Chin Lee Ong

Microscale Adiabatic Gas–Liquid Annular Two-Phase Flow: Analytical Model Description, Void Fraction, and Pressure Gradient Predictions

The study is devoted to the modeling of microscale adiabatic gas–liquid annular two-phase flow. The turbulent diffusion of momentum in the annular liquid film is assumed to be governed by the conditions near the channel wall, in analogy with single-phase turbulent bounded flow. This allows the universal velocity profile for single-phase turbulent flow to be extrapolated to the annular liquid film for the prediction of the local velocity. Conservation of mass applied to the liquid film allows the calculation of the average liquid film thickness, which in turn yields the void fraction. Once the void fraction is known, conventional one-dimensional, two-fluid modeling can be applied to predict all the relevant hydrodynamic parameters, an approach applied previously to macrochannel two-phase flow that in the present article is extended to microchannels. In the article, the analytical model is described and applied to an experimental database containing about 1100 data points for refrigerants R-134a and R245fa flowing through three horizontal circular glass microchannels of inner diameters 0.52 mm, 0.80 mm, and 1.0 mm, respectively. The database includes the pressure drop, mass flow rate, and vapor quality and covers operating pressures from 155 to 877 kPa, mass fluxes from 277 to 2026 kg m−2 s−1 and vapor qualities from 0.07 to 0.92. In particular, the analytical results regarding the void fraction are shown to compare favorably with macroscale empirical correlations extrapolated to microchannels, while the two-phase friction factor is successfully correlated using just one dimensionless flow parameter (defined as the ratio of a liquid film Reynolds number to a gas core Weber number), allowing a satisfactory prediction of the measured pressure gradients.

Thermal Design of a Hierarchical Radially Expanding Cavity for Two-Phase Cooling of Integrated Circuits

A major challenge in the implementation of evaporative two-phase liquid-cooled ICs with embedded fluid microchannels/cavities is the high pressure drops arising from evaporation-induced expansion and acceleration of the flowing two-phase fluid in small hydraulic diameters. Our ongoing research effort addresses this challenge by utilizing a novel hierarchical radially expanding channel networks with a central embedded inlet manifold and drainage at the periphery of the chip stack. This paper presents a qualitative description of the thermal design process that has been adopted for this radial cavity. The thermal design process first involves construction of a system-level pressure-thermal model for the radial cavity based on both fundamental experiments as well as numerical simulations performed on the building block structures of the final architecture. Finally, this system-level pressure-thermal model can be used to identify the design space and optimize the geometry to maximize thermal performance, while respecting design specifications. This design flow presents a good case study for electrical-thermal co-design of two-phase liquid cooled ICs.Copyright

Two-phase mini-thermosyphon electronics cooling, Part 4: Application to 2U servers

This paper is the fourth part of the present study on two-phase mini-thermosyphon cooling. As mentioned in the first three parts, gravity-driven cooling systems using microchannel flow boiling can become a long-term scalable solution for cooling of datacenter servers. Indeed, the enhancement of thermal performance and the drastic reduction of power consumption together with the possibility of energy reuse and the inherent passive nature of the system offer a wide range of solutions to thermal designers. While Part 1 presented the first-of-a-kind low-height microchannel two-phase thermosyphon test results and Parts 2 and 3 showed the system scale steady and dynamic modeling and simulation results associated with this design using our inhouse simulator, Part 4 deals here with an end-user application, i.e. the cooling of a 2U server. The dynamic code of Part 3 is used to model the behavior of a mini-thermosyphon that would fit within the height of a 2U server (8.9cm high), while respecting the other geometric constraints (positions of the processors, distance of the processors to the back of the blade, etc.). Thus, the simulated system consists of two parallel multi-microchannel evaporator cold plates on the top of two chips of about 11cm2, a riser, a common water-cooled micro-condenser at the back of the blade, a liquid accumulator and a downcomer (including the piping branches to/from the two cold plates). First, an analysis of the steady-state operation highlights multiple solutions from which one is stable and one is unstable. Then, the influences of few parameters such as refrigerants, piping diameters, water coolant inlet temperature and flow rates, filling ratio and heat flux are evaluated. Simulations with unbalanced heat loads on the two chips being cooled in parallel then show the desirable flow distribution obtained in such gravity-driven systems. Finally, temporal heat load and water coolant flow rate disturbances are simulated and discussed. Noting all of these numerous influences on optimal mini-thermosyphon operation, the need for a accurate and detailed simulation code, benchmarked versus actual system tests, is seen to be imperative for attaining a good, reliable, robust design.

Two-phase mini-thermosyphon electronics cooling, Part 1: Experimental investigation

Efficient, small, state-of-the-art passive cooling two-phase systems, i.e. advanced micro-thermosyphon cooling systems, are viable solutions for high performance datacenter servers and power electronics cooling applications. The objective of this study is to push through the “two-phase threshold” that seems to be hindering the application of this cooling technology by offering here proven experimental results (Part 1), validated steady-state and transient simulation tools (Parts 2 and 3) and a server case study (Part 4). The experimental investigation in Part 1 presents the thermal-hydraulic performance of a mini-thermosyphon loop with a small riser height, Hriser = 15.0 cm. The thermosyphon loop has a multi-microchannel copper evaporator, mounted on top of a pseudo-chip CPU emulator (heat source). Experimental results for R134a, acquired under both pumped flow and passive thermosyphon driven flow (for direct comparison) for mass flow rates up to 10 kg/hr, uniform heat fluxes, q of up to 61.4 W/cm2 and refrigerant filling ratios up to 83% were obtained. An innovative thermal calibration method, developed as a non-intrusive mass flow measurement technique, has also been implemented to monitor the thermosyphons operation. Summarizing in brief, the two-phase thermosyphon loop with an integrated in-line liquid accumulator offered a very sustainable cooling performance for the microchannel/pseudo-CPU package, and is a first step forward in our effort towards the integration of such two-phase passive cooling devices for data center servers and other electronic devices at heat flux of up to 80 W/cm2 (or more).

Benchmarking Study on the Thermal Management Landscape for Three-Dimensional Integrated Circuits: From Back-Side to Volumetric Heat Removal

An overview of the thermal management landscape with focus on heat dissipation from three-dimensional (3D) chip stacks is provided in this study. Evolutionary and revolutionary topologies, such as single-side, dual-side, and finally, volumetric heat removal, are benchmarked with respect to a high-performance three-tier chip stack with an aggregate power dissipation of 672 W. The thermal budget of 50 K can be maintained by three topologies, namely: (1) dual-side cooling, implemented by a thermally active interposer, (2) interlayer cooling with four-port fluid delivery and drainage at 100 kPa pressure drop, and (3) a hybrid approach combining interlayer with embedded back-side cooling. Of all the heat-removal concepts, interlayer cooling is the only approach that scales with the number of dies in the chip stack and hence enables extreme 3D integration. However, the required size of the microchannels competes with the requirement of low through-siliconvia (TSV) heights and pitches. A scaling study was performed to derive the TSV pitch that is compatible with cooling channels to dissipate 150 W/cm per tier. An active integrated circuit (IC) area of 4 cm was considered, which had to be implemented on the varying tier count in the stack. A cuboid form factor of 2 mm 4 mm 2.55 mm results from a die count of 50. The resulting microchannels of 2 mm length allow small hydraulic diameters and thus a very high TSV density of 1837 1/mm. The accumulated heat flux and the volumetric power dissipation are as high as 7.5 kW/cm and 29 kW/cm, respectively. [DOI: 10.1115/1.4032492]

Two-phase mini-thermosyphon electronics cooling, Part 3: Transient modeling and experimental validation

This paper is the third part of the present study on two-phase mini-thermosyphon cooling systems. As mentioned in the first two parts, gravity-driven cooling systems using microchannel flow boiling are a very promising long-term viable solution for electronics cooling and more specifically for datacenter servers. Indeed, the enhancement of thermal performance and the drastic reduction of power consumption together with the possibility of energy reuse and the inherent passive nature of the system offer a wide range of solutions to thermal designers. In order to design this new type of cooling system, a new novel simulation code specifically developed for this purpose is required. While Part 2 dealt with a steady-state simulation code, the present Part 3 considers the dynamic nature of the system. The dynamic simulator is a set of connected partial differential equations (temporal and spatial) solved for the four components of the thermosyphon, meaning for the micro-evaporator, riser, condenser and downcomer. Thermal inertia of the electronic package and role of a liquid accumulator are also accounted for. Predicted steady states obtained for 6 different heat fluxes (from 15.2 to 33.1 W/cm2) are compared to experimental results obtained with the test loop presented in Part 1 in terms of chip temperature and system pressure. Mean errors of 2.9 and 3.1% are respectively found and good performances of the heat transfer prediction methods used in the simulator are emphasized. Additionally, the dynamic response to a heat load disturbance is compared with experimental results in terms of chip temperature. Two variations with different time constants are both observed experimentally and predicted numerically. Finally, the predicted mass flow rate variations are discussed.

Two-phase mini-thermosyphon electronics cooling, Part 2: Model and steady-state validations

In the present study, a simulation code specifically developed to evaluate the thermal-hydraulic performance of thermosyphon cooling loops is validated through the experimental results obtained in the Part 1. It considers levels of heat load conventionally observed in real servers of datacenters, which means idle, normal and maximum clock speed of actual microprocessors. The thermosyphon is a very compact unit with a height of 15 cm and capable of safely operating up to a heat flux of 80 W cm-2. The loop basically is comprised of a riser, a downcomer, a micro-evaporator and a counter flow tube-in-tube condenser. The latter is cooled by cold water whose mass flow rate can be controlled through an external pump (speed control), so that parameters such as saturation temperature and/or condenser outlet subcooling can be adjusted for a pre-defined set point, and thus increasing the range of operability of the cooling loop. Other parameters were also explored experimentally, cooling looping overall performance, chip (junction) temperature, whilst the critical heat flux was estimated from a leading CHF method. Finally, the study showed that the passive two-phase closed loop thermosyphon cooling system is a safe and energetically viable technology solution for the next generation of datacenters.

Ultra‐High‐Concentration Photovoltaic‐Thermal Systems Based on Microfluidic Chip‐Coolers

The electrical efficiency of a photovoltaic‐thermal system for coolant inlet temperatures ranging from 25 °C to 75 °C and concentrations from 500 to 1500 suns was investigated experimentally and theoretically. In this system absorbed radiation and thermal losses from the electric circuit are collected in a thermal circuit. This allows one to directly drive a thermal desalination process thereby contributing to an improved system efficiency. A triple‐junction solar cell was tested in two different configurations. At 1500 suns the electric efficiency of a silicon microchannel cooler package exceeded the efficiency of a reference package with a copper cooler by 2% and it remained fully functional up to concentrations of 4930 suns. We present a general model for concentrated photovoltaic‐thermal systems in which the standard efficiency modeling approaches for triple‐junction cells are extended by temperature and concentration dependencies. The currents were modeled both following the Shockley‐Queisser and a “...

Experimental Adiabatic Two-Phase Pressure Drops of R134a, R236fa and R245fa in Small Horizontal Circular Channels

Experimental adiabatic two-phase pressure drops data for refrigerants R134a, R236fa and R245fa during flow boiling in small channels with internal diameters of 1.03, 2.20 and 3.04 mm are presented. The main purpose was to investigate the effects of channel confinement on adiabatic two-phase pressure drops. Thus, the two-phase pressure drop trends were systematically investigated over a wide range of test conditions for all three refrigerants and channel sizes. Statistical comparisons have also been made by comparing the experimental pressure drop data database with various macroscale and microscale prediction methods from the literature. The comparison showed relatively moderate accuracy for three prediction methods developed for macroscale flows, i.e. Baroczy and Chisholm, Friedel and the homogeneous model with the Cicchitti et al. viscosity relation. As for microscale prediction methods, the Cioncolini et al. annular flow model worked best with 68.5% of the data within ± 30%, followed by the Sun and Mishima and the Zhang et al. methods. Combining this database with the LTCM lab’s earlier database for 0.509 and 0.790 mm channels, there appears to be no evidence of a macro-to-microscale transition, at least with respect to two-phase pressure drops.Copyright

Benchmarking Study on the Thermal Management Landscape for 3D ICs: From Back-Side to Volumetric Heat Removal

An overview of the thermal management landscape with focus on heat dissipation from 3D chip stacks is provided in this study. Evolutionary and revolutionary topologies, such as single-side, dual-side and, finally, volumetric heat removal, are benchmarked with respect to a high-performance three-tier chip stack with an aggregate power dissipation of 672 W. The thermal budget of 50 K can be maintained by three topologies, namely, 1) dual-side cooling, implemented by a thermally active interposer, 2) interlayer cooling with 4-port fluid delivery and drainage at 100 kPa pressure drop, and 3) a hybrid approach combining interlayer with embedded back-side cooling.Of all the heat-removal concepts, interlayer cooling is the only approach that scales with the number of dies in the chip stack and hence, enables extreme 3D integration. However, the required size of the microchannels competes with the requirement of low TSV heights and pitches. A scaling study was performed to derive the TSV pitch that is compatible with cooling channels to dissipate 150 W/cm2 per tier. An active IC area of 4 cm2 was considered, which had to be implemented on the varying tier count in the stack. A cuboid form factor of 2 mm × 4 mm × 2.55 mm results from a die count of 50. The resulting microchannels of 2 mm length allow small hydraulic diameters and thus a very high TSV density of 1837 1/mm2. The accumulated heat flux and the volumetric power dissipation are as high as 7.5 kW/cm2 and 29kW/cm3, respectively.Copyright