Research On Vector-controlling Characteristic Of Dual Synthetic Jets And Its Applications In Heat Transfer Enhancement

Variable and controllable flow is the development trend of fluid mechanics,and is also a core technology for active flow control(AFC).It is a key for improving the control system capacity and optimizing the control law to expand the actuator controllability.The combination of AFC and heat-transfer enhancement technology has been a research topic in the interdisciplinary area of fluid mechanics and engineering thermal physics.It has significant application value to explore and exploit a compact,highly efficient heat-dissipation technology based on AFC.For a dual-synthetic-jet actuator(DSJA),theoretical analyses and experimental studies are executed from the structure and function design,to the evaluation of performance and energy conversion efficiency,and then to the construction of a closed-loop control system.The aim is to breakthrough the dual-synthetic-jet(DSJ)technology in practical applications.The design and optimization analysis are carried out for a DSJA to manufacture a vectoring DSJA.By comparing intelligent materials and micro-stepper motor,a modularity and unitization vectoring DSJA is designed based on a micro-stepper motor.An optimal design for a DSJA applied in enhancing the mixing and heat transfer is carried out by combining with orthogonal experiment design and variance analysis.As a result,an optimized parameter combination is obtained,and the corresponding evaluation index increases more than 11.4%.The vibration,flow and energy characteristics of DSJA are studied.The effects of driving parameters on the vibration and cavity pressure are analyzed.Two jet velocity peaks are obtained at vibrating membrane frequency and Helmholtz resonance frequency.The effects of structure and flow field parameters on the intensity and distribution of vorticity are investigated by particle image velocimetry(PIV)experiments.Moreover,the proper orthogonal decomposition(POD)and reconstruction are adopted to analyze the decomposition and reconstruction flow field,as well as the frequency characteristic of each mode.A calculation model for the energy conversion efficiency(ECE)is established.The ECE reaches a maximum near two resonance frequencies.The ECE of a DSJA is about two times of a conventional synthetic jet(CSJ)actuator.Although the increase of driving voltage improves the jet velocity,the maximum ECE decreases.The dissertation presents a vectoring evaluation method based on the potential core,which can effectively reduce the error due to the field flow fluctuation.Then the effects of driving parameters,slider position and structure on the vectoring angle of DSJ are experimentally studied using schlieren and PIV.The velocity of the primary jet and the vectoring angle varying with the slider position are analyzed in detail,and the physical mechanism of vectoring deflection is revealed.The results indicate that DSJ always deflects toward the slot with a greater width,and there is an optimal value for driving voltage and frequency to obtain a larger vectoring angle.The vectoring angle can be adjusted bidirectionally within a linear interval by controlling the slider movement.Additionally,the increase of slider chamfer angle will extend the variation range and linear interval of vectoring angle,which is beneficial to optimizing the control law of AFC.The heat dissipation of LED(Light-emitting diode)arrays is experimentally studied.An active-passive combined cooling method is proposed based on DSJ and a small-scale fin.The heat-dissipation characteristics and installation-space reductions of different cooling methods are comparatively analyzed.The effects of jet impingement spacing,angle and slot configuration on the heat-dissipation performance of the combination structure are investigated.The results indicate that the heat-dissipation performance of DSJ is better than that of CSJ under the same cooling method,and the combined structure(vertical layout)can make the stable temperature drop 15? and the installation space decreases 72.3% comparing to the commercial fin,while installation-space reduction reaches 81% in parallel layout which is helpful for the high-power LED package.Moreover,a closed-loop control system based on DSJ is developed,which achieves the autonomous control within a set temperature range.A duty cycle parameters for evaluating actuator's performance is established.With the increase of jet impingement spacing,there is a optimal spacing at which the duty cycle is lowest at 29.1%,and the working duration and power consumption are least.For large-area electronics cooling,a ?sweeping flow? method based on vectoring DSJ is proposed,and the flow field and heat-dissipation characteristics of vectoring DSJ impinging on a plate are studied.The results indicate that the jet center point and the stagnation region move away from the plate center,which can improve the local convective heat transfer coefficient of the border region.When the impingement spacing is small,the ?self-support? phenomenon of DSJ makes its heat-dissipation performance be worse than CSJ.However,with the increase of impingement spacing,the heat-dissipation performance of DSJ is improved rapidly and exceeds CSJ.Moreover,the optimal impingement spacing of DSJ is smaller than CSJ,which can effectively reduce the installation space.The position changes of slider will influence the impingement strength and angle of vectoring DSJ,causing three local peaks of average Nusselt number(Nuavg).The moving speed of slide has little effect on Nuavg.For different moving speeds,Nuavg first increases and then decreases with the increase of impingement spacing.The lateral average temperatures vary periodically with the reciprocating motion of slider,which can avoid a long duration of the border region at high temperature and realize a large-area,uniform heat dissipation.