Rheological Analysis of Thermally Aged Natural Ester Fluid Using Nonlinear Least Square Technique

In the present study, rheological studies dealing with the flow behavior of thermally aged natural ester fluid under laboratory-controlled conditions are performed. Visual observation of gelling is witnessed due to thermal aging. An enhancement in viscosity of about 188% is observed with the 500-h aged fluid compared to the nonaged fluid. It is estimated that the flow behavior altered by gel follows non-Newtonian behavior with a shear-thinning effect on applied shear rate conditions. The dependency of storage and loss modulus to amplitude and frequency sweep under oscillatory shear flow was investigated. The viscous state of gel dominates over its initial elastic state, beyond the gel point. The flow models that govern the behavior of the gel were applied, adopting nonlinear least square methods to investigate the rheological parameters in terms of flow behavior index and yield stress. The results of the thermally aged natural ester fluid demonstrate a positive correlation with Bingham's model and Mizrahi-Berk's model based on the yield stress and flow behavior index, respectively. The phase transition of gel investigated in the present study can be used as a time-based preventive measure, by the transformer manufacturer, in the heat transfer performance of natural ester-filled transformers.