Concurrent programming is increasingly important for achieving performance gains in the multi-core era, but it is also a difficult and error-prone task. Concurrency bugs are particularly difficult to avoid and diagnose, and therefore in order to improve methods for handling such bugs, we need a better understanding of their characteristics. In this paper we present a study of concurrency bugs in MySQL, a widely used database server. While previous studies of real-world concurrency bugs exist, they have centered their attention on the causes of these bugs. In this paper we provide a complementary focus on their effects, which is important for understanding how to detect or tolerate such bugs at run-time. Our study uncovered several interesting facts, such as the existence of a significant number of latent concurrency bugs, which silently corrupt data structures and are exposed to the user potentially much later. We also highlight several implications of our findings for the design of reliable concurrent systems.

A miniature solid-state fan has been developed. The solid-state fan blows air like a typical rotary fan but without any moving parts. It uses the principle of electro- aerodynamic pumping. These fans have been shown to produce a pressure head of up to 42 Pa. Operable fans as small as 1 mm thick have been demonstrated. These fans are intended to provide air flow to cool a 20 Watt chip in an integrated, chip-scale cooling system or to replace the blower in a laptop cooling system.

Induction electrohydrodynamics (EHD) has been investigated as a possible means of pumping liquids through microchannel heat sinks for cooling microprocessors. A pump utilizing induction EHD has been microfabricated and tested. The experimental results matched the predictions from correlations to within 30%. Based on this, a micropump has been designed which is miniaturizable to a level where it can be integrated into the microchannels. The micropump utilizes a vibrating diaphragm along with induction EHD for pumping. The vibrating diaphragm does not cause any net flow by itself but causes high local bulk fluid velocities which lead to an increase in the power drawn from the electrodes and an increase in efficiency of EHD, both of which lead to a higher flow rate. The performance of the pump is predicted using an experimentally validated numerical model. The numerical model solves the three-dimensional transient fluid flow and charge transport problem due to simultaneous actuation of EHD and the vibrating diaphragm. Numerical results for micropumps integrated into trapezoidal microchannels are presented. The results indicate that the proposed micropump design has significant potential for microelectronics cooling applications: It is easy and inexpensive to fabricate, needs no added space, and can achieve the high flow rates needed.

Liquid-cooled microchannel heat sinks are regarded as being amongst the most effective solutions for handling high levels of heat dissipation in space-constrained electronics. However, obstacles to their successful incorporation into products have included their high pumping requirements and the limits on available space which precludes the use of conventional pumps. Moreover, the transport characteristics of microchannels can be different from macroscale channels because of different scaling of various forces affecting flow and heat transfer. The inherent potential of microchannel heat sinks, coupled with the gaps in understanding of relevant transport phenomena and difficulties in implementation, have guided significant research efforts towards the investigation of flow and heat transfer in microchannels and the development of microscale pumping technologies and novel diagnostics. In this paper, the potential and capabilities of microchannel heat sinks and micropumps are discussed. Their working principle, the state of the art, and unresolved issues are reviewed. Novel approaches for flow field measurement and for integrated micropumping are presented. Future developments necessary for wider incorporation of microchannel heat sinks and integrated micropumps in practical cooling solutions are outlined.

Liquid-cooled microchannel heat sinks are regarded as being amongst the most effective solutions for handling high levels of heat dissipation in space-constrained electronics. However, obstacles to their successful incorpora- tion into products have included their high pumping require- ments and the limits on available space which precludes the use of conventional pumps. Moreover, the transport character- istics of microchannels can be different from macroscale channels because of different scaling of various forces affecting flow and heat transfer. The inherent potential of microchannel heat sinks, coupled with the gaps in understanding of relevant transport phenomena and difficulties in implementation, have guided significant research efforts towards the investigation of flow and heat transfer in microchannels and the development of microscale pumping technologies and novel diagnostics. In this paper, the potential and capabilities of microchannel heat sinks and micropumps are discussed. Their working principle, the state of the art, and unresolved issues are reviewed. Novel approaches for flow field measurement and for integrated micropumping are presented. Future developments necessary for wider incorporation of microchannel heat sinks and integrated micropumps in practical cooling solutions are outlined.

Integrated microchannel cooling systems, with micropumps integrated into microchannels, are an attractive alternative to stand-alone micropumps for liquid-cooled microchannel heat sinks. A new micropump design capable of integration into microchannels and especially suited for electronics cooling is presented. It combines induction electrohydrodynamics (EHD) with a valveless nozzle-diffuser micropump actuated using a vibrating diaphragm. A comprehensive numerical model of the micropump has been developed to study the combined effect of EHD and valveless micropumping. The numerical model has been validated using theoretical and experimental results from the literature. The flow rate achievable from the new micropump is presented and the effect of several key parameters on the micropump performance investigated.

The efficiency of conversion of electrical power into fluidic power in an electrohydrodynamic (EHD) pump depends on the bulk fluid velocity. An analytical formulation is developed for calculation of the efficiency of an EHD pump, with and without the presence of a superimposed flow due to an externally imposed pressure gradient. This formulation is implemented into a numerical model, which is used to investigate the effect of bulk fluid velocity on the efficiency of the EHD action. In particular, the net flow due to the combined action of EHD and a positive or negative external pressure gradient is computed. Both ion-drag pumps and induction EHD pumps are considered. Pumps based on the ion-drag principle that are studied include a one-dimensional pump, a two-dimensional pump driven by a stationary potential gradient, and another driven by a traveling potential wave. Two-dimensional repulsion-type and attraction-type induction pumping caused by a gradual variation in the electrical conductivity of the fluid is also investigated. The efficiency of EHD pumps exhibited a strong dependence on bulk fluid velocity: for the two-dimensional steady ion-drag pump, for example, the efficiency increased from less than 2% to 22% under the influence of an external pressure gradient. The corresponding increase in efficiency for a two-dimensional repulsion-type EHD pump was from 0.26% to 24.5%.

A predictive model for estimating thermal contact conductance between two nominally flat metallic rough surfaces has been developed and experimentally validated. The predictive model consists of two complementary parts, the first of which is a surface deformation analysis to calculate the actual area of contact for each contact spot, while the second accounts for the effects of constriction resistance and gas gap conductance between the contacting surfaces. A surface characterization technique is developed which generates an equivalent 3-D surface profile from multiple 2-D profiles and determines the unique wavelengths of importance for the surface deformation and constriction resistance models. For given surface profiles and material properties of two contacting surfaces, and a specified contact pressure, the surface characterization technique filters out non-essential wavelengths on the surface, after which the surface deformation analysis calculates the deformation and contact area of each contacting asperity by considering three different modes of deformation, namely, elastic, elastic–plastic, and plastic. The constriction resistance model is then used to calculate the constriction resistance for each contacting asperity based on the area of contact and radius of curvature of the asperity. The constriction resistance values for all the contacting asperities are then used to calculate the total thermal contact conductance. An experimental facility has also been constructed to measure thermal contact conductance of interfaces to verify the results of the predictive model. Good agreement has been found between the model predictions and experimental measurements, validating the modeling approach.

Thermal interface materials (TIMs) are used in electronics cooling applications to decrease the thermal contact resistance between surfaces in contact. A methodology to determine the optimal volume fraction of filler particles in TIMs for minimizing the thermal contact resistance is presented. The method uses finite element analysis to solve the coupled thermo-mechanical problem. It is shown that there exists an optimal filler volume fraction which depends not only on the distribution of the filler particles in a TIM but also on the thickness of the TIM layer, the contact pressure and the shape and the size of the filler particles. A contact resistance alleviation factor is defined to quantify the effect of these parameters on the contact conductance with the use of TIMs. For the filler and matrix materials considered-platelet-shaped boron nitride filler particles in a silicone matrix-the maximum observed enhancement in contact conductance with the use of TIMs was by a factor of as much as nine.

Microchannel heat sinks are widely regarded as being amongst the most effective heat removal techniques from space-constrained electronic devices. However, the fluid flow and heat transfer in microchannels is not fully understood. The pumping requirements for flow through microchannels are also very high and none of the micropumps in the literature are truly suitable for this application.A wide-ranging research program on microchannel heat sinks and micropumps is underway in the Electronics Cooling Laboratory atPurdue University. This article provides an overview of the research being conducted to understand fluid flow and heat transfer in microchannels and to identify pumping requirements and suitable mechanisms for pumping in microchannels.

A novel micropump design for electronics cooling applications capable of integration into microchannel heat sinks is presented. The pumping action is due to the combined action of Coulomb forces due to induction electrohydrodynamics (EHD) and a vibrating diaphragm with nozzle-diffuser elements for flow rectification. A comprehensive numerical model of the micropump accounting for transient charge transport and vibrating diaphragm deformation is developed. Each component of the model is validated by comparing to analytical, numerical or experimental results from the literature. It is shown that the flow rate achieved by the micropump with combined action of EHD and vibrating diaphragm can be higher than the sum of flow rates achieved from the action of the EHD and the vibrating diaphragm, independent of each other.

A review of the state of the art in micropumping technologies for driving fluid through microchannels is presented with a particular emphasis on small-scale cooling applications. An extensive variety of micropumping techniques developed over the past fifteen years in the literature is reviewed. The physical principles, engineering limitations, and advantages of approximately twenty different kinds of micropumps are reviewed. The available micropumping techniques are compared quantitatively, primarily in terms of the maximum achievable flow rate per unit cross-sectional area of the microchannel and the maximum achievable back pressure. A concise table is developed to facilitate the convenient comparison of the micropumps based on different criteria including their miniaturization potential, size (in-plane and out-of-plane), actuation voltage and power required per unit flow rate, ease and cost of fabrication, minimum and maximum frequency of operation, and suitability for electronics cooling. Some important performance characteristics of the micropumps, which are likely to be decisive for specific applications, are also discussed. The current state of the art in micropump design and fabrication is also comprehensively reviewed. There are 171 references cited in this review article.

Flow characteristics of low Reynolds number laminar flow through gradually expanding conical and planar diffusers were investigated. Such diffusers are used in valveless micropumps to effect flow rectification and thus lead to pumping action in one preferential direction. Four different types of diffuser flows are considered: fully developed and thin inlet boundary layer flows through conical and planar diffusers. The results from the numerical analysis have been quantified in terms of pressure loss coefficient. The variation of pressure loss coefficient with diffuser angle is presented for Reynolds numbers of 200, 500 and 1000. The pressure loss coefficients have been used to calculate the diffuser efficiency for two different types of nozzle-diffuser elements. The general trend of variation of pressure loss coefficient with diffuser angle was found to be similar to that for high Reynolds number turbulent flow. However, unlike at high Reynolds numbers, pressure loss coefficients at low Reynolds numbers vary significantly with Reynolds number. It was also observed that trends of variation in the pressure loss coefficient with Reynolds number are different for small and large diffuser angles. Also, at low Reynolds numbers, the pressure loss coefficients for a thin inlet boundary layer are not always smaller than those for fully developed inlet boundary layer, in contrast to the behavior for high Reynolds number flows. Contrary to past claims, flow rectification is shown to be indeed possible for laminar flows. The two different types of nozzle-diffuser elements considered led to pumping action in opposite directions. Further, it was observed that flow rectification properties of both kinds of nozzle-diffuser elements improved with increasing Reynolds number.

The constriction resistance of a semi-infinite cylinder terminating in the frustum of a cone in a gaseous environment is analyzed numerically. The variation of constriction resistance with the ratio of contact radius to cylinder radius, the cone angle, and the gas-to-substrate thermal conductivity ratio is investigated. Nonlinear curve fitting is used to develop a comprehensive predictive correlation for the constriction resistance as a function of these parameters. The parameters are investigated over a wide range of values covering the range expected in practical applications. The correlation could be used in conjunction with an appropriate surface deformation model to predict the thermal contact resistance between rough metallic surfaces.