Raffaele L. Amalfi

Two-phase liquid cooling system for electronics, part 4: Modeling and simulations

The fourth article of the present four-part series presents the details of a modeling study using a proprietary two-phase flow simulation code, developed in-house at the LTCM, to predict thermal-hydraulic performance of the pump-driven loop discussed in Part 1 and thermosyphon loop introduced in Parts 2 and 3. The simulation code incorporates LTCMs proven methods for predicting local flow boiling heat transfer coefficients and two-phase pressure gradients in multi-microchannels coupled with a thermosyphon code to simulate all components of the systems together with their operational characteristics and thermal performance. The present simulation tool also provides important guidance for the design of the 18 microcooling zones of the cold plate and louver fin flat tube air-cooled condenser as well as to understand the transient and steady-state contributions of other system components to the overall thermal-hydraulic performance. A comprehensive overview of the modeling and the associated flow charts are first addressed in the present work. Then, the simulation results are compared against the experimental database at the same test conditions resulting in a good agreement. Finally, a sensitivity analysis is performed to investigate the effects of working fluid, thermosyphon height and riser/downcomer diameters on the thermosyphon cooling performance.

Two-phase liquid cooling system for electronics, part 3: Ultra-compact liquid-cooled condenser

An experimental study to investigate the thermal performance of a two-phase thermosyphon for electronics cooling is presented in this article. In this study, the thermosyphon evaporator was connected via a riser and a downcomer to an ultra-compact condenser operating as a refrigerant-to-refrigerant counter flow heat exchanger. The secondary side of the condenser evaporated R134a from a “bus line” to condense the working fluid in the thermosyphon. Experiments were carried out for filling ratios ranging from 60% to 76%, heat loads from 102 W to 1841 W, secondary side mass flow rates from 40 kg/h to 120 kg/h, inlet subcoolings from about 0 K to 5 K, and saturation pressures from 600 kPa to 730 kPa. Robust thermal performance was observed for the entire range of operating test conditions. In particular, at the optimum filling ratio of 65%, secondary side mass flow rate of 80 kg/h, inlet subcooling close to 0 K and saturation pressure of 600 kPa, the mean temperature difference from the evaporator to inlet coolant was only 9.4 K. Experimental results demonstrated an increase of 14X in heat density dissipation, 19X increase in energy efficiency and a virtually noiseless system compared to the air-cooled thermosyphon discussed in Part 2.

Two-phase liquid cooling system for electronics, part 1: Pump-driven loop

An experimental study to analyse the thermal performance of a two-phase pump-driven loop for electronics cooling is presented, with the target application being a telecommunications equipment shelf having multiple circuit pack cards each dissipating several hundred Watts of power. The upward flow boiling heat transfer and pressure drop of R134a within an evaporator prototype fabricated with 18 individual microcooling zones to cool multiple electronics heat sources was investigated. The electronic heat sources were emulated by multiple copper heater blocks with embedded cartridge heaters, where each heat source was capable of dissipating more than 100 W, for a total power dissipation larger than 1800 W. Experimental results demonstrated the best cooling capability at a mass flow rate of 140 kg/h, uniform heat load of 1800 W to the 18 microcooling zones, system pressure of 600 kPa and inlet subcooling of 2 K in which the temperature difference between the evaporator and coolant inlet was 7.1 K with a uniform flow distribution within the evaporator.

Two-phase liquid cooling system for electronics, part 2: Air-cooled condenser

An experimental study investigating the thermal performance of a two-phase thermosyphon for electronics cooling is presented in this article. Two-phase cooling implemented using a gravity-driven thermosyphon-based system represents an efficient solution for dissipating high power densities compared to traditional air-cooling approaches, allowing for increased reliability and reduced power consumption. The thermosyphon-based system consists of an evaporator with 18 individual microcooling zones connected via riser and downcomer tubes to an air-cooled condenser. Experiments were carried out with working fluid R134a for filling ratios ranging from 45% to 65%, heat loads from 102 W to 1841 W and air flow rates from 516 m3/h to 1404 m3/h. Robust thermal performance was observed for the entire range of operating conditions. In particular, at the optimum filling ratio of 50%, minimum air flow rate of 516 m3/h and uniform heat load of 1841 W, the temperature difference between the evaporator and ambient air was less than 20 K with a COP of 102, while at the highest fan speed of 1404 m3/h this temperature difference was reduced to 8.9 K, with a reasonable CoP of 11. The test results show the high efficiency of the current hybrid air- and liquid-based cooling technology for removing heat from electronics to the ambient.

Role of a Liquid Accumulator in a Passive Two-Phase Liquid Cooling System for Electronics: Experimental Analysis

Gravity-driven two-phase liquid cooling systems using flow boiling within micro-scale evaporators are becoming a game changing solution for electronics cooling. The optimization of the systems filling ratio can however become a challenging problem for a system operating over a wide range of cooling capacities and temperature ranges. The benefits of a liquid accumulator to overcome this difficulty are evaluated in the present paper. An experimental thermosyphon cooling system was built to cool multiple electronic components up to a power dissipation of 1800 W. A double-ended cylinder with a volume of 150 cm(3) is evaluated as the liquid accumulator for two different system volumes (associated to two different condensers). Results demonstrated that the liquid accumulator provided robust thermal performance as a function of filling ratio for the entire range of heat loads tested. In addition, the present liquid accumulator was more effective for a small volume system, 599 cm(3), than for a large volume system, 1169 cm(3), in which the relative size of the liquid accumulator increased from 12.8 % to 25% of the total systems volume.