## 4 Results and Discussion  
Figure 8 presents the change in Rayleigh, Grashof and Prandtl numbers through time. The temporal changes in the Rayleigh number for the test fluids show a rapid increase during the initial phase of heating, followed by saturation in the later phase. The Rayleigh number is a dimensionless parameter that is critical to the calculation of natural convection. The onset of convection occurs when the Rayleigh number reaches a critical value. Thus, it is an important indicator for determining whether natural convection is laminar or turbulent. It is simply the product of the Grashof number and the Prandtl number. Figure 8: Time variation in (**a**) Rayleigh number, (**b**) Grashof number, and (**c**) Prandtl number in measurements. In this research, it should be noted that the Rayleigh number for test fluid A reaches approximately \(10^{6}\) in the later stages of heating, while it is about \(10^{5}\) for test fluid B, and about \(10^{4}\) for test fluid C.  
The Grashof number is a parameter that helps us understand the turbulent regime within the fluid. A higher Grashof number indicates a turbulent boundary layer, while a lower Grashof number indicates a laminar boundary layer [26]. It is important to note that the Grashof number for liquid A exceeds \(10^{2}\) towards the end of the heating process. This means that after the eighth minute, thermal convection becomes more dominant than thermal conduction. Furthermore, the Prandtl number is another significant parameter in our study. Smaller Prandtl numbers signify a greater effectiveness of thermal convection, while larger Prandtl numbers indicate a higher effectiveness of thermal conduction [27]. Our experimental findings demonstrate that as heating time increases, Prandtl numbers decrease, indicating that thermal convection becomes more dominant. Moreover, as the viscosity of the fluid decreases, the strength of thermal convection also increases. Figure 9 confirms significant temperature variations between thermocouples in both the thermal conduction and convection experiments. These experimental results align with the mathematical outcomes based on Rayleigh, Prandtl and Grashof numbers. As the heating period increases, the temperature differences among the thermocouples of Fluid A become significantly smaller compared to Fluid B and Fluid C, indicating a decrease in the Prandtl number. With the Grashof number surpassing \(10^{2}\) for Fluid A, thermal convection becomes more dominant, resulting in smaller temperature fluctuations. Figure 9: Temperature assessment of **(a)** Fluid A (100 cSt); **(b)** Fluid B (1000 cSt); and **(c)** Fluid C (10,000 cSt). Similarly, Fluid B and Fluid C exhibit smaller Rayleigh numbers, which are not significant enough to initiate turbulent flow within the fluids. As a result, laminar flow becomes dominant and causes larger temperature fluctuations between the thermocouples. On the other hand, the Rayleigh number of Fluid A is powerful enough to induce turbulent flow, leading to smaller temperature variations among the thermocouples. Meanwhile, the Rayleigh number of Fluid A is powerful enough to cause turbulent flow, representing smaller temperature variations between the thermocouples. As a reminder, the measurement of temperature deviations is crucial for this research, as temperature deviations also mean deviations in viscosity. Similar outcomes are presented from the flow visualization experiment. Figure 10 illustrates the temporal evolution of temperatures for the test fluids in the test cup, as well as the heating water, as recorded by the thermocouples. All temperatures gradually rise as time (\(t\)) progresses following the initiation of heating. Notably, the temperatures of the test fluid and the heating water are initially maintained at the same level during the onset of heating. While the relationship between temperature and time in the heating water remains unaffected by fluid viscosities, the temperatures (\(T_{c}\) and \(T_{b}\)) within the cup are influenced by the viscosity of the fluid.