Citation: Cati, Y.; Wiesche, S.a.d.; Düzgün, M. Investigation of Convective Heat Transfer and Stability on a Rotating Disk: A Novel Experimental Method and Thermal Modeling. Fluids 2024, 9, 167. https://doi.org/10.3390/ fluids9070167 Academic Editors: Patrice Estellé, Lioua Kolsi and Walid Hassen Received: 4 June 2024 Revised: 15 July 2024 Accepted: 16 July 2024 Published: 22 July 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). fluids Article Investigation of Convective Heat Transfer and Stability on a Rotating Disk: A Novel Experimental Method and Thermal Modeling Yusuf Cati 1, * , Stefan aus der Wiesche 2 and Mesut Düzgün 3 1 Research and Technology Center, Turkish State Railways, 06590 Ankara, Turkey 2 Department of Mechanical Engineering, Muenster University of Applied Sciences, Stegerwaldstr. 39, 48565 Steinfurt, Germany; wiesche@fh-muenster.de 3 Faculty of Technology, Automotive Engineering Department, Gazi University, Taskent Building Floor 3, No. 318, 06500 Ankara, Turkey; mduzgun@gazi.edu.tr * Correspondence: yusuf.cati@gazi.edu.tr Abstract: Experimental and numerical investigations are conducted on a rotating disk from the perspective of convective heat transfer to understand the effect of heating on the stability of flow. A non-invasive approach with a thermal camera is employed to determine local Nusselt numbers for different rotational rates and perturbation parameters, i.e., the strength of the heat transfer. A novel transient temperature data extraction over the disk radius and an evaluation method are developed and applied for the first time for the air on a rotating disk. The evaluation method utilizes the lumped capacitance approach with a constant heat flux input. Nusselt number distributions from this experimental study show that there is a good agreement with the previous experimental correlations and linear stability analysis on the subject. A significant result of this approach is that by using the experimental setup and developed approach, it is possible to qualitatively show that instability in the flow starts earlier, i.e., an earlier departure from laminar behavior is observed at lower rotational Reynolds numbers with an increasing perturbation parameter, which is due to the strength of heating. Two experimental setups are modeled and simulated using a validated in-house Python code, featuring a three-dimensional thermal model of the disk. The thermal code was developed for the rotating disks and brake disks with a simplified geometry. Experimentally evaluated heat transfer coefficients are implemented and used as convective boundary conditions in the thermal code. Radial temperature distributions are compared with the experimental data, and there is good agreement between the experiment and the model. The model was used to evaluate the effect of radial conduction, which is neglected when using the lumped capacitance approach to determine heat transfer coefficients. It was observed that the radial conduction has a slight effect. The methodology and approach used in this experimental study, combined with the numerical model, can be used for further investigations on the subject. Keywords: rotating disk flow; experimental analysis; convective heat transfer; flow stability; transient thermal model 1. Introduction Convective flow over a rotating disk has been the subject of a wide range of research studies [1–5]. The subject has also been an active R&D area in turbomachinery, brake disks and other related areas. Convective heat transfer on a rotating disk has been investigated by several authors for a long time with different crossflow combinations, such as axial flow and parallel flow on a rotating disk. Convective heat transfer studies were mainly aimed at the determination of local or average Nusselt number dependence on rotational Reynolds ( Re ω = ω r 2 / ν ) and Prandtl numbers for different fluids [6–8]. It is important to emphasize that De Vere’s doctoral dissertation [9] contains comprehensive and relevant Fluids 2024, 9, 167. https://doi.org/10.3390/fluids9070167 https://www.mdpi.com/journal/fluids