| Summary: | 碩士 === 國立中興大學 === 機械工程學系 === 91 === In this study, convective boiling and two-phase flow phenomena in T shaped microchannels was experimentally studied. Microchannels with cross sections of 600umX40um, 200umX40um, 500umX40um, and 400umX40um were fabricated by standard wet etching on a silicon wafer and anodic bonding. The length of inlet and outlet channel were 15mm and 30mm, respectively. An electric heating strip with area of 6.4mmX6.4mm was placed near the entrance of the microchannel to generate the phase change inside the channel. Deionized water and ethyl alcohol were employed as the working fluids. The flow rate was set in the range of 0.1 to 0.3ml/min and controlled by a constant flow rate syringe pump. A CCD camera was integrated with an axial lighting source and a microscope for visualizing the boiling and two-phase flow from the top of the channel. Pressure drops and bottom wall temperature variation along the channel were also measured by pressure transducers and thermocouples, respectively.
The results showed that bubble shapes and flow pattern depend strongly on the heating rate. At low heating power, both water and alcohol microchannel flows were in single phase, pressure drop between inlet and exit of the microchannel decreases with the increasing heating power because of decrease in fluid viscosity. Beginning with the heating rate of 5W, bubble formation was first observed for the water flow. At this heating rate, we were able to observe bubble coalescence phenomena and vapor condensation on the ceiling of channel to form small droplets. When the heating rate was increased beyond 6W, a large bubble slug was formed inside the channel. Depending on the heating power, the upstream front of the vapor slug was found to fluctuate inside the channel with certain frequency. The upstream front of vapor slug can move as far as into the inlet channel when heating power is large. At the downstream front of the vapor slug, a periodic bubble cutting process, induced by thickening of liquid film was found. Spectrum analyses were performed on the inlet pressure signal in order to explore the physical insight of flow patterns.
Because of the differences in the thermophysical properties, two-phase flow patterns found for the alcohol flow were somewhat different from those of water flow. Bubble activities started when the heating rate was increased to 3W. Similar to the water flow, large vapor slug can also be found inside the microchannel for heating power in the range of 3.5W to 5W. Beyond this heating power, annular flow was found although heating is supplied only locally.
|
|---|
