National Science Foundation
Cooling Technologies Research Center
Purdue University - School of Mechanical Engineering logo

CTRC Breakthroughs


Microscale Transport and Microchannels

Evaluation of 3D Printing Technologies for Microchannel Heat Sinks


3D Microchannels breakthroughs figure Additive manufacturing (AM) technology has the potential to produce heat sinks for high-power, compact cooling applications with geometric feature complexity beyond those produced by convention methods. This project aims to develop new heat sink topologies that take advantage of these fabrication capabilities to demonstrate enhancement performance. A novel, permeable membrane microchannel (PMM) heat sink was designed and the performance compared to a benchmark straight microchannel (SMC) heat sink. The PMM offers improved performance when operating at a low pressure drop is a high priority. From am applications perspective, this means that the PMM is preferred for large heater foot-print areas, small available heat sink heights, high viscosity coolants, and at high flow rates. Additionally, the AM process was tuned to fabricate smaller-than-achievable channel sizes compared to quoted machine capabilities, which has potential performance benefits across any type of heat sink design. The outcomes of this project will be leveraged in future work in the CTRC to achieve even higher performance heat sink designs by implementing state-of-the art design and optimization methodologies that further leverage the 3D printing technology.



Investigation of Two-Phase Microchannel Flow Structures for Improved Predictive Models


Flow Structures breakthroughs figure Multiphase microchannel heat sinks are an attractive option for cooling of high power electronics due to their compact size and high rate of heat extraction.    One of the limiting factors is the difficulty in predicting performance, which hampers the design of application-specific cooling systems.  For this reason, it is desirable to develop mechanistic models that capture the underlying physics of the flow. By using optical microscopy in conjunction with advanced image processing techniques, this project achieved the most detailed characterization of liquid-gas interface structures in a multiphase microchannel environment to date.  Additionally, parametric investigations into dependence of interface shape on operating conditions within the annular flow regime were conducted.




Effects of Non-Uniform Heating on Two-Phase Flow through Microchannels

Asymmetric Heating breakthroughs figure Understanding non-uniform heating effects on microchannel cooling will enable better assessment of existing heat transfer models for prediction of realistic non-uniform heating profiles in any system. The developed simple computational model allows for an easy analysis of different designs for cooling high-power electronics. The results from this project enable the prediction of realistic non-uniform heating profiles on any microchannel heat sink. Images extracted from high-speed videos of four different hotspot heating conditions. The total input power to the microchannel heat sink is approximately the same in all cases. The red lines indicate the heated area of the device and the flow goes from left to right. Despite the same total power applied to the heat sink, the location and degree of boiling is different for each configuration.



Thickness in Slug and Annular Flow for Heat Transfer Model 

Film Thickness breakthroughs figure This project developed a new three-dimensional measurement technique applicable to boiling of coolant in microchannel heatsinks.  The novel, non-intrusive technique developed will allow for investigations that were hitherto not possible in terms of in-situ measurements in liquid-vapor phase-change applications. The figure shows a 400 µm diameter capillary tube and a fluorescent image of a planar cross section within the tube.



Optimization of Manifold Microchannel Heatsinks

Manifold Microchannel breakthroughs figureDeterministic as well as probabilistic optimization is performed to arrive at robust, reliable designs. Further, a response surface, capturing the dependence of outputs on various inputs is developed, that can be used as a cost-effective tool for future optimization purposes. A porous medium model is also proposed to obtain the system level probabilistic optimization at a reduced computational cost. Manifold microchannel heat sinks involve the use of a manifold system arranged in a transverse manner over conventional microchannels, for distributing the flow through multiple alternating inlet and exit pairs. Such an arrangement leads to a reduction in the incurred pressure drop, while improving the overall heat transfer performance.  The figure shows a schematic diagram of flow through a manifold mcirochannel heat sink. The inset shows the unit cell computational domain and boundary conditions used in present study.


Flow Regime-Based Predictive Model for Flow Boiling in Microchannels

Regime Based Modeling breakthroughs figureA comprehensive understanding of flow boiling heat transfer and pressure drop in microchannels for various liquid/surface combinations, across a wide range of operating conditions, has been developed. A large database of boiling flow pattern visualizations is obtained for a wide range of channel dimensions and flow parameters, leading to a good understanding of microscale flow regimes. This gallery of visualizations has been widely disseminated through the Cooling Technologies Research Center website for researchers and practitioners worldwide to use. The effects of microchannel geometry on flow boiling and surface roughness on boiling heat transfer enhancement has also been established. Based on the experimental results, a new transition criterion is developed which predicts the conditions under which microscale confinement effects are exhibited in flow boiling. The figure shows a high-speed flow visualization of bubbly flow and slug flow in 400 µm-wide microchannels

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Boiling Heat Transfer Enhancement in Microchannels Using Surface Treatment

Microchannel Surfaces breakthroughs figureOne strategy for removing large amounts of heat in electronics is the utilization of the highly effective heat transfer mechanisms associated with the liquid-vapor phase change process. The present work seeks to characterize and predict the enhancement to boiling heat transfer provided by arbitrarily rough surfaces in microchannel heat sinks. Results indicate that increasing the surface roughness by a factor of 3 yields a 30% enhancement in the amount of heat that can be removed while keeping the heat sink temperature constant. Further increases in surface roughness appear to be of little additional benefit. Boiling from a flat, polished surface in a pool of water is shown in the figure.


Two-Phase Transport in Microchannels

Microchannels brekathroughs figureTransport through microchannels that range in width from 100 to 400 micrometers in copper and silicon substrates is experimentally characterized. Experiments include high-speed flow visualization, and local and global temperature and pressure measurements. A predictive model is also being formulated to aid in the design and optimization of microchannel heat sinks.