Synthetic Jet Research Articles

A novel synthetic jet based heat sink with PCM filled cylindrical fins for efficient electronic cooling

The current electronic industries strive to build an effective cooling solution for modern compact electronic gadgets. The current experimental study, thus, focused on assessing the performance of a heat sink that combines the benefit of active and passive cooling utilizing the synthetic jet and Phase change material (PCM), respectively. The PCM offers a high latent heat value, whereas the synthetic jet induces vortices-based turbulent flow. The cooling characteristics of the heat sink filled with PCM and without PCM are evaluated when subjected to three different operating conditions, which are (i) natural convection, (ii) forced convection using DC fan, and (iii) forced convection using a synthetic jet. The PCM selected for the current study are Eicosane and 1-hexadecanol. It is observed that the inclusion of PCM into the heat sink improved its performance. The multiple-orifice synthetic jet outperformed the single-orifice synthetic jet. The steady-state base temperature attained by multiple orifice synthetic jet is 5 °C lower than that of the synthetic jet with a single orifice. The heat sink cooling performance is best when it is operated at its optimum height measured from the heat sink tip. Heat sink filled with Eicosane subjected to synthetic jet with multiple orifice showed best results, the thermal resistance, heat transfer coefficient and COP is found to be 0.91 K/W, 420.4 W/m2, and 2.1, respectively. The synthetic jet with multiple orifices performed better than the DC (Direct Current) fan operated at the same electrical power input of 0.5 W.

Jet Velocity and Acoustic Excitation Characteristics of a Synthetic Jet Actuator

The effect of the excitation frequency of synthetic jet actuators on the mean jet velocity issuing from an array of circular orifices is investigated experimentally, focusing on the acoustic excitation characteristics of the actuator’s cavity. Two cavity configurations are considered. In the first configuration, synthetic jets are generated by exciting a single, large cavity having an array of sixteen orifices via sixteen piezoelectric elements. In the second configuration, the cavity volume of the first configuration is divided into eight isolated compartments, each with two orifices and two piezoelectric elements. Several distinct resonant peaks were observed in the frequency response of the synthetic jet actuator built with a single large-aspect-ratio cavity, whereas the case of compartmentalised cavities exhibited a single resonant peak. Acoustic simulations of the large-aspect-ratio-cavity volume showed that the multiple peaks in its frequency response correspond to the acoustic standing-wave mode shapes of the cavity. Due to its large aspect ratio, several acoustic mode shapes coexist in the excitation frequency range aside from the Helmholtz resonance frequency. When the actuator’s cavity volume is compartmentalised, only the Helmholtz resonance frequency is observed within the excitation frequency range.

Dual Synthetic Jets Actuator and Its Applications—Part III: Impingement Flow Field and Cooling Characteristics of Vectoring Dual Synthetic Jets

In order to understand the impingement flow field and cooling characteristics of vectoring dual synthetic jets (DSJ), an experimental investigation was performed to analyze the parameter effects. With the variation of the slot location, the vectoring angle of DSJ can be adjusted from 34.5° toward the left to 29.5° toward the right. The vectoring function can greatly extend the length of impingement region. There are three local peaks both for the local cooling performance (Nu) and the whole cooling performance (Nuavg). Although the peak Nu at a certain location of the slider is higher than that at the center, the corresponding Nuavg is lower. As for different driving frequencies, the vectoring angle reaches its minimum of 9.7° at 350 Hz, but the Nu is obviously improved. There is one local peak of Nuavg values at 350 Hz rather than three local peaks at 250 Hz and 450 Hz. The slot locations where the Nuavg of 250 Hz and 450 Hz reach maximum are different. With the increase in driving voltage from ±100 V to ±200 V, the vectoring angle drops from 46.9° to 22.2°, but both Nu and Nuavg are improved. The maximum Nuavg of each driving voltage occurs at the center location of the slider. The choking effect and the cross flow have dominated the vectoring angle and the cooling performance of impingement DSJ. Vectoring DSJ will give impetus to the thermal management of large-area electric devices in spaced-constrained cooling and removing dynamic hotspots.

Higher-order dynamic mode decomposition on-the-fly: A low-order algorithm for complex fluid flows

This article presents a new method to identify the main patterns describing the flow motion in complex flows. The algorithm is an extension of the higher-order dynamic mode decomposition (HODMD), which compresses the snapshots from the analysed database and progressively updates new compressed snapshots on-the-fly, so it is denoted as HODMD on-the-fly (HODMD-of). This algorithm can be applied in parallel to the numerical simulations (or experiments), and it exhibits two main advantages over offline algorithms: (i) it automatically selects on-the-fly the number of necessary snapshots from the database to identify the relevant dynamics; and (ii) it can be used from the beginning of a numerical simulation (or experiment), since it uses a sliding-window to automatically select, also on-the-fly, the suitable interval to perform the data analysis, i.e. it automatically identifies and discards the transient dynamics. The HODMD-of algorithm is suitable to build reduced order models, which have a much lower computational cost than the original simulation. The performance of the method has been tested in three different cases: the axi-symmetric synthetic jet, the three-dimensional wake of a circular cylinder and the turbulent wake behind a wall-mounted square cylinder. The obtained speed-up factors are around 7 with respect to HODMD; this value depends on the simulation and the configuration of the hyperparameters. HODMD-of also provides a significant reduction of the memory requirements, between 40−80% amongst the two- and three-dimensional cases studied in this paper.

Experimental Investigations of the LED Lamp with Heat Sink Inside the Synthetic Jet Actuator

The paper presents the experimental research on the thermal management of a 150 W LED lamp with heat sink inside a synthetic jet actuator. The luminous flux was generated by 320 SMD LEDs with a nominal luminous efficacy equal to 200 lm/W mounted on a single PCB. Characteristic temperatures were measured with three different measurement techniques: thermocouples, infrared camera, and an estimation of the junction temperature from its calibrated dependence on the LED forward voltage. The temperature budget between the LED junction and ambient as well as the thermal resistance network was determined and analyzed. The energy balance of the LED lamp is presented along with the values of the heat flow rate and heat transfer coefficient in different regions of the LED lamp surface. For an input power supplied to the SJA equal to 4.50 W, the synthetic jet dissipated approximately 89% of the total heat generated by the LED lamp. The heat from the PCB was transferred through the front and rear surfaces of the board. For the input power of 4.50 W, approximately 91% of the heat generated by LEDs was conducted by the PCB substrate to the heat spreading plate, while the remaining 9% was dissipated by the front surface of the PCB, mostly by radiation. The thermal balance revealed that for the luminous efficacy of the investigated LEDs, approximately 60% of the electrical energy supplied to the LED lamp was converted into heat, while the rest was converted into light.

Influences of trailing-edge synthetic jets on longitudinal aerodynamic characteristics of a flying wing aircraft

For realizing the nice aerodynamic maneuverability of flying wing aircrafts (FWAs), a longitudinal aerodynamic control technology based on circulation control (CC) using trailing-edge synthetic jet actuators was proposed. Influences on longitudinal characteristics of a FWA were investigated. Results show that synthetic jets could improve the lift, drag, and nose-down moment, having potential of flight control at entire area of attack of angles (AOAs). Cl increment dips and then rises with the growth of AOAs, reaching the minimum at AOA of 12°. The maximum percentage of Cl enhancement and ΔCl/Cμ is separately 64.5% and 50.74%, respectively. Before 12°, synthetic jets could narrow the area of “dead zone,” improve flow velocities along the upper surface, and then move the trailing-edge separation point and the leading-edge (LE) stagnation point downward, enhancing the circulation. Moreover, synthetic jets grow rapidly through entrainment of the local flow, leading to the improvement of valid camber and, hence, the increase in Cl. CC efficiency decreases with the augmentation of AOAs, and leading-edge vortex (LEV) is weakened, causing the drop of Cl increment. After 12°, synthetic jets could enhance the longitudinal velocity of LEV and reduce the swirling number, improving the strength and stability of LEV, which results in larger suction near the leading edge. Moreover, the strengthened LEV could promote flow mixing and then weaken reverse pressure gradients along the wing section, thus improving flow velocities and CC efficiency at the wing section. It is above two factors that make Cl increment rise after 12°.

Boundary layer transition induced by low-speed synthetic jets

The effect of low-speed, circular synthetic jets (SJ) on the turbulent transition of a laminar boundary layer is studied through direct numerical simulations. The SJ capability in fixating the streamwise location of transition onset is analyzed in terms of its operation parameters (reduced frequency F+ and momentum coefficient Cμ). The effect of free-stream turbulence (FST) on the near-wall vortical structures generated by the synthetic jet is analyzed as well, to mimic the actual operation of the control system. Velocity spectra, phase portraits, and dynamic mode decomposition allow us to investigate flow unsteadiness and transition to a chaotic state. In most of the investigated cases, SJs successfully promote transition, as the result of varicose-symmetric hairpin-like vortices generated at the jet exits. In particular, it is found that increasing the momentum coefficient always reduces the size of the laminar region; a non-monotonic behavior of the laminar fetch is noted as the reduced frequency is increased, suggesting the existence of an optimal frequency value. Combination of FST and SJ actuation results in spanwise-asymmetric vortical structures, with little difference in the location of the transition onset as compared to the previous case. The present analysis can be used to gather information on the practical implementation of low-speed SJ actuators as active turbulators.

Plasma kernel model and energy transformation characteristic of plasma synthetic jet actuator

This article conducted a theoretical, experimental, and numerical investigation to clarify the plasma kernel model and energy transformation characteristics of the plasma synthetic jet actuator (PSJA). Plasma kernel and blast wave theory were used to describe the formation and evolution of the arc-discharge energy deposition process and build a plasma kernel model. Schlieren experiment visualized the formation and evolution of the synthetic jet flow and used it as a validation of the numerical simulation. Five plasma synthetic jet actuators with different cavity volumes (128–512 mm3) and different discharge energy (2.8–11.3 mJ) were modeled numerically to investigate the energy transformation characteristic of PSJA. Results showed that plasma kernel radius and formation time could be theoretically predicted with specific deposition energy and correspond well with simulation results. The peak pressure and temperature rise in the cavity can also be calculated. Moreover, the proportion of kinetic energy increases linearly with non-dimensional deposition energy, while potential energy has a reverse tendency with non-dimensional cavity volume and deposition energy.

Active control of flow past an elliptic cylinder using an artificial neural network trained by deep reinforcement learning

The active control of flow past an elliptical cylinder using the deep reinforcement learning (DRL) method is conducted. The axis ratio of the elliptical cylinder Γ varies from 1.2 to 2.0, and four angles of attack α = 0°, 15°, 30°, and 45° are taken into consideration for a fixed Reynolds number Re = 100. The mass flow rates of two synthetic jets imposed on different positions of the cylinder θ1 and θ2 are trained to control the flow. The optimal jet placement that achieves the highest drag reduction is determined for each case. For a low axis ratio ellipse, i.e., Γ = 1.2, the controlled results at α = 0° are similar to those for a circular cylinder with control jets applied at θ1 = 90° and θ2 = 270°. It is found that either applying the jets asymmetrically or increasing the angle of attack can achieve a higher drag reduction rate, which, however, is accompanied by increased fluctuation. The control jets elongate the vortex shedding, and reduce the pressure drop. Meanwhile, the flow topology is modified at a high angle of attack. For an ellipse with a relatively higher axis ratio, i.e., Γ ⩾ 1.6, the drag reduction is achieved for all the angles of attack studied. The larger the angle of attack is, the higher the drag reduction ratio is. The increased fluctuation in the drag coefficient under control is encountered, regardless of the position of the control jets. The control jets modify the flow topology by inducing an external vortex near the wall, causing the drag reduction. The results suggest that the DRL can learn an active control strategy for the present configuration.

Effects of dual synthetic jets on longitudinal aerodynamic characteristics of a flying wing layout

A novel method of flight control based on dual synthetic jets (DSJ) is proposed and applied to a large-sweep flying wing aircraft with a hybrid flow mode of leading-edge vortex (LEV) and spanwise separation. An array of dual synthetic jet actuators (DSJAs) is placed along the leading edge to explore its aerodynamic control characteristics and control mechanism. The comparison with synthetic jets (SJ) is implemented to show the differences. Results show that DSJ could delay stalling angle of 6° and promote Cl with greater increments at large attack of angles (AOAs). At AOA of 36°, there was an obvious increase of 23.12% in Cl. Cd decreases at small AOAs, while augments sharply when AOA is larger than 26°. Nose-up moment could be improved. Above features indicate that leading-edge DSJ is not only a nice method of lift enhancement but also an efficient way of flight control. In the fuselage section, DSJ has functions of strengthening LEV, delaying the LEV breakdown and even regenerating LEV at large AOAs, which enhances the bleeding and improves the ability of resisting the reverse pressure gradient, thus totally suppressing the separation and forming the larger leading-edge suction. In the wing section, DSJ acts as a “LEV generator”. Reconstructed LEV could interact with the initial spanwise separation, emerging a hybrid flow mode of “streamers” and “rollers”, in which flow energy could be augmented and separation is partly suppressed. The comparison with SJ supports that the larger leading-edge suction and stronger LEV could be achieved by DSJ, which generate greater increments of Cl and Cm. Maximum Cl increments achieved by DSJ and SJ are respectively 23.12% and 16.91% of Cl without control, meaning DSJ has the higher applying potential in flight control with the same energy consumption.

Numerical study of a liquid cooling device based on dual synthetic jets actuator

This study focuses on an efficient and innovative cooling technology to improve thermal performance of high heat flux electronics. This paper proposes a novel liquid cooling active heat dissipation device based on a dual synthetic jets actuator (DSJA). The heat transfer mechanism and flow mechanism of DSJA in the channel are studied through 3D CFD numerical simulation. Then the influence of inlet flow rate, jet orifice diameter, and diaphragm frequency on the cooling device are analyzed. Heat transfer and pressure drop are significantly influenced by the inlet flow rate when DSJA is off. DSJA has an excellent cooling capacity in the inlet flow range of 0.6–6 L/min after DSJ is on. The actuator with a water inlet turned on shows better heat transfer than the actuator without a water inlet. Increasing the diaphragm frequency from 15 Hz to 80 Hz enhances the heat transfer slightly and has little effect on pressure drop. The hydraulic thermal performance of DSJ before and after opening is 1.263, which verifies the high efficiency of DSJA for liquid cooling.

Characteristics of a Fluidic Oscillator with Low Frequency and Low Speed and Its Application to Stall Margin Improvement

Active flow control methods are commonly used in expanding the operating range of compressors. Indeed, unsteady active control methods are the main focus of researchers due to their effectiveness. For constructing an unsteady active control system, reliable actuators are significant. To compare with conventional actuators such as synthetic jet actuators and rotating valves, fluidic oscillators have structurally robust characteristics and can generate self-excited and self-sustained oscillating jets, which leads to its higher applicability in compressors under severe working conditions. Thus, to explore the feasibility of unsteady active control systems by the usage of fluidic oscillators, a low-frequency and low-speed oscillator is first designed and experimentally studied for improving the stability of a low-speed axial flow compressor. During the experiments, a special casing is designed to install 15 uniformly distributed oscillators in the tip region of compressor. Based on the unsteady micro injections of the rotor tip with rotor rotation frequency, the results indicate that the frequency/period of oscillators are flexible, in which the values are decoupled with the variation of inlet pressure. When the inlet-to-outlet pressure ratio of the oscillator is in the range of 1.1~2.0, the maximum velocity ranges from 30 m/s to 80 m/s. Moreover, the mass flow rate of the single oscillator only varies from 0.017‰ to 0.059‰ from the designed compressor mass flow rate. For the improvement of the compressor stall margin, the value is 3.45% when the total mass flow of oscillators is 0.08% of the designed compressor mass flow.

Lagrangian analysis on structure evolution and mass transport of circular and noncircular turbulent synthetic jets

Lagrangian analysis on structure evolution and mass transport of circular and noncircular turbulent synthetic jets

Local Heat Transfer Measurements for an Impinging Synthetic Jet

Abstract Research results demonstrate the heat transfer effectiveness of an impinging synthetic jet toward cooling a plane normal to it. The utility of the synthetic jet lies in that the supply of coolant comes from the device itself as an alternating jetting flow that emerges from a plenum followed by a sink flow that returns to that same plenum. Experiments reported herein were conducted with the synthetic jet driven by an oscillating diaphragm powered by a rotating cam to expel fluid from the plenum out of a single hole, then return it through the same hole. The frequency of diaphragm oscillation and the distance from the synthetic jet's orifice to the surface being cooled are varied in the test program to determine their effects on cooling performance. A numerical study agrees with the results given by the experiment and flow visualization utilizing a smoke generator supports the data and numerical results. The local, time-average Nusselt numbers were measured in the experiment using the thermochromic liquid crystal technique and air as coolant. The color display of each test case was recorded with a fisheye camera. In the case of the highest frequency and shortest distance from orifice to cooled plate, a Nusselt number of nearly 40 was achieved within the central region of the cooled plate when the Reynolds number based upon jet maximum velocity and orifice diameter was 7500 and the distance from the orifice to cooled plate was 3.2 orifice diameters.

Numerical simulation investigation on the suction stroke and blowing stroke of synthetic jet circulation control

Numerical simulation investigation on the suction stroke and blowing stroke of synthetic jet circulation control

Flow and heat transfer characteristics of single and multiple synthetic jets impingement cooling

In this paper, the comparison between synthetic jet actuator with single axisymmetric orifice and multiple axisymmetric orifices is presented. The aim of this paper is to compare the flow and heat transfer characteristics of single and multiple synthetic jet impingements cooling. The multiple orifice configurations had a central orifice and a satellite orifices placed in the pitch circle. The synthetic jet actuator had central orifice with 0, 3, 4, 5 and 6 orifices placed in the diameters of the relative pitch circle p/d = 2.5, 3 and 4. A total of 13 various geometric configurations were tested experimentally. In the multiple orifice configurations, the total cross-section area was approximately equal to the cross-section area of the single orifice configuration. The input power delivered to the synthetic jet actuator was maintained constant and equal to 3.00 W. The synthetic jet actuator energetic efficiency, Reynolds number, equivalent Reynolds number, radial and axial distributions of pressure coefficient, and heat transfer characteristic were presented. The synthetic jet issuing from multiple orifice plate merged at some axial distance, creating a single axisymmetric jet. The merging jet axial distances were determined for all investigated configurations. In the present paper, the integral quantities averaged over all orifices cross section area, or over impingement plate much larger than total orifices area, were analyzed for the purpose of objective comparison. From all various investigated parameters obtained from five different and independent experimental methods, the following conclusion may be made: if the total orifice cross section area in the case of single and multiple orifice synthetic jet is similar and real power deliver to the SJA is maintained at the same level the resulting equivalent Reynolds number and equivalent dimensionless stroke length achieved similar values and the impinging jet equivalent Nusselt number also achieved comparable values and distributions.

Large eddy simulation of temperature-variation effect of impinging planar lobed synthetic jet on flat plate and the semi-cylindrical concave plate

Temperature-variation becomes increasingly evident when an impinging synthetic jet is restricted and confined excessively by the target plate. However, the confinement effect of the target plate on the temperature-variation effect during impinging heat transfer by a synthetic jet has not been clarified. In this study, impinging heat transfer by a synthetic jet induced by a planar lobed orifice on a flat plate and a semi-cylindrical concave plate is investigated by means of Large Eddy Simulation. The planar lobed synthetic jet is characterized by the geometry of the lobed nozzle, and the velocity along its major diameter is larger than that along its minor diameter. As the mainstream jet moves downstream, the difference between the major and minor diameters decreases gradually. At dimensionless jet-to-surface distance of H/d = 8, the flow field attached to the flat plate along the major diameter is similar to that along the minor diameter. The results show that two types of mechanisms have induced the temperature-variation. For the flat plate, its temperature-variation comes from the suction of the wall jet near the stagnation region confined by the target plate, which becomes more conspicuous particularly at small dimensionless jet-to-surface distance. By contrast, the temperature-variation is less conspicuous at large dimensionless jet-to-surface distance because the inhaling region is distant from the target plate. For the concave plate, its temperature-variation is due to the large recirculation zone along the curvilinear direction. During the suction process of the cycle, the heated wall jet attached to the plate surface is re-inhaled into the cavity through the large recirculation zone, and the temperature of the air inside the cavity increases. During the ejection process of the cycle, the wall jet is entrapped into the jet mainstream and then returns to the target plate. Correspondingly, the mean velocity of the wall jet and the wall shear stress of the target plate are less than those of the flat plate. Time and frequency analyses had revealed the characteristics of inherent aggressive pulsation inside the planar lobed synthetic jet. The velocity in the primary frequency dominates the planar lobed synthetic jet, whereas higher-order velocities are negligible.

Investigations on the Interfering Factor of Single Synthetic Jet Actuator on Improving the Efficiency of Wing Control Surface

After the deflection of the wing control surface, flow separation is easily generated at the trailing edge of the wing, which will reduce the lift coefficient and the control surface efficiency. The rudder of the wing is aileron. If the lift generated by the wing is used to improve the efficiency of the control surface, the flow separation caused by the deflection of the control surface must be restrained. Using synthetic jet to change the flow state of boundary layer is the main method to solve the problem of flow separation. Synthetic jet actuator (SJA) has the advantages of no energy loss and simple structure. In this paper, a method of using synthetic jet actuator to suppress the flow separation at the rear of the wing when the aileron deflects is proposed, and the lift coefficient is obtained. The increase of aileron efficiency is calculated by the change of lift coefficient. The EPPLER555 wing with aileron deflection angle of 3°~9° is simulated, and the changes of lift coefficient and aileron efficiency under corresponding working conditions are obtained. The results show that the average lift coefficient of the wing is 0.5 when the deflection angle of the aileron is 3°~9° without SJA. After SJA employed, the lift coefficient will be greatly improved, and the control surface efficiency of EPPLER555 wing will be effectively improved, the lift coefficient will increase by about 20% to 0.6-0.7. For example, when the deflection angle of aileron is 4°, using a SJA with a maximum outlet velocity of 200m/s and an excitation frequency of 400/2π, the effective lift coefficient generated by the wing is 0.5931. Under the effect of SJA, the control surface efficiency of EPPLER555 wing will be effectively improved. The lift coefficient is reflected by the ratio of the change of lift coefficient after SJA employed to the lift coefficient without synthetic jet actuator.

Experimental study on airfoil flow separation control via an air-supplement plasma synthetic jet

An air-supplement plasma synthetic jet (PSJ) actuator increases the air supplemental volume in the recovery stage and improves the jet energy by attaching a check valve to the chamber of a conventional actuator. To explore the flow control effect and mechanism of the air-supplement actuator, via particle image velocimetry experiments in a low-speed wind tunnel, the flow field and boundary layer characteristics of a two-dimensional airfoil surface under different actuation states were compared for different attack angles and jet orifices. The experimental results show that, compared with the conventional actuation state, the jet energy of the air-supplement PSJ is higher and the indirect mixing effect of the counter-vortex sequence produced by the jet-mainstream interaction is stronger. Furthermore, the boundary layer mixing effect is better, which can further suppress flow separation and improve the critical flow separation attack angle. Moreover, increasing the jet momentum coefficient can enhance the flow control effect. The findings of this study could provide guidance for the flow control application of air-supplement PSJs.

A compacted non-pump self-circulation spray cooling system based on dual synthetic jet referring to the principle of two-phase loop thermosyphon

A traditional spray cooling system usually requires a pump to circulate the fluid, and the kinetic energy of high-temperature steam is wasted. A two-phase loop thermosyphon (TPLT) which can circulate its working fluid is thermally driven. However, heat leakage may occur for low heating power. This paper proposes a compacted non-pump self-circulation spray cooling system named active two-phase loop thermosyphon (ATPLT). Dual synthetic jet integrated with spray cooling (DSJS) is used to enhance the performance of evaporator of ATPLT. Without an external pump, the waste heat and evaporation of liquid spray drive the system internal pressure to increase, which pumps the water to the reservoir as well as avoid heat leakage. It only needs a little working fluid and a little energy consumption for actuator, but can maintain hundreds of Watts of heat dissipation capability for a long time, which can facilitate the development of the energy systems. The performance of ATPLT is studied through temperature, pressure, laser particle size and particle image velocimetry experimental researches. The experimental results show that the cooling capability of ATPLT is mainly influenced by Re, We and Ja, and finally a correlation for ATPLT cooling is established with relative errors within ±18%.