Bioinspired Heat Exchangers: A Multi-Scale Review of Thermo-Hydraulic Performance Enhancement

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

Hyunsik Yang et al. have written a comprehensive review paper on bio-inspired structures for increasing the energy efficiency of heat exchangers. The focus is particularly on publications that appeared after 2020 in order to highlight the latest developments in this emerging field. The text is clearly structured and deals in particular with micro- and nanostructures such as those found on the surfaces of various leaves, textures such as those found primarily in the animal world, e.g., in fish, which influence vortices and drag reduction, and bio-inspired networks for the efficient distribution of fluids.

Due to the novelty of the topic, it is understandable that the scope of the literature evaluated is somewhat smaller than in comparable review articles. Nevertheless, the publication offers a comprehensive starting point for the bio-inspired optimization of heat exchangers.

I would like to see the authors expand on the examples already listed and discuss the extensively researched noses of seals and other marine mammals, which can serve as models in terms of their controlled heat exchange and ability to retain humidity. One example of this is the publication by Cheon et al. "Structure-function relationships in the nasal cavity of Arctic and subtropical seals" DOI: 10.1016/j.bpj.2023.11.012

Author Response

1) Reviewer’s comment: I would like to see the authors expand on the examples already listed and discuss the extensively researched noses of seals and other marine mammals, which can serve as models in terms of their controlled heat exchange and ability to retain humidity. One example of this is the publication by Cheon et al. "Structure-function relationships in the nasal cavity of Arctic and subtropical seals" DOI: 10.1016/j.bpj.2023.11.012

Author’s response: Thank you for your insightful suggestion. We have maintained the overall structure of the manuscript so as to preserve its strengths as you noted. We agree that the nasal structures of seals and other marine mammals provide a well-established biological model for controlled heat exchange and moisture (humidity) retention. Accordingly, we have added a brief discussion of mammalian nasal-cavity architectures—including the study by Cheon et al. (DOI: 10.1016/j.bpj.2023.11.012)—and highlighted the engineering implications for heat and moisture recovery in biomimetic heat-exchanger design in Section 4.2.

4.2. Complementary Evaluation Frameworks Beyond Conventional Thermo-Hydraulic Metrics

To date, research on biomimetic heat exchangers has largely advanced around the trade-off between improved heat-transfer performance and increased pressure drop, and heat–fluid-based performance metrics such as PEC have provided an effective criterion for comparing different design concepts. However, because these are transfer-performance-centered metrics defined under specific operating conditions, it may be useful to complement them with a system-level metric such as energy production. Accordingly, drawing on evaluation frameworks from other fields may offer additional insights that complement existing metrics and help guide future design.

In this regard, a series of studies on mammalian nasal cavities interpreted heat and moisture exchange using entropy production as a system-level metric, thereby analyzing exchange efficiency from the perspective of irreversible losses. For example, Magnanelli et al. (2016) constructed a dynamic model that reflects the geometric morphology of the reindeer nasal cavity and quantified geometric effects by comparison with a simple cylindrical structure having the same total volume and contact area. This study suggests that the complex nasal-cavity structure of mammals goes beyond merely increasing surface area, and can distribute entropy production more uniformly.

Studies on marine mammals likewise provide a similar perspective. Cheon et al. (2023) extracted the cross-sectional area of the flow path and the perimeter of the air–tissue interface from CT-based nasal-cavity geometries of Erignathus barbatus and Monachus monachus, and applied them to a quasi-1D dynamic model based on mass and energy balances. The analysis showed that, under identical environmental conditions, entropy production decreases significantly as the perimeter of the maxilloturbinate increases, and they discussed that, in terms of energy dissipation, the Arctic species is relatively advantageous due to geometric differences. Subsequently, Flekkøy et al. (2023) demonstrated, through a thermo-hydrodynamic model, that ice formation must be considered to reproduce expiratory temperatures in low-temperature environments. Furthermore, Cheon et al. (2024) evaluated entropy production under multiple environmental temperatures, relative humidities, and physiological conditions using a CFD model for the maxilloturbinate structure of seals.

Although these studies are not cases directly implemented as engineering heat exchangers, they are meaningful as conceptual reference cases showing that minimizing irreversible losses can be considered a design objective in the process of evaluating heat-exchange performance. This evaluation framework, rather than replacing existing heat–fluid performance metrics, provides applicability as a complementary perspective for future biomimetic heat-exchanger design and analysis.

The biological systems mentioned in this section are not presented as direct design templates for heat exchangers, but are introduced as conceptual reference cases to present an evaluation framework and interpretive perspective that can supplement existing heat–fluid performance metrics.

Taken together, future research should prioritize the scaling-up of biomimetic designs and the experimental validation of real-world performance to effectively enhance heat exchanger efficiency through nature-inspired strategies. In particular, further investigation is needed in areas such as operating condition optimization, techno-economic feasibility in fabrication and maintenance, material compatibility and long-term durability, and the scalability of biomimetic designs for use in general-purpose industrial heat exchangers. In this process, supplementing existing performance-evaluation metrics using indicators such as entropy production is expected to help refine comparisons among design alternatives and the specification of design targets.

Author Response File: Author Response.pdf

Reviewer 2 Report

Comments and Suggestions for Authors

The submitted manuscript is fascinating to review. However, several points need to be addressed before making the final decision.

 

1- I could not find any critical review of the applications "surface scale, texture scale, and network scale", PCM, the design, and many other criteria that have been addressed in this manuscript. The authors just reviewed the previous study without proper discussion and critical analysis.

2- The authors claimed that this manuscript has been reviewed in several articles using major academic publisher platforms, including "MDPI, ScienceDirect, Wiley Online Library, SpringerLink, and Scopus". Additionally, they checked Google Scholar; therefore, I expected to find at least 100 articles that have been reviewed to analyse this work.

3- Some studies show that the work has been done based on CFD simulation, while the authors did not provide any solid analysis of these studies, with their outcomes.

4- Tables 1 and 2 show a poor comparison of the data provided; nothing new has been addressed from their side. The authors should discuss the outcome of these references within the table. The purpose of the review paper is to critique the articles and make it easier for future scholars.

5- In the conclusion section, the authors cover the limitations and the future work, which is good. However, I would suggest placing the limitations and future work in a new section before the conclusion and enhancing the conclusion of this review.

Comments on the Quality of English Language

The language may need improvement.

Author Response

Reviewer #2:

1) Reviewer’s comment: I could not find any critical review of the applications "surface scale, texture scale, and network scale", PCM, the design, and many other criteria that have been addressed in this manuscript. The authors just reviewed the previous study without proper discussion and critical analysis.

Author’s response: Thank you for this important comment. We agree that a review article should provide critical discussion and synthesis, rather than merely listing prior studies. Accordingly, in the revised manuscript we strengthened the critical analysis by systematically identifying commonalities and differences across the reviewed works and using these comparisons to derive broader technical insights.

This critical discussion is incorporated throughout the three main scales (Sections 3.1, 3.2, and 3.3) as well as within each corresponding subcategory (Sections 3.1.1–3.1.2, 3.2.1.1–3.2.1.2, 3.2.2–3.2.3 and 3.3.1–3.3.3). In addition, we explicitly summarize the key insights for each scale in Sections 3.1.3, 3.2.4, and 3.3.4, while the main takeaways for each subcategory are highlighted at the end of the relevant subsection.

3.1.3. Surface-scale critical synthesis and design guidelines

Surface-scale bioinspired technologies can be interpreted as an approach that secures operational stability by suppressing performance degradation caused by condensation, icing, and contamination, rather than directly increasing the heat transfer coefficient. Both lotus-based superhydrophobic surfaces and Nepenthes-inspired SLIPS enhance droplet mobility and reduce liquid-film/frost accumulation, but their effectiveness is strongly dependent on operating conditions and surface states.

Lotus-inspired superhydrophobic surfaces can induce dropwise condensation and delay initial icing by suppressing droplet spreading via micro-/nanostructures and low surface energy. However, in high-humidity environments, ice adhesion can increase due to enhanced condensation nucleation or moisture entrapment associated with the Cassie–Wenzel transition. Therefore, superhydrophobic surfaces should be evaluated within a performance window defined by humidity, temperature, and condensation intensity.

In addition, the static contact angle does not sufficiently explain real operational performance. From a heat-exchanger perspective, the key evaluation metrics are drainage-dynamics indicators such as sliding angle, contact angle hysteresis, and the critical departure diameter/period. Performance evaluation of surface technologies should be reformulated away from static water repellency and toward dynamic drainage characteristics under condensation/icing conditions.

SLIPS can complement the vulnerable regime of superhydrophobicity by suppressing droplet/frost adhesion under humid, low-temperature conditions through a lubricant layer and ultra-low hysteresis. However, the key bottleneck is long-term stability due to oil depletion, and practical adoption is governed more by lubricant-layer retention/replenishment and maintenance strategies than by initial performance.

In summary, for heat-exchanger applications, surface modification can inversely limit performance through added thermal resistance (coatings/layers), air entrapment, and accumulated wear/contamination. Accordingly, it should be treated as a multi-objective optimization problem that includes thermal resistance, wear resistance, contamination resistance, and repeated-defrost durability, in addition to water repellency and drainage performance. Ultimately, the selection criteria between superhydrophobicity and SLIPS should be organized not by water repellency itself, but by a performance-window and life-cycle design perspective that reflects the operating humidity range, icing/defrost conditions, contamination/wear environment, and maintainability.

 

2) Reviewer’s comment: The authors claimed that this manuscript has been reviewed in several articles using major academic publisher platforms, including "MDPI, ScienceDirect, Wiley Online Library, SpringerLink, and Scopus". Additionally, they checked Google Scholar; therefore, I expected to find at least 100 articles that have been reviewed to analyse this work.

Author’s response: Thank you for your comment. Our primary objective was to capture the most recent nature-inspired studies that are directly relevant to improving heat-exchanger performance within the specific scope of this review. While we conducted literature searches across major academic platforms (e.g., MDPI, ScienceDirect, Wiley Online Library, SpringerLink, Scopus, and Google Scholar), a substantial portion of “bio-inspired” studies identified in these databases falls outside our defined scope (e.g., studies not addressing heat-exchanger performance enhancement in a manner aligned with the focus of this manuscript). Therefore, rather than aiming for a very large number of broadly related papers, we intentionally prioritized a more targeted set of publications and provided deeper critical synthesis across them.

To complement this targeted approach, we also included a separate discussion (Section 4.2) that introduces additional evaluation perspectives which, although slightly beyond the strict scope, may offer useful performance-assessment insights for future biomimetic heat-exchanger design.

4.2. Complementary Evaluation Frameworks Beyond Conventional Thermo-Hydraulic Metrics

To date, research on biomimetic heat exchangers has largely advanced around the trade-off between improved heat-transfer performance and increased pressure drop, and heat–fluid-based performance metrics such as PEC have provided an effective criterion for comparing different design concepts. However, because these are transfer-performance-centered metrics defined under specific operating conditions, it may be useful to complement them with a system-level metric such as energy production. Accordingly, drawing on evaluation frameworks from other fields may offer additional insights that complement existing metrics and help guide future design.

In this regard, a series of studies on mammalian nasal cavities interpreted heat and moisture exchange using entropy production as a system-level metric, thereby analyzing exchange efficiency from the perspective of irreversible losses. For example, Magnanelli et al. (2016) constructed a dynamic model that reflects the geometric morphology of the reindeer nasal cavity and quantified geometric effects by comparison with a simple cylindrical structure having the same total volume and contact area. This study suggests that the complex nasal-cavity structure of mammals goes beyond merely increasing surface area, and can distribute entropy production more uniformly.

Studies on marine mammals likewise provide a similar perspective. Cheon et al. (2023) extracted the cross-sectional area of the flow path and the perimeter of the air–tissue interface from CT-based nasal-cavity geometries of Erignathus barbatus and Monachus monachus, and applied them to a quasi-1D dynamic model based on mass and energy balances. The analysis showed that, under identical environmental conditions, entropy production decreases significantly as the perimeter of the maxilloturbinate increases, and they discussed that, in terms of energy dissipation, the Arctic species is relatively advantageous due to geometric differences. Subsequently, Flekkøy et al. (2023) demonstrated, through a thermo-hydrodynamic model, that ice formation must be considered to reproduce expiratory temperatures in low-temperature environments. Furthermore, Cheon et al. (2024) evaluated entropy production under multiple environmental temperatures, relative humidities, and physiological conditions using a CFD model for the maxilloturbinate structure of seals.

Although these studies are not cases directly implemented as engineering heat exchangers, they are meaningful as conceptual reference cases showing that minimizing irreversible losses can be considered a design objective in the process of evaluating heat-exchange performance. This evaluation framework, rather than replacing existing heat–fluid performance metrics, provides applicability as a complementary perspective for future biomimetic heat-exchanger design and analysis.

The biological systems mentioned in this section are not presented as direct design templates for heat exchangers, but are introduced as conceptual reference cases to present an evaluation framework and interpretive perspective that can supplement existing heat–fluid performance metrics.

Taken together, future research should prioritize the scaling-up of biomimetic designs and the experimental validation of real-world performance to effectively enhance heat exchanger efficiency through nature-inspired strategies. In particular, further investigation is needed in areas such as operating condition optimization, techno-economic feasibility in fabrication and maintenance, material compatibility and long-term durability, and the scalability of biomimetic designs for use in general-purpose industrial heat exchangers. In this process, supplementing existing performance-evaluation metrics using indicators such as entropy production is expected to help refine comparisons among design alternatives and the specification of design targets.

 

3) Reviewer’s comment: Some studies show that the work has been done based on CFD simulation, while the authors did not provide any solid analysis of these studies, with their outcomes.

Author’s response: Thank you for raising this important point. We agree that many studies—particularly those in the texture-scale category—derive their conclusions primarily from CFD simulations, and that a review should critically assess the reliability and implications of such outcomes. To address this, we added/strengthened critical discussion for CFD-based studies across the relevant subsections (Sections 3.2.1.1–3.2.1.2, 3.2.2–3.2.3, and 3.3.1–3.3.2). In each subsection, we summarize our assessment of CFD validation considerations and the interpretation of key outcomes at the end of the section. In addition, we provide an overarching guideline for CFD result validation and analysis in Section 2.3.

3.2.1.1 Drag Reduction and Flow Stabilization (Excerpt)

The CFD in these studies generally quantifies performance through direct comparisons against a reference, and, particularly when simultaneous thermo-hydraulic optimization is required, uses performance metrics with explicitly stated baselines, such as normalized indices of friction factor and Nusselt number, such that the setup of comparison targets is relatively clear. However, although Sasamori et al. (2017) have the advantage of comparing 3D sinusoidal riblets with optimal 2D riblets and wavy riblets including geometric parameters, caution is needed when extrapolating to the operating regime of heat exchangers because the drag-reduction effect can vary as Re increases. In the case of Martin et al. (2016), they present an optimal wall-unit range, but because it is sensitive to wall-unit matching normalized by friction velocity, a limitation remains in that, under conditions where friction velocity varies with operating conditions as in practical heat exchangers, the geometry must be redesigned in wall units. Wu et al. (2017) selected the RNG k–ε turbulence model in consideration of separation and recirculation, and ensured consistency to enable comparison between fish-scale and flat surfaces, but since the chosen model is not one that directly reproduces the microscale flow on actual surfaces, it is safer to interpret the reported reductions as indicative of feasibility. 

2.3. Performance Evaluation and Validation of Numerical Analysis

This review evaluates each study by focusing on the performance bottlenecks and operating windows beyond which the improvement diminishes. It emphasizes that, even for the same biomimetic geometry, superiority can be reversed depending on the selection of the baseline (comparative reference geometry), and it critically examines the risk of overinterpretation when improvement figures presented under different metrics and baselines are compared across studies.

In particular, it is necessary to exercise caution regarding this tendency in studies that conduct only numerical analyses. Some of the studies reviewed in this paper mention only numerical-analysis results without experimental outcomes. These results should be interpreted as evidence of trends and relative superiority under identical conditions, rather than as absolute performance conclusions. In practical applications, real-world factors such as clogging in compact structures, flow non-uniformity, and manufacturing deviations may reverse the performance ranking.

Therefore, to mitigate these issues, the validation frameworks were evaluated for some topics in which many studies are based on numerical analyses. Assuming grid independence and adequate near-wall resolution/modeling, it was investigated to what extent the numerical analyses reflected actual heat exchangers with rigor, and the reproducibility of numerical simulations was checked through benchmarking against smooth channels and baseline geometries, or correlations/legacy data. As a result, this paper distinguishes the reliable and vulnerable ranges of CFD results, guiding readers to interpret the literature based not on simple performance figures, but on design feasibility that includes fabrication and operation.

 

4) Reviewer’s comment: Tables 1 and 2 show a poor comparison of the data provided; nothing new has been addressed from their side. The authors should discuss the outcome of these references within the table. The purpose of the review paper is to critique the articles and make it easier for future scholars.

Author’s response: Thank you for this insightful comment. We agree with the reviewer that the previous versions of Tables 1 and 2 were largely limited to listing performance metrics, which provided insufficient cross-study comparison and critical interpretation. To address this limitation, we revised both tables to include within-table discussion of the key outcomes and constraints reported in the cited studies.

In the revised Table 1, we explicitly specify the baseline (i.e., the non-biomimetic reference case) for each study and summarize, within the table, the applicable operating/experimental conditions and the main limitations as trade-offs or boundary conditions. In Table 2, we moved beyond a “maximum PEC”–centered summary by presenting, in parallel, the heat-transfer gains relative to the baseline together with the corresponding hydraulic costs/benefits. We also added a validity/trade-off column to discuss baseline dependence, sensitivity, and restrictions in applicability and operating range. As a result, the two tables now serve not merely as descriptive summaries but as critical comparative matrices, enabling future researchers to quickly assess generalizability and make informed design choices based on the reported evidence.

 

5) Reviewer’s comment: In the conclusion section, the authors cover the limitations and the future work, which is good. However, I would suggest placing the limitations and future work in a new section before the conclusion and enhancing the conclusion of this review.

Author’s response: Thank you for this helpful suggestion. In response, we reorganized the original Conclusion section into two separate sections to improve clarity and strengthen the overall message. Specifically, we created a new Section 4 (“Limitations and Future Work”) to explicitly discuss the limitations of the reviewed studies and to propose directions for addressing these limitations in future research. We then revised Section 5 (“Conclusion”) to focus more directly on summarizing the key takeaways and contributions of this review.

4. Limitations and Future Perspectives

4.1. Current Limitation and practical challenges

This review has certain limitations, which stem not only from the constraints of the individual studies reviewed but also from methodological boundaries inherent to the review process. First, many studies have been conducted using small-scale specimens and under single operating conditions. While these studies demonstrate the feasibility of biomimetic applications in heat exchangers, they lack verification of long-term durability and performance under real operating environments. For example, in the surface scale, key issues include wear resistance, chemical stability, lubricant replenishment, and practical performance under environmental conditions. In the network scale, challenges remain regarding structural reliability and stability of optimization under operational stresses. In the texture scale, the high parameter sensitivity of geometric variables makes it difficult to achieve both drag reduction and heat transfer enhancement simultaneously, thereby limiting the generality and applicability of the results.

Second, most studies have been conducted with specific industrial conditions or application purposes in mind. As a result, their findings often exhibit limited generality, and because the experiments were not performed under identical boundary conditions, it remains difficult to directly compare performance outcomes across different studies. In particular, for the studies reviewed under the network scale, the optimization objectives and heat exchanger configurations varied significantly, making it impossible to directly compare their performance results.

Taken together, the limitations identified in this review are not attributable solely to the experimental scale or validation scope of individual studies, but are also closely related to the way performance itself is defined and compared. Although studies conducted under different operating conditions and objective functions each present meaningful results, constraints remain in generalizing them under a common standard or comparing them directly. This suggests that, in future research on biomimetic heat exchangers, there is a need not only for refined designs and validation under real-world conditions, but also for complementary discussions on the perspective itself through which heat-exchange performance is interpreted and evaluated.

4.2. Complementary Evaluation Frameworks Beyond Conventional Thermo-Hydraulic Metrics

To date, research on biomimetic heat exchangers has largely advanced around the trade-off between improved heat-transfer performance and increased pressure drop, and heat–fluid-based performance metrics such as PEC have provided an effective criterion for comparing different design concepts. However, because these are transfer-performance-centered metrics defined under specific operating conditions, it may be useful to complement them with a system-level metric such as energy production. Accordingly, drawing on evaluation frameworks from other fields may offer additional insights that complement existing metrics and help guide future design.

In this regard, a series of studies on mammalian nasal cavities interpreted heat and moisture exchange using entropy production as a system-level metric, thereby analyzing exchange efficiency from the perspective of irreversible losses. For example, Magnanelli et al. (2016) constructed a dynamic model that reflects the geometric morphology of the reindeer nasal cavity and quantified geometric effects by comparison with a simple cylindrical structure having the same total volume and contact area. This study suggests that the complex nasal-cavity structure of mammals goes beyond merely increasing surface area, and can distribute entropy production more uniformly.

Studies on marine mammals likewise provide a similar perspective. Cheon et al. (2023) extracted the cross-sectional area of the flow path and the perimeter of the air–tissue interface from CT-based nasal-cavity geometries of Erignathus barbatus and Monachus monachus, and applied them to a quasi-1D dynamic model based on mass and energy balances. The analysis showed that, under identical environmental conditions, entropy production decreases significantly as the perimeter of the maxilloturbinate increases, and they discussed that, in terms of energy dissipation, the Arctic species is relatively advantageous due to geometric differences. Subsequently, Flekkøy et al. (2023) demonstrated, through a thermo-hydrodynamic model, that ice formation must be considered to reproduce expiratory temperatures in low-temperature environments. Furthermore, Cheon et al. (2024) evaluated entropy production under multiple environmental temperatures, relative humidities, and physiological conditions using a CFD model for the maxilloturbinate structure of seals.

Although these studies are not cases directly implemented as engineering heat exchangers, they are meaningful as conceptual reference cases showing that minimizing irreversible losses can be considered a design objective in the process of evaluating heat-exchange performance. This evaluation framework, rather than replacing existing heat–fluid performance metrics, provides applicability as a complementary perspective for future biomimetic heat-exchanger design and analysis.

The biological systems mentioned in this section are not presented as direct design templates for heat exchangers, but are introduced as conceptual reference cases to present an evaluation framework and interpretive perspective that can supplement existing heat–fluid performance metrics.

Taken together, future research should prioritize the scaling-up of biomimetic designs and the experimental validation of real-world performance to effectively enhance heat exchanger efficiency through nature-inspired strategies. In particular, further investigation is needed in areas such as operating condition optimization, techno-economic feasibility in fabrication and maintenance, material compatibility and long-term durability, and the scalability of biomimetic designs for use in general-purpose industrial heat exchangers. In this process, supplementing existing performance-evaluation metrics using indicators such as entropy production is expected to help refine comparisons among design alternatives and the specification of design targets.

5. Conclusions

As demonstrated throughout this review, biomimetic design has emerged as a promising and innovative approach for enhancing the performance and reliability of modern heat exchangers. These systems inherently face challenges such as frost formation, fouling, and pressure drop, yet bioinspired technologies provide effective solutions across multiple scales, offering tailored strategies to mitigate these fundamental limitations.

For instance, applying superhydrophobic properties inspired by lotus leaves or slippery surface characteristics modeled after Nepenthes pitcher plants to heat exchanger surfaces can enable self-cleaning functionality, effectively preventing fouling. In particular, for low-temperature heat exchangers, such surface modifications can suppress frost formation, mitigating potential efficiency losses. Similarly, by replicating the scale textures of fish or the micro-patterns of various plant and animal surfaces, the frictional resistance between the fluid and the wall can be minimized, thereby reducing pressure drop without compromising heat transfer performance—an improvement that can be quantitatively evaluated using the PEC.

At the macroscopic level, depending on industrial requirements, overall heat exchanger geometry and flow channel arrangements can also be optimized to improve thermal performance. Such designs draw inspiration from branching vein structures in leaves and trees, TPMS, and the gas exchange mechanism of the human lung. As illustrated by these recent studies, biomimetic technologies hold significant promise for achieving advanced performance enhancement and energy efficiency optimization in next-generation heat exchangers.

Recent research trends clearly demonstrate the broad applicability and potential of bioinspired technologies across multiple scales for improving the energy efficiency of heat exchangers. Once commercial-scale demonstrations generate sufficient operational data, these technologies are expected to drive transformative advancements in thermal energy management within modern industry.

6) Reviewer’s comment: The language may need improvement.

Author’s response: Thank you for your comment. We have carefully revised the manuscript to improve readability and overall language quality. In particular, we streamlined overly complex sentence structures, reduced unnecessary length, and replaced awkward or non-standard terminology with clearer academic expressions to ensure that our key messages are communicated more clearly throughout the manuscript.

Author Response File: Author Response.pdf

Round 2

Reviewer 2 Report

Comments and Suggestions for Authors

The article has been revised, while the critical review is still not strong enough.

Also, the authors mentioned that the CFD analysis has been addressed in the article, but I found only the model without any CFD analysis. The authors should show the CFD outcome and explain the results in details. Also, the authors should provide proper comparison with exsiting work to show the improvement of this work.

A review paper usually requires more references, 50 references to cover the topic still poor for a review paper. I would suggest adding more and try to reach at least 80 references.

More critical review similar to Tables 1 and 2 is required.

Comments on the Quality of English Language

The language may need improvement.

Author Response

Responses to the Associate Editor’s and Reviewers’ Comments (2nd Round)

 

09 January 2026

 

Dear editor and reviewer,

We sincerely appreciate the thorough consideration and review of our manuscript, “Bioinspired Heat Exchangers: A Multi-Scale Review of Thermo-Hydraulic Performance Enhancement”. The reviewer’s additional comments in this second round helped us to better understand and address the key issues in the paper. We have carefully addressed the remaining concerns and revised the manuscript accordingly. We appreciate the reviewer’s and editor’s continued efforts, which have helped improve the manuscript. All changes in the revised manuscript are underlined and highlighted in blue. A point-by-point response to the reviewer’s comments is provided below.

 

1) Reviewer’s comment: The article has been revised, while the critical review is still not strong enough.

Author’s response: Thank you for the constructive comment. We agree that the manuscript previously lacked sufficient critical synthesis. To strengthen the critical review, we have added an explicit, design-oriented perspective that translates bio-inspired concepts into actionable guidance for practical heat-exchanger design. Specifically, for each scale, we identified representative and promising approaches and critically summarized their key characteristics, applicability, and limitations in consolidated tables. These revisions can be found in Sections 3.1.3, 3.2.4, and 3.3.4.

 

2) Reviewer’s comment: Also, the authors mentioned that the CFD analysis has been addressed in the article, but I found only the model without any CFD analysis. The authors should show the CFD outcome and explain the results in details. Also, the authors should provide proper comparison with existing work to show the improvement of this work.

Author’s response: Thank you for the insightful comment. We acknowledge that our previous wording could have led readers to interpret that we performed CFD in this study. To avoid this misunderstanding, we have revised the manuscript to clearly state that we did not conduct new CFD simulations, and that CFD-related content is discussed only as part of the literature review.

Regarding the reviewer’s request to show CFD outcomes and explain the results, we have strengthened the review sections by explicitly summarizing and interpreting the CFD results reported in the key studies covered in our manuscript (e.g., representative flow/thermal fields and the associated performance implications). At the same time, we clarify that direct quantitative comparison across CFD results is inherently limited because the simulation setups vary substantially among studies.

In addition, to address the request for comparison with existing work, we have improved the cross-study comparison by extracting consistent performance metrics and reporting conditions wherever possible, and by organizing the prior work into a structured comparison that highlights what each approach improves (and under which conditions) as well as the remaining limitations. These revisions are incorporated in the revised manuscript.

2.3 Performance Evaluation and Validation of Numerical Analysis (excerpt)

Therefore, to mitigate these issues, the validation frameworks were summarized for some topics in which many studies are based on numerical analyses. Based on the validation information reported in each study, this review assessed whether the numerical analyses represent practical heat-exchanger conditions. Accordingly, this paper classifies computational fluid dynamics (CFD)-based claims reported in individual studies as more and less robust, guiding readers to interpret the literature not merely by performance figures, but by design feasibility considering fabrication and operation.

 

3) Reviewer’s comment: A review paper usually requires more references, 50 references to cover the topic still poor for a review paper. I would suggest adding more and try to reach at least 80 references.

Author’s response: Thank you for the suggestion. We agree that adequate coverage is essential for a review paper; however, we also believe that the number of references should be driven by scope and relevance rather than by a target count. In the revision, we conducted an additional literature screening and incorporated 18 further studies that are directly aligned with the specific scope of this review, increasing the total number of references from 50 to 68. We deliberately excluded tangential or redundant citations to maintain a focused and critical synthesis within our three-scale framework. We therefore consider the current reference set to provide sufficient and representative coverage of the topic as defined in this manuscript.

3.1.1 Lotus Leaves (excerpt)

Xing et al. (2024) sought to mitigate the frost growth issue on the outdoor heat exchanger of an ASHP by following the trajectory of nature-inspired superhydrophobic surface research. They fabricated a superhydrophobic surface with a contact angle of 156.7°, and comparatively evaluated, through visualization experiments, the frost growth and melting characteristics on finned tubes. As a result, they reported performance gains, with the frost build-up reduced by about 18.9% and the defrosting efficiency enhanced by 21.2%. Furthermore, they analyzed fin pitch, fin surface temperature, and wind speed as parameters affecting frosting characteristics, and organized the data to identify under which operating and geometric conditions the effect becomes larger. However, the evidence presented in this paper is focused on frost growth/defrosting performance and an early-stage mechanistic model. From a practical application perspective, follow-up validation is still needed to link the concurrent effects of fouling, reproducibility under fluctuations in humidity, wind speed, and temperature, and the long-term durability of the superhydrophobic surface.

Lotus leaf–inspired superhydrophobic surface modification applied to heat exchangers can address the problem of increased thermal resistance caused by frost, fouling, and corrosion. However, prior superhydrophobic studies have remained focused on materials and surface-treatment mechanisms, leaving the utility and durability unclear when applied to actual heat-exchanger fins. To resolve this issue, Li et al. (2022) conducted process optimization and durability validation. As a result, they secured superhydrophobicity with a contact angle of 166.9°, and applied this process to a full-size fin-and-tube heat exchanger to demonstrate enhanced hydrophobicity through droplet behavior. In this way, they established a package of a process applicable to heat-exchanger components, wettability, and durability. Further evaluation is needed on how much this package improves actual heat-exchange performance.

Accordingly, lotus-leaf-inspired surface mimicry suppresses droplet spreading and film formation through microscale surface structures, and consequently promotes roll-off and dropwise condensation, reducing liquid-film thermal resistance and frost accumulation. Although the contact angle is predominantly used for performance comparisons, dynamic metrics that critically affect real operation, such as sliding angle, contact angle hysteresis, and the droplet departure period and diameter, were insufficiently reported. The aforementioned studies commonly point out limitations that the structures are vulnerable to wear and contamination, and that the experimental conditions are static. From a practical design perspective, lotus-leaf-inspired surface mimicry is effective under conditions where suppressing initial icing translates into efficiency, such as ASHP fins with a high defrost-cycle burden. However, under conditions with substantial outdoor contamination or wear, and under conditions dominated by thermal-load fluctuations, the simultaneous optimization of durable fabrication processes, thermal resistance, ice resistance, and cleanability is essential.

 

4) Reviewer’s comment: More critical review similar to Tables 1 and 2 is required.

Author’s response: Thank you for the constructive comment. We agree that the critical review needed to be strengthened in a format comparable to Tables 1 and 2. To address this, we expanded the table-based synthesis for each scale to provide a more compact yet critical, design-oriented summary. Specifically, the added/updated tables consolidate key design-relevant aspects, thereby offering actionable guidance rather than a purely descriptive listing. These revisions can be found in Sections 3.1.3, 3.2.4, and 3.3.4.

 

5) Reviewer’s comment: The language may need improvement

Author’s response: Thank you for the comment. We have carefully revised the English throughout the manuscript to improve clarity, readability, and scientific precision. Specifically, we corrected grammatical issues, refined wording for conciseness, and improved consistency in terminology and style, so that the intended meaning is communicated more clearly and rigorously.

 

 

General changes:

To ensure copyright compliance, we replaced the previous figure(s) with author-drawn framework schematics and revised the corresponding captions and in-text descriptions accordingly. Figure 4a image was generated using a generative AI tool. In these cases, the authors first developed the complete conceptual framework and layout based on a thorough understanding of the content, and the generative AI tool was used only for graphic rendering and post-processing. The scientific content, structure, and interpretation presented in the figures were determined entirely by the authors.

Figures 2(g–j) were removed, and the corresponding text was revised to ensure that the deletion does not affect the consistency or clarity of the manuscript.

The abbreviations were reorganized and clarified to improve readability and facilitate easier understanding for readers.

We disclose the limited use of a generative AI tool for creating an illustrative figure in the Methods and Acknowledgments, in accordance with MDPI’s policy.

Author Response File: Author Response.pdf

Round 3

Reviewer 2 Report

Comments and Suggestions for Authors

Good improvement. No further comments.