*3.2. LED Thermal Performance Experiment*

For a single LED, the operating temperature is approximately −40 ◦C to 80 ◦C, yet the temperature of the light-emitting crystal can reach about 120 ◦C which has a tremendous impact on the luminous efficiency and the lifespan, so the temperature must be kept under 80 ◦C. The temperature of the LED module will be stable after 30 min; therefore, it was recorded every 40 min in this experiment. The original design of LED was for 3 W in the present study. However, the natural temperature of the jet exceeds 80 ◦C at 4 W, so the wattage supply was no longer increased. Since the power supply can not directly set the wattage and only the voltage and current can be altered, these were adjusted to a similar wattage. The energy supplied to the LED will not be completely converted to light; currently, the light efficiency of white LEDs is about 30%, while the other 70% is converted to heat. The devices of PAJ used in the experiment were tested under the same operating conditions; the ambient temperature was 25.3 ◦C, the input voltage was 30 V, the voltage frequency was 300 Hz, and they were, respectively, placed at distances of 5 mm, 10 mm, 15 mm, 20 mm and 25 mm to analyze the different cooling effects by observing the changes in thermal resistances. Figure 11a compares the position and the total thermal resistance. Among the five designs, case2 has the best cooling effect when placed at 20 mm, reducing the LED module temperature to 36.4 ◦C, which is lower than the natural convection when supplied 1 W. Therefore, it can continuously increase the wattage of LED lights to provide better illumination capability. The worst cooling effects were achieved when placing the device at 5 mm in case4 and at 25 mm in case3. It can be seen that even though the device of PAJ consisted of small pieces of piezoelectric sheets with a diameter of 31 mm and had a flared jet path and smaller spacing, its cooling effect is not ideal from the present findings. This is mainly because the measured amplitude of displacement in the vibration experiment based on a diameter of 31 mm is small. The volume flow rate of the air that can be forced into the cavity is low and the jet strength is weak so that there was no significant improvement in convection. When the device of PAJ was placed at 5 mm in case5, it lowered the temperature of the LED module to 26.3 ◦C, whereas case1 reduced the temperature to 27.1 ◦C. The results show that when increasing the distance between the piezoelectric sheets, the cooling capability of the device of PAJ deteriorates.

**Figure 11.** *Cont.*

**Figure 11.** 3W LED experimental data. (**a**) Position vs. total thermal resistance (**b**) Position vs. heat convection coefficient.

Figure 11b compares the position and heat convection coefficient. The heat convection coefficient for case1 is 129 W/m2K placed at 5 mm. When the placing distance increased to 25 mm, the heat convection coefficient reduced to 81.7 W/m2K. The efficiency of the heat convection decreased as distance increased. The heat convection coefficient of the case2 is 132.9 W/m2K placed at 5 mm. In addition, it can upgrade to three times if placed at 20 mm. The heat convection coefficients of case3 and case4 are about 48 to 98 W/m2K, respectively. The enhanced heat convection efficiency is not large compared to other devices of piezo actuation jet, yet it still can be applied to the heat source with low wattage. Case5 can enhance its convection efficiency to between 157% and 288% compared to natural convection. The case1, case3, and case5 can be applied to the heat source at a short distance due to the experimental results and previous analysis. Since they have larger opening lengths and areas, they can inhale a wider range of the air jet around with large air volume but low wind speed. If placed too far away from the heat source, then it can not achieve its cooling effect. The PAJ device placed with 5 mm had the best cooling performance among these five locations. Besides, case2 and case4 had the best heat dissipation effect at 20 mm. case2 and case4 have higher wind speeds, increasing distance is conducive to sucking in the surrounding fluid and to avoid hot air is sucked back again. However, the strength of the airflow is limited. When the distance increases to 25 mm, the strength of the airflow will decrease and is not conducive to heat dissipation.

## **4. Conclusions**

The present study designed a novel type of piezoelectric fan and investigated operating input conditions such as voltage frequency, placement distance, spacing between piezoelectric sheets, piezoelectric sheet size, and noise produced. The experimental results show that when a device of piezo actuation jet is placed too close to the heat source, the high temperature will suck back the surrounding fluids, causing the fluid chamber temperature to rise and the cooling effect to be reduced. However, the preferred design and operating conditions can fully make use of the device with a better cooling effect than traditional rotary fans. Additionally, the device should be placed 10 to 20 mm from the heat source to ensure that the temperature of the air returned to the chamber will be able to achieve an

optimal cooling effect. However, if the device of the PAJ is placed too far away, it will not be able to send wind to the heat source and will not be able to effectively dissipate heat. The device of 41 mm increases the opening area and the spacing of the piezoelectric sheet, the performance is still better than the device consisting of 31 mm piezoelectric sheets. Moreover, the power consumption comparison results show that the power needed for the device of piezo actuation jet to reduce temperature 1 ◦C is only 10% to 25% of the power needed of a rotary fan and the device of piezo actuation jet has lower costs. Finally, about 25 devices of PAJ can be connected in series while taking up no more space, which can offer above twenty-five times the number of heat convection effects.

**Author Contributions:** Conceptualization, Y.-T.C. and J.-C.W.; Data curation, Y.-T.C., R.-T.W. and J.-C.W.; Investigation, Y.-T.C., J.-C.W. and R.-T.W.; Methodology, Y.-T.C. and J.-C.W.; Supervision, R.-T.W. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** All data are offered by the authors for reasonable request and the novel device of piezo actuation jet (PAJ) are available from the authors.

**Conflicts of Interest:** The authors declare no conflict of interest.
