Convective Heat Transfer in an Impinging Synthetic Jet: A Numerical Investigation of a Canonical Geometry

2013 ◽  
Vol 135 (8) ◽  
Author(s):  
Luis A. Silva ◽  
Alfonso Ortega
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Stokes Equations ◽  
Heated Surface ◽  
Vortex Pair ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Navier Stokes ◽  
Scaling Analysis ◽  
Dependent Flow

Synthetic jets are generated by an equivalent inflow and outflow of fluid into a system. Even though such a jet creates no net mass flux, net positive momentum can be produced because the outflow momentum during the first half of the cycle is contained primarily in a vigorous vortex pair created at the orifice edges; whereas in the backstroke, the backflow momentum is weaker, despite the fact that mass is conserved. As a consequence of this, the approach can be potentially utilized for the impingement of a cooling fluid onto a heated surface. In previous studies, little attention has been given to the influence of the jet's origins; hence it has been difficult to find reproducible results that are independent of the jet apparatus or actuators utilized to create the jet. Furthermore, because of restrictions of the resonators used in typical actuators, previous investigations have not been able to independently isolate effects of jet frequency, amplitude, and Reynolds number. In the present study, a canonical geometry is presented, in order to study the flow and heat transfer of a purely oscillatory jet that is not influenced by the manner in which it is produced. The unsteady Navier–Stokes equations and the convection–diffusion equation were solved using a fully unsteady, two-dimensional finite volume approach in order to capture the complex time dependent flow field. A detailed analysis was performed on the correlation between the complex velocity field and the observed wall heat transfer. Scaling analysis of the governing equations was utilized to identify nondimensional groups and propose a correlation for the space-averaged and time-averaged Nusselt number. A fundamental frequency, in addition to the jet forcing frequency, was found, and was attributed to the coalescence of consecutive vortex pairs. In terms of time-averaged data, the merging of vortices led to lower heat transfer. Point to point correlations showed that the instantaneous local Nusselt number strongly correlates with the vertical velocity v although the spatial-temporal dependencies are not yet fully understood.

Numerical Simulation of Local Heat Transfer and Scaling of a Synthetic Impinging Jet in a Canonical Geometry

2011 ◽  
Author(s):  
Luis Silva ◽  
Alfonso Ortega
Keyword(s):  
Heat Transfer ◽  
Stokes Equations ◽  
Heated Surface ◽  
Vortex Pair ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Synthetic Jets ◽  
Navier Stokes ◽  
Dependent Flow ◽  
Finite Volume Approach

Synthetic jets are generated by an equivalent inflow and outflow of fluid into a system. Even though such a jet creates no net mass flux, net positive momentum can be produced because the outflow momentum during the first half of the cycle is contained primarily in a vigorous vortex pair created at the orifice edges whereas in the backstroke, the backflow momentum is weaker, despite the fact that mass is conserved. As a consequence of this, the approach can be potentially utilized for the impingement of a cooling fluid over a heated surface. In the present study, a canonical geometry is presented, in order to study the flow and heat transfer of a purely oscillatory jet that is not influenced by the manner in which it is produced. The unsteady Navier-Stokes equations and the convection-diffusion equation were solved using a fully unsteady, two-dimensional finite volume approach in order to capture the complex time dependent flow field. A detailed analysis was performed on the correlation between the complex velocity field and the observed wall heat transfer. A fundamental frequency, in addition to the jet forcing frequency, was found, and was attributed to the coalescence of consecutive vortex pairs. In some instances, this vortex pairing can lead to zones of low heat transfer. Two point correlations showed that the Nusselt number Nu, showed stronger correlation with the vertical velocity v although the spatial-temporal dependencies are not yet fully understood. It was found that the Reynolds number and the Strouhal number, are sufficient to successfully scale the problem at larger dimensions and this is presently being exploited in order to design validation experiments using jets large enough to allow careful local measurements.

CFD Analysis of the Vortex Dynamics Generated by a Synthetic Jet Impinging on a Heated Surface

2011 ◽  
Author(s):  
Luis Silva ◽  
Alfonso Ortega
Keyword(s):  
Heat Transfer ◽  
Stokes Equations ◽  
Heated Surface ◽  
Target Surface ◽  
Synthetic Jet ◽  
Navier Stokes ◽  
Secondary Vortex ◽  
Two Dimensional ◽  
Vortex Identification ◽  
Dependent Flow

A canonical geometry has been used to investigate the flow and heat transfer of a purely oscillatory jet that is not influenced by the manner in which it is produced. Such a jet has been popularly termed a synthetic jet in the literature, and recently has been investigated for thermal management of electronics by causing the jet to impinge onto the heated surface. Because of its oscillatory nature, the impinging jet thus formed is dominated by vortices that are advected towards the surface. This surface-vortex interaction is key to understanding the fundamental mechanisms of convective heat transfer by the impinging synthetic jet and hence is the subject of the current investigation. The unsteady two-dimensional Navier-Stokes equations and the convection-diffusion equation were solved using a fully unsteady, two-dimensional finite volume approach in order to capture the complex time dependent flow field. Various vortex identification methods were investigated for proper identification of the train of vortices emanating from the jet and their evolution and eventual dissipation. Intuitive definitions of vortices such as spiraling streamlines, pressure minima and isovorticity surfaces suffer from inaccuracies. In the present work, the vortex-identification criteria employed was the Q-criterion (Hunt et al. 1988), which defines vortices as connected fluid regions with positive second invariant of the velocity gradient tensor. By tracking vortices, it was found that a primary vortex advecting parallel to the target surface gives rise to a secondary vortex with opposite net vorticity. It was found that the secondary vortex is largely responsible for enhancement of the heat transfer within the wall jet region. In addition it was found that in some situations vortex coalescence or pairing occurs, leading to degradation in the heat transfer enhancement due to the reduction in the frequency of vortices interacting with the surface.

scholarly journals Effects of Frequency and Jet to Surface Spacing on Synthetic Jet Impingement Heat Transfer

2020 ◽  
Vol 9 (5) ◽  
pp. 655-660
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Heated Surface ◽  
Low Frequency ◽  
Jet Impingement ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Nozzle Geometry ◽  
Ansys Fluent ◽  
Piston Cylinder

Synthetic jet is a new technique for electronic chip cooling, which combines stagnant air to form a jet resulting from periodic diaphragm oscillations in a cavity. In this work, the heat transfer characteristics of a synthetic jet are investigated experimentally and numerically. A Piston-cylinder arrangement powers the synthetic jet through a circular orifice for the impingement of jet on the heated surface. Air is considered as the cooling medium. The major parameters identified to describe the impinging jet heat transfer are Reynolds number, frequency, ratio of jet spacing to diameter(Z/D) and nozzle geometry. Numerical studies have been carried out using the finite volume based commercial software ANSYS Fluent. The turbulent model used is k-ω model. The dimensionless distance between the nozzle and plate surface is in the range 2 to 16. Numerical results are in fair agreement with experimental results. As the frequency increases the average Nusselt number increases. High frequency synthetic jets were found to remove more heat than low frequency jets for small Z/D ratio, while low frequency jets are more effective at larger Z/D ratio. Nusselt number is maximum at the stagnation point and there occurs a secondary peak at lower Z/D ratios. Synthetic jet with rectangular orifice is more effective as compared to circular and square geometries.

Micro Fluidic Jets for Thermal Management of Electronics

Volume 4 ◽  
2004 ◽  
Author(s):  
Jivtesh Garg ◽  
Mehmet Arik ◽  
Stanton Weaver ◽  
Seyed Saddoughi
Keyword(s):  
Heat Transfer ◽  
Natural Convection ◽  
Heated Surface ◽  
Turbulent Jets ◽  
Harmonic Signal ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Small Scale ◽  
Air Cooling ◽  
Infrared Thermal Imaging

Micro fluidics devices are conventionally used for boundary layer control in many aerospace applications. Synthetic Jets are intense small scale turbulent jets formed from entrainment and expulsion of the fluid in which they are embedded. The idea of using synthetic jets in confined electronic cooling applications started in late 1990s. These micro fluidic devices offer very efficient, high magnitude direct air-cooling on the heated surface. A proprietary synthetic jet designed in General Electric Company was able to provide a maximum air velocity of 90 m/s from a 1.2 mm hydraulic diameter rectangular orifice. An experimental study for determining the thermal performance of a meso scale synthetic jet was carried out. The synthetic jets are driven by a time harmonic signal. During the experiments, the operating frequency for jets was set between 3 and 4.5 kHz. The resonance frequency for a particular jet was determined through the effect on the exit velocity magnitude. An infrared thermal imaging technique was used to acquire fine scale temperature measurements. A square heater with a surface area of 156 mm2 was used to mimic the hot component and extensive temperature maps were obtained. The parameters varied during the experiments were jet location, driving jet voltage, driving jet frequency and heater power. The output parameters were point wise temperatures (pixel size = 30 μm), and heat transfer enhancement over natural convection. A maximum of approximately 8 times enhancement over natural convection heat transfer was measured. The maximum coefficient of cooling performance obtained was approximately 6.6 due to the low power consumption of the synthetic jets.

Experimental Investigation of the Effect of Synthetic Jets in Minichannels With Subcooled Boiling Flow

2013 ◽  
Author(s):  
David M. Sykes ◽  
Andrew L. Carpenter ◽  
Gregory S. Cole
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Local Heat Transfer ◽  
Local Heat ◽  
High Heat ◽  
Synthetic Jet ◽  
Synthetic Jets ◽  
Transfer Coefficients ◽  
Heat Transfer Coefficients ◽  
Heat Transfer Augmentation

Microchannels and minichannels have been shown to have many potential applications for cooling high-heat-flux electronics over the past 3 decades. Synthetic jets can enhance minichannel performance by adding net momentum flux into a stream without adding mass flux. These jets are produced because of different flow patterns that emerge during the induction and expulsion stroke of a diaphragm, and when incorporated into minichannels can disrupt boundary layers and impinge on the far wall, leading to high heat transfer coefficients. Many researchers have examined the effects of synthetic jets in microchannels and minichannels with single-phase flows. The use of synthetic jets has been shown to augment local heat transfer coefficients by 2–3 times the value of steady flow conditions. In this investigation, local heat transfer coefficients and pressure loss in various operating regimes were experimentally measured. Experiments were conducted with a minichannel array containing embedded thermocouples to directly measure local wall temperatures. The experimental range extends from transitional to turbulent flows. Local wall temperature measurements indicate that increases of heat transfer coefficient of over 20% can occur directly below the synthetic jet with low exit qualities. In this study, the heat transfer augmentation by using synthetic jets was dictated by the momentum ratio of the synthetic jet to the bulk fluid flow. As local quality was increased, the heat transfer augmentation dropped from 23% to 10%. Surface tension variations had a large effect on the Nusselt number, while variations in inertial forces had a small effect on Nusselt number in this operating region.

Effect of Contact Angle on the Heat Transfer to an Evaporating Meniscus on a Moving Heated Surface

2005 ◽  
Author(s):  
A. Mukherjee ◽  
S. G. Kandlikar
Keyword(s):  
Heat Transfer ◽  
Contact Angle ◽  
Stokes Equations ◽  
Contact Region ◽  
Heated Surface ◽  
Contact Angles ◽  
Navier Stokes ◽  
Wall Heat Transfer ◽  
Moving Wall ◽  
Liquid Circulation

Numerical simulation is carried out to study a 2D evaporating meniscus formed on a moving wall. The complete Navier-Stokes equations along with continuity and energy equations are solved. The liquid vapor interface is captured using the level set technique. The meniscus is fed with saturated water from the top whereas the bottom wall is maintained at a higher temperature and is also imparted with a velocity. The meniscus attains a steady shape when all the incoming liquid gets evaporated due to heat transfer from the wall. The advancing and receding contact region of the meniscus are provided with different contact angles. Results indicate that the average heat flux at the meniscus base increases with increase in contact angle. The primary reason for heat transfer from the wall is attributed to the liquid circulation inside the meniscus and the corresponding transient conduction from the wall. As the meniscus contact angle increases the liquid circulation is found to disturb the thermal boundary layer more effectively thereby resulting in increased wall heat transfer. The effect of contact angle on wall heat transfer to the moving and evaporating meniscus is compared to partial nucleate pool boiling.

Effect of Heat Transfer on Magnetohydrodynamic Axisymmetric Flow Between Two Stretching Sheets

2010 ◽  
Vol 65 (11) ◽  
pp. 961-968 ◽  
Author(s):  
Tasawar Hayat ◽  
Muhammad Nawaz
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Stokes Equations ◽  
Axisymmetric Flow ◽  
Skin Friction Coefficient ◽  
Navier Stokes ◽  
Tabular Form ◽  
Analysis Method ◽  
Navier Stokes Equations ◽  
Homotopy Analysis

This investigation describes the effects of heat transfer on magnetohydrodynamic (MHD) axisymmetric flow of a viscous fluid between two radially stretching sheets. Navier-Stokes equations are transformed into the ordinary differential equations by utilizing similarity variables. Solution computations are presented by using the homotopy analysis method. The convergence of obtained solutions is checked. Skin friction coefficient and Nusselt number are given in tabular form. The dimensionless velocities and temperature are also analyzed for the pertinent parameters entering into the problem.

Modeling of Continuous and Intermittent Gas Jet Impingement and Heat Transfer on a Solid Surface

2001 ◽  
Author(s):  
A. K. Chaniotis ◽  
D. Poulikakos
Keyword(s):  
Heat Transfer ◽  
Stokes Equations ◽  
Heated Surface ◽  
Quantitative Description ◽  
Jet Impingement ◽  
Local Heat Transfer ◽  
Local Heat ◽  
Navier Stokes ◽  
Planar Jet ◽  
Particle Hydrodynamics

Abstract The present work focuses on the effect of flow pulsation on the characteristics of the planar jet impingement normally on a heated surface. Specifically, the influence of frequency, amplitude and Reynolds number of the jet is examined, concerning the instantaneous and time average convective heat transfer. The simulations are conducted using a novel, improved Smooth Particle Hydrodynamics (SPH) methodology that is based on particle discretization of the governing compressible Navier-Stokes equations. The simulation of jet impingement focuses on the quantitative description of the flow field and the energy exchange between jet and surface. The strong aerodynamic and thermal interaction that exists between the gaseous jet and the impingement surface greatly enhances the local heat transfer in the stagnation and wall jet regions as well as the average heat transfer over the surface. This study is the first step toward modeling the same process but in the presence of chemical reactions and ablation between the gaseous jet and the plate.

Heat Transfer due to the Impingement of an Axisymmetric Synthetic Jet Emanating From a Circular Orifice: A Numerical Investigation of a Canonical Geometry

2013 ◽  
Author(s):  
M. Ebrahim ◽  
L. Silva ◽  
A. Ortega
Keyword(s):  
Heat Transfer ◽  
Reynolds Number ◽  
Stokes Equations ◽  
Heated Surface ◽  
Three Dimensional ◽  
Jet Impingement ◽  
Synthetic Jet ◽  
Heated Wall ◽  
Driving Frequency ◽  
Surface Distance

Synthetic jets are produced by periodically injecting and ejecting fluid from an orifice. The mass flow rate is conserved in such a jet but net momentum flux is created due to the difference in the fluid dynamics at the orifice between the ejection and suction parts of each cycle. When pointed towards a heated surface, the synthetic jet can be used for cooling using the well-known advantages of jet impingement. In the present work, we have created a “canonical” jet in order to investigate the flow and heat transfer of a purely periodic synthetic jet which is not influenced by the manner in which it is generated. As such the “canonical” jet and the resulting heat transfer, can be considered to be dependent solely on the driving suction/ejection mechanisms at the orifice and thus can be examined independently of the actuator. The unsteady Navier-Stokes equations and the convection-diffusion equation were solved using a fully unsteady, laminar, three-dimensional axisymmetric finite volume approach in order to capture the complex time-dependent flow field created by different frequencies. The influence of jet-to-surface distance, Reynolds number, and driving frequency on heat transfer were investigated. Both stagnation and averaged Nusselt numbers were observed to be less dependent on frequency. Heat transfer was found to be higher at high Re numbers and low jet-to-surface distance. Results were compared with the steady continuous jet, experimental data of previous studies and the canonical slot synthetic jet at the same Reynolds number. A circular jet was found to be less efficient in removing heat over the heated wall than a slot synthetic jet.

Computational Analysis of Conjugate Heat Transfer in Gaseous Microchannels

2015 ◽  
Vol 137 (4) ◽  
Author(s):  
Giulio Croce ◽  
Olga Rovenskaya ◽  
Paola D'Agaro
Keyword(s):  
Heat Transfer ◽  
Nusselt Number ◽  
Mach Number ◽  
Heat Sink ◽  
Conjugate Heat Transfer ◽  
Stokes Equations ◽  
Heat Transfer Analysis ◽  
Navier Stokes ◽  
Flow Conditions ◽  
Navier Stokes Equations

A fully conjugate heat transfer analysis of gaseous flow in short microchannels is presented. Navier–Stokes equations, coupled with Maxwell and Smoluchowski slip and temperature jump boundary conditions, are used for numerical analysis. Results are presented in terms of Nusselt number, heat sink thermal resistance, and resulting wall temperature as well as Mach number profiles for different flow conditions. The comparative importance of wall conduction, rarefaction, and compressibility are discussed. It was found that compressibility plays a major role. Although a significant penalization in the Nusselt number, due to conjugate heat transfer effect, is observed even for a small value of solid conductivity, the performances in terms of heat sink efficiency are essentially a function only of the Mach number.

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