Tingting Hao

Flow patterns and transition characteristics for steam condensation in silicon microchannels

This study investigated the two-phase flow patterns and transition characteristics for steam condensation in silicon microchannels with different cross-sectional geometries. Novel experimental techniques were developed to determine the local heat transfer rate and steam quality by testing the temperature profile of a copper cooler. Flow regime maps for different microchannels during condensation were established in terms of steam mass flux and steam quality. Meanwhile, the correlation for the flow pattern transition was obtained using different geometrical and dimensionless parameters for steam condensation in microchannels. To better understand the flow mechanisms in microchannels, the condensation flow patterns, such as annular flow, droplet flow, injection flow and intermittent flow, were captured and analyzed. The local heat transfer rate showed the nonlinear variations along the axial direction during condensation. The experimental results indicate that the flow patterns and transition characteristics strongly depend on the geometries of microchannels. With the increasing steam mass flux and steam quality, the annular/droplet flow expands and spans over a larger region in the microchannels; otherwise the intermittent flow occupies the microchannels. The dimensionless fitting data also reveal that the effect of surface tension and vapor inertia dominates gravity and viscous force at the specified flow pattern transitional position.

Experimental Study on Steam Condensation in Horizontal Microchannels

Experiment was conducted to study steam condensation in horizontal microchannel etched on silicon wafer and bonded by a Pyrex glass plate from the top. The trapezoidal microchannel had a top width of 7943.11μm and depth of 81.77μm with hydraulic diameter of 161.49μm. The experiments were performed at different steam mass fluxes ranged from 128 kg m−2 s−1 to 320 kg m−2 s−1 , while the inlet temperature of cooling water was fixed at 30°C or 50°C. Characteristics of condensation heat transfer and pressure drop in microchannels were measured and discussed. The temperature of Pyrex glass and local steam quality were also tested. It is found that the heat transfer flux, heat transfer coefficient and pressure drop depend greatly on steam mass flux and cooling water temperature.© 2009 ASME

Effects of Surface Wetting Characteristics on Start-Up Performance of an Oscillating Heat Pipe

A simultaneous visualization and heat performance of oscillating heat pipes (OHPs) were performed. Experiments were performed under different surface wetting characteristics. Results showed that the start-up performance was improved on hydrophilic OHP as opposed to the copper OHP. A small bubble grew quickly and became a vapor plug in the evaporation section with hydrophilic surface. The process of vapor expansion and contraction accompanying liquid slug movement upward and backward continued to occur as a spring, and the OHPs started up. However, the hydrophobic OHP failed to start up. For the superhydrophobic OHP, nucleate boiling took place in the evaporation section, and the bubble expansion and contraction phenomenon were not observed. Heat transfer results showed that wall temperature fluctuations were observed at the start-up stage. The start-up time for the hydrophilic OHP was lowest and the amplitudes of temperature oscillations were increased in hydrophilic OHP compared to the copper OHP.Copyright