Synthetic Jet Flow Research Articles

Enhancing heat transfer with a synthetic jet for thermal management applications

AbstractThis article explores the utilization of a synthetic jet as an approach to cool microelectronic devices, addressing their thermal management needs. The study includes both experimental measurements and numerical simulations to gain a comprehensive understanding of the heat transfer characteristics and fluid flow patterns generated by the synthetic jet actuator. The average Nusselt number (Nu) of the synthetic jet impinging flow with the dimensionless separation distances of the orifice to the heated surface (H/D) is investigated at different Reynolds numbers. A dynamic mesh scheme is employed in performing the simulations of the fluid domain to showcase the diaphragm's vibration and its deformation over time. The velocity profiles exhibit that the synthetic jet flow prompts the formation of two countervortices during every vibrating cycle of the diaphragm. The experimental results align closely with the predicted outcomes, indicating that the synthetic jet significantly enhances heat transfer by 3.1 times relative to the natural convection in the case of (H/D = 8.4) across different Reynolds numbers while maintaining low power consumption, a compact size, and a noise‐free operation.

A numerical study on the effect of rib size and location on thermo-fluid performance of synthetic jet

A two-dimensional, axisymmetric numerical model is used to study the effect of synthetic jet flow structure and thermal performance, validated with experimental data. The Reynolds number is set at 5473, and orifice-to-surface spacing (z/d) varies (2, 6, 10). Three square cross-section ribs at different radial positions are analyzed. Results show that ribs reduce heat transfer at the stagnation point, with a maximum decrease of 74.37% at z/d=10 for a 5mm×5mm rib at r/d=0.5 and a minimum of 47.70% at z/d=6 for the same rib. Heat transfer increases near the rib's leading edge but drops before and after the rib. No area-averaged heat transfer improvement is observed, with a maximum reduction of 69.24% at z/d=10 for a rib at r/d=2 and a minimum of 2.43% at z/d=6 for a rib at r/d=1. Stagnation pressure increases by 24.42% at z/d=2 and decreases by 15.66% at z/d=10 due to a rib at r/d=2.

Experimental investigation of the heat transfer characteristics of a synthetic annular jet impingement on a flat surface

ABSTRACT Annular impinging jets create a more uniform flow on the impact surface compared to circular impinging jets, allowing the surface to cool better. Additionally, periodic flow oscillations significantly increase heat transfer by reducing the thermal resistance on the surface. Therefore, this study experimentally investigated the heat transfer characteristics of a synthetic annular jet impinging on a flat surface with constant heat flux. In the experiments, the jet-target surface distance (H/D), jet Reynolds number (Re j), oscillation amplitude (A o), and Womersley number (Wo) were changed. In contrast, the Prandtl number (Pr) and other geometric parameters were kept constant. The effects of these parameters on heat transfer were analyzed and the results were compared with continuous circular and annular impinging jets. Local temperature values on the target surface were obtained for different parameters and heat transfer from the surface was calculated. Experimental results showed that heat transfer increased with decreasing H/D ratio for all jet types. The highest heat transfer on the surface was achieved in synthetic jet flow. Heat transfer increased as the oscillation amplitude decreased. It was observed that there is a specific value for the Womersley number (Wo = 94) and that the heat transfer decreases after this value. For Re = 50000 and H/D = 2, the annular jet provided 27% higher heat transfer than the circular jet. In synthetic jet flow, heat transfer at H/D = 2 was improved by 21% compared to H/D = 8 for Re = 6000, A o = 1.87 and Wo = 163.

Visualizing three-dimensional effects of synthetic jet flow control

Visualizing three-dimensional effects of synthetic jet flow control

A computational study of particle dynamics in synthetic jet flow field

This computational study aims to investigate the transport and dispersion of solid particles in a synthetic jet flow field. Solid spherical particles were injected into the jet at a single point, and their behavior was observed until they left the domain. The interaction between the particles and the vortex rings in the jet was compared for synthetic jets, with different Strouhal numbers (0.05, 0.1, and 0.2). The Reynolds number (Re = 150), the mass of injected particles, particle diameter distribution, and particle Stokes number at injection were kept constant. The results showed that the jet's momentum mainly transported the particles, while the vorticity of the vortex rings caused them to spread. The particle Stokes number affected the distribution of particles within the jet, with a higher Stokes number particle concentrating at the jet's center and lower Stokes number particles being carried within the vortex ring. For the low Strouhal number jet, the combined effect of the fast decline in the particle Stokes number, low vorticity of vortex ring, significant distance between successive vortices, and high jet momentum resulted in a higher spread of particles and rapid transport across the domain. As the Strouhal number increased, the particle Stokes number decreased slowly downstream; moreover, the vorticity of the vortex ring increased, the distance between vortices decreased, and low momentum imparted by the jet led to less particle spread and slower transport. Overall, the spread and transport of particles were most effective in the synthetic jet, with the low Strouhal number.

The flow and heat transfer behavior of synthetic jets with star shaped orifice of different lobes

In the present experimental investigation, the flow and heat transfer characteristics of synthetic jet (SJ) issuing from different lobed (3–10) star-shaped orifices are studied. Tests are performed by actuating the SJ actuator at constant input power of 4Vrms, with varied range of orifice to surface spacings (1–16) and numerous values of Reynolds number (8692–10746). The synthetic jet's flow characteristics are studied using hot-wire anemometry, while the thermal characteristics are analyzed using an IR camera. Distinct peaks in turbulence intensity are observed in major and minor planes for different lobed (3–6) star orifices; this may be due to the development of two shear layers in such orifices, which consequently merges into a single shear layer for higher lobed (8–10) star orifice. The 6-lobed star orifice is found to exhibit maximum heat transfer performance at z/d varying between 2 and 14. At z/d = 2, the 6-lobed star orifice exhibit 7.6% and 10.1% improvement in average Nusselt number compared to circular and 10-lobed star orifice respectively, which may be due to formation of vortical structures along the lobed edges and subsequent switching of axes between the major and minor planes. To comprehend the energy associated with the issuing vortices, the power spectral density of the velocity signal at the orifice exit is examined. A unified correlation for the average Nusselt number is also derived for various star-shaped orifices.

Policy transfer of reinforcement learning-based flow control: From two- to three-dimensional environment

In the current paper, the zero-mass synthetic jet flow control combined with a proximal policy optimization (PPO) algorithm in deep reinforcement learning is constructed, and a policy transfer strategy which is trained in two-dimensional (2D) environment and migrated to three-dimensional (3D) environment is proposed and analyzed. By policy, we mean the flow control strategy of the agent learned by interacting with environment through deep reinforcement learning (DRL) algorithm. Through comprehensive evaluations of vortex separation in the cylindrical boundary layer and wake region at different Reynolds (Re) numbers, the PPO model trained in the 2D environment can reduce the drag coefficient by approximately 6.3%, 18.6%, and 23.7% at Re = 100, 200, and 300, respectively, when the spanwise length of the 3D environment is equal to the cylinder's diameter. Moreover, when the spanwise length is three times the diameter, the drag reduction capability is about 5.8%, 15.4%, and 13.1% at the three Re numbers, respectively. Additionally, the PPO model trained in the 2D environment also demonstrated outstanding migration learning capability in a new 3D flow field environment with varying Re numbers, successfully suppressing vortex shedding and reducing drag coefficient. Furthermore, the results illustrate that the model trained at high Re numbers could still reduce the drag coefficient in the 3D environment with low Re numbers, while the model trained at low Re numbers was not as effective at achieving drag reduction in the environments under high Re numbers. Overall, the proposed policy transfer strategy has been proven to be an effective method applying DRL agent trained in 2D flow to a new 3D environment.

Entrainment mechanism of turbulent synthetic jet flow

Synthetic jets have received extensive attention due to their superior mixing property. However, its mechanisms have not been investigated from the perspective of the turbulent/non-turbulent interface (TNTI). To shed new light on this issue, the entrainment and TNTI properties of a synthetic jet are experimentally investigated and compared with a continuous jet at $Re_j = 3150$ . The fuzzy clustering method is applied to select an appropriate vorticity threshold to detect the TNTI. Statistically, it is revealed that the entrainment coefficients of the two jets significantly differ in the near field, while they become almost identical in the far field. Instead of the vortex ring, the ‘breakdown of the vortex ring’ enhances the entrainment in the present synthetic jet. Instantaneously, the TNTI more violently fluctuates in the near field of the synthetic jet, which leads to a larger fluctuation in the TNTI radial position, a higher fractal dimension and enhanced local entrainment. Moreover, the transition of the probability density function of the TNTI orientation from the unimodal distribution in the near field to the bimodal distribution in the far field is found in both jets. The multi-scale analysis reveals a new mechanism for the bimodal distribution caused by the TNTI-thickness-scale structures.

Synthetic jet flow driven by a standing-wave thermoacoustic heat engine

This paper presents an experimental investigation of the formation of synthetic jets into external quiescent environment for cooling purposes using a thermoacoustic engine which converts heat into acoustic oscillations. Thermoacoustic technology is proposed in this study as an option for enhancing the cooling effect in equipment normally relying on natural convection. For this purpose, a standing-wave thermoacoustic heat engine was modelled and built and its ability to generate synthetic jet flow structures as well as their propagation were examined. It was found that the engine is able to drive synthetic jets through 10 orifices at a temperature difference less than 150 °C. The working medium is CO2 at ambient pressure. In experiments, the performance of thermoacoustic engine was evaluated in terms of jet velocity and the required temperature difference across the stack by varying the heating power.

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.

Effect of orifice shape on impinging synthetic jet

The present experimental study investigated the synthetic jet flow characteristics issued from four different orifice shapes (Re = 4739–5588). The flow dynamics of the jet is examined using the particle image velocimetry technique for circular, rectangular, square, and elliptical orifice shapes and two different surface spacings (z/d = 3 and 8). The results are presented in terms of streamwise distribution of velocity, normal stress, and shear stress. Furthermore, the most dominant modes of higher energy containing structures are obtained using proper orthogonal decomposition and compared for different orifice shapes. The results show that for a lower nozzle to surface spacing, vortices formed in the wall jet from the elliptical orifice contain higher energy than the other orifice shapes. The higher energy-containing vortices cause a delay in attaining self-similarity. Therefore, the self-similarity in the wall jet for the elliptical orifice is delayed than that of the other orifice shapes. Also, the elliptical orifice shape has relatively higher normal and shear stresses than that of other orifice shapes. The elliptical orifice shows 30% and 17% higher crosswise normal and shear stress than that of the circular orifice, respectively. However, for the larger nozzle to surface spacing, the least dominant mode of the structure is observed for the rectangular orifice shape, which results in a shifting of the self-similarity location toward the stagnation point. The findings from the present work are used to explain the variation of the heat transfer rate from a synthetic jet having different orifice shapes and impinging at different surface spacings.

The interactions of a circular synthetic jet with a turbulent crossflow

The three-dimensional flow of a circular synthetic jet interacting with a turbulent crossflow is investigated with unsteady Reynold-Averaged Navier–Stokes simulations. The effects of jet momentum are examined using three blowing ratios (CB=0.32, 0.67, and 1.10) at constant actuation frequency (f=300 Hz), approach crossflow Reynolds number, Reθ=900, and boundary layer thickness, δ/d=7.25, where d is the jet diameter. The results showed that the expelled jet is accompanied by a reverse flow region on the downstream side which undergoes alternating expansion and contraction during the jet cycle. The size of the reverse flow region and depth of penetration increased with increasing jet momentum. For the low momentum jet, the expelled flow structure evolved into a hairpin vortex which significantly enhanced the wall shear stress in the spanwise direction and near the jet exit. The higher momentum jets, on the other hand, exhibited strong vortex loops around the expelled jet column that transitioned into trailing vortex pairs and a tilted vortex ring further downstream. Along the wall, both horseshoe vortex upstream of the jet exit and tertiary vortices downstream were found attached to the wall. The strong tertiary vortices promoted downwash of fluids which significantly enhanced the wall shear stress along the symmetry plane for the medium and high momentum jets. The impact of the tertiary vortices increased with the jet momentum, offering greater potential for flow separation control.

Influence of secondary flow with a Coanda surface on the direction of jets

Abstract We propose a method for fluidic thrust vectoring by studying the effect of excited secondary Coanda flow on the direction of jets. The primary flow is the steady continuous jet, while the the secondary flow is synthetic jet or continuous flow. In this experiment, speaker is used to adjusted the frequency and velocity amplitude of synthetic jets, while a blower is used to adjust the continuous jet and suction flow.We visualize and compare the primary flow under the influence of various secondary flows. Additionally, computational fluid dynamics is used to investigate the flow characteristics, including the deflection angle. The main results of this study is that both synthetic jets and continuous suction flow are capable of deflection.

On the topology patterns and symmetry breaking in two planar synthetic jets

This article studies the flow structures and main patterns driving the flow dynamics in one and two planar synthetic jets. We perform numerical simulations at different Reynolds numbers (Re), for a similar forcing frequency, to model the synthetic jet flow and the two planar synthetic jets, which present a movement in-phase (synchronous jets) and out-of-phase (asynchronous jets). We identify two types of flow regimes as function of the Reynolds number: (i) the flow is symmetric and (ii) the symmetry is broken at Re ≈110 and Re ≈140 for the single and the synchronous jets, respectively. On the contrary, the flow is always asymmetric in the two asynchronous jets. We calculate the thrust produced by the several jet configurations, finding that the thrust produced by a single jet is always half of the thrust produced by the two synchronous jets; however, this quantity is much smaller in the asynchronous jets. Finally, we use higher order dynamic mode decomposition to identify the main patterns driving the flow dynamics. The solution is periodic in the single and two synchronous jets, with the forcing frequency (St*) as the dominant mode. The emerging rise in amplitude of a low-frequency mode (St0 = St*/6) that sub-harmonic of the forcing frequency as the Reynolds number increases suggests a connection between this mode and the symmetry breaking. A new mode is identified in the asynchronous jets, breaking the flow periodicity.

A Critical Review on Flow and Heat Transfer Characteristics of Synthetic Jet

A Critical Review on Flow and Heat Transfer Characteristics of Synthetic Jet

Computational and Experimental Study of Heat Transfer on the heat sink with an impinging synthetic jet under Various Excitation Wave

This work presents an investigation of an impinging synthetic jet produced by a continuous movement of an oscillating piezoelectric membrane in a synthetic jet actuator (SJA). The jet stream blows from the top of the heat sink attached to an electrically heated mat, and is used to enhance heat transfer to simulate a microelectronic component cooling system. Experimental measurements and numerical simulation were conducted to elucidate the distribution of forced convective heat transfer. The membrane's movement was set to produce a synthetic jet for a complete cycle to investigate flow dynamics in suction and blowing streams. The purpose of this research is to characterize synthetic jet membrane vibrations using sinusoidal, square, and triangular wave excitation modes to find which mode is best for vibrating the synthetic jet membrane. The membrane was excited with various functions, including sinusoidal, triangular, and square waveforms with frequency of 80 Hz, 120 Hz, and 160 Hz. The synthetic jet flow simulation utilized a commercial CFD software, FLUENT and DesignModeler to generate an element meshing with total mesh volume of 537.422. The amplitude for all membrane waves is assumed to be 2 mm/s. A k-ω Shear Stress Transport turbulence model was used to complete the simulation. A user-defined function model was developed to drive the piezoelectric membrane using various sinusoidal and non-sinusoidal excitation waves. Measurements were conducted using thermocouples with an automated data acquisition system to obtain temperature data and derive heat transfer coefficients.The conclusion from this research is that the 120 Hz square wave excitation mode produces the best heat transfer value. This research can contribute to the development of synthetic jet models for the microelectronic component cooling system industry.

Model-free short-term fluid dynamics estimator with a deep 3D-convolutional neural network

Deep learning models are not yet fully applied to fluid dynamics predictions, while they are the state-of-the-art solution in many other areas i.e. video and language processing, finance, robotics . Prediction problems on high-dimensional, complex dynamical systems require deep learning models devised to avoid overfitting while maintaining the required model complexity. In this work we present a deep learning prediction model based on a combination of 3D convolutional layers and a low-dimensional intermediate representation that is specifically designed to forecast the future states of this type of dynamical systems. The model predicts p future velocity-field time-slices (samples) based on k past samples from a training dataset consisting of a synthetic jet in transitional regime. The complexity of this flow is characterized by two topology patterns that are periodically changing, making this flow as a suitable example to test the performance of deep learning models to predict time states in complex flows. Moreover, the wide number of applications of synthetic jets (i.e.: fluid mixing, heat transfer enhancement, flow control), points out this example as a reference for future applications, where modeling synthetic jet flows with a reduced computational effort is needed. This work additionally opens up research opportunities for other areas that also operate with complex and high-dimensional time-series data: future frame video prediction, network traffic forecasting, network intrusion detection .The proposed model is presented in detail. A comprehensive analysis of the results is provided. The results are based on a strict validation strategy to ensure its generalization. The model offers an average symmetric mean absolute error (sMAPE) and a relative root mean square error (RRMSE) of 1.068 and 0.026 respectively (one order of magnitude improvement over low-rank approximation tools), using 10 past samples and predicting 6 future samples of a two-dimensional velocity field on a 70x50 point matrix associated to a synthetic jet dataset.

An experimental and numerical study on heat transfer enhancement of a heat sink fin by synthetic jet impingement

Compared to traditional continuous jets, synthetic jets (jets with oscillatory flow such that the time-average velocity is zero) have specific advantages, such as lower power requirement, simpler structure and the ability to produce an unsteady turbulent flow that is known to be effective in augmenting heat transfer. This study presents experimental and computational results that document heat transfer coefficients associated with impinging a synthetic jet flow onto the tip region of a longitudinal fin used in an electronics cooling system. The effects of different parameters, such as amplitude and frequency of diaphragm movement and jet-to-cooled-surface spacing, are recorded. The computational results show a good match with experimental results. In the experiments, an actual-scale (1 mm jet orifice) system is introduced and, for finer spatial resolution and improved control over geometric and operational conditions, a large-scale mock-up (44 mm jet orifice) is applied in a dynamically-similar way, then tested. Results of the experiments at the two scales, combined with the computational results, describe fin heat transfer coefficients on and near the jet impingement stagnation point. A linear relationship for heat transfer coefficient versus frequency of diaphragm movement is shown. Heat transfer coefficient values as high as 650 W/m2K are obtained with high-frequency diaphragm movement. Cases with different orifice shapes show how jet impingement cooling performance changes with orifice shape.

Research on Heat Dissipation Enhancement of Venturi Effect Based on Synthetic Jet Technology

Convection is the main way of cooling and heat dissipation of mechanical devices. Because of its easy realization and high heat efficiency, convection is widely used in various electronic integrated devices or other purely mechanized devices. However, for some occasions with narrow space and high thermal radiation frequency, convective heat dissipation is often difficult to achieve good results. In this paper, the synthetic jet mode of dual mode double cavity resonance is studied, and combined with the Venturi tube, the flow of synthetic jet is supplemented by the Venturi effect to enhance the convection frequency and flow rate. Through the CFD optimization design and experimental verification, the Venturi tube outlet flow can be increased by 16.7%, and it has wide application occasions and good heat dissipation benefits.

Experimental investigation on the performance of a novel dual synthetic jet actuator-based atomization device

A synthetic jet actuator is a zero net mass flux device. In order to improve the compactness of spray cooling system, a novel dual synthetic jet actuator-based atomization device (DSJAA) is proposed and designed. In this atomization system, water is atomized by the oscillating dual synthetic jets instead of high-pressure air source. Moreover, no extra power for supplying water is needed. The effects of driving voltage, driving frequency, fluid volumetric flow rate and spray height are investigated on atomization rate, water droplet size, surface temperature and heat flux. The maximum atomization rate is 3.4 L/h at driving frequency of 700 Hz and driving voltage of 210 V. The dimensionless correlations between the physical properties of dual synthetic jet and the size of water droplet are presented, which provides guideline for the design of a dual synthetic jet actuator-based fluid atomizer. The experimental results show that when the resonance frequency of piezoelectric diaphragm is equal to half the Helmholtz frequency of DSJAA, the atomization performance is the best by setting driving frequency to resonance frequency. The best spray height is equal to the fluid stroke length of DSJAA. The cooling capability is mainly influenced by the Reynolds number of dual synthetic jet and volumetric flow rate of water. A correlation for DSJAA cooling is established. A maximum heat flux of the novel spray cooling method reaches as high as 59 W/cm2 in the experiments.