Experimental Characterization of Different Condenser Technologies in a Passive Two-Phase Cooling System for Thermal Management of Electronics

Passive two-phase, micro-channel cooling for thermal management of electronics represents an efficient and viable solution to augment conventional air-cooling systems with the potential for higher power dissipation densities, increased reliability, reduced power consumption and decreased noise levels. This paper focuses on the development of a novel thermosyphon-based cooling system for electronics. The target application is a telecommunications equipment shelf unit that comprises 18 circuit pack cards. The envisioned cooling system presented here is an infrastructure-independent, air-cooled thermosyphon loop consisting of an evaporator (cold plate), manufactured with 18 individual micro-channel zones (one per circuit pack card), and connected via riser and downcomer tubes to an air-cooled condenser. The total height of the thermosyphon loop, from mid-evaporator to mid-condenser, is approximately 50 cm. The main objective of this work is to assess the effect of different condenser technologies on the thermal-hydraulic performance of the thermosyphon; prior work examining the optimal design of the evaporator, riser and downcomer tubes has already been discussed in our previously published studies. A comprehensive experimental program was conducted to evaluate two different air-cooled condensers in a front-to-back air flow configuration: a single-pass, louvered-fin, flat-tube design and a multiple-pass, wavy-fin, circular-tube design. Thermosyphon experiments were carried out with R134a as the working fluid for filling ratios from 45% to 65%, imposed uniform and non-uniform heat loads from 102 W to 1023 W, fan speeds from 7302 min(-1) to 15480 min(-1), and fan tray configurations ranging from 5 to 7 fans. Test results demonstrate that the louvered-fin flat-tube condenser provides low liquid-side frictional pressure drops and thus high refrigerant mass flow rate in the loop, which is ideal for increasing power densities as it is more conservative with respect to dry-out phenomenon. On the other hand, the multiple-pass, wavy-fin, circular-tube condenser offers low thermosyphon thermal resistances in the entire region of heat loads, primarily due to the low air-side pressure drop.

Simulation and experimental validation of pulsating heat pipes

Philippe Aubin, Filippo Cataldo, Jackson Braz Marcinichen, John Richard Thome

The one-dimensional transient numerical code of JJ Cooling Innovation has been significantly improved to simulate the thermal/hydraulic performance of pulsating heat pipes (PHPs). Using a mechanistic approach, the liquid slug and vapor plug flows and their interaction with the surrounding wall are modeled. The heat transfer is predicted using the Three-Zone Model of flow boiling in microchannels, which includes liquid film evaporation and liquid film condensation of vapor plugs and convection of liquid slugs. Discretizing the entire PHP serpentine into one continuous grid, the code solves all variables based on local conditions. The code allows the user to define the working fluid, the channel's shape, material and wall roughness, the serpentine's pitch, the geometrical parameters of evaporator, adiabatic region and condenser zones, and the operating parameters. The boundary conditions in the evaporator zone can either be constant heat flux or constant temperature, while on the condenser side, the coolant temperature is constant as well as the heat transfer coefficient. The code works without any fitting factor and predicts all local variables in the 1D grid as a function of time, including the liquid film thickness and the onset of boiling for the formation of new vapor plugs within the evaporating and adiabatic zones, if local nucleation conditions are reached. Growth, collapse and coalescence of generated bubbles are taken into account. The film thermal resistance is corrected to a radial conduction model. The implementation of the initial liquid film thickness laid behind a liquid slug is updated to include the effect of the bubble velocity and acceleration as well as the length of the leading slug. The nucleation of new vapor plug has also been updated. The onset of nucleation is only dependent on the local thermodynamic state. If the threshold is reached, a new method is used to simulate micro-bubbles formation and coalescence to a channel-size vapor plug. These modifications greatly improve the code accuracy and stability allowing simulations with high pressure fluid rather than ethanol only. Furthermore, in the on-going EU project Pulsating Heat Pipes for Hybrid Propulsion Systems (PHP2), a new PHP test bench has been designed and built at Provides Metalmeccanica. A copper tube PHP with a 19-turn of 2.0 mm internal diameter and 12 mm tube pitch was tested over a wide range of imposed heat loads. A microchannel water-cooling plate was used at the condenser side with an inlet temperature of 20 degrees C. Then, tests were done for vertical and horizontal orientations with R1233zd(E) as the working fluid. The code, which was previously validated for ethanol PHP test data from KAIST, is now successfully validated for the present experimental data points. Accurate results were obtained from low heat loads up to 250 W for a set of 49 data points, predicting 97% within an error bandwidth of 30%.

PERGAMON-ELSEVIER SCIENCE LTD2021

State of the Art of High Heat Flux Cooling Technologies

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

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.

2007

Design Of Passive Two-Phase Thermosyphons For Server Cooling

Raffaele Luca Amalfi, Filippo Cataldo, Jackson Braz Marcinichen, John Richard Thome

The main objective of this paper is to utilize an improved version of the simulator presented at InterPACK 2017 to design a thermosyphon system for energy-efficient heat removal from 2-U servers used in high-power datacenters. Currently, between 25% and 45% of the total energy consumption of a datacenter (this number does not include the energy required to drive the fans at the server-level) is dedicated to cooling, and with a predicted annual growth rate of about 15% (or higher) coupled with the plan of building numerous new datacenters to handle the "big data" storage and processing demands of emerging 5G networks, artificial intelligence, electrical vehicles, etc., the development of novel, high efficiency cooling technologies becomes extremely important for curbing the use of energy in datacenters. Notably, going from air cooling to two-phase cooling, not only enables the possibility to handle the ever higher heat fluxes and heat loads of new servers, but it also provides an energyefficient solution to be implemented for all servers of a datacenter to reduce the total energy consumption of the entire cooling system. In that light, a pseudo -chip with a footprint area of 4 x 4 cm2 and a maximum power dissipation of 300 W (corresponding heat flux of about 19 W/cm2), will be assumed as a target design for our novel thermosyphon-based cooling system. The simulator will be first validated against an independent database and then used to find the optimal design of the chip's thermosyphon. The results demonstrate the capability of this simulator to model all of the thermosyphon's components (evaporator, condenser, riser and downcomer) together with overall thermal performance and creation of operational maps. Additionally, the simulator is used here to design two types of passive two-phase systems, an air- and a liquid-cooled thermosyphon, which will be compared in terms of thermal -hydraulic performance. Finally, the simulator will be used to perform a sensitivity analysis on the secondary coolant side conditions (inlet temperature and mass flow rate) to evaluate their effect on the system performance.