Bubble dynamics in microchannel flow boiling

Microchannel flow boiling is a very promising solution to the thermal management problem facing the microprocessor industry. In macroscale systems, such as nuclear reactors, flow boiling has been successful in removing large amounts of power. Unfortunately, microscale flows deviate from conventional theory. Large fluctuations in pressure, temperature, and flow distribution are generated by single bubbles due to channel confinement.; Difficulties instrumenting microchannels for experimental studies require many standard parameters to be inferred from external sensors. Using a novel experimental design, the dominant forces in bubble departure and convection in high aspect ratio channels are identified from scaling analyses. Form drag due to increased liquid velocities surrounding the bubbles opposes surface tension during departure, while the convection velocity is determined by the balance of this form drag and viscous shear in thin liquid films between the bubble and the walls. Film thickness has a significant effect on convection velocity and determines whether the bubbles move faster or slower than the superficial liquid velocity.; Since the earliest studies in microchannel flow boiling, researchers have believed there are fundamental differences in the bubble dynamics from larger channels. Modeling efforts failed to identify these differences. Conventional cavitation and water hammer theories are leveraged to characterize confined bubble growth. Single bubbles under practical thermal loads create water hammers inside the channel. Propagating pulses create local depressions in the static liquid pressure causing bubble nucleation at lower wall temperatures. Channel confinement inhibits bubble growth as a result of the additional water hammer pressure, thereby inhibiting heat transfer in nucleate boiling. Based on this model, a non-dimensional number is proposed for channel classification.; Major contributions to microchannel flow boiling will come from the development of new metrologies. Non-invasive techniques to measure local liquid pressure, flow rate, temperature, and void fraction need to be developed to validate models of the physical interactions. Fluorescent thermometry is extended to microscale two-phase flows using a two-dye/two-color technique. This metrology provides an optical method to measure local liquid temperature and liquid fraction. Arrays of these sensors will also allow bubble size, frequency, velocity and perhaps growth rate to be measured.