From Chip to Cooling Tower Data Center Modeling:

The chiller cooled data center environment consists of many interlinked elements that are usually treated as individual components. This chain of components and their influences on each other must be considered in determining the benefits of any data center design and operational strategies seeking to improve efficiency, such as temperature controlled fan algorithms. Using the models previously developed by the authors, this paper extends the analysis to include the electronics within the rack through considering the processor heat sink temperature. This has allowed determination of the influence of various cooling strategies on the data center coefficient of performance. The strategy of increasing inlet aisle temperature is examined in some detail and found not to be a robust methodology for improving the overall energy performance of the data center, while tight temperature controls at the chip level consistently provide better performance, yielding more computing per watt of cooling power. These findings are of strong practical relevance for the design of fan control algorithms at the rack level and general operational strategies in data centers. Finally, the impact of heat sink thermal resistance is considered, and the potential data center efficiency gains from improved heat sink designs are discussed

HT2008-56195 AN ANALYSIS OF THE FLOW FIELDS WITHIN GEOMETRICALLY-SIMILAR MINIATURE SCALE CENTRIFUGAL PUMPS

ABSTRACT Thermal management has become a key constraint in the development of contemporary electronics systems. It is evident that heat fluxes are currently approaching the limits of conventional forced air cooling, and that liquid cooling technologies are now under consideration. As the space available to incorporate a pump is often limited, miniaturescale pumps are required. Because such pumps operate at low Reynolds numbers, their operation may deviate from that predicted from the conventional pump scaling laws, and their efficiencies reduced. This paper investigates such deviations, and reduced efficiency, through experimental measurements of the performance of two geometrically-similar pumps of -a fully scaled pump of diameter 34.3mm, and a half scale version of the same construction. A facility for the measurement of bulk pressure-flow performance characteristics is described. Particle Image Velocimetry (PIV) measured velocity profiles were extracted at varying radii in the blade passages, and at varying angular positions in the volutes. The absolute, relative, radial and whirl velocity vectors were evaluated for each flow field at three operating points and compared with conventional pump theory. The data was plotted non-dimensionally to investigate points of similitude. Fluidic phenomena occurring in the impeller passage at both pressure and suction sides of the impeller blades are addressed. The theoretical velocity triangles occurring at the impeller tip are compared with the experimental data. The blade angle at inlet and discharge are found to have a large bearing on the poor pumping performance. The quantitative velocity characteristics are discussed in the context of efficiency degradation at decreasing Reynolds numbers. KEYWORD

The influence of surface roughness on the flow fields generated by an oscillating cantilever

With the current trend of miniaturisation of electronic devices, piezoelectric fans have attracted increasing interest as a means of inducing forced convection cooling, instead of traditional rotary solutions. Although there exists an abundance of research on various piezo-actuated flapping fans in the literature, the geometries of these fans all consist of a smooth rectangular cross section with thicknesses typically of the order of 100 μm. The focus of these studies has primarily been on variables such as frequency, amplitude and, in some cases, resonance mode. It is generally noted that the induced flow dynamics are a direct consequence of the pressure differential at the fan tip as well as the pressure driven ‘over the top’ vortices generated at the upper and lower edges of the fan. Rough surfaces such as golf ball dimples or vortex generators on an aircraft wing have proven to be beneficial by tripping the boundary layer and energising the adjacent airflow. This paper aims to examine the influence of surface roughness on the airflow generation of a flapping fan, and to determine if the induced wake can be manipulated or enhanced by energising the airflow around the fan tip. Particle-Image Velocimetry (PIV) is carried out on mechanically oscillated rigid fans with surfaces consisting of protruding pillars and dimples. A smooth rigid fan surface is also investigated as a control. No significant difference was noted between the smooth and roughened fans through observation of the induced flow fields. Both fans produced results that were largely consistent with the existing literature on oscillating cantilevers. The results of this paper may be used to inform the design of piezoelectric fans and to aid in understanding the complex aerodynamics inherent in flapping wing flight

Two-degree-of-freedom velocity-amplified vibrational energy harvester for human motion applications

Conventional vibrational energy harvesters (VEHs) are gen erally based on a linear mass-spring oscillator model that features narrow bandwidth and high resonant frequencies at small scales. To overcome these limitations, a two-degree-of-freedom (2-Dof) velocity amplified VEH was developed. The harvester comprises two masses, relatively oscillating one inside the other, between four sets of magnetic springs. Impacts between the two masses are allowed, and momentum is transferred from the larger outer mass to the smaller inner mass, thereby providing velocity amplification. Electromagnetic transduction was chosen because it can be easily implemented in a velocity-amplified VEH. The harvester was tested with harmonic excitation of different amplitudes and two peaks of similar heights were observed at arms = 0.6g, resulting in a -3dB bandwidth of 10 Hz. The VEH was also tested under human motion and at a running speed of 10 km/h the harvester generated P = 0.44 mW, a power level that could be accumulated in a storage medium over time and used for powering wireless sensor nodes

The use of segregated heat sink structures to achieve enhanced passive cooling for outdoor wireless devices

Environmental standards which govern outdoor wireless equipment can stipulate stringent conditions: high solar loads (up to 1 kW/m(2)), ambient temperatures as high as 55 degrees C and negligible wind speeds (0 m/s). These challenges result in restrictions on power dissipation within a given envelope, due to the limited heat transfer rates achievable with passive cooling. This paper addresses an outdoor wireless device which features two segregated heat sink structures arranged vertically within a shielded chimney structure: a primary sink to cool temperature-sensitive components; and a secondary sink for high power devices. Enhanced convective cooling of the primary sink is achieved due to the increased mass flow within the chimney generated by the secondary sink. An unshielded heat sink was examined numerically, theoretically and experimentally, to verify the applicability of the methods employed. Nusselt numbers were compared for three cases: an unshielded heat sink; a sink located at the inlet of a shield; and a primary heat sink in a segregated structure. The heat sink, when placed at the inlet of a shield three times the length of the sink, augmented the Nusselt number by an average of 6 4% compared to the unshielded case. The Nusselt number of the primary was found to increase proportionally with the temperature of the secondary sink, and the optimum vertical spacing between the primary and secondary sinks was found to be close to zero, provided that conductive transfer between the sinks was suppressed

Benchmarking of a novel contactless characterisation method for micro thermoelectric modules (μTEMs)

Significant challenges exist in the thermal control of Photonics Integrated Circuits (PICs) for use in optical communications. Increasing component density coupled with greater functionality is leading to higher device-level heat fluxes, stretching the capabilities of conventional cooling methods using thermoelectric modules (TEMs). A tailored thermal control solution incorporating micro thermoelectric modules (mu TEMs) to individually address hotspots within PICs could provide an energy efficient alternative to existing control methods. Performance characterisation is required to establish the suitability of commercially-available TEMs for the operating conditions in current and next generation PICs. The objective of this paper is to outline a novel method for the characterisation of thermoelectric modules (TEMs), which utilises infra-red (IR) heat transfer and temperature measurement to obviate the need for mechanical stress on the upper surface of low compression tolerance (similar to 0.5N) mu TEMs. The method is benchmarked using a commercially-available macro scale TEM, comparing experimental data to the manufacturer's performance data sheet

The identification of period-doubling in a nonlinear two-degree-of-freedom electromagnetic vibrational energy harvester

Common vibrational energy harvesters are generally based on a linear mass-spring oscillator model, and these typically show narrow bandwidth and high resonant frequency at small scales. To overcome these problems, a two-degree-of-freedom nonlinear velocity-amplified energy harvester has been developed. The device comprises two masses, relatively oscillating one inside the other between four sets of magnetic springs. The magnetic springs introduce nonlinear effects, such as period doubling, that can be exploited to enhance the output power and bandwidth of the harvester. This article studies the dynamics of the harvester when a key geometrical parameter (the height of the device) is varied. For large height values, a significant increase of the output power in regions far from the resonant frequency of the device is observed, and the associated period-doubling effect was verified through high-speed imaging. It is demonstrated that nonlinear effects can be used to enhance the bandwidth of the device in order to harvest energy in regions far from resonance

Nonlinear analysis of a two-degree-of-freedom vibration energy harvester using high order spectral analysis techniques

Conventional vibration energy harvesters are generally based on linear mass spring oscillator models. Major limitations with common designs are their narrow bandwidths and the increase of resonant frequency as the device is scaled down. To overcome these problems, a two-degree-of-freedom nonlinear velocity-amplified energy harvester has been developed. The device comprises two masses, oscillating one inside the other, between four sets of nonlinear magnetic springs. Impacts between the masses allow momentum transfer from the heavier mass to the lighter, providing velocity amplification. This paper studies the nonlinear effects introduced by the presence of magnetic springs, using high order spectral analysis techniques on experimental and simulated data obtained for a range of excitation levels and magnetic spring configurations, which enabled the effective spring constant to be varied. Standard power spectrum analysis only provide limited information on the response of nonlinear systems. Instead, bispectral analysis is used here to provide deeper insight of the complex dynamics of the nonlinear velocity-amplified energy harvester. The analysis allows identification of period-doubling and couplings between modes that could be used to choose geometrical parameters to enhance the bandwidth of the device