Showing papers by "Terrence W. Simon published in 2012"

High frequency, large displacement, and low power consumption piezoelectric translational actuator based on an oval loop shell

Abstract: Compact piezoelectric actuators based on an oval loop shell structure were fabricated and their vibration characteristics were investigated. The actuators can successfully create a translational motion at a high frequency and a large displacement working distance at the second resonant mode of the shell structure. As a result of a shot peening process to harden the surface and increase the strength of the shell structures, fatigue limits were enhanced. The highest operating frequency of 1444 Hz was achieved with about 1.5 mm translational displacement under an applied voltage of 100 VAC. The largest amplified displacement of 2.1 mm was obtained at a resonant frequency of 961 Hz. Displacement amplification ratios between static and resonance conditions are presented and compared. A theoretical approach was provided to estimate the natural frequencies of the oval loop shell actuators. The estimated natural frequencies of the actuators agreed with experimental values to within 12%. In addition, load bearing capacity and efficiency of one of the shell actuators was evaluated with an experimental method. The calculated actuator efficiency is around 55% when 3.1 g of mass is loaded to the actuator and an applied voltage of 140 V is applied. A possible application of the actuator, a cooling device, was demonstrated by providing its configuration and test results.

Hot embossing at viscous state to enhance filling process for complex polymer structures

Abstract: In this paper, a new hot embossing process, molding at the viscous state, for fabrication of complex polymer structures at the micro and millimeter scale is presented. Polymer deformability is enhanced due to its low viscosity and is increased by an inner pressure from confinement of the polymer flow. Various millimeter-scale polymer structures with high aspect ratios and complex features were hot embossed. In addition, typical microstructures were achieved. This new approach promises the advantages of a broad process capability and strong compatibility with conventional hot embossing processes.

Heat Transfer in a Long, Thin Tube Section of an Air Compressor: An Empirical Correlation from CFD and a Thermodynamic Modeling

Abstract: Heat transfer during compression of air in a long, thin tube is studied by CFD. The tube represents one of the many in a honeycomb geometry inserted in a liquid piston air compressor to minimize temperature rise. A dimensionless number for the heat flow rate that includes the changing heat transfer area between the tube wall and air during compression is used. From the CFD results, alinear relation between the inverse of this dimensionless heat flow rate and the Stanton number is found. Using thisrelation, the transient volume-averaged temperature, and heat flow rate from the air can be well predicted by thermodynamic modeling.With the heat transfer model, a non-linear ODE is solved numerically todetermine the average temperature and pressure. The application of this study can be found in liquid piston air compressors for compressed air energy storage systems.Copyright © 2012 by ASME

Storage Power and Efficiency Analysis Based on CFD for Air Compressors Used for Compressed Air Energy Storage

Abstract: The compression process in a piston cylinder devic e in a Compressed Air Energy Storage (CAES) system is studied computationally. Twelve different cases featuring f our different compression space length-to-radius aspect ratios an d three different Reynolds numbers are studied computationally using the commercial CFD code ANSYS FLUENT. The solutions show that for compression with a constant velocity, the compression can be approximated by a polytropic pressure vs. volume relation. The polytropic exponent, �, characterizes the heat transfer and temperature rise of the air being compressed. For the cases computed, it varies from 1.124 to 1.3 05 and is found to be more affected by Reynolds number and less by the length-to-radius ratio. Since the efficiency and st orage power of the compressor depend on pressure vs. volume trajec tory during compression, they are written as functions of the p ressure rise ratio and the polytropic exponent, �. The efficiency is high at the beginning of the compression process, and decre ases as the compression proceeds. The effect of temperature ris e, or heat transfer, on efficiency and storage power is shown by comparing the efficiency and storage power vs. volume curves having

Microfabrication of short pin fins on heat sink surfaces to augment heat transfer performance

Abstract: Plate-fin heat sinks have been a successful technology in electronics cooling. Thermal performance of such heat sinks, however, has been driven to improve due to increasing heat generation in modern electronics devices. This paper proposes to introduce short pin fins on surfaces of plate-fin heat sinks to address such challenges. A microfabrication approach based on photolithography and electroplating technologies is devised to fabricate short copper pin fins on copper plates. The photolithography implements desired patterns of pin fins, and the electroplating enables pin fins to directly grow out of the base plate. A series of pin-fin coupons were fabricated using the devised method. A heat transfer test was designed to evaluate heat transfer augmentation by the pin fins. Fabricated coupons were tested in a rectangular channel and their thermal conductance and channel pressure drop were measured. A Design of Experiments (DoE) procedure via the Taguchi method was employed to find the influence of four factors: pin-fin height, diameter, spacing, and cross sectional shape, on the combination of thermal conductance and channel pressure drop for the coupons of different pin-fin parameters. Compared with similar plain coupons, pin-fin coupons of the best design parameters increase the thermal conductance by 78.3 % with only 7.8% increase of channel pressure drop. The devised micro-pin-fin fabrication has been proved as an effective approach to augmenting heat transfer of air-cooled plate-fin heat sinks.

An Active Heat Sink System With Piezoelectric Translational Agitators and Micro Pin Fin Arrays

Abstract: Conventional heat sink systems with blowers or fans are approaching maximum thermal management capability due to dramatically increased heat dissipation from the chips of high power electronics In order to increase thermal performance of air-cooled heat sink systems, more active or passive cooling components are continually being considered One technique is to agitate of the flow in the heat sinks to replace or aid conventional blowers In the present study, an active heat sink system that is coupled with a piezoelectric translational agitator and micro pin fin arrays on the heat sink surfaces is considered The piezoelectric translational agitator generates high frequency and large displacement motion to a blade It is driven by an oval loop shell that amplifies the small displacement of the piezo stack actuator to the several-millimeter range The blade, made of carbon fiber composite, is easily extended to a multiple-blade system without adding much mass The micro pin fin arrays were created with the LIGA photolithography technique The cooling performance of the heat sink system was demonstrated in single-channel and multiple-channel test facilities The singlechannel test results show that the active heat sink with the agitator operating at a frequency of 686 Hz and peak-to-peak displacement of 14 mm achieved a low thermal resistance of 0053 C/W in a channel with a 79 m/sec flow velocity Different configurations of the translational agitator with multiple blades were fabricated and tested in a 26-channel, full-size heat sink Vibrational characteristics are also provided© 2012 ASME

Convective Heat Transfer Enhancement With Micro Pin-Fin Surfaces Cooled by a Piezoelectrically-Driven Translational Agitator

Abstract: Air cooling of electronic equipment continues to hold many advantages over liquid cooling in terms of simplicity, reliability, cost, etc. Many active and passive air cooling techniques have been developed to meet the thermal challenges of modern, high-power electronics. Active cooling includes such features as piezoelectric flapping fans and synthetic jets that could directly break down and thin the thermal boundary layers on heated surfaces. A microchannel bank of fins, micro pin-fin surfaces, etc. are passive methods for increasing heat transfer area. In the current study, both active and passive methods, piezoelectric translational agitators and micro pin fin arrays, are employed to dramatically enhance convective heat transfer rates. A piezoelectric stack actuator coupled with an oval loop shell displacement amplifier was utilized to generate high-frequency and large-displacement translational agitation over the micro pin fin surface. Two different micro pin-fin surfaces were fabricated using copper and the LIGA process. Heat transfer experiments were performed in a single channel that houses a one-sided, heated surface with attached micro pin fins. The piezoelectric translational agitator oscillates at a high frequency of 596 Hz with a large displacement of up to 1.8 mm. The heat transfer coefficients on the micro pin-fin surface cooled by the agitator and various channel through-flows were compared with those of plain surfaces under the same channel flow rates. A maximum improvement of 222% in the heat transfer rate was achieved when the agitator was operated, the micro pin-fin surface was in place and the channel flow velocity was 11.6 m/sec, compared to that of a non-agitated plain surface case with the same flow rate.Copyright © 2012 by ASME

An Experimental Study on the Effects of Agitation in Generating Flow Unsteadiness and Enhancing Convective Heat Transfer

Abstract: Agitation is produced inside a channel by a plate that is periodically oscillating normal to the channel side walls. The test channel is a rectangular cavity open on one end to allow inflow and outflow of air, as driven by the plate movement. Heat transfer and velocity measurements are made within different regions of the channel to study the effectiveness of agitation in promoting heat transfer from the channel side wall. The purpose of agitation is to strongly mix the near-wall flow, to thin the thermal boundary layer and increase the convective heat transfer coefficient. Velocity measurements using laser Doppler velocimetry are made to document the fluctuations of velocity within the agitated cavity. Variations of ensemble-averaged velocity throughout the cycle identify the unsteady sloshing of the flow. Cycle-to-cycle variations about the ensemble mean computed as an RMS and resolved in time within the cycle period present the changing turbulence levels throughout the agitation cycle. The ensemble-averaged mean velocity variations show periods of acceleration, deceleration and flow reversal during a cycle as a result of agitator movement. Turbulence is found to increase toward the end of the acceleration phase and persist through the deceleration phase. Intensities of sloshing and turbulence are used to explain the measured convective heat transfer coefficients. ANSYS FLUENT simulations supply velocity contours and flow visualization. This study finds application in electronics cooling where agitation can be used inside air-cooled heat sinks to enhance heat transfer to through-flow driven by a fan.Copyright © 2012 by ASME

Comparison of Heat Transfer Enhancement by Actuated Plates in Heat-Sink Channels

Abstract: This paper investigates heat transfer enhancement of an air-cooled plate-fin heat sink by introducing actively-driven agitating plates within its channels. The investigation was computationally conducted with a single actuated plate in a single channel constructed as two fin wall surfaces and one fin base surface. As air flows through the channel, the plate is vibrated transversely to agitate the channel flow and thereby enhance heat transfer. The channel flow and the actuated plate are considered to be driven by a fan and a piezoelectric stack, respectively. A Coefficient of Performance (COP), ratio of total heat dissipated from the fin channel to total electric power to drive the fan and the agitator plate, is employed to evaluate overall heat transfer enhancement. A short plate, i.e. a plate is only placed at the entrance of the channel, has been shown to possess higher COP than a longer plate, i.e. a plate that is extended to be over most of the channel. For the short plate, COP is higher when it is actuated than when it is stationary. Detailed turbulence-kinetic-energy contours indicate that the higher COPs are due to turbulence generated along the plate edges and streamwise acceleration and deceleration of the bulk channel flow; both are induced by the vibration of the plate. Within regions where the plate is present, the generated turbulence and the acceleration and deceleration augment heat transfer. For a short plate, the turbulence and unsteadiness are transported downstream of the actuated plate to increase heat transfer in that region. However, such turbulence and unsteadiness are drawn out of the channel without full benefit of agitation and heat transfer enhancement when the plate is long, as the plate’s trailing edge is already close to the channel exit. This leads to a conclusion that the short plate is a better choice for active heat transfer enhancement.Copyright © 2012 by ASME

Fluid Damping and Power Consumption of Active Devices Used in Cooling Electronics

Abstract: Active devices, such as synthetic jets and oscillating plate agitators were found to be effective in cooling of high-heat-flux electronics. These devices generate unsteady flows, thinning the thermal boundary layer and enhancing turbulent transport. However, the active devices cause extra power consumption due to flow friction and separation. It is important to understand the factors influencing power consumption in these devices if they are to be applied in cooling system designs. The present study analyzes fluid damping and power consumption in high-frequency (about 1000 Hz) synthetic jets and oscillating plate agitators driven by piezoelectric stacks. This analysis is done numerically, since it is difficult to measure fluid damping. In the simulations, the moving part of the active device is modeled with the moving wall boundary condition. The mesh is updated and the flow is calculated every time the moving part changes its position. The coherent vortex structures generated by theses active devices, like vortices in the synthetic jet cavity or in the oscillating plate tip gap region, are found to cause fluid damping and power consumption. Fluidic power consumption levels with different geometries and different operating frequencies and amplitudes are studied. A correlation is developed to predict fluidic power consumption at different operating conditions.Copyright © 2012 by ASME

Heat Transfer Enhancement by Synthetic Jet Arrays in Air-Cooled Heat Sinks for Use in Electronics Cooling

Abstract: A synthetic jet is an intermittent jet which issues through an orifice from a closed cavity over half of an oscillation cycle. Over the other half, the flow is drawn back through the same orifice into the cavity as a sink flow. The flow is driven by an oscillating diaphragm, which is one wall of the cavity. Synthetic jets are widely used for heat transfer enhancement since they are effective in disturbing and thinning thermal boundary layers on surfaces being cooled. They do so by creating an intermittently-impinging flow and by carrying to the hot surface turbulence generated by breakdown of the shear layer at the jet edge. The present study documents experimentally and computationally heat transfer performance of an array of synthetic jets used in a heat sink designed for cooling of electronics. This heat sink is comprised of a series of longitudinal fins which constitute walls of parallel channels. In the present design, the synthetic jet flow impinges on the tips of the fins. In the experiment, one channel of a 20-channel heat sink is tested. A second flow, perpendicular to the jet flow, passes through the channel, drawn by a vacuum system. Surface- and time-averaged heat transfer coefficients for the channel are measured, first with just the channel flow active then with the synthetic jets added. The purpose is to assess heat transfer enhancement realized by the synthetic jets. The multiple synthetic jets are driven by a single diaphragm which, in turn, is activated by a piezoelectrically-driven mechanism. The operating frequency of the jets is 1250 Hz with a cycle-maximum jet velocity of 50 m/s, as measured with a miniature hot-film anemometer probe. In the computational portion of the present paper, diaphragm movement is driven by a piston, simulating the experimental conditions. The flow is computed with a dynamic mesh using the commercial software package ANSYS FLUENT. Computed heat transfer coefficients show a good match with experimental values giving a maximum difference of less than 10%. The effects of amplitude and frequency of the diaphragm motion are documented. Changes in heat transfer due to interactions between the synthetic jet flow and the channel flow are documented in cases of differing channel flow velocities as well as differing jet operating conditions. Heat transfer enhancement obtained by activating the synthetic jets can be as large as 300% when the channel flow is of a low velocity compared to the synthetic jet peak velocity (as low as 4 m/s in the present study).Copyright © 2012 by ASME

Noise Measurements and Reduction for High-Frequency Vibrating Devices in the Application of Cooling Electronics

Abstract: Traditional heat sinks for electronics cooling have become ever more difficult to design to meet the high dissipation rate of modern high-heat-flux electronics. Active devices, especially devices operating at a high frequency, show promise toward enhancing heat transfer performance. However, active devices generate noise that may not be acceptable to personnel. The present work studies acoustic characteristics of piezoelectrically-driven synthetic jets and oscillating plate agitators operating at high frequency (around 1000 Hz) employed in an electronics cooling module for heat transfer enhancement purposes. The A-weighted noise level from such actuators is measured and found to increase with increases of driving voltage and operational frequency. The measured sound pressure level of the active devices used in our present enhanced heat transfer module can be as high as 100 dB. Through a power spectrum analysis, we find that most acoustic energy is in a narrow frequency band close to the operating frequency of the active device. To decrease the noise level, a muffler, which also allows cooling air to recirculate through the equipment cabinet, has been designed and tested. An analytical model is employed to select the geometry of the muffler for optimal performance based on acoustic characteristics of the active devices and the through-flow pressure drop. The muffler having this optimal design is fabricated and tested and found to be able to decrease the noise level generated by two actuators from 83 dB to 64 dB.Copyright © 2012 by ASME

Development of Synthetic Jet Arrays for Heat Transfer Enhancement in Air-Cooled Heat Sinks for Electronics Cooling

Abstract: Synthetic jet arrays driven by a piston-diaphragm structure with a translational motion were fabricated. A piezo-bow actuator generating large translational displacements at a high working frequency was used to drive the jets. Vibration analysis with a laser vibrometer shows the peak-to-peak displacement of the piston inside the jet cavity of about 0.5 mm at the second resonant vibrational frequency of 1,240 Hz. In this driving condition, the peak velocity of a 20-orifice jet array reaches 45 m/s for each orifice with a total power consumption of 1.6 W. Heat transfer performance of the jet array was tested on a 100-mm-long single channel of a 26-channel heat sink. The synthetic jet flow impinges on the tips of the fins. A cross flow through the channel enters from the two ends of the channel, and exits from the middle. Results show that the activation of jets generates a unit-average heat transfer enhancement of 9.3% when operating with a channel flow velocity of 14.7 m/s, and 23.1% when operating with a channel flow velocity of 8 m/s. The effects of various choices for orifice configuration and different dimensionless distances from the fin tips, z/d, on jet performance were evaluated. By decreasing the length of the fin channel from 100 mm to 89 mm and reducing the orifice number of the jet array from 20 to 18, jet peak velocities of about 54 m/s can be obtained with the same power consumption, and a heat transfer enhancement of 31.0% from the jets can be achieved on the 89-mm-long heat sink channel with a flow velocity of 8 m/s.Copyright © 2012 by ASME