David W. Fogg

Advanced cooling technologies for microprocessors

Recent trends in processor power for the next generation devices point clearly to significant increase in processor heat dissipation over the coming years. In the desktop system design space, the tendency has been to minimize system enclosure size while maximizing performance, which in turn leads to high power densities in future generation systems. The current thermal solutions used today consist of advanced heat sink designs and heat pipe designs with forced air cooling to cool high power processors. However, these techniques are already reaching their limits to handle high heat flux, and there is a strong need for development of more efficient cooling systems which are scalable to handle the high heat flux generated by the future products. To meet this challenge, there has been research in academia and in industry to explore alternative methods for extracting heat from high-density power sources in electronic systems. This talk will discuss the issues surrounding device cooling, from the transistor level to the system level, and describe system-level solutions being developed for desktop computer applications developed in our group at Stanford University.

Bubble-Induced Water Hammer and Cavitation in Microchannel Flow Boiling

While microchannel flow boiling has received much research attention, past work has not considered the impact of acoustic waves generated by rapidly nucleating bubbles. The present work provides a theoretical framework for these pressure waves, which resembles classical “water hammer” theory and predicts a strong influence on bubble nucleation rates and effective convection coefficients. These pressure waves result directly from confinement in microchannel geometries, reflect from geometrical transitions, and superimpose to create large transients in the static liquid pressure. Feedback from the pressure waves inhibits bubble growth rates, reducing the effective heat transfer. Pressure depressions generated by the propagating pressure pulses can cause other bubbles to grow at lower than expected wall temperatures. The additional nucleation enhances heat transfer over short times but increased flow instability may inhibit heat transfer over longer periods. The limited quantitative measurements available in the literature indicate confined bubble growth rates in microchannels are significantly lower than those predicted by the classical Rayleigh‐Plesset equation. The present model predicts confined bubble growth rates to within 20%. A nondimensional number indicative of the relative magnitude of the water hammer pressure to bubble pressure is proposed to characterize the transitions from conventional to microchannel flow boiling. DOI: 10.1115/1.3216381

Two-phase microfluidics for semiconductor circuits and fuel cells

Industrial trends are presenting major challenges and opportunities for research on two-phase flows in microchannels. Semiconductor companies are developing 3D circuits for which multilevel microfluidic cooling is important. Gas delivery microchannels are promising for PEM fuel cells in portable electronics. However, data and modeling are needed for flow regime stability, liquid entrainment/clogging, and bubble inception/departure in complex 2D and 3D geometries. This paper provides an overview of the Stanford two-phase microfluidics program, with a focus on recent experimental and theoretical progress. Microfabrication technologies are used to distribute heaters, thermometers, pressure sensors, and liquid injection ports along the flow path. Liquid PIV quantifies forces on bubbles, and fluorescence imaging detects flow shapes and liquid volume fraction. Separated flow models account for conjugate conduction, liquid injection, evaporation, and a variety of flow regimes. This work benefits strongly from interactions with semiconductor and fuel cell companies seeking validated models for product design.

Piranha Pin-Fins (PPF): Voracious boiling heat transfer by vapor venting from microchannels - system calibration and single-phase fluid dynamics

A novel approach to embedded electronics cooling with a multi-phase microfluidic heat sink termed the Piranha Pin-Fin (PPF) is presented. Several first-generation PPF devices, as well as plain-channel and solid pin-fin heat sinks, have been fabricated and experimentally tested under single-phase adiabatic conditions. Details of the PPF device geometry and microfabrication process are provided. Plots showing pressure drop and friction factor are also provided. Numerical fluid dynamics modeling has been performed in parallel to the experiments. Modeling data presented includes fractional flow through the pins, predicted pressure losses, fluid streamlines and velocity gradients under several operating conditions. Additionally, micro-particle image velocimetry (μPIV) measurements have been performed. The velocity fields are used to provide further insight into the fluid mechanics within the heat sink as well as to validate the models. Velocity field measurements are included for various operating conditions.

Boiling Flow Interaction Between Two Parallel Microchannels

Microchannel heat exchangers predominately use a parallel channel configuration to maximize heat transfer with minimal pump demand. Previous work optimized bulk performance of liquid flow heat exchangers but noted that upon boiling, flow redistributed among parallel channels, and they ultimately found that this instability caused an uncontrollable operating condition. This work predicts and measures fully coupled boiling flow interaction in a simplified two microchannel system. A series of silicon microfabricated devices enable piecewise study of the coupled fluidic and heat transfer interactions, first uniting the fluid inlets of thermally isolated channels, then connecting neighboring channel walls to allow heat transfer between channels. Multiple combinations of boiling and liquid flow, each satisfying system boundary conditions, are identified using flow demand curves assembled from single channel data. Each unique flow condition is experimentally demonstrated and found to be heavily dependent on the prior state of the channels. Connecting channel walls, thermally, is shown to lessen the number of allowed solutions and increase instability in the two channel system, allowing distinction between purely fluidic instabilities and fluidic instabilities coupled to heat transfer between channels. This work in describing interaction between two channels is a necessary step as work continues toward characterizing flow boiling in more complex parallel channel heat sinks.Copyright

MICROCHANNEL EXPERIMENTAL STRUCTURE FOR MEASURING TEMPERATURE FIELDS DURING CONVECTIVE BOILING

This work designs and fabricates a microchannel structure for measurement of wall temperature fields in two-phase flow. The microchannel with hydraulic diameter of 100 micrometers is etched into a suspended beam of silicon with three independently heated regions and integrated doped silicon resistors sensitive to channel temperature. Doped silicon resistors are also sensitive to strain in the silicon caused by pressure transients in the channel, so sensors are designed with two different orientations and thus two different piezoresistive coefficients to allow decoupling of pressure and temperature effects. Use of a 400 micrometer wide suspended beam reduces side-wall conduction compared to a bulk sample and provides better opportunities to measure the influence of flow regimes on heat transfer coefficients in future work. Use of the central heater reduces fluid preheating in the inlet plenum. The measured temperature distributions at flowrates up to 0.25 ml/min with heat fluxes into the silicon beam up to 78 W/cm 2 show initial capabilities of the structure.

Numerical Simulation of Transient Boiling Convection in Microchannels

Two-phase microchannel heat exchangers are receiving increasing attention from the microprocessor industry as power density levels in microchips increase. Previous numerical investigations of convective boiling in microchannels assumed steady flow within the channels. However, experimental data shows that two-phase flows in microchannels are highly transient even under steady heat loads. Little work has been done to model the dynamics associated with vapor generation in microchannels. The present work simulates the periodic distribution of vapor within microchannels filled with water by solving one-dimensional homogeneous equations for the mass, momentum and energy transport in conjunction with a transient wall conduction equation. A wall superheat constraint is incorporated to account for the excess superheat temperature required for bubble nucleation. Boiling events reduce the local wall temperature and change the pressure and enthalpy distributions within the flow. The transient pressure fluctuations predicted here are consistent with those observed in experiments. This study provides insight into the significance of bubble nucleation for forced convective boiling in microchannels and will be useful for the optimization of microchannel heat exchangers.Copyright

Development and Calibration of a Two-Dye Fluorescence System for Use in Two-Phase Micro Flow Thermometry

The increasing need for more effective cooling in electronic devices has led to research into the use and modeling of two-phase cooling strategies in micro-scale geometries. In order to verify these models it is necessary to reliably determine local parameters such as fluid temperature in boiling flows, which cannot be easily obtained due to micro-scale geometries. A convenient and non-contact method of thermometry is the use of fluorescence where the emitted intensity is a function of the local temperature. Previous work has verified the ability to use single dyes to measure void fraction in isothermal cases and suggested the possibility of simultaneous thermometry using a system of two dyes. In the present work, we verify the ability to measure the temperature of a single-phase liquid system using pairs of dyes and to this end also determine if two-dye temperature-intensity calibration curves could be accurately constructed from single dye calibration curves. The experimental set-up and procedure yield calibration results from 300K to 400K for Stilbene 420, Kiton Red, Rhodamine B and Fluorescein. Two-dye calibration curves are constructed from single-dye calibration curves and compared experimentally to a system containing two dyes in mixture. The small variation in predicted and actual responses suggest that two-dye systems should be calibrated in mixture form, and if done accurately, have the potential to measure the liquid temperature of a two-phase system

The Effects of Liquid Compressibility on the Nucleation and Growth of Bubbles in Forced Convective Flow Boiling Within Microchannels

Forced convective flow boiling in microchannels is characterized by the nucleation and rapid growth of vapor bubbles in confined geometries. Experimental studies of these flows have been limited to the measurement of wall temperature, inlet liquid velocity, inlet and outlet pressures, and high speed imaging forcing analysts to infer the conditions inside the channel from measured external values. The present study examines the evolution of the pressure field during bubble growth prior to bubble departure using a one-dimensional fully compressible Lagrangian-Eulerian model. Numerical results for a single bubble growing from a nucleation site for both constant pressure and constant volumetric flow rate conditions demonstrate the magnitude of the pressures generated and bound the magnitude of the reflected pulses from the channel ends. The reflected pulses can locally decrease the pressure in the channel below the levels predicted by incompressible models. Additional simulations predict nucleation and growth of bubbles at sites that would be inactive if liquid compressibility is neglected. The results indicate the acoustic characteristics of microchannels for flow boiling can not be neglected and will be important in the optimization of microchannel designs.Copyright

Bubble Departure and Convection in Large Aspect Ratio Microchannels

The growth and departure of vapor bubbles governs pressure drop and thermal resistance of two-phase microchannel heat sinks. Little data is available for the growth, departure, and convection of bubbles in microchannels. The current study uses isothermal air injection to simulate the nucleation and growth of bubbles in high aspect microchannels with Dh ≈48μm and aspect ratios from 20 to 40 with 1 < ReH < 10. Liquid pressure drop and flow rate are measured during bubble growth along with the time history of the bubble geometry obtained from a high speed video imaging system at rates up to 50,000 frames per second. Bubble departure is found to vary linearly with aspect ratio divided by inlet Reynolds number, while the convection velocity depends on the normalized bubble width and normalized liquid film thickness. A scaling analysis identifies the increase in axial pressure drop due to bubble confinement as the driving force for both bubble departure and convection.Copyright