Research On Convective Heat Transfer Characteristic Of Piezoelectric Fan Inside Forced Flow Passage

Piezoelectric fan is a solid-state device which generally consists of a patch of piezoelectric material and a flexible blade.It employs the reversed piezoelectric effect to make the piezoelectric patch expand and contract periodically,driving the attached flexible blade to oscillate at the same frequency.Due to the oscillatory motion of flexible blade,the neighboring fluid is periodically excited and thus a pseudo-jet or streaming flow is produced shedding along the fan tip.On account of its some features,such as simple structure,low power consumption,and easy controllability,piezoelectric fan has recently gained much attention in the practical applications,such as electronics cooling,energy harvesting,biomimetic robotic propulsion,etc.Innovation on heat transfer enhancement with avtive flow control technology is an frontier issue aspect facing to the engineering thermal sicence.In order to further understant the flow and heat transfer features as well as the correspondingly affecting mechanism of piezoelectric fan inside a forced flow passage,four aspects are outlined in the current work.Firstly,the experiments on the vibration performances of a single piezoelectric fan under different boundary conditions,as well as the effects of dual fans interaction were carried out by using Laser Doppler Vibrameter(LDV).The influences of operating voltage,channel flow velocity,phase difference and the dimensionless pitch of dual fans on the vibration performances are illustrated.Based on this,the displacement functions describing the vibrating piezoelectric fan are established.The results show that in a forced channel flow the aerodynamic damping of a single fan is mainly influenced by the angle of the mainstream direction and the vibration direction(α)and the main velocity.The larger α is and the smaller the mainstream speed is,then the smaller the aerodynamic damping of the fan is.For the dual fans,the aerodynamic damping is tightly dependent on the array configuration,the dimensionless pitch(PN=P/W)and phase difference.In an edge-to-edge configuration,when the dual fans are arranged with a little dimensionless pitch(PN<1.2),the aerodynamic damping slightly increases for in-phase and slightly decreases for out-of-phase.However,once the dimensionless pitch(PN>1.2)is beyond 1.2,the phase difference has nearly no influence on the aerodynamic damping.In a face-to-face configuration,when the dimensionless pitch is less than 1(PN<1),the aerodynamic damping slightly decreases for in-phase and slightly increases for out-of-phase.Once the dimensionless pitch is greater than 1(PN>1),the effects of phase difference on the aerodynamic damping disappears.Secondly,an experimental and numerical investigation were performed to further explore the convective heat transfer performance by a single fan and dual fans in static environment.The convection heat transfer characteristics of the streaming flow induced by fan,the main affecting factors on this and the evolution mechanism of vortex structures are revealed.The results show that for a single fan two sets of horse shoe type vortex are formed from both edges of vibrating fan,moving towards the fan tip and rolling up over the fan tip periodically.Due to the existence of the heated surface,the vortical structures excited by the piezoelectric fan behave somewhat differently from those observed in the free space.The shedding vortex is easier to be broken down in relation to the case in the free space.The near-wall flow field induced by the piezoelectric fan demonstrates obvious lateral flow parallel to the fan,and suction flow on both sides of the fan.The local convective heat transfer in the fan-tip vibration envelope is effectively enhanced.Dumbbell-shaped distribution of local convective heat transfer around the fan-tip vibration envelope is demonstrated.For the edge-to-edge dual fans,the horse shoe type vortex formed from both edges of vibrating fan are merged together,forming a stronger vertical flow impingement as well as near-wall flow in outward direction in-phase.By comparison with single piezoelectric fan,the edge-to-edge dual fans in-phase enhances the peak local convective heat transfer coefficient but the edge-to-edge dual fans out-of-phase reduces the peak local convective heat transfer coefficient.The heat transfer enhancement of the edge-to-edge dual fans in-phase is much better than the edge-to-edge dual fans out-of-phase.For the face-to-face dual fans,the vertical flow is stronger out-of-phase than in-phase.By comparison with single piezoelectric fan,the face-to-face dual fans enhances the peak local convective heat transfer coefficient both in-phase and out-of-phase.The heat transfer enhancement of the face-to-face dual fans out-of-phase is better than that in-phase.Thirdly,the flow and heat transfer characteristics of the streaming flow induced by a single fan or dual fans in the presence of channel flow were studied.The convection heat transfer characteristics of the coupling flow with streaming flow and channel flow are analyzed to illustrate its flow mechanism.The results show that for a single piezoelectric fan the vortical structures at the edges of vibrating fan are certainly suppressed in the presence of channel flow,and the mutual interaction of both flows is tightly dependent on velocity ratios.Under small velocity ratios,the impingement role of streaming flow along fan tip is still dominated and simultaneously the channel flow passing through the vibration envelope is effectively pulsated.Therefore,combined flows generally produce heat transfer enhancement around the fan vibration envelope related to the pure vibrating fan,especially at a small non-dimensional tip-to-surface gap.At a large tip-to-surface gap,the peak local convection coefficient is moved a little downwards by the presence of channel flow.While under large velocity ratios,the impingement role of streaming flow induced by a vibrating fan is seriously weakened by the strong channel flow.The convective heat transfer produced by combined flows in the fan vibration envelope is generally reduced in comparison with pure piezoelectric fan.On the other hand,the combined flows effectively improve the convective heat transfer related to the pure channel flow,especially downstream of the fan vibration envelope.For the dual fans arranged in the edge-to-edge mode,the shedding vortical structures induced by the upstream piezoelectric fan is moved downwards by the presence of channel flow and mixed together with the vortical structures induced by the downstream piezoelectric fan.The convective heat transfer in the region between two adjacent fans under the out-of-phase situation is significantly less than that under the in-phase situation.For the dual fans arranged in the face-to-face mode,as the shedding vortical structures under the out-of-phase situation is easier to mix and develop than that under the in-phase situation.The convective heat transfer downstream the fan-tip vibration envelope under the out-of-phase situation is higher than that under the in-phase situation.Finally,an experimental investigation was carried out to analysis the heat transfer performance of two typical heat sinks(micro pin-fin heat sink and finned heat sink)integrating piezoelectric fan in the presence of channel flow.The variation laws of the heat transfer performance of the heat sinks with relevant factors are obtained.Results show that the micro pin-fin heat sink integrating piezoelectric fan has a highly efficient heat transfer enhancement.The vibrating fan could enhance the heat transfer performance of the micro pin-fin heat sink significantly,especially under a high Reynolds number of streaming jet and small fan tip-to-surface distance.The linear relationships of heat transfer coefficient and overall thermal performance parameter are derived with respect to the total Reynolds number combining the channel flow and piezoelectric fan.It is demonstrated that the influence weighting of piezoelectric fan on the overall thermal performance parameter is weaker than that on the pure thermal performance of micro pin-fin heat sink.The heat transfer performance of the finned heat sink integrating piezoelectric fan is tightly dependent on the channel flow velocity and piezoelectric fan orientation.The piezoelectric fan indeed plays a role on improving the thermal performance of finned heat sink,especially when the piezoelectric fan is located tightly close to the front edge of heat sink and the channel flow velocity is low.The vertical or horizontal arrangement of piezoelectric fan shows different influence,dependent on its relative location to the finned heat sink.