Condensation heat transfer in microchannels has important applications in space exploration as well as in terrestrial applications such as in micro heat pipes and micro fuel cells. In this thesis, a visualization study together with temperature measurements was carried out to investigate the condensation of steam in an array of 10 parallel microchannels. The microchannels were fabricated on a silicon wafer with trapezoidal cross-section having a hydraulic diameter of 82.9μm. Pyrex glass wafer was bonded on top of the silicon wafer for visualization. Superheated steam at 3.45x10⁵Pa (50psi) generated from boiler was directed to flow through these microchannels. To enhance the condensation process along the microchannels, the bottom surface of the silicon surface was cooled by forced flow...[ Read more ]

Condensation heat transfer in microchannels has important applications in space exploration as well as in terrestrial applications such as in micro heat pipes and micro fuel cells. In this thesis, a visualization study together with temperature measurements was carried out to investigate the condensation of steam in an array of 10 parallel microchannels. The microchannels were fabricated on a silicon wafer with trapezoidal cross-section having a hydraulic diameter of 82.9μm. Pyrex glass wafer was bonded on top of the silicon wafer for visualization. Superheated steam at 3.45x10⁵Pa (50psi) generated from boiler was directed to flow through these microchannels. To enhance the condensation process along the microchannels, the bottom surface of the silicon surface was cooled by forced flow of water at 8°C. Four experimental runs with average mass flux maintained at 47.49g/cm²s, 31.98g/cm²s, 24.31g/cm²s and 19.33g/cm²s respectively were performed. As the mass flux was reduced, condensation flow patterns were observed to change in three stages: from a fully dropwise flow to dropwise condensation- injection-plug flow, and finally a totally plug flow. Among these variations of flow pattern, injection flow was first observed in this experiment, and the position of this injection was found to be shifting toward the inlet as the mass flux was reduced. This injection flow pattern was classified as two-phase flow instability by the superposition of condensation film thickness and Kelvin-Helmholtz Instability. Local temperature variations along the microchannels at five different positions were also measured by thermocouples located at the bottom of the silicon wafer. It was found that temperature fluctuation became large due to the change in flow patterns.

Keywords: Condensation, microchannel, steam, injection flow, Kelvin-Helmholtz Instability