Mark T. North

Heat and Mass Transport in Heat Pipe Wick Structures

temperature field,areobtainedfortheporouswicksundertheactionofadiscreteheatsource(evaporator)mounted on one end. The working fluid, supplied from a condenser pool, evaporates from the wick surface primarily in the evaporator region and is condensed and collected into a container separate from the pool, to yield mass flow rates. Thus the liquid-pumping capability of the wick, coupled with flow impedance, is measured as a function of applied heat flux.Repeatableresultswithlowuncertaintyareobtained.Acarefulanalysisofthetransportpathsforheatand masstransferin thewickstructure confirmsthatmasstransfer duetovaporization oftheworking fluidisthelargest contributor to heat dissipation from the wick. The expected and measured values of evaporation rate are in good agreement. Results are also presented in terms of overall effective conductance based on measured temperatures.

Modeling and Design Optimization of Ultrathin Vapor Chambers for High Heat Flux Applications

Passive phase-change thermal spreaders, such as vapor chambers have been widely employed to spread the heat from small-scale high-flux heat sources to larger areas. In this paper, a numerical model for ultrathin vapor chambers has been developed, which is suitable for reliable prediction of the operation at high heat fluxes and small scales. The effects of boiling in the wick structure on the thermal performance are modeled, and the model predictions are compared with experiments on custom-fabricated vapor chamber devices. The working fluid for the vapor chamber is water and a condenser side temperature range of 293 K-333 K is considered. The model predictions agree reasonably well with experimental measurements and reveal the input parameters to which thermal resistance and vapor chamber capillary limit are most sensitive. The vapor space in the ultrathin devices offers significant thermal and flow resistances when the vapor core thickness is in the range of 0.2 mm-0.4 mm. The performance of a 1-mm-thick vapor chamber is optimized by studying the variation of thermal resistance and total flow pressure drop as functions of the wick and vapor core thicknesses. The wick thickness is varied from 0.05 to 0.25 mm. Based on the minimization of a performance cost function comprising the device thermal resistance and flow pressure drop, it is concluded that the thinnest wick structures (0.05 mm) are optimal for applications with heat fluxes below 50 W/cm2 , while a moderate wick thickness of 0.1 mm performs best at higher heat flux inputs 50 (>;W/cm2).

Development of Micro/Nano Engineered Wick-Based Passive Heat Spreaders for Thermal Management of High Power Electronic Devices

Spreading of high-flux electronics heat is a critical part of any packaging design. This need is particularly profound in advanced devices where the dissipated heat fluxes have been driven well over 100W/cm2 . To address this challenge, researchers at Raytheon, Thermacore and Purdue are engaged in the development and characterization of a low resistance, coefficient of thermal expansion (CTE)-matched multi-chip vapor chamber heat spreader, which utilizes capillary driven two-phase heat transport. The vapor chamber technology under development overcomes the limitations of state-of-the-art approaches by combining scaled-down sintered Cu powder and nanostructured materials in the vapor chamber wick to achieve low thermal resistance. Cu-coated vertically aligned carbon nanotubes is the nanostructure of choice in this development. Unique design and construction techniques are employed to achieve CTE-matching with a variety of device and packaging materials in a low-profile form-factor. This paper describes the materials, design, construction and characterization of these vapor chambers. Results from experiments conducted using a unique high-heat flux capable 1DSS test facility are presented, exploring the effects of various microscopic wick configurations, CNT-functionalizations and fluid charges on thermal performance. The impacts of evaporator wick patterning, CNT evaporator functionalization and CNT condenser functionalization on performance are assessed and compared to monolithic Cu wick configurations. Thermal performance is explained as a function of applied heat flux and temperature through the identification of dominant component thermal resistances and heat transfer mechanisms. Finally, thermal performance results are compared to an equivalent solid conductor heat spreader, demonstrating a >40% reduction in thermal resistance. These results indicate great promise for the use of such novel vapor chamber technology in thickness-constrained high heat flux device packaging applications.Copyright

Enhancing Heat Transfer of Air-Cooled Heat Sinks Using Piezoelectrically-Driven Agitators and Synthetic Jets

Air-cooled heat sinks prevail in microelectronics cooling due to their high reliability, low cost, and simplicity. But, their heat transfer performance must be enhanced if they are to compete for high-flux applications with liquid or phase-change cooling. Piezoelectrically-driven agitators and synthetic jets have been reported as good options in enhancing heat transfer of surfaces close to them. This study proposes that agitators and synthetic jets be integrated within air-cooled heat sinks to significantly raise heat transfer performance. A proposed integrated heat sink has been investigated experimentally and with CFD simulations in a single channel heat sink geometry with an agitator and two arrays of synthetic jets. The single channel unit is a precursor to a full scale, multichannel array. The agitator and the jet arrays are separately driven by three piezoelectric stacks at their individual resonant frequencies. The experiments show that the combination of the agitator and synthetic jets raises the heat transfer coefficient of the heat sink by 80%, compared with channel flow only. The 3D computations show similar enhancement and agree well with the experiments. The numerical simulations attribute the heat transfer enhancement to the additional air movement generated by the oscillatory motion of the agitator and the pulsating flow from the synthetic jets. The component studies reveal that the heat transfer enhancement by the agitator is significant on the fin side and base surfaces and the synthetic jets are most effective on the fin tips.Copyright

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

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

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 1.4 mm achieved a low thermal resistance of 0.053 C/W in a channel with a 7.9 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

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

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

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

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

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

A Computational Study of Active Heat Transfer Enhancement of Air-Cooled Heat Sinks by Actuated Plates

Heat transfer performance of air-cooled heat sinks must be improved to meet thermal management requirements of microelectronic devices. The present paper addresses this need by putting actuated plates into channels of a heat sink so that heat transfer is enhanced by the agitation and unsteadiness they generate. A proof-of-concept exercise was computationally conducted in a single channel consisting of one base surface, two fin wall surfaces, and an adiabatic fourth wall, with an actuated plate within the channel. Air flows through the channel, and the actuated plate generates periodic motion in a transverse direction to the air flow and to the fin surface. Turbulence is generated along the tip of the actuated plate due to its periodical motion, resulting in substantial heat transfer enhancement in the channel. Heat transfer is enhanced by 61% by agitating operation for a representative situation. Translational operation of the plate induces 33% more heat transfer than a corresponding flapping operation. Heat transfer on the base surface increases sharply as the gap distance between it and the plate tip decreases, while heat transfer on the fin wall surface is insensitive to the tip gap. Heat transfer in the channel increases linearly with increases of amplitude or frequency. The primary operational parameter to the problem is the product of amplitude and frequency, with amplitude being slightly more influential than frequency. The analysis shows that the proposed method can be used for modern levels of chip heat flux in an air-cooled model forestalling transition to liquid or phase-change cooling.Copyright