# Comparison of the Axial Fan and Synthetic Jet Cooling Systems
## 3 Results and Discussion
### Comparision of Thermal Parameters of Different Cooling Devices
For the input power of \(P=7.0\) W, the fan-cooled (upward flow) heat sink under a temperature difference of \(\Delta T=60\) K dissipated the thermal power of \(Q_{\text{HS}}=487\) W. For the same input power and temperature difference, the heat sink cooled with synthetic jet dissipated the thermal power of \(\dot{Q}_{\text{HS}}=339\) W, while under natural convection, it dissipated the thermal power of \(\dot{Q}_{\text{HS}}=57\) W (Figure 7). Thus, the heat transfer enhancement with fan cooling  
relative to natural convection is equal to 8.5, while the enhancement with synthetic jet cooling relative to natural convection is equal to 6.0. Figure 8a shows the average heat transfer coefficient as a function of the power supplied to the cooling device. The heat transfer coefficient increased with the power and was the highest in the case of the fan \"upward flow\" configuration. The value of the coefficient was the lowest for the configuration referred to as SJ ([PERSON] et al. [27]). The highest observed value of the heat transfer coefficient was equal to \(h_{avg}=86\) W/(m\({}^{2}\)K). For reference, the average heat transfer coefficient of the heat sink for natural convection conditions was approximately \(h_{avg}=8\) W/(m\({}^{2}\)K) (Figure 8b). The heat transfer coefficient in the case of synthetic jet-cooling increased with the increasing input power \(P\) due to the higher values of the Reynolds number [26]. The heat transfer coefficient observed in the case of the fan cooling also increased due to higher values of the airflow with increased input power [11]. The average heat transfer coefficient under active cooling conditions was almost independent of the heat sink base temperature (and consequently temperature difference). On the contrary, the heat transfer coefficient under passive cooling conditions depended strongly on natural convection and radiation, both enhanced with increasing heat sink base temperature (Figure 8b). Figure 6: The thermal resistance of the heat sink as a function of (**a**) the power supplied to the cooling device; (**b**) temperature difference \(\Delta T=(T_{\text{b}}-T_{\infty})\) at the power of \(P=7.0\) W. Data from: [PERSON] et al. [27], [PERSON] and [PERSON] [28]. Figure 7: The thermal power dissipated by the heat sink as a function of temperature difference \(\Delta T=(T_{\text{b}}-T_{\infty})\) at the power of \(P=7.0\) W. Data from: [PERSON] et al. [27], [PERSON] and [PERSON] [28]. It is important to note that the average heat transfer coefficient measured in the present work, in the cases of both fan and SJ, was quite low compared to some results reported in the literature. It may be explained by the fact that the value reported in the present paper is an average from the entire surface area \(S\) = 0.10 m\({}^{2}\) of the heat sink, while in some papers, the value of the heat transfer coefficient corresponds to the stagnation point or some other characteristic surface area. The coefficient of performance is one of the most important parameters of cooling devices, and the methods to increase its value are constantly being sought after [33,34]. The COP* of investigated cooling devices is presented in Figure 9.  
Figure 9a shows that the highest COP* was observed in the case of the fan \"upward flow\" configuration and the lowest in the case of SJ ([PERSON] et al. [27]). The highest difference between COP*'s of both cooling systems occurred for the low input power of \(P\) = 0.5W: the heat sink cooled with synthetic jet (S) [PERSON] & Wilk [28]) had the COP* value equal to 5.0, while the configuration with fan cooling (upward flow) achieved the COP* of 8.8. The fan cooling system thus reached almost a 76% better cooling efficiency than synthetic jet cooling. For the higher input power of \(P\) = 7.0 W, the relative difference in COP* decreased to the value of 48%. The COP* characteristics resulted from the thermal resistance characteristics presented in Figure 6a: the highest coefficient of performance occurred for the case with the lowest thermal resistance.