Heat Transfer Analysis in a Channel Mounted with In-Line Downward-Facing and Staggered Downward-Facing Notched Baffles

Round 1

Reviewer 1 Report

Comments and Suggestions for Authors

This manuscript provides a valuable experimental analysis of heat transfer and flow resistance in rectangular channels with different baffle configurations. The comparison among transverse, in-line notched, and staggered notched baffles, and the parametric study of pitch-to-baffle height ratio, are particularly meaningful for the heat transfer enhancement community. The results are well organized and clearly presented. However, several technical aspects could be strengthened or clarified to improve the scientific rigor and usefulness of the study. 
1.The manuscript would benefit from a detailed discussion of the experimental uncertainties and potential sources of error in key measured variables (e.g., Nusselt number, friction factor, Reynolds number). Please specify how uncertainties were estimated and their typical magnitudes. Discuss how measurement errors might affect the main conclusions.
2.The channel walls are stated to be insulated, but no quantitative assessment of possible heat losses is provided. Please clarify whether any heat losses (radial or axial) were quantified and how they were accounted for in the calculation of the heat transfer coefficient. This is important for the reliability of the Nusselt number values, especially at high Reynolds numbers.
3.While the manuscript presents friction factor results, a more in-depth discussion of the practical implications of the increased pressure drop—such as the impact on required pumping power and system efficiency—would be useful. 
4.The results show that the P/e = 6.0 ratio yields optimal heat transfer for all configurations, but it is not clear whether this result is expected to be universal or if it might depend on the specific channel geometry and operating conditions. Discuss the sensitivity of this optimum to channel size, fluid properties, or baffle dimensions, and provide any guidance for practical design.
5.The manuscript states that dead zones are reduced in the notched and staggered configurations, but this claim is primarily qualitative (based on flow patterns and color maps). If possible, please provide a quantitative analysis (e.g., area or volume fraction of recirculation zones, or temperature uniformity metrics) to support this statement.

 

Author Response

The authors would like to thank the editors and reviewers for their thorough review and constructive comments. The specific changes and clarifications requested are outlined below.

 

Manuscript Number: eng-3768518

Heat Transfer Analysis in a Channel Mounted with In-Line Downward-Facing and Staggered Downward-Facing Notched-Baffles

 

Comments and Suggestions for Authors

Reviewer #1:

This manuscript provides a valuable experimental analysis of heat transfer and flow resistance in rectangular channels with different baffle configurations. The comparison among transverse, in-line notched, and staggered notched baffles, and the parametric study of pitch-to-baffle height ratio, are particularly meaningful for the heat transfer enhancement community. The results are well organized and clearly presented. However, several technical aspects could be strengthened or clarified to improve the scientific rigor and usefulness of the study.

 

1. The manuscript would benefit from a detailed discussion of the experimental uncertainties and potential sources of error in key measured variables (e.g., Nusselt number, friction factor, Reynolds number). Please specify how uncertainties were estimated and their typical magnitudes. Discuss how measurement errors might affect the main conclusions.

Response: We sincerely thank the reviewer for this valuable comment. The uncertainties of key measured and derived variables are now described in Section 3 “Experimental Facility Setup”. The evaluation was performed using the root-sum-square (RSS) method, based on instrument specifications and repeated measurements, following the methodology developed by Kline and McClintock [25]. Typical uncertainties of the primary measurements were ±0.1 °C for temperature, ±1.0% psi for pressure drop, and ±2.5% L/s for flow rate, resulting in propagated errors of approximately ±3.54% for Re, ±3.10% for Nu, and ±5.34% for f. The main sources of error included thermocouple/TLC calibration drift, flow-rate fluctuations, pressure sensor resolution, and heat losses to the surroundings.

 

These uncertainty values are below ±6%, which is generally regarded as reliable for experimental thermofluid research, and are smaller than the observed differences between configurations (>3.10% for Nu and >5.34% for f). This confirms that the reported trends and conclusions remain valid despite measurement variability. The reported values are also consistent with those from similar experimental studies, reinforcing the credibility and robustness of the results.

 

Additional references:

[25] S.J. Kline, F.A. McClintock, Describing uncertainties in single sample experi-ments, Mech. Eng. 75 (1953) 3-8.

 

2. The channel walls are stated to be insulated, but no quantitative assessment of possible heat losses is provided. Please clarify whether any heat losses (radial or axial) were quantified and how they were accounted for in the calculation of the heat transfer coefficient. This is important for the reliability of the Nusselt number values, especially at high Reynolds numbers.

Response: The issue is described in section 2 (Theoretical Aspects)

 

“In the experimental setup, the channel walls were wrapped with multi‑layer insulation to minimize radial heat losses. A quantitative heat‑loss assessment was conducted, which showed that heat losses were within 5% of the total heat input across the tested temperature range. These small losses were accounted for by subtracting them from the net heat input in the calculation of the heat transfer coefficient. Given that the losses are well below the observed variations in Nusselt number, the reported values remain reliable, even at high Reynolds numbers.”

 

3. While the manuscript presents friction factor results, a more in-depth discussion of the practical implications of the increased pressure drop—such as the impact on required pumping power and system efficiency—would be useful.

Response: We appreciate the reviewer’s suggestion. In the revised manuscript, we have added a discussion of the practical implications of the increased friction factor in Section 4.2.

 

“The higher friction factor directly leads to a higher pressure drop (ΔP), which in turn increases the required pumping power (P) according to P =ΔP×Q, where Q is the volumetric flow rate. While this results in a rise in energy consumption, the corresponding heat transfer enhancement outweighs the pumping penalty, yielding an overall improvement in system thermal efficiency. We have highlighted that in practical applications, the trade‑off between enhanced heat transfer and additional pumping power should be considered when selecting the optimized configuration.”

 

4. The results show that the P/e = 6.0 ratio yields optimal heat transfer for all configurations, but it is not clear whether this result is expected to be universal or if it might depend on the specific channel geometry and operating conditions. Discuss the sensitivity of this optimum to channel size, fluid properties, or baffle dimensions, and provide any guidance for practical design.

Response: The discussion is given in Section 4.4.  In this study, the ratio P/e = 6.0 consistently provided the highest thermal performance among the tested configurations; however, this optimum is not expected to be universal. The optimal spacing can vary with channel size and aspect ratio, as smaller hydraulic diameters or high‑aspect‑ratio ducts alter recirculation patterns and vortex reattachment lengths. It may also depend on fluid properties and Reynolds number, since changes in viscosity or thermal conductivity affect the balance between heat transfer and pressure loss. Furthermore, baffle dimensions, including height, thickness, and tip angle, influence vortex strength and flow disruption, potentially shifting the preferred pitch‑to‑height ratio. For practical design, P/e ≈ 6.0 can serve as a robust starting point, but adjustments through CFD simulations or preliminary experiments are recommended for channels or operating conditions that deviate significantly from those examined in this study.

 

5. The manuscript states that dead zones are reduced in the notched and staggered configurations, but this claim is primarily qualitative (based on flow patterns and color maps). If possible, please provide a quantitative analysis (e.g., area or volume fraction of recirculation zones or temperature uniformity metrics) to support this statement.

Response: We appreciate the reviewer’s insightful observation. At present, the statement regarding reduced dead zones in the notched and staggered configurations is based on qualitative analysis from flow patterns and temperature contour maps. A detailed quantitative evaluation, such as calculating the recirculation zone area/volume fraction and temperature uniformity indices, has not yet been performed for this study. We acknowledge that such an analysis would strengthen the claim, and we plan to conduct this quantitative assessment in future work to provide a more rigorous confirmation.

 

We sincerely appreciate the reviewer’s comments and suggestions, which contributed significantly to strengthening the manuscript.

 

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Author Response File: Author Response.docx

Reviewer 2 Report

Comments and Suggestions for Authors

The literature review lacks reference to at least one work from the Eng journal that provides thematic continuity in the field of heat transfer and heat exchanger research.

The description of the test stand does not explain how it was possible to simultaneously insulate the channel, heat it and observe the color map in order to determine the temperature using the TLC method.

Section 3 does not provide the accuracy of the temperature, pressure and volume flow measurement sensors.

Has the thermochromic liquid crystal (TLC) measurement method been calibrated?

Was the influence of sensor errors on the final determined values analyzed?

In the description of the results for at least one case, it would be good to provide the measured values, including the determined wall temperature distribution, which is the basis for determining the local Nuselt number.

Author Response

As the equations cannot be displayed normally on the web page, please review the complete content in the attached file.

The authors would like to thank the editors and reviewers for their thorough review and constructive comments. The specific changes and clarifications requested are outlined below.

 

Manuscript Number: eng-3768518

Heat Transfer Analysis in a Channel Mounted with In-Line Downward-Facing and Staggered Downward-Facing Notched-Baffles

 

Comments and Suggestions for Authors

Reviewer #2:

1. The literature review lacks reference to at least one work from the Eng journal that provides thematic continuity in the field of heat transfer and heat exchanger research.

Response: In the revised manuscript, we have incorporated an additional reference from the Engineering journal to strengthen the thematic continuity in the area of heat transfer and heat exchanger research. This reference is now cited in the literature review section to better align our study with prior developments in the field.

“Complementing these strategies, Kouidri et al. [6] developed AI‑based models to estimate fouling resistance in shell‑and‑tube heat exchangers, providing a data‑driven approach to maintain thermal-hydraulic performance and link long‑term operation with enhancement techniques. This addition reinforces the thematic continuity in heat‑transfer and heat‑exchanger research, bridging energy‑efficiency efforts with practical system performance.

 

Additional references:

[6] Kouidri, I.; Dahmani, A.; Furizal, F.; Ma’arif, A.; Mostfa, A.A.; Amrane, A.; Mouni, L.; Sharkawy, A.-N. Artificial Intelligence‑Based Techniques for Fouling Resistance Estimation of Shell‑and‑Tube Heat Exchanger: Cascaded Forward and Recurrent Models. Eng 2025, 6(5), 85.”

 

 

 

 

 

 

2. The description of the test stand does not explain how it was possible to simultaneously insulate the channel, heat it and observe the color map in order to determine the temperature using the TLC method.

Response: We sincerely thank the reviewer for this comment. The description has now been clarified in Section 3 “Experimental Facility Setup”. In our experimental setup, the channel was wrapped with a multi-layer thermal insulation to minimize radial and axial heat losses, while leaving a dedicated observation window directly above the test section. This window allowed optical access for capturing the thermochromic liquid crystal (TLC) color patterns without compromising the overall insulation.

 

Heating was provided by a polyimide electric film heater installed on the bottom wall of the test section, delivering a uniform heat flux controlled by a variac transformer. The TLC sheet was applied to the heated surface inside the observation window, and its color changes were recorded using a high-resolution digital camera.

 

Prior to the experiments, the TLC sheet was calibrated under a constant wall temperature condition using a controlled heating setup. The calibration was performed under the same environmental and operational conditions as the actual experiments to ensure consistency in the hue–temperature relationship.

 

This configuration enabled simultaneous heating, insulation, and temperature mapping using the TLC method, while the prior calibration ensured accurate temperature measurements. The combined setup maintained both the accuracy and repeatability of the measurements.

 

3. Section 3 does not provide the accuracy of the temperature, pressure and volume flow measurement sensors.

Response: We thank the reviewer for this observation. The accuracy of the temperature, pressure, and volumetric flow measurement sensors has now been included in the revised manuscript.

 

4. Has the thermochromic liquid crystal (TLC) measurement method been calibrated?

Response: The TLC sheet used in this study was calibrated prior to experiments using a controlled water‑bath setup, following standard procedures to establish the relationship between hue value and surface temperature. The calibration confirmed that the measurement uncertainty remained within ±0.2 °C in the working range.

 

5. Was the influence of sensor errors on the final determined values analyzed?

Response: We sincerely thank the reviewer for this valuable comment. The uncertainties of key measured and derived variables are now described in Section 3 “Experimental Facility Setup”. The evaluation was performed using the root-sum-square (RSS) method, based on instrument specifications and repeated measurements, following the methodology developed by Kline and McClintock [25]. Typical uncertainties of the primary measurements were ±0.1 °C for temperature, ±1.0% psi for pressure drop, and ±2.5% L/s for flow rate, resulting in propagated errors of approximately ±3.54% for Re, ±3.10% for Nu, and ±5.34% for f. The main sources of error included thermocouple/TLC calibration drift, flow-rate fluctuations, pressure sensor resolution, and heat losses to the surroundings.

 

These uncertainty values are below ±6%, which is generally regarded as reliable for experimental thermofluid research, and are smaller than the observed differences between configurations (>3.10% for Nu and >5.34% for f). This confirms that the reported trends and conclusions remain valid despite measurement variability. The reported values are also consistent with those from similar experimental studies, reinforcing the credibility and robustness of the results.

 

Additional references:

[25] S.J. Kline, F.A. McClintock, Describing uncertainties in single sample experi-ments, Mech. Eng. 75 (1953) 3-8.

 

6. In the description of the results for at least one case, it would be good to provide the measured values, including the determined wall temperature distribution, which is the basis for determining the local Nuselt number.

Response: We sincerely thank the reviewer for the insightful comment. The revised manuscript now provides a clearer description of the methodology for determining the local Nusselt number (Nu_x) in the Results section. Specifically, the following clarifications have been added:

• The governing equation for calculating Nu_x.
• The relevant parameters, including the local surface heat flux (q_x), local wall temperature (T_w,x), local bulk air temperature (T_b,x), and the thermal conductivity of air (k).
• The specific axial (x) and transverse (y) positions along the absorber plate where Nu_x was evaluated.

For the notched-baffles configuration, the local convective heat transfer coefficient (h_x) was calculated using:

(7)

 

Direct measurement of T_b,x at every axial position was not feasible. Therefore, it was assumed that the variation of bulk air temperature along the test section was linear, which is reasonable under the conditions of (1) constant wall heat flux, (2) a specified Reynolds number range, and (3) uniform airflow without significant heat storage in the system. Under this assumption, T_b,x can be estimated as:

(9)

 

This approach allows for an accurate yet practical determination of Nu_x, providing a reliable spatial characterization of the heat transfer performance.

 

In the revised manuscript, the methodology and measured wall temperature distributions for a representative case have been added in Section 2 “Theoretical Aspects”, as the basis for determining the local Nusselt number. This section details the step-by-step procedure for deriving the local heat transfer coefficient from the measured temperature data and subsequently calculating the local Nusselt number, providing direct experimental evidence and ensuring transparency in the reported heat transfer analysis.

 

We sincerely appreciate the reviewer’s comments and suggestions, which contributed significantly to strengthening the manuscript.

 

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Author Response File: Author Response.docx

Round 2

Reviewer 1 Report

Comments and Suggestions for Authors

The revised manuscript has undergone a thorough evaluation and is found to have addressed all previously raised comments in full. Methodological rigor has been enhanced, the presentation is clear and well-structured, and the work now conforms to the journal’s formatting, ethical, and reporting guidelines. 
Accordingly, the manuscript now meets publication standards of the journal and is suitable for acceptance.

Reviewer 2 Report

Comments and Suggestions for Authors Most of my comments have been incorporated into the revised version of the paper. I think the article is ready for publication.