Small Science Big Impact
Published Nov. 19, 2010
Researchers at RIT's Thermal Analysis, Microfluidics, and Fuel Cell Laboratory have gained international recognition for their fundamental research. With nearly two dozen student researchers, the laboratory immerses students in real-world research that establishes students within the industry long before graduation.
Fundamental Research Fuels Future Technologies
The saying goes, "Don't sweat the small stuff," but at RIT's Thermal Analysis, Microfluidics, and Fuel Cell Laboratory (TAμFL), it's the small stuff that has resulted in industry breakthroughs and earned the laboratory international recognition. Nearly 20 years ago, Dr. Satish Kandlikar, professor of mechanical engineering, founded the laboratory to focus on understanding the fundamentals of microfluidics and the phenomena of heat transfer. The research conducted is driven by actual industry challenges that allow students to learn at a level that will enable them to be innovators and influence future technologies.
The lab's early research focused on microscale fluid mechanics and heat transfer and positioned it as an international leader. Today, the lab has expanded to encompass a range of microfluidic applications, including pool boiling, heat transfer on a silicon chip, and water management of proton exchange membrane (PEM) fuel cells.
Revealing Roughness Effects in Microflows
The effect of surface roughness on internal fluid flow has been apparent since 1933, but until recently the effect at the microscale was never fully characterized. In 2005, the lab opened a new area of study looking at the roughness effects on flow at the microscale. "Things are pretty well known for macroflows, but when you go down to microfluid flows—in channels smaller than a few hundred micrometers—things start to look different. By characterizing the microflow, it bridges the gap between macro and micro into one theory," explains Dwight Cooke, a master's degree student in mechanical engineering.
Roughness Effects Enhance Heat Transfer
Numerical and experimental studies show rougher microchannels have the ability to enhance heat transfer, but at the same time increase pressure drop. This is believed to be the first effort to systematically characterize the effect of structured roughness on fluid flow and heat transfer.
After carefully studying the literature published over 60 years, Kandlikar developed a modified Moody diagram that provides an accurate, yet very simple, way of calculating pressure drop in pipes at the macroscale, as well as at the microscale. Derek Schmidt and Tim Brackbill, master's degree graduates from the lab, confirmed the validity of the theory based on carefully conducted microscale experiments on rough tubes.
"Roughness inside the walls is on the order of micrometers—at the same size of the channels," says Viral Vinodray Dharaiya, a master's student who is building on to the research. "We can't neglect how the roughness can affect the fluid flow and heat transfer."
Dharaiya is conducting numerical simulations for both smooth and triangular ribbed geometries and analyzing the resulting pressure drop through a National Science Foundation (NSF) sponsored project. The numerical simulation results are validated in an experimental investigation being conducted by graduate students Rebecca Wagner and Nicholas Schneider. The research shows rougher channels have enhanced heat transfer, but at the same time increased the pressure drop. This work is believed to be the first of its kind to systematically characterize the effect of structured roughness on fluid flow and heat transfer. The goal is to develop designer surfaces that will provide the desired thermohydraulic characteristics at microscale using roughness structures.
The modified Moody Diagram has positioned the group as leaders within the engineering community. Additionally, they are recognized for coining the industry- and academia-accepted definitions for microchannels, which are 200 micrometers and smaller in diameter; minichannels, which range from 200 micrometers to 3 millimeters; and nanochannels, which are below 1 micrometer.
Understanding Bubbles
Another key area of research the lab focuses on is boiling heat transfer and chip cooling. Recently, the lab partnered with IBM through a sequence of three (maximum possible) IBM Faculty Awards to conduct fundamental research that aided in the development of a proprietary chip design capable of cooling chips at approximately 700 watts per square centimeter. This work builds on some of the lab's pioneering research in the fundamental understanding of the boiling phenomena. More recently the research focuses on enhancing heat transfer on chips through the analysis of fundamental mechanisms of pool boiling and critical heat flux using high-speed cameras and microscopes.
Predicting Bubble Behavior
In-depth analysis of the pool boiling visualizations uncovered a bubble nucleating on the bottom surface (a-b), then moving to the top fin of the channel where it attaches itself and rapidly grows (c-d). The lab continues to investigate this finding to better predict bubble behavior and develop an optimal surface for heat transfer.
"In pool boiling there is a stagnant liquid that is heated up, as opposed to flow boiling, which requires a more active system," explains Cooke. "This approach has the potential to remove a large amount of heat resulting from the evaporation of liquid into vapor phase with little pressure drop penalty."
Using high-speed video imagery, as well as quantitative data, the lab is investigating the effects of different geometries on the pool boiling performance. "Studies have been conducted on roughening up the channels to promote bubble growth, but it's not understood how the bubbles emerge, how they interact, and what actually makes it better," says Cooke.
Five particular chips are being investigated, each having overall dimensions of 20 mm x 20 mm with a heated micromachined area of 10 mm x 10 mm and either 200 or 100 micrometer channels. The visualizations are captured at 1,000 frames per second, then closely analyzed to understand the bubble nucleation and growth in microchannels. All of the chips show an increase in performance compared to a plain chip, which may be explained by the increased surface area of the etched chips that allows for more heat transfer. Additionally, the structure of the chips affects the fundamental mechanics of bubble dynamics in pool boiling.
After watching countless hours of footage, Cooke also began to notice an extraordinary pattern. "I discovered a bubble nucleate on the bottom surface, then remarkably move to the fin—or top—of the channel, where it attaches itself and rapidly grows," Cooke explains. The group is taking a closer look at these findings to develop an optimal setup and better predict the bubble behavior. The work is also being extended to the application of nanotubes to enhance heat transfer during boiling on a silicon chip in collaboration with Dr. Yen-Wen Lu at the National Taiwan University under another National Science Foundation grant.
An Industry Partnership to Advance Water Management in PEM Fuel Cells
One of the lab's most successful areas of research deals with water management in PEM fuel cells. Through a series of Department of Energy (DOE) and New York State Energy Research and Development Authority grants, the lab works intimately with General Motors to obtain a fundamental understanding of transport phenomena, which helps optimize material selection and product design.
Water management within a PEM fuel cell is critical to optimizing the cell's performance and longevity, especially under cold weather and freezing conditions. Without sufficient hydration, proton conductivity of the membrane cannot be maintained, but an excess amount of water can lead to flooding, which blocks reaction sites and hinders the flow of reactants. The lab is conducting visualization studies to provide a fundamental understanding of water transport within the fuel cell and identify the optimal balance of water. The process can be investigated at any level of the cell. At TAμFL, one group is looking at the channel level two-phase flow, while another group focuses on the gas diffusion layer (GDL).
Due to the complexity of a fuel cell, there are limited techniques available for characterizing the flow inside the cell. In the RIT lab, researchers are using optical imaging, which requires the development of a modified cell with transparent components. Jacqueline Sergi, currently a mechanical engineering master's degree student, helped to design a transparent fuel cell while on co-op at General Motors.
Optical Visualization
A transparent fuel cell designed at the lab represents actual auto motive geometry and allows for simultaneous visualization of the anode and cathode. Highspeed imaging is used to analyze the twophase flow and water structures inside the cell's microchannels (A). A MATLAB algorithm developed by Jacqueline Sergi, a mechanical engineering master's degree student, allows the water inside the microchannels to be automatically detected and quantified (B).
Using high-speed cameras, optical visualization studies are conducted at TAμFL to analyze the flow right down to the structure of the water. The fuel cell is transparent on both sides, which allows the anode and cathode flow channels to be viewed simultaneously. During operation, two-phase flow within the cell is captured using highspeed cameras. The flow is then characterized, and the frequency of different flow types is identified.
Sergi also developed an imageprocessing algorithm in MATLAB, which automatically detects pixels in the videos that represent liquid water. This processing technique enables the quantification of water inside the microchannels.
"Satish is internationally recognized for his expertise in this area. By partnering with his lab, we are able to gain fundamental knowledge to understand why something works and receive accurate parameters to help optimize our designs," says Jon Owejan, senior research engineer at General Motors.
Complementary work is conducted by General Motors using neutron imaging, which allows a fuel cell to be imaged without any modification to the standard cell materials. Because neutrons are able to pass through fuel cell materials with little attenuation but are heavily scattered by hydrogen, liquid water accumulation inside an operating fuel cell can be probed without modification to the design or materials. The capability was developed jointly by General Motors and the National Institute of Standards and Technology (NIST), led by Owejan and Thomas Trabold, former laboratory group manager at General Motors and now associate research professor at the Golisano Institute for Sustainability.
Michael Daino, a microsystems doctoral student, is investigating the GDL, a porous carbon fiber paper critical to water management. The GDL distributes the reactants throughout the cell and transports product water from the reaction sites into the gas channels. In recent years, advancements to this fibrous material have improved water transport dramatically. However, the transport mechanisms continue to be debated because of the inability to probe the opaque material with high spatial resolution. Daino's research utilizes a confocal laser scanning microscope to examine the 3-dimensional microstructure of the GDL and reveal pore-scale properties that affect water transport. He also observes the GDL cross-section in an operating fuel cell using a digital microscope and high-resolution infrared camera.
The high-resolution cross-sectional imagery reveals anode GDL water transport is dominantly in the vapor phase as opposed to the cathode GDL where liquid water is routinely observed. The infrared imaging is utilized to determine the temperature gradient across the 200-micrometer-thick material, which affects the water transport through the GDL. This novel study allows for the direct comparison of GDL thermal properties to fuel cell performance.
"This partnership with General Motors provides an invaluable experience for our students," says Kandlikar. "They are able to participate in research using the most sophisticated experimental techniques and have a direct influence on industry even before they graduate."
Research Experience Provides a World of Opportunities
"Students are the centerpiece of the research at TAμFL. It is amazing to witness the dramatic transformation as students unravel a new dimension of their talent and are contributing at the cutting edge of the technology. They are on their way to become the top researchers in their chosen field with an unparalleled combination of fundamental and applied science perspective," says Kandlikar.
Every student in the lab publishes at least one journal paper before graduation—many become top-cited in his or her particular field of research. Dr. Mark Steinke, the university's first microsystems engineering doctoral graduate and now a thermal engineer at IBM, published over 12 conference and journal papers.
This summer Kandlikar hosted the 8th annual International Conference on Nano channels, Microchannels, and Minic hannels in Montreal. Kandlikar is also regularly invited to give keynote and plenary lectures around the world, including engagements in Montreal, Washington, D.C., La Grande-Motte, Moscow, and Fukuoka.
Under Kandlikar's leadership his students are immersed early on into the global research community. "This experience has allowed me to see that my research is relevant on a global level and has exposed me to a whole world of opportunities," says Sergi.
Thermal Analysis, Microfluidics, and Fuel Cell Laboratory
Led by Dr. Satish Kandlikar, TAμFL provides students with real-world research challenges that prepare them to be leaders in their industry. Students use the most sophisticated experimental techniques and are making significant contributions to industry long before graduation.
