosity. Is the model ignoring the change of fluid viscosity applicable here? Can youauthors verify it?
3. Thevapor phase fraction is not uniform as shown in Fig. 10. Then what statistical means is used to obtain the vapor phase fraction at each time point in Fig. 11, 12, 13, such as mean, maximum or the value of a specific point? Please make additional remarks.

Author Response

Please see the attachment.

Author Response File: Author Response.pdf

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.

Round 1

Reviewer 1 Report

The manuscript can be recommended for pulication

Reviewer 2 Report

Corrections have been made, but they deal only with style and not with the substance of the work. The results are useful only for this particular geometry and operating conditions. Without a more detailed validation of the numerical methodology, the reliability of the results is poor.

Still rejected.

Reviewer 3 Report

This paper studies the heat exchanges involved during the precooling of hydrogen necessary for hydrogen refueling station. The work is a computational-based investigation aiming to simulate the thermodynamics of hydrogen filling process. The article is interesting and well written, but results are not sufficiently discussed and comparisons with literature (although mentioned several times throughout the paper) are rather limited (only 24 papers overall, among which only few are used for actual comparisons with results reported elsewhere). Although interesting, the current study shows results that are somehow expected. For instance, as summarized in the abstract and later on in the conclusions, “the higher the inlet temperature, the higher the outlet temperature” and “the higher the refrigerant flow rate the lower the hydrogen outlet temperature”. These facts seem straight forward, and follow well-known laws of thermodynamics and fluid mechanics. Such results could be foreseen, thus the use complex simulations to reach the same conclusions does not appear justified (those are just two examples among findings shown here). Simulations are very important since they bring insights that experimental studies cannot provide. Therefore, the Authors should further delve in heat exchange mechanisms, emphasize the importance of their findings and provide extensive literature background for a better overview. Both the originality and the actual contribution  of the work should be further justified. Also please find some additional comments below:

P1 line 36: delete “Strategic development direction.”

 

P1 line 40: “Hydrogen has low density under normal pressure”. Please add the density value at a defined pressure. “Normal pressure” and “low density” are meaningless without clarifying the pressure and temperature conditions.`

 

P3 line 120; Please further describe the type of flower used in this study. Also revise “in [19] Type baffle”

 

P11 line 342: “has a very good effect”. Please revise or consider removing.

 

Please revise the reference list: sometimes the full authors list is provided and sometimes it is just the first author with "et al.". Please use a uniform format.