Study of Transient Phase Change Heat Transfer in Thermal Microfluidic Systems

Microscale phase change heat transfer exists extensively in different kinds of microfluidic systems such as microreactor, microfuelcell, microevaporator, microcondenser, micro heat pipe, microbubble actuator. It is important to study the microscale phase change phenomena in microfluidic systems in order to design and operate these systems well. In the present study, three typical microscale phase change phenomena including microscale flow boiling, microscale flow condensation and microbubble dynamics on a microheater were studied at length. All the test sections used in present study were fabricated by standard MEMS technology, which ensures the smooth surface instead of rough surface by the common mechanical fabrication process. Considering the extensive usage of microchannels with complicated structures, a microfluidic chip consisting of intercrossed microchannels was designed and fabricated to investigate the microscale flow boiling phenomenon using acetone as the working fluid. It was found that all the microchannels repeated the flow patterns in a timescale of milliseconds, with three substages: single phase liquid refilling stage, transient stratified flow stage and partial dry-out stage. In a separated microchannel zone, the liquid film behaved a non-uniform distribution along the flow direction due to the large evaporation momentum force and dry-out always took place from the upstream to the downstream. Stratified flow was observed inside the microchannels due to the low liquid Froude numbers. The temperatures on the backside surface of the chip could almost keep synchronous with those on the inner wall surface in that the Biot number was small. Although the IR camera was not fast enough to catch up with the transient flow patterns, it could still figure out the temperature fluctuations during the periodical flow pattern transitions. A single channel micro condenser with a low aspect ratio was fabricated in order to acquire the dynamic flow behaviors during water steam condensation. Two different cooling rates were used: cooling by air natural convention and cooling by chilled water with a temperature of zero centigrade. It was found that the flow patterns inside the microchannel were also cyclic with the cycle period of milliseconds. There existed a quasi-stable elongated bubble slug attached at the channel inlet that pinched off into a train of bubbles at the upstream. Enhancing the cooling rate would result in large frequencies of the bubble emission and small diameters of the detached bubbles. The periodical bubble emission from the upstream elongated bubble slug was due to the large Weber number at the vapor/liquid interface. The nearly stable bubble diameter during the bubble moved toward the outlet was because the resident time for the bubble moving inside the microchannel was only one percent of the total time when the bubble was fully condensed. A microcondenser with three parallel microchannels was designed and fabricated in order to clear up the multichannel effect on microscale flow condensation. It was found that the flow patterns in all the three micochannles were similar to those in the single channel microcondenser, i.e. there were nearly stable elongated bubble slugs at the upstream and bubble trains in the downstream. Three different modes of vapor bubble detachment were identified: single vapor thread break-up mode, dual vapor threads synchronous break-up mode and dual vapor threads asynchronous break-up mode. The three parallel microchannels had different temperature distributions due to the heat conduction of silicon substrate. In the center channel, the temperature distribution was symmetric against its centerline, while for the side channels, the high temperature zones deviated toward the center channel. The temperature difference on the whole microcondenser was small due to the good heat conduction performance of silicon, but this small temperature difference could still lead to large temperature gradient along the channel width direction. Under this condition, the dual vapor threads in the center channel was always symmetric against the centerline of the channel and broken up synchronously; the dual vapor threads in the side channels were always deviated toward the center channel and broken up firstly from the side adjacent to the center channel. The marangoni convection effect induced by the temperature gradient in the channel width direction made the bubble moving toward the zones with high temperatures after its detachment from the elongated bubble slug. During the investigation of microbubble dynamics on a microbubble actuator, a thin Platinum film microheater of was fabricated. It was found that the boiling incipience on this Platinum film microheater reached the superheat limit, indicating the homogeneous nucleation. In a wide range of the pulse heating parameters, there existed three different type of bubble patterns :( 1) explosive boiling and quick shrinking at boiling incipience, followed by the bubble regrowth and recondensation; (2)explosive boiling, bubble break-up, attraction and coalescence ;(3)periodical oscillation of bubble size followed by continuous growth and obtaining a stable diameter eventually. In the first type bubble patterns, the heat stored inside the glass substrate during the heating process played an important role in the bubble regrowth. For the second type of bubble patterns, the marangoni convection effect led to the bubble attraction and coalescence. In the third type of bubble patterns, the bubble diameter reached stable in that the vapor mass generated inside the vapor bubble was balanced by the vapor mass condensed on the vapor/liquid interface. The results of this paper will provide a scientific guidance for the design and operation of the microfluidic systems with phase change phenomenon such as microreactor, microfuelcell, micro heat exchanger, microbubble actuator.