Study of the effects of external corrosion on the thermal-hydraulic performance of brazed aluminum microchannel heat exchangers

Wang, Yupeng

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https://hdl.handle.net/2142/127496 Copy

Description

Title

Study of the effects of external corrosion on the thermal-hydraulic performance of brazed aluminum microchannel heat exchangers

Author(s)

Wang, Yupeng

Issue Date

2024-12-05

Director of Research (if dissertation) or Advisor (if thesis)

Zhao, Hui

Doctoral Committee Chair(s)

Miljkovic, Nenad

Committee Member(s)

• Wang, Xiaofei
• Zhang, Yuanhui

Department of Study

Mechanical Sci & Engineering

Discipline

Mechanical Engineering

Degree Granting Institution

University of Illinois at Urbana-Champaign

Degree Name

Ph.D.

Degree Level

Dissertation

Keyword(s)

• Microchannel heat exchanger
• Corrosion
• Thermal-hydraulic performance

Abstract

Brazed aluminum microchannel heat exchangers (MCHEs) are widely used in heating, ventilation, and air-conditioning (HVAC) systems owing to their lightweight, relatively low fabrication cost, compact design, and high thermal performance. External corrosion of brazed microchannel heat exchangers (MCHEs) can cause serious reduction of both mechanical integrity and thermal-hydraulic performance, especially when exposed to harsh working environments. Significant R&D effort has been devoted to improving the corrosion resistance of the HXs, especially to prevent the microchannel tube from fast perforation and reduce the risk of refrigerant leak. However, the corrosion-related change in thermal-hydraulic performance lacks in-depth explorations. A good understanding of the effect of external corrosion on the performance of the MCHEs can provide effective mitigation strategies and significantly save the cost of system troubleshooting and repair. This study explores the correlation between the thermal-hydraulic performance of brazed aluminum MCHEs and their external corrosion behaviors. The initial stage of this study develops a reliable experimental procedure that prepares corroded brazed aluminum HXs for performance evaluation using a laboratory accelerated corrosion test, ASTM G85-11 Standard Practice for Modified Salt Spray (Fog) Testing Annex 3 (SWAAT). To evaluate an effective post-SWAAT cleaning method, a series of AA3003 coupons are exposed to SWAAT. The effects of the cleaning agent (nitric acid solution with different concentrations), cleaning frequency, and the duration of the cleaning procedure are evaluated. Immersion in 5wt.% nitric acid solution is found to be effective in salt and corrosion product removal and does not cause aggressive aluminum substrate loss. To develop a suitable experimental procedure for HXs with complex geometries, two brazed aluminum HXs are exposed to SWAAT for 10 days. One HX is cleaned periodically during SWAAT, and the other is cleaned only once after SWAAT. Thermal-hydraulic performances of the HXs are measured before and after SWAAT, with and without cleaning. It is found that the salt and corrosion product deposition on the HX surface significantly increases its airside pressure drop. The increase is measured to be as much as 112% after ten days of exposure to SWAAT without cleaning. An almost linear relation between the increase in pressure drop and the increase in HX mass, as the result of deposit accumulation, is obtained. The UA of both HXs, with or without cleaning, decreased after the SWAAT tests, but by less than 18%. The decrease in UA of the cleaned HX is almost linearly correlated with the mass loss of the HX. The periodically cleaned HX shows larger mass loss and a relatively larger reduction in UA than the HX cleaned only once after SWAAT. From this comprehensive experimental study, a feasible procedure is developed to evaluate the performance of corroded aluminum HXs after exposure to SWAAT without the interference of the salt deposition caused by the laboratory accelerated corrosion test. The established test procedure is applied to the experimental study of three MCHEs that are identical except for different anti-corrosion coatings. These MCHEs are exposed to SWAAT for ten days, followed by the cleaning procedures developed in the previous stage of the study. The thermal-hydraulic performance of each MCHE is evaluated before and after SWAAT. After the corrosion test, the airside pressure drop of the bare HX increased by up to 13% while its airside thermal resistance increased by around 350%. The airside pressure drop of the TCP-coated HX does not change significantly after the corrosion test. However, around 200% increase in airside thermal resistance is found after the corrosion test. The e-coated HX shows insignificant changes in thermal-hydraulic performance after the ten-day SWAAT exposure. The corrosion behavior of the MCHEs is studied with visual inspection. Metallurgical analysis of samples taken from the bare HX is performed. Severe corrosion of fin-tube brazed joints is observed on both bare and TCP-coated HXs. The metallurgical analysis of the bare HX samples shows that 0% to 16% of the fin-tube brazed joints still exist on the examined cross-sections. Corrosion of fins on these HXs is generally not severe compared to the brazed joint. No obvious sign of corrosion is found on the post-SWAAT e-coated HX. The study of the three MCHEs shows a significant impact of the loss of fin-tube joints on the thermal performance degradation of MCHEs. A theoretical model is developed and experimentally validated to quantitatively correlate the fin-tube brazed joint loss and HX thermal performance degradation. The model predicts the airside thermal resistance of corroded HXs based on the reduction in fin efficiency caused by the loss of fin-tube joints. The model is validated with a novel experimental method. Three aluminum MCHEs are brazed with designated fin-tube brazed joint quantity, size, and distributions to mimic various scenarios of corrosion damage on the joints, which includes: (1) a baseline case with no joint corrosion; (2) loss of around 80% fin-tube joints; (3) and reduction in joint size. The experimental evaluation of the specially fabricated MCHEs shows that around 80% reduction in joint quantity contributes to around 100% increase in airside thermal resistance. The model’s prediction on the airside thermal resistance of this HX is up to 4.4% larger than the experimental value. It shows a good agreement between model prediction and the experiment evaluation. The experiment also demonstrates that an approximately 40% reduction in joint fillet width does not significantly affect thermal performance. Using the validated model, a range of possible joint quantities can be predicted by also considering the effect of the distribution of remaining fin-tube brazed joints. A further examination of the model is performed using three HX brazed with different material combinations. After exposure to SWAAT for 6 days, the material difference leads to different corrosion morphology of the external heat transfer components, including louvered fins and fin/tube joints. The airside thermal resistance of the HXs increases, corresponding to the severity of visually observed fin-tube joint corrosion conditions. The model predicts that the HX with up to 45% increase in airside thermal resistance has a joint loss rate of 30% to 54%. However, the metallurgical examination on a corroded dummy sample that replicates the material of this HX suggests that the fin-tube joint corrosion may not be as severe. The other two HXs, whose airside thermal resistance increases by more than 400% and 600%, are predicted to have a joint loss rate of over 84% and over 95%, respectively. Metallurgical examination of the representative corroded dummy samples evidences the loss of joint and severe fin-tube joint corrosion. Some fin damages due to corrosion are observed from the metallurgical analysis of all samples. Since the model assumes that all performance degradation is caused by the loss of fin-tube joints, the application of the model on these HXs will not capture the effect of the fin corrosion on the performance and may be over-predicting the percentage of the lost fin-tube joint. The experimental results with these HXs provide more data for future model improvement.

Graduation Semester

2024-12

Type of Resource

Thesis

Handle URL

https://hdl.handle.net/2142/127496

Copyright and License Information

Copyright 2024 Yupeng Wang

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