Review Reports - A Numerical Analysis of the Cooling Performance of a Hybrid Personal Cooling System (HPCS): Effects of Ambient Temperature and Relative Humidity

Round 1

Reviewer 1 Report

Introduction:

Page 2, Line 71: Author could explain how insulation pads could help to improve the performance of HPCS. How insulation pads were integrated, what was the configuration of PCM, Insulation, and clothing layers.

Page 3, Line 96: Why authors choose these temperature and RH (if it is in section 2.3, authors need not to mention the temperature and RH here). What about air velocity. It is also an ambient environmental parameter could affect the performance. Authors need to discuss about this specially when outdoor clothing.

Methodology:

Section 2.1: What were the key criteria for this design. Why author thinks that 24 PCM packs are optimum for the comfort. Why author integrated 4 fans in the lower back and pelvis region. What kind of 100% cotton fabrics (construction, design of the fabrics) are used for this clothing. Why author used only 100% cotton, generally due to low wicking cotton could absorb quite a lot of moisture. Any insight on this? What about the wash ability of the PCM packs.

Page 4, Line 126: 24 packs was 1.54 kg: it seems an extra burden to the workers. How authors will justify this.

Figure 2: is the ventilation layer is the microclimate, be consistent in Fig 2 (a) and (b). Authors discussed few information in this section that could be the part of the literature review. Please organize the content accordingly.

Section 2.2: When somebody will work in 40C/90%RH, may be sweat dipping could occur, does the developed model considered anything about sweat dipping?

Section 2.3, Line 198: Why author choose this numerical simulation scenarios? Is it established scenarios in the literature?

Line 209: author could explain this 50, 10, and 6 discreate nodes in detail.

Results and Discussion:

In section 3.1, 3.2, and 3.3, authors discussed about three conditions associated with temperature and RH. In each scenario, author discussed about 8 graphs individually. However, these discussions are mainly what is demonstrating in the graphs. As it is a comparison study under different conditions, it is expected that author should prepare a summary table or diagram in order to effectively compare the conditions. It could give more insight about the implementation of this study.

Is it possible to validate these model findings, through human trial or manikin experiments? Although author validated their model in previous study, it is with one temperature and humidity (as indicated in line 86). Author could use this study for the validation of the model in different conditions.

Conclusion: This section looks like the summary of the results and discussion. Conclusion section could be more generalized, for example 40C & 90RH could cause hyperthermia in 38-40 min. In which conditions, new HPCS could be used or cannot be used.

Author Response

[REPLY] Thanks, we have now explained how insulation pads could improve HPCS performance (i.e., providing wearers extended cooling duration). Also, we described how the insulation was incorporated in to the system in the revised manuscript. The configuration was described in Figure 2.

[REPLY] Thanks, we moved the description to section 2.3. The air velocity is minimal, about 0.15 m/s.

Methodology:

[REPLY] Generally, the larger the cooling area, the better the cooling effectiveness. However, the weight issues might be considered. The 24 locations were chosen because these body parts are normally in close contact with clothing. PCMs must be closely contact with the body so that they could draw body heat effectively through conduction.

The fabric is twill weave. Cotton is hygroscopic and good at absorbing moisture. PCM packs are washable.

Page 4, Line 126: 24 packs was 1.54 kg: it seems an extra burden to the workers. How authors will justify this.

[REPLY] To provide good cooling effect, a small amount of PCMs would not function well. Compared to many documented personal cooling system, a weight of 1.54 kg seems acceptable. In fact, many published work on cooling clothing showed that the personal cooling systems are normally greater than 5 kg. See the below reference

Chan APC, Song W, Yang Y (2015) Meta-analysis of the effects of microclimate cooling systems on human performance under thermal stressful environments: potential applications to occupational workers. Journal of Thermal Biology, 49-50:16-32.

[REPLY] Thanks, we revised the ventilation layer to microclimate. We carefully read the whole section again and we feel the description of the model should be located in section 2.2. Moving them to literature review seems inappropriate.

Section 2.2: When somebody will work in 40C/90%RH, may be sweat dipping could occur, does the developed model considered anything about sweat dipping?

[REPLY] Yes, the model has considered the sweat dripping. This has been described in our work published in Int J Heat Mass Transfer (volume 126, page 641).

Section 2.3, Line 198: Why author choose this numerical simulation scenarios? Is it established scenarios in the literature?

[REPLY] Thanks, the scenario has been widely used in our previous work.

Line 209: author could explain this 50, 10, and 6 discreate nodes in detail.

[REPLY] PCM layers, fabric layers and insulation layers were calculated through finite difference method. Physically, the temperature distribution should be continuous. However, in the numerical method, the continuous space of PCM layers, fabric layers and insulation layers was divided into different numbers of elements by 50, 10, and 6 discrete nodes, respectively. The node temperature of each element represents the temperature within the whole element. Therefore, the temperature distribution could be characterized by the nodes’ temperatures.

Results and Discussion:

[REPLY] Thanks, a table (Table 2) was added to show the maximal core temperature and mean skin temperature observed in the various studied environmental conditions.

[REPLY] Thanks, this is possible. However, additional human trials are required to validate the model.

[REPLY] Thanks, this section has been revised based on your suggestions.

Reviewer 2 Report

For the near future, the combination of PCM packs and ventilation fans would be closely chosen. Please update this problem for covid19.

Section 2.1 must be oriented to explain the thermal influence zones for the HPCS. It is not clear a constant thermal distribution heat as function of human body. Please add a delivered heat study for several human activities.

The one - dimensional analysis from reference 25th is a good approach but for the schematic of figure 1, another one-dimension thermal analysis is missed. Please add from the end of the phase change material the thermal distribution as function of surrounding distance.

Results and discussion is very good section and the values match with the parameters and equations; exception at 70 minutes for experimental inflected behavior. Please verify Figure 10 has an unexpected gain variation.

Conclusion section is good enough.

Author Response

For the near future, the combination of PCM packs and ventilation fans would be closely chosen. Please update this problem for covid19.

[REPLY] Updated, thanks.

[REPLY] Thanks for your comments. As per our reply to Review 1, PCM packs must be placed as close as possible to the body, and preferably, they should be in tight contact with the body. These 24 PCM packs were located to body site where they could be in direct contact with the body.

[REPLY] We are not sure which one-dimensional thermal analysis is missing, please advise.

[REPLY] We double checked Figure 10. The temperature gain in Fig.10(a) after 70th min is because the totally melted PCM has low thermal conductivity, which added thermal resistance for skin heat dissipation. The temperature gain in Fig.10(b) after 70th min at RH = 30% is induced by lack of sweat evaporation. When RH = 50%, 70% and 90%, the moderate work leads to much sweat generation which could not evaporate completely during the moderate work period. When the moderate work terminated at 70th min, the sweat evaporates quickly at RH=50% which overweighs the heat generation, and thus the skin temperature decreases. At RH=70% and 90%, the sweat evaporation was suppressed, which almost equal to the heat generation after 70 min, so the skin temperature has very small variation. At RH=30%, the sweat evaporates quickly during the moderate work period, which is equivalent to the body heat dissipation, and thus there is no extra sweat on the skin at 70 min. When the moderate work terminated, the skin temperature will increase due to the lack of extra sweat evaporation.

Conclusion section is good enough.

[REPLY] Thanks.

Reviewer 3 Report

I read your paper. It was great research on human body cooling system. I recommend few things to improve your paper.

Your system is accompanied flow passage. But it was not clear how air flow i s circulating or flowing.
You showed mathematical equation to solve your problem, but the simulation procedure by Matlab can not be caught.
Also air flow should be explained by flow rate and velocity. These are not clear.
Mainly air flow will be the main influencing factor. It should be clearly explained.
PCM packs are placed in some places in body, possibly randomly. Is there any reasons for each position. (Even if you described this facts in Line 55-60, but it is not enough.)
Figs. 8 & 9 are not clear with PCM or not.?
In Fig.9, (c) and (d) can be kind of energy balance. What is the system usefulness between Input from environment and output from the body in terms of PCM and air flow ?

Author Response

Your system is accompanied flow passage. But it was not clear how air flow i s circulating or flowing.

[REPLY] Thanks, the text on the air flow circulation was added.

You showed mathematical equation to solve your problem, but the simulation procedure by Matlab can not be caught.

[REPLY] The ambient air was blown into the clothing microclimate and the air could only go out of the new HPCS’s microclimate through sleeve openings. Detailed air flow direction could be found in the reference [33].

Choudhary, B.; Udayraj; Wang, F.; Ke, Y.; Yang, J., Development and experimental validation of a 3D numerical model based on CFD of the human torso wearing air ventilation clothing. Int. J. Heat Mass Transfer 2020, 147, 118973.

Also air flow should be explained by flow rate and velocity. These are not clear.

[REPLY] The flow rate for each ventilation fan was 0.02 m³/s, which was measured using an air flow meter.

Mainly air flow will be the main influencing factor. It should be clearly explained.

[REPLY] Thanks, air flow direction and methods on how to prevent air leakage were added.

PCM packs are placed in some places in body, possibly randomly. Is there any reasons for each position. (Even if you described this facts in Line 55-60, but it is not enough.)

[REPLY] Thanks, as per our reply to the review 1, the 24 locations were chosen because these body parts are normally in close contact with clothing. PCMs must be closely contact with the body so that they could draw body heat effectively through conduction.

Figs. 8 & 9 are not clear with PCM or not.?

[REPLY] Figures 8 & 9 are the results with PCM. Figure 8 showed the core temperature and the mean skin temperature. Figure 9 showed the data at the chest segment covered with PCMs.

In Fig.9, (c) and (d) can be kind of energy balance. What is the system usefulness between Input from environment and output from the body in terms of PCM and air flow ?

[REPLY] Generally, the PCM could absorb a large amount of heat due to high latent heat. Our system added an insulation layer outside the PCM, which reduces the PCM heat adsorption from environment. The environmental RH has little effect on the PCM heat adsorption from environment at RH = 30%, 50% and 70%, because the temperature on the outer surface of insulation layer is lower than the saturation vapor temperature, so no condensation could take place and the heat adsorption is only caused by natural convection. When the environmental RH reached 90%, environmental vapor will be condensed on the outer surface of the insulation layer, which increased the PCM heat absorption from environment. Therefore, our system could enlarge the working range of the environmental RH. As for the air flow, although it decreased the melting time of the PCM, it could improve the thermal comfort before and after the PCM melting. Initially, the PCM temperature is very low leading the thermal discomfort, which could be improved by the air flow. Moreover, when the PCMs are totally melted, the air flow could increase the evaporative heat dissipation and thus improve the thermal comfort again.

Round 2

Reviewer 3 Report

Thank you for your response.

1. Simulation is much difficult without clear system configuration. In your manuscript. the system is very simplified and not clear.

2. Your simulation model was described in section 2.2, but this is just defining your model. It is important to describe your simulation logic based on your system. Specially in 1-d model, this logical calculation should be clearly defined.

Author Response

We have added an equation (Eg.7) to show how the skin accumulation at the skin surface was calculated and also to explain how sweat was absorbed by the underwear in detail. Besides, a flow chart to illustrate the entire operation of the sequence of the HPCS-thermoregulatory model was added. Thank you.