UC Merced

Varnish is an oxidative byproduct of lubrication that builds up on mechanical components during operation. Varnish must be removed to enable optimal performance of a mechanical system. Many mechanical systems reach very high temperatures; for example, gas turbine engines have areas of extreme temperature up to the 2000˚C. This means that oxidation and oxidative byproducts are a constant issue in these systems. There are many ways to mitigate varnish buildup and control oxidative byproducts, but chemical flushes are necessary if the varnish buildup reaches a critical point. This research investigated the effectiveness of different formulations of chemical cleaners in a standardized low flow test and characterized the effects of flow rate, temperature and concentration for one of those cleaners. The comparative cleaner tests showed that many commercial cleaners did not remove substantial varnish at 0.5 GPM and 90° C. Only two fluids were capable of removing significant varnish during the 50-hour test. One of those fluids was then tested over a range of temperatures, flow rates and cleaner concentrations. The temperature tests revealed that removal rate increases with increasing temperature. The effect of temperature was described by a polynomial trend that suggested an effective operating temperature range for varnish removal. The concentration of chemical cleaner additives had a large effect on the removal rate, with low concentrations removing very little varnish even after extended periods of time. This effect was governed by a power law relationship with diminishing returns at the higher concentrations. Flow rate tests showed a distinct trend of removal rate increase with flow rate increase. The relationship between varnish removal and flow rate was described by a power law where the effect of increasing flow rate diminished at higher flow rates. Flow rate also affected the size and distribution of varnish particles collected on a downstream filter and the results showed that the filter size needed for optimal varnish removal should be flow rate dependent. The effect of flow rate was further analyzed using computational fluid dynamics models. The simulations showed that increasing flow rates cause higher shear stress at the surface of the varnish, facilitating varnish removal. Simulations also showed that geometric features on the varnish can affect shear rates and in turn the rate of local varnish removal. Overall, this study showed the importance of operating conditions and their impact on varnish removal and provided information that can help in the formulation of new cleaners as well as selection of operating conditions and filter media for the most efficient varnish removal.