Milnes David

VAPOR-VENTING, MICROMACHINED HEAT EXCHANGER FOR ELECTRONICS COOLING

The increasing complexity of modern integrated circuits and need for high-heat flux removal with low junction temperatures motivates research in a wide variety of cooling and refrigeration technologies. Two-phase liquid cooling is especially attractive due to high efficiency and low thermal resistances. While two-phase microfluidic cooling offers important benefits in required flow rate and pump size, there are substantial challenges related to flow stability and effective superheating. This work investigates the use of hydrophobic membrane to locally vent the vapor phase in microfluidic heat exchangers. Previous work has demonstrated selective venting of gas in microstructures and we extend this concept to two-phase heat exchangers. This paper details the design, fabrication and preliminary testing of the novel heat exchanger. Proof-of-concept of the device, carried out using an isothermal air-water mixture, found the air-mass venting efficiency exceeding 95%. Two-phase, thermal operation of the heat exchanger found the pressure-drop to be smaller compared to a two-phase, non-venting model. The paper also includes a discussion of design challenges such as membrane leakage and optical inaccessibility. The favorable results demonstrated in this first-generation, vapor-venting, micromachined, heat exchanger motivates further study of this and other novel microstructures aimed at mitigating the negative effects of phase-change. With continued research and optimization, we believe two-phase cooling is a viable solution for high heat flux generating electronics.Copyright

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

Hydrodynamic and Thermal Performance of a Vapor-Venting Microchannel Copper Heat Exchanger

Two-phase microfluidic cooling has the potential to achieve low thermal resistances with relatively small pumping power requirements compared to single-phase heat exchanger technology. Two-phase cooling systems face practical challenges however, due to the instabilities, large pressure drop, and dry-out potential associated with the vapor phase. Our past work demonstrated that a novel vapor-venting membrane attached to a silicon microchannel heat exchanger can reduce the pressure drop for two-phase convection. This work develops two different types of vapor-venting copper heat exchangers with integrated hydrophobic PTFE membranes and attached thermocouples to quantify the thermal resistance and pressure-drop improvement over a non-venting control. The first type of heat exchanger, consisting of a PTFE phase separation membrane and a 170 micron thick carbon-fiber support membrane, shows no improvement in the thermal resistance and pressure drop. The results suggest that condensation and leakage into the carbon-fiber membrane suppresses venting and results in poor device performance. The second type of heat exchanger, which evacuates any liquid water on the vapor side of the PTFE membrane using 200 ml/min of air, reduces the thermal resistance by almost 35% in the single-phase regime in comparison. This work shows that water management, mechanical and surface properties of the membrane as well as its attachment and support within the heat exchanger are all key elements of the design of vapor-venting 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

Application of a Modified Quality Function Deployment Method for MEMS

Quality Function Deployment (QFD) has long been used as a successful design methodology in the heavy industrial and automotive industries. QFD helps designers utilize the ‘voice of the customer’, or customer requirements, to determine which engineering metrics or product specifications are the most essential [1]. This prioritization helps designers know what part of the product or process is most beneficial to focus on during design, resulting in products that better meet customer requirements and generate increased commercial success. QFD and most other design methodologies have rarely been applied to MEMS products [2]. In the case of QFD, the structure of the most common format of the tool dictates that engineering metrics should be related to parts characteristics in the second step of applying QFD. This causes difficulties in using the tool for MEMS as most MEMS do not have physical ‘parts’ that are assembled into a final device. Rather, MEMS have product specifications and a manufacturing process used to create the product. Generally there is a tight link between product and process in MEMS. This link has been utilized in creating a modified version of QFD that relates engineering metrics to design concepts, including product conceptualization and manufacturing process. The modified QFD utilizes aspects of Pugh Concept Selection, and differs from typical QFD primarily in consideration of product idea and manufacturing process in the early phases of product definition. The modified QFD was applied to a MEMS project whose goal was to develop a handheld device that allows users to control the selection and release of a variety of stored scents. The technique was also applied to a microscale heat exchanger for integrated circuits. The scent dispenser and heat exchanger were designed and prototyped at Stanford University in 2005 and 2006, respectively. The modified version of QFD gave insight early in the product definition phase on which design concept to pursue to prototype. Use of this and other design methodologies in the MEMS field could shorten the time it takes to progress through product development to volume manufacturing, and increase confidence in the marketability of the chosen design and manufacturing process. A case study demonstrating the effects of using modified QFD Phase II to assist in finding a good fit between technical capabilities and market application was performed by the author on an acoustic sensor technology [3].Copyright

Enhanced gaseous natural convection from micro-scale fin structures using acoustic stimulation

This paper investigates the effect of acoustic stimulation on natural convection heat transfer in micro-scale fin structures. Enhancement of heat transfer from electronic components enclosed within compact geometries is a key goal in thermal management. Among the techniques to enhance heat transfer, extended surface is a simple approach to achieve the goal. However, micro scale fin heat sinks have little effect on natural convection heat transfer. Previous work has shown that heat transfer via flow-induced vibration in a micro fin array can be enhanced, but only in a forced convection scheme with high flow rates. Acoustic stimulation has also been investigated in boiling and we extend this method of stimulation to enhance gaseous natural convection. The acoustic stimulation is generated using a speaker placed close to the micro fin array device. Waves of varying amplitude and frequency are generated and device surface temperatures recorded along with the speaker velocities, measured using Laser Doppler Velocimetry. The results obtained suggest that natural convection heat transfer can be enhanced using acoustic stimulation in micro-scale fin structures where the fins contribute negligible effect on overall heat transfer, and the enhancement varies with fin geometries. The results indicate that gaseous natural convection is feasible as a thermal management scheme in micro fin structures for low heat flux applications. Future works will be devoted to finding the resonance frequency of natural convection in micro-scale fin structures that can enhance heat transfer dramatically with little additional power consumption.

3-D Visualization of Flow in Microscale Jet Impingement System

This paper presents our recent work on the reconstruction of three-dimensional (3-D) flow fields in a microscale jet impingement cooling flows. We use micron-resolution PIV to capture successive two-dimensional (2-D) “slices” of the flow field in a 3-D structure, and compare with models of flow near microscale jets. This approach is specifically constructed to determine the z-component of the flow velocities, which play a key role in enhancing heat transfer at the impingement surface. These new results will enable a predictive design capability for microjet impingement cooling structures for high power electronics.© 2009 ASME