Vishal Singhal

Microscale pumping technologies for microchannel cooling systems

A review of the state of the art in micropumping technologies for driving fluid through micro-channels 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 micropump-ing 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 micro-pumps based on different criteria including their miniaturization potential, size ͑in-plane and out-of-plane͒, 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.

On-Chip Thermal Management With Microchannel Heat Sinks and Integrated Micropumps

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

Single-Phase Flow and Heat Transport and Pumping Considerations in Microchannel Heat Sinks

Microchannel heat sinks are widely regarded as being among 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 is truly suitable for this application. Results are reported from a wide-ranging research program being conducted on microchannel heat sinks and micropumps to understand fluid flow and heat transfer in microchannels and to identify pumping requirements and suitable mechanisms for pumping in microchannels. In particular, experiments have been performed to show that conventional correlations for fluid flow and heat transfer adequately predict the behavior in microchannels of hydraulic diameters as small as 250 μ m. Pumping requirements of microchannel heat sinks have been analyzed, and the size of the microchannels have been optimized for minimum pumping requirements. Results are also provided from a comprehensive review of micropumping technologies in the literature.

Optimization of thermal interface materials for electronics cooling applications

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.

A novel valveless micropump with electrohydrodynamic enhancement for high heat flux cooling

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.

Influence of Bulk Fluid Velocity on the Efficiency of Electrohydrodynamic Pumping

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.

Analysis and prediction of constriction resistance for contact between rough engineering surfaces

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.

Single-Phase Flow and Heat Transport in Microchannel Heat Sinks

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 at Purdue 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.Copyright

Analysis of Pumping Requirements for Microchannel Cooling Systems

Large pressure drops, and the associated pumping requirements, are often considered the most critical factor hindering widespread commercial use of microchannel heat sinks. Analytical methods are used in the present work to arrive at the pumping requirements for any given microchannel heat sink. A graphical method to check the suitability of a pump to a microchannel heat sink application has been devised. The size of the microchannels is also optimized so that for a specified heat removal rate, the pumping requirements are minimized. A number of commercially available pumps as well as several micropumps presented in the literature are compared based on their flow rate, pressure head and physical size to assess their suitability for a specific representative cooling application.Copyright

A Novel Micropump for Electronics Cooling

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.Copyright