Experimental evaluation of the thermal performances of a thermosyphon cooling system rejecting heat by natural and forced convection

In the present paper, a closed-loop thermosyphon cooling system for power electronics has been developed and tested. The evaporator is a multi-microchannel heat sink designed for high heat fluxes. The power electronics module is emulated by four independent electrical heaters, which can generate a non-uniform heat flux on the evaporator. The condenser is air-cooled and is tested in both natural convection and forced convection modes. R1234ze has been selected as the working fluid. Although in natural convection mode the cooling system experiences a maximum thermal resistance of approximately 0.6 K/W, the complete passiveness makes such a system very attractive. With values of heat fluxes of 107 W/cm(2) and 25 W/cm(2) equally distributed on the four heaters, the maximum heater temperature measured is 53 degrees C at a saturation temperature of 45 degrees C. In forced convection mode, two different values of air flow have been tested. A reduction of 50% on the overall thermal resistance has been measured with an imposed air flow rate of 87.8 m(3)/h, consuming only 5.29 W to drive the fan for dissipating 70.2 W. At this condition, the maximum heater temperature is reduced to 36 degrees C. In summary, it has been demonstrated that a thermosyphon coupled with air cooling can dissipate high heat fluxes of power electronics with no or little energy consumption. (C) 2017 Elsevier Ltd. All rights reserved.

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

Convective boiling heat transfer in a single micro-channel

Lorenzo Consolini

The current industrial need for compact high heat density cooling devices has conveyed an increasing interest on convective boiling in micro-channels. Although there is general agreement that these systems may be able to dissipate potentially very high heat fluxes, their heat transfer characteristics are still unclear and require investigation. The present study aims at providing further insight on two-phase single micro-channel heat transfer, through a sensitivity analysis on the effect of the different operational parameters, fluid properties, and channel size on thermal performance. The current database includes results for three different refrigerants, R-134a, R-236fa and R-245fa, two channel diameters, 510 and 790µm, and a multitude of heat fluxes and mass velocities, for a total of over 1800 data points. Two-phase flow instability, which represents one of the factors that has prevented a well-established understanding of the micro-scale phenomenon, has been also investigated, with results showing the potential error it may induce on the estimation of the heat transfer coefficients. Finally, a one-dimensional analysis based on the effect of interfacial shear on the temporal evolution of film thickness is proposed for a heat transfer model for slug flow, which has been shown to be the dominant flow mode at low and intermediate vapor qualities in micro-channel evaporators.

EPFL2008

Innovative low-mass cooling systems for the ALICE ITS Upgrade detector at CERN

Manuel Gómez Marzoa

The Phase-1 upgrade of the LHC to full design luminosity, planned for 2019 at CERN, requires the modernisation of the experiments around the accelerator. The Inner Tracking System (ITS), the innermost detector at the ALICE experiment, will be upgraded by replacing the current apparatus by new silicon pixels arranged in 7 cylindrical layers. Each layer is composed by multiple independent modules, named staves, which provide mechanical support and cooling to the chips. This thesis aims to develop and validate experimentally an ultra-lightweight cooling system for the ITS Upgrade. The moderate thermal requirements, with a nominal power density of 0.15 W/cm^2 and a maximum chip temperature of 30°C, are counterweighted by extreme low-mass restrictions, obliging to resort to lightweight, non-metallic materials, such as carbon fibre-reinforced polymers and plastics. Novel lightweight stave concepts were developed and experimentally validated, meeting thermal requirements with minimal material inventory. The proposed staves are made of thin, layered, composite high thermal conductivity carbon fibre plates, with dimensions up to 1502 × 30 mm, and innovative polyimide cooling channels, with inner diameters (IDs) ranging from 1.024 mm to 2.667 mm and thin walls. Six different stave layouts were tested with water at sub-atmospheric pressure (leak-less cooling), showing excellent cooling performances: the temperature differences between the heated surfaces and the coolant are below 7 K at 0.15 W/cm^2, with low temperature gradients. Cooling tests with evaporative C4F10 refrigerant were performed on the baseline stave prototypes, yielding similar thermal performance as with water. This indicates that the thermal resistance of conduction in the stave dominates over the convective one between the channel wall and the coolant. A two-phase coolant would display low material inventory thanks to the presence of the vapour phase, lighter than liquid. An experimental study on the two-phase coolant inventory was carried out in one stave. The results are compared with void fraction prediction methods integrated along the cooling channel length, to find the best one for calculating the material budget of the two-phase coolant. An experimental study on the two-phase pressure drop and flow boiling heat transfer in a 2.689 mm ID water-heated polyimide channel was performed, using R245fa as operating fluid. Mass fluxes ranging from 100 to 500 kg/(m^2 s^1), heat fluxes from 15 to 55 kW/m^2, vapour qualities between 0.05 and 0.80, and saturation temperatures of 35, 41 and 47°C were considered in the 300-point experimental database. The influence of the two-phase flow parameters on the mean heat transfer coefficient was analysed parametrically, being its lack of dependence on the heat flux the major finding. Finally, the results were compared with other experimental data and with prediction methods in the literature.