## 4 Concluding Remarks  
The thermal performance of alternative oils versus mineral cooling oils on a reference case of a transformer winding geometry was investigated by means 2D axisymmetric simulation using a conjugate heat transfer solver. Four oils were considered, namely two mineral oils, a natural ester oil (Midel 1215) and a synthetic ester (Midel 7131). The comparison was carried out by considering the same mass flow rate for all the oils; while such a hypothesis can be, to a certain extent, unrealistic in practical cases, it still allows for a meaningful comparison on the same reference test case, assuming, for instance, that the oil flow is directed in the windings by a pump.  
Results show that, generally, the natural ester (Midel 1215) not only determines a more stable flow, arguably thanks to its higher Prandtl number, but also exhibits the lowest overall maximum temperature in the conductors. A significantly poorer performance, with a much higher (by more than 20 \({}^{\circ}\)C) hot-spot temperature, is observed for the Nytro Taurus mineral oil. The behavior of the synthetic ester (Midel 7131) is similar to the one of the natural esters in the lower part of the domain, while it is aligned to the mineral oil in the upper passes, where oil temperatures rise, and the value of Pr rapidly decreases. All in all, it can be concluded that the influence of Pr is of primary concern for the comparison between these oils.  
Another important aspect concerns the position of the hot spot, which is considered, by technical standards, to occur in the very last discs, for disc-type winding arrangements. Not only this does not happen in any of the cases considered here (except for the Midel 7131 oil), but for one of the two mineral oils (the Nytro Taurus), it does not even occur in the last pass of the zigzag cooling duct. Such a result underlines the importance of the use of advanced predictive tools for transformer cooling design.  
Finally, even in this limited number of cases, a variety of dynamical behaviors, including instabilities and flow reversals, were observed, prompting future analyses relating these occurrences to the values of the main dimensionless flow parameters.  
Figure 9: Time-averaged horizontal velocity profiles normalized by the inlet velocity across different locations of selected channels in the upper half of Pass 4 for the cases of Midel 7131 (magenta lines) and Nytro Taurus (green lines).  
**Author Contributions:** Conceptualization, E.S. and D.A.; methodology, E.S., A.L., M.G. and D.A.; software, E.S., A.L. and M.G.; data curation, E.S. and A.L.; visualization, E.S. and A.L.; writing--original draft preparation, E.S., A.L. and D.A.; writing--review and editing, M.G., D.A. and M.A.C.; supervision, D.A. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
**Institutional Review Board Statement:** Not applicable.  
**Informed Consent Statement:** Not applicable.  
**Data Availability Statement:** The raw data supporting the conclusions of this article will be made available by the corresponding author on request.  
**Conflicts of Interest:** The authors declare no conflicts of interest.