Hybrid Synthetic Jet Research Articles

A novel three-dimensional center energy deposition model for the hybrid synthetic jet actuator

The low energy conversion efficiency of the plasma synthetic jet actuator (PSJA) and the misfiring problem caused by the low gas backfill rate have hindered the application of the PSJA in supersonic aircraft for fast response aerodynamic control. Although the hybrid synthetic jet actuator (HSJA) based on the PSJA has the potential to solve these problems, the heat and mass transfer mechanism of hybrid synthetic jets is still unclear. According to the capacitive discharge process, a novel three-dimensional center energy deposition model was proposed in this paper. The accuracy of the three-dimensional center energy deposition model was verified by comparing it with the experimental results. Considering the damped oscillation of the discharge intensity, the effect of nonconstant center energy deposition on the heat and mass transfer inside the cavity was further discussed. The simulation results show that the electrodes have an impact on the creation and evolution of the vortices inside the cavity. And the three-dimensional nonconstant center energy deposition model reveals the real heat and mass transfer mechanism of the HSJA, which provides the theoretical foundation for improving energy conversion efficiency and gas backfill rate of the HSJA.

Active flow control for supersonic aircraft: A novel hybrid synthetic jet actuator

In order to improve the performance of traditional synthetic jet actuators in active flow control for supersonic aircraft, a novel millimeter-scale hybrid synthetic jet actuator (HSJA) is proposed in this paper. It provides additional power for the jet spout stage and gas refresh stage by adding a piezo-driven diaphragm at the bottom of the plasma synthetic jet actuator (PSJA). On the one hand, the problem of the low jet velocity of the piezo-driven synthetic jet actuator (PDSJA) could be avoided due to the heating and pressurization of gas by arc discharge. On the other hand, more external gas would be refilled during the gas refresh stage due to the restoration of the piezo-driven diaphragm compared with the plasma synthetic jet actuator. In an attempt to optimize the structure of the novel hybrid synthetic jet actuator, the flow field of the actuator is numerically simulated. The results show that the peak velocity of synthetic jet and the gas refilled rate are both increased compared with the plasma synthetic jet actuator. The effectiveness of the proposed hybrid synthetic jet actuator is verified.

Impingement Cooling Performance Analysis and Improvement of Synthetic Jets for Electronic Devices

With the development of miniaturization, high power density, and a high degree of integration of electronic devices, heat dissipation has become a major factor constraining its development. As an active flow control technology, synthetic jets can significantly enhance heat transfer and cooling ability. In this paper, an unsteady numerical model of heat transfer is established to explore the impingement cooling performance of synthetic jets. The effect of nozzle-to-surface distance on the cooling performance is analyzed. The results show that the cooling ability of synthetic jets first increases and then decreases with an increasing nozzle-to-surface distance. At small nozzle-to-surface distances, there is a significant recirculation phenomenon of hot air, leading to a notable deterioration of heat transfer and cooling performance. To solve this issue at small nozzle-to-surface distances, a hybrid synthetic jet based on a fluid diode is designed. The further simulation results show that the proposed hybrid synthetic jet can effectively avoid recirculation of hot air and significantly improve the impingement cooling ability at small nozzle-to-surface distances. The temperature of the target surface under the action of the proposed hybrid synthetic jet is decreased by about 10°C in comparison with the conventional synthetic jet without fluid diodes.

Cooling performance improvement of impingement hybrid synthetic jets in a confined space with the aid of a fluid diode

For the conventional synthetic jet in a confined space, the recirculation of heated air at a small nozzle-to-surface spacing results in deterioration of its impingement cooling ability. To overcome this drawback, various novel fluid diodes are designed and introduced to form hybrid synthetic jets in this paper. Numerical analyses are carried out to reveal the cooling performance enhancement mechanism of the hybrid synthetic jets with the aid of fluid diodes. Instantaneous images of velocity contour and streamline at one cycle are comparatively presented to elaborate on their flow characteristics. Based on the simulation results, the volumetric efficiency, air temperature at the nozzle exit, average temperature on the heated surface and average Nusselt number are further calculated to quantify the improvements of the proposed hybrid synthetic jets compared to the conventional synthetic jet. The hybrid synthetic jet with a convergence nozzle generates the best cooling ability among various configurations. This study provides significant guidance for the design of novel hybrid synthetic jets with high-efficiency impingement cooling performance.

Energetic efficiencies of synthetic and hybrid synthetic jet actuators driven by electrodynamic transducers

The paper investigates synthetic and hybrid synthetic jet actuators that are based on similar geometries. The actuators are supplied by a harmonic power input and are operated close to the first resonance frequency. An energetic efficiency is applied. The maximal outlet characteristic velocities and efficiencies of both actuators are studied as functions of the input real power. Theoretical approximations of these functions were derived for the synthetic jet actuator and successfully compared with experimental hot-wire results.The paper demonstrates a usefulness of the applied definition of the energetic efficiency. Moreover, the advantages of the hybrid synthetic jet actuator were quantified. The hybrid synthetic jet actuator achieves a 15–23% increase in the velocities and a 41–48% increase in the energetic efficiency, both in comparison with the ordinary synthetic jet actuator working at the same input real power.

Numerical study of nozzle design for the hybrid synthetic jet actuator

This paper presents a numerical study on the influence of the geometrical arrangement of the hybrid synthetic jet (HSJ) actuators. We study five variants of the HSJ actuators. The laboratory experiment by using water as the working fluid has been conducted for comparison and validation of the numerical simulation. The numerical results reveal the formation and propagation of vortices during the operating cycle in detail. It is identified that the internal vortex within the HSJ actuator plays essential roles on the flow mass exchange during the operating cycle. Our results show that the flow pattern of synthetic jet is highly sensitive to the internal geometrical design, which leads to significant difference in efficiency for different actuators.

Impingement heat/mass transfer to hybrid synthetic jets and other reversible pulsating jets

This study focused on round, hybrid synthetic (non-zero-net-mass-flux) jets impinging on a wall. To complete this study, two additional variants of reversible pulsating jets were investigated, namely synthetic (zero-net-mass-flux) jets and mixed pulsed jets (pulsed jets containing an additional blowing component). For comparison purposes, the continuous jet was used. The working fluid was air. The Reynolds numbers ranged from 4000 to 6000, the dimensionless stroke lengths were 14–16, and the dimensionless orifice-to-wall distances were 2–16 (both related to the orifice exit diameter of 8mm). The experiments used flow visualization, single-sensor hot-wire measurements, and mass transfer measurements on the wall using the naphthalene sublimation technique. The local heat transfer coefficient, expressed as a Nusselt number, was evaluated using the heat/mass transfer analogy.All tested jets exhibited relatively flat, nearly top-hat velocity profiles. An increase of the Reynolds number by an additional blowing component resulted in a heat/mass transfer enhancement. The flow oscillations (for the present geometry and driven parameters) caused a heat/mass transfer enhancement of 12–40%.The main outcome was that the 26% larger flow rate of the hybrid synthetic jet, versus the conventional synthetic jet, resulted in an 18% increase in the heat transfer rate. These gains were caused by a partial rectification effect of incorporated fluidic diodes, without consuming any additional energy and without introducing movable parts.

Visualization study of hybrid synthetic jets

A synthetic jet (SJ) is a fluid flow that is created from an oscillatory process of suction and blowing. A hybrid synthetic jet (HSJ) combines this principle with fluidic pumping through a valveless pump. The present study addresses round HSJs issuing into quiescent surroundings from an actuator orifice 8 mm in diameter. For comparison purposes, a common (zero-net-mass-flux) SJ is used. The working fluid is air, and the maximum Reynolds numbers are 11,000 and 9,000 for HSJs and SJs, respectively. The following five experimental methods are employed: flow visualization using a smoke wire technique, velocity measurements using a hot-wire anemometer, velocity measurements using a Pitot tube, impedance measurements of the actuators, and measurements of the jet momentum using precision scales. Flow visualization demonstrates phase-locked flow fields. The first resonance frequencies are theoretically derived to be 79 and 98 Hz for an SJ and HSJ, respectively. These values are confirmed by all of the experimental methods used. The results demonstrate the advantages of HSJs. The tested HSJ achieves a 24 % higher pumped volume flow rate in comparison to the SJ at a maximum volumetric efficiency of 33 %. Moreover, the overall energy efficiency of the HSJ actuator is 1.8 times higher than that of the SJ actuator. These promising HSJ features, including significantly higher efficiencies, can be useful for various heat transfer applications such as the cooling of highly loaded electronic devices.

Impinging jet-based fluidic diodes for hybrid synthetic jet actuators

A new variant of a fluidic diode for hybrid synthetic jet actuators (HSJAs) is introduced in this paper, namely the diode in the form of a conical duct. The periodic jet flow from the diode impinges on the wall and the resulting flow differences during blowing and suction increase the volumetric efficiency of the actuator. Two alternative definitions of the volumetric efficiency are used. The HSJs velocities and volumetric efficiencies were experimentally evaluated. The experiments were performed using a phase-locked smoke visualization and hot-wire anemometry with air as the working fluid. The diode-to-wall distance and the driving frequency were varied, and the combination of parameters that provided the highest HSJ velocity and highest volumetric efficiency was obtained.

A new method for fluid input into a hybrid synthetic jet actuator

A new principle of flow rectification for hybrid synthetic jet actuators is introduced in this paper. As is well known, the flow rectification can be best accomplished by means of fluidic diodes. Novelty of the present study are fluidic diodes with two mutually opposed nozzles. Interaction between the periodic jet flows from the nozzles causes a difference between the blowing and suction strokes, resulting in a particularly efficient rectification effect. The distance between the nozzle exits as well as the oscillation frequency were the parameters, which were varied during hot-wire measurements. The combination of those parameters achieving the highest volumetric effciency was identified.

Comparison of double-acting and single-acting synthetic jets

This paper deals with an experimental comparison of double-acting synthetic jets (DASJs) with commonly used single-acting synthetic jets (SASJs). The DASJ is a fluid jet resulted from interactions of two pulsatile jets operating in anti-phase, i.e., a central round SASJ and an annular hybrid synthetic jet in coaxial arrangement. The experiments were performed by using water as the working fluid with actuation frequencies from 2Hz to 12Hz. The phase-locked measurements by means of particle image velocimetry (PIV) have shown the basic difference of the time-averaged flow-field and demonstrated the characteristic changes in a cycle of synthetic jet. The velocity and vorticity fields of DASJ and SASJ are compared. It shows that the former has in general higher vorticity (more than two times) and narrower jet width (only 43.7%) of the latter. A saddle point on the jet axis exists at the distance about one diameter of central nozzle of the actuator, i.e., the flow velocity is always positive for x/Dc>1. In short, DASJ has revealed good potential with significant vorticity enhancement for the design of synthetic jet in heat transfer applications.

Novel Fluidic Diode for Hybrid Synthetic Jet Actuator

This paper deals with a new design of a hybrid synthetic jet actuator (HSJA), which is based on a novel fluidic diode. Two fluidic diodes were tested using pressure-drop measurements with air as the working fluid, and their diodicities were evaluated. A greater diodicity was achieved with the new diode design. Two outlet nozzles of the HSJA were tested (shorter and longer), and the velocity resonance curves were evaluated using hot-wire measurements at the outlets of the nozzles. Volumetric efficiency of the HSJA was evaluated as function of the operating frequency. The greatest efficiency was achieved at the second resonant frequency of the actuator with the longer nozzle.

Acoustic compressor coupled with fluidic diodes

Performance of an acoustic compressor coupled with a fluidic diode is studied. The acoustic compressor works on the idea of Resonant Macrosonic Synthesis (RMS) technology demonstrated by Lawrenson et al. The main idea is to replace non-return valves by no moving part fluidic device. Fluidic diode rectifies an oscillatory flow analogous to rectifying an electric AC to obtain DC output. Synthetic jets analogous to AC current falls in the category of jet-driven acoustic streaming, which has a zero mean mass flux. The synthetic jet can be combined with no-moving part fluidic device to generate Hybrid Synthetic Jets with non zero mean mass flux. Non-zero mass flow rate was achieved by coupling fluidic diode and the resonator. In the present study, better rectification is achieved by having number of fluidic diodes in series. Experiments were done for the case of 3 and 4 diodes in series. In the case of three diodes in series the mass flow was 4.6 l/min and in case of four diodes it was 4.9 l/min. Full paper will present the pressure and mass flow measurements for 1 to 5 diodes in series. Thus, RMS cavities with fluid diodes can work as a pump.

Effective hydraulic resistance of a nozzle in an electrodynamic actuator generating hybrid-synthetic jet – Part I: Data acquisition

Hybrid-synthetic jet actuators combine the advantages of synthetic jets and steady jets in flow control applications. For the design–especially of no-moving-part fluidic actuator versions – it is important to know the properties of the jet generating nozzle. Its effective time-mean hydraulic resistance with superposed oscillatory flow is higher, sometimes substantially, than in the steady flow. In a recent publication [18], a law governing this increase was derived from experimental data obtained with piston-type actuator at small oscillation amplitudes (smaller than – or at most equal to - the steady flow component). In the present paper, the investigations are extended to the large-amplitude conditions, with nozzle flow reversal in a part of the cycle - and to the case of an electrodynamic actuator with compliant membrane.

Effective hydraulic resistance of a nozzle generating hybrid-synthetic jet in an electrodynamic actuator – Part II: Analysis and correlations

Due to nonlinearity of their characteristics, nozzles exhibit an increase in time-mean pressure drop across them if an alternating flow component is superposed on the basic steady flow. In a recent publication (Tesař [15]) a law governing this increase was derived from experimental data. The validity of this law is limited to pulsation amplitudes small enough to avoid velocity reversals. In the present paper, consisting of two parts, investigations were extended beyond this small-amplitude limit. Experimental data and characterisation parameters used in processing were discussed in Part I. Now this Part II presents analysis and derivation of a universal correlation, useful for engineering purposes.

Two forward-flow regimes in actuator nozzles with large-amplitude pulsation

Extensive systematic investigations were made of pulsatile flows in a wide range of frequencies (extending over more than a decimal order) in a small nozzle of simple shape and size representative of actuator nozzles generating hybrid-synthetic jets – i.e. with return flow into the nozzle during a part of the cycle. In a synergetic approach, data from hot-wire anemometer measurements are supported and explained by results of numerical flowfield computations. Several effects not previously mentioned in literature were discovered. Particularly interesting among them is the existence of two different characters of the flowfield inside the nozzle in the forward-flow part of the period. In the transition between them, the initial annular-flow dominated configuration changes at a later stage into central-dominated flow.

Hybrid Liquid Immersion and Synthetic Jet Heat Sink for Cooling 3-D Stacked Electronics

This paper focuses on the design and parametric numerical study of a hybrid heat sink combining a liquid thermal interface with an array of synthetic jet actuators for 3-D chip stack cooling. The air-side heat sink exploits enhanced localized heat transfer achieved via a central array of synthetic jet actuators. The key focus of this paper is the numerical simulation of the dielectric liquid interface used to efficiently transmit the heat from the high-power 3-D stacked electronics to the hybrid heat sink base. The coupled natural convection in the fluid and conduction in solid spreaders sandwiched between the tiers of the stack form a novel efficient, passive, and scalable thermal management solution for 3-D stacked die structures. It is shown that this heat sink with a footprint of 76-mm square × 51-mm height can dissipate a total of 41 W of heat/power from the stack for a 44°C average chip temperature rise above ambient (an Rja of ~ 1.06 K/W obtained passively).

Mechanism of pressure recovery in jet-type actuators

Jet-type actuators generating synthetic jets became recently very important for control of flow past bodies. In contrast to mechanical pulsators usually used in them, a number of advantages especially for the hybrid-synthetic jet versions bring fluidic no-moving-part valves. The paper presents an example of such a valve used for flow separation control on a wing wind tunnel model. Usual fluidic valves have large and long collector diffusers, which cause problems with stowage inside the model but so far were considered necessary for the required pressure recovery. Somewhat surprisingly, this study demonstrates that a substantial amount of the pressure recovery actually takes place before the flow enters the diffuser. This is an important information for actuator designers, who – it seems – have so far concentrated their attention in valve development and improvement at a wrong part of the devices.

Quasi-similarity model of synthetic jets

Actuators generating synthetic jets (and related hybrid-synthetic jets) are currently of increasing importance and it is useful to have for their design a simple model evaluating spatial distributions of various quantities of interest in the generated jet. In this paper, such a model – a set o simple ordinary differential equations – is derived and shown to describe synthetic jets remarkably well. It uses the Tollmien hypothesis of the length scale of the vortices proportional to the local jet diameter. The experiments show that for the best fit the proportionality multiplier should vary with the streamwise distance X 1 * from the nozzle. This variation leads to the new concept of quasi-similarity. The model was tested by application to synthetic jet data obtained at large distances X 1 * where an interesting change occurs in the character of the dependence.

Enhancing impinging jet heat or mass transfer by fluidically generated flow pulsation

Periodic pulsation of high intensity generated by a no-moving-part fluidic oscillator located upstream from jet generating nozzles was demonstrated to enhance heating (or cooling and, because of the analogy of governing equations, also drying) by impinging hybrid-synthetic jets. The test setup was built for an envisaged application in food processing by baking or roasting. It has two nozzles, each connected to one of the two outlets of the diverter-type oscillator. The nozzles were of annular exit cross-section and operated in anti-parallel (outflow from one nozzle simultaneous with suction into the other nozzle). The investigation, focused on study of the temperature fields on a plane impingement wall visualised by thermochromic liquid crystals, revealed some unexpected effects.