Roger D. Flynn

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.

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.

Temperature-Dependent Permeability of Microporous Membranes for Vapor Venting Heat Exchangers

Improved flow regime stability and lower pressure drop may be possible in two-phase microfluidic heat exchangers through the use of a hydrophobic membrane for phase separation. Past research on vapor-venting heat exchangers showed that membrane mechanical and hydrodynamic properties are crucial for heat exchanger design. However, previous characterizations of hydrophobic membranes were primarily carried out at room temperatures with air or nitrogen, as opposed to liquid water and steam at the elevated operating temperature of the heat exchangers. This work investigates laminated PTFE, unlaminated PTFE, and nylon membranes and quantifies the permeability of the membranes to air and steam. The pressure drop across the membrane as a function of fluid flow rate and temperature characterizes the membrane permeability. This work will facilitate more focused experimental work and predictive modeling on optimizing membrane properties and will help with the development of more effective vapor venting heat exchangers.Copyright

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.

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

Convective Heat Transfer of Nanofluids (DI Water-Al2O3) in Micro-Channels

The convection performance of nanofluids in microchannels has received relatively little attention. This work reports convective heat transfer experiments of deionized water/Al2 O3 nanofluids using 200μm hydraulic diameter MEMs fabricated microchannel structures and a stainless steel tube with 250μm inside diameter. The tube wall is heated electrically producing a constant heat flux boundary condition and an infrared camera is used to measure the outside tube wall temperature. A full numerical conjugate analysis of the apparatus is used to infer the fluid thermal conductivity from the temperature measurements. The effective thermal conductivity of nanofluids increased only by 4% for 4% volume concentration nanofluids in the MEMs fabricated microchannel and 5% for 3% volume concentration in the stainless steel tube under laminar flow conditions. The effective viscosity of the nanofluids increased 12% for 2% volume concentration. A dynamic light scattering system was used to measure the effective particle diameter and particle size distributions of nanoparticles with various pH values and surfactants. The measured mean diameter of Al2 O3 nanoparticle is 170 nm, which is larger than the 40–50 nm nominal size.Copyright

Decoupled Thermal and Fluidic Effects on Hotspot Cooling in a Boiling Flow Microchannel Heat Sink

Heat sinks for next generation microprocessors must remove increasing levels of power with non-uniform spatial distribution (hotspots). Two-phase convection promises strongly reduced pump size but is challenging because of boiling flow instabilities. This work studies parallel microchannel boiling stability using a series of dual channel devices which are fabricated with varying lateral thermal resistance and integrated heaters and thermometers. The data are consistent with a demand curve analysis predicting flow distribution and wall temperatures in thermally isolated parallel channels with strictly fluidic interaction. Increasing the thermal resistance between two parallel channels is shown to strongly influence the onset of instabilities and adversely increase the peak temperature. These dual-channel experiments capture the key physics of multichannel instabilities and provide the foundation for improved design of two-phase microfluidic heat sinks.Copyright

Convective Boiling in Silicon Microchannels With Localized Heating and Thermometry

While silicon microchannel heat sinks are promising for high heat flux integrated circuits, they have not reached their potential because microscale convective boiling is poorly understood. Previous work integrated sensors and heaters into a silicon chip to provide distributed thermometry, but did not specifically examine hotspots or thoroughly treat experimental uncertainty. This work microfabricates a single channel in a thinned silicon beam, instrumented with doped sensors and aluminum heaters, to study the wall temperature and fluidic response to flow boiling induced by non-uniform heating. Uncertainty analysis shows a need for better measurements of the fabricated channel including channel cross section and surface roughness. Refined data from this work will suggest improvements to existing boiling flow models, which may then be implemented into a design tool for optimizing boiling flow microchannel heat sinks.Copyright