Cooling performance and characteristics of metal piezoelectric fans in a heat sink-equipped handheld projector

In this study, the thermal performance of several pin-fin arrays in a horizontally-oriented pico projector cooled with and without a metal piezoelectric fan was evaluated at several heating powers. The vibrating frequency of the 20-Vrms Al piezoelectric fan and the 30-Vrms stainless steel piezoelectric fan ranged from 242 Hz to 257 Hz, and from 171 Hz to 186 Hz, respectively. The results showed that the thermal resistance of the fin array cooled with a piezoelectric fan vibrating at a specific frequency in the pico projector would not monotonically reduce with the increase of the heating power. At a heating power of 7 W, the thermal resistance of S1.5 fin array cooled with a 251-Hz Al and 180-Hz stainless steel piezoelectric fan was approximately 14.8 K/W and 12.2 K/W, respectively, while the average heat transfer coefficient of S1.5 fin array cooled with a 251-Hz Al and a 180-Hz stainless steel piezoelectric fans achieved approximately 21 W/m2 K and 31.5 W/m2 K, respectively. The results also reported that the increase of the fan tip velocity resulted in the increase of both the average heat transfer coefficient of the fin arrays and the power consumption of the piezoelectric fan. An empirical correlation for estimating Nusselt number of those pin fin heat sinks was developed in a form of Nu¯=0.1715Remax0.3314Pr0.34 with mean deviation of 8.59% between the predicted value and the measured one for all conditions. Using a piezoelectric fan having high Reynolds number that can be achieved by increasing both the first natural frequency and the vibrating amplitude of the piezoelectric fan to cool a dense pin fin array is suggested.