# Comparison of the Axial Fan and Synthetic Jet Cooling Systems
## 3 Results and Discussion
### Comparision of Thermal Parameters of Different Cooling Devices  
Figure 6a presents the thermal resistance of the heat sink as a function of the power supplied to the cooling device. The thermal resistance in the case of SJ was higher than in the case of a fan, and the \"upward flow\" configuration with an axial fan exhibited lower thermal resistance than the \"downward flow\" fan configuration. The thermal resistance also depends on the difference between the heat sink temperature and ambient temperature; however, this dependence is weak (Figure 6b). The lowest value of the thermal resistance in the entire experiment of \(R=0.12\) K/W was observed in the fan \"upward flow\" configuration at \(P=7.0\) W. The thermal resistance \(R\) of the heat sink decreased with the increased power \(P\) delivered to the synthetic jet actuator due to the increase in the synthetic jet velocity and Reynolds number [26]. With the increased synthetic jet Reynolds number, the heat transfer enhancement occurred. Thus the thermal resistance of the radial heat sink decreased. A similar effect was observed for fan cooling: with the increased DC voltage delivered to the fan, the input power \(P\) increased, which, in turn, caused the higher rotational speed of the fan and, in consequence, higher airflow. Increased airflow through the heat sink channels enabled heat transfer enhancement, thus reducing heat sink thermal resistance. Comparing the cooling characteristic of the best synthetic jet case referred to as SJ (Gil and Wilk) [28] with upward flow fan cooling, it may be concluded that the results are similar to those presented by [PERSON] and [PERSON] [11]. The highest difference between the thermal resistance of both cooling systems occurred for the low input power of \(P=0.5\) W: the heat sink cooled with a synthetic jet had the thermal resistance of \(R=0.39\) K/W while the same heat sink cooled with a fan achieved \(R=0.23\) K/W. Thus, the fan cooling performance was almost 70% better than the performance of synthetic jet cooling. For a higher input power of \(P=7.0\) W, the relative difference decreased to the value of 42%. [PERSON] and [PERSON] [11] also observed a lower difference between synthetic jet and fan cooling performance of 6% at higher levels of input power. The main problem with such a comparison is the quality and performance of the fan used. [PERSON] and [PERSON] [11] used a Delta Model AFB0612E fan with external dimensions of 60 mm \(\times\) 60 mm \(\times\) 25.4 mm and a maximum flow rate of 65 m\({}^{3}\)/h (at input power of \(P=4.56\) W), while in the present paper, the AABCOOLING Black Jet Fan with external dimensions of 120 mm \(\times\) 120 mm \(\times\) 25 mm and a maximum flow rate of 170 m\({}^{3}\)/h (at input power \(P=3.84\) W) was used. Indeed, fan maximum flow rate alone is insufficient for objective comparison because it is necessary to know the fan pressure-flow rate characteristic as well as heat sink flow resistance to determine the fan-heat sink flow operating point. Figure 7 presents the thermal power dissipated by the heat sink for natural and forced convection as a function of the temperature difference. All the results presented in Figure 7 were recorded at the power of \(P=7\) W supplied to SJA/fan. As expected, the lowest value of the dissipated thermal power occurred in the case of natural convection and was much higher for the active cooling methods. The highest value was measured in the case of the fan \"upward flow\" configuration. In the cases of fan \"downward flow\" and SJ ([PERSON] and [PERSON] [28]), the values of dissipated thermal power were similar. With the increasing heat sink base temperature \(T_{\text{b}}\) at constant ambient temperature \(T_{\infty}\), the dissipated thermal power for active cooling methods increased almost linearly.