State of the Art of High Heat Flux Cooling Technologies

Bruno Agostini, Bruno Michel, Jung Eung Park, John Richard Thome, Leszek Wojtan
2007
Article

Résumé

The purpose of this literature review is to compare different cooling technologies currently in development in research laboratories that are competing to solve the challenge of cooling the next generation of high heat flux computer chips. Today, most development efforts are focused on three technologies: liquid cooling in copper or silicon micro-geometry heat dissipation elements, impingement of liquid jets directly on the silicon surface of the chip, and two-phase flow boiling in copper heat dissipation elements or plates with numerous microchannels. The principal challenge is to dissipate the high heat fluxes (current objective is 300 W/cm2) while maintaining the chip temperature below the targeted temperature of 85°C, while of second importance is how to predict the heat transfer coefficients and pressure drops of the cooling process. In this study, the state of the art of these three technologies from recent experimental articles (since 2003) is analyzed and a comparison of the respective merits and drawbacks of each technology is presented. The conclusion is that two-phase flow boiling in microchannels is the most promising approach; impingement cooling also has good prospects but single-phase liquid cooling is probably only a short-term solution. As an example of the state of the first technology, the Heat and Mass Transfer Laboratory at Ecole Polytechnique Fédérale de Lausanne has already achieved 200 W/cm2 of cooling in a first prototype, with a low pumping power, good temperature uniformity, and at the required maximal operating temperature.

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https://infoscience.epfl.ch/record/89737?ln=fr

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Bruno Agostini, Bruno Michel, Jung Eung Park, John Richard Thome, Leszek Wojtan
2007
Article

Résumé

Official source

https://infoscience.epfl.ch/record/89737?ln=fr

À propos de ce résultat

Concepts associés (18)

Concepts associés

Chargement

Publications associées (82)

Publications associées

Chargement

Cooling High Heat Flux Micro-Electronic Systems using Refrigerants in High Aspect Ratio Multi-Microchannel Evaporators

Etienne Costa-Patry

Improving the energy efficiency of cooling systems can contribute to reduce the emission of greenhouse gases. Currently, most microelectronic applications are air-cooled. Switching to two-phase cooling systems would decrease power consumption and allow for the reuse of the extracted heat. For this type of application, multi-microchannel evaporators are thought to be well adapted. However, such devices have not been tested for a wide range of operating conditions, such that their thermal response to the high non-uniform power map typically generated by microelectronics has not been studied. This research project aims at clarifying these gray areas by investigating the behavior of the two-phase flow of different refrigerants in silicon and copper multi-microchannel evaporators under uniform, non-uniform and transient heat fluxes operating conditions. The test elements use as a heat source a pseudo-chip able to mimic the behavior of a CPU. It is formed by 35 independent sub-heaters, each having its own temperature sensor, such that 35 temperature and 35 heat flux measurements can be made simultaneously. Careful measurements of each pressure drop component (inlet, microchannels and outlet) found in the micro-evaporators showed the importance of the inlet and outlet restriction pressure losses. The overall pressure drop levels found in the copper test section were low enough to possibly be driven by a thermosyphon system. The heat transfer coefficients measured for uniform heat flux conditions were very high and typically followed a V-shape curve. The first branch was associated to the slug flow regime and the second to the annular flow regime. By tracking the minimum level of heat transfer, a transition criteria between the regimes was established, which included the effect of heat flux on the transition. Then for each branch, a different prediction method was used to form the first flow pattern-based prediction method for two-phase heat transfer in microchannels. A non-uniform heat flux creates important temperature gradients in the evaporator, such that the data reduction procedure needs to be adapted to include heat spreading within the evaporator. To do so, a robust multi-dimensional thermal conduction scheme was developed. Once these effects were taken into consideration, the local heat transfer coefficients provided by two-phase flow were found to be the same for uniform and non-uniform heat fluxes, allowing the flow pattern-based method to be extended to non-uniform heat flux conditions. Lastly, with proper control of the mass flow, transient heat flux situations were well handled by the micro-evaporators.

EPFL2011

Chargement

A theoretical and numerical model to predict film condensation heat transfer in mini, micro and ultra micro-channels of different internal shapes is presented in this thesis. The model is based on a finite volume formulation of the Navier-Stokes and energy equations and it includes the contributions of the unsteady terms, surface tension, axial shear stresses, gravitational forces and wall thermal conduction. Notably, interphase mass transfer and near-to-wall effects (disjoining pressure) are also included. This model has been validated versus various benchmark cases and versus published experimental results from three different laboratories, predicting micro-channel heat transfer data with an average error of 20 % or better. The conjugate heat transfer problem arising from the coupling between the thin film fluid dynamics, the heat transfer in the condensing fluid and the heat conduction in the channel wall has been studied and analyzed. The work has focused on the effects of three external wall boundary conditions: a uniform wall temperature, a non uniform wall heat flux and single-phase convective cooling. The thermal axial and peripheral conduction occurring in the wall of the channel can affect the behavior of the condensate film, not only because it redistributes the heat, but also because the annular laminar film condensation process is dependent on the local saturation to wall temperature difference. When moving from mini to micro and ultra-micro channels, the results shows that the axial conduction effects can become very important in the prediction of the wall temperature profile and they can not be ignored. Under these conditions, the overall performances of the heat exchanger become dependent not only on the fluid properties and the operative conditions but also on the geometry and wall material. Results obtained for steady state conditions are presented for circular, elliptical and flattened shape cross sections for R-134a and ammonia, for hydraulic diameters between 10 µm and 3 mm. Microscale condensation finds applications in heat pipes and compact heat exchangers for electronic equipment or spacecraft thermal control, in automotive condensers, in residential air conditioning and in refrigeration applications: the influence of the steady or unsteady gravitational field and of the inertia forces on the flow field and consequently on the heat transfer performances is investigated allowing the model to be applied as a design and optimization tool for enhanced heat exchangers.

EPFL2010

Chargement

Macro-to-Microchannel Transition in Two-Phase Flow and Evaporation

Chin Lee Ong

Continuous development of highly efficient and compact cooling elements are driven by extensive developments of Micro-Electro-Mechanical Systems (MEMS) devices such as microprocessors or micro high-powered laser systems. As microprocessors continue to head in the direction of miniaturization, it was quickly realized that the major obstacle to overcome was "heat". This justified the need for the development of a new generation of liquid cooling technologies that involves phase-change processes. However, significant differences in two-phase transport phenomena have been reported in the microscale sized channels as compared to conventional macroscale channels. The classification of macroscale, mesoscale and microscale channels with respect to two-phase processes is still an open question. This research project focuses on investigating the macro-to-microscale transition during flow boiling in small scale channels of various size and refrigerants to investigate the effects of channel confinement on flow boiling heat transfer, two-phase flow patterns, pressure drop, critical heat flux and liquid film stratification in a single circular horizontal channel. In this project, we systematically investigated the trends observed in the heat transfer data, pressure drop, critical heat flux and film stratifications for wide range of conditions as a function of flow pattern, creating a uniquely comprehensive experimental database for R134a (high pressure), R236fa (medium pressure) and R245fa (low pressure) in the 1.03, 2.20 and 3.04 mm channels. From a heat transfer viewpoint, the prediction of the heat transfer requires first the prediction of the two-phase flow regimes in these small scale channels. So, in that respect an improved flow pattern map has been proposed and by determining the flow patterns transitions existing under different conditions, the effects of the flow regimes and channel dimensions on flow boiling heat transfer is thus explained. A flow pattern based heat transfer prediction model, i.e. the three-zone model showed good agreement in the prediction of heat transfer coefficients corresponding to the isolated bubble (IB) and coalescing bubble (CB) flows. As for the two-phase pressure drops, various statistical comparisons have been made by comparing the pressure drop database with various macroscale and microscale prediction methods, finding that three macroscale and two microscale methods predicted the pressure drop database with good accuracy. From this research, a distinctive peak in the CHF has been identified when comparing the experimental CHF database for 0.50, 0.80, 1.03, 2.20 and 3.04 mm channels (the first two from previous study). A new CHF prediction method was developed that is applicable for the CHF prediction of single, square multi-microchannels and split flow multi-channels. Finally, the processing of the high speed flow visualization images provided the interpretation of the lower threshold of the macro-microscale transition. As an approximate rule, the lower threshold of macro-scale flow is Co= 0.3-0.4 while the upper threshold of the symmetric microscale flow is Co≈1.

EPFL2010

Chargement

Cooling High Heat Flux Micro-Electronic Systems using Refrigerants in High Aspect Ratio Multi-Microchannel Evaporators

Etienne Costa-Patry

EPFL2011

EPFL2010

Macro-to-Microchannel Transition in Two-Phase Flow and Evaporation

Chin Lee Ong

EPFL2010