Iron oxides have been investigated by employing mostly Fourier-transform infrared spectroscopy (FTIR). However, absorption spectra sometimes are difficult to obtain in situ, where accessibility to the location is limited. In this paper, we propose using emission spectra to investigate the purity of hematite (α-Fe₂O₃) and magnetite (Fe₃O₄). We used an infrared beam (wavelength of 1064 nm) from a pulsed laser to obtain emission spectra of the α-Fe₂O₃ and Fe₃O₄ for two different purities (99.99% and 96%) ranging from visible to near-infrared regions. The average values of full width at half maximum (FWHM) for the 99.99% purity were found to be 0.15 nm and 0.17 nm for the hematite and the magnetite, respectively; the average values of FWHM for the 96% purity were found to be 0.19 nm and 0.23 nm for the hematite and the magnetite, respectively. In addition, it is found that lower purity iron oxides exhibited a higher amplitude in the broader emission spectrum when compared to the iron oxides with high purity under the same experimental conditions. Therefore, by observing the amplitude of the broader emission spectra, it is possible to differentiate qualitatively between hematite and magnetite purities; moreover, from the average value of the FWHM of the spectral lines, it is possible to evaluate percentage purity content of hematite and magnetite, which are useful for different mechanical and biomedical applications. FTIR measurements also confirmed that the purity of the oxides can be found by measuring the peaks’ shifts. However, the proposed technique can be used in place of FTIR, where accessibility to the measurement’s location becomes problematic.

All-aluminum microchannel heat exchangers have recently gained popularity in the heating, ventilation, and air conditioning industry. Despite their attractive thermal performance design, these heat exchangers make coils used in automotive, commercial, and residential applications prone to crevice corrosion. This study uses high-fidelity conjugate heat transfer simulations to model a micro channel heat exchanger system that includes fins and tubes with crossflow to compare their thermal effectiveness to gain insight into potential crevice corrosion of the MCHE alloy. This study considers three fin geometries (louver, step, and saw) with the same tube and circular shape microchannel and identifies the corrosion hot spot and thermal effectiveness. A predicted flow field also identifies crevices between fins and tube surfaces as critical corrosion hot spots often associated with low-velocity regions. The crevice volumes for the louver, step, and saw fin shapes are calculated as respectively. Results also show that the same circular microchannel louver shape fin has higher effectiveness than the step and saw shape fin. The thermal effectiveness for microchannel tubes with louver, step, and saw shape fins are 0.337, 0.20737, and 0.2895, respectively.

In this study, high-fidelity conjugate heat transfer simulations are used to model a micro-channel heat exchanger (MCHE) in a crossflow to study its thermal-hydraulic performance. Three different microchannel geometries, namely circular, triangular, and square with louver-shaped fins, are considered. The local flow field showed a strong coupling between the microchannel flow, solid domain, and crossflow. The flow separation and wake regions formed near MCHE resulted in a large variation in the velocity field and temperature in the crossflow. The interaction of the wake region with the fins introduces significant spanwise variations, leading to variations in the thermal boundary layer. The heat conduction in the solid structure provided a non-uniform temperature field with a higher temperature near the microchannel and a slightly lower temperature near the surface exposed to the crossflow. The microchannel flow analysis showed that the internal geometry affects the pressure drop, which is highest for the triangular MCHE and lowest for the circular MCHE. Based on the effectiveness and performance index calculations, the circular MCHE exhibited the best performance, while the rectangular MCHE showed higher heat transfer effectiveness.

All aluminum micro-channel heat exchanger (MCHE) is used in automobiles and HVAC because of their low density and relatively high strength. While exhibiting significant general corrosion resistance, the MCHE or aluminum alloy is susceptible to various forms of localized corrosion, such as crevice and pitting corrosion. Crevice corrosion also occurs on the contact surface of the fin and tube for an all-aluminum microchannel heat exchanger. An electrochemical impedance spectroscopy (EIS) examination was performed on aluminum MCHE in corrosive conditions with varying sodium chloride and ammonium sulfate concentrations. The electrolytes discussed in this book chapter have pH values between 4.0 and 5.8, closer to neutral conditions commonly seen in various atmospheres. This work employs high-fidelity conjugate heat transfer simulations to analyze a microchannel heat exchanger system with fins and tubes in crossflow to investigate the possibility of crevice corrosion in the MCHE alloy. A predicted flow field can identify crevices between fins and tube surfaces. These crevices are typically linked with regions of low velocity.

In this study, high-fidelity conjugate heat transfer simulations are used to model a micro-channel heat exchanger (MCHE) in a crossflow to study its thermal-hydraulic performance. This study considers three different microchannels (internal flow) geometries (circular, triangular, and square) with louver-shaped fins. The local flow field showed a strong coupling between the microchannel flow, solid domain, and crossflow. The flow separation and wake regions formed near MCHE resulted in a large variation in the velocity field and temperature in the crossflow. The wake region had a significant spanwise variation due to its interaction with fins, which also causes variations in the thermal boundary layer. The heat conduction in the solid structure provided a non-uniform temperature field with a higher temperature near the microchannel and a slightly lower temperature near the surface exposed to the crossflow. The microchannel flow analysis showed that the internal geometry affects the pressure drop, which is highest for the triangular MCHE and lowest for the circular MCHE. However, the microchannel flow temperature change was relatively similar for all microchannels. Results showed that for the same volume of the microchannel, the circular shape microchannel has a higher performance index value than the triangular and square shapes. This study also considers three different fin (external flow/crossflow) geometries (louver, step, and saw) with the same tube and circular shape microchannel and identifies the corrosion hot spot. Crossflow shows higher temperatures near the boundary layer of the tube, which results in higher corrosion rates. A predicted flow field also identifies crevices between fins and tube surfaces as critical corrosion hot spots often associated with low-velocity regions. Electrochemical impedance spectroscopy (EIS) analysis was done on AA3102 (Alloy used in the circular channel and louver fin) alloy in corrosive environments containing low and high concentrations of the combination of sodium chloride and ammonium sulfate. Electrolytes used in this research have pH values ranging from 4.0 to 5.8, closer to nearly neutral environments encountered in many atmospheres. EIS results are presented, including Rsol, Rpore, and Rct of AA3102 with very thin arc evaporated porous Zinc film on AA 3102 along with their equivalent circuit.