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In light of diminishing natural resources, global climatic change and increased environmental sensitivity, renewable-based lubricants are being considered potential alternatives to petroleum-based lubricants. Understanding the tribological performance of vegetable-based lubricants in relation to their chemical composition is essential for their industrial implementation. This study focuses on the friction and abrasion rate characteristics of soybean and sunflower oils in comparison to a base mineral oil under sliding wear at ambient conditions for various applied loads. It was found that the abrasion rate and friction were the least severe for the soybean, followed by the sunflower oil. The observed trends were attributed to differences in their fatty acid compositions, in particular, a lower percentage of linoleic and oleic acids within the soybean oil.

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A combined numerical and experimental study was performed to determine the turbulent heat transfer on a stationary disk, which is situated in a close distance from a rotating disk. The RNG k-e model and the steady-state liquid crystal technique were employed respectively in the numerical simulation and the experiment. In the range of the rotational Reynolds number from 1:42 Â 10 5 to 3:33 Â 10 5 , the heat transfer rate on the stator and the flow characteristics in the gap between the disks are presented. The results revealed that there exists an optimum rotor-stator distance for a given Reynolds number, at which the average heat transfer on the stator reaches maximum. When the Reynolds number increases, the maximum shifts towards smaller disk-distances.

In general, vegetable oils exhibit superior lubrication properties but lack the thermal stability of petroleum base stocks. However, vegetable oils could make an ideal candidate as a base stock for lubrication applications involving high humidity levels, such as marine and offshore applications. This study focuses on the friction and wear rate of unmodified soybean and sunflower oils in comparison with an unformulated mineral oil at various levels of relative humidity, ranging from 10% to 98% RH. It was observed that the vegetable oils retain their friction and wear reducing capabilities much better than the mineral oil at high humidity levels. This was attributed to their inherent ability to react with the metallic contacting surfaces and form multilayers of soap films. Furthermore, the soybean oil provided a superior level of wear resistance when compared with the sunflower oil at extremely high levels of relative humidity due to its lower viscosity and differences in chemical compositions.

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