To investigate the response of material to cavitation erosion, a comparative work was carried out on three materials, aluminum, copper alloy and titanium. Ultrasonic cavitation erosion was produced as the specimen was submerged in the deionized water. Within a cavitation erosion period of 120 min, the cumulative mass loss was measured at certain time intervals. Surface structure and cavitation damage patterns were observed for the three materials. Microhardness was measured and compared. The results indicate that the cumulative mass loss of aluminum is the highest among the three materials, while the slightest material removal is associated with titanium, which is still in the initial stage of cavitation erosion after 120 min of cavitation erosion. The surface of the aluminum specimen is eroded rapidly after the cavitation erosion commences. Large erosion pits dominate the eroded surface as the cavitation erosion progresses. The surface of the titanium specimen manifests needle-like erosion pits and cleavage cracks. Even at the later stage of the cavitation erosion, non-eroded surface elements are identifiable. The cavitation erosion pattern on the copper alloy specimen surface is related to the twin-phase crystal structure and large erosion pits are produced at the later stage of cavitation erosion. The highest resistance to the cavitation erosion of titanium is related to the close-packed hexagonal structure and the weak slip effect associated.
A combinative use of the photovoltaic modules and roof coating on building was implemented for an institutional hall in Ghana. Computational fluid dynamics simulation was performed for both coated and uncoated roofs to investigate the heat transfer on the roof with regards to the temperature reduction. Numerical results were compared with the field data. The performance of the solar photovoltaic module deployed on the rooftop was evaluated as well. The results demonstrate that the coated roof gives rise to a significant reduction in temperature, which enhances the thermal comfortability in the building. With the participation of the solar photovoltaic module, a total energy of 427.670 MW h/year could be fed into the national grid. The reduction in the power generation cost is accomplished with the joint application of the solar photovoltaic system and the roof coating.
Applications of the Savonius rotor have been extended in recent years, necessitating an in-depth investigation on flow characteristics of such a fluid energy converting device. For the wake flow downstream of the Savonius rotor, studies have been reported extensively. Nevertheless, literature specifically devoted to the upstream flow of the Savonius rotor can rarely be found. This review collects and compiles findings from relevant studies to prove the significance of upstream flow patterns to the operation of the Savonius rotor. Then attempts from experimental and numerical aspects to substantiate the important effect of the upstream flow are implemented. Based on practical cases and laboratory works, upstream flow patterns for the Savonius rotor are divided into four types, namely uniform flow, guided flow, rotor wake flow and oscillating flow. Accordingly, conditions under which these upstream flow patterns arise are analyzed respectively. Experimental and numerical results are presented to clarify the influential factors underlying diverse upstream flow patterns. Furthermore, the relationship between the performance of the Savonius and the upstream flow is elucidated, facilitating the development of techniques of controlling the upstream flow. This review provides a systematic reference for the control of the upstream flow for the Savonius rotor, which has the tendency of developing into an independent technical branch.
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