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The project investigated two-phase pressure drop, flow boiling heat transfer, and flow instability associated with flow boiling of methanol-water binary mixtures in micro-channels.  While pure liquid flow boiling in micro-scale structures has been studied quite extensively in the past, binary mixture flow boiling in similar geometries has received little research attention so far and could behave quite differently due to the effect of mixture composition (mixture effect).

The work has a broader impact on a variety of mechanical and chemical applications that involve binary mixture flow boiling in micro-scale structures.  For instance, the research results 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.  The experimental system 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.

An experimental study was performed to investigate flow boiling characteristics of methanol-water mixtures in the micro-channels.  Tests are conducted with pure water and methanol as well as five methanol-water binary mixtures with methanol molar fraction ranging from 16% to 82% over a mass velocity range of 160–510 kg/m²s and at an inlet temperature of 30 ^oC.  The testing results show 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 decrease with increasing methanol molar fraction.  Flow boiling heat transfer coefficient decrease with increasing methanol molar fraction for a fixed vapor quality. A new correlation of the flow boiling heat transfer coefficient was proposed based on the present methanol-water mixture data. An assessment of experimental pressure drops with different methanol molar fractions across the micro-channel test section was conducted. Two-phase pressure drop across the micro-channels increases appreciably once flow boiling occurred in micro-channels.  The methanol-water binary mixtures offers better two-phase flow stability than the pure water with less inlet pressure fluctuation in micro-channels.

The project allows the PI to further advance the frontier of the field of micro-scale thermal/fluid transport processes.  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.  Results from this research enable the PI to secure a research grant from the U.S. National Science Foundation (NSF) to further explore the complex transport phenomena.  Detail of the NSF grant is provided below.

Investigation of Binary Mixture Flow Boiling in Micro-Scale Structures 

Weilin Qu (PI)

National Science Foundation (NSF)

Award Number: CBET-1034242

Amount: $299,984.00

Project period: 08/01/10 - 07/31/13

The project positively influences graduate student education through research activities.  The research is an integral part of Chun Ka Kwok’s master program.  Chun was 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.  Chun eventually landed an engineering job in Schlumberger.