Porous Media Modeling of Microchannel Cooled Electronic Chips with Nonuniform Heating

<p>Microchannels are used for the cooling of electronic chips. However, the three-dimensional computational fluid dynamics modeling of the large number of channels in a full chip requires a huge number of meshes and computation time. Although porous media modeling of microchannels can significantly reduce the effort of simulation, most previous porous media models are based upon the assumption that the surface heat flux or temperature is uniform on the chip. In reality, the heat flux on the chip is usually highly nonuniform. In the present study, the porous media model considers the simultaneously developing entrance effect at the microchannel inlet and the thermally developing entrance effect due to the severe heat flux variation along the channel. Duhamel’s integral is used to provide the Nusselt number distribution corresponding to the nonuniform heat flux distribution along the channel. The computing cost of this modeling method is only about 1% of the three-dimensional conjugate simulation. This porous media thermal modeling method is applied to model two full-scale electronic chips with realistic power distributions on the surfaces, and temperature maps are generated. The porous media thermal modeling offered by this study is an accurate and efficient alternative for modeling the electronic chips cooled by microchannels.</p

hermal-Aware Microchannel Cooling of Multicore Processors: A Three-Stage Design Approach

<p>This study goes beyond the common microchannel cooling system composed of uniform parallel straight microchannels and proposed a three-stage design approach for spatially thermal-aware microchannel cooling of 2D multicore processors. By applying effective strategies and arranging key design parameters, stronger cooling is provided under the high power core area, and less cooling is provided under the low power cache area to effectively save the precious pumping power, lower the hot spot temperature and lower temperature gradients on chip. Two microchannel cooling systems are specifically designed for a 2 core 150 W Intel Tulsa processor and an 8 core 260 W (doubled power) Intel Nehalem processor with single phase HFE7100 as coolant. For the Tulsa processor, a strategy named strip-and-zone is used. The final design leads to 30 kPa pressure drop and 0.094 W pumping power while maintains the hot spot temperature to be 75 °C. For the Nehalem processor, a split flow microchannel system and a widen-inflow strategy are applied. A design is achieved to cost 15 kPa pressure drop and 0.0845 W pumping power while maintains the hot spot temperature to be 82.9°C. The design approach in this study provides the basic guide for the industrial applications of effective multicore processor cooling using microchannels.</p

A mechanistic model of critical heat flux under subcooled flow boiling conditions for application to one- and three-dimensional computer codes

Based on a review of visual observations at or near critical heat flux (CHF) under subcooled flow boiling conditions and consideration of CHF triggering mechanisms, presented in a companion paper [Le Corre, J.M., Yao, S.C., Amon, C.H., 2010. Two-phase flow regimes and mechanisms of critical heat flux under subcooled flow boiling conditions. Nucl. Eng. Des.], a model using a two-dimensional transient thermal analysis of the heater undergoing nucleation was developed to mechanistically predict CHF in the case of a bubbly flow regime. The model simulates the spatial and temporal heater temperature variations during nucleation at the wall, accounting for the stochastic nature of the boiling phenomena. It is postulated that a high local wall superheat occurring underneath a nucleating bubble at the time of bubble departure can prevent wall rewetting at CHF (Leidenfrost effect). The model has also the potential to evaluate the post-DNB heater temperature up to the point of heater melting. Validation of the proposed model was performed using detailed measured wall boiling parameters near CHF, thereby bypassing most needed constitutive relations. It was found that under limiting nucleation conditions; a peak wall temperature at the time of bubble departure can be reached at CHF preventing wall cooling by quenching. The simulations show that the resulting dry patch can survive the surrounding quenching events, preventing further nucleation and leading to a fast heater temperature increase. The model was applied at CHF conditions in simple geometry coupled with one-dimensional and three-dimensional (CFD) codes. It was found that, within the range where CHF occurs under bubbly flow conditions (as defined in Le Corre et al., 2010), the local wall superheat underneath nucleating bubbles is predicted to reach the Leidenfrost temperature. However, a better knowledge of statistical variations in wall boiling parameters would be necessary to correctly capture the CHF trends with mass flux (or Weber number).</p

Two-phase flow regimes and mechanisms of critical heat flux under subcooled flow boiling conditions

A literature review of critical heat flux (CHF) experimental visualizations under subcooled flow boiling conditions was performed and systematically analyzed. Three major types of CHF flow regimes were identified (bubbly, vapor clot and slug flow regime) and a CHF flow regime map was developed, based on a dimensional analysis of the phenomena and available experimental information. It was found that for similar geometric characteristics and pressure, a Weber number (We)/thermodynamic quality (x) map can be used to predict the CHF flow regime Based on the experimental observations and the review of the available CHF mechanistic models under subcooled flow boiling conditions, hypothetical CHF mechanisms were selected for each CHF flow regime, all based on a concept of wall dry spot overheating, rewetting prevention and subsequent dry spot spreading. Even though the selected concept has not received much attention (in term or theoretical developments and applications) as compared to other more popular DNB models, its basis have often been cited by experimental investigators and is considered by the authors as the “most-likely” mechanism based on the literature review and analysis performed in this work. The selected modeling concept has the potential to span the CHF conditions from highly subcooled bubbly flow to early stage of annular flow and has been numerically implemented and validated in bubbly flow and coupled with one- and three-dimensional (CFD) two-phase flow codes, in a companion paper. [Le Corre, J.M., Yao, S.C., Amon, C.H., in this issue. A mechanistic model of critical heat flux under subcooled flow boiling conditions for application to one and three-dimensional computer codes. Nucl. Eng. Des.].</p

Numerical and experimental analysis of composite fouling in corrugated plate heat exchangers

<p>This paper provides a numerical and experimental analysis on precipitation and particulate fouling in corrugated plate heat exchangers with different geometric parameters which are plate height, plate spacing, and plate angle. The Realizable <em>κ</em>–<em>ε</em>model with non-equilibrium wall functions is used in the 3D numerical simulation considering the realistic geometries of the flow channel to obtained Nusselt number and wall shear stress, while Von-Karman analogy is used to obtain mass transfer coefficient. Numerical analysis is verified by experimental study. The predicted influence of fluid velocity in fouling resistance is compatible with experimental data that it can help to optimize the design of plate heat exchangers. This investigation significantly simplifies the fouling analysis of complex flow fields and can be used to assess the fouling potential of corrugated plate heat exchangers.</p

Multiphysics modeling of a micro-scale Stirling refrigeration system

<p>A new micro-scale refrigeration system composed of arrays of silicon MEMS cooling elements that operate on the Stirling cycle has been designed. In this paper, we describe a multiphysics computational approach for analyzing the system performance that considers compressible flow and heat transfer with a large deformable mesh. The regenerator pressure drop and effectiveness are first explored to determine the optimal porosity. A value near 0.9 is found to maximize the coefficient of performance. To overcome the computational complexity brought about by the fine pillar structure in the regenerator, a porous medium model is used to allow for modeling of a full element. Parametric studies demonstrate the effect of the operating frequency on the cooling capacity and the coefficient of performance.</p

Design and modeling of a fluid-based micro-scale electrocaloric refrigeration system

<p>A refrigeration system composed of silicon MEMS cooling elements is designed based on the electrocaloric (EC) effect in a P(VDF–TrFE–CFE) terpolymer, poly(vinylidene fluoride–trifluoroethylene–chlorofluoroethylene) 59.2/33.6/7.2 mol%. Each cooling element includes two diaphragm actuators fabricated in the plane of a silicon wafer, which drive a heat transfer fluid back and forth across terpolymer layers that are placed between them. In the EC effect, reversible temperature and entropy changes related to polarization changes appear in a material under the application and removal of an electric field. Finite element simulations are performed to explore the system performance. The effect of the applied electric field is studied, and the time lag between the electric field and the diaphragm motion is found to significantly affect the cooling power. A parametric study of the operating frequency, externally-applied temperature span, and the electric field amplitude are conducted. The results indicate that when the system is operated at a temperature span of 15 K, a cooling power density of 3 W/cm²and a percent of Carnot <em>COP</em> of 31% are achieved for one element.</p