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The primary objective of the project is to conduct a fundamental study of the various transport phenomena (flow boiling heat transfer, two-phase pressure drop, flow pattern, and flow instabilities) associated with flow boiling of methanol-water binary mixtures in multiple parallel micro-channels with characteristic size ranging from 10 to 1000 microns. Flow boiling of binary mixtures in micro-scale geometries has received little research attention so far and could be quite different from flow boiling of pure liquids in identical geometries due to the effect of mixture composition (mixture effect).

In addition to its fundamental merits, the project will have a broader impact on a variety of chemical and mechanical applications that involve binary mixture flow boiling in micro-scale geometries. For instance, the outcome of the research may be applied to the thermal design of miniature chemical reactors. Miniature chemical reactors refer to devices that integrate the various transport and chemical processes into a flow system composed of micro-channels. Flow boiling of binary and multi-component mixtures is a common process in these devices, as feedstock to many gas-phase reactors are mixtures in liquid phase. This can be better illustrated by using an example of micro methanol steam reformers to produce hydrogen gas. In these devices, liquid water and liquid methanol were first mixed to form a binary mixture in the mixer. The binary mixture was then converted to vapor phase in the vaporizer through flow boiling before entering the catalytic reformer for chemical reaction. Knowledge of micro-scale mixture flow boiling will help better design the vaporizer, which may lead to improved reactor performance.

During the past project period, an experimental system has been designed and constructed in our research laboratory. Using the experimental system, a primary experimental study has been performed on methanol-water mixture flow boiling heat transfer, two-phase pressure drop, and two-phase flow instabilities in micro-channels having a 240-micron by 640-micron cross-section. Key results up to date are summarized below.

The experimental system that was developed is composed of a two-phase flow loop, a micro-channel test module, and instrumentation. The flow loop was configured to supply methanol-water mixtures to the micro-channel test module at desire testing conditions. The micro-channel test module consisted of an oxygen-free copper micro-channel test section, a G-7 fiberglass housing, a transparent polycarbonate plastic cover plate, and eight cartridge heaters. The top surface of the test section measured 1.0 cm in width and 4.0 cm in length. Twenty-two rectangular 240-micron wide and 630-micron deep micro-slots were formed within the 1-cm width of the test section top surface. Eight holes were drilled into the test section bottom surface to accommodate the cartridge heaters that provided heat power during flow boiling tests. The test section was inserted into the central portion of the housing. The cover plate was then bolted atop to form closed micro-channels. The transparent cover plate facilitated direct visual access to the flow boiling process in these micro-channels. Key instrumentation included rotameters to measure mixture flow rate, thermocouples to measure mixture inlet and outlet temperatures as well as temperatures inside the micro-channel test section, pressure transducers to measure pressure drop and pressure level, and a precision power meter to measure heating power input.

Primary experimental studies were performed using both pure water and methanol-water mixtures as testing fluids. The pure water experimental data served as the baseline against which the “mixture effect” can be assessed. After tests with pure water were complete, two methanol-water binary mixtures with methanol molar fraction of 36% and 63% were prepared and tested. Tests with each testing fluid were conducted over a mass velocity range of 160 - 500 kg/m2s and at an mixture inlet temperature of 30 oC.

The testing results showed a number of intriguing characteristics of micro-scale mixture flow boiling. Both dissipative heat flux and micro-channel wall temperature at the onset of flow boiling in the micro-channels decreased with increasing methanol molar fraction. For a given dissipative heat flux, micro-channel temperature decreased with increasing methanol molar fraction. Two-phase pressure drop across the micro-channels increased appreciably once flow boiling occurred in micro-channels. For a given dissipative heat flux, pressure drop increased with increasing methanol molar fraction. The methanol-water binary mixtures offered better two-phase flow stability than the pure water. Further in-depth analysis is being conducted to explore these unique parametric trends.

The project allows the PI to further advance the frontier of the field of micro-scale flow boiling. While the subject matters of binary mixture flow boiling in conventional millimeter or larger diameter channels as well as pure liquid flow boiling in micro-scale structures have been studied quite extensively in the past, thermal and hydraulic aspects of binary mixture flow boiling in micro-scale structures remain largely unexplored. The coupled effect of micro-scale flow passage size (micro-scale effect) and mixture composition (mixture effect) on the flow boiling process will be fully explored by the end of the project period. Possible heat transfer degradation associated with mixture flow boiling will be examined with its mechanism fully identified.

The project positively influences graduate student education through research activities. The research is an integral part of the student' master program. The graduate student is able to gain valuable research experience and develop broader engineering science knowledge as well as specific insights into the cutting-edge micro-scale flow boiling processes.