Analysis of the Thermodynamic Characteristics of a Hyper-Compressor through Numerical Simulation and Experimental Investigation

Round 1

Reviewer 1 Report

1. In the abstract, the deviation is mentioned as 1.77%. Mention which parameter this deviation belongs to.

2. Cite the source of equation 2.

3. Reasons for the assumptions made in section 2.2 should be mentioned. Especially the 2nd one as a good amount of heat generation would be expected at these high pressures.

4. Conclusions can have more quantitative results. 

5. In a few sections, English sentence construction needs to be improved for the benefit of the reader so as to not miss such an important work.

Author Response

Point 1: In the abstract, the deviation is mentioned as 1.77%. Mention which parameter this deviation belongs to.
Response 1: The parameter is the indicated power
Point 2: Cite the source of equation 2.
Response 2: The revised manuscript has cited the source. 
Point 3:  Reasons for the assumptions made in section 2.2 should be mentioned. Especially the 2nd one as a good amount of heat generation would be expected at these high pressures.
Response 3: 1)The main leakage channel of the hyper-compressor is through the central valve and piston ring. If both are considered, a very small leakage channel needs to be left at the piston ring and valve clearance when the model is established. This increases the difficulty of discretizing the physical model, and at the same time, the number of mesh will increase exponentially, leading to a significant increase in calculation time. Therefore, this model didn't consider the leakage of the compressor; 2)The compressor speed is 200rpm. The heat exchange between the gas in the working chamber and the outside during one cycle is negligible, so the working process inside the cylinder can be regarded as an adiabatic process. 3) The suction buffer tank can effectively attenuate pressure pulsation, so the effect of gas pulsation before the buffer tank on the gas flow inside the hyper-compressor can be ignored.
Point 4: Conclusions can have more quantitative results. 
Response 4:  The revised manuscript has added more results.
Point 5: In a few sections, English sentence construction needs to be improved for the benefit of the reader so as to not miss such an important work.
Response 5: This paper has undergone English language editing. The text has been checked for correct use of grammar and common.

Reviewer 2 Report

Please submit the draft using the right format that way reviewer can better keep in track and indicate paragraph efficiently.

There are many grammatical errors in this article such as:

'Second, the one-dimensional model based on the acoustic wave theory and transfer matrix used to calculate the pressure pulsation [10].'; 'However, this method is lower accuracy as ignoring the mutual effect of pressure changes in the working chamber and internal plant piping.'

 

'Thus, Zhan Liu [14] proposed a hybrid method coupled compressor and pipeline.' This is not a complete sentence, by how the author combined compressor and pipeline? 

From 'In addition, a small 0.05mm gap is reserved to a...' to 'The inlet and outlet boundaries were positioned before the suction buffer' should be in your numerical section.  

What is 2-1A and 2-1B type? And I can not understand 'Considering the 2-1A and 2-1B hyper-compressors share one intake buffer tank, and then the mutual effect of the pressure pulsation of 2-1A and 2-1B, the dynamic pressure and temperature monitored at the same position of 2-1B transformed 180°were loaded to the 2-1A as the boundary condition, to achieve both highly accurate results and avoiding unnecessary computational resources. '

In the assumptions, 'The heat exchange during the operating cycle was neglected. ' I am wondering compress the ethylene at such a high temperature pressure, is the heat exchange during the process negligible?

I cannot understand the logic of this 'A mesh independence check must be conducted to assess the suitable mesh density for achieving highly precise results while minimizing computational resources. The numerical model (Figure 7) was rebuilt with three structured mesh levels, and the simulation results for mass flow rate and indicated power are presented in Table 1. Clearly, the difference in the indicated power and mass flow between tests 2 and 3 was less than 1%. The mesh of item2 was then utilized.' Why test 2 is used? Can I claim that test 3 is less than 1% difference than test 2 and test 3 can be used as reference? 

Please rewrite this paragraph 'To validate the simulation results, non-destructive test (NDT) was built to measure the piston rod strain, the top dead center (TDC) signal, and the sucpipe and dispipe strain (Figure 9), and then the p − diagram and pressure pulsation were reconstructed by the measured data.'

This should be in your hardware or experiment setup section 'The hyper-compressor is a two-stage, eight-cylinder, opposed-balanced, four-row, large-scale plunger compressor system. The running speed is 200rpm and exhaust pressure is 180Mpa. The details of the hyper-compressor are provided in Table 2'. Please pay attention to the organization of the article, the experiment setup, numerical method and simulation setup are messed up. 

I cannot understand 'The thickness components and safety devices in case of leakage around cylinder cannot permit to install a pressure sensor measuring the dynamic pressure in cylinder. The pressure in the working chamber equals F g divided by A p , and then the F g can be reconstructed based on the main acting forces analysis in the piston (Eq. 4). '

 'Thus, the crank angle would set 0°at this time and the operating time can be converted to crank angle for sections 3.2 and 3.3.' How to determine the crank at other angles? 

Figure 11 is unnecessary. 

Figure 12. How is this experiment conducted?

'The indicated power of simulation and experiment were 547.85kW and 557.73kW respectively, demonstrating a small deviation error. The estimated discharge power loss was 108.33kW, which was 7.58% off from the experimental value (100.70kW).' Are they running at a constant power? 

'The pressure ratio in this experimental test was 1.8. However, the actual pressure ratio was about 2.2, and the maximum can reach 2.6. ' What do you mean by actual pressure? 

Author Response

Point 1: There are many grammatical errors in this article such as:

Second, the one-dimensional model based on the acoustic wave theory and transfer matrix used to calculate the pressure pulsation [10].'; 'However, this method is lower accuracy as ignoring the mutual effect of pressure changes in the working chamber and internal plant piping.'

Response 1: This paper has undergone English language editing. The text has been checked for correct use of grammar and common.

 

Point 2: 'Thus, Zhan Liu [14] proposed a hybrid method coupled compressor and pipeline.' This is not a complete sentence, by how the author combined compressor and pipeline?

Response 2: Zhan Liu [14] proposed a hybrid method where the compressor was modeled based on the LPM and the pipeline was described using a gas dynamic model.

 

Point 3: From 'In addition, a small 0.05mm gap is reserved to a...' to 'The inlet and outlet boundaries were positioned before the suction buffer' should be in your numerical section. 

Response 3: This part described the detail of the numerical model. This section has been renamed the physical and numerical model.

 

Point 4: What is 2-1A and 2-1B type? And I can not understand 'Considering the 2-1A and 2-1B hyper-compressors share one intake buffer tank, and then the mutual effect of the pressure pulsation of 2-1A and 2-1B, the dynamic pressure and temperature monitored at the same position of 2-1B transformed 180°were loaded to the 2-1A as the boundary condition, to achieve both highly accurate results and avoiding unnecessary computational resources. '

Response 4: 2-1A and 2-1B are two opposed-balanced hyper-compressors, which share one suction buffer tank. The crank angle of 2-1A is 180 difference of 2-1B.

 

Point 5: In the assumptions, 'The heat exchange during the operating cycle was neglected. ' I am wondering compress the ethylene at such a high temperature pressure, is the heat exchange during the process negligible?

Response 5: The compressor speed is 200rpm. In addition, the components around the working chamber are very thick. The heat exchange between the gas in the working chamber and the outside during one cycle is negligible, so the working process inside the cylinder can be regarded as an adiabatic process.

 

Point 6: I cannot understand the logic of this 'A mesh independence check must be conducted to assess the suitable mesh density for achieving highly precise results while minimizing computational resources. The numerical model (Figure 7) was rebuilt with three structured mesh levels, and the simulation results for mass flow rate and indicated power are presented in Table 1. Clearly, the difference in the indicated power and mass flow between tests 2 and 3 was less than 1%. The mesh of item2 was then utilized.' Why test 2 is used? Can I claim that test 3 is less than 1% difference than test 2 and test 3 can be used as reference?

Response 6: The deviations of mass flow rate and indicated power between test 2 and 3 were negligibly small. The mesh density of test 2 can ensure both the mesh-independent simulation results and less calculation time.

Point 7: Please rewrite this paragraph 'To validate the simulation results, non-destructive test (NDT) was built to measure the piston rod strain, the top dead center (TDC) signal, and the sucpipe and dispipe strain (Figure 9), and then the  diagram and pressure pulsation were reconstructed by the measured data.'

Point 8: This should be in your hardware or experiment setup section 'The hyper-compressor is a two-stage, eight-cylinder, opposed-balanced, four-row, large-scale plunger compressor system. The running speed is 200rpm and exhaust pressure is 180Mpa. The details of the hyper-compressor are provided in Table 2'. Please pay attention to the organization of the article, the experiment setup, numerical method and simulation setup are messed up.

Response 7 & 8: An experimental study has to be performed to validate the numerical model, before the simulation results can be used to analyze the thermodynamic characteristic of the hyper-compressor.  As shown in Figure 9 and Table 2, a two-stage, eight-cylinder, opposed-balanced, four-row, large-scale hyper-compressor system was modified for the test. The hyper-compressor was equipped with a data acquisition system along with several sensors that recorded the signals of piston rod and pipe strain and, piston position. Then, these signals were reconstructed based on the non-destructive test method (NDT) to obtain the  p-theta diagram and pipe dynamic pressure.

 

Point 9: I cannot understand 'The thickness components and safety devices in case of leakage around cylinder cannot permit to install a pressure sensor measuring the dynamic pressure in cylinder. The pressure in the working chamber equals F g divided by A p , and then the F g can be reconstructed based on the main acting forces analysis in the piston (Eq. 4). '

Response 9: The thickness components and safety devices in case of leakage around the cylinder do not permit the installation of a pressure sensor measuring the dynamic pressure in the cylinder. The non-destructive test (NDT) based on the force-induced strain on the piston was used to measure and reconstruct the p-θ diagram.

The following functions show the relationship between the pressure in the working chamber and the strain.

 

Point 10:  'Thus, the crank angle would set 0°at this time and the operating time can be converted to crank angle for sections 3.2 and 3.3.' How to determine the crank at other angles?

Response 10: The crank at other angles can be obtained by the following funtion: θ=360/T*t

 

 

Point 11: Figure 11 is unnecessary.

Response 11: Figure 11 has beed deleted.

 

Point 12: Figure 12. How is this experiment conducted?

Response 12: Due to the extremely high pressure, pressure sensors cannot be installed on the test system by drilling. Therefore, a non-destructive method measuring the dynamic pressure has been proposed. To verify this method can accurately measure the p-θ diagram and pipe pressure pulsation , strain gauges were installed near the pressure sensor, which was already installed during the pipeline construction phase. The results of the two tests were compared in Figure 12. Figure 12 revealed that the NDT was valid and the pressure reconstructed by the strain outer the pipe can be used to analyze the pressure pulsation of the hyper-compressor system.

 

Point 13: 'The indicated power of simulation and experiment were 547.85kW and 557.73kW respectively, demonstrating a small deviation error. The estimated discharge power loss was 108.33kW, which was 7.58% off from the experimental value (100.70kW).' Are they running at a constant power?

Response 13: Ideally the hyper-compressor runs at a constant power under certain substance, speed, suction and discharge pressure and temperature. However, due to factors such as component wear and micro-polymer adhering to the inner surface, the indicated power may vary within a small range.

 

Point 14: 'The pressure ratio in this experimental test was 1.8. However, the actual pressure ratio was about 2.2, and the maximum can reach 2.6. ' What do you mean by actual pressure?

Response 14: Ethylene needs to be compressed to 220-260 MPa before entering the reaction kettle for polymerization in LDPE industrial production. The inlet pressure of the hyper-compressor is generally 100 MPa, so the actual compression ratio is 2.2-2.6.

Author Response File: Author Response.docx

Reviewer 3 Report

Generaly it's a very good paper. There is one typo on page three (Phycial). Please add information about the software used to build the mesh and perform numerical calculations. Was the software commercial or self-made?

Author Response

Point 1:  Please add information about the software used to build the mesh and perform numerical calculations. Was the software commercial or self-made?
Response 1: The softwares used to build mesh and perform numerical calculation are ICEM and Fluent, respectively. They are commerical softwares.

Round 2

Reviewer 2 Report

Thank you for the authors' response. The quality of English has improved significantly, but I would suggest running another round of editing. Organization and presentation wise, I have a few suggestions that may help readers better understand the scope and importance of this work. 

1. The main topic of this paper is to provide a CFD model that can capture the thermodynamic characteristics of a hyper-compressor, and a non-destructive method is studied to measure the experiment data. The conclusion is that the CFD model results match with the experiment result. So, I suggest that the authors focus on these two main topics. Usually, a well-presented paper includes no more than 8 figures. Figures 1, 3, 6, 7, 8, 9,10 are not needed. Some figures of how to set the mesh and model could be combined into one figure. Otherwise, it is more like a scientific report instead of a research paper. 

Content wise, please address the issues mentioned below: 

1. In the numerical methodology session, for a Non-Newtonian fluid, how do you set up the RANS model? I think this is a very critical part, and the authors need to be more specific about describing how the model is set up since the main purpose of this work is to prove that CFD model can capture real-life physics. 

2. In equation 2, is theta a function of time? I am assuming this is the crank angle you mentioned later in this work. Please clarify that. Following the crank angle I asked about in revision 1, is this a linear function? How could the authors use one single data point to capture whole P-theta graph? 

3. The reason for NDT is high pressure working conditions restrict holes on a plant pipe. How is the pressure sensor installed and how is the result compared with the strain gauge data? 

4. The authors did not answer the question why in the experiment the measured pressure ratio is 1.8 while the authors claim that the real pressure ratio is 2.2 to 2.6.

Author Response

Point 1: The main topic of this paper is to provide a CFD model that can capture the thermodynamic characteristics of a hyper-compressor, and a non-destructive method is studied to measure the experiment data. The conclusion is that the CFD model results match with the experiment result. So, I suggest that the authors focus on these two main topics. Usually, a well-presented paper includes no more than 8 figures. Figures 1, 3, 6, 7, 8, 9,10 are not needed. Some figures of how to set the mesh and model could be combined into one figure. Otherwise, it is more like a scientific report instead of a research paper.

Response 1: Thank you for your suggestion. Some details of the paper have changed. Figures 1, 6, 8, 10 were deleted. Original Figures 2, 3 and 4, 5 were combined into Figure 2 and Figure 3, respectively.

 

Point 2: In the numerical methodology session, for a Non-Newtonian fluid, how do you set up the RANS model? I think this is a very critical part, and the authors need to be more specific about describing how the model is set up since the main purpose of this work is to prove that CFD model can capture real-life physics.

Response 2: The numerical calculation was performed by the commercial software Fluent. The help of this software describes the specific RANS model. I think I can’t use this in my paper.

 

Point 3: In equation 2, is theta a function of time? I am assuming this is the crank angle you mentioned later in this work. Please clarify that. Following the crank angle I asked about in revision 1, is this a linear function? How could the authors use one single data point to capture whole P-theta graph?

Response 3: Theta in equation 2 is the crank angle. This experiment monitored and recorded the strain and time t (the change of strain with time). The speed n of the hyper-compressor is 200rpm. The period T=n/60. One period corresponds to an angle of 360° or 2π. After confirming the initial angle, other angles can be determined using the formula: theta = 360/T * t. Then the whole P-theta graph can be captured.

 

Point 4: The reason for NDT is high pressure working conditions restrict holes on a plant pipe. How is the pressure sensor installed and how is the result compared with the strain gauge data?

Response 4: The pressure sensor was installed at the stage of pipe fabrication, but its sampling frequency and location did not meet the experimental requirements. The manufacturer didn’t allow drilling holes in the pipeline during the experiment. Some strain gauges were installed near the pressure sensor. The results of the two tests were compared to verify the NDT. Other strain gauges were used to analyse the pressure pulsation.

 

Point 5: The authors did not answer the question why in the experiment the measured pressure ratio is 1.8 while the authors claim that the real pressure ratio is 2.2 to 2.6.

Response 5: The experiment was completed during the test run after the hyper-compressor was shut down for maintenance. The pressure ratio during the test run was 1.8, but during the actual production of LDPE, the pressure ratio was 2.2-2.6.

Author Response File: Author Response.docx