An Integrated Cooling Jet and Air Curtain System for Stadiums in Hot Climates

Reviewer 1 comments

Response to review comments

Authors presented a detail works on CFD simulation to evaluate cooling on football stadium, to analyse comfort in it while estimating electricity consumption.

Research work is original, well-detail and explain. I would suggest (if it is possible) to short it in some chapters (e.g. Introduction and Literature review).

I suggest the paper for publication.

The authors would like to thank the reviewer’s comments.

Agreed. We have shortened the abstract, introduction and literature review as well as the conclusion section.

Reviewer 2 comments

Response to review comments

In general, the abstract needs to undergo some substantial revision. As it is presently, it does not provide with clarity insights into what the paper is about and especially with respect to the paper’s title and objectives.

The authors would like to thank for the comments from the reviewer.

Agreed. We have modified the abstract and made it more relevant to the paper’s title and objectives.

In line 11, the statement ‘results of thermal and wind environment’ appears to be incomplete. Is it wind environment modelling?

Agreed. The statement is modified as ‘results of thermal and wind environment modelling’.

In line 13, it would be helpful to mention the specific simulation tool that was used in the study.

Agreed. The simulation tool ANSYS Fluent is mentioned in the following sentence ‘This paper presents comparisons among the results of thermal and wind environment modelling of a semi-outdoor stadium under three different cooling configurations and a baseline configuration without cooling using Computational Fluid Dynamics (CFD) tool ANSYS Fluent 18.2’

In line 14, it would also be helpful if a description of the experimental configurations could be provided early on.

Agreed. Descriptions of each configuration have been added as: ‘The three cooling configurations are: (1) vertical jets only above upper tiers (2) vertical jets above upper tiers and horizontal jets at the back of lower tiers and around the pitch (3) integrated vertical jets above upper tiers, horizontal jets at the back of lower tiers and air curtains at gates.’

In line 22, we do not know the difference between configurations 1, 2 and 3. This statement does not help much on the clarity of the presentation.

Agreed. We have explained the difference between the three configurations in the texts.

In line 53, what was the context of the other studies that investigated wind flow without linking it to cooling performance? Was it pollutant dispersion, for instance? Actually, from your literature review, it appears that these studies that you claim did not address cooling performance, in fact, did address it. The use of natural ventilation is one of the strategies that can be employed in moderating the cooling load. Perhaps the authors may wish to develop another way to demonstrate the novelty of their present work.

Sorry for the confusion about this point. The point we actually want to express is that lots of current stadium studies focus on the assessments of wind flow inside/around stadiums or wind comfort, but much less existing stadium studies investigated cooling performance. The other studies don't refer to the studies investigated wind flow without linking it to cooling performance, as what the reviewer understood. In addition, these wind flow studies aimed to improve the wind environment conditions or wind comfort, rather than to reduce the cooling load. The impact of indoor natural ventilation on the cooling performance or temperature distributions of stadiums was not mentioned in these literatures. Moreover, natural ventilation can’t moderate the cooling load in hot humid climate but will increase the cooling load since the outdoor air is hot and humid. Therefore, mechanical cooling is necessary to be used in stadiums in hot climates, in order to maintain thermal comfort for spectators and players. The cooling configurations proposed in the present work aim to show the effect of them on the thermal conditions inside the stadium compared with the baseline configuration which only uses natural ventilation.

To avoid the confusion, we removed the literature review for cooling load studies from the former paragraph and combined them into the next paragraph starting at line 123 which reviews the research work related to evaluations of thermal conditions inside the stadium and stadium cooling configurations.

In line 80, which authors?

The authors have been clarified. ‘Sofotasiou et al. [16] also conducted a study on the influence of the stadium orientation on the cooling load and found that the monthly cooling load could be reduced by up to 1.48 MWh with a modification in the orientation of the stadium.’

In line 85, how different is Ghani’s study from the present one?

Ghani’s work measured and predicted the temperature distributions on a playing field by experiments and numerical simulations, respectively. The temperature measurements of grass, subsoil, ambient air and running track surface can not only be used for validating the numerical results but also used for estimating boundary conditions for CFD simulations. However, cooling method/technology was not incorporated in Ghani’s study. That study only shows the thermal performance of a playing field under natural ventilation. The present study provides thermal conditions on both the spectator tiers and the pitch of the stadium under three different cooling configurations and a baseline configuration (natural ventilation), which is more detailed and holistic in analyzing the thermal performance of the entire stadium and also covers the impacts of cooling configurations on the thermal performance. This is clarified in the manuscript.

In line 93, Zhong’s study also appears similar to the present study. Perhaps the authors would wish to clearly explain the differences between their present work and that what has been done previously

The present study is based on the previous Zhong’s study but actually provides a solution (using air curtains) to the limitation of the previous study where the penetration of outside hot air through gates of the stadium led to higher temperatures of at least 35.3 °C at some local regions on the pitch. Therefore, the thermal environment is improved by the integration of cooling jets and air curtains compared with the configurations of (1) vertical jets only and (2) vertical jets above upper tiers and horizontal jets at the back of lower tiers and around the pitch proposed in the previous work.

In addition, the present work made improvements on the boundary conditions used in CFD simulations. It predicted the radiative heat fluxes on the ground, stadium and surrounding buildings by carrying out de-coupled solar radiation simulations, in order to capture more realistic thermal boundary conditions for the simulations which assess the thermal performance of the stadium under cooling configurations and a baseline configuration.

Moreover, this study presents parametric analysis for supply velocity magnitudes of jets of different locations and air curtains. Three scenarios were proposed for each velocity parameter and their impacts on pitch thermal conditions were compared. Thus, this study could provide an insight about the influence on the cooling performance caused by adjusting supply velocity magnitudes for jets of different locations and air curtains for the industry practitioners, engineers, stadium designers, etc., which is still limited in the current stadium cooling field.

In line 98, again, without earlier description of these configurations, this statement is of no benefit to the reader.

The descriptions of these configurations were provided in the above context as: ‘(1) vertical jets above the upper tiers only, (2) combined vertical and horizontal jets at tiers with only an array of jets around the pitch, (3) combined vertical and horizontal jets at tiers with three arrays of jets around the pitch.’ These configurations present in the literature review refer to the configurations proposed in the previous study, not for the present study.

Between line 104 and 106, the transition between the paragraphs rather comes through as being ad-hoc. The authors may wish to ensure a smooth transition that helps to make logical sense in the flow.

Agreed. A transitioning sentence has been added at the end of the former paragraph starting at line 150: ‘To provide a solution to this limitation, the use of mechanical ventilation technique(s) at gates of the stadium is recommended to explore.’

In line 104, what is hot thermal performance? The authors may wish to find a more appropriate technical way to present this one.

Agreed. That expression has been modified as: ‘resulted in higher temperatures of at least 35.3 °C at some local regions on the pitch.’

In line 108, which previous study? The line comes through as being a case of copy & paste. Please, check it.

The previous study refers to Zhong et al. [19]. We have clarified it as ‘the previous study of Zhong et al. [19]’.

In line 163, again, a description of these configurations early on would be helpful.

Agreed. The description of these configurations has been added in the section 1 Introduction and Literature Review: ‘The three cooling configurations are: (1) vertical jets only above upper tiers (2) vertical jets above upper tiers and horizontal jets at the back of lower tiers and around the pitch (3) integrated vertical jets above upper tiers, horizontal jets at the back of lower tiers and air curtains at gates.’

In line 340-345, merely mentioning that this method was validated in another study is not good enough. It must be demonstrated that the method has been validated for the purposes of this present study just like the authors have validated the simulation in Section 3.1.

We have added the relevant validation for the cooling jets modelling method in the section 3.2: ‘The characteristics of cooling jets were validated based on previous experimental measurements [48] and CFD simulation results [49]. The cooling jet was modelled as a reduced-scale spray nozzle with diameter of 4 mm in a simplified computational domain of 0.585 m ´ 0.585 m ´ 1.9 m (Fig. 7a), which was used as the wind tunnel test section in the experiment [48]. The boundary conditions of the domain, the characteristics of the nozzle and the Fluent setup were based on [49] and therefore not repeated in this paper. The dry-bulb temperatures (DBTs) were measured by nine measurement points (at the center of each 3 ´ 3 grid) (Fig. 7b) on the outlet plane of the domain from current CFD simulation and were compared with the measurements on the same points from the previous experimental testing [48] and CFD simulation results [49]. The error between current CFD results and experimental measurements on each measurement point was plotted in Figure 7c. It is observed that the maximum absolute error is 10% and the absolute deviations between the two results for most of the points are within 5%. The mean absolute error between the two results is calculated as 4.55%, which shows a good agreement between the current CFD results and previous experimental results. Apart from this, the graph plotting the relationship between the DBTs from current CFD simulations and the previous experimental testing [48] (Fig. 7c) is compared with the graph presenting the relationship between the DBTs from previous CFD simulations [49] and experimental testing [48]. The maximum absolute error is around 7% and the absolute errors for the remaining points are within 5%, which are close to those from the current study. But the trends of data points are slightly different between the two graphs. The DBTs from current simulations are generally lower than those from previous simulations, which also results in the fact that the DBTs from current simulations are overall lower than those from the previous experimental measurements. These discrepancies maybe caused by the different discrete phase model conditions set for the walls of the domain. The ‘reflected’ boundary condition was used in the previous simulations while the ‘escape’ condition was set in the current simulations considering the larger domain of the stadium environment than the simplified domain used in the previous simulations. Overall, the validity of the characteristics of cooling jets were verified.’

Reviewer 3 comments

Response to review comments

The study focuses on the space cooling of a football stadium in a hot and humid climate using cooling jets and air curtains. I consider this study to be very good engineering work. The paper is written clearly and is easy to read. My main concerns are related to the level of originality and scientific contribution. First, about half of the paper (methodology and validation) has been already presented in a preceding paper published in a scientific journal. Compared to the published paper, the configuration of the jets was modified, and air curtains were added at the entrances. In my opinion, the overlap with the previous investigations is considerable.

The authors would like to thank the reviewer’s comments.

The present study is based on the previous work of Zhong et al. [19] but actually provides a solution (using air curtains) to the limitation of the previous study where the penetration of outside hot air through gates of the stadium led to higher temperatures of at least 35.3 °C at some local regions on the pitch. Therefore, the thermal environment is improved by the integration of cooling jets and air curtains compared with the configurations of jets only proposed in the previous work.

Although a portion of the methodology and validation are similar to the previous work, the methodology and validation are still improved based on the previous work. The boundary conditions used for the ground, stadium and surrounding buildings are updated in the present study to capture more realistic conditions for the CFD model using de-coupled solar radiation simulations. The results for cooling jets validation are also updated by comparing the current CFD simulation results with the previous experimental measurements, which are not presented in the previous work. In addition, the validation for air curtains is also not presented in the previous work. The authors have to repeat a part of the methodology and validation results, which are used in the previous work, to ensure the consistency of the CFD models for the same case study stadium. This will be helpful for the practitioners, engineers and designers to better follow the authors’ work, in order to let them make the best use of the outcome of the research work.

Moreover, this study presents parametric analysis for supply velocity magnitudes of jets of different locations and air curtains. Three scenarios were proposed for each velocity parameter and their impacts on pitch thermal conditions were compared. Thus, this study could provide an insight about the influence on the cooling performance caused by adjusting supply velocity magnitudes for jets of different locations and air curtains for the industry practitioners, engineers, stadium designers, etc., which is still limited in the current stadium cooling field.

Second, both the cooling jets (that have been investigated in the previous paper) and the air curtains are well-know technologies. Air curtains are commonly being used as a thermal barrier and combined with air cooling systems. Due to the combination of these facts, in my opinion, the study lacks scientific originality.

The combination of cooling jets and air curtains are indeed commonly used in enclosed buildings. But the use of this combination in stadiums or semi-outdoor structures is still limited. The impact of this combination on the thermal conditions of stadiums in hot humid climate is still required to be assessed to verify the feasibility and reliability of this integrated system for providing thermally comfortable environment inside the stadium, which is also lacked in the stadium cooling field.

·         The abstract should be shorter.

Agreed. The abstract has been made brief.

·         The conclusion is way too long and it only summarizes the results. A certain level of generalization of the results would be beneficial.

Agreed. We have shortened the conclusion and generalized the key findings from the results.

·         The description of the energy calculations is brief. Performing complex energy calculations properly is not an easy task and would deserve a more thorough explanation (although I know that it could make the paper much longer).

Agreed. The governing equations for heat transfer processes from this tool are given in the section 2.5. The heat balance equations which are used to calculate energy consumptions are also added in the section 2.5.

Reviewer 4 comments

Response to review comments

In this study, the thermal and wind environment of a semi-outdoor stadium under three different cooling configurations and a baseline configuration has been studied using Computational Fluid Dynamics (CFD) simulations. The study is within the scope of this journal. I have the following suggestions for further improvement.

The authors would like to thank the reviewer’s comments and suggestions.

Abstract:

“Doha under fair weather conditions” – In this statement, what is fair weather condition? Can you please mention the air-temperature, RH%, and airspeed, etc.

Sorry for the confusion of this point. Actually ‘fair weather conditions’ is a particular solar irradiance calculation method in ANSYS Fluent. It enables the solar radiation model to use reasonable values of solar irradiance data rather than the theoretical maximum values which are unlikely to be experienced in practice due to atmospheric conditions. For further information, the reviewer could access the Fluent Users Guide ‘Solar Load Model’ section.

To avoid the confusion on this point, the statement “Doha under fair weather conditions” has been modified as “Doha under fair weather conditions method in Fluent”.

Abstract:

“Based on the set conditions, the results showed that air curtains, employed in the 21 configurations 3 are effective in preventing the penetration of outside hot air of 34.2 °C and 5.3 m/s 22 through the gates of the stadium.” --- What is condition 3 here? It will be better if you mention the features of the three conditions/scenarios investigated in this study and then discuss the findings.

Agreed. Descriptions of the three cooling configurations have been added in the abstract as following: ‘The three cooling configurations are: (1) vertical jets only above upper tiers (2) vertical jets above upper tiers and horizontal jets at the back of lower tiers and around the pitch (3) integrated vertical jets above upper tiers, horizontal jets at the back of lower tiers and air curtains at gates.’

Abstract:

PPD and PMV are used first time here so kindly use the full form.

Agreed. These two terms should be modified to their full forms as ‘Predicted Percentage of Dissatisfied (PPD)’ and ‘Predicted Mean Vote (PMV)’. For shortening the abstract, the relevant sentences are removed.

Abstract:

“Future work is recommended 29 to investigate the operation conditions of air curtains for stadiums and analyze the influence of air 30 curtain parameters on its performance.” --- My suggestion is to remove this statement from here and put it inside the conclusion section.

Agreed. We have moved this statement from the abstract to the conclusion section.

Introduction:

“Thus, to deliver energy-saving and thermally comfortable stadiums for the event, the country should seek solutions from energy-efficient cooling systems or passive cooling techniques for stadiums” --- You can strengthen your literature by citing some of the novel or relevant cooling method/technology here to create a base of your study such as “Adaptable cooling coil performance during part loads in the tropics”, and “Computation of zone-level ventilation requirement based on actual occupancy, plug, and lighting load information” etc.

Agreed. We have cited some novel or relevant literatures on cooling systems/technologies in the text ‘Thus, to deliver energy-saving and thermally comfortable stadiums for the event, the country should seek solutions from energy-efficient cooling systems [7-10] or passive cooling techniques for stadiums.’ and added the corresponding references in the references section:

7. Boero, A.; Agyenim, F. Modeling and simulation of a small-scale solar-powered absorption cooling system in three cities with a tropical climate. Int. J. Low-Carbon Technol. 2020, Vol. 15, Issue 1, pp. 1-16. https://doi.org/10.1093/ijlct/ctz040

8. Sekhar, C.; Anand, P.; Schiavon, S.; Tham, K.W.; Cheong, D.; Saber, E.M. Adaptable cooling coil performance during part loads in the tropics—A computational evaluation. Energy Build. 2018, Vol. 159, pp. 148-163. https://doi.org/10.1016/j.enbuild.2017.10.086

9. Anand, P.; Cheong, D.; Sekhar, C. Computation of zone-level ventilation requirement based on actual occupancy, plug and lighting load information. Indoor Built Environ. 2020, Vol. 29, Issue 4, pp. 558–574. https://doi.org/10.1177/1420326X19875802

10. Irshad, K.; Habib, K.; Basrawi, F.; Saha, B.B. Study of a thermoelectric air duct system assisted by photovoltaic wall for space cooling in tropical climate. J. Energy 2017, Vol. 119, pp. 504-522. https://doi.org/10.1016/j.energy.2016.10.110

Methodology: 

The methodology section is very well explained. However, kindly mention the computational time and system configuration used for CFD simulation.

Agreed. The computational time and system configuration used for CFD simulations have been added in the section 2.4 Fluent setup: ‘The Fluent calculations were implemented using parallel processing on a HP Z620 Workstation which contains dual Octa-Core Intel Xeon E5-2690 2.90 GHz processors and 64 GB DDR3 memory.’ ‘…and an average computational time of 24 hours per simulation was consumed.’

Conclusion:

Kindly reduce the text of the conclusion. It is too wordy and too elaborative. If you are not able to condense it then go for bullet points.

Agreed. We have shortened the conclusion and only pointed out the key findings from results.