# Comparison of the Axial Fan and Synthetic Jet Cooling Systems
## 4 Conclusions  
The synthetic jet actuator with the heat sink mounted inside the cavity was investigated as a cooling device and compared with the heat sink attached to the axial fan. The fan was operated in two configurations: with upward and downward flow.  
The highest difference between the thermal resistance of both cooling systems occurred for a 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 the synthetic jet cooling. For a higher input power of \(P=7.0\) W, the relative difference decreased to the value of 42%.  
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 \(\dot{Q}_{\mathrm{HS}}=487\) W. For the same input power and temperature difference, the heat sink cooled with a synthetic jet dissipated the thermal power of \(\dot{Q}_{\mathrm{HS}}=339\) W while under natural convection, it dissipated the thermal power of \(\dot{Q}_{\mathrm{HS}}=57\) W. Thus, the heat transfer enhancement with fan cooling relative to natural convection was equal to 8.5, while enhancement with a synthetic jet cooling relative to natural convection was equal to 6.0.  
The highest difference between the COP* of both cooling systems occurred for a low input power of \(P=0.5\) W: the heat sink cooled with a synthetic jet 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 76% better cooling efficiency than synthetic jet cooling. For a higher input power of \(P=7.0\) W, the relative difference in COP* decreased to the value of 48%.  
The fan-cooled heat sinks achieved higher heat dissipation than the SJ-cooled heat sinks for the same temperature difference and the same volume in the present investigation. The SJ cooling devices increased the \(\varepsilon\) by around two times relative to the natural convection, while the fan cooling increased it by around eight times.  
The fan with the upward flow configuration provided better heat transfer conditions than the fan with the downward flow configuration in the present investigation.  
The \(\varepsilon\) of the SJA may be increased by the use of more efficient loudspeakers, increasing the heat transfer area of the heat sink or by improving the design of the SJA dedicated to cooling a specific device.  
Conceptualization, supervision, project administration, funding acquisition, P.G.; methodology, formal analysis, R.G. and P.G.; software, validation, resources, visualization, L.P.; data curation, writing--original draft preparation, E.S.; investigation, writing--review and editing, E.S., R.G. and P.G. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Center for Research and Development, Poland Grant No.: LIDER/6/0024/L-10/18/NCBR/2019.  
Not applicable.  
**Data Availability Statement:** Some or all data generated or used during the study are available from the corresponding author by request.